Modules for Introducing Organometallic Reactions: A Bridge ... · Modules for Introducing Organometallic Reactions: A Bridge between Organic and Inorganic Chemistry Supporting Information

SI-1

Modules for Introducing Organometallic Reactions: A Bridge

between Organic and Inorganic Chemistry

Supporting Information

Chris P. Schaller,* Kate J. Graham, Brian J. Johnson

Department of Chemistry, College of Saint Benedict and Saint John’s University, Saint Joseph,

MN, 56374, USA

TABLE OF CONTENTS

Organometallic Topics within a traditional curriculum 2

Organometallic Topics within CSB/SJU Courses 3

Modules:

Coordination Compounds: Structure and Geometry 4

Coordination Compounds: Electron Counting 7

Coordination Compounds: Isomerism 13

Reaction Mechanisms: Lewis Acid/Lewis Base 19

Reaction Mechanisms: Ligand Exchange 23

Reaction Mechanisms: Insertions/Eliminations 27

Reaction Mechanisms: Oxidative Addition/Reductive Elimination 39

Catalytic Cycles 43

Olefin Metathesis 58

Sample Quizzes

Geometry, Ligands & e- count 64

Insertion and Elimination 66

Catalytic Cycles 69

SI-2

ORGANOTRANSITION METAL CHEMISTRY WITHIN A TRADITIONAL CURRICULUM

General Chemistry 1

• Matter & Measurement

• Atoms, Molecules and Ions

• Stoichiometry

• Aqueous Reactions

• Thermochemistry

• Electronic Structure of Atoms

• Periodic Properties of the Elements

• Basic Concepts of Bonding

• Molecular Geometry

• Coordination Compounds: Geometry and Bonding

• Gases

• Intermolecular Forces, Liquids and Solids

General Chemistry 2 (or Gen Chem 1 “Atoms First”)

• Modern Materials

• Properties of Solutions

• Chemical Kinetics

• Chemical Equilibrium

• Acid-Base Equilibria

• Additional Aspects of Aqueous Equilibria

• Chemistry of the Environment

• Chemical Thermodynamics

• Electrochemistry

• Nuclear Chemistry

• Metals and Metallurgy

• Coordination Compounds: Electron Counting

Organic Chemistry 1

• Reactivity: Acid-Base Chemistry

• Coordination Compounds: Lewis Acid Base Reactions

• Structural Topics – Bonding and Conformation of Hydrocarbons

• Stereochemistry

• Coordination Compounds: Isomers

• Association/Dissociation Mechanisms

• Substitution and Elimination Mechanisms

• Redox reactions (organic and inorganic)

• Oxidative Addition and Reductive Elimination

• Electrophilic Addition

• Introduction to Polymers (Cationic Polymerizations)

Organic Chemistry 2

• Spectroscopy

• Conjugated Systems

• Aromaticity

• Electrophilic Aromatic Substitution

• Nucleophilic Additions to Carbonyls

• Nucleophilic Substitutions to Acid Derivatives

• Organometallic Insertion/Elimination Reactions

• Organometallic Catalytic Cycles

• Radicals

• Radical Polymerizations

• Pericyclic Reactions

• Rearrangements

• Olefin Metathesis

SI-3

ORGANOTRANSITION METAL CHEMISTRY WITHIN CSB/SJU FOUNDATIONAL COURSES

Chem 125: Structure and Properties

• Atomic Structure and Periodic Trends

• Metallic Structure and Properties

• Ionic Structure and Properties

• Molecular Structure: Bonding and Geometry

• Molecular Structure: Isomers and Stereochemistry

• Intermolecular Forces

• Biological Structures and Biopolymers

• Network Solids

• Coordination Compounds: Electron Counting, Geometry and Isomers

• Molecular Bonding revisited: Molecular Orbitals

• Reactivity: Acid-Base Chemistry

Chem 250: Reactivity 1

• Nucleophilic Additions to Carbonyls

• Nucleophilic Substitutions to Acid Derivatives

• Introduction to Thermodynamics of Reactions

• Ligand Field Theory

• Organometallic Insertion/Elimination Reactions

• Conjugate Additions

• Enzyme Catalysis

• Glycolysis, TCA Cycle and Fatty Acid Biosynthesis

• Thermodynamics of Pathways

• Enzyme Regulation

• Introduction to Condensation Polymers


• Introduction to Kinetics & Collision Theory

• Association/Dissociation Mechanisms

• Substitution and Elimination Mechanisms

• Substitution in Synthesis

• Oxidative Addition and Reductive Elimination

• Organometallic Catalytic Cycles

• Enzyme Kinetics

• Electrophilic Addition

• Electrophilic Aromatic Substitution

• Cationic Polymerizations

• Eyring Plots


• Redox reactions

• Reduction Potentials & Cell Potentials

• Biological Redox: Binding & Reduction of O2 and N2

• Radicals

• Radical Polymerizations

• Oxidative Phosphorylation

• Atmospheric Chemistry

• Photochemistry (inorganic and organic compounds)

• Pericyclic Reactions

• Rearrangements

• Olefin Metathesis

SI-4

COORDINATION COMPOUNDS: BONDING AND GEOMETRY

Coordination Compounds: Structures of Transition Metals

In a coordination compound, a central metal ion is attached to a group of surrounding

molecules or ions (called ligands).

Practice Predicting Geometry and Drawing Coordination Complexes

Conventions for Representing Coordination Complexes:

◊ The coordination complex is inside the brackets.

◊ A ligand that binds to the metal is inside the parentheses.

◊ In front of the brackets is a cation. Behind the brackets are negative anions.

Cations and anions float nearby but do not affect the coordination number.

• What would the coordination number be for these different compounds?

• Draw each of these the compounds in the predicted geometry using wedges and

dashes.

g. [Cu(NCCH3)4]BF4 h. K3[Fe(CN)6]

SI-5

Ligand Binding

A dative bond (also called a coordinate covalent bond) is a bond where one species

provides both electrons for the covalent bond.

◊ A ligand donates at least one pair of electrons to the metal.

◊ Ligands may be anions or neutral.

• Draw structures of these neutral ligands and circle the lone pair that will be donated:

H2O (aqua) NH3 (ammine) CO (carbonyl)

(pyridine, py) PPh3(triphenylphosphine)

• Draw structures of these anionic ligands and circle the lone pair that will be donated:

Cl- (chloride) CH3- (methyl) OH- (hydroxide) H- (hydride)

CH2CHCH2- (allyl) PhO- (phenoxide)

SI-6

Polydentate Ligands

Denticity is the number of donor atoms per ligand. If there is only one donor

atom the ligand is monodentate (meaning “one – toothed”).

Polydentate ligands are also known as chelating agents. These agents bond to

metal ions and “trap” them as very stable complexes.

• What would bidentate imply?

• What would tridentate imply?

• What would tetradentate imply?

• What would polydentate imply?

• Ethylenediamine (H2NCH2CH2NH2 , often abbreviated as en) is a common

ligand. What are the donor atoms on this ligand? How many times will this ligand

bind?

• Draw Ni(en)(H2O)4+2

• Draw Mg+2 complexed with propylenediamine (H2NCH2CH2CH2NH2). What

shape will this complex adopt?

• Hydrazine (NH2NH2) has two e- pairs but is NOT bidentate. Explain why.

• Determine how many times each of these ligands might bind a metal:

o Label the donor atoms:

SI-7

COORDINATION COMPOUNDS: ELECTRON COUNTING

Electron Counting on Coordination Complexes

Electron count on a metal in a complex can be accomplished as follows:

1. Decide whether any anionic ligands are present (just cover up the metal, assume the donor atom in the ligand has an octet, and determine whether that atom has a formal charge). If so, adjust the charge on the metal atom to keep the overall charge balanced.

2. Count the number of electrons on the metal, given its oxidation state (charge on the metal).

3. Count the number of electrons donated by ligands. 4. Total the electrons.

A worked example: (PPh3)4Pd

• Draw this compound.

• Disconnect the ligands assuming the pair in the dative bond remains with the

ligand.

o Calculate the charge on each ligand.

• Determine the charge on the metal.

• Fill in the following table:

Charge on the ligands: _______

Valence electrons on Metal: 10

Charge on the Metal: 0

Revised Count on the Metal (accounting for charge): 10

Number of electrons donated from the ligands: ______

Two per dative bond to the metal

Total electrons in this complex: ______

SI-8

A more complicated example: K[Ru(CN)4(NH3)2]

• Draw this compound (remember the

complex is inside the brackets).

• Disconnect the ligands assuming the pair in the dative bond remains with the

ligand.

o Calculate the charge on each ligand.

NH3 ________

CN ________

Total charge on ligands: -4

• Determine the charge on the metal.

Charge on complex = charge on metal + charge on ligands

-1 = ________ + (-4)

• Fill in the following table:

Valence electrons on Metal: _______


Charge on the Metal: _______

Revised Count on the Metal (accounting for charge):______


Two per dative bond to the metal


SI-9

• Count the electrons on the metals in these complexes.

[Mo(CO)6]




Revised Count on the Metal (accounting for charge): ______



[Ni(CO)4]







[Co(NH3)6] (Cl)3







SI-10

[(PPh3)2Rh(CO)Cl]







K3[Fe(C2O4)3] (hint: bidentate oxalate ligands)







[Ni(en)2(Cl)2]







[Re(CO)5(PF3)]Cl







SI-11

[(PPh3)3RhCl] (Wilkinson's catalyst)







[(NH3)2PtCl2] (cis-platin)







[Pd(en)2(NO2)2](PF6)2







[Co(NH3)3Cl3]







SI-12

18 Electron Rule

Although the majority of metal complexes do not satisfy the 18-electron rule, the rule

predicts formulas for many organometallic complexes of the Cr, Mn, Fe, and Co triads.

• Compounds that obey the 18 VE rule are typically "exchange inert." Complexes

with fewer than 18 valence electrons tend to show enhanced reactivity. Explain.

Complexes with bulky ligands often do not complete the 18 e- configuration.

• Draw these complexes in the correct geometry and count the electrons.

Co(norbornyl)4 Pt(PtBu3)2 ((CH3)3CCH2)3TaCl2

Note:

• What prevents these compounds from binding to more ligands and achieving a

full valence shell?

SI-13

COORDINATION COMPOUNDS: ISOMERISM

Isomers of Coordination Complexes

Linkage isomers occur when there is a choice between connecting the metal to one

atom or another atom in the same ligand.

• Draw the two best Lewis structures for SCN - (same connectivity) and assign

formal charges to each atom.

• In SCN -, which atoms are most likely to donate a pair of electrons. Explain your reasoning.

• Show two pictures of SCN- binding to a metal (M).

Geometric Isomers in Coordination Complexes

We have looked at cis and trans before in the context of carbon compounds.

• Define cis and trans:

This same type of relationship can also be applied to transition metal complexes.

• Draw the two geometric isomers of the square planar complex Pt(NH3)2Cl2

• Which structure would be considered cis and which one trans? Why did you

label them as such?

• Draw the two geometric isomers of the octahedral complex [Co(NH3)4Cl2].

• Which structure would be considered cis and which one trans? Why did you

label them as such?

SI-14

Another diastereoisomeric relationship is fac and mer. These occur when there are

three identical ligands

• Draw the geometric isomers of the octahedral complex [Co(NH3)3Cl3].

• How would you describe the relationship between the two (i.e. what is different

between the two)?

SI-15

Stereoisomers

• Review: Define enantiomer.

Square Planar vs Tetrahedral

Compare the two mirror images of a metal complex in four coordinate shapes --

tetrahedral and square planar.

• Label the tetrahedral pair and the square planar pair.

• Are these pairs of enantiomers? Or pairs of identical complexes?

• If achiral, show a plane of symmetry.

SI-16

Octahedral Stereoisomers

• Will any of these pairs of octahedral complexes be chiral?

• Draw a plane of symmetry through any complex that is achiral.

SI-17

Additional Practice with Types of Isomers

• Label the type of isomeric relationship between these compounds (fac/mer,

cis/trans, enantiomers, linkage or identical)

SI-18

Finding all the isomers in Octahedral Transition Metal Complexes

To determine the number of possible isomers (geometric and optical) for an octahedral

transition metal complex using the “trans pair” naming system method.

See Structure and Reactivity, SC18 for more detail: See Structure and Reactivity, SC18

for more detail:

http://employees.csbsju.edu/cschaller/Principles%20Chem/stereochem/stereo_enantio

mersOh.htm

1. Draw 1 isomer with the ligands arranged in any order. o Remember bidentate ligands can only occupy cis sites.

2. Write out the 3 pairs of trans ligands.

3. Switch a pair of cis ligands and again write out the 3 pairs of trans ligands. o If the 2 isomers do not have the same pairs of trans ligands then they are

geometric isomers of one another.

4. Examine each geometric isomer for an internal plane of symmetry (or draw mirror

image). o If there is an internal plane, or its mirror image is superimposable, then it does

not have an enantiomer.

o If there is no internal plane of symmetry, or its mirror image is non-

superimposable, then it does have an enantiomer.

• Determine the number of geometric isomers for [Cr(H2O)3(OH)2Cl].

• Which of these isomers have enantiomers?

SI-19

REACTION MECHANISMS: LEWIS ACID/LEWIS BASE REACTIONS

Definitions and Introduction to Lewis Acids and Bases

Lewis Acid: electron acceptor

Lewis Base: electron pair donor

Lewis Acid/Base Complexes (Dative bonds)

• What structural feature would be required for a Lewis Base?

• What structural feature(s) would be required for a Lewis Acid?

• Which of these compounds is most likely to behave as a Lewis acid?

a) BH3 b) NH3 c) CH4 d) H2O

• Which of the central atoms is most likely to behave as a Lewis base?

a) BH3 b) BeH2 c) CH4 d) H2O

• For the molecules below: o Add lone pairs of electrons o Circle those that are Lewis acids o Underline those that are Lewis Bases o Explain your choices

SI-20

Strengths of Lewis Acids

• Rank the following Lewis acids in order of increasing acidity and explain your ranking:

Fe+3 Fe+2 Fe+1

• Same question, new molecules:


MO of Lewis Acid-Lewis Base Reactions

• Draw an MO diagram showing the overlap of these two orbitals resulting in the sigma

and sigma* orbitals on the product.

SI-21

Curved Arrow Notation for Lewis Acid-Lewis Base Reactions

Rules for Arrows:

Arrows always begin at the source of electrons (nucleophile or Lewis Base). Arrows always end at the electrophilic (electron deficient or Lewis Acid).

This accounting of bonds formed and broken is called a reaction mechanism.

• Predict the products for the following Lewis Acid-Lewis Base Reactions.

• Use curved arrows to show the mechanism of the following reactions).

• Label the Nucleophile (Lewis Base) and the Electrophile (Lewis Acid) in

the starting materials.

SI-22

Two Step Lewis Acid-Lewis Base Reactions

Sometimes there are two arrows for these Lewis acid:Lewis base reactions.

• Use curved arrows to show the mechanism of the following reactions.

• Propose a reason why these reactions need a second arrow.

SI-23

REACTION MECHANISMS: LIGAND EXCHANGES

Association / Dissociation

One ligand in a transition metal

complex is replaced by another ligand.

• Draw the possible mechanisms for the ligand substitution.

Associative:

Dissociative:

• Label the two mechanisms above with molecularity.

Challenge: • Draw a third possible mechanism for the ligand substitution where the bond-making and

bond-breaking steps occur simultaneously. This is called Ia (Associative Interchange).

• What would the reaction potential diagram look like?

• What would the molecularity for this reaction be?

Factors Affecting Ligand Substitution Mechanism Type

Pd

PPh3

Ph3P

Cl

Cl

NaSH

Pd

PPh3

Ph3P

Cl

SH NaCl

SI-24

Transition metal complexes have flexible geometry / coordination environment, so

predicting a mechanism can be difficult. However, there are some general

observations.

1. Electron count

The order of steps may be influenced by electron saturation at the metal.

• Explain how the electron count in an 18-electron complex may influence the order of

steps in a ligand substitution.

• Explain how things may be different in a 16-electron complex.

• Draw square planar (NH3)2PtCl2.

• Count the electrons on the metal in the complex.

• Replace one of the amines with PH3. Show a mechanism with arrows.

2. Electron Filling

Complexes containing metals such as Cr+3 or Mo+3 are often resistant to association.

• Provide an octahedral ligand splitting diagram for Cr(acac)3 (acac is a bidentate anionic ligand).

• Using the splitting diagram, explain why this complex may not be willing to bind another

ligand.

SI-25

3. Geometry

The order of steps may be influenced by the geometry at the metal.

• Four coordinate complexes of Ni2+ and Pt2+ often have two different geometries. What is

the geometry of each? Why?

• Which of these two metal ions might be more likely to undergo substitution via

association followed by dissociation: (NH3)2NiCl2 or (NH3)2PtCl2? Why?

4. Sterics

Order of steps may also be influenced by crowding at the metal.

• What is the electron count of the metal in the tetrahedral complex, (PPh3)4Pd?

• Replace a triphenylphosphine with a tricyclohexylphosphine. Show a mechanism with

arrows.

5. Metal Lability

The order of steps may be influenced by the lability of the metal.

• If a metal is labile, the complex would be more likely to undergo an ( associative / dissociative ) mechanism.

• Which of these two metal ions might be more likely to undergo substitution via

association followed by dissociation: L6V2+ or high spin L6Fe2+? Show why using a d

orbital splitting diagram.

SI-26

6. Ligand Lability

The order of steps may be influenced by the coordinating ability of the ligand.

• If a ligand is labile, the complex would be more likely to undergo an

( associative / dissociative ) mechanism.

• Compare the coordinating ability of water to carbon monoxide.

• Which of these two ligands might be more likely to undergo substitution via dissociation

followed by association?

7. Jahn-Teller

Jahn-Teller distortions can lead to increased lability and, hence increased ligand

exchange, in some complexes.

• Explain why [Cr(H2O)6]2+ undergoes substitution more easily than [Cr(H2O)6]

3+.

SI-27

REACTION MECHANISMS: INSERTIONS/ELIMINATIONS

Carbonyl Binding and Migratory Insertion

Binding CO to transition metal

• Draw the Lewis structure for CO.

• Show, with mechanism arrows, CO binding to a nickel atom.

• What type of ligand (σ only, π acceptor or π donor) is CO?

CO binding usually involves two events:

a) donation of lone pair electrons to an empty metal p orbital (σ bond)

b) donation of metal d electrons to a pi* orbital (π bond)

• Sketch these two orbital interactions as two separate drawings.

• Label the type of bond formed in each picture.

• Explain what happens to the π−bond when the CO binds.

• Is this consistent with your above drawing of the bound CO? Draw a new Lewis structure to support the MO picture of bonding.

Ni(CO)4 absorbs near 2057 cm-1.

C-O absorbs around 1250

C=O absorbs around 1700

C=O absorbs around 2150

• When CO is bound to the metal, approximately what is the Bond Order?

1 1.5 2 2.5 3 • Why?

SI-28

Migratory Insertion or 1,1-Insertion

Review: A more familiar reaction is addition to carbonyl.

• Draw the product.

• Identify the nucleophile and the electrophile in the above reaction.

A migratory insertion is related to the reaction above. Fill in the boxes below and follow

directions.

M

H C OM C

H

OMigratory Insertion

SI-29

Migratory insertion is the addition of an alkyl or hydride to a coordinated carbonyl.

• Draw mechanism arrows in the drawing below.

• Draw mechanism arrows and fill in the products of the following reactions.

Mn

CH3

CO

OC

OC

CO

CO

Cp

UCp

Cp

RR' N C

complex binds the isonitrile

insertion

SI-30

1,2-Insertion and ββββ-Elimination

– Insertion is the addition of a metal-hydride to a π bond. – Elimination is the reverse reaction, from right to left.

Insertion: Carbonyls

The following scheme describes the reaction of methanal with a metal hydride to give a

metal methoxide.

• Fill in the intermediate, the product and mechanism arrows.

β-Elimination: Carbonyls

• Show the reaction of methoxide ion with a metal cation to give methanal and a metal hydride (the reverse of the above reaction).

M

R H2C O R CH2

O

1,2 Insertion: Carbonyls

M

M

H

H2C OH CH2

O

β-Elimination

M

SI-31

Representations of Metal Alkene Binding

• Show mechanism arrows for the following step and show the product, with formal charges.

Sometimes, a bound alkene is shown this way, as a metallacyclopropane.

• Can you explain why using Lewis structures?

Alkene binding usually involves two events:

a) donation of pi electrons to an empty metal p orbital

b) donation of metal d electrons to a pi* orbital.

• Sketch these two orbital interactions as two separate drawings.

• Explain what happens to the pi bond when the alkene binds. How is this consistent with the above drawings of the bound alkene?

SI-32

Insertion: Alkenes:

• Alkenes are not usually electrophiles. Why is the alkene bound to a metal an electrophile?

• In the following drawing, complete the reaction by showing mechanism arrows for each step, showing the intermediate as a metallacyclopropane and filling in the product.

M

H H2C CH2 M CH2

CH2

1,2 Insertion: Alkenes

H

SI-33

Application Problems

1. Hydride transfer reduction

Catalytic reductions are ubiquitous in industrial reactions.

• Show how, in the presence of a weak amine base, benzaldehyde can be reduced to benzyl alcohol. This is hydrogen transfer reduction, and it takes place in alcohol solvent, such as isopropanol.

• Fill in the boxes in the following scheme.

H

O

cat. HRu(PPh3)4

Et3N(CH3)2CHOH

H

OH

H

Hydride Transfer

SI-34

Asymmetric 1,2-insertion

Review: Cation Stability (from acid-base chemistry)

• Rank the following Lewis acids in order of increasing stability and explain your ranking:


Cationic Intermediates in 1,2-insertion

• Draw the bound complex and the insertion product for the generic reaction below. Two different insertion products are possible. Show them. Is there a preferred product? Why or why not?

• Provide an explanation based on steric effects (suppose the M is Wilkinson's

catalyst, (PPh3)3RhCl):

• Provide an explanation based on electronic effects / ion stabilities:

R

R"

R'

M H

binding

alkene insertion

bindingalkene insertion

SI-35

Practice Problems

• Circle the more stable cation intermediate (consider resonance structures).

• Draw the insertion product.

• Is there a preferred product? Why or why not?

• Draw the bound complex and the insertion product.

• Is there a preferred product? Why or why not?

M H

OH

O

bindinginsertion

MH

OH

O

M H

OH

O

binding

Circle the more stable cation.

insertion

R

R"

R'

M H

binding

alkene insertion

SI-36

Hydroformylation can be performed using HCo(CO)4 as a catalyst. However, some

branching of the longer-chain aldehyde occurs.

To limit branching and encourage straight-chain aldehyde formation, promoters such as

Ph3P are sometimes added to the reaction.

• Explain, using the pictures of the cationic intermediates, how that change promotes formation of a straight-chain aldehyde.

SI-37

The Heck Reaction Mechanism

• Draw arrows where indicated.

• Fill in boxes with appropriate products.

• Provide a name for Complex A.

• Count the electrons on Complex B.




Electrons on Metal ion: _______

Ligand electrons donated: _______

Total electron count: _______

• Geometry of Complex B: Use LFT splitting diagrams, to explain why this complex is square planar NOT tetrahedral.

Heck Reaction

O

OMe cat. Pd(PPh3)2

Br O

OMe

Pd

BrPh3P

Ph3PPd

PPh3

Ph3P

Pd

BrPh3P

HPh3P

HBr

"oxidative addition"

provide arrows

"reductive elimination"

Br

O

OMe

Pd

Ph3P

BrPh3P

O

OMe

Pd

Ph3P

BrPh3P O

OMe

H

product!

"beta-elimination"

provide arrows

Complex AComplex B

Complex C

H

HH

SI-38

Application of The Heck Reaction

The alkaloid, huperzine A, is isolated from the clubmoss, Lycopodium. It has shown

potential as a treatment for Alzheimer’s disease which has heightened synthetic interest

in this family of natural products.

Bisai and Sarpong, “Methoxypyridines in the Synthesis of Lycopodium Alkaloids: Total Synthesis of

Lycoposerramine R”, Org. Lett., 2010, 12(11), 2551-2553.

• Fill in the blanks in the synthesis shown below.

• Don’t use the same reagent twice.

O

i-BuO

LDA,

NBr

Br

OMe

i-BuO O

N

Br

OMe

i-BuO OH

N

Br

OMe

O

N

Br

OMe

HClN OMe

O

N OMe

O

1. LDA2. MeI

H

N OMe

HO

Hmultiple steps

N OMeH

O

O

N OMeH

HN

Lycoposerramine R

SI-39

REACTION MECHANISMS: OXIDATIVE ADDITION/REDUCTIVE ELIMINATIONS

Oxidative Addition and Reductive Elimination

Oxidative addition is the addition of a bond to a metal center.

Reductive elimination is the reverse reaction.

• Why is the forward reaction called an oxidative addition?

• What is the oxidation state on the metal in the reactants and the products of this reaction?

Ni + Cl2 �

• Explain why this is an oxidation (remember: lose e- oxidize, gain reduce).

• Draw the reverse reaction.

• On the previous page, label the oxidative addition in the cycle.

• On the previous page, label the reductive elimination in the cycle.

SI-40

Oxidative Addition Mechanisms

There are two possible mechanisms.

1. The metal can donate a pair of electrons to one of the atoms, displacing the other atom as an ion. This ion recombines with the metal.

2. The addition can be concerted.

1. Stepwise Polar Addition

• This mechanism is really an example of an SN2 mechanism. BUT in this reaction, the metal is the nucleophile. That should seem very strange. Why?

• To do an SN2 on a substrate, what structural characteristics does the substrate need?

• Show the mechanism with arrows for addition of a) HCl and b) CH3CH2Br to nickel.

a.

b.

• Polar Oxidative Additions are much faster in polar solvents. Propose a reason for this

phenomenon.

• The addition of H2 to nickel does not follow this mechanism. Explain why.

SI-41

2. Concerted Oxidative Addition

Understanding how the alternative, concerted mechanism works (both atoms add at

the same time) may require some MO theory.

• Draw an occupied molecular orbital on hydrogen (H2) and its interaction with an

unoccupied p orbital on nickel.

• Draw an occupied metal d orbital and its interaction with an empty molecular orbital on

hydrogen (H2).

• Explain how these interactions combine to break the H-H bond.

• Draw a mechanism for this reaction using arrows. The mechanism should reflect what

you know about the MO situation.

SI-42

Practice Problems

• Choose the most likely mechanism for the following cases (one-step or two-step).

• Explain why you chose this mechanism for these cases.

SI-43

CATALYTIC CYCLES

Catalytic Cycles

Catalytic Cycle is a term for a multistep reaction mechanism that involves a catalyst.

Since catalysts are regenerated, catalytic cycles are usually written as a sequence of

chemical reactions in the form of a loop.

• Fill in the blanks on the cycle below:

HRh(CO)2(PPh3)2Precatalyst:

What is the charge (oxidation state) on the metal?

What is the d electron count on the metal?

What is the total count on the complex?

Is this complex likely to undergo association? be a catalyst?

CO

RhPh3P

OC PPH3

H

Charge:

e- count?

H2CCH2 Rh

Ph3P

PPh3

CO

H

Charge:

e- count?

Charge:

e- count?

Reaction Type:

CO

Charge:

e- count?

Reaction Type:

Charge:

e- count?

Charge:

e- count?

H2

RhOC

Ph3P PPh3

H

H

O

SI-44

Classic Organometallic Catalytic Cycles

DuPont Hydrocyanation (nylon synthesis)

• Count electrons for each metal complex in the cycle shown below.

• Label each step with the type of reaction (association, dissociation, migratory insertion,

1,2-insertion, β-elimination, oxidative addition, reductive elimination).

• Draw the arrows for each step.

• How many turns on the cycle to make the precursor for Nylon-6,6?

(PPh3)4Ni

DuPont Hydrocyanation

HCN

NCCN

R

NiH(CN)L2NiL4HCN

- 2 L

+ 2 L

HCN

NiL2

RCN

RCH2CH2Ni(CN)L2

R H

NiH(CN)L2

SI-45

Reppe Carbonylation (Very similar to DuPont Hydrocyanation)

• Fill in the missing metal complexes.



• Predict the product if ethanol (CH3CH2OH) is used rather than H2O.

HH CO H2ONi(CO)4 H

CO2H

H

H2O- 2 L

+ 2 L

H2O

H

CO2H

HH

H

Ni(CO)4

CO

1,2 insertion

1,1 insertion

SI-46

Monsanto Acetic Acid Process

Efforts to establish organic syntheses on the basis of C1 building blocks from coal

resources include the Monsanto Acetic Acid Process. This cycle involves

homogeneous catalysis: carbonylation of methanol.





• Fill in missing products.

CH3OH

CH3COOH

HI H2ORh

I

I CO

CO

-1

Rh

I

I CO

CO

-1CH3

I

Rh

I

I COCH3

CO

-1

I

COH3C

O

I

Monsanto Acetic Acid Process

CO

cis-K[Rh(CO)2I2], HI

CH3OHOH

O

SI-47

Sonogashira Coupling

Sonogashira (1975) developed a method to form a carbon-carbon bond between a

terminal alkyne and an aryl or vinyl halide.





• Label the cis-trans isomerization step.

• Why did the square planar Pd complex have to undergo isomerization for this cycle?

Br

(PPh3)4Pd, Et3N, CuI

Sonogashira Coupling

CH3

SI-48

Sonogashira Application

Sonogashira reaction also has a carbonylation variation. The following example shows

the synthesis of a pyridone in which the carbonylation and cyclization occur in tandem

(Kalinin, Tet. Lett., 1992, 33, 373).

• Propose a cycle for this process.

SI-49

Buchwald-Hartwig Coupling




Cl

(PPh3)4Pd, Et3N

Buchwald-Hartwig Coupling

NH2

HN

Na(O-t-Bu)

R1CH2NH2

NaBrt-BuOH

PdAr

L L

NHCH2R1

ArNHCH2R1

ArBr

Association

Oxidative Addition

Reductive Elimination

PdL

L L

L

- 2 L

SI-50

Stille Coupling




Br SnMe3

(PPh3)4Pd

Stille Coupling

R1X

PdX

R1 L

L

R2SnBu3SnBu3X

L

L

R2SnBu

Bu

Bu

Pd

L L

R1X

Pd

R2

R1

L

cis-trans isomerism

SI-51

Hydrogenation

In a hydrogenation reaction, two hydrogen atoms are added across the double bond of an alkene. Hydrogenation is used in the food industry to make a large variety of manufactured goods, like Crisco or margarine from liquid oils. Hydrogenation is also used in coal processing. Solid coal is converted to a liquid through the addition of hydrogen. Liquefying coal makes it available to be used as fuel. The catalyst can be a homogeneous catalyst or a heterogeneous catalyst. Homogeneous Catalytic Cycle:

• Fill in the blanks on this cycle using a Pd catalyst.

• Calculate charges on each metal complex.

• Label each step as: binding, reductive elimination, oxidative addition, or 1,2-insertion.

Alkene Hydrogenation

H2

Metal Catalyst

SI-52

The rate of reduction is partially based on the availability of the π-bond electrons and the stability of the alkene.

• Rank the following π-bonds in order of π-electron availability (1 = most reactive).

• Based on your answer to the above question, how could an alkyne be selectively hydrogenated to an alkene. (i.e. How strong of a catalyst would the Lindlar catalyst be?)

Strong Medium Weak

• What strength of a catalyst would be needed to reduce a tetrasubstituted alkene? Circle one.

Strong Medium Weak

SI-53

Application Problems 1. Compare the selectivity of these two catalysts.

• Which Catalyst is stronger?

Crabtree’s Catalyst OR Wilkinson’s Catalyst

• Draw both catalysts.

• Draw both catalysts after the H2 is added.

• Provide the geometry and electron count for each catalyst (before and after H2 addition).

• COD is a sacrificial ligand. What happens to the COD on Crabtree's catalyst with the hydride? Draw the product of this reaction. Now what is the geometry and electron count?

• Explain the difference in selectivity between the two hydrogenation catalysts.

SI-54

Heterogeneous Catalytic Hydrogenation

On solid metal catalysts, the accepted mechanism is the Horiuti-Polanyi mechanism.

Some of the most common metal catalysts are:

– Pt, Pd, and Ni

– These are usually placed on a highly dispersed powder support with large

surface area (Carbon or alumina)

• With this mechanism, explain the following results.

H H H H H2C CH2H2C CH2H H

H3C CH2H3C CH3 H

Catalyst Surface

SI-55

Hydrogenation Practice Problems

• Fill in the products (be sure to include stereochemistry).

Ph

H2, Pd/C

H2, Pd/CPh

Ph

H2(PPh3)3RhCl

H2[(COD)Ir(PCy3)py]PF6

H2, Pd/CH2

[(COD)Ir(PCy3)py]PF6

Ph

H2(PPh3)3RhCl

H2, Lindlar cat.

SI-56

Electrophilic Addition with Catalytic Cycles

Suzuki

First published in 1979 by Akira Suzuki, the Suzuki reaction couples boronic acids

(containing an organic substituent) to halides (2010 Nobel Prize). • Provide a mechanism for the reaction of catecholborane with the alkyne.

• Fill in the blanks on the following catalytic cycle for the Suzuki coupling.

B(OH)2 Br

(PPh3)4Pd

Suzuki Coupling

OTBS

Br

Ph3P Pd PPh3

Ph3P

PPh3

Ph3P Pd

PPh3

PPh3

PPh3

provide arrows

Ph3P Pd

Ph3P

PPh3

TBSO

Br

Na OHprovide arrows

Pd

Ph3P

PPh3

TBSO

Br

provide arrows

Pd

Ph3P

PPh3

TBSO

OH

PPh3

Pd

Ph3P

PPh3

TBSO

Br

OH

provide arrows

Br

Pd

Ph3P

PPh3

TBSO

O

B

X X

H

Pd

PPh3

TBSO

PPh3

PPh3

provide arrows

provide arrows

Pd

PPh3

TBSO

PPh3

Ph3P

BX2

Formation of nucleophilic borane complex

provide arrows

provide arrows

Mechanism Type:

Mechanism Type:

SI-57

Roadmap with Suzuki

White and Choi, (-)-Ibogamine Org. Lett., 2000, 2(15), 2373-2376.

• Fill in the boxes in the following synthesis of Ibogamine.

Br

OTBS

Pd(PPh3)4, NaOEt OTBS

+

O

O

TiCl4 (Lewis Acid), toluene

OH

OH

OTBS

H

H

Hint: No OH-, H2O2

H2/Pd

PDC, CH2Cl2

O

OTBS

H

HMeO OMe

N

OTBS

H

HMeO OMe

HO

OTBS

H

H

HN

OMeMeO

O pTsCl, Et3N, DMAP

N

MeO

MeO

O

Acetone, pTsOH

1. PhNHNH2, ACOH2. NaBH4, BF3-OEt2

N

O

NH

(-)-Ibogamine

O

BH

O

Bu4NF

SI-58

OLEFIN METATHESIS

Olefin Metathesis

In olefin (or alkene) metathesis, a C=C bond is split in half and reattached with

another partner.

One application of olefin metathesis is found in the Shell Higher Olefin Process

(SHOP), which supplies raw materials for making soaps (sodium lauryl sulfate, etc).

• Provide a mechanism for the oligomerization step.

• Describe what is happening in the metathesis step.

SHOP allows the statistical distribution of olefins produced in oligomerization to be

adjusted and optimized. The desired fractions are then subject to hydroformylation,

reduction, and sulfonation. • Suggest reagents to complete the preparation of sodium lauryl sulfate.

CH34

CH34

O CH34

HO

CH34

OO3S

Na

SI-59

Olefin Metathesis: Overall Reaction

Yves Chauvin, Dick Schrock and Bob Grubbs shared the 2005 Nobel Prize in Chemistry

for their work in this area.

• Show the products of the following olefin metatheses:

Chauvin was working as a bachelor’s-level chemist at the French Petroleum Institute in

the 1960’s. He was an avid reader of the chemical literature, and these two reports

caught his eye: • Fill in the products of the olefin metathesis reactions.

MLn

MLn

MLn

W(CO)6 / Al2O3

90-315 oC

WCl6 / AlEt3

Banks & Bailey, Phillips Petroleum Co., 1964

Giulio Natta, Politecnico di Milano, 1964 (Nobel Prize for polymer chemistry, 1963)

SI-60

Olefin Metathesis: Making Better Catalysts on the East Coast

By 1980, Schrock had moved from DuPont to MIT, where his lab was developing better

metathesis catalysts.

• Propose reagents for each step in the synthesis.

Compounds of this type (lower left) are effective metathesis catalysts. The analogous

molybdenum compounds are among the fastest metathesis catalysts; options are

available that offer excellent control over cis vs. trans bond formation.

Similar catalysts were developed for alkyne metathesis.

• Propose a synthesis of the catalyst from WCl6.

• Complete the following synthesis of the perfume, civetone.

W

Cl

MeO Cl

OMeMeO

ClW W

O

Cl

Cl

O

Cl

Me

Me

W

O

NH

Cl

O

Cl

Me

Me

iPr

iPr

W

O

N

Cl

O

Cl

Me

MeiPr

iPr

WNO

O iPr

iPr

O

WO O

O H2 / Pd / Pb(SO4)2

SI-61

Olefin Metathesis: Making Better Catalysts on the West Coast

Meanwhile, Bob Grubbs at CalTech sought metathesis catalysts that were more tolerant

of air, moisture and oxygen/nitrogen-containing functional groups. • Classify in terms of hard/soft acid/base

Tungsten Molybdenum Ruthenium Oxygen Nitrogen

• Explain briefly why unwanted hard/soft acid/base interactions may interfere with

catalysis.

The new catalyst was based on a precursor, below right, from Geoff Wilkinson’s lab at

Imperial College. Wilkinson shared Fischer’s 1973 Nobel Prize.

• What is the oxidation state of ruthenium (a) before and (b) after the reaction?

• What is the electron count on ruthenium in the brown dichloride complex? Show your

work.

• Triphenylphosphine is a new ligand in the reaction. What is its other role?

The first generation of catalysts from the Grubbs lab are stable to air and water. Their

ease of use quickly made them the preferred catalysts worldwide for routine

metatheses.

Olefin Metathesis: Application

SI-62

1. Professor Hoye and others (Organic Letters, 1999, 2, 277-279) were looking at

synthesizing a natural product (±)-Differolide (compound 1).

A) The first step is an olefin metathesis mechanism. It is postulated that it can

occur two different ways (“yne-then-ene” or “ene-then-yne”). Evidence for the

“ene-then-yne” mechanism is the production of an aromatic side product.

Propose a mechanism (catalytic cycle) for this “ene-then-yne” reaction. Hint:

Compound 4 is an initiator and two molecules of compound 3 are needed to

generate compound 2.

B) What is the structure of the aromatic side product that is produced from this

reaction?

C) Compound 4 is referred to as a carbene complex. Draw a resonance

structure of 4 and explain how it is a carbene.

SI-63

A) The last step in this mechanism to produce compound 1 is a dimerization of compound 2. Propose a mechanism for this last step.

B) Compound 3 can be synthesized as outlined below. Provide reagents for the steps.

O

O

3

HO

O

Cl

O

SI-64

SAMPLE QUIZ: GEOMETRY, LIGANDS AND ELECTRON COUNTS

Quiz: Coordination Compounds

Chem 125: Chemical Structure and Properties

1. Coordination Compounds

• (2 pts each) Make a perspective drawing for each of the following coordination compounds.

• (1 pts each) Provide the name of the geometry for each compound.

• (5 pts each) Count the valence electrons on each metal and the total electrons donated by the ligands in the tables provided.

a. Na[AuCl4] b. K2[Zn(CN)4] c. [Ni(ox)2(H2O)(NH3)]

a. Geometry:_______________ Valence electrons on Metal:_______ Charge on the ligands: _______ Charge on the Metal: _______ Electrons on Metal ion: _______ Ligand electrons donated: _______ Total electron count: _______

c. Geometry:_______________ Valence electrons on Metal:_______ Charge on the ligands: _______ Charge on the Metal: _______ Electrons on Metal ion: _______ Ligand electrons donated: _______ Total electron count: _______

b. Geometry:_______________ Valence electrons on Metal: _______ Charge on the ligands: _______ Charge on the Metal: _______ Electrons on Metal ion: _______ Ligand electrons donated: _______ Total electron count: _______

_______/24

65

2. Stereochemistry of Coordination Compounds

a. (4 pts) Draw the fac and mer geometric isomers of the octahedral

complex [Mn(NH3)3(F)3]. Label them.

b. (4 pts) Draw the two geometric isomers of the square planar complex

[Rh(Cl)2(PPh3)2]. Label them as cis and trans.

c. (2 pts) What is the relationship between these two compounds?

Enantiomers Diastereomers Identical Constitutional Isomers

/10

66

SAMPLE QUIZ: INSERTIONS AND ELIMINATIONS

Quiz: Insertions and Eliminations Chem 250: Reactivity 1

Synthesis of Methoxyfumimycin

Gross, Hartmann, Nieger and Brase, J. Org. Chem., 2010, 75, 229-232.

Due to bacterial resistance to common classes of antibiotics, a continuing

discovery of antibiotics with new modes of action is critical. One target of special

interest is the bacterial peptide deformylase (PDF). In the course of screening

for PDF inhibitors, fumimycin was isolated from cultures of the mildew Aspergillus

fumisynnematus F746.

• (2 pts) Fill in the boxes with appropriate reagents or products.

______/10

O

MeO OTBS

N

CO2Et

P

O

Ph

PhO

MeO OTBS

HN

CO2Et

P

O

Ph

PhBu4NF O

MeO OH

HN

CO2Et

P

O

Ph

Ph

Base

HO

MeO

HNP

O

Ph

Ph

OO

HO

MeO

HNP

O

Ph

Ph

O

O

RhCl3

EtOH

HO

MeO

NH2

OO

HO

MeO

HN

OO

O

O

OtBu

HO

MeO

HN

OO

O

O

OH

Fumimycin

67

In one of the key steps of the synthesis of fumimycin, the alkene is isomerized with a rhodium catalyst.

The rhodium trichloride reacts with the EtOH to form the following catalyst.

• (2 pts) What is the geometry of the rhodium in this complex?

Geometry: _________________

• (4 pts) What is the total electron count on Rh in this complex?




Revised Count on the Metal (accounting for charge): _______

Number of electrons donated from the ligands: _______

Total electrons in this complex: _______

• (3 pts) Draw a d orbital splitting diagram (see Appendix).

• (2 pts) Will this complex be

High Spin OR Low Spin

______/11

68

In one of the key steps of the synthesis of fumimycin, the alkene is isomerized with a rhodium catalyst.

The rhodium trichloride reacts with the EtOH to form the following catalyst.

• (6 pts) In this isomerization reaction, propose a mechanism for how this catalyst effects this reaction.

• (3 pts) Label each of the steps of your mechanism with the

appropriate type of mechanism (migratory insertion, 1,2-insertion, β-elimination, association, dissociation).

• (2 pts) Considering structure and thermodynamics, why is this the final product?

______/11

69

SAMPLE QUIZ: CATALYTIC CYCLES

Quiz: Catalytic Cycles Chem 251: Reactivity 2

Hydrosilylation with Biscarbene Rhodium Complexes

Gigler, Bechlars, Herrmann and Kühn, J. Am. Chem. Soc., 2011, 133, 1589-1596.

Rhodium complexes are commonly used catalysts for the hydrosilylation of ketones. Only a few mechanistic studies on rhodium-based hydrosilylation reactions have been published. Ojima proposed a mechanism shown below.

• (2 pts each) Label the mechanistic steps with the type of reaction

(association, dissociation, migratory insertion, 1,2-insertion, β-elimination,

oxidative addition, reductive elimination).

• (2 pts) Fill in the blank box for the product.

• (2 pts each) Provide the charge and e- count for each of the metal

complexes:

o Complex 1: charge ______ e- count___________




_______/18

RhPh3P

Ph3PCl

R3SiH

Mechanistic Step A: ______________ Mechanistic Step B:

______________

Mechanistic Step D:______________

RhPh3P

Ph3P Cl

H

SiR3

1

2

3

4

O

RhPh3P

Ph3P Cl

H

SiR3

O

Mechanistic Step C:______________

RhPh3P

Ph3P Cl

H

O

SiR3

70

A different mechanism was proposed by Zheng and Chan where the ketone is thought to coordinate to the silicon atom rather than the Rh.

• (6 pts) Propose a catalytic cycle for the Zheng mechanism.

• (1 pt) In the Ojima mechanism, when is the C-H bond formed? 3 � 4 OR 4 � 1

• (1 pt) In contrast, when is the C-H bond formed in the Zheng mechanism? 3 � 4 OR 4 � 1

_______/8

RhPh3P

Ph3PCl

R2SiH2

1

2

3

4

O

Documents

Modules for Introducing Organometallic Reactions: A Bridge ... · Modules for Introducing Organometallic Reactions: A Bridge between Organic and Inorganic Chemistry Supporting Information