University of Groningen Monodentate secondary phosphine ... · PDF fileand nitrile hydrolysis ... Chlorobenzene is normally not active in cross-coupling reactions ... isolated yields

University of Groningen

Monodentate secondary phosphine oxides (SPO's), synthesis and application in asymmetriccatalysisJiang, Xiaobin

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Chapter 2

Chapter 2

Secondary phosphine oxides (SPO’s) and nitrile hydrolysis

The first part of this chapter gives an introduction to the synthesis and properties of secondary phosphine oxides (SPO’s), including their use as ligands in homogeneous catalysis. The second part describes methods of nitrile hydrolysis. The original aim of the work, the transition metal catalyzed hydrolysis of nitriles using enantiopure SPO’s as ligands and the outline of the thesis is introduced. Contents 2.1 Introduction to secondary phosphine oxides (SPO’s) 18

2.2 Examples of the use of SPO’s as ligands in catalysis 19

2.2.1 Hydroformylation 20

2.2.2 Cross-coupling reaction 21

2.2.3 Asymmetric allylic substitution 21

2.2.4 Nitrile hydrolysis 22

2.3 Introduction to nitrile hydrolysis 22

2.3.1 Classical methods—strong acid or base 23

2.3.2 Enzymatic methods 27

2.3.3 Catalysis with transition metals 29

2.4 Aims and outline of this thesis 33

2.5 References and notes 33

17


2.1 Introduction to secondary phosphine oxides (SPO’s) The name phosphine oxide is derived from the words “phosphine” and “oxygen”, which means these compounds stem from phosphine compounds upon reacting with molecular oxygen. This type of compounds can be divided into two classes, depending on the number of substituents on the phosphorus atom. When there are three substituents, it is called a tertiary phosphine oxide and when there are two substituents, it is called a secondary phosphine oxide. It is necessary to point out that the two substituents on the phosphorus should be carbon. If the two substitutes are oxygen, the compounds are called phosphites and have different properties compared to the phosphine oxides (Figure 2.1). In this thesis, only secondary phosphine oxides (SPO’s) are discussed.

R1P

R2

O

R3

Tertiary phosphine oxide

R1P

R2

O

HSecondary phosphine oxide

R1OP

R2O

O

H

Phosphites

Figure 2.1 Structure of phosphine oxides Secondary phosphine oxides (SPO’s) are also called phosphinous acids or phosphinic acids. They have been known for many years.1 There is an acidic proton in these compounds; most of them are not very stable to strong basic conditions, which can lead to the formation of phosphines or phosphoric acids. They are generally stable towards air and moisture, thus easy to handle and purify. The compounds can be purified by vacuum distillation or flash column chromatography (SiO2). Most of them have very high boiling points due to the presence of hydrogen bonds. At temperatures above 180 oC, SPO’s decompose to the corresponding phosphoric acids and phosphines in a few minutes even under inert atmosphere (Scheme 2.1). At room temperature, this process is rather slow. It has been observed in our lab that storage after 1.5 year at RT * , 80% of t-BuPhPHO self-disproportionated to the corresponding phosphoric acid as shown in scheme 2.1. At 0 oC or lower temperatures, this process is quite slow. Thus pure SPO’s should be kept at low temperatures (below 0 oC at least).

P OHR

R'P

O

R'

R

HP H

R

R'P

O

R'

R

OH

2 2 +

Scheme 2.1 Self-disproportionation of SPO’s

There is equilibrium between the secondary phosphine oxide form (pentavalent) and the phosphinite form (trivalent) (Scheme 2.2). There is an energy barrier between these forms. * See appendix, RT = room temperature.

18

Chapter 2

For example, when R = t-Bu, R’ = Ph, the calculated energy barrier under vacuum is about 60 kcal/mol.3b At room temperature, the phosphine oxide form is the stable one. Upon coordination with metal precursors, the phosphinite form is exclusively present in the metal complexes.2

R1 PR2

OHR1

PR2

O

H

M R1 PR2

OH

M

Scheme 2.2 Tautomeric forms of SPO’s and their metal complexes

In principle, when R≠R’, the compound can be chiral, depending on the stability of the enantiomers. Most SPO’s are easily prepared in one or two steps, and are stable to air and water at RT. The two enantiomers of t-BuPhPHO are configurationally stable and can be separated by classical resolution (chiral acids)3 or a 3-steps resolution reported by Haynes and co-workers 4b (Figure 2.2). In the presence of metal precursors, the two enantiomers are still stable both in the solution and solid state. They do not racemize under these conditions.8 Only at high temperatures or strong basic conditions, they can racemize and decompose easily.

S-(-)

t-BuP

O

HPh

R-(+)

t-BuP

O

HPh

Figure 2.2 Enantiomers of t-BuPhPHO

Generally, SPO’s are used as important intermediates for the synthesis of phosphorus- containing compounds, such as phosphines and tertiary phosphine oxides (Scheme 2.3).4 In the sequence of reactions leading to these products, the acidic proton is first removed by a base (such as n-BuLi or LDA at –78 oC) to form anions followed by the addition of electrophiles to form tertiary phosphine oxides, followed by the reduction to the corresponding phosphine compounds. In this procedure, the chirality on phosphorus is fully retained, which means that no racemization happens. In this manner, many achiral and chiral phosphorus compounds could be synthesized efficiently.4

R1P

R2

O

H

R1P

R2

O -R1

PR2

O

R3

..R1P

R2 R3

ReductionR3XBase

Scheme 2.3 SPO’s as intermediates for the synthesis of phosphines

19


2.2 Examples of the use of SPO’s as ligands in catalysis The platinum and palladium complexes of SPO’s have been reported and have found some applications in catalytic reactions.2,5-9 The platinum complexes C4.15 and C4.2 were made from Pt(PPh3)4 with excess Me2PHO and Ph2PHO, respectively. The Pt(0) species was oxidized to Pt(II) by SPO’s in situ. The driving force of this reaction might be the poor solubility of these complexes in most organic solvents (Scheme 2.4). For details see chapter 4.

O PPt

POH

P

H

R R

R R

RR

OHPt(PPh3)4 PO

H

R

R

R=Me, C4.1R=Ph, C4.2

+Tol., RT

R=Me, L3.1R=Ph, L3.2

Scheme 2.4 The synthesis of platinum and SPO’s complexes The palladium SPO complexes C2.1, C2.2 and C2.3 were made from Pd(COD)Cl2 and (t-Bu)2PHO with different metal/ligand ratios in 1,4-dioxane (Scheme 2.5). The structures of these three complexes were elucidated by X-ray analysis. They have been used successfully in catalytic cross-coupling reactions. 7

1,4-dioxane

RT

PPd

Cl

ClPd

Pt-Bu

t-Bu

OH

OHt-Bu

t-BuCl

Cl

C2.3

P OH

t-Bu

t-Bu

P PdOH

t-Bu

t-Bu Cl

Cl

C2.2

PPd

P

Cl

ClPd

P

PO

t-But-Bu t-But-Bu

O

O

t-Bu t-Bu

O

t-Bu t-Bu

HH

C2.1

Pd(COD)Cl2

(t-Bu)2PHO

+

Scheme 2.5 The synthesis of palladium and (t-Bu)2PHO complexes All these complexes with SPO’s are stable towards air and moisture, thus easy to handle. They have proven to be effective catalysts in the following catalytic reactions. 2.2.1 Hydroformylation reaction6

Complexes based on platinum and Ph2POH (C4.2) or other SPO’s have been used as

20

Chapter 2

catalysts in hydroformylation reaction of terminal and internal alkenes in 1990. The products found were mainly linear aldehyde (>90%). The reaction intermediates were characterized by NMR and X-ray analysis. 2.2.2 Cross-coupling reaction7

Dimeric and monomeric palladium complexes C2.1-C2.3 with (t-Bu)2POH as ligand have been used in various C-C cross-coupling reactions such as Heck,7a, f Kumada,7b, c, e Suzuki, 7d, e Negishi7d and Stille7f reactions with chlorobenzene or substituted chlorobenzene as substrates (Scheme 2.6). Chlorobenzene is normally not active in cross-coupling reactions when palladium is used in combination with triarylphosphine ligands. Catalyst C2.1-C2.3 were also found to be active in various C-N7e, f and C-S7d, e, f bond formation reactions with chlorobenzene as substrate. The nucleophiles in all these reactions can be olefins,7a, f boronic acids,7d, e amines,7e, f thiols,7d, e, f organozinc 7d, tin7f and Grignard reagents7b, c, e. The isolated yields are quite good in most cases (approx. 80%). A sulfur analogue of SPO, (t-Bu)2PHS has also been used as ligand in the nickel- catalyzed Kumada reaction.7b

X

R'Nucleophiles

X = C, N, S

Cl

R R

+C2.1-C2.3

NaOt-Bu, toluene

Nucleophiles = amines, thiols, boronic acids, organic zinc, tin and Grignard reagents

Cl R'

R R

R'+C2.1-C2.3

NaOt-Bu, toluene

Scheme 2.6 Palladium SPO complex catalyzed cross-coupling reactions

2.2.3 Asymmetric allylic substitution Recently, Dai and co-workers 8 reported the palladium catalyzed asymmetric allylic substitution with enantiopure t-BuPhPHO as ligand (Scheme 2.9). The catalyst derived from [Pd(C3H5)Cl]2 induced the highest e.e. up to 80%. The use of NaOAc led to better e.e.’s compared to KOAc and LiOAc. The bridged dimeric palladium complex C2.4 was isolated and characterized by X-ray analysis.8

Pd(COD)Cl2 PO

HPh

PdCl

ClPd

Cl

Pt-Bu

OHCl

Pt-Bu

OH

Ph

Ph

C2.4

+

(R)-(+)-L3.4

21


OAcCOOMeMeOOC

*

[Pd(C3H5)Cl]2

e.e. up to 80%

3 eqs. of BSA

3 eqs. of dimethyl malonateCat. NaOAc

(R)-(+)-L3.4

5 mol%Pd/L = 1/2

Scheme 2.9 Palladium catalyzed asymmetric allylic substitution 2.2.4 Catalytic nitrile hydrolysis9 The platinum complexes of Me2POH and Ph2PHO, C4.1 and C4.2, have proven to be highly active catalysts in nitrile hydrolysis under neutral conditions. Details can be found in the following paragraph 2.3.3. 2.3 Introduction to nitrile hydrolysis The hydrolysis of nitriles to amides and carboxylic acids are very important transformations in organic chemistry.10 Many industrial examples are known, such as the hydrolysis of amino nitriles to amino acids,11 acrylonitrile to acrylamide and acetone cyanohydrin to the corresponding amide,12 en route to methyl methacrylate. Only using specific conditions, it is possible to stop the hydrolysis at the amide stage. Frequently used methods for nitrile hydrolysis to amides use strong acid (96% H2SO4)13 or base (50% KOH /t-BuOH).14 However, in general, selective hydrolysis of nitriles to amides is troublesome and yields are reasonable at best due to two reasons: (1) It is difficult to stop the hydrolysis at the amide stage and further hydrolysis to the carboxylic acid often takes place, as the rate constant of amide hydrolysis is usually larger than that for nitrile hydrolysis, especially under dilute acidic or basic conditions. However, in concentrated acid or base the relationship is reversed (Scheme 2.10).15

R CN RNH2

OR

OH

O

slow fast+ H2O

Scheme 2.10 Nitrile hydrolysis (2) Since the nitrile group is not very reactive, harsh conditions using strong acids or bases at high temperatures are generally required, which precludes the presence of acid or base sensitive functional groups. In addition to strong base or acid, enzymes and transition metal catalysts are also used to convert a variety of nitriles to amides. With enzymes, asymmetric hydrolysis or dynamic kinetic resolution of nitriles is possible. Below these three different are reviewed.

22

Chapter 2

2.3.1. Classical methods --- Strong acid or base This is one of the most frequently used general methods present in the literature. In order to activate nitrile groups, strong inorganic acid/alkaline base and high temperatures are normally needed. Yields range from poor to good depending on the substrates and conditions. As stoichiometric amounts of acid or base are used, large amounts of salts are formed after work-up, which is neither economic nor environmental friendly. As mentioned above, chemoselective hydrolysis to amides is a problem. Using concentrated acid or base like 96% H2SO4 or 50% KOH / t-BuOH, it is possible to control the nitrile hydrolysis to amides selectively to some extent. Nitriles vary greatly in their ease of hydrolysis; sterically hindered nitriles are extremely difficult to hydrolyze. Some examples are presented below: (1) Basic conditions: (a) 50% KOH in t-BuOH or other alcoholic solutions14a

With 50% KOH in boiling t-BuOH solution, some nitriles can be hydrolyzed to amides selectively. The scope of substrates is rather broad and yields range from 54% to 90% (R= phenyl, benzyl, t-butyl) with long reaction times (up to 20 h) (Scheme 2.11). Nitriles can also be converted to substituted amides via a tandem reaction. The nitriles are first hydrolyzed to primary amides and subsequently alkylated with an alkyl halide to provide the substituted amide. In this case, the alkyl-substituents can be R1 = benzyl, n-Pr; R2 = Me, n-Pr. The yields range from 52% to 78% (Scheme 2.11).14b

RCN RCONH2 RO

NHR2R2X, 4 h

KOH

t-BuOH1h

Scheme 2.11 Nitrile hydrolysis with 50% KOH in t-BuOH

In these procedures, 50% KOH has to be used in refluxing t-BuOH, which excludes the presence of most other functional groups. NaOH also can be used as base in this reaction. Other alcohols such as ethanol, 2-propanol, n-BuOH could also be used as solvents. (b) H2O2 in alkali solution16

H2O2 in dilute aqueous alkali can also be used to hydrolyze nitriles to the amides and finally to the acids.17 This procedure can be stopped at the amide stage by using different alkaline conditions with H2O2 in aqueous ethanol.16a Compared to method a (50% KOH in refluxing t-BuOH), the reaction conditions are much milder. In this case, R can be c-hexyl, c-pentyl, PhCH2CH2, PhCH=CH, p-NO2-C6H4-CMe2 with yields exceeding 80% (Scheme 2.12). The effective species is thought to be HO2

-, which is a much stronger nucleophile than hydroxide ion (α-effect nucleophile).

23


This reaction can also be performed in DCM with the help of a phase transfer catalyst (n-Bu4NHSO4) and excess H2O2. In this case, R can be phenyl, Ph(CH2)3, c-hexyl, c-pentyl, (E)-PhCH=CHCH2 with yields exceeding 80% (Scheme 2.12).18

RCN RCONH2

NaOH, aq. EtOH

H2O2 (2 eq.), 50oC

RCN RCONH2

n-Bu4NHSO4 / CH2Cl2

20% aq. HO- / excess H2O2

Scheme 2.12 Nitrile hydrolysis with H2O2 and alkali More recently, a simple and versatile procedure was reported by Katritzky and coworkers19 who used a combination of 30% H2O2 / K2CO3 / DMSO at 0 oC to obtain high yields of pure amides. This is one of the most effective methods reported in the literature. The substituted group R can be phenyl, 4-ClC6H4, 2-pyridyl, 2-MeOC6H4CH2, n-octyl, PhCH2CH2, 4-MeO2CC6H4, etc. The reaction is very fast (5-30 min) and high yields are obtained in the range of 65% to 99% (Scheme 2.13). However, we found that this procedure fails for sterically hindered nitriles like tertiary nitriles.

RCN RCONH2

30% H2O2, 0 oC

DMSO / K2CO3

Scheme 2.13 The hydrolysis of nitriles to amides with H2O2/K2CO3 in DMSO

A method based on a combination of urea / H2O2 / K2CO3 was also reported providing reasonable yields of amides (50-60%).20

(2) Acidic conditions: (a) H2SO4 or other inorganic acids This is an often-used method to hydrolyze nitriles to amides. Aqueous H2SO4 (75%),21 aqueous H2SO4 (70%) in acetic acid22 or 96% H2SO4

13 at temperatures of 60-150 oC can be used to hydrolyze nitriles to amides effectively without significant further hydrolysis to the acids. A long reaction time is normally required to give reasonable yields. Quite large amounts of side products are often found in this reaction due to the harsh conditions. After basic work-up, amides are obtained with yields in the range from poor (18%) to excellent (90%) depending on the substrate and conditions (Scheme 2.14). Other strong acids like HCl can also be used. Some sterically hindered nitriles such as tertiary nitriles can be hydrolyzed to the corresponding amides with this method. Even formamide can be prepared in this way from HCN. In the presence of a tertiary alcohol like t-BuOH or the related olefins (2-methyl-1-propene), mono-substituted amides are produced (Ritter reaction).23

24

Chapter 2

R S

CN

R S

NH2

O

RCNR

O

NH2

R

O

NH

H2O+96% H2SO4

t-BuOH

Thiocyanate

Thiocarbamate

Scheme 2.14 Nitriles to amides with 96% H2SO4

Thiocyanates can be viewed as a special kind of nitrile with a sulfur atom attached to the cyano group. These compounds can be hydrolyzed to the corresponding thiocarbamate by concentrated H2SO4 in high yields.24 For example, when R is phenyl, the isolated yield of the thiocarbamate is 87%. In the presence of alcohols (MeOH, EtOH, etc), mono N-alkylated thiocarbamates are obtained in high yields similar as in the Ritter reaction (Scheme 2.14). (b) Formic acid or formic acid and HCl (HBr) Formic acid can hydrolyze nitriles to amides at high temperature (180-250 oC) in a silver (Ag) or tantalum (Ta) vessel. The disadvantages of this reaction are high temperature and the use of the expensive vessels. The reason why good yields of amides are only obtained in tantalum or silver vessels is not clear. R can be c-hexyl, 3-Cl-C6H4, 2-Me-C6H3, 1-naphthyl with yields exceeding 87% (Scheme 2.15).25 With the combination of formic acid and HCl or HBr, the reaction temperature can be as low as 40 oC and there is no need for a silver or tantalum vessel. R can be c-hexyl, phenyl, 4-Me-C6H3, benzyl, 2-OH-C3H6 with yields exceeding 85% (Scheme 2.15).26

RCN RCONH2

HCO2H, 180-250oC

Ta or Ag vessel, 2 h

RCN RCONH2

HCO2H / HCl

40oC

Scheme 2.15 Nitrile hydrolysis to amides with formic acids (c) TiCl4 and glacial acetic acid27

This method was developed for the hydrolysis of carbohydrate derived nitriles. The reaction is performed at 0 oC with the yields approx. 80%. R are different carbohydrate moieties. No epimerization was found on the stereogenic centers of the sugar units (Scheme 2.16).

25


RCN RCONH2

TiCl4

Glacial acetic acid, 0oC

Scheme 2.16 Nitrile hydrolysis to amides with TiCl4 (3) Miscellaneous (a) BF3*Et2O It was found that some specific tertiary nitriles can be hydrolyzed with BF3*Et2O in methanol under mild conditions. No racemization was found with respect to the stereogenic center when using enantiopure nitriles. Yields are approx. 70%, although the scope of this procedure has not been established yet (Scheme 2.17).28

Sn-Bu

O

Tol

CN

Sn-Bu

O

Tol

ONH2BF3.OEt2

MeOH, 78%

Scheme 2.17 Nitrile hydrolysis with BF3*Et2O

(b) Aqueous borax (Na2B4O7)29

The hydrolysis of aldehyde cyanohydrins to α-hydroxyamides can be achieved by treatment with aqueous borax or alkaline borates at 80 oC (Scheme 2.18). The reaction is finished in 1-8 h under basic conditions (pH 9-11).

R

OH

CNR

OH

NH2

Oaq. Na2B4O7 (borax)

80 oC

Scheme 2.18 Cyanohydrin hydrolysis with aqueous borax

Results are shown in table 2.1. In some cases, the yields could be increased by the addition of KCN.

Table 2.1 Results of α-cyanohydrin hydrolysis with aq. borium reagents

Borate R Yield (%) NaB(OH)4 H 39

Borax (Na2B4O7) n-hexyl 67 Borax Me 72

KB(OH)4 Me 75 Borax Ph 73 Borax i-Pr 86 Borax MeSCH2CH2 79

26

Chapter 2

(c) MnO2 on SiO230

Some nitriles can be hydrolyzed to amides with MnO2 on a SiO2 support, which is prepared by treating SiO2 with Mn(SO4)2 and aqueous KMnO4. The reaction is performed in refluxing n-octane for 48 h and isolated yields range from 35% (R = benzyl) to 83% (R = t-Bu). (d) Active Al2O3 A combination of active Al2O3 and KF in refluxing t-BuOH can be used to hydrolyze some nitriles to amides. Yields range from 10% to 98% depending on the substrate.31 Another combination of active Al2O3 and CF3SO3H can also be applied in this process at 60 oC with yields in the range of 18%-92%.32 For example, when R is t-Bu, the isolated yield is 87% (Scheme 2.19).

RCN RCONH2

KF / Al 2O3 / t-BuOH

reflux

RCN RCONH2

Al2O3 / CF 3SO 3H

60oC to 100 oC

Scheme 2.19 Nitrile hydrolysis to amides with active Al2O3 (e) Silane reagents Some silyl reagents can hydrolyze nitriles to amides. For example, when t-BuCN (R= t-Bu) is treated with potassium trimethylsilanoate in refluxing toluene for 16 h, trimethyl acetamide is isolated in 32% yield as a side-product.33 With the use of dimethylphenyl silane lithium salt, following by the addition of 3 M aq. HCl, some nitriles can be hydrolyzed to the amides with the yields ranging from 10% to 95%.34

2.3.2. Enzymatic methods35

Enzymes are known that can convert nitriles to amides or acids in an asymmetric 35, 39-45 or non-asymmetric 36 way depending on the substrates and enzymes. The enzymes used in the hydrolysis of nitriles are called “nitrile hydrolases”. They can be divided into two classes. The first one, nitrile hydratases (NHase) catalyzes the hydrolysis of a nitrile to the corresponding amide, which is then converted by an amidase to the acid and ammonia. The second class is called “nitrilases”; it catalyzes the direct hydrolysis of a nitrile to the corresponding acid and ammonia (Scheme 2.20).

R CN R COOH+2H2O

NitrilaseNH3+

R CN R CONH2 R COOH NH3++H2O

Nitrile hydratase (NHase)

+H2O

Amidase

Scheme 2.20 Two pathways for nitrile hydrolysis by enzymes

27


By the use of nitrile hydratases, some non-chiral aliphatic nitriles like acrylonitrile and hetero-aromatic nitriles with base or acid sensitive groups can be successfully hydrolyzed to the corresponding amides.36 Two examples of industrial processes based on the use of nitrile hydratases illustrate the successful application in the hydrolysis of nitriles to the corresponding amides. The first is the hydrolysis of acrylonitrile to acrylamide (Nitto)37 and the second the hydrolysis of 3-cyanopyridine to nicotinic acid (Lonza).37, 38 Other examples are the dynamic kinetic resolution of racemic nitriles such as 2.1 (with pseudomonas putida 5B)39 and 2.2 (with agrobacterium tumefaciens d3)40 to enantiopure amides 2.3 and 2.4, respectively, in approx. 30-40% conversions with e.e. > 90% (Scheme 2.21).

R'

R

CN

R'

R

O NH2

H2O

pseudomonas putida 5B oragrobacterium tumefaciens d3

R=Cl, R’= i-Pr, 2.1 2.3 R=H, R’= Et, 2.2 2.4 Scheme 2.21 Kinetic resolution of nitriles with enzymes

Next to the examples mentioned above, the hydration of achiral di-nitriles such as 2.5 to 2.6 has proven a particularly attractive strategy for obtaining chiral synthons (Scheme 2.22).41

Enantioselectivities are normally very high (up to >99%).

R R

CN

CN R'

CONH2

R' = CN, COOHrhodococcus sp. CGMCC0497

H2O

2.5 2.6

Scheme 2.22 Asymmetric hydrolysis of di-nitriles

Frequently used enzymes in the asymmetric hydrolysis or dynamic kinetic resolution of nitriles are obtained from rhodococcus rhodochrous IFO15564 Nhase;41a-b rhodococcus sp. N-771;42 rhodococcus sp. R-312;43 pseudomonas putida 5B; agrobacterium tumefaciens d3; bacillus pallidus Dac521; 44 rhodococcus sp. DSM11397; 45 rhodococcus sp. CGMCC0497 41f etc. Among these enzymes, bacillus pallidus Dac521 is a special one. It is thermostable and only works with aliphatic nitriles like acetonitrile and acrylonitrile, whereas for cyclic, hydroxy nitriles, di-nitriles or aromatic nitriles, no conversions are found. Thiocyanate hydrolase, isolated from thiobacillus thioparus THI115, can catalyze the hydrolysis of thiocyanate to thiocarbamate and finally to carbonyl sulfide (Scheme 2.23).46 However, it does not work with other nitriles such as acetonitrile and propionitrile.

28

Chapter 2

S C OS CN- +H2O

-NH3

SO

OH

-SO

NH2

- +H2O

-OH -

+H2O

Scheme 2.23 The hydrolysis of thiocyanate with thiocyanate hydrolase In spite of many successful examples summarized above, there are still some disadvantages associated with enzyme- catalyzed nitrile hydrolysis. Most enzymes are quite sensitive to the structural variation of nitriles and reaction conditions. Slow reactions and low conversion are also problems associated with enzymatic methods, whereas sometimes reproducibility is a problem. 2.3.3. Catalysis with transition metals (1) Heterogeneous catalysts47

(a) “Reduced Cu”48

Nitriles can be hydrolyzed to amides under neutral conditions by the use of a black copper powder prepared from CuSO4 and NaBH4 in aqueous NaOH solution. Yields are in the range of 50-95% depending on the substrates. The active catalyst is a reduced Cu(0) species. The structure of this catalyst is unknown. (b) Zeolites and metal oxides49

NaY zeolite can catalyze the hydrolysis of nitriles to amides. It is found that only aromatic nitriles can be hydrolyzed, whereas aliphatic nitriles do not react. Yields are not high (30-40%).50 A mechanistic study of this transformation has been performed using Zn2+ zeolite.51 Some metal oxides can also catalyze this transformation. For example, MnO2, CuO, Co3O4 are found to be active and selective for the hydrolysis of acrylonitrile to acrylamide, although yields (<20%) are too low for any synthetic application. 49

(c) Supported metals It is found that some metals like Cu, Ni, Ag, Pd supported on poly (4-vinylpyridine), zeolite X, zeolite Y, SiO2, Celite, TiO2, γ-Al2O3, ZnO or a MgO surface can catalyze the hydrolysis of nitriles, such as acrylonitrile to acrylamide.52 Among them, the combination of Cu on MgO surface is the most active catalyst. However, compared to other methods, the yields are rather low (20-50%). (2) Homogeneous catalysts53

Homogeneous catalysts have also been used in the hydrolysis of nitriles to amides and

29


proved to be much more effective and selective than heterogeneous catalysts. For an efficient homogeneous catalyst there are three basic requirements: 1. There should be site (s) that can accommodate the substrates and bring them in close proximity to the metal center. 2. Bond formation between the substrate(s) and catalyst can occur. 3. The newly formed complexes between the products and the catalyst can be released so that the catalytic cycle can be repeated. Some catalysts are not effective for nitrile hydrolysis, due to strong binding of the amides to the catalysts, which makes it difficult for the product to be released, thus blocking the catalytic cycle.54

Below are summarized successful catalysts used in the hydrolysis of nitriles. (a) Rh(I) complexes55 The rhodium complex trans-[Rh(OH)(CO)(PPh3)2] and the complex made in situ from [Rh(COD)Cl]2 and TPPTS56 have been used as catalysts in the hydrolysis of nitriles to amides. (b) Cu(II),60a-b Ni(II),60a-b Zn(II),60a-b, 57 Fe(II),60a Ru(III)59g, Rh(III)59g , Ir(III)58 & Co(III)

complexes Some nitriles with additional coordination sites such as 2-cyanopyridine, 2-cyano-1,10- phenanthroline can be hydrolyzed to amides using Cu(II),60a-b Ni(II),60a-b Zn(II)60a-b, 57 and Co(III),60e-f, 59 Ru(III),60g Ir(III) and Rh(III) complexes under basic conditions.60 (c) Ru(II)-complex [Ru(II)(tpy)(bpy)](PF6)2 was used to hydrolyze benzyl nitrile to benzyl amide under basic conditions (tpy = 2,2': 6',2"-terpyridine). The active species is thought to be [Ru(tpy)(bpy) (OH)]+.61 Another Ru(II) catalyst, [RuH2(PPh3)4] 62was reported for the preparation of N- alkylated amides from nitriles and amines at 160 oC in DME. [(NH3)5RuCl]Cl2 63 can be used stoichiometrically in the hydrolysis of some nitriles to amides. (d) [MeCp)2Mo(OH)(H2O)]+ complex64

A new molybdenum-based catalyst for nitrile hydrolysis has been developed recently, which shows quite high activities in the hydrolysis of a variety of simple nitriles and nitriles with functional groups. (e) Pd (II) complexes PdCl2 can be used stiochiometrically to hydrolyze nitriles to amides.65 Pd (II) complexes such as cis-[PdL(H2O)2]2+ (L = En, Met-OMe, dtcol, dien, H2O); 66 [PdCl(OH)(bipy) (H2O)]; 67 a combination of [K2PdCl4] / bipy / NaOH68 and a di-nuclear palladium complex,

30

Chapter 2

[C23H29N4O2SPd2] 69 have been used for the hydrolysis of nitriles to amides under basic conditions. The structures of these ligands are depicted in figure 2.3. Most of the Pd-complexes have been used in kinetic studies of the nitrile hydrolysis process.

NH2

NH

NH2

NH2

NH2

SNPd

O

N PdN

N

O

En dien [C23H29N4O2SPd2]

NH

NH

NH

NH

S

NH2

CH3OOC

S

SOHP

O

H

phospholane (C4H8POH) Met-OMe dtcol cyclen

Figure 2.3 Structure of some ligands and

a bis-Pd complex used in metal catalyzed hydrolysis En=ethane-1,2-diamine; Met-OMe=methionine methyl ester; dtcol=1,5-dithia-cyclo-octan -3-ol; dien=diethylenetriamine; cyclen=1,4,7,11-tetraazacyclododecane; C4H8POH= phospholane; bipy =2,2’-bipyridine; TPPTS = triphenylphosphine trisulfonate. (f) Pt (II) and Pt (0) complexes The platinum catalysts such as trans-[PtH(H2O)(PR3)2X] (R = Me, Et; X = OH, Cl) are able to hydrolyze simple nitriles to amides under basic conditions. 70 Other Pt(0) complexes such as [PtC6H8(PPh3)2],55a [Pt(PEt3)3], [Pt(PiPr3)3] and [PtP(c-C6H11)3] show much lower activities under similar conditions.71

A breakthrough in this field has been made by Parkins and co-workers who developed a Pt(II) complex, [PtH(PMe2OH)(PMe2O)2H] which can catalyze the hydrolysis of various nitriles to amides with high activities under neutral conditions. The applied secondary phosphine oxide ligands (SPO’s) are possibly involved in the hydrolysis reaction by intramolecular nucleophilic attack on the coordinated nitrile. 9

A comparison of most homogeneous catalysts for the hydrolysis of nitriles is listed in table 2.2.

31


Table 2.2 A comparison of homogeneous catalysts for nitrile hydrolysis a

Catalyst T (oC) TOF TON [PtH(PMe2OH)(PMe2O)2H] 9 90 380 5700 [PtH(PPh2OH)(PPh2O)2H] 9 90 23 369 [PtCl(PMe2OH)(PMe2O)2H] 9 90 488 1464 [PtCl(PPh2OH)(PPh2O)2H] 9 90 20 100 [PtCl(PMe2Ph)(PMe2O)2H] 9 90 186 558 [PtCl(C4H8POH)(C4H8PO)2H] 9 90 90 450 [Pt(PEt3)3] 80 2.7 54 Pt[P(c-C6H11)3] 80 26.7 405 trans-[PtH(H2O)(PMe3)2][OH] 25 21.5 n.r. trans-[PtH(H2O)(PMe3)2][OH] 78 178.4 6000 btrans-[PtH(H2O)(PEt3)2][OH] 78 69.9 6000 b[PtC6H8(PPh3)2] 80 16.7 58 [Pt(NHCOMe)Ph(PEt3)2]72 80 2.2 102 PdCl2 60 n.r. 1 c [C23H29N4O2SPd2] 80 n.r. 4000 bCis-[Pd(En)(H2O)2]2+ 66 40 n.r. 5 Cis-[Pd(Met-OMe)(H2O)2]2+ 40 n.r. 5 Cis-[Pd(dtcol)(H2O)2]2+ 66 40 n.r. 5 Cis-[Pd(dien)(H2O)2]2+ 66 40 n.r. 5 [Pd(H2O)4]2+ 66 40 n.r. 5 [PdCl(OH)(bipy)(H2O)] 76 29.4 294 [K2PdCl4] / bipy / NaOH 76 8.8 230 [Co(cyclen)(OH2)2]3+ 40 n.r. 10 trans-[Rh(OH)(PPh3)2(CO)] 80 50.0 150 [Rh(COD)Cl]2 / TPPTS / NaOH 80 295 934 [RuH2(PPh3)4] 160 n.r. 30 [(NH3)5RuCl]Cl2 80 n.r. 1 c[(MeCp)2Mo(OH)(H2O)]+ 64 80 4.8 114 Zn(NO3)2 + ketoxime 80 45 c 1000 dNaOH 78 0.4 1 c

(a). TOF (mol/mol of catalyst h), TON (mol/mol of catalyst), in most cases, the nitrile is acetonitrile, n.r. = not reported. (b). From kinetic experiments under basic condition, TON might be not accurate. (c). Stoichiometric amount. (d). For p-OCH3C6H4CH2CN. From this table it can be seen several catalysts are quite active, however, some of the data (TOF and TON) were obtained from kinetic studies and the selectivity is also a major problem for functionalized nitriles such as acrylonitrile. Most of the reactions were performed under basic conditions. It is clear from the table that platinum SPO’s complexes are the most effective and selective catalysts for the hydrolysis of nitriles to amides under neutral condition.

32

Chapter 2

2.4 Aims and outline of this thesis The main goals of the proposed research are the following:

(1) Explore the possibility to affect kinetic resolution of racemic nitriles via their hydrolysis using chiral platinum SPO complexes.

(2) Explore the possibility to deracemise meso-dinitriles using the same catalysts. (3) Explore other applications of chiral secondary phosphine oxides as ligands in

asymmetric homogeneous catalysis. Possible reactions are: a. Hydrogenation b. Hydroformylation c. Allylic substitution d. Epoxide ring-opening

The thesis is composed of the following chapters: Chapter 1 and 2 are introductory chapters, describing the state-of-the-art in asymmetric catalysis, asymmetric hydrogenation and methods of nitrile hydrolysis as well as an introduction to SPO ligands. Chapter 3 deals with the design and synthesis of racemic and enantiopure SPO’s. In chapter 4, the characterization of platinum SPO’s complexes with the aid of NMR and X-ray analysis is described. The hydrolysis of various nitriles catalyzed by platinum or palladium SPO complexes is described and a possible mechanism is discussed. Chapter 5 describes the applications of enantiopure SPO’s as ligands in iridium- catalyzed asymmetric imine hydrogenation. Studies on the optimization of several parameters such as solvents, metal precursors, hydrogen pressure, temperatures, additives, ligands and substrates are presented. In chapter 6, the application of SPO’s as ligands in the hydrogenation of α- or β-dehydroamino acids, N-acetyl enamides, itaconic acids and enol carbamate catalyzed by rhodium and iridium are described. Optimization of the reaction conditions for different substrates are also given. A study of applications in asymmetric allylic substitution is also described. Chapter 7 draws conclusion on the results of the research presented in this thesis. The future prospects of SPO’s are also discussed. 2.5 References and notes

1 For general information about secondary phosphine oxides (SPO’s), see: (a). Klein, H. J. in Methoden der Organischen Chemie (Houben-Weyl), 4 Auflage, Regitz, M. ed., Band E1, Georg Thieme Verlag, Stuttgart, 1982, p 240. (b). Sasse, K. in Methoden der Organischen Chemie (Houben-Weyl), 4 auflage, Muller, E. ed., Band XII/1, Teil 1, Georg Thieme Verlag, Stuttgart, 1963, p 193. (c). Gallagher, M. J. in The Chemistry of the Organophosphorus Compounds Vol II, Phosphine Oxides, Sulfides, Selenides and Tellurides, Hartley, F. R. Ed.,

33


Wiley, Chichester, 1992, p 53. 2 (a). Goerlich, J. R.; Fischer, A.; Jones, P. G.; Schmutzler, R. Z. Naturforsch., B: Chem. Sci. 1994, 49, 801. (b). Heuer, L.; Jones, P. G.; Schmutzler, R. New J. Chem. 1990, 14, 891. (c). Dixon, K. R.; Tattray, A. D. Can. J. Chem. 1971, 49, 3997. (d). Arif, A. M.; Bright, T. A.; Jones, R. A. J. Coord. Chem. 1987, 16, 45. 3 (a). Drabowicz, J.; Lyzwa, P.; Omelanczuk, J.; Pietrusiewicz, K. M.; Mikołajczyk, M. Tetrahedron: Asymm. 1999, 10, 2757. (b). Wang, F.; Polavarapu P. L. J. Org. Chem. 2000, 65, 7561. However, it was found later that this procedure was not reproducible by the same group. 4 (a). Pietrusiewicz, K. M.; Zablocka, M. Chem. Rev. 1994, 94, 1375. (b). Haynes, R. K.; Au-Yeung, T. L.; Chan, W. K.; Lam, W. L.; Li, Z. Y.; Yeung, L. L.; Chan, A. S. C.; Li, P.; Koen, M.; Mitchell, C. R.; Vonwiller, S. C. Eur. J. Org. Chem. 2000, 3205. (c). Haynes, R. K.; Lam, W. W.; Yeung, L. L. Tetrahedron Lett. 1996, 37, 4729. (d). Haynes, R. K.; Lam. W. W.; Williams, I. D.; Yeung, L. L. Chem. Eur. J. 1997, 3, 2052. (e). Kawashima, T.; Iwanga, H.; Okazaki, R. Chem. Lett. 1993, 1531. (f). Kawashima, T.; Iwanga, H.; Okazaki, R. Heteroatom Chem. 1995, 6, 235. 5 Beaulieu, W. B.; Rauchfuss, T. B.; Roundhill, D. M. Inorg. Chem. 1975, 14, 1732. 6 (a). Van Leeuwen, P. W. N. M.; Roobeek, C. F.; Wife, R. L.; Frijns, J. H. G. J. Chem. Soc., Chem. Commun. 1986, 31. (b). Van Leeuwen, P. W. N. M.; Roobeek, C. F.; Frijns, J. H. G.; Orpen, A. G. Organometallics 1990, 9, 1211. 7 (a). Li, G. Y.; Zheng, G.; Noonan, A. F. J. Org. Chem. 2001, 66, 8677. (b). Li, G. Y.; Marshall, W. J. Organometalics 2002, 21, 590. (c). Li, G. Y. J. Organomet. Chem. 2002, 653, 63. (d) Li, G. Y. J. Org. Chem. 2002, 67, 3643. (e). Li, G. Y.; Fagan, P. J.; Watson, P. L. Angew. Chem., Int. Ed. 2001, 40, 1106, 1153. (f). Wolf, C.; Lerebours, R. J. Org. Chem. 2003, 68, 7077. (g). Patent, Li, G. Y. US6124462, 2000; WO01040147, 2001; WO 01079213, 2001; WO02000574, 2002. 8 Dai, W. M.; Yeung, K. K. Y.; Leung, W. H.; Haynes, R. K. Tetrahedron: Asymm. 2003, 14, 2821. 9 (a). Ghaffar, T.; Parkins, A. W. Tetrahedron Lett. 1995, 36, 8657. (b). Ghaffar, T.; Parkins, A. W. J. Mol. Catal. A 2000, 160, 249. (c). Cobley, C. J.; van den Heuvel, M.; Abbadi, A.; de Vries, J. G. Tetrahedron Lett. 2000, 41, 2467. 10 For classical methods of nitrile hydrolysis, see: (a). Dopp, D.; Dopp, H. eds., Methoden der Organischen Chemie (Houben-Weyl), Thieme: Stuttgart, 1985; Vol. E5 (2), p 1024. (b). Brown, B. R. ed.; The Organic Chemistry of Aliphatic Nitrogen Compounds, Oxford University Press, Oxford, 1994; p 217; 342. 11 Mckenzie, B. F.; Kendall, E. C. in Gilman, H.; Blatt, A. H. eds., Org. Synth. Coll., Vol. 1, 1941, 21. 12 Bauer Jr., W. in Ullmann's Encyclopedia of Industrial Chemistry, 5th ed., Wiley, New York, Vol. A16, 1990, p 441. 13 (a). Li, L.; Lin, K. H.; Hung, Y. T.; Kang, S. A. J. Chin. Chem. Soc. 1942, 9, 1; 14; 31; Chem. Abstr. 1944, 335. (b). Westfahl, J. C.; Gresham, T. L. J. Am. Chem. Soc. 1955, 77,

34

Chapter 2

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35


32 Wilgus, C. P.; Downing, S.; Molitor, E.; Bains, S.; Pagni, R. M.; Kabalka, G. W. Tetrahedron Lett. 1995, 36, 3469. 33 Merchant, K. J. Tetrahedron Lett. 2000, 41, 3747. 34 Fleming, I.; Solay, M.; Stolwijk, F. J. Organomet. Chem. 1996, 521, 121. 35 For a recent review about enzyme-catalyzed hydrolysis of nitriles, see: Kobayashi, M.; Shimizu, S. Curr. Opinion Chem. Biol. 2000, 4, 95. 36 (a). de Raadt, A.; Klempier, N.; Faber, K.; Griengl, H. J. Chem. Soc., Perkin Trans. I 1992, 137. (b). Klempier, N.; de Raadt, A.; Griengl, H.; Heinisch, G. J. Heterocycl. Chem. 1992, 29, 93. (c). de Raadt, A.; Griengl, H.; Klempier, N. J. Org. Chem. 1993, 58, 3179. 37 (a). Yamada, H.; Shimizu, S.; Kobayashi, M. The Chemical Record 2001, 1, 152. (b). Patent, Nitto Chemical, EP0707061, B1, 1996. 38 (a). Patent, Lonza AG; Robins Karen Tracey; Nagasawa, WO02055670, 2002. (b). Patent, Lonza AG, WO9905306, 2002. 39 Fallon, R. D.; Stieglitz, B.; Turner, I. Appl Microbiol Biotechnol. 1997, 47, 156. 40 (a). Stolz, A.; Trott, S.; Binder, M.; Bauer, R.; Hirrlinger, B.; Layh, N.; Knackmuss, H.-J. J. Mol. Catal. B 1998, 5, 137. (b). Wang, M. X.; Lu, G.; Ji, G. J.; Huang, Z. T.; Otto, M.-C.; Colby, J. Tetrahedron: Asymm. 2000, 11, 1123. 41 (a). Yokoyama, M.; Imai, N.; Sugai, T.; Ohta, H. J. Mol. Catal. B 1996, 1, 135. (b). Yokoyama, M.; Nakatsuka, Y.; Sugai, T.; Ohta, H. Biosci. Biotech. Biochem. 1996, 60, 1540. (c). Kakeya, H.; Sakai, N.; Sano, A.; Yokoyama, M.; Sugai, T.; Ohta, H. Chem. Lett. 1991, 1823. (d). Yokoyama, M.; Sugai, T.; Ohta, H. Tetrahedron: Asymm. 1993, 4, 1081. (e). Beard, T.; Cohen, M. A.; Parratt, J. S.; Turner, N. J.; Crosby, J.; Moilliet, J. Tetrahedron: Asymm. 1993, 4, 1085. (f). Wu, Z. L.; Li, Z. Y. Chem. Commun. 2003, 386. 42 (a). Nagashima, S.; Nakasako, M.; Dohmae, N.; Tsujimura, M.; Takio, K.; Odaka, M.; Yohda, M.; Kamiya, N.; Endo, I. Nat. Struct. Biol. 1998, 5, 347. (b). Nakasako, M.; Odaka, M.; Yohda, M.; Dohmae, N.; Takio, K.; Kamiya, N.; Endo, I. Biochemistry 1999, 38, 9887. (c). Tsujimura, M.; Dohmae, N.; Odaka, M.; Chijimatsu, M.; Takio, K.; Yohda, M.; Hoshino, M.; Nagashima, S.; Endo, I. J. Biol. Chem. 1997, 272, 29454. 43 Huang, W.; Jia, J.; Cummings, J.; Nelson, M.; Schneider, G.; Lindqvist, Y. Structure 1997, 5, 691. 44 (a). Cramp, R. A.; Cowan, D. A. Biochim. Biophys. Acta 1999, 1431, 249. (b). Pereira, R. A.; Graham, D.; Rainey, F. A.; Cowan, D. A. Extremophiles 1998, 2, 347. (c). Padmakumar, R.; Oriel, P. Appl. Biochem. Biotechnol. 1999, 77, 671. 45 Layh, N.; Willetts, A. Biotechnol. Lett. 1998, 20, 329. 46 Katayama, Y.; Matsushita, Y.; Kaneko, M.; Kondo., M.; Mizuno, T.; Nyunoya, H. J. Bacteriol. 1998, 180, 2583. 47 Zilberman, E. N. Russ. Chem. Chem. Rev. 1984, 53, 900. 48 Ravindranathan, M.; Kalyanam, N.; Sivaram, S. J. Org. Chem. 1982, 47, 4812. 49 (a). Miura, H.; Sugiyama, K.; Kawakami, S.; Aoyama, T.; Matsuda, T. Chem. Lett. 1982, 183. (b). Nozaki, F.; Sodesawa, T.; Yamamoto, T. J. Catalysis 1983, 84, 267. 50 Milic, D. R.; Opsenica, D.M.; Adnadevic, B.; Solaja, B. A. Molecules 2000, 5, 118.

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University of Groningen Monodentate secondary phosphine ... · PDF fileand nitrile hydrolysis ... Chlorobenzene is normally not active in cross-coupling reactions ... isolated yields