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Chemistry 125: Lecture 49February 10, 2010
Electrophilic Addition to Alkenes with
Nucleophilic Participation This
For copyright notice see final page of this file
Larger than H,Hg bridges near
the middle, preventing cation rearrangement.
Subsequent reduction
by BH4- completes
Markovnikov hydration without rearrangement.
electrophileLUMO (*)
Ethylene
HOMO ()
bonding
antibonding
+
-HOMO (p)
HOMO-2(where the electrons went)
Chlorine
Two Cl-CBonds?
“oxidizing agent”
removes e- from alkene
poor overlap
LUMO
+
-
H2C CH2
:Cl
ClCl-
Cl-
H2C CH2
Cl+ H2C CH2
Cl
Cl
“Electrophilic” Addition of Cl2 to an alkene is both electrophilic
and nucleophilicsimultaneously.
Bromonium Regiochemistry p. 420How do we know the ion is bridged?
“Halohydrin”shows
MarkovnikovRegiochemistry
(like oxymercuration)
Bromonium Stereochemistry p. 419
H2O attacks *C-Br from backside.
(Unbridged C+ would have been attacked from both sides.)
Anti Addition
Other “Simultaneous” ReagentsCl2C: (Carbene)
R2BH (Hydroboration)
CH2I2 Zn/Cu (Carbenoid)
O3 (Ozonolysis)
H-metal (Catalytic Hydrogenation)
R-metal (Metathesis, Polymerization)
RC (Epoxidation)OOH
O
Jack Hine, Ph.D. (1950)
“about as earthy a research project as could be appropriate for a postdoctoral”
-Elimination
Section 10.4d (431-2)
Carbenes
X2C:X3CX3C H O-C(CH3)3
K+
( X = Cl,Br )
X
Reaction of CHnCl4-n with HO-
CHCl3 CH2Cl2 CHCl3 CCl4
fast slow v.slowfast
Reaction of CHnCl4-n with PhS- fast slow v. slowbase
nucleophile
C
C
HH
HH
LUMO
HOMO••CClCl
Most reactions in this lecture use analogous LUMOs and HOMOs to mix with the HOMO and * LUMO of H2C=CH2,
respectively, so as to form two bonds simultaneously.
But both pairs are orthogonal!
+
+
Subsequent CH3BH2-O-OH rearrangement is like SN2
Transition State
HOMOs
very similar
Transition State
LUMOs
very similar
Why do Hydroboration/Oxidationif it just adds H and OH to C=C?
Product is syn (H,OH from same face of C=C)
and anti-Markovnikov (less substituted at C-OH)!
(Contrast with acid-catalyzed hydration)
Simmons-Smith“Carbenoid”
Metal R-X Metal+
R-X
single-electrontransfer(SET)
e
Metal+
R X Metal
R-M X +
The next three slides suggest a plausible, but incorrect, two-step mechanism for addition of ICH2ZnI to H2C=CH2
Model forI-Zn-CH2I
Cl Zn CH3
CH2 CH2
Cl Zn CH3I Zn CH2 I
HOMOZn-C
LUMO
C-I
ZnI2CH2
“SN2”
If it were the diiodide instead of the model…
But these two transition states were just guessed, not calculated quantum mechanically…
End of Lecture 49Feb. 10, 2010
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The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0