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CHEM 261: Organic Chemistry II
C
C BondFormation
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To form the carbon skeletons of complex molecules, organic
chemistsneed an extensive repertoire of carboncarbon bond forming
reactions.
(1) Organocuprate coupling reactions,
(2) Suzuki reaction,
(3) Heck reaction,
(4) Dihalocarbene addition reaction,
(5) Simmons-Smith reaction.
CarbonCarbon Bond FormingReactions
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Methyl, 1, cyclic 2, vinyl, and aryl halides can be used.
Reactions with vinyl halides are stereospecific.
The halogen (X) may be Cl, Br, or I.
Tertiary (3) halides are too sterically hindered to react.
General Features of OrganocuprateCoupling Reactions
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Suzuki Reactions
The Suzuki reaction is a palladium-catalyzed coupling of an
organichalide (RX) with an organoborane (RBY2) to form a product
(RR) witha new CC bond.
Pd(PPh3)4 is the typical palladium catalyst.
The reaction is carried out in the presence of a base such as
NaOH orNaOCH2CH3.
Vinyl or aryl halides are most often used, and the halogen is
usually Bror I.
The Suzuki reaction is completely stereospecific.
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Examples of the Suzuki Reaction
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The Heck Reaction
The Heck reaction is a Pd-catalyzed coupling of a vinyl or aryl
halidewith an alkene to form a more highly substituted alkene with
a new CC
bond.
One H atom of the alkene starting material is replaced by the R
group ofthe vinyl or aryl halide.
Palladium(II) acetate [Pd(OAc)2] in the presence of a
triarylphosphine[P(o-tolyl)3] is the typical catalyst.
The reaction is carried out in the presence of a base such
astriethylamine.
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Examples of the Heck Reaction
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Carbenes and CyclopropaneSynthesis:
A carbene, R2C:, is a neutral reactive intermediate thatcontains
a divalent carbon surrounded by six electrons: thelone pair, and
two each from the two R groups.
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Carbenes and CyclopropaneSynthesis:
Dihalocarbenes, :CX2, are especially useful
reactiveintermediates since they are readily prepared
fromtrihalomethanes (CHX3) by reaction with strong base.
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Examples of the DihalocarbeneReaction
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The Simmons-Smith Reaction:
Non-halogenated cyclopropanes can be prepared by the reaction of
analkene with di-iodo-methane, CH2I2, in the presence of a
copper-activated zinc reagent called zinc-copper couple [Zn(Cu)].
This is knownas the Simmons-Smith reaction.
The reaction is stereospecific.
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