Page 940 - Advanced Organic Chemistry Part B - Reactions & Synthesis

916 The addition reaction of alkenes and phenylmercuric bromide typically occurs at about

80 C. Phenylmercuric iodides are somewhat more reactive and may be advantageous
CHAPTER 10 in reactions with relatively unstable alkenes. 167
Reactions Involving
Carbocations, Carbenes,
and Radicals as Reactive
Intermediates 10.2.3. Addition Reactions
Addition reactions with alkenes to form cyclopropanes are the most studied
reactions of carbenes, both from the point of view of understanding mechanisms and
for synthetic applications. A concerted mechanism is possible for singlet carbenes. As
a result, the stereochemistry present in the alkene is retained in the cyclopropane. With
triplet carbenes, an intermediate 1,3-diradical is involved. Closure to cyclopropane
requires spin inversion. The rate of spin inversion is slow relative to rotation about
single bonds, so mixtures of the two possible stereoisomers are obtained from either
alkene stereoisomer.

R' R' R' R'
H R
C: H R
C C + R' 2 C: H R
R H R H
singlet R H
mechanism

H R H . R' R' R' R'
. H R H H
C C + R' 2 C . RC . CR' 2 +
R H CH
R H R R
triplet R
mechanism

Reactions involving free carbenes are very exothermic since two new bonds are
formed and only the alkene bond is broken. The reactions are very fast and, in fact,
theoretical treatment of the addition of singlet methylene to ethylene suggests that there
is no activation barrier. 168 The addition of carbenes to alkenes is an important method
for synthesis of many types of cyclopropanes and several of the methods for carbene
generation listed in Scheme 10.8 have been adapted for use in synthesis. Scheme 10.9,
at the end of this section, gives a number of specific examples.

10.2.3.1. Cyclopropanation with Halomethylzinc Reagents. A very effective means
for conversion of alkenes to cyclopropanes by transfer of a CH unit involves
2
reaction with methylene iodide and a zinc-copper couple, referred to as the Simmons-
Smith reagent. 169 The reactive species is iodomethylzinc iodide. 170 The transfer of
methylene occurs stereospecifically. Free :CH is not an intermediate. Entries 1 to 3
2
in Scheme 10.9 are typical examples.

167
D. Seyferth and C. K. Haas, J. Org. Chem., 40, 1620 (1975).
168
B. Zurawski and W. Kutzelnigg, J. Am. Chem. Soc., 100, 2654 (1978).
169 H. E. Simmons and R. D. Smith, J. Am. Chem. Soc., 80, 5323 (1958); H. E. Simmons and R. D. Smith,
81, 4256 (1959); H. E. Simmons, T. L. Cairns, S. A. Vladuchick, and C. M. Hoiness, Org. React., 20,
1 (1973); W. B. Motherwell and C. J. Nutley, Contemporary Org. Synth., 1, 219 (1994); A. B. Charette
and A. Beauchemin, Org. React., 58, 1 (2001).
170
A. B. Charette and J.-F. Marcoux, J. Am. Chem. Soc., 118, 4539 (1996).