Page 931 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Table 10.2. Classification of Carbenes on the Basis 907
of Reactivity toward Alkenes a
SECTION 10.2
Nucleophilic Ambiphilic Electrophilic
Reactions Involving
(CH 3 O) 2 C CH 3 OCCl Cl 2 C Carbenes and Related
Intermediates
CH 3 OCF PhCCl
CH 3 OCN(CH 3 2
CH 3 CCl
BrCCO 2 C 2 H 5
a. R. A. Moss and R. C. Munjai, Tetrahedron Lett., 4721 (1979); R. A.
Moss, Acc. Chem. Res., 13, 58 (1980); R. A. Moss, Acc. Chem. Res.,
22, 15 (1989).

+ – : : – +
:
:
CH 3 O C : O CH 3 CH 3 O : C : O : CH 3 CH 3 O : C O : CH 3
:
:
:
Absolute rates have been measured for some carbene reactions. The rate of
addition of phenylchlorocarbene shows a small dependence on alkene substituents, but
as expected for a very reactive species, the range of reactivity is quite narrow. 119 The
rates are comparable to moderately fast bimolecular addition reactions of radicals (see
Part A, Table 11.3).

OC H C H O C CO C H (CH ) CH C H O C CO 2 C H
2 5 2 5 2 2 2 5 2 3 3 2 5 2 2 5
CO C H 5
2
2
3.32 x 10 6 2.24 x 10 6 1.10 x 10 6 1.54 x 10 5
9.97 x 10 6
Absolute rate of addition of
–1 –1
phenylchlorocarbene, k M s
The rates of phenylchlorocarbene have also been compared with the fluoro and
bromo analogs. 120 The data show slightly decreased rates in the order Br > Cl > F. The
alkene reactivity difference is consistent with an electrophilic attack. These reactions
have low activation barriers and the reactivity differences are dominated by entropy
effects.

CH 3 CH 3
(CH ) CH 3
2 3
CH 3 CH 3
4.0 x 10 6
8
PhCBr 3.8 x 10 6
PhCCl 2.8 x 10 8 2.2 x 10
PhCF 1.6 x 10 8 9.3 x 10 5
–1 –1
Absolute rate of addition, k M s

119 N. Soundararajan, M. S. Platz, J. E. Jackson, M. P. Doyle, S.-M. Oon, M. J. H. Liu, and S. M. Anand,
J. Am. Chem. Soc., 110, 7143 (1988).
120
R. A. Moss, W. Lawrynowicz, N. J. Turro, I. R. Gould, and Y. Cha, J. Am. Chem. Soc., 108, 7028
(1986).