Page 947 - Advanced Organic Chemistry Part B - Reactions & Synthesis

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SECTION 10.2
Reactions Involving
Carbenes and Related
97.2 Intermediates

1.946 1.941
Cu
139.1 123.6

1.782
Cu
65.7

O

Fig. 10.7. Computational (B3LYP/6-31G(d)) minimum-energy
structure of carbomethoxycarbene derivative of copper N,N -

dimethylpropane-1,3-diimine. Reproduced from J. Am. Chem.
Soc., 123, 7616 (2001), by permission of the American
Chemical Society.

The metal-carbene complexes are electrophilic in character. They can, in fact, be
represented as metal-stabilized carbocations.

– + –N R R
M +R C N N 2 : C + M C
:
M
2
R R
In most transition metal–catalyzed reactions, one of the carbene substituents is a
carbonyl group, which further enhances the electrophilicity of the intermediate. There
are two general mechanisms that can be considered for cyclopropane formation. One
involves formation of a four-membered ring intermediate that incorporates the metal.
The alternative represents an electrophilic attack giving a polar species that undergoes
1,3-bond formation.
R R
M CR 2 M CR 2 C
C C C C C C
R R
:
M CR 2 M CR 2
or C
+
C C C C C C