Page 929 - Advanced Organic Chemistry Part B - Reactions & Synthesis

When a reaction appears to involve a species that reacts as expected for a carbene 905
but must still be at least partially bound to other atoms, the term carbenoid is used.
Some carbenelike processes involve transition metal ions. In many of these reactions, SECTION 10.2
the divalent carbene is bound to the metal. Some compounds of this type are stable, Reactions Involving
Carbenes and Related
whereas others exist only as transient intermediates. In most cases, the reaction involves Intermediates
the metal-bound carbene, rather than a free carbene.

M C

metal-bound carbene

The stability and reactivity of metallocarbenes depends on the degree of back donation
from the metal to the carbene. If this is small, the metallocarbenes are highly reactive
and electrophilic in character. If back bonding is substantial, the carbon will be less
electrophilic, and the reactions are more likely to involve the metal.

:
M C + M C
dominant electron dominant electron
distribution in electrophilic distribution in metallo-
metallo-carbenes carbenes with strong back-bonding

Carbenes and carbenoids can add to double bonds to form cyclopropanes or insert
into C−H bonds.

X X X Y
R C–H R C CR 2
2
3
R C C Y :C Y
3
R R
H R R
insertion addition
These reactions have very low activation energies when the intermediate is a “free”
carbene. Intermolecular insertion reactions are inherently nonselective. The course
of intramolecular reactions is frequently controlled by the proximity of the reacting
groups. 113 Carbene intermediates can also be involved in rearrangement reactions.
In the sections that follow we also consider a number of rearrangement reactions
that probably do not involve carbene intermediates, but lead to transformations that
correspond to those of carbenes.

10.2.1. Reactivity of Carbenes

From the point of view of both synthetic and mechanistic interest, much attention
has been focused on the addition reaction between carbenes and alkenes to give
cyclopropanes. Characterization of the reactivity of substituted carbenes in addition
reactions has emphasized stereochemistry and selectivity. The reactivities of singlet
and triplet states are expected to be different. The triplet state is a diradical, and would
be expected to exhibit a selectivity similar to free radicals and other species with
unpaired electrons. The singlet state, with its unfilled p orbital, should be electrophilic
and exhibit reactivity patterns similar to other electrophiles. Moreover, a triplet addition

113
S. D. Burke and P. A. Grieco, Org. React., 26, 361 (1979).