Ylide formation triplet carbenes

Relative rates of some prototypical carbenes, obtained by Stem-Volmer methods, are listed in Table 2. Although many of these carbenes have triplet ground states, reaction with nucleophiles Y occurs prior to spin equilibration. Most often, ylide formation with solvent molecules was analysed in terms of Eq. 3. The pyridine-ylide served as the probe for 154. 

The transient ketocarbene is also able to effect both intramolecular and intermolecular H-abstraction. The fact that a triplet state of the carbene is often implicated follows from observations that sensitized photolyses frequently enhance the abstraction pathway. Abstraction and radical combination results in the formal insertion of the carbene into the C—H (or S—H, N—H, etc.) bond. Insertions may also occur into C—C, C—O, C—S, C—Hal, etc. bonds the mechanisms are often not known, but a singlet state of the ketocarbene is probably involved in many cases. Reactions at soft basic centers (e.g. —S or —Br ) generally proceed by the preliminary formation of an ylide. The multiplicity of the possible pathways is illustrated in Scheme 7, equation (14) (photolysis in dioxane affords only 27, 57% and 28, 43%), and Scheme 8. ... 

Whereas free singlet carbenes are rather unselective with respect to formation of cyclopropane 22 or ylide 23 and the cyclopropane is favored under conditions that populate the triplet state of a carbene (see Section I.2.I.2.4.2.6.2.), the metal carbenes generated with copper or rhodium catalysts display a selectivity for functional groups which are more nucleophilic than a double bond. Thus, no cyclopropanes are obtained from dialkylallylamines allyl sulfides -allyl dithioacetals , and allyl selenides under carbenoid conditions (copper or rhodium catalysts). 

---