Carbenes 1.2- acyl shift

A laser flash photolysis study of phenylacyloxycarbene (75) allowed the measurement of the rate of 1,2-acyl shift of between 105 and 106 s 1 in pentane at room temperature.82 The activation parameters were in good agreement with calculations. High-level calculations supported a carbanion-like attack by the carbene lone pah on the carbonyl (76) whereas the effects of substituents on the rate suggested an acyl anion-like transition state (77). The electron-donating, stabilizing effect of OAc slows the 1,2-C and 1,2-H shifts in alkylacetoxycarbenes (78),83 allowing 1,2-acetyl shifts to compete. 

Irradiation of 100 in methanol gives in addition to decarbonylation and 1,3-acyl shift products also the cyclic acetals 133 and 136. The formation of these compoimds is best interpreted as involving the formation of the oxa-carbenes 134 and 135, followed by addition of solvent. 

Analogous oxa-carbene intermediates arc also indicated in the irradiation of 15b and 34 in methanol. 2 .3T) In these cases the first step in the reaction is probably the 1,3-acyl shift to 16b and 101 followed by a-cleavage and carbene formation. This type of reaction is not restricted to p,Y-unsaturated ketones but is observed also in many saturated ketones. 9 )... 

Scheme 77 1,2-Acyl shift followed by diazoalkane carbene coupling...

Similarly, photolysis of 3,3-dimethoxy-5-norbornen-2-one 109 in various solvents (dioxane, acetonitrile, and acetone) gave the 1,3-acyl shift product 110 in minor amounts and the decarbonylated product 111 as the major product (Scheme 25). Irradiation of 109 in methanol gave a complex mixture of products arising through an oxa-carbene formed from the biradical generated after initial a-cleavage. It was also observed that the decarbonylated product 111 was obtained at the expense of the initially formed 1,3-acyl shift product 110. 

Nakajima has shown that a-cyclopropyl acyl silane (23) results from reaction of 1-trimethylsilyl cyclopropyl lithium with dichloromethyl methyl ether at low temperature in THF solution, in a reaction said to involve a carbene intermediate and a 1,2-silicon shift (Scheme 56)147. 

A-heterocyclic carbenes (NHC) are also efficient organocatalytic tools for generating homoenolate equivalents from a,P-unsaturated aldehydes. These reactive intermediates display a versatile reactivity in a number of catalytic transformations attesting to an important synthetic potential [38]. Recently, Scheldt et al. [39a] accomplished the first enantioselective protonation of a homoenolate species generated by a chiral NHC precursor 93 in the presence of DIE A and an excess of ethanol as the achiral proton source (Scheme 3.46). The suggested mechanism involves an initial addition of NHC 93 to the enal 89 followed by a formal 1,2-proton shift resulting in the formation of the chiral homoenolate equivalent 91. A diastereose-lective P-protonation/tautomerization sequence leads to the acyl triazolinium inter-... 

---