Sigmatropic rearrangements carbene complexes

Ketenes react with tertiary allylic amines in the presence of Lewis acids to give zwitterionic intermediates which undergo [3,3]-sigmatropic rearrangement [119]. Photolysis of chromium carbene complexes in the presence of tertiary amines results in similar chemistry [120]. Cyclic (Table 21) and strained allylic amines (Eq. 34) work best, while acylic amines are less reactive (Eq. 35). 

The major reaction pathways for sulfonium ylide formation generated from a metal carbene complex and sulfide are [2,3]-sigmatropic rearrangement and [l,2]-shift, similar to those of the oxonium ylide formation. 

In the past 10 years, studies on the reaction of ylides generated from metal carbene complexes have witnessed tremendous progress. In particular, synthetic applications of the three most common reactions of ylides, namely [2,3]-sigmatropic rearrangement, [l,2]-shift, and 1,3-dipole cycloaddition, have gained considerable success to demonstrate... 

Several related carbene complexes 28 with M = Cr, Mo, or W, have been shown to undergo photocycloadditions with alkenes, in particular acrylates, to furnish a series of 1 -pyrrolines 29 in moderate yields. The mechanism is believed to involve an initial cyclopropanation of the alkene, followed by a light-induced [l,3]-sigmatropic rearrangement of an intermediate N-cyclopropylimine <020M4076>. 

Rhodium(I)-catalyzed reaction of phenyl 2-propylcycloprop-2-en-yl ketone with terminal alkynes gives 2-alkyl-4-propyl-7-phenyloxepines (Scheme 13). The reaction involves the formation of a rhodium-carbene complex, which undergoes a [2 + 2] cycloaddition with a terminal ethyne the resultant rhodacycle rearranges by a 1,5-sigmatropic shift, followed by reductive elimination of rhodium <92JA588l>. 

The process has been extended to the die molybdenum carbene 4 at 60 results in a <. 3.3]sigmatropic rearrangement to hexahydr analogous chromium complex with 3 also re lower yield. An electron-withdrawing group 1... 

Other reactions of sulphonium ylides include o /9 -elimination,metal-mediated carbene-transfer to olefins, insertion into aromatic C—H bonds or other carbenoid-type processes, formation of Pd" complexes, addition to enones (forming cyclopropyl ketones or heterocycles ), reaction with isoquinoline 2-oxide, and [2,3]-sigmatropic rearrangements " [as in the case of (12) 1 or... 

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