Photocycloaddition chromium complex

Upon irradiation with ultraviolet light Fischer chromium-carbene complexes 16 react as if they were ketenes, although evidence for the generation of free ketenes has not been observed (see Section 1.4.2.). Photocycloadditions of these complexes onto chiral enamines show significant asymmetric induction.22... 

Another more versatile method involves the photocycloaddition of chromium carbene complexes to alkenes. These transformations can be formally regarded as [2 + 1 + 1] cycloadditions since these carbene complexes are derived from [1 + 1] carbanion insertion reactions to hexacarbonylchromium. 

The advantage of using the photocycloaddition of pentacarbonylcarbenechromium complexes over the ketene cycloaddition method is the absence of ketene dimerization and the avoidance of use of excess alkene in the former method. Also, the mild reaction conditions associated with the use of chromium carbene complexes avoids epimerization and thermodynamic equilibration of 2-monosubstituted cyclobutanones. 

Amido-substituted methylenecyclopropanes can be used in [2 + 2] photocycloadditions with chromium-alkoxycarbene complexes. (5)-3-(2-Methylenecyclopropyl)-4-phenyloxazolidin-2-one (14) was prepared from pentacarbonyl[A-phenylglycinyl(cyclopropyl)carbene]chro-mium(O) by ring closure with sodium hydride and diphenyl carbonate. Photolysis of a variety of chromium-alkoxycarbene complexes 15 in the presence of two equivalents of optically active enecarbamate under carbon monoxide produced optically active cyclobutanones. The cis- and trani-products, 16 and 17, were formed with a high degree of asymmetric induction at the position a to the oxygen. ... 

Desulfurization. The scope, selectivity, and stereochemistry of this method (C-S - C-H) has been studied using a variety of organosulfur compounds as substrates. /3-Lactams unsubstituted at C-4 are formed in a two-step process involving photocycloaddition of chromium carbene complexes and RN=C(SMe)2 followed by desulfurization. ... 

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