Generation of Alkyl and Cycloalkyl Carbenes

Adamantyldiazirine is the first diazirine for which a fluorescence lifetime has been measured. Primary decay occurs with a lifetime of about 240 ps at ambient temperature, and increases at lower temperatures. The results support 

The carbene, 2-methylcyclohexanylidene, has been generated by photolysis of the corresponding diazirine (20) in a-, 3- and y-cyclodextrins, to determine what effect such constraints would have on the reaction pathway. Only products from 1,2-H shifts were observed (1- and 3-methylcyclohexene), and encapsulation of the diazirine in this way seemed to have little effect on product ratios. 

Rate constants have been measured by flash-photolysis techniques for the reactions of cyclopentadienylidene and fluorenylidene with alcohols and other quenchers. The rates increase with increasing acidity of the alcohol, and alcohols react faster than likely ylide-forming reagents, such as pyridine or THF. In contrast, tetrachlorocyclopentadienylidene reacts most rapidly with the least acidic alcohol studied, and reacts more rapidly with pyridine and THF than with methanol. 

Photolysis of the silacyclobutyl diazomethane (21) in /-butanol gave three different /-butoxy-substituted (silylalkyl)silanes, e.g. (Me3CO)Me2SiCHPh-SiMe3. The migratory aptitudes of substituents from the Si atoms to the carbene centre were determined as Ph Me = 3.8 1.0. 

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Generation of Alkyl and Cycloalkyl Carbenes - Photolysis or thermolysis of a series of alkylchlorodiazirines (16) (Scheme 7) in the presence of alkenes, such as tetramethylethene, results in 1,2-H shifts, giving the corresponding vinyl chorides (18), in competition with additions of the carbenes (17) to the alkenes, yielding cyclopropanes (19). The mechanism of these reactions is discussed in the light of results obtained from photoacoustic calorimetry, and the ratio of vinyl chloride to cyclopropane seems to depend on the excited states of the carbene precursors and also on carbene-alkene complexes. Similar reactions of related diazirines have been investigated by flash photolysis. 

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