Markovnikov reactions carbenes

The elusive diazoalkenes 6 and 14 are unlikely to react with methanol as their basicity should be comparable to that of diphenyldiazomethane. However, since the formation of diazonium ions cannot be rigorously excluded, the protonation of vinylcarbenes was to be confirmed with non-nitrogenous precursors. Vinyl-carbenes are presumedly involved in photorearrangements of cyclopropenes.21 In an attempt to trap the intermediate(s), 30 was irradiated in methanol. The ethers 32 and 35 (60 40) were obtained,22 pointing to the intervention of the al-lylic cation 34 (Scheme 10). Protonation of the vinylcarbene 31 is a likely route to 34. However, 34 could also arise from protonation of photoexcited 30, by way of the cyclopropyl cation 33. The photosolvolysis of alkenes is a well-known reaction which proceeds according to Markovnikov s rule and is, occasionally, associated with skeletal reorganizations.23 Therefore, cyclopropenes are not the substrates of choice for demonstrating the protonation of vinylcarbenes. 

In 1982, Casey and co-workers reported the first reactions that could be considered hydrocarbations because they involved the direct C-H bond addition across the C-C double bond of alkenes. They showed that the cationic bridging iron methylidyne complex undergoes this type of reaction with alkenes with anti-Markovnikov regioselectivity. No other hydrocarbation reactions had been reported until recently, when Kubiak and co-workers reported hydrocarbation reactions of a nickel carbene complex with alkenes. Thus, the dicationic aminocarbene complex 31 reacts with ethylene, resulting in a complete conversion to the ethylcarbene complex (Scheme 1). 

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