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DOI: 10.1055/sos-SD-044-00163

Gandon, V.; Malacria, M.Science of Synthesis, (200844215.

1-Chlorovinyl 4-tolyl sulfoxides such as 105 are readily available from ketenes in three steps using chloromethyl 4-tolyl sulfoxide.[‌143‌] A magnesiumsulfoxide exchange reaction allows the formation of a (1-chlorovinyl)magnesium species, which can also be depicted as an alkylidene carbene 106.[‌104‌,‌143‌‌145‌] Treatment of these carbenoids with α-sulfonyl carbanions gives allenes, e.g. 108, in moderate to good yields (Scheme 63). Overall, this three-component process involving ketones, chloromethyl 4-tolyl sulfoxide, and sulfones allows the rapid construction of tri- and tetrasubstituted allenes. Optimized reaction conditions can be summarized as follows: tert-Butylmagnesium chloride (0.5 equiv) does not react with sulfoxides but is used to remove traces of moisture from tetrahydrofuran in order to avoid the fast reaction of the carbenoid with water; the sulfoxide magnesiumexchange is achieved in one minute by either ethylmagnesium chloride or isopropylmagnesium chloride (3 equiv) keeping the temperature at 78°C, otherwise significant degradation of the carbenoid occurs. A solution of a lithium α-sulfonyl carbanion (3 equiv) is added at 78°C; this anion attacks the electron-defficient carbon and the resulting vinyl anion, e.g. 107, undergoes β-elimination of the sulfonyl group to give the allene.

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