Simmons-Smith methylenating agents

For reasons not yet understood, the Simmons-Smith methylenating agent [CH2Br2, Zn(Cu)] brings about cyclopropanation, instead, to provide cyclopropylaluminum derivatives in good yield (equation 60). ... 

Methylenation of ketonesP The combination Zn-TiCU transforms CH2I2 into a methylenating agent. The Wittig-like reaction is accelerated by PbCh. Traces of lead are found to suppress the reactivity of Zn toward CH2I2, but the reactivity (for the Simmons-Smith reaction) is recovered by adding Me SiCI. 

In the Wittig-type olefination of 4-phenyl-2-butanone or dodecanal using the combination CH2X2-TiCl4-Zn, a catalytic amount of lead or lead chloride (0.5 mol%) has been found to be essential to achieve efficient reaction (Scheme 22.4) the formation of an intermediate organolead species was postulated to accelerate the formation of the active methylenating agent, CH2(ZnI)2. This is in marked contrast to Simmons-Smith cyclopropana-tions, for which trace amounts of lead(O) effectively blocked reaction. " ... 

Methylene ( CH2), the simplest carbene, can be prepared by decomposition of the highly toxic and explosive reagent diazomethane (CH Nj). However, more easily used reagents have been developed that, while they do not produce methylene directly, function as methylene transfer agents. They are called carbenoid species because they react like carbenes. lodomethylzinc iodide, known as the Simmons-Smith reagent, is a carbenoid. In the Simmons-Smith method, diiodomethane reacts with a zinc-copper alloy to produce an intermediate I—CH Zn—I compound. 

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