Dibromomethane, carbenoids from

Since the free carbene ( CH2) is not thought to be present, the reagent (40) is termed a carbenoid. The activity of the zinc surface is of crucial importance to the smoothness and success of the reaction. Failure to ensure appropriate activation procedures may cause the reaction to fail, or the onset of the exothermic reaction may be delayed and then proceed with excessive vigour. The recent use of ultrasonic activation of the zinc surface from the outset of the reaction results in a smoother, less unpredictable reaction rate, and leads to satisfactory yields.118 Furthermore, sonication enables the cheaper dibromomethane to be employed.1 lb... 

Considering the selectivity of this reaction (terminal vs. 1,2-disubstituted alkenes) and the fact that an electron-rich alkene such as isobutyl vinyl ether does not undergo cyclopropanation, it seems that the reactive species formed from the lithiated sulfone and the nickel catalyst does not behave as an electrophilic carbenoid. In this respect, one should note that the Simmons-Smith reagent is electrophilic whereas the methylene transfer reagent arising from treatment of dibromomethane with nickel(O) can achieve cyclopropanation of electron-deficient alkenes only. ... 

Reactions with cyclohexene illustrate the simple diastereoselectivity of halogen-substituted carbenes. Carbenoids generated from fluorodiiodomethane or dichloroiodomethane by di-ethylzinc treatment provide erafo-cycloadducts with moderate to good preference4, while chlorocarbene, obtained by thermolysis of (dichlornmethyl)phenylmercury 3, and bromocar-bene, liberated from dibromomethane by base6, are considerably less selective. 

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