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Abstract
The reaction of excess TMSCl and LiCCl2 Br at low temperature is a technically simple high yield route to TMSCCl2 Br. The latter is a stable source of the dichlorobromomethide carbanion, which undergoes 1,4-addition with cyclic nitroalkenes and (E)-fumarates leading to dichlorocyclopropanes after bromide expulsion. For nitrostyrenes the reaction arrests at the 1,4-addition product. Low temperature NMR spectroscopy studies and DFT calculations suggest the formation of an "ate" species [(nitronate)SiFMe3 ](-) which, upon boil-off of TMSF at 10-20 °C, yields the cyclopropane. DFT calculations also support the experimental differences between fluoride and acetate as promotors.
Highlights
Dihalocyclopropane motifs are attractive targets frequently associated with biological activity, especially in insects.[1]
In seeking for a user-friendly approach to dichlorocyclopropanation we had settled on use of TMSCCl3 and recently could demonstrate both its high yield synthesis and its use in the conjugate addition of trichloromethyl units (A Scheme 1).[8]
Cyclic nitroolefin (2a) was picked as the model alkene for the reaction, as nitroolefins have been reported as potent Michael acceptors for TMSCCl3.[8a,c] Table 1 outlines the effects of different promotors on its cyclopropanation reaction with TMSCCl2Br 1
Summary
Dihalocyclopropane motifs are attractive targets frequently associated with biological activity, especially in insects.[1]. Electron deficient alkenes are insufficiently nucleophilic[5] for such strategies. While some difluorocyclopropanes have been attained from electron-poor alkenes, high temperature reactions of TMSCF3 or related analogues[6] were necessary. For the dichloro- and dibromocyclopropanes typically the only generic viable methodology employs stoichiometric quantities of the highly toxic Seyferth reagents PhHgCX3 (X = Cl, Br).[7] Clearly, the latter are undesirable on environmental grounds. In seeking for a user-friendly approach to dichlorocyclopropanation we had settled on use of TMSCCl3 and recently could demonstrate both its high yield synthesis and its use in the conjugate addition of trichloromethyl units (A Scheme 1).[8] under the published conditions[8a,c] base-induced closure of A to the dichlorocyclopropanes B could lead to poor yields or the formation of alternative (elimination) by-products. Approaches to dichlorocyclopropanes based on TMSCCl2X reagents (X = Cl, Br)
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