Alkenes, dihalogen complexes

In accordance with an earlier suggestion by Olah and Hockswender,240 the initial 1 1 adducts have been shown to be n complexes572 and a second species, a 2 1 bromine-alkene complex was directly observed.573 In the course of the bromination of crowded alkenes, such as ( )-2,2,3,4,5,5-hexamethylhex-3-ene (46), the bro-monium ion intermediate was shown to be reversibly formed.574 However, steric hindrance prevents the formation of the usual dihalogenated product. Instead, it loses a P proton to give an allylic bromo derivative, which is then dehydrobromi-nated to diene 47 ... 

Synthetically useful precursors of oxygen-substituted carbenes (see Houben-Weyl Vol. E19b. pp 1628-1682) are diazirines (photolysis or thermolysis), a-halogen ethers (base treatment), a-dihalogen ethers (treatment with alkyllithium compounds), and stable carbene complexes of the Fischer type (thermolysis). Only the mechanism-based stereoselectivity and the simple diastcreoselectivity of their reactions with alkenes have been studied to date. 

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