Allyl bromides bond formation

Scheme 2. Use of allylic bromides in CrC -mediated bond formations.

The reaction of benzaldehyde with unsymmetrical allylic bromides in the ionic liquid bmim also proceeds regioselectively to realize a carbon-carbon bond formation at the more substituted allylic carbon (Scheme 5). In the coupling of crotyl bromide with benzaldehyde, the product is a nearly 50 50 mixture of antilsyn diastereomers. Cinnamyl bromide predominantly gives the //-diastereomer. The regio- and diastereoselectivity is similar to that observed for these in aqueous media.104... 

The reaction of dibromocarbene with haloalkenes (vinyl halides, alkenes with a halogen more distant from the double bond) leads to the formation of 1,1-dibromocyclopropanes in poor to good yields. However, reactions with allyl halides, particularly substituted allyl bromides, require comment. These alkenes furnish, apart from 1,1-dibromocyclopropanes 1, the alkylation products of tribromomethyl anion 2 and, occasionally, the products of their further transformations (dibromocarbene adducts 3, products of elimination of hydrogen bromide 4 etc.) if they react with bromoform under phase-transfer catalysis conditions (Houben-Weyl, Vol.E19b, pi620). 

The BiBrs-Sm binary reagent promotes reductive C-S and C-Se bond formation between benzyl and allyl bromides and diorganyl disulfides and diselenides in aqueous media, affording the corresponding sulfides and selenides, respectively (Scheme 14.105) [217, 218]. Intramolecular reductive C-S bond formation by use of a BiCl3-M (M = Sn, Zn) redox system is used in the synthesis of 3-hydroxyceph-ems and 2-exo-methylenepenams (Scheme 14.106) [219]. Alkyl and arylsulfonyl chlorides couple with allylic halides in the presence of Bi to afford the corresponding allylic sulfones [220]. 

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