A new strategy is described for the Lewis base-catalyzed
bromochlorination
of unsaturated systems that is mechanistically distinct from prior
methodologies. The novelty of this method hinges on the utilization
of thionyl chloride as a latent chloride source in combination with
as little as 1 mol % of triphenylphosphine or triphenylphosphine oxide
as Lewis basic activators. This metal-free, catalytic chemo-, regio-,
and diastereoselective bromochlorination of alkenes and alkynes exhibits
excellent site selectivity in polyunsaturated systems and provides
access to a wide variety of vicinal bromochlorides with up to >20:1
regio- and diastereoselectivity. The precision installation of Br,
Cl, and I in various combinations is also demonstrated by simply varying
the commercial halogenating reagents employed. Notably, when a chiral
Lewis base promoter is employed, an enantioselective bromochlorination
of chalcones is possible with up to a 92:8 enantiomeric ratio when
utilizing only 1–3 mol % of (DHQD)2PHAL.
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