Effect of pyridine on the regio- and stereo-chemistry in the addition of bromine chloride to α,β-unsaturated aldehydes and ketones

The addition of bromine chloride (BrCl) in methylene dichloride (CH2Cl2) in the presence and absence of acid scavengers such as pyridine to the following α,β-unsaturated aldehydes, ketones and esters is described: acrylaldehyde 1, methyl vinyl ketone 2, phenyl vinyl ketone 3, (E)-crotonaldehyde 4, (E)-pent-3-en-2-one 5, (E)-4-phenylbut-3-en-2-one 6, 4-methylpent-3-en-2-one 7, methyl isopropenyl ketone 8,3-phenylbut-3-en-2-one 9, cyclohex-2-enone 10, methyl acrylate 11, (E)-methyl crotonate 12 and methyl methacrylate 13. The majority of the aldehydes and ketones gave primarily anti-Markovnikov (AM) bromo chloride regioisomer in the absence of pyridine. In most cases the Markovnikov (M) regioisomer increased significantly in the presence of pyridine. Addition of BrCl to 4, 5, 6 and 10 became stereospecific (erythro) in the presence of pyridine. The stereospecificity of addition to ester 12 was high with or without pyridine. These data were interpreted as follows: in the absence of an acid scavenger, an acid-catalysed reaction is involved, initiated by attack of proton on the carbonyl oxygen. When traces of acid are removed by an acid scavenger, the reactions proceed through a bromonium ion–chloride ion intermediate. The esters reacted only via a bromonium ion giving essentially the same mixture of regioisomers with and without acid scavenger.