EFFECT OF PYRIDINE ON THE REGIO-CHEMISTRY AND STEREO-CHEMISTRY IN THEADDITION OF BROMINE CHLORIDE TO ALPHA,BETA-UNSATURATED ALDEHYDES AND KETONES

The addition of bromine chloride (BrCl) in methylene dichloride (CH2CI 2) in the presence and absence of acid scavengers such as pyridine to the following alpha,beta-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-phe nylbut-3-en-2-one 9, cyclohex-2-enone 10, methyl acrylate 11, (E)-meth yl crotonate 12 and methyl methacrylate 13, The majority of the aldehy des and ketones gave primarily anti-Markovnikov (AM) bromo chloride re gioisomer in the absence of pyridine, In most cases the Markovnikov (M ) regioisomer increased significantly in the presence of pyridine, Add ition of BrCl to 4, 5, 6 and 10 became stereospecific (erythro) in the presence of pyridine, The stereospecificity of addition to ester 12 w as high with or without pyridine, These data were interpreted as follo ws: 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 regio isomers with and without acid scavenger.