SYN STEREOSPECIFICITY IN THE SN2' REACTION OF AN ACYCLIC ALLYLIC CHLORIDE WITH SECONDARY-AMINES

Previous theoretical and experimental investigations of the stereochemistry of the SN2’ reaction (bimolecular nucleophilic substitution with allylic rearrangement) are inconclusive and often contradictory. The reaction of isotopically labeled α-methylallyl chloride with three secondary amines is now shown to be stereospecifically syn. (R)-(-)-3-Chloro-(Z)-1-butene-l-d (5) reacts with diethylamine to give a 95 : 5 mixture of (R)-(E)- and (S)-(Z)-allylic amines 14 which is reduced by diimide to (R)-(+)- and (S)-(-)-N, N-diethyl-l-aminobutane-l-d (9). An authentic sample of (R)-(+)-9 from yeast reduction of butanal-Y-rf shows an unusually large specific rotation, α25D +5.66° (ether). Comparison of the specific rotation of 9 from reduction of the SN2’ product with that of optically pure material reveals that the substitution process is stereospecific : nucleophile attacks the allylic system on the face bearing the leaving group. Similarly, reaction of the enantiomeric chloride (S)-(+)-6 with both dimethylamine and piperidine proceeds with syn stereospecificity. © 1979, American Chemical Society. All rights reserved.