The Stereochemistry of S_RN1 Reactions in Nitroacenaphthenes

The reaction of the 2-chloro-l, l-dialkyl-6-nitroacenaphthenes (1)-(3) with azide and p- toluenethiolate ions takes place by the S_RN¹mechanism, to give the substitution products (27)-(32), despite the fact that the nitro group and the chlorine-bearing benzylic carbon are attached to different aromatic rings. The reaction of the stereoisomers (2) and (3) of 2-chloro- 1- ethyl-l-methyl-6-nitroacenaphthene takes place through an effectively planar benzylic radical (35), which is preferentially attacked from the face remote from the a-ethyl group. The presence of geminal alkyl groups a to the reaction site in the rigid acenaphthene system restricts the scope of the substitution processes to sterically unhindered nucleophiles, and the reaction fails with reagents such as sodium p-toluenesulfinate (23) and the salts, (24) and (25), derived from 2-ethylmalononitrile and 2-nitropropane.