HOMOTROPYLIUM CATIONS . SUBSTITUENT CONTROL ON THEIR GENERATION AND SUBSEQUENT REACTIONS

When cyclooctatetraene and a number of its monosubstituted derivatives are treated with chlorosulfony] isocyanate, 1,4 cycloaddition to the eight-membered ring occurs. The structures of the resulting N-(chlorosulfonyl) lactams and their derived products reveal that the electrophilic isocyanate attacks the individual polyene at the carbon atom adjacent to the substituent-bearing carbon to give the more stable classical cation in each instance. By analogy to earlier protonation and chlorination studies, the isocyanate is considered to approach from the endo surface of the tub conformation of the polyene. Energy considerations indicate that the intermediate classical cations pass readily by way of low-energy conformational changes to 1-substituted homotropylium cations which subsequently cyclize by means of C-N bond formation. The control which the various substituents at position 1 of the homoaromatic cation exert on the directional specificity of this cyclization reaction is discussed. The conclusions are reached that groups attached to C-l are unable to interact with the homotropylium cation by resonance and that the inductive effect of the particular substituent is responsible for the appreciable differences in the ratios of the cycloaddition products. © 1969, American Chemical Society. All rights reserved.