Addition of amine nitrogen to an unactivated double bond. The mechanisms of the reverse Hofmann elimination

The tertiary amino-group of amine (7) adds to the transannular double bond with a half-life of 3.3 s at 25°. Reaction is general acid catalysed, and a mechanism is proposed in which nucleophilic addition of amine nitrogen to one end of the double bond is concerted with the transfer of a proton from the general acid to the other. The reaction is observed only with systems in which substantial ground state strain is relieved on cyclisation. It gives the products of antiperiplanar addition, and represents the microscopic reverse of the Hofmann elimination. The reverse Hofmann reaction as normally carried out under acidic conditions is a much slower reaction, much less sensitive to structure, which goes by a quite different mechanism. The cyclisation of amine (8), for example, is specific acid catalysed and probably involves a carbonium ion intermediate.