Berson-Salem subjacent orbital effect

The degenerate thermal rearrangement of methylenecyclobutanes has been examined using the MINDO/2 semi-empirical MO method. The retention of configuration of the migrating centre can be attributed to subjacent orbital control (of the kind described by Berson and Salem) and to antarafacial participation of the allyl unit because of Jahn-Teller instabilities in the biradical transition state. The course of the reaction should be very sensitive to substituent effects. 

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