Carbocations cyclobutenyl

Cyclobutenyl carbocations have been under study in our laboratory for some years (11). In (34), unusually strong temperature dependence of l3C chemical shifts (up to 0.077 ppm/K) was observed for long-lived cations 17a-c. The authors suggested that this resulted from a very low inversion barrier for the four-membered ring (Scheme 13). 

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