Irans-3,4-Dimethylcyclobutene

TS energies are likely to be adequately estimated, especially the trends among related inward versus outward TS for a series of substituted cyclobutenes. They first examined the ring opening of iran -3,4-dimethylcyclobutene (116) and irani -B.d-dihydroxycyclobutene (117).2° °2 The outward rotation is favored over the inward for both cases. More importantly, while the inward barriers are similar, the outward barrier for 117 is 17 kcal mol lower than for 116, a difference not readily amenable to rationalization based solely on substituent size. 

Under thermal conditions, (2 , 4Z)-hexadiene cyclizes to cw-3,4-dimethylcyclobutene, and (2 , 4 )-hexadiene cyclizes to iran -3,4-dimethylcyclobutene. 

These processes may be designated conrotatory (16) and disrotatory (17). In practice the isomerization of the appropriately substituted cyclobutenes follow a conrotatory pathway. Thus ci5-3,4-dimethylcyclobutene yields only cia-irans-2,4 hexadiene, and iraws-3,4-dimethyleyclobutene yields only transition state suggested previously, the conrotatory process is in fact the one to be expected. However, the situation is not quite as simple as here implied. By similar arguments the thermal cyclization of hexatrienes would also be expected to be conrotatory, whereas in fact it is disrotatory, viz. ... 

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