Dominant Disrotatory Double Rotation in the Thermally Induced 1,2-Dimethylspiropentane Geometric Isomerization

Article abstract of DOI:10.1021/ja00545a025

Pyrolysis of optically active trans-1,2-dimethylspiropentane and syn-4,4-dideuterio-cis-1,2-dimethylspiropentane at 290.0 deg C in the vapor-phase allowed determination of k_T->C + k_C->T = 3.43E-5 /s, K_eq(T/C) = 2.41, k_Tact->Trac = 1.36E-5 /s, and k_CS->CS+CA = 8.76E-5 /s.This data indicates preferred double over single rotation in the cleavage of the C-1, C-2 bond, particularly with the cis isomer.The greater preference for double inversion over single inversion from the cis relative to the trans isomer suggests that the double inversion occurs by a disrotatory pathway.Pyrolysis of optically active trans,medial- and trans,proximal-1,2,4-trimethylspiropentanes at low conversions revealed that the previously observed interconversions of these isomers occur predominantly by C-4, C-5 bond cleavage and not by double rotation after C-1, C-2 bond fission.These observations reinforce the conclusion above that the dominant pathway for double rotation in spiropentane isomers is disrotatory.