Trimethylene intermediates

Dynamics calculations have also provided new approaches to the stereochemical modes through which cyclopropanes and trimethylene intermediates may be related. Full quantum dynamics calculations for the trimethylene diradical based on a reduced dimensionality model that followed wave packet densities and time constants for formation of products led to the conclusion that conrotatory and disrotatory double rotations of both terminal methylene groups are favored over a single rotation of just one by a 2.2 1 ratio." °... 

At a temperature of 407 °C (or, very nearly, at 422.5 °C) the assumption that A AT may be used as a fair approximation for AAG and application of the standard steady-state approximation for the trimethylene intermediate leads to an overall estimation of one-... 

Calculations based on the steady-state treatment of trimethylene intermediates, and one-center epimerization as well by way of EF(ts) structures, provided kn kD values of 1.293 and 1.324 for k] and kn for 1,2-drcyclopropane, and of 1.328 and 1.368 for 1,2,3-d3-cyclopropane the balance given by the ratio kx kn, according to these calculations, is not very sensitive to deuterium labeling, for the ratios are 1.06,1.08 and 1.09 for unlabeled, 1,2-d2- and l,2,3-d3-cyclopropanes. Thus the theoretically based kt kl2 ratios for cyclopropanes and these deuterium-labeled cyclopropanes agree closely with the experimentally secured ratio k] kn = 1.0 0.2 found for the 1- C-l, 2, 3-d3-cyclopropanes166. 

All of the experimental and theoretical work on the stereomutations of cyclopropanes and vinylcyclopropanes covered above seems consistent with and understandable in terms of kinetically significant involvements of Cj(ts), Cs(ts) and EF(ts) structures and partitionings of EE trimethylene intermediates resulting in the formation of klt k2 and kl2 products at comparable rates. For trans-1,2-disubstituted cyclopropanes, neither the Smith mechanism (one-center stereomutations only) nor any two-center-only formulation can be correct, as demonstrated by Crawford and Lynch in 1968143 and reinforced by numerous subsequent studies (Figures 2 and 3). 

Thermal deazetization of pyrazolines results in the formation of cyclopropanes and alkenes, illustrated in Figure 44 for the parent compound (18). This reaction is of interest in that one could imagine that it would involve the same trimethylene biradical (19) proposed to be an intermediate in cyclopropane stereomutation (Section III.A). Supporting this notion is the observation that the parent pyrazoline gives 89% cyclopropane and 11 % propylene at 250° C. If one took this product ratio as a reflection of the branching ratio from a common trimethylene intermediate, it should then be possible to compare these figures with the relative rates of stereomutation and propylene formation from cyclopropane-d2 . Interestingly, they are identical. 

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