Secondary deuterium isotope effect, cyclopropane

These recent calculations for cyclopropanes and trimethylene transition structures, and for isotopically labeled analogs, have provided vibrational frequencies from which secondary deuterium isotope effects have been calculated. Getty, Davidson and Borden found that reactions dependent upon C,(ts) at 422.5 °C should be associated with secondary deuterium kinetic isotope effects favoring access to 1,2-d2-Cj(ts) over l,3-d2-C (ts) structures by a factor of 1.13269. 

As usual, the singly subscripted rate constants refer to one-center epimerizations and the doubly subscripted rate constants refer to two-center epimerizations at the carbons indicated by the numbers. There should, in principle, be a secondary deuterium isotope effect included in the mechanistic rate constants comprising but this would have its largest effect on k 2 and 23 and the previous experience in cyclopropane stereomutations suggests that these are likely to be minor contributors at best. 

Thus the secondary and primary deuterium isotope effects determined in this study also indicate that entropy and zero-point energy factors associated with breaking of the C—D bond and migration in the activated complexes are important for the structural isomerization to yield butene-1 and butene-2, and reaction path B (equation 197) must be included in the mechanistic considerations concerning the cyclopropane isomerization. But the higher activation energy for isobutane formation (g = 64.3 kcal mol" ) than that for butene-2 or butene-1 (Q = 62.0 0.6 kcal mol" ) indicates also that the rupture of... 

A different experimental approach to the relative importance of one-center and two-center epimerizations in cyclopropane itself was based on the isomeric l-13C-l,2,3-d3-cyclopropanes165"169. Here each carbon has the same substituents, one hydrogen and one deuterium, and should be equally involved in stereomutation events secondary carbon-13 kinetic isotope effects or diastereotopically distinct secondary deuterium kinetic isotope effects may be safely presumed to be inconsequential. Unlike the isomeric 1,2,3-d3-cyclo-propanes (two isomers, only one phenomenological rate constant, for approach to syn, anti equilibrium), the l-13C-l,2,3-d3-cyclopropanes provide four isomers and two distinct observables since there are two chiral forms as well as two meso structures (Scheme 4). Both chiral isomers were synthesized, and the phenomenological rate constants at 407 °C were found to be k, = (4 l2 + 8, ) = (4.63 0.19)x 10 5s l and ka = (4kl2 + 4, ) = (3.10 0.07) x 10 5 s 1. The ratio of rate constants k, kl2 is thus 1.0 0.2 both one-center and two-center... 

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