Calculations of the effect of tunneling on the Swain-Schaad exponents (SSEs) for the 1,5-hydrogen shift in 5-methyl-1,3-cyclopentadiene. Can SSEs be used to diagnose the occurrence of tunneling?

MPW1K density functional calculations, carried out with the 6-31+G(d,p) basis set, have been combined with canonical variational transition state theory (CVT) and small-curvature tunneling (SCT) corrections in order to compute the primary kinetic isotope effects for rearrangement of 5-methyl-1,3-cyclopentadiene (1) to 1-methyl-1,3-cyclopentadiene (2). The Swain-Schaad exponents, SSE = ln(kH/kT)/ln(kD/kT), for this reaction have been computed over the temperature range 100-600 K. Tunneling results in both large positive and large negative deviations from the value of SSE = 3.26, expected from consideration of only the effect of the isotopic mass on passage over the reaction barrier. In the rearrangement of 1 to 2, SSE approximately 3.26, not only at temperatures >400 K, where tunneling is relatively unimportant, but also around 170 K, where tunneling by both H and D is the dominant mode of reaction. Thus, from an experimental finding that SSE approximately 3.26 at a single temperature, it cannot be rigorously concluded that tunneling is unimportant. Measurement of SSEs over a broad temperature range is advisable; but measurement of the temperature dependence of just kH/kD can be used to establish more unequivocally whether tunneling is important, without the necessity of measuring kT.

Oxidation of tertiary silanes by osmium tetroxide.

In the presence of an excess of pyridine ligand L, osmium tetroxide oxidizes tertiary silanes (Et(3)SiH, (i)Pr(3)SiH, Ph(3)SiH, or PhMe(2)SiH) to the corresponding silanols. With L = 4-tert-butylpyridine ((t)Bupy), OsO(4)((t)Bupy) oxidizes Et(3)SiH and PhMe(2)SiH to yield 100 +/- 2% of silanol and the structurally characterized osmium(VI) mu-oxo dimer [OsO(2)((t)Bupy)(2)](2)(mu-O)(2) (1a). With L = pyridine (py), only 40-60% yields of R(3)SiOH are obtained, apparently because of coprecipitation of osmium(VIII) with [Os(O)(2)py(2)](2)(mu-O)(2) (1b). Excess silane in these reactions causes further reduction of the OsVI products, and similar osmium "over-reduction" is observed with PhSiH(3), Bu(3)SnH, and boranes. The pathway for OsO(4)(L) + R(3)SiH involves an intermediate, which forms rapidly at 200 K and decays more slowly to products. NMR and IR spectra indicate that the intermediate is a monomeric Os(VI)-hydroxo-siloxo complex, trans-cis-cis-Os(O)(2)L(2)(OH)(OSiR(3)). Mechanistic studies and density functional theory calculations indicate that the intermediate is formed by the [3 + 2] addition of an Si-H bond across an O=Os=O fragment. This is the first direct observation of a [3 + 2] intermediate in a sigma-bond oxidation, though such species have previously been implicated in reactions of H-H and C-H bonds with OsO(4)(L) and RuO(4).

Tunneling in the 1,5-hydrogen shift reactions of 1,3-cyclopentadiene and 5-methyl-1,3-cyclopentadiene.

MPW1K/6-31+G(d,p) calculations which include the effects of small curvature tunneling find that, around room temperature, thermally activated tunneling dominates the 1,5-hydrogen shift reactions of 1,3-cyclopentadiene (2a) and 5-methyl-1,3-cyclopentadiene (2c). The calculated temperature dependence of the H/D kinetic isotope effect (KIE) for the latter rearrangement agrees well with experimental measurements that were published nearly 40 years ago. It is argued that the experimental KIEs provide prima facie evidence for tunneling in this reaction. The calculations also predict that it should be possible, at least in principle, to confirm this conclusion by observing curvature in the Arrhenius plot for the rearrangement of 2c.

Why does perfluorination render bicyclo[2.2.0]hex-1(4)-ene stable toward dimerization? Calculations provide the answers.

B3LYP calculations with two different basis sets have been performed to understand why bicyclo[2.2.0]hex-1(4)-ene (1a) undergoes dimerization with DeltaH(++) = 11.5 kcal/mol, but dimerization of perfluorobicyclo[2.2.0]hex-1(4)-ene (1b) has never been observed. The former reaction is computed to be exothermic by 37.2 kcal/mol, whereas the latter is calculated to be endothermic by 7.4 kcal/mol. The 44.6 kcal/mol difference between the enthalpies of these two reactions can be dissected into contributions of 24.5 kcal/mol for the difference between the enthalpies for forming diradical intermediates 2a and 2b and 20.1 kcal/mol for cyclization of 2a and 2b to, respectively, 3a and 3b. The latter enthalpy difference is largely attributable to repulsions between the endo-fluorines in the dimer, although the exo-fluorines also are found to contribute. The former enthalpy difference is attributable to the difference between the dissociation enthalpies of the pi bonds in 1a and 1b, which is shown to amount to 16 +/- 1 kcal/mol. About 25% of the stronger pi bond in fluoroalkene 1b is found to be due to hyperconjugation of the eight C-F bonds in 1b with the filled pi orbital. However, the major contributor to the stronger pi bond in 1b is shown to be the unfavorable interaction that results when a pyramidalized radical center is syn to a C-F bond. Both of these effects, which contribute to the greater strength of the pi bond in 1b, relative to that in 1a, are analyzed and discussed.

Cooperative and competitive effects of substituents at C1 and C4 on the barriers to ring inversion of 5,5-difluorobicyclo[2.1.0]pentanes.

To identify the reasons for the very low barrier that has been measured for ring inversion of 1,4,5,5-tetrafluorobicyclo[2.1.0]pentane (deltaG(double dagger) = 6.8 +/- 0.2 kcal/mol), CASSCF and CASPT2 calculations have been performed on ring inversion in this and other bicyclo[2.1.0]pentanes. The results of the calculations show that a cooperative interaction between the geminal fluorines at C2 and the fluorines at C1 and C3 in the singlet cyclopentane-1,3-diyl transition structure (TS) contributes 3.7 kcal/mol to lowering the barrier to ring inversion in the tetrafluoro compound. In contrast, a competitive substituent effect in the TS for ring inversion of 1,4-dicyano-5,5-difluorobicyclo[2.1.0]pentane is predicted to raise the barrier height by 6.1 kcal/mol. The origin of these cooperative and competitive substituent effects is discussed.

Rate constants for hydrogen abstraction from alkoxides by a perfluoroalkyl radical. An oxyanion accelerated process.

A combination of laser flash photolysis and competitive kinetic methods has been used to measure the absolute bimolecular rate constants for hydrogen atom abstraction in water from a series of fluorinated alkoxides and aldehyde hydrates by the perfluoroalkyl radical, *CF2CF2OCF2CF2SO3- Na+. The bimolecular rate constants observed for the beta-fluorinated alkoxides were in the 10(5) M(-1) s(-1) range, such rates representing enhancements (relative to the respective alcohols) of between 100 and almost 1000-fold, depending on the reactivity of the alkoxide. Likewise, the monobasic sodium salts of chloral and fluoral hydrate exhibit similar rate enhancements, relative to their respective hydrates.

Computational discovery of a novel automerization process for 1-fluorocyclopropene.

[reaction: see text] Both DFT and CCSD and CCSD(T) computational methods indicate that 1-fluorocyclopropene can undergo an unprecedented "electrocyclic" automerization process involving a full 180 degrees rotation of its methylene group, without the formation of an intermediate carbene or diradical.