Showing papers by "K. N. Houk published in 2008"
TL;DR: The overall results indicate that the lack of polymerizability of gamma-butyrolactone should be attributed to the low strain of the ring, which shows much less geometric distortion in the ester group than delta-valerolactone, and the notable stability of the coiled conformations found in model compounds of poly-4-hydroxybutyrate.
Abstract: γ-Butyrolactone, unlike δ-valerolactone, does not polymerize despite a strain energy of ∼8 kcal mol-1 which could be relieved by opening the s-cis lactone ester bond to an s-trans ester bond in the polymer. To explain this anomaly, we have applied quantum mechanical methods to study the thermochemistry involved in the ring-opening reactions of γ-butyrolactone and δ-valerolactone, the conformational preferences of model molecules that mimic their corresponding homopolyesters, and the variation of enthalpy associated to the polymerizability of such two cyclic lactones. The overall results indicate that the lack of polymerizability of γ-butyrolactone should be attributed to the low strain of the ring, which shows much less geometric distortion in the ester group than δ-valerolactone, and the notable stability of the coiled conformations found in model compounds of poly-4-hydroxybutyrate.
153 citations
TL;DR: The computed reductive elimination transition structures show that the participation of the second π-orbital in acetylene and allene reduces the barrier by 9∼15 kcal/mol, compared to ethylene, for which no such interactions are possible.
Abstract: Rhodium dimer [Rh(CO)2Cl]2 efficiently catalyzes the intra- and intermolecular (5 + 2) reactions of vinylcyclopropanes with alkynes and allenes, but not alkenes. This difference in reactivity is attributed to the difficulty of reductive elimination for the alkene. The computed reductive elimination transition structures show that the participation of the second π-orbital in acetylene and allene reduces the barrier by 9∼15 kcal/mol, compared to ethylene, for which no such interactions are possible.
129 citations
TL;DR: Computational studies provide support for the involvement of intermolecular pi-interactions in the chiral recognition of secondary benzylic alcohols by the enantioselective acyl transfer catalyst CF3-PIP.
Abstract: Computational studies provide support for the involvement of intermolecular π-interactions in the chiral recognition of secondary benzylic alcohols by the enantioselective acyl transfer catalyst CF3−PIP.
94 citations
TL;DR: Although azide cycloadditions are suitable for irreversible chemistry and are typically unsuitable for reversible applications, theoretical procedures established by these studies have provided guidelines for the prediction of useful reversible libraries.
Abstract: Substituent effects in 1,3-dipolar cycloadditions of azides with alkenes and alkynes were investigated with the high-accuracy CBS-QB3 method. The possibilities for noncatalytic activation and the reversibility or irreversibility of these reactions was explored; the possibilities for uses in dynamic combinatorial chemistry (DCC) or click chemistry were explored. The activation enthalpies for reactions of ethylene and acetylene with hydrazoic acid, formyl, phenyl-, methyl-, and methanesulfonylazides exhibit modest variation, with ΔH⧧ ranging from 17 to 20 kcal/mol. A detailed study of formylazide cycloadditions with various alkenes and alkynes reveals a narrow range of activation enthalpies (17−21 kcal/mol). The activation enthalpies for the reactions of azides with alkenes and alkynes are similar. FMO theory and distortion/interaction energy control have been used to rationalize the rates and regiochemistries of cycloadditions involving alkene dipolarophiles. Significantly, triazoles, formed from alkynes, ...
70 citations
TL;DR: B3LYP/6-31G* and CASMP2 calculations have been employed to study the ene reaction of singlet oxygen with trans-cyclooctene, predicting that the reaction involves a perepoxide intermediate, whereas alkenes such as tetramethylethylene are predicted by the same methods to occur by a two-step no-intermediate mechanism.
Abstract: B3LYP/6-31G* and CASMP2 calculations have been employed to study the ene reaction of singlet oxygen with trans-cyclooctene. These methods predict that the reaction involves a perepoxide intermediate, whereas alkenes such as tetramethylethylene are predicted by the same methods to occur by a two-step no-intermediate mechanism, with no perepoxide intermediate. The change in mechanism arises because the trans-cyclooctene imposes a substantial strain in the transition state for hydrogen abstraction. The perepoxide is formed through a polarized diradical intermediate that can lead to the observation of alkene isomerization. The polarized diradical also becomes a minimum because of the barrier to abstraction.
46 citations
TL;DR: CBS-QB3 calculations on isodesmic reactions for transfer of groups from an alkane to an ester show that a cyclopropyl group provides hyperconjugative stabilization.
44 citations