Showing papers by "K. N. Houk published in 1993"

Hetero-Diels-Alder reaction transition structures: reactivity, stereoselectivity, catalysis, solvent effects, and the exo-lone-pair effect

Abstract: Transition structures for the Diels-Alder reactions of butadiene with formaldehyde, thioformaldehyde, formaldimine, N-methylformaldimine, diaxene, nitrosyl hydride, singlet oxygen, BH 3 -coordinated formaldehyde and formaldimine, and protonated formaldimine have been located with ab initio molecular orbital calculations using the 3-21G basis set. Activation energies have been evaluated with MP2/6-31G * calculations on these geometries. Formaldehyde and the imines are predicted to be comparable in reactivity to ethylene, whereas the other dienophiles are predicted to be more reactive

Electrocyclic reactions of 1-substituted 1,3,5,7-octatetraenes. An ab initio molecular orbital study of torquoselectivity in eight-electron electrocyclizations

Abstract: The effects of substituents on the conrotatory electrocyclizations of 1-substituted cis,cis-1,3,5,7-octatetraenes have been studied with ab initio molecular orbital theory. The results are compared to the conrotatory electrocyclic ring openings of 3-substituted cyclobutenes. Geometry optimizations employed restricted Hartree-Fock calculations and the 3-21G basis set. Electron correlation energies were calculated using second-order MOller-Plesset theory and the 6-31 G * basis set. The transition structure for the conrotatary electrocyclization of cis,cis-1,3,5,7-octatetraene has a helical structure

Reagent-specific electrostatic effects on electrophilic .pi.-facial stereoselectivity. Interpretation of the stereochemical variation of electrophilic additions to 7-isopropylidenebenzonorbornenes

Abstract: Electrostatic interactions account for the stereochemical variations in electrophilic additions to 7-isopropylidenebenzonorbornenes. Didferent electrophilic reagents have very different electrostatic profiles. This conclusion is supported by electrostatic potential analyses of 7-methylenenorbornene and 7-methylenebenzonorbornene or by calculations on the interactions of point charges with the two faces of the π system

Experimental and theoretical studies of acid-catalyzed oxygen-18 exchange rates of conformationally rigid ketones. Is the antiperiplanar effect important?

Abstract: The relative rates for the acid-catalyzed addition of H 2 18 O to the carbonyl group of the conformationally fixed bridged biaryl ketone, 1, and several derivatives bearing methyl, chloro, and methoxy substituents in axial-like and equatorial-like orientations have been measured. The effects of axial- versus diaxiallike methyl and chloro substituents produce rate decreases which are consistentwith the antiperiplanar (hyperconjugative) interaction proposed by Cieplak; however, the negligible influence of a methoxy group when antiperiplanar is not consistent