Showing papers by "K. N. Houk published in 2000"
112 citations
TL;DR: Predictions for ligand-tuning to improve catalyst efficiency of fenchone-based ligands in dialkylzinc additions to aldehydes are enabled.
Abstract: Three (−)-fenchyl alcohol derivatives, {(1R,2R,4S)-exo-(2-Ar)-1,3,3-trimethylbicyclo[2.2.1] heptan-2-ol, Ar = o-anisyl (2), 2-N-methylimidazolyl (3), 2-N,N-dimethylbenzylamine (4)} were synthesized, characterized by X-ray analyses, and employed as precatalysts in diethyl zinc additions to benzaldehyde. Directions and relative degrees of enantioselectivities are rationalized by QM/MM ONIOM computations of μ-O transition structure models. Enantioselectivities arise from repulsive interactions between “transferring” or “passive” alkyl groups at the zinc centers and the substituents at donor groups or the bicyclo[2.2.1]heptane moieties. These results enable predictions for ligand-tuning to improve catalyst efficiency of fenchone-based ligands in dialkylzinc additions to aldehydes.
104 citations
TL;DR: In this paper, the first study of the pressure dependence of the OH radical yield for ozonolysis of ethene, propene, 1-butene, trans-2-butenes, and trans-3-hexene and cyclopentene over the range 20−760 Torr was performed in a long-path evacuable FTIR cell or a steady-state flow-tube reactor.
Abstract: The OH radical is the key oxidizing agent in the troposphere, and ozone−alkene reactions appear to be a significant and sometimes dominant source of new HOx radicals in urban and rural air. In this work, we report the first study of the pressure dependence of the OH radical yield for the ozonolysis of ethene, propene, 1-butene, trans-2-butene, and 2,3-dimethyl-2-butene over the range 20−760 Torr and of trans-3-hexene and cyclopentene over the range 200−760 Torr. Low-pressure experiments were performed in a long-path evacuable FTIR cell or a steady-state flow-tube reactor in series with a gas chromatograph/flame ionization detector and FTIR cell. We have also investigated the effect of adding SF6 at atmospheric pressure for ethene, 1-butene, and trans-2-butene, in a collapsible Teflon chamber. OH formation increased almost 3-fold for ethene at low pressures, from 0.22 ± 0.06 at 760 Torr to 0.61 ± 0.18 at 20 Torr, and increased somewhat for propene from 0.33 ± 0.07 at 760 Torr to 0.46 ± 0.11 at 20 Torr. A p...
91 citations
TL;DR: In this paper, the ozone reaction with cycloalkenes was measured using small amounts of fast-reacting aromatics and aliphatic ethers to trace ozone radical formation.
Abstract: OH radical formation yields from the reaction of ozone with several cycloalkenes were measured using small amounts of fast-reacting aromatics and aliphatic ethers to trace OH formation. Measured OH yields are much higher than for acyclic analogues. The yields are 0.62 ± 0.15, 0.54 ± 0.13, 0.36 ± 0.08, and 0.91 ± 0.20 for cyclopentene, cyclohexene, cycloheptene, and 1-methylcyclohexene, respectively. Density functional theory calculations at the B3LYP/6-31G(d,p) level are presented to aid in understanding the trends observed. Theory indicates that the OH production from cycloalkenes is largely controlled by the transition states for the cycloreversion of the primary ozonide.
65 citations
61 citations
TL;DR: The potential energy surfaces of the ground state (S0) and triplet ππ* (T1) state for the cycloaddition of acrolein to ethylene have been mapped with ab initio CASSCF calculations and the 6-31G* ba... as mentioned in this paper.
Abstract: The potential energy surfaces of the ground state (S0) and triplet ππ* (T1) state for the cycloaddition of acrolein to ethylene have been mapped with ab initio CASSCF calculations and the 6-31G* ba...
48 citations
47 citations
TL;DR: Hybrid density functional theory calculations with the B3LYP/6-31G ∗ method were used to calculate the activation barriers of nitrile oxide cycloadditions to the unsymmetrical alkynes cyanoacetylene and methyl propiolate as mentioned in this paper.
33 citations
31 citations
24 citations
TL;DR: In this paper, the relationship between number of rings and singlet-triplet gaps was explored by hybrid density functional theory for a series of heterocyclic polyacenes.
TL;DR: Anh and Eisenstein this article solved a classic problem in organic stereochemistry and simultaneously provided the impetus and a model for the use of quantum mechanics to understand a wide range of organic chemical phenomena in a style accessible to experimental chemists.
Abstract: This paper describes how Anh and Eisenstein’s publication in 1977 solved a classic problem in organic stereochemistry and simultaneously provided the impetus and a model for the use of quantum mechanics to understand a wide range of organic chemical phenomena in a style accessible to experimental chemists.
TL;DR: In this article, the rates of anionically promoted oxy-Cope rearrangement of five structurally related exo-norbornanols have been determined, and it is shown that the presence of the PhS substituent causes rearrangements to occur approximately 103 times faster by lowering the activation energy to the extent of 3−4 kcal/mol.
Abstract: The rates of anionically promoted oxy-Cope rearrangement of five structurally related exo-norbornanols have been determined. A critical difference in the substitution pattern of these substrates is the presence or absence of oxygen or sulfur functionality at the terminal carbon of the bridgehead vinyl group and the geometric nature of the olefinic center. The results convincingly establish that although an OPMP substituent exerts, at best, a very modest decelerative kinetic effect, the presence of the PhS substituent causes rearrangement to occur approximately 103 times faster by lowering the activation energy to the extent of 3−4 kcal/mol. This remarkable divergence in behavior is shown to correlate well with the highly dissociative nature of the transition state that is predicted computationally.
TL;DR: Anh and Eisenstein this article solved a classic problem in organic stereochemistry and simultaneously provided the impetus and a model for the use of quantum mechanics to understand a wide range of organic chemical phenomena in a style accessible to experimental chemists.
Abstract: This paper describes how Anh and Eisenstein’s publication in 1977 solved a classic problem in organic stereochemistry and simultaneously provided the impetus and a model for the use of quantum mechanics to understand a wide range of organic chemical phenomena in a style accessible to experimental chemists.
TL;DR: In this article, the [2+2++2]-cycloreversion reactions of cyclohexane and ten fused cycloshexanes were studied computationally with B3LYP/6-31G* and CASSCF methods.
Abstract: [2+2+2]-Cycloreversion reactions of cyclohexane and ten fused cyclohexanes were studied computationally with B3LYP/6-31G* and CASSCF methods. Reactions involving cleavage of bonds in three- and five-membered rings show distinctly lower barriers to cycloreversion than cleavage of four-membered rings. The lower activation energies for the cleavage of odd-membered rings arise from interactions of the sigma framework of the odd-membered ring with the orbitals of the breaking bond. NICS values were calculated to determine the aromaticity of the different rings involved in bond cleavage. In addition to concerted mechanisms, the stepwise diradical pathways for the [2+2+2]-cycloreversions of cyclohexane and cis-tris-cyclopropacyclohexane were studied.