Brett R. Beno

EXPLORATION OF PERICYCLIC REACTION TRANSITION STRUCTURES BY QUANTUM MECHANICAL METHODS : COMPETING CONCERTED AND STEPWISE MECHANISMS

Abstract Density functional theory and multiconfigurational SCF calculations have been applied to a number of pericyclic reactions, including cycloadditions, electrocyclizations and sigmatropic shifts. Emphasis is on the competition between concerted and stepwise mechanisms, comparisons of computed and experimental activation energies and isotope effects, and the performance of MP2, CASSCF and DFT calculations. Various diradical processes, such as the vinylcyclopropane rearrangement and cyclobutane isomerizations, were also studied to test the performance of DFT with diradical processes.

Concerted and stepwise mechanisms in cycloaddition reactions: potential surfaces and isotope effects

CASSCF/6-31G* calculations have been performed on concerted and stepwise Diels–Alder reactions of butadiene with ethene, the dimerization of butadiene and the dimerization of cyclobutadiene. The relative energies of concerted and stepwise mechanisms are compared, and the factors influencing these “energies of concert” are discussed. The comparison of calculated isotope effects to experimental data provides support for theoretical results.

Isotope effects for Lewis acid catalyzed Diels-Alder reactions. The experimental transition state

Abstract 2 H and 13 C kinetic isotope effects were determined for all positions on isoprene in its reactions with methyl vinyl ketone, ethyl acrylate, and acrolein catalyzed by Et 2 AlCl. The results are taken as supporting a highly asynchronous concerted [4+2] cycloaddition. Comparison of the experimental isotope effects with calculated values supports this conclusion and the accuracy of the calculated transition structure.

Pressure-Induced Cycloadditions of Dicyanoacetylene to Strained Arenes: The Formation of Cyclooctatetraene, 9,10-Dihydronaphthalene, and Azulene Derivatives; A Degenerate [1,5] Sigmatropic Shift—Comparison between Theory and Experiment

The reaction of dicyanoacetylene (DCA) with the partially hydrogenated dimethano-bridged anthracene and pentacene derivs. I and II lead to the (1:1) Diels - Alder adducts III and IV, resp., and to the 9,10-dihydronaphthalene derivs. V and VI, resp. These compds. are intensely colored, most likely due to a charge-transfer absorption. Photolysis of the (1:1) adducts III and IV produces the corresponding cyclooctatetraene derivs. , which are not planar despite the torsional constraints caused by the fusion of the eight-membered ring to norbornane and norbornene units. The mechanisms of formation of the dihydronaphthalene derivs. V and VI were elucidated by the use of high pressure; this allowed the (2:1) adducts VII and VIII to be detected as intermediates in the reaction of DCA with III or IV to give V and VI, resp. A degenerate rearrangement consisting formally of a [1,5] vinyl shift in V or VI could be detected by their temp.-dependent 1H NMR spectra. The mechanisms of the [1,5]-sigmatropic shift reactions in V and IX have been examd. at the Becke3LYP/6-31G* level of theory. Stepwise mechanisms are predicted, and the calcd. activation barrier is in good agreement with the measured DH.thermod.. The finding that the irreversible rearrangement of V accompanied by the elimination of HCN to give the azulene deriv. proceeds only on heating of V to 80 Deg in a polar solvent is good evidence for a polar mechanism in this case.

THEORETICAL STUDIES OF NOVEL AROMATIC MOLECULES AND TRANSITION STATES

Monocyclic conjugated molecules have stabilities, bond lengths, and magnetic properties following expectations from Huckels Rule. Two hydrocarbons which seem to deviate from these generalizations, s-indacene and cyclohepta(def)fluorene, were studied with density functional theory. The former has characteristics expected for both aromatic and antiaromatic molecules; the latter is a ground state triplet. Potentially aromatic transition states with 10 electrons were also investigated. The mechanism of the 5,5-sigmatropic shift is predicted to be stepwise involving diradical intermediates.

Retro-cycloadditions and sigmatropic shifts: the C 7 H 8 and C 7 H 10 potential energy surfaces

Retro-cycloadditions and thermal rearrangements of hydrocarbons have been investigated with density functional theory (B3LYP) and complete active space multiconfiguration self consistent field theory (CASSCF and CASPT2). We review recent results from our laboratories. Subjects are the laser-induced retro-Diels- Alder reactions of norbornene, thermal retro-Diels-Alder reactions of norbornenes and isopropylidenorbornene, 1,3-sigmatropic shifts of vinylcyclopropane and bicyclo(3.2.0)heptenes, and the 1,5-sigmatropic shifts of norcaradienes. The competition between concerted and stepwise processes and the nature of diradicals formed in these processes are described.

Cycloaddition of o-benzyne to benzene and the inner phase of a hemicarcerand

Becke3LYP/6-31G* calculations predict a small energy of concert for the Diels–Alder reaction of o-benzyne with benzene, and force-field calculations indicate that steric interactions and transannular strain are responsible for the regiospecificity of the Diels–Alder reaction between o-benzyne and its host hemicarcerand.