Studies on cycloaddition reactions of ketenes: Further investigations of pseudopericyclic reaction mechanisms

Cycloaddition reactions of ketenes are widely used in organic synthesis. The cycloadditions of ketene with ethylene, formaldimine and formaldehyde were studied theoretically using B3LYP/6-31G*, MP2/6-31G* and MCSCF/6-31G* methods. A pseudopericyclic mechanism was proposed for the reactions between ketene with ethylene and formaldehyde. A pseudopericyclic transition state was located for the reaction of ketene with formaldimine as well even though it has a higher energy barrier than a conrotatory electrocyclization transition state, which is a concerted in the gas phase and a stepwise in solvent confirmed by IRC calculations.;The cycloadditions of formaldimine with conjugated ketenes were studied theoretically at the B3LYP/6-31G* level. The concerted and stepwise [4+2], [2+2] cycloaddition reactions were examined systematically. For the reaction of formaldimine with vinylketene, the stepwise [2+2] and the concerted [4+2] are all pericyclic cycloaddition reactions and have similar energy barriers. For the cycloaddition reactions between formaldimine with imidoylketene and formylketene, the stepwise [4+2] pathways are the lowest energy barrier ones. The concerted [4+2] and the second step of the stepwise [4+2], electrocyclization steps are pseudopericyclic with dramatically low barriers. In addition, the choice of formaldimine led the 1,5- and 1,3-hydrogen shift reaction possible in some zwitterions intermediate, therefore transition states of them were located. These transition states have low energy barriers and planar geometries. Thus these reactions are best interpreted as pseudopericyclic as well.;The least developed conjugated ketenes, imidoylketenes were studied more experimentally and theoretically. For the first time, a novel reaction condition was developed to generate a variety of substituted imidoylketenes. The first biomolecular reactions observed are dimerizations of imidoylketenes, which were studied experimentally and theoretically. A series of trapping reactions with other reagents were studied as well. In addition, a new method to generate oxoketenes from beta-keto carboxylic acids were investigated and some experimental evidence for the pseudopericyclic reaction mechanism of the thermal cheletropic reactions were found by analyzing the X-ray structures of ground states of pyrrolediones, furandiones and their derivatives. Finally, sequential transition states were located in the formation of a semibullvalene and the valley-ridge inflection point was located on its potential energy surface as well.*.;*This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Microsoft Office.