Theoretical and experimental studies on the reactivities of conjugated ketenes
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The reactivity of imidoylketene was examined using ab initio molecular orbital theory. MP4(SDQ)/6-31G*//MP2/6-31G* calculations on the conformations of imidoylketene as well as transition states for several of its reactions show parallels between the reactivity of imidoylketene and its oxygen analog formylketene. All reactions proceed via concerted, planar (or nearly so) transition structures regardless of the number of electrons involved. Calculated activation energies are remarkably lower than those for a pericyclic process, as expected from the case of formylketene. The reactions are interpreted in light of their pseudopericyclic orbital topology. N-Propylacetacetimidoylketene was produced by the solution pyrolysis of t-butyl N-propyl 3-amino-2-butenoate. Selectivities of acetimdoylketene toward various polar reagents were measured for the first time in a series of competitive trapping reactions. Significant steric and electronic discriminations of this ketone were observed, suggesting further synthetically useful applications. These experimental reactivity trends indirectly provide support for the planar, pseudopericyclic transition structures predicted by ab initio calculations. The mechanism of the reactions of nitrosoketene to form cyclic nitrones (which leads stereoselective synthesis of a-amino acids) was investigated using ab initio molecular orbital theory (MP4(SDQ)/6-31G*//MP2/6-31G* + ZPE). The direct [3+2] cycloadditions of nitrosoketene with ketones are calculated to be favored over the alternative [4+2] pathway via concerted, asynchronous, pseudopericyclic transition states. The detailed conformations and the reactivity of nitrosoketene toward sterically and electronically different ketones render useful information of the synthetic route for the biologically important reactions. Transition structures for a series of eight cheletropic decarbonylations were optimized at the MP4(SDQ)/D95**//MP2/6-3lG* + ZPE level. Dramatic differences in activation energies and in exothermicities are discussed in terms of the molecular orbital topology. A fundamental question regarding pseudopericyclic orbital overlap is addressed, specifically, how many and what type of orbital orthogonahties in the reaction sites are needed for a reaction to be pseudopericyclic. Generalizations regarding the characteristics of the pseudopericyclic reactions are made to provide a better understanding of the "allowedness" and "favoredness" of the orbital topologies.