CHEMISTRY OF N-HETEROCYCLIC CARBENE–BORANE COMPLEXESSolovyev, Andrey (2012) CHEMISTRY OF N-HETEROCYCLIC CARBENE–BORANE COMPLEXES. Doctoral Dissertation, University of Pittsburgh. (Unpublished)
AbstractStudies on chemistry of N-heterocyclic carbene-borane complexes (NHC-boranes), a new class of organic reagents, are reported. The radical chain mechanism of xanthate reduction with NHC-boranes has been established by the evaluation of rate constants of hydrogen abstraction, the isolation of boron-derived by-products, and EPR spectroscopic studies. NHC-BH3 complexes have been found to react with many electrophilic compounds. They reduce alkyl halides and sulfonates by an ionic mechanism. The boron products of their reactions with halogenation agents, Brønsted and Lewis acids were isolated and characterized. NHC-boryl iodide and triflate complexes undergo nucleophilic substitutions at the boron atom. In this way, a variety of substituted boranes with unusual and unprecedented structural motifs were prepared, including boryl azides, nitrosooxyborane, and nitroborane. The reaction with phenoxides in tetrahydrofuran (THF) afforded unexpected products of THF ring opening. A compound with a novel dihydroxyborenium cation was obtained from certain disubstituted NHC-boranes under acidic conditions. Reduction of NHC-boryl iodide complex with lithium di-tert-butylbiphenylide (LDDB) gave a reactive NHC-boryl anion. This was trapped with several electrophiles to obtain boron-substituted complexes, including acyl boranes. Deprotonation of imidazol-2-ylidene-boranes with BuLi and subsequent reactions with electrophiles allowed us to prepare ring-functionalized NHC-borane complexes. The resulting complexes of substituted NHC and BH3 can be converted to corresponding substituted imidazolium salts by a simple deboronation protocol. Share
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