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Oxa- and azarhodacyclobutanes as potential intermediates in novel catalytic transformations Dauth, Alexander


Oxa- and azametallacyclobutanes are commonly invoked as reactive intermediates in transition-metal catalyzed reactions. However, experimental and theoretical support for their involvement in certain transformations is often scarce. In recent years, our group has been investigating the chemistry of oxa- and azarhodacyclobutanes to gain a deeper understanding of their formation and reactivity. The reactivity of oxarhodacyclobutanes (rhodaoxetanes) with organometallic reagents was investigated. Successful transmetalation with a variety of organoboron nucleophiles was achieved in good yields. This constitutes the first step in a proposed carbohydroxylation protocol. Studies towards reductive elimination have been performed but have to date remained unsuccessful. Rhodaoxetanes were also found to undergo migratory insertion with electron deficient alkynes. The resulting six-membered oxarhodametallacycles were fully characterized. Substituted rhodaoxetanes were prepared from the corresponding rhodium-olefin complexes. Exclusive selectivity for incorporation of the oxygen atom on the more substituted olefin carbon was achieved. Oxidation of rhodium ethylene complexes with a selection of nitrene precursors led to formation of azarhodacyclobutanes as two isomers. Appropriate choice of solvent and oxidant allowed for the selective preparation of either isomer. Preliminary reactivity studies revealed the high thermal and chemical stability of the product complexes. Similar to rhodaoxetanes, substituted azarhodacyclobutanes were formed by oxidation of rhodium complexes bearing substituted olefins. In this case, one isomer was formed as the major or sole product with good-to-excellent selectivity for incorporation of the nitrogen atom on the less substituted olefin carbon. Preliminary mechanistic considerations suggest that two independent mechanisms are operative including oxidation of a coordinated olefin and cycloaddition with a free olefin in solution.

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