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| - Reaction Profiles and Substituent Effects for the Reduction ofCarbonyl Compounds with Monomer and Dimer Simple MetalHydride Reagents
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| - Reaction profiles and energetics for the reactions of substituted benzaldehydes with a series ofdifferent simple metal hydrides (BH3, BMeH2, BMe2H, AlH3, and AlMe2H) are examined computationally. B3LYP/6-31G* optimizations and MP2/6-311G* single point energy calculations revealedthat the Al reagents are more reactive than B reagents. Replacement of H with Me on BH3 or AlH3makes the reduction transition state (TS) less stable. Accordingly the overall reactivity is in theorder AlH3> AlMe2H > BH3> BMe2H. The Hammett ρ value for substituted benzaldehydes (BAs)is negative for the initial complex formation and positive for the hydride-transfer step. The sizeand the sign of the apparent ρ value depend on the relative stabilities of the separated reactantsand the complex. The TS structures vary according to the Hammond postulate for substituted BAsand the variation is reflected in carbonyl-carbon and aldehyde-deuterium isotope effects. Comparisonof the reaction profiles of the monomer and dimer reagents reveals that the real reacting speciesis the monomer in the gas phase for BH3 but the dimer for BMe2H.
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