Quantum chemical modeling of chiral catalysis. Part 19. Strain and stability - oxazadiboretanes potentially involved in the enantioselective reduction of ketones promoted by chiral oxazaborolidines

  • Nevalainen V
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The relative stabilities of 2,7-dioxa-5-aza-1,6-diborabicyclo[3.2.01,5]heptane (e.g. 4???a) and 2,10-dioxa-8-aza-1,9-diboratricyclo[6.2.01,8.04,8]decane derivatives (e.g. 4???b) were studied by means of ab initio MO (RHF) methods. The stabilities were assessed in the light of energies of reactions leading to the opening of the oxazadiboretane ring system of the derivatives. Opening reactions of the tricyclo derivatives giving rise to the regeneration of the catalyst were found to require ??? 20 kJ mol-1 (MP2/6-31G//6-31G) more energy than those of the corresponding bicyclo derivatives. Cleavages of the tricyclo systems leading to the rupture of both the oxazaborolidine and oxazadiboretane rings were found to require ??? 30 kJ mol-1 (MP2/6-31G//6-31G) more energy than those of the bicyclo ones. The stability of oxazadiboretane adducts increases with the increasing angle strain related to the partial BN ??-bond of the parent oxazaborolidines. The role of electron correlation in the description of the stability of oxazadiboretanes (relative to the parent amino- and alkoxyboranes) was found to be more significant than that of polarization functions. ?? 1994.

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  • Vesa Nevalainen

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