Lewis acids, boranes

Brown proposed a mechanism where the enolate radical resulting from the radical addition reacts with the trialkylborane to give a boron enolate and a new alkyl radical that can propagate the chain (Scheme 24) [61]. The formation of the intermediate boron enolate was confirmed by H NMR spectroscopy [66,67]. The role of water present in the system is to hydrolyze the boron enolate and to prevent its degradation by undesired free-radical processes. This hydrolysis step is essential when alkynones [68] and acrylonitrile [58] are used as radical traps since the resulting allenes or keteneimines respectively, react readily with radical species. Maillard and Walton have shown by nB NMR, ll NMR und IR spectroscopy, that tri-ethylborane does complex methyl vinyl ketone, acrolein and 3-methylbut-3-en-2-one. They proposed that the reaction of triethylborane with these traps involves complexation of the trap by the Lewis acidic borane prior to conjugate addition [69]. 

The Lewis basic carbonyl group forms a complex with the empty p orbital of the Lewis acidic borane. Hydride transfer is then possible from anionic boron to electrophilic carbon. The resulting tetrahedral intermediate collapses to an iminium ion that is reduced again by the borane. 

Amine-boranes are coordination complexes formed by combination of an amine Lewis base with a tricoordinate Lewis acid borane fragment (equation 1), and as noted in Figure 1, they are analogs of alkanes. 

Hydroboration of coordinated alkenes has been achieved with the phosphine-stabilized olefin complexes, (775-C5H5)2Zr(772-CH2=CHR)(PPh2Me) (R = H, 78 Et, 79 Ph, 80), upon addition of the Lewis-acidic borane, HB(C6F5)2 (Equation (5)).37,65 Solid-state characterization indicates a weak interaction between the formally positively charged zirconium center and the carbon adjacent to the borate anion. This interaction is maintained in solution, as an upheld shifted 13C NMR resonance is observed for this carbon, which is in agreement with previous reports of metal-carbon interactions of this type. 

Masamune and coworkers [212, 583] designed boranes 2.14 (R = H) bearing a C2 axis of symmetry for asymmetric hydroboration. In the presence of catalytic amounts of 2.14 (R = MeSOj), which behaves as a Lewis acid, boranes 2.14 (R = H) are very efficient reagents for asymmetric reductions of ketones [87], A drawback to the use of boranes 2.14 (R = H) is their poor stability. In practice, they must be generated just prior to use from a precursor 2.14 (R = OMe). 

Strong Lewis acidic boranes such as FAB can abstract alkyl groups from metal complexes other than group 4. Puddephatt et al. " showed that platinum-(II) cations (67) can be generated via a methide abstraction from the complex (dbbipy)Pt(CH3)2 (eq 37). 

Boron trifluoride (BFg) is a classic Lewis acid. Borane, BHg, is a highly reactive boron compound that also functions as a Lewis acid in the presence of a suitable electron-donating species. 

In comparison with the A-nnsnbstitnted pyridine the hyperpolarizabilities by insertion of the Lewis acidic borane increase by a factor of approximately two. The (3 values determined by EFISH experiments ranged between 30 and 72.5 x 10 ° esn for the pyridine/BFj and BlCgFjlj Lewis adducts. 

M. J. G. Lesley, A. Woodward, N. J. Taylor, T. B. Marder, I. Cazenobe, I. Ledoux, J. Zyss, A. Thornton, D. W. Bruce, A. K. Kakkar, Lewis acidic borane adducts of pyridines and stilbazoles for nonlinear optics. 

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