| Research Abstract |
Comparative studies were made of the use as acyl acceptor of organosilicon compounds [Me_3Si(CH_2)_nOH] and the corresponding carbon compounds [Me_3C(CH_2)_nOH] in the stereoselective esterification of 2-(4-chlorophenoxy)propanoic acid by the use of lipase OF 360 of candida cylindracea in water-saturated benzene. The organosilicon compounds were effectively used as the substrates for the esterification of D-acid enantiomer. Of the organosilicon compounds of different chain-length between the silicon atom and the hydroxyl group, trimethylsilylmethanol (n=1) enabled the esterification reaction to be both fast and highly stereoselective, which was difficult with conventional substrates such as its carbon counterpart. On the other hand, no difference was observed between trimethylsilylethanol (n=2) and its carbon analogue (3,3-dimethylbutanol) in the enzymatic activity and the stereoselectivity. These results indicate that the silicon atom behaved as a mimic carbon atom for lipase in the c
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ase of trimethylsilylethanol but was effective in enhancing the reactivity of trimethylsilylmethanol. The difference could be explained on the basis of the properties of silicon atom, such as its low electronegativity and big atomic radius compared with the carbon atom.
Among the commercially available alcohol dehydrogenases from different sources tested, equine lliver alcohol dehydrogenase (HLADH) recognized several organosilicon compounds, sila-substituted alcohols, as a substrate and exhibited the oxidative activity. Especially, large differences in catalytic rates were observed between trimethylsilyl alcohols [Me_3Si(CH_2)_nOH] but not between the carbon analogues. Thus, the activity of HLADH was extremely and peculiarly affected by trimethylsilyl group in the organosilicon compounds [trimethylsilylmethanol (n=1), 27% ; trimethylsilylethanol (n=2), 212% ; trimethylsilylpropanol (n=3), 65% ; ethanol, 100%]. These differences might result from beta-effect of trimethylsilyl group (stabilization of a negative charge in the carbon to the silicon and ・ a positive charge on the carbon to the silicon) by considering the possible mechanisms for hydrogen transfer at HLADH active site. As a result of kinetic investigations with HLADH, trimethylsilylethanol was found to show high affinity compared with the others. Furthermore, enhancement of HLADH activity derived from sila-substitution was not easily provided even by the use of the corresponding carbon analogues with electron attractive group (for example, 3-methoxy-3-methylbutanol). Less
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