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| - Electron-Donating and -Accepting Strength of Enoxysilanes andAllylsilanes in the Reaction with Aldehydes
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| - The reactivity of enoxysilanes against aldehydes has beendiscussed theoretically in terms of the localelectron-accepting ability and the Lewis acidic hardness of the siliconcenter. Those quantities have been evaluatedby choosing a Si 3p atomic orbital that will play the major role in theformation of a new bond with the aldehydeoxygen and by projecting out the reactive unoccupied orbital for eachenoxysilane molecule that shows the maximumamplitude on that AO. They change significantly depending on thearrangement of bonds around the silicon center.The activation energy calculated for the reaction of enoxysilaneswith formaldehyde correlates well with both of thetwo quantities, showing that 1-enoxysilacyclobutane having a smallC−Si−C angle is a stronger Lewis acid thanmethyl-substituted enoxysilanes. The acidic hardness of silicon inenoxysilane has been found to be very similar inmagnitude to that of silicon in the corresponding allylsilane. Incontrast, the γ-carbon of an enoxy group has ahigher electron-donating ability than that in an allyl group,suggesting that enoxysilanes are more reactive againstaldehydes.
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