Calculations at the MP2/6-31G** level of theory show that the activation barrier of the allylation reaction between benzaldehyde and allyltrichlorosilane with the aid of a neutral Lewis base, N,Ndimethylformamide (DMF) or hexamethylphosphoric triamide (HMPA), is considerably lower than that of the reaction in the absence of the Lewis base. An orbital analysis indicates that the electrondonating ability of the γ-carbon in the allyl group is enhanced by the coordination of a DMF or HMPA molecule. The silicon having a hexacoordinated bond arrangement shows an electronaccepting ability or Lewis acidity significantly stronger than that in silicon having a pentacoordinated structure to facilitate the coordination of DMF or other neutral Lewis bases. On the other hand, the electron-accepting level of the orbital that is left for the bond formation with the attacking aldehyde is elevated by the preceding coordination of a neutral base. The orbital is seen, however, to be localized more efficiently on the silicon center in the hexacoordinated transition state. The two Si-Cl bonds that are approximately perpendicular to the Si-O bond being formed and the Si-C bond being broken are strongly polarized at the transition state, reducing the repulsive interaction with the attacking aldehyde. These make up for the weakened electron-accepting ability of the silicon and, therefore, the combined coordination of the aldehyde and a neutral base recovers most of the destabilization associated with the changes in reagent and reactant structures to give the hexacoordinated transition state.