The formation of charge-transfer complexes of iodine and of iodine monobromide with alcohols and alkoxysilanes has been established spectroscopically, and the formation constants of iodine–ethoxytriethylsilane and iodine–diethoxydimethylsilane complexes has been determined as 0.55±0.01 and 0.61±0.02, respectively. On the basis of these observations and the kinetic information recently reported, the previously proposed mechanism for the iodine or iodine monobromide catalyzed alkoxy–alkoxy exchange reactions of alkoxysilanes is discussed afresh. It has been confirmed that a mechanism involving a four-centered transition state containing a CT-complex is most favorable.
Iodine or iodine monobromide was found to be an especially favorable catalyst for the study of the alcoholysis equilibria of dialkoxydimethylsilanes by gas chromatography. Iodine monobromide was employed to promote the reactions associated with tertiary alcohols, and iodine was used for the other reactions. Eight alcohols were allowed to react with diethoxydimethylsilane, and successive equilibrium constants, K1 and K2 were determined at 20 °C. K1=2.36–2.75 and K2=0.66–0.69 (by the use of primary alcohols), K1=1.34–1.38 and K2=0.28–0.30 (secondary alcohols), and K1=0.15–0.16 (tertiary alcohols) were observed. These values express precisely the difference of reactivities of the three groups of alcohols with diethoxydimethylsilane. Out of the dialkoxydimethylsilanes used in this study, eight unreported compounds were isolated and characterized.
The equilibrium constants K of alcoholysis of triethylalkoxysilanes were determined at 20 °C and 40 °C. Iodine monobromide was used to promote the reactions associated with the tertiary alkoxyl groups, while the other reactions proceeded in the presence of iodine. The K values of the reaction systems with ethoxyl or propoxyl—primary, secondary, and tertiary alkoxyl pairs were 1 or above, about 0.5, and about 0.05, respectively. These values reflect the extent of the binding abilities of the alkoxyl groups to silicon, which is in the expected order of primary>secondary>tertiary alkoxyl groups. A mechanism is postulated for the reaction which involves participation by the iodine or iodine monobromide.
Iodine monobromide is an extremely effective catalyst for the redistribution of alkoxyl groups on silicon atoms in dimethyldiakoxysilanes. The equilibrium constants of these reactions were determined at 40 °C by means of gas chromatography. Assuming that the reaction intermediate is the dative form of a dimethyldialkoxysilane–iodine monobromide complex, a mechanism of the reaction was proposed.
In order to appreciate the excellent catalytic effect of iodine on the alcoholyses of alkoxysilanes more precisely, the rates of the reaction, Et3SiOBun + BusOH ⇌ Et3SiOBus + BunOH, were determined at various iodine concentrations.
Both forward and reverse reactions are first order with respect to butoxysilane and to butanol, and pseudo first‐order rate constants were measured at 40°, 30°, and 20°C on reaction mixtures containing both butanols in excess by means of gas‐liquid chromatography. The observed rate constants as a function of iodine concentration gave linear relationships, and from these data the catalytic coefficients of iodine were evaluated:
The enthalpies and the entropies of activation were estimated to be 53.2 kJ mol−1, −103 J K−1 mol−1 (forward, 30°C) and 51.8 kJ mol−1, minus;100 J K−1 mol−1 (reverse, 30°C).
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