Silane disproportionation results in spontaneous ignition
Stephen L. Buchwald
Chemical & Engineering News (29 Mar 1993) Vol. 71, No. 13, pp. 2.
We have recently reported a catalyst system for the conversion of esters to alcohols that functions in an air atmosphere using a catalytic amount of titanium isopropoxide (Ti[OCH(CH3)2]4) and triethoxysilane (HSi[OCH2CH3]3) as the stoichiometric reductant. In our communication [J. Org. Chem., 57, 3751 (1992)], we warned that, in the absence of substrate and under an inert atmosphere, HSi(OCH2CH3)3 is disproportionated by Ti(OCH(CH3)2)4 to form SiH4, a pyrophoric gas. Such disproportionations, catalyzed by many metal complexes, have been reported in the literature [Adv. Organomet. Chem., 19, 213 (1981); Can. J. Chem., 68, 471 (1990)].
It has come to our attention that silane production may occur with our catalyst system even in the presence of substrate, especially if the reaction is run on a large scale or a large excess of silane is used. A user of the procedure has informed us that he attempted to perform the reduction of a methyl ester (90 millimoles) using HSi(OCH2CH3)3(313 millimoles, 3.5 equivalents). He first added the reagents to the reaction vessel, which was flushed with nitrogen, and the reaction was vented to an oil bubbler. After heating to 50 C, he reported that an exothermic reaction started and the temperature rose to 75 C. The reaction was then cooled to 40 C and on removing the cooling bath, the temperature of the reaction mixture rapidly rose to about 90 C. During this time, an extremely pyrophoric gas (probably SiH4) was given off, which resulted in several fires and an explosion.
At Massachusetts Institute of Technology, we have run this reaction
on scales of up to 50 millimoles, but without event when run under
an atmosphere of dry air, which we surmise may quench any SiH4 as
soon as (or before) it is formed. When the reaction is run under an
inert atmosphere, the SiH4 can build up, especially after the
reduction reaction is complete, leading to fires when the mixture is
finally exposed to air. We strongly urge those who are contemplating
the use of this procedure to be alert to the possibility of SiH4
formation and possible exotherms and to take suitable precautions.
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