10.1002/anie.202002062
Angewandte Chemie International Edition
COMMUNICATION
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product of the process. Although disiloxanes are generally
considered as terminal wastes in hydrosilylation reactions, we
envisioned the possible recycling HMDSO into 2b, as both
compounds feature a strong Si–O bond. Although HMDSO has
been used for the silylation of carboxylic acids, in the presence of
H2SO4 as a catalyst,[20] this procedure could not be transposed to
convert FA to 2b presumably because (i) sulphuric acid readily
decarbonylates FA[21] and (ii) FA and water form an azeotrope
mixture, that prevents shifting the equilibrium towards the
consumption of FA by water distillation. This difficulty was
overcome through a first activation of HMDSO with sulfuric acid
(Scheme 6a), which affords after 5.5 h at reflux of toluene the
solid bis(trimethylsilyl)sulfate 8 in 89 % isolated yield. The
formulation of 8 as SO4(SiMe3)2 was confirmed by X-Ray
diffraction (see SI), formation of which was postulated by
Dunogues et al. nearly 55 years ago.[22] Interestingly, 8 features
highly electrophilic silicon atoms that readily react with a variety
of nucleophiles.[23] Accordingly, 8 reacted with sodium formate to
give silyl formate 2b in 70 % isolated yield (92 % purity), after
separation from the solid by-product Na2SO4 and the dibutyl ether
solvent by distillation.
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The two-step protocol depicted in Scheme 6a affords an
efficient recycling procedure for the conversion of the siloxane by-
product to the starting trimethylsilyl formate 2b. The net reaction
balance involves the utilization of sodium formate and sulphuric
acid to obtain methanol and sodium sulfate in 77 % yield in
nonane (44 % isolated yield), while 77 % of the silicon compounds
can be recycled (45 % isolated yield) (Scheme 6b). This methanol
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yield outperforms
most of
the state-of-the-art
FA
disproportionation protocols and compares well to the recent
report of Himeda, Laurenczy et al., who obtained 75 % MeOH
yield in D2O, using stoichiometric quantities of H2SO4.[10] The
overall process also affords CO2 as a byproduct (2 equiv. relative
to methanol), which is inevitably produced during the
disproportionation step and can be recycled to formates by a 2-
electron reduction.
These results hence show how shutting down the
dehydrogenation of formic acid by replacing a proton with a silyl
group can provide a high yielding, globally redox-neutral and
operationally simple disproportionation route to methanol. While
hydrosilylation chemistry with genuine hydrosilanes is hampered
by the generation of siloxane wastes,[6] this work also
demonstrates for the first time that reductive chemistry with
silicon-based reductants can be amenable to renewability via the
recycling of silyl formates from spent siloxanes.
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[21] R. E. DeRight, J. Am. Chem. Soc. 1933, 55, 4761–4764.
[22] N. Duffaut, R. Calas, J. Dunoguès, Bull. Soc. Chim. Fr. 1963, 1, 512.
[23] S. N. Borisov, M. G. Voronkov, E. Y. Lukevits, in Organosilicon Deriv.
Phosphorus Sulfur, Springer US, Boston, MA, 1971, pp. 157–333.
Keywords: disproportionation • formic acid • silyl formate •
methanol • ruthenium
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