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In summary, we have developed a new protocol for efficient
generation
of
silyl
radicals
through
photo-induced
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decarboxylation of silacarboxylic acids, which can be easily
prepared in high yield on a gram scale and can be stored for a
long period even in air. In the presence of a commercially
available photo-catalyst, decarboxylation of silacarboxylic acid
proceeded smoothly under mild conditions, affording silyl
radicals that can be used for silylation of alkenes with broad
functional group compatibility. The current protocol can be
performed with D2O as the deuterium source even in the
presence of water, enabling deuterosilylation of alkenes in high
yield and with a high deuteration ratio. It is also applicable for
the generation of germyl radicals with equally high efficiency.
This work provides a mild, selective and synthetically useful
method for the generation of reactive hetero-radical
intermediates, exemplifying the value of photocatalysis for
efficient synthetic transformations of functional elements. We
also confirmed the feasibility of a cascade involving both
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a HAT process. Thus, the present
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trialkylsilyllithium remains
a big challenge in organosilicon
chemistry, and this also limits the ability to synthesize trialkyl-
substituted silacarboxylic acids. Work to find solutions to these
problems, as well as to extend the scope of this reaction and to
apply it for the synthesis of group-14-element-containing
functional molecules, is in progress.
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Acknowledgements
This work was supported by JSPS Grants-in-Aid for Scientific
Research No. 17H05430 and No. 17H06173 (to M.U.) and No.
18K06544 (to C.W.)
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Keywords: silyl radical • decarboxylation • photo-catalysis •
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[17] Silacarboxylic acid is decomposed by treatment with some bases.
Some silacarboxylate anion species, such R3SiCOOLi (i.e. the product
of the reaction of R3SiLi and CO2 before quenching) or R3SiCOO–
(present in the dissociation equilibrium of R3SiCOOH in neutral solution),
are rather stable, due to the strong interaction between the COO–
moiety and Li+ or an adjacent proton. But, when such interactions are
weakened, large amounts of “naked” silacarboxylate anion are formed,
uncovering the high basicity/nucleophilicity of oxygen. Then,
intramolecular degeneration can take place through 1,2-Brook
rearrangement followed by extrusion of CO. On the other hand, some
bases can also act as nucleophiles to attack silicon, leading to
intermolecular degradation pathways. For decomposition of
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