Carboxylate-Assisted β‑(Z) Stereoselective Hydrosilylation of Terminal Alkynes Catalyzed by
a Zwitterionic Bis-NHC Rhodium(III) Complex
Posted on 2020-06-22 - 17:04
The zwitterionic
compound [Cp*RhCl{(MeIm)2CHCOO}] is
an efficient catalyst for the hydrosilylation of terminal alkynes
with excellent regio- and stereoselectivity toward the less thermodynamically
stable β-(Z)-vinylsilane isomer under mild
reaction conditions. A broad range of linear 1-alkynes, cycloalkyl
acetylenes, and aromatic alkynes undergo the hydrosilylation with
HSiMe2Ph to afford the corresponding β-(Z)-vinylsilanes in quantitative yields in short reaction times. The
reaction of aliphatic alkynes with HSiEt3 is slower, resulting
in a slight decrease of selectivity toward the β-(Z)-vinylsilane product, which is still greater than 90%. However,
a significant selectivity decrease is observed in the hydrosilylation
of aromatic alkynes because of the β-(Z) →
β-(E) vinylsilane isomerization. Moreover,
the hydrosilylation of bulky alkynes, such as t-Bu–CCH
or Et3SiCCH, is unselective. Experimental evidence
suggests that the carboxylate function plays a key role in the reaction
mechanism, which has been validated by means of density functional
theory calculations, as well as by mass spectrometry and labeling
studies. On the basis of previous results, we propose an ionic outer-sphere
mechanism pathway in which the carboxylate fragment acts as a silyl
carrier. Namely, the hydrosilylation mechanism entails the heterolytic
activation of the hydrosilane assisted by the carboxylate function
to give the hydrido intermediate [Cp*RhH{(MeIm)2CHCOO–SiR3}]+. The transference of the silylium moiety from
the carboxylate to the alkyne results in the formation of a flat β-silyl
carbocation intermediate that undergoes a hydride transfer from the
Rh(III) center to generate the vinylsilane product. The outstanding
β-(Z) selectivity results from the minimization
of the steric interaction between the silyl moiety and the ligand
system in the hydride transfer transition state.
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Puerta-Oteo, Raquel; Munarriz, Julen; Polo, Víctor; Jiménez, M. Victoria; Pérez-Torrente, Jesús J. (2020). Carboxylate-Assisted β‑(Z) Stereoselective Hydrosilylation of Terminal Alkynes Catalyzed by
a Zwitterionic Bis-NHC Rhodium(III) Complex. ACS Publications. Collection. https://doi.org/10.1021/acscatal.0c01582
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AUTHORS (5)
RP
Raquel Puerta-Oteo
JM
Julen Munarriz
VP
Víctor Polo
MJ
M. Victoria Jiménez
JP
Jesús J. Pérez-Torrente