Carboxylate-Assisted β-(Z) Stereoselective Hydrosilylation of Terminal Alkynes Catalyzed by a Zwitterionic Bis-NHC Rhodium(III) Complex

Abstract

The zwitterionic compound [Cp*RhCl{(MeIm)2CHCOO}] is an efficient catalyst for the hydrosilylation of terminal alkynes with excellent regio-and stereoselectivity towards 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 towards the β-(Z)-vinylsilane product, which is still greater than 90%. However, a significant selectivity decrease is observed in the hydrosilylation of aromatic alkynes due to the β-(Z)β-(E) vinylsilane isomerization. Moreover, the hydrosilylation of bulky alkynes, such as t-Bu-C≡CH or Et3SiC≡CH, is unselective.Experimental evidences suggest that the carboxylate function plays a key role in the reaction mechanism, which has been validated by means of DFT calculations, as well as by mass spectrometry and labelling studies. On the basis of previous results, we propose an ionic outersphere 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 -silyl carbocation intermediate that undergoes a hydride transfer from the Rh(III) center to generate the vinylsilane product. The outstanding -(Z) selectivity results from the minimization of the steric interaction between the silyl moiety and the ligand system in the hydride transfer transition state.Regional Governments of Aragón/FEDER 2014-2020 "Building Europe from Aragón" (group E42_17R). In addition, the resources from the supercomputer "Memento", technical expertise and assistance provided by BIFIZCAM (Universidad de Zaragoza) are acknowledged.

Activation of Si–H and B–H bonds by Lewis acidic transition metals and p-block elements: same, but different

“…In a recent contribution, Pérez-Torrente, Jiménez, et al have reported zwitterionic rhodium complex [Cp*RhCl-{(MeIm) 2 CHCOO}], 40 , which catalyses the regio- and stereoselective hydrosilylation of terminal alkynes, yielding β-( Z )-vinylsilanes (Scheme 41). 91 They provided evidence (by means of DFT calculations, isotopic labelling and mass spectrometry) for a catalytic system working through an outer-sphere mechanism. The carboxylate group plays a key role by temporarily trapping an “R 3 Si + ” fragment from a σ-SiH iridium complex (intermediate T , η 1 -coordination mode) and transferring it to the terminal alkyne.…”

Transition-Metal-Catalyzed Stereo- and Regioselective Hydrosilylation of Unsymmetrical Alkynes

“…The β-Z selective hydrosilylation of terminal alkynes were dominated by noble metal catalysts, especially Rh ( 3, entries 9 and 10). [78][79] Both Ir(I) and Ir(III) complexes were prove to be suitable catalysts for β-Z selective hydrosilylation of alkynes (Table 3, entries 11-16). [80][81][82][83][84][85][86] The hard-basic ligands with N, O, C as coordinative groups seems suitable for Ir catalysts.…”

Heterolytic cleavage of a Si–H bond by a metal–ligand cooperation of a cationic iridium amido complex and hydrosilylation of aldehydes

“…Although the reactivity and mechanism of metal–ligand cooperative Si–H bond cleavage have thus far been intensively studied, reports on its application in catalytic reactions are still limited, such as hydrosilylation of acetophenone by group 9 metal thiolates, other reactions catalysed by a cationic ruthenium thiolate complex, and hydrosilylation of terminal alkynes catalysed by a bis-NHC rhodium complex. 9–12…”