Site‐Selective Aliphatic C−H Silylation of 2‐Alkyloxazolines Catalyzed by Ruthenium Complexes

Abstract

The Ru‐catalyzed dehydrogenative silylation of 2‐alkyloxazolines with 1,1,1,3,5,5,5‐heptamethyltrisiloxane took place site‐selectively at methyl C(sp3)−H bonds located γ to the nitrogen atom of the oxazolyl groups. Pyridine and pyrazole rings could also be used as a directing group. A catalytic mechanism based upon successive σ‐bond metathesis is proposed.

A comprehensive overview of directing groups applied in metal-catalysed C–H functionalisation chemistry

“…Finally, a C(sp 3 )–H silylation on 2-ethyloxazoline derivatives was reported by Murata and co-workers in 2016 ( Scheme 160I ). 990 The reaction was performed at 180 °C with HSi(Me)(OSiMe 3 ) 2 . This reaction did not require the use of a hydrogen acceptor, probably due to the high temperature used.…”

A well-defined NHC–Ir(iii) catalyst for the silylation of aromatic C–H bonds: substrate survey and mechanistic insights

“…The mechanistic knowledge of this type of reaction is mainly restricted to the experimental study by Hartwig et al 21 on the Rh( i )-catalyzed silylation of arenes, and the theoretical calculations reported by Murata and co-workers on a Ru-catalysed process. 29 A plausible mechanism for an Ir( iii )-catalysed silylation reaction was proposed by Mashima et al , 19 b however, no kinetic or theoretical support for this postulation has been presented so far.…”

Metal-catalyzed silylation of sp3C–H bonds

“…Several other catalytic dehydrogenative silylations of sp 3 C-H bonds have been directed by the nitrogen atom of heterocycles such as oxazolines, azoles and quinolines with Ru(II), Ir(III) and Ru(III) respectively, for which the nature of key ligands on the metal catalysts play a crucial role. [47][48][49] An excellent example was described by Murata 47 in 2016 who showed that the oxazoline nitrogen atom could direct the silylation of sp 3 C-H bond of the 2-alkyl group but regioselectively at the β- .nH2O also display a rather good catalytic activity for this reaction which also allows disilylation at the alkyl β-carbon atoms. (Scheme 11) 47 However the nature of the silane is crucial as HSiEt3 and HSiMe2(OMe) are not efficient for this reaction.…”

“…[47][48][49] An excellent example was described by Murata 47 in 2016 who showed that the oxazoline nitrogen atom could direct the silylation of sp 3 C-H bond of the 2-alkyl group but regioselectively at the β- .nH2O also display a rather good catalytic activity for this reaction which also allows disilylation at the alkyl β-carbon atoms. (Scheme 11) 47 However the nature of the silane is crucial as HSiEt3 and HSiMe2(OMe) are not efficient for this reaction. The silylation can be extended efficiently to 2-ethylpyridine (89%) and 1-ethylpyrazole (68%).…”

“…To understand the mechanism DFT calculations were made using HSiMe3 and Ru-H as the initial reagents. 47 They showed the easy interaction of one β-C-H bond with H-Ru-N(heterocycle) intermediate to eliminate H2 and to form a Ru(II)-cyclometalate. By contrast the oxidative addition of the H-Si bond to the (cyclometalate)Ru species and β-Carbon silylation require higher energy (ΔG ≠ = 35.9 kcal.…”