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Intermolecular C-H activation effected by CP*W(NO)-containing complexes Tsang, Jenkins Yin Ki

Abstract

Thermolysis of Cp*W(NO)(CH₂CMe₃)₂ (2.1) in halo, methoxy, or phenylethynyl-substituted benzenes leads to the formation of the alkylidene intermediateCp*W(NO)(=CHCMe₃) which selectively activates ortho C-H bonds of the organicsubstrates. The ortho-regioselectivity diminishes as the size of the substituent increasesfrom F (97 %) to C-=CPh (51 %). In the solid-state structure of all complexes the ortho-substituent is not coordinated to the metal centre; rather, the metal centre is engaged inagostic interactions with a neopentyl methylene C-H bond. Mechanistic studies on the chlorobenzene reaction reveal that the ortho-C-H-activation product is preferentially formed via thermal isomerization from the meta / para-C-H-activation isomers. Reactions between Cp*W(NO)(CH₂EMe₃)Cl (E = C or Si) and a variety of bis(allyl)magnesium reagents lead to the expected formation of Cp*W(NO)(alkyl)(allyl)complexes. Cp*W(N0)(CH₂CMe₃)(η³-CH₂CHCH₂) (3.5), Cp*W(N0)(CH₂CMe₃)(η³-CH₂CMeCH₂) (3.6), Cp*W(N0)(CH₂CMe₃)(η³-CH₂CHCHMe) (3.7),Cp*W(N0)(CH₂CMe₃)(η³-CH₂CHCHPh) (3.8) and Cp*W(N0)(CH₂SiMe₃)(η³-CH₂CHCHMe) (3.9) have thus been synthesized in moderate yields. The solid-state molecular structures of 3.5 and 3.7-3.9 feature a σ-π distorted ally! ligand in the endoconformation. Complex 3.5 reacts with pyrrolidine at RT to form Cp*W(NO)(NC₄H8)(CHMeCH₂NC₄H8) (3.10), a nucleophilic-attack product. Complexes 3.6-3.9 effect the concurrent N-H and α-C-H activation of pyrrolidine at RT and form alkyl-amido complexes analogous to the previously known Cp*W(N0)(CH₂EMe)(NC₄H₇-2-CMe₂CH=CH₂) (3.12). Thermolysis of Cp*W(N0)(CH₂CMe₃)(η³-CH₂CHCHMe) (3.7) at RT leads to the loss of neopentane and the formation of the η²-diene intermediate Cp*W(N0)(η²-CH₂=CHCH=CH₂) (A) which has been isolated as a PMe₃ adduct. In the presence of saturated organic substrates, C-H activation occurs exclusively at the methyl positions of the molecule. Reactions between intermediate A and unsaturated substrates lead to coupling between the coordinated η²-diene and the unsaturation on the organic molecule.Treatment of Cp*W(N0)(n-C₅H₁₁)(η³-CH₂CHCHMe) (4.1) with I₂ at -60 °C produces n-C₅H₁₁ I in moderate yields. Thermolysis of Cp*W(N0)(CH₂CMe₃)(η³-CH₂CHCHPh) (3.8) in benzene at 75 °C for one day leads to the exclusive formation of Cp*W(N0)(H)(η³-PhCHCHCHPh) (5.1).Trapping, labelling, and monitoring experiments suggest that 5.1 is formed via 1) the loss of neopentane and the generation of the allene intermediate Cp*W(N0)(η²-CH₂=C=CHPh), 2) the C-H activation of benzene resulting in a phenyl phenylallyl complex, and 3) the thermal isomerization of this latter species to 5.1.

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