Ortho-Selective C−H Activation of Substituted Benzenes Effected by a Tungsten Alkylidene Complex without Substituent Coordination

Abstract

Gentle thermolysis of the bis(neopentyl) complex Cp*W(NO)(CH2CMe3)2 (1) at 70 °C in various substituted benzenes results in the loss of neopentane and the generation of the transient alkylidene complex Cp*W(NO)(CHCMe3) (A), which subsequently effects single C−H bond activations of the benzenes. These activations exhibit a pronounced selectivity for the C−H linkages ortho to the benzene substituents. Thus, thermal reactions of 1 with C6H5X lead to the preferential formation of the corresponding Cp*W(NO)(CH2CMe3)(o-C6H4X) complexes, the ortho-selectivity, i.e., X = F > OMe > Cl > Br > C⋮CPh, diminishing as the steric demands of the substituents increase. Consistently, thermolyses of 1 in o-C6H4X2 afford Cp*W(NO)(CH2CMe3)(2,3-C6H3X2) complexes with 98% selectivity when X = F and 82% selectivity when X = Cl. Similarly, the principal organometallic products resulting from the thermolyses of 1 in m-C6H4X2 are the ortho-para-activated isomers Cp*W(NO)(CH2CMe3)(2,4-C6H3X2) with 84% selectivity when X = F and 89% selectivity when X = Cl. Finally, thermolyses of 1 in para-disubstituted benzenes p-C6H4XY (X = F or OMe, Y = Cl or OMe) again reveal that the ortho-directing abilities of the substituents diminish in the order F > OMe > Cl. Some mechanistic insights into these activation processes have been obtained by monitoring of the early stages of the thermolysis of 1 in chlorobenzene by 1H NMR spectroscopy. This monitoring reveals that the selectivity of the alkylidene intermediate A for forming the ortho-activated isomer is thermodynamic rather than kinetic in nature. Thus, the meta- and para-activated isomers are formed initially, but then convert to the ortho-activated product. The principal organometallic complexes resulting from ortho-C−H activations are formally 16-electron species in which there are no interactions between the tungsten centers and the Lewis-basic benzene substituents. Instead, the Lewis-acidic tungsten centers engage in intramolecular agostic interactions with the methylene C−H bonds of their neopentyl ligands to acquire some additional electron density. Being coordinatively and electronically unsaturated, these tungsten complexes react readily with added Lewis bases. Thus, treatment of Cp*W(NO)(CH2CMe3)(o-C6H4F) with Et4NCN and CO produces [Et4N]+[Cp*W(NO)(CH2CMe3)(CN)(o-C6H4F)]- and Cp*W(NO)(η2-C(O)CH2CMe3)(o-C6H4F), respectively, and the solid-state molecular structure of the latter complex has also been established by a single-crystal X-ray crystallographic analysis.

Activation of Anisole by Organoplatinum(II) Complexes: Evidence for Rate-Determining C–H Activation

“…The easiest C–H activation occurs from U to give L ′ via the transition state U † , and reductive elimination from L ′ then gives methane and o - 4b by way of R ′ † and R ′ (Scheme ). The rate-determining step is the C–H activation, consistent with the observed isotope effect k H / k D = 3.6. − , The selectivity of C–H activation of arenes can be determined by either thermodynamic or kinetic effects. − , In the present case, kinetic control is proposed, but we note that the product o - 4b is favored by both kinetic and thermodynamic effects (Figure )…”

“…Several theoretical studies have been made on carbon–hydrogen bond activation of methane or benzene by electrophilic platinum­(II) complexes, − ,,,, but there has been little computational work on the selectivity in anisole activation . A DFT study of the isomers of [PtMe­(anisyl)­(NN)] and of the isomers formed by protonation, namely, [PtHMe­(anisyl)­(NN)] + , [PtMe­(anisole)­(NN)] + , and [Pt­(anisyl)­(CH 4 )­(NN)] + , with NN = bipy or DPK, has therefore been carried out.…”

“…In the successful reactions, toluene gave mostly the m- tolylplatinum complex, while anisole gave mostly the o -anisylplatinum complex . This article reports a detailed study of the reactions with anisole aimed at understanding the selectivity of the reaction and the reasons for the low reactivity when NN = bipy.…”

Synthesis of (Imido)vanadium(V) Alkyl and Alkylidene Complexes Containing Imidazolidin-2-iminato Ligands: Effect of Imido Ligand on ROMP and 1,2-C–H Bond Activation of Benzene

“…More recently, we realized that 1,2-C–H (or 1,2-C–D) activation took place in the thermolysis of the imidazolin-2-iminato-modified dialkyl precursor (Scheme ), , as exemplified by the reaction of V(NAr)(CH 2 SiMe 3 ) 2 [1,3-Ar′ 2 (CHN) 2 CN] ( A ; Ar = 2,6-Me 2 C 6 H 3 ; Ar′ = 2,6- i Pr 2 C 6 H 3 ), in C 6 H 6 (or C 6 D 6 ) in the presence of PMe 3 . We thus demonstrated that an exclusive formation of the phenyl analogues V(NAr)(CH 2 SiMe 3 )(C 6 H 5 )[1,3-Ar′ 2 (CHN) 2 CN] ( C ) by 1,2-C–H (or 1,2-C–D) bond activation of C 6 H 6 (or C 6 D 6 ) with the dialkyl analogue ( A ) takes place via a vanadium(V) alkylidene intermediate, and a subsequent thermolysis in C 6 H 6 (or C 6 D 6 ) afforded the diphenyl complexes V(NAr)(C 6 H 5 ) 2 [1,3-Ar′ 2 (CHN) 2 CN] ( D ). ,− The corresponding vanadium(V) alkylidene complex B (proposed as a key intermediate in the reaction of the dialkyl analogue in C 6 D 6 or C 6 H 6 ) and vanadium(V) benzyne complex E (assumed as an intermediate in the latter reaction affording the diphenyl complexes) could be isolated from the reaction mixture by trapping with PMe 3 , and their structures were determined by X-ray diffraction analysis…”

Thermal Chemistry of a Tungsten Trimethylsilylallyl Complex in Benzene and Fluorobenzenes

“…5 Activation at position 4 on o-difluorobenzene, farthest from the fluorine atoms, is contrary to some of our previous research, during which a preference for activation at the o-F position was documented during the activation of fluorobenzene by Cp*W(NO)(Npt) 2 . 20 Indeed, in most cases of C−H activation of fluorobenzenes, the C−H bond ortho to a C−F bond is the favored site for activation. 21−23 This preference has been attributed to a thermodynamic stabilizing effect by the o-F, since the resulting M−C bond is strengthened more by the ortho C−F bond than the C−H bond in the substrate.…”