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

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.