C-C and C-N Couplings Following Hydride Addition on Isocyanide Cyclopolyenyl Dimolybdenum Complexes to Give Tethered Aldimine and Aminocarbene Derivatives.

Reaction of [Mo Cp (μ-κ :κ ,η -PMes*)(CO) ] with S or Se followed by protonation with H(OEt ) gave the cationic derivatives Mo Cp {μ-κ :κ ,η -EP(C H tBu )}(CNR)(CO) (E=S; R=tBu, iPr, Ph, 4-C H OMe, Xyl; or E=Se; R=tBu; Ar'=3,5-C H (CF ) ). Reaction of the latter with K[BHsBu ] yielded the aldimine complexes [Mo Cp {μ-κ :κ ,η -SP(C H tBu (CHNR))}(CO) ] and their aminocarbene isomers [Mo Cp {μ-κ :κ ,η -SP(C H tBu (NRCH))}(CO) ] (R ≠ Xyl), following C-C and C-N couplings, respectively. Monitoring of these reactions revealed that the initial H attack takes place at a Cp ligand to give cyclopentadiene intermediates [Mo Cp{μ-κ :κ ,η -SP(C H tBu )}(η -C H )(CNR)(CO) ], which then undergo C-H oxidative addition to give the hydride isomers [Mo Cp {μ-κ :κ ,η -SP(C H tBu )}(H)(CNR)(CO) ]. In turn, the latter rearrange to give the aldimine and aminocarbene complexes. DFT calculations revealed that the hydride intermediates first undergo migratory insertion of the isocyanide ligand into the Mo-H bond to give unobservable formimidoyl intermediates, which then evolve either by nucleophilic attack of the N atom on the C ring (C-N coupling) or by migratory insertion of the formimidoyl ligand into the C ring (C-C coupling). Our data suggest that increasing the size of the substituent R at the isocyanide ligand destabilizes the aldimine isomer to a greater extent, thus favoring formation of the aminocarbene complex.