Organometallic intermediates in the group 4 metallocene-catalyzed dehydrocoupling of organosilanes. 2. Variations in the intermediates of dimethyltitanocene reactions with several silanes

Abstract

2980 (1992). As previously described, organosilanes react with dirnethyltitanocene (DMT) to give three reduced titanium intermediates, CpzTi(k-SiRRIH)(p-H)TiCpz, 1, [Cp.Ti(k-SiRR'H)],, 2, and [Cp2Ti(H)],(~-H), 3. When R = Me and R' = tz-Bu, the DMT is quantitatively converted to 1, which is persistent at room temperature. When R' = H, DMT is initially converted quantitatively to 1, which then rapidly declines to give an equilibrium mixture of 1, 2, and 3, with 2 as the dominant species. When R = Ph and R' = Ph or Me, 1 is again produced quantitatively, but it rapidly declines to produce an unidentified paramagnetic product, rather than 2 or 3. It is believed that 2 is unstable with respect to dissociation in these cases due to the steric bulk of the substituents. [Traduit par la rkdaction]

Titanocene-Catalyzed Reduction of Lactones to Lactols

“…Brintzinger and Martin studied the reaction of titanocene dichloride 1a with 2 equiv of RMgX or RLi (R = Et or i -Pr) and proposed that the reactive species obtained in this process was consistent with a titanocene(III) hydride in either a monomeric or dimeric form. Similarly, Harrod and co-workers were able to isolate and characterize two binuclear titanium(III) silyl hydride complexes (Figure ) from the reaction of dimethyltitanocene ( 1b ) with phenylsilane, which are proposed to be intermediates in the polymerization of primary silanes . Since a titanium hydride catalyst is implicated in the catalytic cycle of our ester reduction system,6a we speculated that 1b could be used as a precatalyst in the hydrosilylation of lactones.…”

“…Although several titanium hydrides and titanium silyl hydrides have been reported in the literature, 12a, the exact nature of the active species in this system is unclear. A reasonable mechanistic model can be proposed by assuming that titanocene(III) hydride (or its equivalent) is the actual catalytic reducing agent …”

Studies of the Formation and Decomposition Pathways for Cationic Zirconocene Hydrido Silyl Complexes

“…Formation Pathway for 1. Mixtures of neutral Cp 2 MX 2 metallocenes (M = Ti, Zr, Hf; X = Me, H) and primary silanes were studied earlier, and analogs of compounds 1 were isolated from them. ,− However, the combination catalyst Cp‘ 2 ZrCl 2 /2BuLi/B(C 6 F 5 ) 3 behaves differently from Cp 2 ZrX 2 (X = Me, H) in that it has a better selectivity for the production of long linear polymer chains. An even more intriguing observation is the fact that the Cp 2 ZrCl 2 /2BuLi/B(C 6 F 5 ) 3 mixture has a totally different silane dehydrocoupling performance than the almost identical combination catalyst Cp 2 ZrMe 2 /B(C 6 F 5 ) 3 , which is virtually inactive .…”

First-row transition metal catalyzed reduction of carbonyl functionalities: a mechanistic perspective

“…63 The intermediacy of the Ti(III) hydride in the catalytic cycle is still hard to verify despite the fact that (EBTHI)TiH has been independently synthesized and crystallographically characterized as a dimer with the hydride ligands bridging two Ti centers. 64 The chemical complexity of the actual catalytic systems, including various reactions between titanocene complexes and silanes, [65][66][67][68][69] has not yet ruled out silyl Ti(IV) hydrides, bimetallic titanium hydrides, or Ti(III) silyl compounds as the possible intermediates. 56-60 Non-metallocene titanium complexes with chiral diol or bis(oxazoline) ligands have also proved to be active catalysts for the hydrosilylation of ketones, albeit with lower enantioselectivities.…”