Unprecedented H-atom transfer from water to ketyl radicals mediated by Cp2TiCl

Abstract

The H-atom transfer (HAT) from water to ketyl radicals, mediated by titanocene(iii) aqua-complexes, can explain the Ti(III)-promoted reduction of ketones in aqueous medium better than the conventional House mechanism. Moreover, we also report novel evidences supporting the existence of these titanocene(iii) aqua-complexes.

Hydrogen Generation by Water Reduction with [Cp*2Ti(OTf)]: Identifying Elemental Mechanistic Steps by Combined In Situ FTIR and In Situ EPR Spectroscopy Supported by DFT Calculations

“…The last aspect is not only of interest with respect to water splitting, but also in view of a potential application of such titanocene complexes as catalysts for hydrogen-atom transfer (HAT) reactions, being a topic of growing interest in organic synthesis. [8][9][10][11] The open questions raised above can only be answered by a detailed in situ analysis of the system under reaction conditions. Therefore, we have monitored the stepwise reaction of 1 with water using in situ IR spectroscopy in the attenuated total reflection mode (ATR-FTIR) and in situ electron paramagnetic resonance (EPR) spectroscopy under the same conditions.…”

“…Moreover, some important issues of the reaction mechanism are still unclear, including the nature of complex 3 in solution, the likely presence of other intermediates in solution, as well as the question of molecular hydrogen formation from only one H atom per reaction cycle. The last aspect is not only of interest with respect to water splitting, but also in view of a potential application of such titanocene complexes as catalysts for hydrogen‐atom transfer (HAT) reactions, being a topic of growing interest in organic synthesis 8–11…”

Reductive Umpolung Reactions with Low-Valent Titanium Catalysts

“…The origin of the second electron transfer under catalytic conditions has been a topic of discussion. Coordination and SET from a second Ti III species has been proposed . Here, C–C bond formation could occur as a result of a radical combination from two Ti III ‐coordinated molecule fragments.…”

New Advances in Titanium-Mediated Free Radical Reactions

“…If the use of catalytic amounts of titanocenes is attractive for reagent controlled radical reactions [ 88 ], on the other hand this process presents the disadvantage deriving from the generation of stoichiometric amounts of highly toxic reducing waste, such as benzene ( Scheme 26 ). However, it has been demonstrated that titanocene reagents are able to activate water and methanol toward hydrogen atom transfer, by substantially lowering the bond dissociation energy (BDE) of the OH bond [ 89 , 90 , 91 ]. This combination is chemically more efficient and environmentally benign.…”