A Formally Zwitterionic Ruthenium Catalyst Precursor for the Transfer Hydrogenation of Ketones that Does Not Feature an Ancillary Ligand NH Functionality

Abstract

The ruthenium-mediated transfer hydrogenation (TH) of ketones under basic conditions employing iPrOH or other H 2 -donor solvents has emerged as an atom-economical methodology for the synthesis of secondary alcohols.[1] Although the catalytic abilities of Ru complexes supported by a range of structurally diverse ancillary ligands have been evaluated, precatalysts featuring a RuÀNH linkage commonly offer the highest levels of activity and selectivity in the TH of ketones. [1,2] In the case of the landmark {(h 6 -arene)Ru} catalysts featuring chiral diamido ligands developed by Noyori and co-workers, [3] this now well-established N À H effect has been rationalized in terms of an outer-sphere TH mechanism involving the concerted transfer of H 2 from a {(H)RuÀNH 2 R} intermediate to a ketone substrate. [1c, 3] More recently, Baratta and co-workers have reported an alternative class of precatalysts supported by 2-(aminomethyl)pyridine (ampy) coligands that give rise to the most active catalysts known for the TH of ketones.[4] For both of these prominent Ru-based TH catalyst systems, the inferior performance exhibited by the analogous {RuÀNMe 2 R} species underscores the crucial role that the ancillary ligand NH 2 terminus plays in enhancing catalyst activity and selectivity. [3c, 4] Notwithstanding the advancements in Ru-mediated TH that have been enabled through the development of ancillary ligands featuring NÀH donors, such a structural prerequisite limits the future design of alternative classes of Ru-based TH catalysts for use in mediating new and increasingly challenging substrate transformations. In this context, the identification of novel ligation strategies that do not rely on the NÀH effect and which give rise to highly active and selective Ru-based TH catalysts represents an important goal in modern catalysis research. [1,2] Encouraged by the catalytic abilities of Noyoris [(h 6 -arene)Ru(diamido)] complexes, [3] and in light of the observation that the pairing of P-and N-donor ligands is a common feature in several highly effective Ru-based TH catalysts, [1c, 2b, 4] we became interested in developing new non-NÀ H [(h 6 -arene)Ru(k 2 -P, N)(Cl)] precatalysts for use in the TH of ketones. In the context of our research comparing the reactivity properties of late-transition-metal cations and zwitterions derived from 1-H, [5] we identified the cationic complexes 2 + X À and the zwitterion 3 as being worthwhile candidates for catalytic studies (Scheme 1). Herein, we report the synthesis and characterization of 2 + X À and 3 as well as their application as precatalysts in the TH of ketones. Despite lacking an ancillary ligand NÀH functionality, complex 3 is among the most active ketone TH precatalysts known, providing near-quantitative conversions and exhibiting consistently high turnover frequencies (TOFs; up to 220 000 h À1 ) for a diversity of alkyl and aryl ketone substrates at low catalyst loadings. Furthermore, while the anticipated zwitterionic hydrido complex 4 is formed quantitatively upon treatment...

Osmium(II) CNN Pincer Complexes as Efficient Catalysts for Both Asymmetric Transfer and H2 Hydrogenation of Ketones

“…8 10 5 h À1 at 50 % conversion) in the presence of 2 (0.005 mol %) and NaOiPr (2 mol %), while the reaction with 3 goes to completion in 5 min (TOF = 1.3 10 6 h À1 ; Table 1). [8] Surprisingly, this rate is slightly higher than that of the analogous compound [RuCl(CNN)(dppb)] (TOF = 1.1 10 6 h À1 ), [6c] which is one of the most active TH catalysts, [9] and, in contrast to the ruthenium-based catalysts, 3 also displays high activity with only one equivalent of NaOiPr (TOF = 1.0 10 6 h À1 ). [10] are the only other systems which allow the TH of ketones at such low catalyst loadings.…”

Zwitterionic Relatives of Cationic Platinum Group Metal Complexes: Applications in Stoichiometric and Catalytic σ‐Bond Activation

“…In fact, the formation of 100 has been shown to be a catalyst deactivation pathway in ketone transfer hydrogenation reactions employing 98 or 99 as pre-catalysts. [107] Nonetheless, such an observation does not rule out the involvement of alternative hydrido species as active catalysts in reactions employing 99, including those arising by intramolecular CÀH bond activation, as was observed for 77 (Scheme 22) and 93 (Scheme 26). Whilst in a related investigation little difference was observed between the performance of the cationic and zwitterionic iridium precatalysts 68 and 70 (Figure 4) in the transfer hydrogenation of acetophenone (2 h, 0.1 mol % Ir, 93-98 %; 37 000-47 000 h À1 ), these complexes out-performed Crabtrees catalyst (56, Figure 2), which afforded only 29 % conversion under similar experimental conditions.…”

“…[104,105] Divergent catalytic performance was observed in the course of a comparative catalytic investigation to probe the ability of the structurally analogous cationic and zwitterionic (h 6 -arene)Ru complexes 98 and 99 to mediate the transfer hydrogenation of ketones under basic conditions and employing iPrOH as an H 2 donor solvent (Scheme 27). [107] In a preliminary test reaction employing acetophenone (R = Ph in Scheme 27), the cation 98 exhibited only modest activity, with final conversions into 1-phenylethanol in the range of 4-23 %. In stark contrast, the zwitterion 99 afforded 99 % conversion into 1-phenylethanol after only five minutes.…”

Ru(II) pyridyl-based NNN complex catalysts for (asymmetric) transfer hydrogenation of ketones at room temperature

“…Very recently, Morris group [15,16] reported that chiral NNPP ligands and their iron(II) complexes were effective in the ATH of ketones under mild conditions, representing a rare case of using inexpensive iron as the active metal. Although new ligands and their transition metal complexes have been established in this area [17][18][19][20], the desire for more highly active catalytic systems with better stereoselectivity and broader substrate scopes under mild conditions is still strong. We have recently documented that highly active transition metal complex catalysts could be rationally constructed by introducing a benzimidazolyl or pyrazolyl coordinating arm on a pyridyl-based ligand framework [21][22][23][24][25][26][27][28][29][30].…”