Experimental and Computational Investigation of the Aerobic Oxidation of a Late Transition Metal-Hydride

Abstract

The rational development of homogeneous catalytic systems for selective aerobic oxidations of organics has been hampered by the limited available knowledge of how oxygen reacts with important organometallic intermediates. Recently, several mechanisms for oxygen insertion into late transition metal-hydride bonds have been described. Contributing to this nascent understanding of how oxygen reacts with metal-hydrides, a detailed mechanistic study of the reaction of oxygen with the Ir III hydride complex ( dm Phebox)Ir(OAc)(H) (1) in the presence of acetic acid, which proceeds to form the Ir III complex ( dm Phebox)Ir(OAc) 2 (OH 2 ) (2), is described. The evidence supports a multifaceted mechanism wherein a small amount of an initially formed metal hydroperoxide proceeds to generate a metal-oxyl species that then initiates a radical chain reaction to rapidly convert the remaining Ir III −H. Insight into the initiation step was gained through kinetic and mechanistic studies of the radical chain inhibition by BHT (butylated hydroxytoluene). Computational studies were employed to contribute to a further understanding of initiation and propagation in this system.

Air- and Water-Tolerant (PNP)Ir Precatalyst for the Dehydrogenative Borylation of Terminal Alkynes

“…Interestingly, a solution of fully aerobically degraded (PNP)­IrH­(OAc) was also capable of performing DHBTA (Figures S8 and S9). This implies that the mixture of the presumed oxygenated iridium species can be revived by HBpin and reenter the catalytic cycle.…”

“…11 Interestingly, a solution of fully aerobically degraded (PNP)IrH(OAc) was also capable of performing DHBTA (Figures S8 and S9). This implies that the mixture of the presumed 25 In an experiment meant to push the limits of catalytic turnover, 200 pmol of (PNP)IrH(OAc) was added to a 200 μmol sample of 3-dimethylamino-1-propyne and HBpin (under argon, in dry C 6 D 6 ). After ca.…”

Formate-driven catalysis and mechanism of an iridium–copper complex for selective aerobic oxidation of aromatic olefins in water

“…Since metal-hydroperoxide (M-OOH) species have so far been proposed as intermediates in olefin oxidations to produce corresponding ketones using late-transition metal catalysts, we have employed H 2 O 2 as an oxidant instead of O 2 and HCOOH to examine the selective oxidation of StyreneS by 1. [42][43][44][45] Remarkably, AcetophS was not produced in the oxidation of StyreneS when using H 2 O 2 in the place of O 2 and HCOOH (Table S2). Therefore, we conclude that the oxidation of StyreneS performed by 1 involved the formation of Ir(EtBnS)-Cu generated by the reaction between Ir(H)-Cu and StyreneS through olefin insertion into an Ir III -H bond instead of an Ir-OOH species.…”

Lowering the Barrier to C–H Activation at IrIII through Pincer Ligand Design

“…The Ir III complex (Phebox)­Ir­(OAc) 2 (OH 2 ) (Phebox = 2,6-bis­(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl) was reported to require temperatures upward of 160 °C to activate n -octane and form an Ir III -octyl complex . In contrast to the PCP- and POCOP-ligated Ir complexes, such C–H bond activations proceed via concerted metalation–deprotonation (CMD) mechanisms, which maintain a high-valent Ir III oxidation state. − CMD mechanisms for C–H activation are notable in their ability to operate under aerobic conditions. − Further studies demonstrated that (Phebox)­Ir­(Octyl)­(OAc) could undergo β-hydride elimination to form (Phebox)­Ir­(H)­(OAc) and olefin and that the Ir–H product reacts with O 2 and HOAc to regenerate the starting (Phebox)­Ir­(OAc) 2 (OH 2 ) complex at moderate temperatures, establishing a potential cycle for aerobic alkane dehydrogenation. ,− …”