The role of neutral Rh(PONOP)H, free NMe2H, boronium and ammonium salts in the dehydrocoupling of dimethylamine-borane using the cationic pincer [Rh(PONOP)(η2-H2)]+ catalyst

The σ-amine-borane pincer complex [Rh(PONOP)(η1-H3B·NMe3)][BArF4] [2, PONOP = κ3-NC5H3-2,6-(OPtBu2)2] is prepared by addition of H3B·NMe3 to the dihydrogen precursor [Rh(PONOP)(η2-H2)][BArF4], 1. In a similar way the related H3B·NMe2H complex [Rh(PONOP)(η1-H3B·NMe2H)][BArF4], 3, can be made in situ, but this undergoes dehydrocoupling to reform 1 and give the aminoborane dimer [H2BNMe2]2. NMR studies on this system reveal an intermediate neutral hydride forms, Rh(PONOP)H, 4, that has been prepared independently. 1 is a competent catalyst (2 mol%, ∼30 min) for the dehydrocoupling of H3B·Me2H. Kinetic, mechanistic and computational studies point to the role of NMe2H in both forming the neutral hydride, via deprotonation of a σ-amine-borane complex and formation of aminoborane, and closing the catalytic cycle by reprotonation of the hydride by the thus-formed dimethyl ammonium [NMe2H2]+. Competitive processes involving the generation of boronium [H2B(NMe2H)2]+ are also discussed, but shown to be higher in energy. Off-cycle adducts between [NMe2H2]+ or [H2B(NMe2H)2]+ and amine-boranes are also discussed that act to modify the kinetics of dehydrocoupling

From Ruthenium to Iron for the Catalytic Reduction of Ketones: Catalysis and Mechanistic Insights

A range of air- and moisture-stable phosphonium salts was prepared. Compounds were isolated in high yield and fully characterized. The properties of these compounds and the nature of their formation were explored. The phosphonium salts react with base to give phosphino-aldehydes which are important building blocks in the synthesis of PNNP ligands. The condensation reaction between phosphino-aldehydes and a diamine usually employed for the preparation of PNNP ligands was not applicable to the phosphino-aldehydes derived from these phosphonium salts as a result of the high reactivity of the nucleophilic phosphorus causing uncontrollable side-reaction. In order to resolve this problem, a template reaction with iron(II) Lewis acid was used to suppress the reactivity of the phosphorus via coordination. The reaction was successful and gave rise to bis-tridentate complexes with PNN ligands ([Fe(Ph2PCH2CH=N---NH2)2][BPh4]2, where N---NH2 depends on diamine used) as the kinetic product and to desired tetradentate complexes with PNNP ligands (trans-[Fe(Ph2PCH2CH=N---N=CHCH2PPh2)(CH3CN)2][BPh4]2, where N---N depends on diamine used) as a thermodynamic product of the reaction. The reaction appeared to be very general; complexes iii with various diamines incorporated in the ligand backbone were prepared in high yield and fully characterized. Mono-carbonylation reaction of the complexes containing tetradentate PNNP ligands resulted in the formation of the precatalysts with a general formula (trans-[Fe(Ph2PCH2CH=N---N=CHCH2PPh2)(CO)(Br)][BPh4]. These precatalysts give active (TOF up to 28000 h-1) and enantioselective (up to 95 % ee) catalytic systems for the ATH of ketones when activated with base in a solution of 2-propanol as the reducing agent. On the basis of a kinetic study and other evidence, we propose a mechanism of activation and operation of the catalytic system involving the precatalyst trans-[Fe(CO)(Br)(Ph2CH2CH=N-((S,S)-C(Ph)H-C(Ph)H)-N=CHCH2PPh2)][BPh4] and acetophenone as a model substrate. We determined that the activation of the precatalyst to the active species involves the stereoselective reduction of one imine group of the ligand, since when the active species are quenched with acid, the complex trans-[Fe(CO)(Cl)(Ph2CH2CH-(H)N-((S,S)-C(Ph)H-C(Ph)H)-N=CHCH2PPh2)][BPh4] containing amine and imine functionalities in the backbone is produced.Ph

Developing asymmetric iron and ruthenium-based cyclone complexes : complex factors influence the asymmetric induction in the transfer hydrogenation of ketones

The preparation of a range of asymmetric iron and ruthenium-cyclone complexes, and their application to the asymmetric reduction of a ketone, are described. The enantioselectivity of ketone reduction is influenced by a single chiral centre in the catalyst, as well as by the planar chirality in the catalyst. This represents the first example of asymmetric ketone reduction using an iron cyclone catalyst

Asymmetric catalysis using iron complexes : 'ruthenium lite'?

A review of recent developments in the use of iron catalysts for asymmetric transformations, including hydrogenations, transfer hydrogenation, hydrosilylation and oxidation reactions