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

Catalyst-Directed Chemoselective Double Amination of Bromo-chloro(hetero)arenes: A Synthetic Route toward Advanced Amino-aniline Intermediates

A chemoselective sequential one-pot coupling protocol was developed for preparing several amino-anilines in high yield as building blocks for active pharmaceutical ingredients (APIs). Site (Cl vs Br on electrophile) and nucleophile (amine vs imine) selectivity is dictated by the catalyst employed. A Pd-crotyl<i>(t-</i>BuXPhos) precatalyst selectively coupled the Ar–Br of the polyhaloarene with benzophenone imine, even in the presence of a secondary amine, while Pd-based RuPhos or (BINAP)­Pd­(allyl)Cl coupled the Ar–Cl site with secondary amines

Asymmetric Transfer Hydrogenation of Ketimines Using Well-Defined Iron(II)-Based Precatalysts Containing a PNNP Ligand

Well-defined iron(II)-based complexes containing PNNP ligands catalyze a highly enantioselective reduction of <i>N</i>-(diphenylphosphinoyl)- and <i>N</i>-(<i>p</i>-tolylsulphonyl)-ketimines. Under mild conditions and low catalyst loading, the ketimines are successfully reduced to the corresponding amines in enantiomeric excess ranging from 94 to 99%

The Mechanism of Efficient Asymmetric Transfer Hydrogenation of Acetophenone Using an Iron(II) Complex Containing an (<i>S</i>,<i>S</i>)-Ph₂PCH₂CHNCHPhCHPhNCHCH₂PPh₂ Ligand: Partial Ligand Reduction Is the Key

On the basis of a kinetic study and other evidence, we propose a mechanism of activation and operation of a highly active system generated from the precatalyst <i>trans-</i>[Fe­(CO)­(Br)­(Ph₂PCH₂CHN-((<i>S</i>,<i>S</i>)-C­(Ph)­H–C­(Ph)­H)-NCHCH₂PPh₂)]­[BPh₄] (<b>2</b>) for the asymmetric transfer hydrogenation of acetophenone in basic isopropanol. An induction period for catalyst activation is observed before the catalytic production of 1-phenethanol. The activation step is proposed to involve a rapid reaction of <b>2</b> with excess base to give an ene–amido complex [Fe­(CO)­(Ph₂PCH₂CHN-((<i>S,S</i>)-C­(Ph)­H–C­(Ph)­H)-NCHCHPPh₂)]⁺ (<b>Fe</b>_<b>p</b>) and a bis­(enamido) complex Fe­(CO)­(Ph₂PCHCH-N-(<i>S</i>,<i>S-</i>CH­(Ph)­CH­(Ph))-N–CHCHPPh₂) (<b>5</b>);<b> 5</b> was partially characterized. The slow step in the catalyst activation is thought to be the reaction of <b>Fe</b>_<b>p</b> with isopropoxide to give the catalytically active amido-(ene-amido) complex <b>Fe</b>_<b>a</b> with a half-reduced, deprotonated PNNP ligand. This can be trapped by reaction with HCl in ether to give, after isolation with NaBPh₄, [Fe­(CO)­(Cl)­(Ph₂PCH₂CH₂N­(H)-((<i>S</i>,<i>S</i>)-CH­(Ph)­CH­(Ph))-NCHCH₂PPh₂)]­[BPh₄] (<b>7</b>) which was characterized using multinuclear NMR and high-resolution mass spectrometry. When compound <b>7</b> is treated with base, it directly enters the catalytic cycle with no induction period. A precatalyst with the fully reduced P-NH-NH-P ligand was prepared and characterized by single crystal X-ray diffraction. It was found to be much less active than <b>2</b> or <b>7</b>. Reaction profiles obtained by varying the initial concentrations of acetophenone, precatalyst, base, and acetone and by varying the temperature were fit to the kinetic model corresponding to the proposed mechanism by numerical simulation to obtain a unique set of rate constants and thermodynamic parameters

The Mechanism of Efficient Asymmetric Transfer Hydrogenation of Acetophenone Using an Iron(II) Complex Containing an (<i>S</i>,<i>S</i>)-Ph₂PCH₂CHNCHPhCHPhNCHCH₂PPh₂ Ligand: Partial Ligand Reduction Is the Key

On the basis of a kinetic study and other evidence, we propose a mechanism of activation and operation of a highly active system generated from the precatalyst <i>trans-</i>[Fe­(CO)­(Br)­(Ph₂PCH₂CHN-((<i>S</i>,<i>S</i>)-C­(Ph)­H–C­(Ph)­H)-NCHCH₂PPh₂)]­[BPh₄] (<b>2</b>) for the asymmetric transfer hydrogenation of acetophenone in basic isopropanol. An induction period for catalyst activation is observed before the catalytic production of 1-phenethanol. The activation step is proposed to involve a rapid reaction of <b>2</b> with excess base to give an ene–amido complex [Fe­(CO)­(Ph₂PCH₂CHN-((<i>S,S</i>)-C­(Ph)­H–C­(Ph)­H)-NCHCHPPh₂)]⁺ (<b>Fe</b>_<b>p</b>) and a bis­(enamido) complex Fe­(CO)­(Ph₂PCHCH-N-(<i>S</i>,<i>S-</i>CH­(Ph)­CH­(Ph))-N–CHCHPPh₂) (<b>5</b>);<b> 5</b> was partially characterized. The slow step in the catalyst activation is thought to be the reaction of <b>Fe</b>_<b>p</b> with isopropoxide to give the catalytically active amido-(ene-amido) complex <b>Fe</b>_<b>a</b> with a half-reduced, deprotonated PNNP ligand. This can be trapped by reaction with HCl in ether to give, after isolation with NaBPh₄, [Fe­(CO)­(Cl)­(Ph₂PCH₂CH₂N­(H)-((<i>S</i>,<i>S</i>)-CH­(Ph)­CH­(Ph))-NCHCH₂PPh₂)]­[BPh₄] (<b>7</b>) which was characterized using multinuclear NMR and high-resolution mass spectrometry. When compound <b>7</b> is treated with base, it directly enters the catalytic cycle with no induction period. A precatalyst with the fully reduced P-NH-NH-P ligand was prepared and characterized by single crystal X-ray diffraction. It was found to be much less active than <b>2</b> or <b>7</b>. Reaction profiles obtained by varying the initial concentrations of acetophenone, precatalyst, base, and acetone and by varying the temperature were fit to the kinetic model corresponding to the proposed mechanism by numerical simulation to obtain a unique set of rate constants and thermodynamic parameters