Direct Zn-diamine promoted reduction of C=O and C=N bonds by polymethylhydrosiloxane in methanol

Ketones and imines are chemoselectively reduced at room temperature in methanol to the corresponding alcohols and amines in high yields in a one-step procedure using polymethylhydrosiloxane (PMHS) and a simple zinc-diamine catalyst.     

New synthesis of 1,4-diketones via rhodium-catalysed 1,4 carbonylative addition of arylboronic acids to alpha,beta-unsaturated ketones

The reaction of various arylboronic acids with alpha,beta-unsaturated ketones under CO pressure and in the presence of rhodium catalyst yields 1,4-diketones.     

Neodymium alkoxides: synthesis,characterization and their combinations with dialkylmagnesiums as unique systems for polymerization and block copolymerization of ethylene and methyl methacrylate

The synthesis and characterization (NMR and X-ray) of a variety of neodymium alkoxides derived from simple and functionalized tertiary monoalcohols, and their application as inorganic precursors in combination with dialkylmagnesium reagents for ethylene and methyl methacrylate (MMA) (co)polymerization have been investigated. Salt metathesis reactions between NdCl(3) and sodium alkoxides in THF led to the formation of trinuclear complexes [Nd(3)(mu(3)-OR)(2)(mu(2)-OR)(3)(OR)(4)(thf)(2)] with R=tBu (1), tAm (2), while aggregate structure [Nd(12)(OtAm)(26)(HOtAm)(2)Cl(11)Na].(OEt(2))(2) (3) was obtained when the synthesis was performed in Et(2)O. [Nd(3)(mu(3)-OtBu)(2)(mu(2)-OtBu)(3)(OtBu)(4)(HOtBu)(2)] (4), prepared by aminolysis of Nd[N(SiMe(3))(2)](3) in hexane, slowly decomposed in toluene into oxo complex [Nd(5)(mu(5)-O)(mu(3)-OtBu)(4)(mu(2)-OtBu)(4)(OtBu)(5)] (5). Finally, the dimer [Nd(2)(mu(2),eta(2)-OR)(2)(eta(2)-OR)(2)(eta(1)-OR)(2)] (OR=OCMe(2)CH(2)CH(2)OMe) (6) was synthesised by aminolysis reaction from the corresponding gamma-donor-functionalized alcohol. Some of these neodymium alkoxides, in particular homoleptic complex 1, when associated in situ to one equivalent of a dialkylmagnesium, allow the formation of an active catalyst for ethylene polymerization. Under mild conditions (0 degrees C, 1 bar), the latter catalyst system exhibited a moderate activity (5-10 kg mol(-1) h(-1) bar(-1)). Effective transfer reactions were observed in the presence of H(2) or PhSiH(3) and renewal/improvement of activity occurred upon extra addition of dialkylmagnesium. The most outstanding feature of this catalytic system lies in the precipitation of the active "Nd-polyethylenyl" species during the ethylene polymerization course as solid S which could be isolated. This heterogeneity was turned to good account, enabling to achieve heterogeneous solid-gas ethylene polymerization and to prepare diblock PE-PMMA copolymers with high diblock efficiency and high molecular weights (M(n) > 200 000). A catalytic cycle for this unique system is proposed based on the isolation of a transmetallation product (7) from a neodymium alkoxide/dialkylmagnesium combination and NMR studies of the latter.     

Unusual product distribution in ethylene oligomerization promoted by in situ ansa-chloroneodymocene–dialkylmagnesium systems

Ethylene polymerization using in situ combinations between a chloroneodymocene precursor and a dialkylmagnesium reagent has been investigated to prepare tailor-made oligomers. Combinations of [Cp^*₂NdCl₂Li(OEt₂)₂] (1) with 40 equiv. of n-butylethylmagnesium (BEM) or di(n-hexyl)magnesium (DHM) gave oligoethylenes with M_n up to 2500 and narrow molecular weight distributions (M_w/M_n<1.10) in moderate activity (A_{1 h}=79 kg/(mol of Nd h atm) at 80 °C, 1 atm). Under these conditions, ethylene polymerization proceeded in a controlled fashion, with a linear growth of M_n vs monomer conversion, ascribed to an effective chain transfer between the Nd and Mg centers. Combinations of [rac-{Me₂Si(η⁵-2-SiMe₃-4-t-Bu-C₅H₂)₂}Nd(μ-Cl)₂Li(THF)₂] (2) with either BEM or DHM (20–40 equiv.) showed decreased activity, suggesting possibly a different rate-determining-step for ethylene polymerization than for that of higher -olefins. The oligoethylenes obtained from combinations based on 2 have narrow molecular weight distributions (M_w/M_n<1.2) but higher contents of vinyl terminations. Monitoring of the reactions showed also a non-linear growth of M_n vs monomer conversion, especially marked when DHM was used as co-reagent. The 2/DHM combination behaves as a “self-correcting” catalyst system that deviates from the calculated M_n values for a controlled-living polymerization in the early stage of the reaction and re-approach them progressively in the second stage.     

Utilisation des ligands aminophosphinephosphinites en hydroformylation catalysée par le rhodium: amélioration de l'énantioselectivité par synthèse selective des hydrures RhH(CO)2L2

Asymmetric hydroformylation of styrene by rhodium catalysts modified with aminophosphinephosphinite ligands has been studied. Use of either Rh₄(CO)₁₂/L₂ mixture or RhClCOL₂ in electroreduction under CO/H₂, gives interesting results in respect of regio- (PhCHMeCHO/PHCH₂CH₂CHO) and enantio-selectivity when the ligands (1R,2S)-PPh₂NMeCHMeCHPhOPPh₂ and (CH₃)₂C(N(Me)PPh₂)-HCH₂OPPH₂ (e.e. > 30%) are used.

¹H and ³¹P NMR spectroscopy has indicated the occurrence of the RhH(CO)₂L₂ hydride precursor, suggested to be responsible for catalysis. It is a trigonal complex having an aminophosphine function in a trans position to the hydrogen, the equatorial position being occupied by the (P---O) group and the two CO moieties.     

Half-lanthanidocenes catalysts via the “borohydride/alkyl” route: A simple approach of ligand screening for the controlled polymerization of styrene

The “borohydride/alkyl” (B/A) route initially reported for isoprene has been applied successfully to the polymerization of styrene. This method provides via an in situ approach an interesting tool for the assessment of the influence of a ligand on the performance of half-lanthanidocene catalysts. All systems lead to well-controlled oligomerization/polymerization processes. This method is thus a convenient tool for the controlled polymerization of styrene starting from a common trisborohydride precursor and commercial ligands. The influence of the nature of several ligands on the activity could be established, with trends corresponding to those obtained starting from the isolated precursors: HCpHCp^Ph₃>HCp^*(Cp=C₅H₅,Cp^Ph₃=1,2,4-Ph₃C₅H₂,Cp^*=C₅Me₅). These results suggest an influence of the electron donating ability of the ligand rather than steric requirements.     

Preparation of pyridinyl aryl methanol derivatives by enantioselective hydrogenation of ketones using chiral Ru(diphosphine)(diamine) complexes. Attribution of their absolute configuration by H NMR spectroscopy using Mosher's reagent

Ruthenium-diamine-diphosphine complexes provide highly efficient catalysts for enantioselective hydrogenation of a series of pyridinyl aryl ketones. The hydrogenation proceeds under mild conditions providing chiral pyridinyl aryl methanol derivatives with consistently high yields and moderate to excellent enantioselectivities (up to 99% ee) according to the structure of the chiral diphosphine. NMR studies, based on Mosher's ester derivatisation, allowed to determine the configuration of the major alcohol obtained during asymmetric hydrogenation.