Alkyne Metathesis Catalysts: Scope And Future

This paper presents the evolution of alkyne metathesis since the early discoveries, essentially from the catalyst point of view. It is shown that although well defined carbynes may be useful for this reaction, further work has been made, aimed at the synthesis of new catalysts or catalytic systems, based on molybdenum precursors , associated or not with phenolic cocatalysts. The major objectives have been to obtain more functional groups tolerants catalysts, for their application in organic synthesis, including RCM for further stereoselective hydrogenation of the triple bond in the cycle, as well as for polymerization of aromatic diynes. Some insights into the possible mechanisms are given, and data relevant to the metathesis of terminal alkynes, a still challenging reaction where polymerization is occurring via the formation of deprotonated metallacyclobutadiene, are also presented and discussed

In situ generated half-lanthanidocene based catalysts for the controlled oligomerisation of styrene: Selectivity, block copolymerisation and chain transfer

International audienceThe Cp*Nd(BH4)2(THF)2/n-butylethylmagnesium combination affords a controlled and syndioselective oligomerisation of styrene. Living oligostyrenes can be used as macromonomers for block copolymerisation, leading to the unprecedented synthesis of a (polystyrene)-block- (1,4-trans polyisoprene) copolymer. Reversible transmetallation between the neodymium and the magnesium atom is further established with a transfer efficiency close to 100%

Controlled Trans-stereospecific Polymerisation of Isoprene with Neodymium (III) Borohydride-dialkylmagnesium system

International audienceNeodymium trisborohydride, associated with MgR2 alkylating agents, affords an efficient initiator for the stereospecific polymerization of isoprene. This system presents good activity (150 kg of polyisoprene/mol of Nd/h; 95% yield in 2 h) and is trans-stereospecific (up to 97.7%), and its polymerization shows a quasi-living character. The molecular weights are controlled by the [Isoprene]/[Nd] ratio, with a polydispersity index lower than 1.6. A full study is presented here, including the influence of the nature of the alkylating reagent (MgR2), the solvent, the reaction temperature, the [MgR2]/[Nd] ratio, and the replacement of neodymium by other lanthanides. This work also includes a kinetic study of the system and a detailed NMR study of the microstructure of the trans-polyisoprene obtained, providing mechanistic insights about the polymerization reaction

One-pot synthesis of an ionic half-sandwich complex of Neodymium. Application to stereospecific isoprene polymerization catalysis.

International audienceThe reaction of one equivalent of Nd(BH4)3(THF)3 with an half equivalent of dialkylmagnesium in the presence of a stoechiometric amount of pentamethylcyclopentadiene cleanly affords a new kind of half-sandwich of neodymium that is stable toward comproportionation. This strategy can be advantageously applied to generate in situ catalysts allowing the controlled polymerisation of isoprene

A New Family of Styrene/Diene Rubbers

International audienceThe insertion of single styrene units into polyisoprene is demonstrated using borohydrido rare earth/dialkylmagnesium systems. This yields a new family of styrene/diene copolymers (SBR rubbers). The resulting poly[(1,4-trans-isoprene)-co-styrene] exhibits quite narrow molecular weight distributions, up to 30% inserted styrene, and a 96–98% 1,4-trans-microstructure. The presence of a bulky and electron-rich ligand in the coordination sphere of the metal leads to an increase of the amount of styrene inserted and narrower chemical composition and molecular weight distributions. The presence of significant quantities of styrene in the medium does not alter the selectivity of the reaction, in contrast with cis-specific polymerizations

New ionic half-metallocenes of early lanthanides

International audienceWe present in this study a new and one-step method allowing the preparation of an unprecedented family of stable half-lanthanidocenes. X-ray analysis shows that the isolated compounds all display the same ionic Ln–Mg bimetallic structure consisting of two anionic (CpR)Ln(BH4)3 species and one cationic Mg(THF)6 (CpR =C5Me5, Ln = Nd, 1a, Ln = La, 1b; CpR =C5H5, Ln = Nd, 2a; CpR =C5H2Ph3, Ln = Nd, 3a). Such complexes display high stability with respect to disproportionation in solution. Combined with dialkylmagnesium, neodymium complexes provide powerful catalysts for stereospecific isoprene polymerisation

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

International audienceThe ‘‘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: HCp = HCpPh3 > HCp* (Cp = C5H5; CpPh3 = 1; 2; 4-Ph3C5H2; Cp* = C5Me5). These results suggest an influence of the electron donating ability of the ligand rather than steric requirements