Polymerization with borohydride complexes

Table 17 Polymerization of isoprene with borohydride complexes and magnesium activator...

Metathesis of the borohydride ligand for cyclopentadienyl in a reaction with KCp or diorganomagnesium compounds gives half sandwich borohydride complexes [Mg(THF)6][Cp Ln(BH4)3] (Cp =Cp, Cp C5H2Ph3, Ln — La, Nd) that are stable to redistribution. Combined with dia-lkylmagnesium, the Nd complexes provide useful catalysts for stereospecific isoprene polymerization.62... 

Tardif reported recently that the cationic half-sandwich lanthanide amido complexes [(Ind)Ln N(SiMe3)2 ][B(C6Fs)4] (29, Fig. 3) were also highly efficient and c/x-l,4-selective for butadiene polymerization [115]. Meanwhile, Visseaux demonstrated that the half-sandwich scandium borohydride complex Cp Sc(BH4)2(THF) (30, Fig. 3) combined with [Ph3C][B(C6Fs)4] and TIBA led to the very active and highly stereoselective isoprene polymerization (>90% c/x-1,4. Table 11) as well as styrene (>99.9% syndio, Table 12). Improvement of the control of the polymerization was performed at lower temperature at — 10°C that the cm-1,4-ratio increased up to 97.2% followed by the decrease of PDI down to 1.7 [116]. This... 

Scheme 61 Polymerization of TMC initiated with samarium borohydride complexes.

A complex of 9-BBN with MMA can be formed and compounded with sodium borohydride [92], Derivatives from the combination of 9-BBN with fatty acid or fatty alcohol give an initiator with improved stability [93], Stability appears to improve with increasing molecular weight, so oligomeric and polymeric analogs... 

Devaky and Rajasree have reported the production of a polymer-bound ethylenediamine-borane reagent (63) (Fig. 41) for use as a reducing agent for the reduction of aldehydes.87 The polymeric reagent was derived from a Merrifield resin and a 1,6-hexanediol diacrylate-cross-linked polystyrene resin (HDODA-PS). The borane reagent was incorporated in the polymer support by complexation with sodium borohydride. When this reducing agent was used in the competitive reduction of a 1 1 molar mixture of benzaldehyde and acetophenone, benzaldehyde was found to be selectively reduced to benzyl alcohol. 

The borohydrides of the tetravalent actinides can be prepared through the reaction of the tetrafluoride with Al(BEl4)3. The products of this reaction are An(BEl4)4 and AlF2(BEt4). The nranium complex is polymeric in the solid state and is similar to the Zr and Hf analogs. The metal center is... 

Ozonolysis of 6 to produce 7 was studied at temperatures ranging from -78 to -0 °C in mixtures of dichloromethane and methanol. The intermediates that formed were found to be stable below -30 °C. Diol 7 was obtained in high yields when the reaction mixture was quenched with aqueous sodium borohydride. If the reaction mixture was warmed to room temperature before the quench, a complex mixture of products formed as indicated by the NMR spectrum. Similarly, only polymeric products were formed when the ozonolysis was carried out without methanol even at -78 °C. When ozonolysis was done at -5 to 0 °C in the presence of methanol, formation of trityl methyl ether was noticed, a result similar to which has been observed before [2], Diol 7 was obtained in near quantitative crude yield when ozonolysis was carried out below -30 °C, followed by the addition of the reaction mixture to cold, aqueous sodium borohydride solution. The product was used without further purification in the next step. 

Polymerization of diallg l vinylphosphonate monomers was nevertheless efficiently carried out in the presence of lanthanide derivatives and especially cyclopentadienyl lanthanide complexes, used both as initiators and catalysts. Very recently, Shen et al performed the synthesis of poly (diethyl vinylphosphonate) using a lanthanide tris(borohydride) below 50 °C. The authors showed that the polymerization eould be controlled and proceeds under pseudo-first-order kinetics, giving rise to high molecular weight polymers, i.e. ranging from 20 to 40 kDa with molecular weight dispersity below 1.7. 

The slow initiation step can be rationalized by a mechanism involving [2+2] cycloaddition of the olefin at the Ru=C bond of the vinylidene ligand, with generation of a new Ru carbene species able to propagate faster polymerization (Scheme 14). This mechanism has been documented by Ozawa" for ROMP of norbomene with [RuCl2(PPh3)2(=C=CHFc)] (Fc = ferrocenyl) and by Kirchner in a stoichiometric reaction using a Tp-coordi-nated Ru complex (Tp = tris(pyrazolyl)borohydride)." ... 

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