Ruthenium based homogeneous olefin metathesis

[摘要] English: The aim of this study was to investigate the decomposition of the methylidenes 4a [Ru(Cl)2(PCy3)2CH2] and 6 [Ru(Cl)2(PCy3)(NHC)CH2] and alkylidene 5 [Ru(Cl)2(PCy3)2CHR], formed in the homogeneous olefin metathesis reaction using either the first generation Grubbs catalyst 2a [Ru(Cl)2(PCy3)2CHPh] or the second generation Grubbs catalyst 3 [Ru(Cl)2(NHC)(PCy3)CHPh]. (NHC = 1,3-dimesityl-4,5-dihydroimidazole-2- ylidene). Grubbs catalyst di-bromide 2b [Ru(Br)2(PCy3)2CHPh] was found to react with chlorinated solvents with the formation of CDCl2Br and CCl3Br, indicating that there is a direct reaction with the solvent itself (CDCl3) and not HCl in the solvent. A mechanism is proposed for this reaction. The intermolecular halide exchange reaction was observed during this study and used to gain insight into the bimolecular decomposition pathway. A mechanism where Grubbs catalyst 2a acts as a ligand in a reaction with the 14 electron intermediate 7b [Ru(Br)2(PCy3)CHPh] is proposed for intermolecular halide exchange (see scheme below). It is proposed that the intermolecular halide exchange reaction occurs continuously in all solutions of Grubbs catalyst, with the formation of the dimeric intermediate 45 (see below). The decomposition of Grubbs catalyst 2a in CDCl3 results in the formation of paramagnetic species and consequently in the loss of the 31 P NMR signal. It has recently been reported that phenol and SnCl2 dramatically increase the life time of Grubbs catalyst 2a. An understanding of the formation and behaviour of dimeric intermediate 45 lead to the proposal of a mechanism for the inhibition of bimolecular decomposition in the presence of phenol and SnCl2 in CDCl3. To inhibit the formation of the dimeric intermediate 45 above, the use of solid and dissolved salts as additives was studied. 31 P and 1 H NMR studies showed that the presence of Bu4NBr dissolved in CDCl3 results in halide substitution at Grubbs catalyst 2a [Ru(Cl)2(PCy3)2CHPh] to form a mixed halide catalyst 2c [RuBrCl(PCy3)2CHPh] and finally Grubbs catalyst di- bromide 2b [Ru(Br)2(PCy3)2CHPh]. Improved conversion and selectivity were obtained when solid and dissolved salts were added to the reaction mixture in the self metathesis of 1-octene. Molecular modelling has indicated that there is a possibility of ethylene induced decomposition of the Grubbs catalyst methylidene 4a [Ru(Cl)2(PCy3)2CH2] with the formation of propene as shown below. 1 H NMR experiments monitoring the ethylene induced decomposition of methylidenes 4a and 6 in C6D6 overnight clearly showed the formation of α - olefins, supporting the proposed β -hydride decomposition pathway. This pathway includes the formation of a hydride species that is highly likely to be isomerisation active and may lead to the decreased selectivity found experimentally. This study was taken further by carrying out ethylene induced decomposition in CDCl3, where it was found that there is no propene formation but rather addition of CDCl3 across the double bond via the Kharasch reaction. This indicates that the β -hydride pathway is inhibited in chlorinated solvents, which may be part of the reason why better selectivities are obtained in the presence of chlorinated solvents.

[发布机构] University of the Free State