Grubbs complexes

The first generation Grubbs complex (PCy3),Cl2Ru(=CHPh) also appeared to be a very active catalyst in these processes, but additional experiments provided evidence for the initiation of Ru-H bond formation on fire Gmbbs catalyst heating at 60 °C [16]. 

It is well known that the reaction of the Grubbs complexes with alkyl vinyl ethers readily occurs at room temperature to yield the ruthenium complex with alkoxy-substituted carbene ligand, via a ruthenacyclobutane intermediate (Scheme 11) [28]. 

An exactly opposite trend was observed in olefin metathesis for which the mechanistic scheme for complexes 4 and 5 postulates the dissociation of a phosphine ligand from the metal centre as the key step in the dominant reaction pathway [23]. For instance, ruthenium benzylidene complexes 5 bearing only one /V-heterocyclic carbene entity were found to be significantly more active than those incorporating two of them (6) and the original Grubbs complex (4). This was clearly substantiated for the conversion of the suitable dienes into dihydropyrrole (Scheme 6, Table 2) [20d], and polyhydroxylated cyclohexene rings [24]. 

The fate of the benzylidene fragment in these ruthenium complexes is another matter of debate [20a, 25]. The thermal stability of benzylidene complexes 4-6 was tested at 85 °C, under conditions mimicking polymerisation of methyl methacrylate. As monitored by H NMR, complete disappearance of the benzylidene fragment of the mixed phosphine/Af-heterocyclic carbene complex 5 (R = Cy, R = (5j-CHMePh) was observed within 20 min, whereas the Grubbs complex, RuCl2(=CHPh)(PCy3)2, showed only 55 % decomposition, and the bis-A/ -heterocyclic carbene ruthenium complex 6 (R = Cy) 88 % decomposition over the same time interval (Figure 6). 

The catalytic inactivity of metallacarbene species e.g. Schrock catalyst [32] and Grubbs complex RuCl2(PPh3)(CHPh) in metathesis of vinyl-trisubstituted silanes and siloxanes also supports such a mechanism. This reaction is also called homo(hetero)coupling or trans-silylation of olefins with vinylsilanes. 

To the solution of the ethoxylated thallium salt was added a solution of 2 mmol Grubbs catalyst (la) in 10 ml THF. The reaction mixture was stirred at room temperature for three hours. After evaporation of the solvent, the residu was dissolved in a minimal amount of benzene and cooled to 0°C. Thalliumchloride was removed via filtration. The desired complex was then washed with cold benzene (10 ml 3) and the filtrate was evaporated. The solid residue was recrystallized from pentane (- 70°C) to give the Shiff base modified Grubbs complex (2) as a brown solid. 

Initiation with ethyl vinyl ether (0,77 mM 1, 0.58M EVE, 20 C), determined by UV— Table 9.6 Solvent effects with the second-generation Grubbs complex 2 [13]. vis at484nm. 

The Grubbs pyridine solvates are the fastest initiators of alkene metathesis and are valuable as synthetic intermediates to prepare other ruthenium carbene complexes. In particular, the 18-electron pyridine solvates 4a,b are very fast initiators that were developed to catalyze difficult alkene metatheses (e.g., the cross metathesis of acrylonitrile) [6]. The rates of initiation for several complexes are provided in Table 9.9. The pyridine solvate 4a has been found to initiate about 105 times faster than the parent Grubbs complex 2 and at least 100 times faster than the second-generation triphenylphosphine variant 26. When compared with the Hoveyda-Blechert complex 3a, 4a initiated about 100 times faster (c entry 3 vs. entry 5). The bromopyridine solvate 4b exceeded all of these in its initiation rate it was at least 20 times more reactive than 4a. 

These studies were extended to the Hoveyda-Grubbs type complexes [59]. Here, Verpoort and coworkers compared the catalytic activity of unsymmetrical complexes 74-79 with the classical Hoveyda-Grubbs complexes 3 and 11 (Figure 11.15). However, no improvement in activity was observed in any of the tested metathesis reactions. In this study, the increase of steric interactions led to a significant decrease in activity thus, 79, the most sterically hindered complex, was the least active catalyst for all olefin metathesis reactions tested. 

It has been shown that Grubbs complexes with benzylidene group (I) are more stable in comparison with other carbenes (Table 1) [4]. ITiis moiety is gradually consuming during initiation step (Scheme 1). It usually leads, especially in case of terminal olefins metathesis or ethenolysis, to the formation of methylidene carbenes (5) and (6a) ... 

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