Ruthenium allenylidene precursors

In the field of alkene metathesis ruthenium-allenylidene precursors have made, since 1998, an important contribution to catalysis [12, 31, 32], for the formation of cycles and macrocycles via RCM, ROMP and acyclic diene metathesis (ADMET) polymerization. 

Scheme 8.5 Enyne metathesis catalyzed by the photochemically activated ruthenium allenylidene precursor la.

Actually, applications of indenylidene-ruthenium complexes for alkene metathesis were reported before, at a time when the action mode of their ruthenium allenylidene precursors was not known. These complexes catalyzed a variety of RCM reactions of dienes and enynes [31, 32, 47] (see Section 8.2.2). 

It is noteworthy that the addition of thiols to form propargylic sulfides is not catalysed by the neutral complex [Cp RuCl(p2-SR)2RuCl(Cp )], but requires the utilization of a cationic precursor such as [Cp RuCl(p2-SMe)2Ru(Cp )(H20)]0Tf [85]. With this catalytic system,propargylic alcohols bearing an internal triple bond are also transformed into propargylic sulfides, which indicates that in this special case, the reaction does not involve a ruthenium allenylidene as an active species. 

Recently, complex Q [103] and other neutral and cationic ruthenium allenylidene complexes (U, V) [104] have been reported as efficient catalyst precursors for the ROMP of cyclic olefins such as cyclooctene and norbor-nene. 

The ruthenium allenylidene complexes W are excellent precursors for the catalytic dimerization of tributyltin hydride under mild conditions [ 109] (Eq. 15). In the presence of Bu3SnH, the hydride addition at Cy provides a catalytically active alkynyl ruthenium-tin species (Scheme 22). 

The synthesis of the first ruthenium indenylidene catalysts arose from a 1998 report by the Fiirtsner and Dixneuf groups [13] that identified ruthenium-allenylidene complexes, easily prepared by the activation and dehydration of propargylic alcohols, as efficient catalysts for RCM reactions. These catalyst precursors were made from readily available arene-ruthenium(II) complexes containing a bulky and electron-donating ligand in the presence of the non- or weakly coordinating anion salt NaX or AgX (Scheme 14.1). 

From these studies, it was demonstrated that the alkene metathesis activity was not due to the allenylidene precursor, but due to the indenylidene ruthenium catalyst 6, which has a structure analogous to the Grubbs I catalyst [15, 17]. Both complexes generate the same RuCl2(=CFl2) intermediate upon reaction with a terminal alkene. 

The homobimetallic, ethylene-ruthenium complex 15, which contains three chloro bridges, was readily obtained from the reaction of [RuCl2(/ -cymene)]2 with 1 atm of ethylene [34]. In 2009, Demonceau and Delaude [34] showed that complex 15 could be a useful precursor to allow subsequent access to the diruthenium vinylidene complex 16, allenylidene complex 17, and indenylidene complex 18 (Scheme 14.8). Upon reaction with propargylic alcohol, complex 15 afforded vinylidene complex 16, which converted into the allenylidene complex 17 in the presence of molecular sieves [34]. As shown in the acid-promoted intramolecular rearrangement of mononuclear ruthenium allenylidene complexes [19, 20, 32], the addition of a stoichiometric amount of TsOH to complex 17 at -50 °C led to the indenylidene binuclear complex 18 [34]. Complex 18 has been well... 

As commented previously, alkenyl(amino)allenylidene ruthenium(II) complexes 41 are easily accessible through the reaction of indenyl-Ru(ll) precursors with ynamines (Scheme 10) [52-54]. Based on this reactivity, an original synthetic route to polyunsaturated allenylidene species could be developed (Scheme 19) [52, 53]. Thus, after the first ynamine insertion, complex 41 could be transformed into the secondary derivative 62 by treatment with LiBHEts and subsequent purification on silica-gel column. Complex 62 is able to insert a second ynamine molecule, via the cyclization/cycloreversion pathway discussed above, to generate the corresponding dienyl(amino)allenylidene species. Further transformations of this intermediate in the presence of LiBHEts and Si02... 

In 1998 it was revealed that allenylidene-ruthenium complexes, arising simply from propargylic alcohols, were efficient precursors for alkene metathesis [12], This discovery first initiated a renaissance in allenylidene metal complexes as possible alkene metathesis precursors, then it was observed and demonstrated that allenylidene-ruthenium complexes rearranged into indenylidene-ruthenium intermediates that are actually the real catalyst precursors. The synthesis of indenylidene-metal complexes and their efficient use in alkene metathesis are now under development. The interest in finding a convenient source of easy to make alkene metathesis initiators is currently leading to investigation of other routes to initiators from propargylic derivatives. 

Allenylidene-Ruthenium Complexes as Alkene Metathesis Catalyst Precursors the First Evidence... 

Now, it has been shown [33, 34] that allenylidene-metal precursors I generate the indenylidene-metal intermediate III which is the real catalyst precursor (Scheme 8.2). Thus, we now understand that to generate the active species III, the poro-cymene ligand is more easily displaced from the ruthenium site and the triflate, which interacts weakly with the mthenium-allenylidene, favors the formation of the indenylidene ligand and arene displacement. 

The allenylidene-ruthenium(arene) catalyst precursors I have been used for the synthesis of macrocycles by the RCM reaction and were revealed as active as the first generation Grubbs catalyst RuCl2(=CHPh)(PCy3)2 [35], depending on the nature of the diene functional groups and macrocyde size [32] (Scheme 8.3).These macrocyde syntheses show that the allenylidene mthenium catalysts I offer functional group tolerance. 

Allenylidene-ruthenium complex Ib readily promotes the ROMP of norbornene, much faster than the precursor RuCl2(PCy3)(p-cymene) [39] (Table 8.1, entry 1). The ROMP of cyclooctene requires heating at 80 °C (5 min), however a pre-activation of the catalyst allows the polymerization to take place at room temperature. The activation consists, for example, in a preliminary heating at 80 °C or UV irradiation of the catalyst before addition of the cyclic aikene, conditions under which rearrangement into indenylidene and arene displacement take place [39] (Table 8.1, entries 2,3). The arene-free allenylidene complexes, the neutral RuCl2(=C=C=CPh2)... 

Kinetic studies of diallyltosylamide RCM reaction monitored by NMR and UV/VIS spectroscopy showed that thermal activation of the catalyst precursors la and Ib (25-80 °C) led to the in situ formation of a new species which could not be identified but appeared to be the active catalytic species [52]. Attempts to identify this thermally generated species were made in parallel by protonation of the catalysts I. Indeed, the protonation of allenylidene-ruthenium complex la by HBF4 revealed a significant increase in catalyst activity in the RCM reaction [31,32]. The influence of the addition of triflic acid to catalyst Ib in the ROMP of cyclooctene at room temperature (Table 8.2, entries 1,3) was even more dramatic. For a cyclooctene/ruthenium ratio of 1000 the TOF of ROMP with Ib was 1 min and with Ib and Sequiv. of TfOH it reached 950min [33]. 

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