Allenylidene-ruthenium rearrangements

Scheme 3.24 Ferrocenylethyl(dimethylamino)allenylidene ruthenium by nucleophilic addition of ferrocenylmethylamine to C3 of 10 and subsequent rearrangement.

Among the R2C(=C) =Ru homologs promoting alkene metathesis the most recent discoveries deal vhth the allenylidene-ruthenium and related pre-catalysts. This chapter is devoted to the class of ruthenium-allenylidene metathesis precatalysts, their intramolecularly rearranged indenylidene catalysts, and their use in... 

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 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)... 

These observations indicate that when the metal complex is electron-rich, the allenylidene-metal complexes are stable (VI and VII), even on heating or protonation [42]. However, with less electron-rich systems (e.g., PPh3 ligands instead of PCy3 or NHC) the corresponding allenylidene complex was never observed, to the profit of the indenylidene complex VIII. These results suggested that the allenylidene-ruthenium complex is a transient species that rearranges into the indenylidene complex VIII, as was observed for a C5 cumulenylidene [48]. 

Le Gendre and Moise [50] produced an allenylidene ruthenium complex analogous to I but with a titanium(IV)-containing phosphine such as XI (Scheme 8.9). Its rearrangement into indenylidene was not observed and its catalyst activity remained moderate. 

Two observations initiated a strong motivation for the preparation of indenylidene-ruthenium complexes via activation of propargyl alcohols and the synthesis of allenylidene-ruthenium intermediates. The first results from the synthesis of the first indenylidene complexes VIII and IX without observation of the expected allenylidene intermediate [42-44] (Schemes 8.7 and 8.8), and the initial evidence that the well-defined complex IX was an efficient catalyst for alkene metathesis reactions [43-44]. The second observation concerned the direct evidence that the well-defined stable allenylidene ruthenium(arene) complex Ib rearranged intramo-lecularly into the indenylidene-ruthenium complex XV via an acid-promoted process [22, 23] (Scheme 8.11) and that the in situ prepared [33] or isolated [34] derivatives XV behaved as efficient catalysts for ROMP and RCM reactions. 

The indenylidene-ruthenium complexes were shown to be the actual alkene metathesis catalysts arising from the addition of propargylic alcohols [15-18]. The Dixneuf group [19, 20] later revealed that the intramolecular rearrangement of allenylidene-ruthenium complexes into indenylidene-ruthenium complexes was... 

The central Co,=Cp double bond of an allenylidene backbone can also react with a variety of dipolar organic substrates to yield cyclic adducts. Most of the cychza-tion processes reported occur in a stepwise manner via an initial nucleophilic attack at the Coi atom and further rearrangement of the molecule involving a coupling with the Cp carbon. Representative examples are the reactions of the electron-poor ruthenium-allenylidene complex 46 with ethyl diazoacetate and 1,1-diethylpropar-gylamine to yield the five- and six-membered heterocyclic compounds 82 and 83, respectively (Scheme 29) [260, 284]. 

A proposed reaction pathway is shown in Scheme 7.29, where either the aromatic carbon or oxygen atom of naphthol may work as a nucleophile. Thus, the first step is the nucleophilic attack of the carbon atom of 1 -position of 2-naphthol on the C. atom of an allenylidene complex A to give a vinylidene complex B, which is then transformed into an alkenyl complex C by nucleophilic attack of the oxygen atom of a hydroxy group upon the Co, atom of B. Another possibility is the nucleophilic attack ofthe oxygen of 2-naphthol upon the Co, atom of the complex A. In this case, the initial attack of the naphthol oxygen results in the formation of a ruthenium-carbene complex, which subsequently leads to the complex B via the Claisen rearrangement of the carbene complex. 

The allenylidene-mthenium complexes I also catalyze the enyne metathesis to alkenylcydoalkenes with a 1,3-diene stmcture. Initial studies showed the transformation of simple enynes with ether function [37] (Scheme 8.5). This reaction was significantly accelerated by initial catalyst photodiemical activation, which is now understood to favor the rearrangement of the allenylidene- into the active indenylidene-ruthenium moiety and arene displacement. 

Other closely related ruthenium-allenylidene were made and evaluated in alkene metathesis [32]. Werner et al. [49] also produced allenylidene complexes of analogous structure to that of the Grubbs catalyst, but containing hemilabile phosphine such as complex X (Scheme 8.9). However, the Ru—O bond may be too stable to initiate the rearrangement into indenylidene, the coordination of alkene and to become a catalyst. 

PCyg (Scheme 14.3) [16,18]. It was observed that the electron density at the ruthenium site brought by the PCyg hgand in complexes 3 and 4 inhibited the rearrangement of the allenylidene into the corresponding indenylidene complex. Although allenyhdene complexes 3 and 4 have been reported to be catalytically active for the RCM reaction of various functional dienes, the catalytic activity of ruthenium indenyhdene complex 6 was not evaluated in this report [18]. 

Dixneufs group [19, 20] has reported the intramolecular rearrangement of a ruthenium-bound allenylidene ligand into an indenylidene ligand. The stoichiometric protonation of arene-ruthenium-allenylidene complexes lla-c with TfOH at -40 °C gave the alkenyl carbyne complex 12, which, upon raising the temperature to -20 °C, completely transformed into the related, isolable arene-ruthenium, indenylidene complexes 13a-c (Scheme 14.6). The protonation of the allenylidene carbon at C2 generates a very electrophilic carbyne carbon at... 

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... 

The transformation 7a -> 32-> 33 offered the first direct evidence step by step of the rearrangement of an allenylidene into an indenylidene. This transformation was suggested by Nolan [49] and Furstner [50] as on attempts to produce ruthenium-allenylidene complexes they obtained the related indenylidene system 34, without observation of the allenylidene intermediate (Scheme 23). 

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