Indenylidene complexes

For a thorough review of Ru-NHC-catalysts for metathesis, see Samojlowicz C, Bieniek M, Grela K (2009) Chem Rev 109 3708-3742 for ruthenium indenylidene-complexes in cross metathesis, see Boeda F, Bantreil X, Clavier H, Nolan SP (2008) Adv Synth Catal 350 2959-2966 For Hll-types systems, see Schrodi Y, Pederson RL (2007) Aldrichimica Acta 40 45-52... 

The objective of this chapter is to report on these various aspects allenylidenes in alkene metathesis, their transformation into indenylidenes, alkene metathesis with indenylidene complexes, other propargylic derivatives as alkene metathesis initiators and their application in alkene metathesis. 

However, the reaction of RuCl2(PPh3)4 with only HC = CCPh20H directly led to the indenylidene complex VIII, which gave, after phosphine exchange with PCy3, the indenylidene complex IX [42]. 

Scheme 8.7 Synthesis of allenylidene and indenylidene complexes with the Grubbs catalyst structure.

At the same time, Fiirstner et al. [43 14] also observed that RuCl2(PPh3)3 on reaction tvith HC = CCPh20H did not lead to the expected allenylidene complex but to the same indenylidene complex VIII (Scheme 8.8) [45]. 

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]. 

This carbyne was shown not to be the RCM active species. At —20 °C it rearranged spontaneously into the indenylidene complex XV with release of TfOH. This intramolecular transformation corresponds to the electrophilic ortho-substitution of one phenyl group by the electrophilic carbyne carbon of XIV. The carbene complex XV was identified as the species thermally formed in situ from the catalyst precursors Ia,b in the range 25-80 °C. 

The catalytic activities for ROMP of cyclooctene of isolated indenylidene complexes XV-XVII were evaluated in chlorobenzene (Table 8.3). 

Table 8.3 Cyclooctene polymerization catalyzed by isolated indenylidene complexes XV—XVII at room temperature".

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 NHC ligand-containing indenylidene complexes XXI, XXII and XXIII, XXIV were readily prepared by reaction of the NHC ligand with complex VIII and IX, respectively [57] (Scheme 8.15). 

The binuclear indenylidene complex XXV was readily obtained on reaction of complex IX with [RuCl2(p-cymene)]2 (Equation 8.5) [58]. 

Scheme 8.16 Preparation of indenylidene complexes with bidentate iminophenolate ligand.

The first efficient application of a well-defined ruthenium indenylidene complex in metathesis was described in 1999 by Frirstner for the total synthesis of a cyclic prodigiosin derivative, a potential lead compound for the development of immunosuppressive agents. The RCM using the ruthenium indenylidene complex DC (10 mol%) as precatalyst leads to the transformation of the N-protonated diene into the desired macrocycle in 65% yield (Equation 8.8) [43]. 

Starting from complex IX, Fiirstner developed a homobimetallic phenylindeny-lidene complex XXV (Equation 8.5), and both of these were used in the cyclization of medium-sized rings by RCM. A series of examples is presented which shows that indenylidene complexes are as good as or superior to the classical Grubbs first generation catalyst in terms of yield, reaction rate, and tolerance towards different functional groups (Scheme 8.17) [58]. 

The same reaction (RCM) was used as the key step for the formation of a family of potent herbicidal 10-membered lactones. An important aspect from the preparative point of view is the control of stereochemical outcome of the RCM by the choice of catalyst. Thus, the use of the ruthenium indenylidene complex IX always leads to the corresponding ( )-alkenes, whereas the second generation of Grubbs catalyst bearing a N-heterocyclic carbene ligand affords the isomeric (Z)-olefin with good selectivity (Scheme 8.19) [64]. 

During the same period, Nolan showed also that indenylidene complexes are active catalyst precursors in the RCM of dienes. The reactions were performed on the NMR scale and moderate to good yields were obtained for diethyl diallylmalonate. 

The pyridine-containing ruthenium-based complex XXVI developed by Nolan [59] from the indenylidene complex DC, promoted the RCM of various dienes (Equation 8.6). Kinetic studies were carried out and showed that, despite a rapid initiation, the presence of pyridine in the reaction mixture has a negative effect on the stability of the active species and only moderate catalytic conversions were obtained [59] (Scheme 8.21). 

Arene)ruthenium-indenylidene complex XV (Scheme 8.11), developed by Dixneuf et al. [33, 34] was also a very efficient catalyst for RCM of dienes and enyne metathesis (Table 8.4). 

One of the latest compounds of this class is the phoban-indenylidene complex XXVII, synthesized by Forman et al. in 2006 [60]. This robust catalyst was tested in self-metathesis and ethenolysis reactions of methyl oleate, giving rise to significantly... 

The ruthenium indenylidene complex XV, prepared in situ from RuCl(ri -p-cymene) (=C=C=CPh2)(PCy3)][CF3S03] Ib vith different acids showed very high activity in the cyclooctene polymerization (Table 8.2) [33]. 

Nitrile rubber polymers, having lower molecular weight have been prepared by metathesis of nitrile butadiene rubber with ruthenium indenylidene complexes [65]. 

Another application of ruthenium indenylidene complexes was the atom transfer radical addition of carbon tetrachloride to vinyl monomers reported by Verpoort [61]. This Kharasch reaction afforded good yields for all substrates tested, especially with the catalyst VIII (Equation 8.11, Table 8.8). 

The indenylidene complexes IX and XXVIIIc were also reported to promote the addition of different carboxylic acids to terminal alkynes to give enol esters, the Markovnikov addition product being the major product with, in some cases, the competing catalytic dimerization of terminal alkynes [61]. 

Table 8.8 ATRA of carbon tetrachloride to various olefins catalyzed by ruthenium indenylidene complexes.

In parallel, since the first preparation of allenylidene-metal complexes in 1976, the formation of these carbon-rich complexes developed rapidly after the discovery, in 1982, that allenylidene-metal intermediates could be easily formed directly from terminal propargylic alcohols via vinylidene-metal intermediates. This decisive step has led to regioselective catalytic transformations of propargylic derivatives via carbon(l)-atom bond formation or alternately to propargylation. Due to their rearrangement into indenylidene complexes, metal-allenylidene complexes were also found to be catalyst precursors for olefin and enyne metathesis. 

As already mentioned, the development of metathesis catalysts that can be easily accessed from simple precursors is necessary if a large-scale application is desired. With this in mind, Forman et al. developed a robust ruthenium-based phoban-indenylidene complex through a simple and relatively inexpensive procedure, if compared to the preparation of C3 [40]. This mthenium alkylidene was tested in the bulk SM of methyl oleate. As a result, they could reach up to 50% conversion with 0.005 mol% catalyst at 50°C. 

The outcome of the RCM experiments employing different catalysts was consistent with the analysis outlined above (Scheme 3). Exposure of diene 29 to the ruthenium indenylidene complex 4 15), a readily available alternative to the classical Grubbs catalyst, afforded the desired ( >configured lactone ( >30 in 69% isolated yield, together with only 9% of the corresponding (Z)-isomer. Importantly, the E Z ratio does not evolve with time, indicating that the observed... 

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