Indenylidene ligands

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. 

These observations constituted the first evidence for intramolecular rearrangement of allenylidene into indenylidene ligand, although the first isolation by... 

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... 

A similar procedure for the two-step synthesis of the first-generation ruthenium indenylidene catalysts has been patented by Umicore (Scheme 14.11) [38]. The main differences from the previously reported route lie in the utilization of diox-ane as the reaction solvent at 90 C in the first step, followed by the addition of HCl for the acid-promoted formation ofthe indenylidene ligand via the alkenyl carbyne [19]. The preparation of the tricyclohexylphosphine complex was carried out in one pot without isolating the intermediate triphenylphosphine complex 5 [38]. 

Ruthenium Catalysts Bearing a Chelating Indenylidene Ligand... 

Scheme 14.27 Ruthenium complexes containing a chelating indenylidene ligand.

Figure 12.11 Ruthenium complexes with S-chelated carbene ligand (46, 47), phosphene chelated carbene ligand (48), and bidentate isopropoxy-indenylidene ligand (49, 50).

A predictable move in the development of NHC-Ru complexes bearing indenylidene ligands was the preparation of the third-generation complex 38 by the groups of Slugovc and Verpoort in 2008. The first team established its X-ray crystallographic structure and demonstrated its aptitude to efficiendy promote the controlled living ROMP of norbomene monomers, while the second one probed its catalytic activity in diverse RCM, CM, and ROMP reactions. ... 

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]. 

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). 

Scheme 8.15 Preparation of indenylidene ruthenium complexes containing a NHC ligand.

The indenylidene catalysts XXVIII a-d, containing a bidentate ligand, were obtained on reaction of complex IX or XXI with the corresponding iminophenol thalium derivative (Scheme 8.16) [61]. 

Scheme 8.16 Preparation of indenylidene complexes with bidentate iminophenolate ligand.

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]. 

Indenyl-fluorenyl systems, propylene polymerization, 4, 1068 Indenylidenes, in ROMP initiation, 11, 633 7]5-Indenyl ligand, in molybdenocene dihalides, 5, 573 Indenyl ligands, in cobalt(III) complexes, 7, 20 7]5-Indenyl ligands, in rhodium alkene complexes, 7, 197 2-(Indenyl)—phenoxo complexes, with mono-Cp Ti(IV),... 

A class of olefin metathesis catalysts that contains phosphite ligands has advantages over current catalysts for some challenging reactions, such as ring-closing metatheses of hindered dienes. Cazin et al. [184] modified an existing ruthenium indenylidene metathesis catalyst with triisopropyl phosphite groups to form cis and trans phosphite complexes. 

The highly conjugated ruthenium indenylidene precatalysts are a unique class of complexes discovered through an alternative synthesis of the carbene moiety (Chapter 14). A wide variety of precatalysts are available with different dissociable L2-type ligands. Recent mechanistic studies have provided initiation rate data that support a dissociative mechanism, with one exception. These initiators have been modified in the aromatic moieties of the H2lMes group, and they show promise in RCM of hindered dienes. 

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). 

A few weeks later, Nolan [16] reported the indenylidene-ruthenium complexes 7-10, which contain a NHC ligand analogous to the Grubbs II catalyst. Using catalysts 6 and 7 -10, excellent activities were shown in the RCM reaction of various... 

---