Indenylidene generation

Fig. 3.28 ROMP initiators, first, second and third generation Grubbs catalysts (complexes 71, 72, and 73, respectively) 74a-c 3-phenyl-indenylidene replaces the benzylidene carbene...

An unusual intramolecular addition of one ortho C-H bond of a phenyl substituent across the C3=C4 bond in 20 (Scheme 3.33) is observed on thermolysis of 20 in CHCI3 [7]. A similar complex is also formed when the corresponding dppm complex is generated in situ from [Cl(dppm)2Ru-C = C-C = C-C(Ph)20SiMe3] and HBF4 in CH2CI2 [30, 31]. This transformation is actually the parent of the later commonly observed allenylidene to indenylidene intramolecular rearrangement. 

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. 

Although the in situ generation and use of XV is convenient, the presence of add also favors slow decomposition of XV. It was thus suitable to isolate catalyst XV by filtration of its addic solution on basic alumina [34]. Thus the indenylidene-ruthenium(arene) complexes containing various phosphines were isolated (Scheme 8.12). 

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]. 

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. 

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... 

While carbyne complex 12a did not effect the ROMP of unstrained cyclooctene, the cationic indenylidene complex 13a was found to catalyze this reaction with a turnover frequency (TOP) of more than 17000min [20]. In s/ta-generated arene-ruthenium indenylidenes 13a-b were also shown to be excellent catalysts for a variety of RCM reactions employing functional dienes and enynes [20]. 

First-Generation Ruthenium Indenylidene Catalysts Bearing Two Phosphine Ligands... 

The iButylPhoban complex 20, which is commercially available, has been used to prepare a second-generation, ruthenium indenylidene catalyst [37]. 

Scheme 14.10 First-generation ruthenium indenylidene complexes.

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]. 

The ruthenium indenylidene complex 6 has also been used as precursor for the first-generation Grubbs- and Hoveyda-type catalysts, as shown in Scheme 14.13. It was shown by Nolan [39] that the reaction of styrene with 6 afforded the first-generation Grubbs catalyst 21 in excellent yield. This procedure benefits from the use of commercially available starting materials and avoids the use of diazo compounds. 

Scheme 14.13 Ruthenium indenylidene complexes as precursors of first-generation ben-zylidene and vinylcarbene complexes.

First-Generation Ruthenium indenylidene Catalysts Bearing a Chelating Ligand... 

When the free carbenes are stable enough, as is the case with most unsaturated imidazolylidene derivatives, they can be isolated and engaged in substitution reactions. The first such preparation using this method was reported in 1999 by Nolan [16], who prepared RuCl2(indenylidene)(imidazolylidene)(PR3) complexes 7—10 from Af,A/ -bis((mesityl)imidazol-2-ylidene) (IMes) and A5A/ -bis((2,6-diisopropylphenyl)imidazolyl-2-ylidene) (IPr) carbenes in toluene at room temperature (Scheme 14.5). Other NHCs substituted on the NHC backbone [48, 49] and various Af-aryl groups [50] have also been prepared and coordinated onto ruthenium indenylidene moieties using the free or in sif -generated carbenes with complex 6. 

Scheme 14.20 Second-generation ruthenium indenylidene complex bearing a chelating...

Scheme 14.21 Second-generation, ruthenium indenylidene complexes bearing a phosphite ligand.

Scheme 14.23 Preparation of the third-generation ruthenium indenylidene catalyst 53.

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