Metathesis complexes

Figure 5.5 Synthetic route to the grafted rhenium metathesis complex and the corresponding triphenylsiloxy analog. (Redrawn from Chabanas et al. [18].)...

Scheme 25 Olefin metathesis complexes bound to (poly)divinylbenzene monoliths...

An alternative solution to ionic tagging to confine a catalyst into an IL phase is that of using a cationic metathesis complex, e.g. ruthenium allenylidene salt (86). Treatment of diallyltosylamide at 80 °C for 5h dissolved in [bmi-m][OTf] in the presence of 2.5 mol.% of pre-catalyst 86 led to the... 

For the synthesis of the second generation ATH catalysts (see Figure 3 for the general structure), the bis(acetonitrile) complexes were dissolved in acetone and placed under CO atmosphere in the presence of KBr to give, after salt metathesis, complexes 64-72 (Scheme 13). " Alternatively, one acetonitrile can be replaced with CO by following the same procedure in the absence of KBr to obtain complex 73, which was tested for ATH with a variety of ketones. ... 

Conversion of this racemic material to a mixture of diastereomeric salts began with anion metathesis. Complex 4.18 in the presence of an excess of [Cinchonidinium] [A-TRISPHAT] was applied to a neutral aluminium column. Subsequent elution by CH2CI2 provided the first yellow band which was collected and identified as fra 5-[(5p,5p))-bis (Cp Ru)carbazolyl] [A-TRISPHAT] (4.18a) and frflMi-[(Rp,Rp)-bis(Cp Ru)carbazolyl] [A-TRISPHAT] (4.18b) (Figure 4.9). These complexes were separated through fractional crystallization from CHCls/ether to give samples 1 and 2. 

In order to improve the catalytic performance of ruthenium-based metathesis complexes, extensive theoretical [9,10] and experimental [11,12] studies were conducted. The generally accepted mechanism involves the formation of a highly active and unstable 14-electron complex through the dissociation of one of the phosphine ligands (Scheme 11.1) [13-17]. This actual catalytic species allows the binding of incoming olefin to form the Chauvin metallacyclobutane [18] intermediate and eventually the reaction products. 

Indenylidene complexes have also made their mark in ruthenium olefin metathesis complexes. First noticed by Nolan and coworkers [48] and Furstner et al. [49], these complexes were developed by Nolan and coworkers [50,51] to afford the very stable second-generation complexes 14 (with NHC ligands). More recently, Schrodi and coworkers [52] and Bruneau and coworkers [53,54] presented chelated indenylidene complexes 15 and 16 (Figure 11.5). The relatively fecile methodology for their synthesis and their increased stability certainly makes the indenylidene complexes another well-studied family in the rutheniiun olefin metathesis catalyst field. 

The large potential unveiled by the introduction of the NHC ligands motivated chemists to expand the NHC toolbox in order to achieve more stable and active catalysts. The following sections account for only a brief survey of some interesting and useful modifications of N-heterocyclic carbene ligands in ruthenium metathesis complexes, illustrated with their activity in various metathesis reactions using standard substrates (Scheme 11.3). 

Figure 11.19 Structure of metathesis complexes bearing fluorinated A/-aryl-substituted NHCs 94-100.

Figure 11.25 Structure of Ru-metathesis complexes 117 and 118 bearing six-membered ring NHCs.

Figure 11.26 Ru-metathesis complexes bearing four- (119) and seven- (120) membered ring...

The study was also extended toward the synthesis of four-membered NH(Zs. In 2005, Despagnet-Ayoub and Grubbs reported the synthesis of a ruthenium metathesis complex 119 featuring a four-membered NHC and compared its activity to that of complexes with five-membered NHCs (Figure 11.26) [102,103]. In all substrates tested, the reactions were found to be slower than those catalyzed by 10 or 11. 

In 2002, Hoveyda and coworkers introduced an alternative concept to install chirality in ruthenium olefin metathesis complexes through a Ci-symmetric bidentate NHC ligand, bearing binaphtholate moieties (Figure 11.33). The NHCs in this type of complexes lacked backbone substitution and it was chelation that prevented free rotation of the ligand [118]. In this case, chiral information installed within the A-substituent was transferred directly to the ruthenium center. Unfortunately, these complexes were found to be less active because of the reduced Lewis acidity at the metal center, mainly due to the exchange of Cl... 

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