Functional Schrock catalysts

The Schrock catalyst has shown to be the most active and it was compatible with a range of functional groups giving good-to-excellent yields (Table 12,... 

Olefin-metathesis is a useful tool for the formation of unsaturated C-C bonds in organic synthesis.186 The most widely used catalysts for olefin metathesis include alkoxyl imido molybdenum complex (Schrock catalyst)187 and benzylidene ruthenium complex (Grubbs catalyst).188 The former is air- and moisture-sensitive and has some other drawbacks such as intolerance to many functional groups and impurities the latter has increased tolerance to water and many reactions have been used in aqueous solution without any loss of catalytic efficiency. 

The Schrock catalysts are more active and are useful in the conversion of sterically demanding substrates, while the Grubbs catalysts tolerate a wide variety of functional groups. 

Around 1990, Richard Schrock developed versatile molybdenum and tungsten catalysts for olefin metathesis that tolerate a wide range of functional groups in the alkylidene fragments of the olefins. The Schrock catalyst shown in Figure 8-10a is now commercially available. The Schrock catalysts tend to be air- and moisture-sensitive, which limits their use in commercial processes. 

Grubbs catalyst 86a efficiently produced 16- or 21-membered lactone 90a-c in good yields (Scheme 14) [59], The method was further applied to the synthesis of (4-)-12-methyl-13-tridecanolide (91), a minor musk-odor component. Lasiodiplo-din (94) was efficiently synthesized via the formation of 12-membered lactone 93 by RCM of 92 with 86a as the key step [60]. Schrock catalyst 85 was successfully applied to the synthesis of fluvirucin Bi (Sch 38516) (97). Reaction of the fully functionalized diene 95 with 85 afforded macrolactam 96 as a single (Z)-olefin [61]. RCM has been also utilized in the total synthesis of epothilones (5) (Section... 

Scheme 31. End-Functionalization of Living Mo-Based Schrock Catalyst-Initiated Polymers ...

Schrock catalysts 22 are highly active, effective catalysts, not only for ROMP and ADMET, but also other useful types of olefin-metathesis reactions. However, due to their relatively high electrophilicity, they do not tolerate polar functions well. Nonetheless, they have been effectively employed in complex organic synthetic applications, and have been modified to effect enantioselective reactions with high selectivity. Ruthenium-based Grubbs catalysts were developed to increase the range of functional groups in which metathesis could be performed. 

Cross-metathesis was recognized as a convenient route to introduce various functionalities into organometallic frameworks, initially including ferrocenes. For example, catalyst with a methylideneferrocenyl ligand was prepared by a stoichiometric reaction of vinylferrocene (4-1) with the Schrock catalyst (Scheme 12.21) [3]. Polymers with one ferrocenyl redox-active end group were obtained with this unique initiator. 

Vinylsilsesquioxanes react similarly with pent-4-en-l-on and 5-brompent-l-ene in the presence of Schrock catalyst [7]. We found that Grubbs catalyst I exhibits high activity in the cross-metathesis of octavinylsilsesquioxanes with olefins (Eq. 3) [8]. The reaction offers possibilities for the synthesis of highly functionalized silsequioxane and spherosihcate frameworks but it can also be a model for vinylsiloxanes and for vinyl-functionalized surface modification. 

The acceptance of a (new) catalytically mediated methodology by the target-directed synthetic community strongly depends on the availability, stability, and functional group tolerance of the respective catalysts. With the commercial availability of Grubbs5 benzylidene ruthenium catalyst A [13] and Schrock s even more active, yet highly air- and moisture-sensitive molybdenum catalyst B [14]... 

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