Schrock molybdenum-alkylidene

Ring-closing metathesis reaction of allenynes occurs at room temperature in the presence of a Schrock molybdenum-alkylidene complex to give ring-closed... 

Although numerous advantages are associated with the use of supercritical carbon dioxide (scC02) as an ecologically benign and user friendly reaction medium, systematic applications to metal-catalyzed processes are still rare. A notable exception is a recent report on the use of scC02 for the formation of industrially relevant polymers by ROMP and the eyelization of various dienes or enynes via RCM [7]. Both Schrock s molybdenum alkylidene complex 24 and the ruthe-... 

Although the Grubbs ruthenium benzylidene 17 has a significant advantage over the Schrock catalyst 3 in terms of its ease of use, the molybdenum alkylidene is still far superior for the cross-metathesis of certain substrates. Acrylonitrile is one example [28] and allyl stannanes were recently reported to be another. In the presence of the ruthenium catalyst, allyl stannanes were found to be unreactive. They were successfully cross-metathesised with a variety of alkenes, however, using the molybdenum catalyst [39] (for example Eq. 20). 

Murakami et al. reported a ring-closing metathesis reaction of allenynes using Schrock s molybdenum alkylidene complex [37]. Treatment of allenynes ISl with a catalytic amount of the complex 15 2 in toluene at rt gave cyclopentene derivatives 1 S3 in good yield. Two possible reaction mechanisms were proposed, one through a vinylidene complex 154 and the other through a carbene complex, but based on several mechanistic studies, they favored the vinylidene complex pathway, which is shown here (Scheme 5.42). 

In order to understand the polymer structures that are obtained in the polymerization of 1,6-heptadiynes, one needs to consider all possible polymerization mechanisms. If 1,6-hep tadiynes are subject to cyclopolymerization using well-defined Schrock catalysts, polymerization can proceed via two mechanisms. One is based on monomer insertion, where the first alkyne group adds to the molybdenum alkylidene forming a disubstituted alkylidene, which then reacts with the second terminal alkyne group to form poly(ene)s consisting of five-membered rings. Analogous to 1-alkyne polymerization, one refers to this type of insertion as a-insertion (Scheme 4). 

Furthermore Grubbs et al. have published water-soluble as well as chiral ruthenium alkylidene complexes based on 16 for ARCM and AROM, whereas Schrock, Hoveyda and coworkers have synthesized a variety of asymmetric molybdenum alkylidene complexes, e.g. (5)-17 17,27 addition Hoveyda et al. have synthesized the achiral ruthenium complex 18 and the chiral complex 19 for ARCM and AROM. 

Other, less acidic alcohols show no reaction, while phenol derivatives result in the formation of dineopentyl derivatives. Finally, binuclear molybdenum alkylidenes are obtained by reaction of a Schrock carbene with a.cu-dienes such as divi-nylbenzene or with octatetraene [91]. 

Fig. 3. Three well-defined metathesis catalysts Schrock s molybdenum alkylidene (1) and Grubbs first generation (2) and second generation (3) benzylidene catalysts.

A significant development for the selective synthesis of alkenes makes use of alkene metathesis. Metathesis, as applied to two alkenes, refers to the transposition of the alkene carbon atoms, such that two new alkenes are formed (2.110). The reaction is catalysed by various transition-metal alkylidene (carbene) complexes, particularly those based on ruthenium or molybdenum. The ruthenium catalyst 84, developed by Grubbs, is the most popular, being more stable and more tolerant of many functional groups (although less reactive) than the Schrock molybdenum catalyst 85. More recently, ruthenium complexes such as 86, which have similar stability and resistance to oxygen and moisture as complex 84, have been found to be highly active metathesis catalysts. 

Schrock et al. developed and explored a wide range of achiral and chiral molybdenum alkylidene compounds that can be employed as well-defined ROMP catalysts. Some examples of... 

Heppekausen J, Piirstner A. Rendering schrock-type molybdenum alkylidene complexes air stable user-friendly precatalysts for alkene metathesis. Angew Chem Int Ed. 2011 50(34) 7829-7832. 

The synthesis of high-oxidation-state molybdenum alkylidenes was reported by Schrock in 1987 [121]. Due to their improved tolerance towards functional groups (table) their better reaction profile and their lower costs well-defined molybdenum based initiators are now preferred over the related systems containing tungsten [122]. 

In spite of its versatility, metathesis could not, until lately, be developed to its full synthetic potential because the traditional catalysts were ill-suited for application being relatively short-hved and susceptible to air, moisture, or side reactions. Schrock was the first to develop an entire family of tungsten-alkylidene and, more importantly, molybdenum-alkylidene complexes with very high activity and selectivity in olefin metathesis. Further on, Grubbs discovered ruthenium (Ru) catalysts which up to now are among the most tolerant of functional groups initiators. ... 

Schrock and co-workers (yclopolymerized diethyl dipropar l-malonate (X = C(C02Et)2) with a well-defined molybdenum alkylidene catalyst [145]. Through NMR spectroscopy, equal... 

These limitations were overcome with the introduction of the well-defined, single-component tungsten and molybdenum (14) alkylidenes in 1990. (Fig. 8.4).7 Schrock s discoveiy revolutionized the metathesis field and vastly increased die utility of this reaction. The Schrock alkylidenes are particularly reactive species, have no side reactions, and are quite effective as polymerization catalysts for both ROMP and ADMET. Due to the oxophilicity of molybdenum, these alkylidenes are moisture and air sensitive, so all reactions using these catalysts must be performed under anaerobic conditions, requiring Schlenk and/or glovebox techniques. 

Initial reports of cross-metathesis reactions using well-defined catalysts were limited to simple isolated examples the metathesis of ethyl or methyl oleate with dec-5-ene catalysed by tungsten alkylidenes [13,14] and the cross-metathesis of unsaturated ethers catalysed by a chromium carbene complex [15]. With the discovery of the well-defined molybdenum and ruthenium alkylidene catalysts 3 and 4,by Schrock [16] and Grubbs [17],respectively, the development of alkene metathesis as a tool for organic synthesis began in earnest. 

Alkylidene carbonyl iridium complexes, reactions, 7, 275 Alkylidene compounds, NLO properties, 12, 121 Alkylidene-containing complexes, in molybdenum complexes, Schrock-type complexes, 5, 524 a-Alkylidene cyclic carbonyl compounds, isomerization,... 

The proposed idea that metal alkyhdene complexes are be able to catalyze olefin metathesis was confirmed in 1980 [8] and consolidated in 1986 by Schrock with the development of the first well-characterized, highly active, neutral tungsten (Cl, Fig. 3) [9] and molybdenum (C2) [10] alkylidene complexes. These complexes were able to catalyze both the metathesis of different olefins and the ROMP of functionalized norbomene to polynorbomene with low polydispersities [11]. Moreover, these catalysts were used by Wagener and coworkers to perform the first quantitative ADMET polymerization [12] and copolymerization [13] of 1,5-hexadiene and 1,9-decadiene. However, the low stability of these catalysts in... 

Fig. 3 Olefin metathesis catalysts Schrock tungsten (Cl) and molybdenum (C2) alkylidene complexes, Grubbs first- (C3) and second-generation (C4) catalysts, Hoveyda-Grubbs second-generation catalyst (C5), and Grubbs third-generation catalyst (C6)...

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