Molybdenum-alkylidene complex

As catalysts, ruthenium- or molybdenum-alkylidene complexes are often employed, e.g. commercially available compounds of type 7. Various catalysts have been developed for special applications. " ... 

The ruthenium carbene catalysts 1 developed by Grubbs are distinguished by an exceptional tolerance towards polar functional groups [3]. Although generalizations are difficult and further experimental data are necessary in order to obtain a fully comprehensive picture, some trends may be deduced from the literature reports. Thus, many examples indicate that ethers, silyl ethers, acetals, esters, amides, carbamates, sulfonamides, silanes and various heterocyclic entities do not disturb. Moreover, ketones and even aldehyde functions are compatible, in contrast to reactions catalyzed by the molybdenum alkylidene complex 24 which is known to react with these groups under certain conditions [26]. Even unprotected alcohols and free carboxylic acids seem to be tolerated by 1. It should also be emphasized that the sensitivity of 1 toward the substitution pattern of alkenes outlined above usually leaves pre-existing di-, tri- and tetrasubstituted double bonds in the substrates unaffected. A nice example that illustrates many of these features is the clean dimerization of FK-506 45 to compound 46 reported by Schreiber et al. (Scheme 12) [27]. 

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-... 

Since the late seventies efforts were directed toward the development of well-defined catalysts that would be active without addition of additives or further modification. A wide variety of tungsten and molybdenum alkylidene complexes have been prepared. Many of them show some activity, but only few are good catalysts. The synthesis is often not straightforward and a range of synthetic procedures varying solvents, alkylating reagents, anions, and alkylidene moieties have to be tried before a desired compound will be obtained. 

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). 

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

Figure 3 Chiral molybdenum alkylidene complexes used in asymmetric ring closing metathesis polymerization...

A series of molybdenum alkylidene complexes react with aldehydes, and in some cases ketones, to give the product of methylenation (equation 33). Some of the examples appear to involve an alkylidene, while others may follow an addition-elimination route typical of the Peterson alkenations. Probably the most interesting aspect of this work is the observation that some of the methylenation reactions can be carried out in aqueous or ethanolic media (equation 33). ... 

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. 

Enolethers can be transformed to dihydrofiirans using a molybdenum alkylidene complex as catalyst <94JOC4029>. 

DFT calculations on the reactivity of d catalysts, carried out on simplified models of bisalkoxy imido molybdenum alkylidene complexes, suggested that the separated reactants achieve metathesis via a Chauvin-type mechanism, with a single step for the metallacyclobutane formation and one for the cycloreversion [5, 36]. Although the two metallacyclobutanes (TBP and SP) are minima of the potential energy surface, only the TBP intermediate is on the reaction pathway. Moreover, calculations showed that the preferential attack of ethene occurs trans to one of... 

Cis/Trans Selectivity in Polymerizations Catalyzed by Imido Molybdenum Alkylidene Complexes... 

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. 

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. ... 

In recent years, catalysts and protocols for the Z-selective formation of alkenes via alkene metathesis have been reported. Most progress has been made in the successful optimization of molecular molybdenum-alkylidene complexes. The substituents at their imido ligand fragment and at the electron-poor alcoholate/... 

Cycloolefin macromonomers have been recently used in ring-opening metathesis polymerization reactions to manufacture block and graft copolymers of novel macromolecular architectures [84]. For this purpose, a- and co-norbornenyl-polybutadiene macromonomers, a-NBPB (R = CH2) and co-NBPB (R = COO), were reacted in the presence of molybdenum alkylidene complex, Mo(NAr)(CHtBu)(OtBu)2, to form polynor-bornene-polybutadiene diblock copolymers (117), with comb-like structure [85] [Eq. (49)]. 

Scheme 11.39 Representative chiral molybdenum alkylidene complexes for enantioselective metathesis.

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