Homogeneous alkylidene complexes

Metathesis is one of the attractive methods for the conversion of alkanes and olefins, and has been developed in both homogeneous and heterogeneous catalysis fields. In homogeneous catalysis, Mo alkyl alkylidene complexes have been known to be active catalysts for metathesis [45-48]. Basset and Coperet et al. [49-53] prepared surface organometallic catalysts, which are simple monoatomic systems with hydrides, alkyl alkylidenes, or alkyl alkylidynes of Group 5 or 6 metal complexes supported on oxide surfaces for alkane and olefin metatheses. 

Undoubtedly the most active homogeneous catalyst systems are the well-defined metal alkylidene complexes synthesised in the laboratories of Schrock, Basset and Grubbs. First examples are the complexes I [29] and II [30]. The bulkiness of imido and aryloxide ligands probably slows down dimerization of these electron-deficient organometallic complexes to inactive complexes and prevents to some extent the coordination of the functional group to the tungsten atom [37]. 

Building off of seminal work on the development of iridium complexes for dehydrogenation - hydrogenation catalysts, an improved, homogeneous, dual-catalytic system for alkane metathesis has recently been discovered [101, 102]. Goldman and Brookhart found that a combination of an Ir-pincer-based catalyst with Schrock-type, Mo- or W-alkylidene complexes (as the olefin metathesis catalyst) transformed alkanes into lower and higher new alkanes (Figure 2.12). 

Also, 77 was observed to be longer lived than the homogeneous control, which provided evidence for the prevention of bimolecular decomposition processes via site-isolation. Shortly after, a similar procedure was employed for the preparation of a related well-defined, silica-supported W-alkylidene complex [72]. 

Discrete alkylidene W and Mo complexes on silica prepared by asset s group such as the silica-supported alkylidyne-alkylidene complex SiO Re - - u H- - u H - - u also catalytieally metathesize entyne. They are intermediate between homogeneous and heterogeneous with the advantages of both types. ... 

Table 3.16. Alkylidene and carbyne complexes as single-component catalysts for homogeneous-phase alkene metathesis.

The alkene metathesis reaction arose serendipitously from the exploration of transition-metal-catalysed alkene polymerisation. Due to the complexity of the polymeric products, the metathetic nature of the reaction seems to have been overlooked in early reports. However, in 1964, Banks and Bailey reported on what was described as the olefin disproportionation of acyclic alkenes where exchange was evident due to the monomeric nature of the products [8]. The reaction was actually a combination of isomerisation and metathesis, leading to complex mixtures, but by 1966 Calderon and co-workers had reported on the preparation of a homogeneous W/Al-based catalyst system that effected extraordinarily rapid alkylidene... 

The synthesis of alkylidenes incorporating late transition metals has resulted in alkene metathesis catalysts having unprecedented functional group tolerance. In particular, the discovery that complexes of Group VIII transition metals were efficient ROMP catalysts introduced several advantages. Relative to their early transition metal counterparts, these classical catalysts functioned well in the presence of a variety of polar and protic functional groups (Table 2), and they functioned homogeneously in water. 

Homogeneous catalysts generally consist of (/) a well-characterised alkylidene (carbene) complex of a transition metal (Ru, Mo or W), or (ii) a combination of a transition metal halide or oxo-halide with an alkylating cocatalyst. 

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