Imido alkylidene complexes

Imido alkylidene complexes were first prepared by a reaction analogous to that shown In equation 6. Recently they have been prepared from imido alkyl complexes by well-behaved a-hydrogen abstraction reactions (16) Imido neopentylidene complexes seem to be more stable than oxo neopentylidene complexes, possibly because the oxo ligand is sterically more accessible to Lewis acids, including another tungsten center. 

The primary reason for attempting to synthesize imido alkylidene complexes, e.g., W(NR)(CH-f-Bu)(0-f-Bu)2, was the belief that the appropriate imido ligand will block bimolecular decomposition reactions more effectively than an oxo... 

Molybdenum imido alkylidene complexes have been prepared that contain bulky carboxylate ligands such as triphenylacetate [35]. Such species are isola-ble, perhaps in part because the carboxylate is bound to the metal in an r 2 fashion and the steric bulk prevents a carboxylate from bridging between metals. If carboxylates are counted as chelating three electron donors, and the linear imido ligand forms a pseudo triple bond to the metal, then bis(r 2-carboxylate) species are formally 18 electron complexes. They are poor catalysts for the metathesis of ordinary olefins, because the metal is electronically saturated unless one of the carboxylates slips to an ri1 coordination mode. However, they do react with terminal acetylenes of the propargylic type (see below). 

A variety of catalytically active five-coordinate tungsten oxo and imido alkylidene complexes also have been prepared that contain some donor amine or pyridine linked either to the imido ligand or to a phenyl ligand bound to the metal (A, Scheme 2) [101-105]. Such species show metathesis activity (e.g., ROMP of norbornene),but there does not appear to be any proof that the integrity of the initiator is maintained. 

Schrock, R. R. The Discovery and Development of High Oxidation State Mo and W Imido Alkylidene Complexes for Alkene Metathesis. 

Connon SJ, Blechert S. Recent developments in olefin crossmetathesis. Angew. Chem., Int. Ed. 2003 42 1900-1923. Schrock RR, Hoveyda AH. Molybdenum and mngsten imido alkylidene complexes as efficient olefin-metathesis catalysts. Angew. Chem. Int. Ed. 2003 42 4592-4633. 

Schrock, R. R. Recent advances in olefin metathesis by molybdenum and tungsten imido alkylidene complexes. J. Mol. Catal. A Chemical... 

Bryan, J. C., Mayer, J. M. Oxidative addition of carbon-oxygen and carbon-nitrogen double bonds to WCl2(PMePh2)4. Synthesis of tungsten metallaoxirane and tungsten oxo- and imido-alkylidene complexes. J. Am. Chem. Soc. 1990,112, 2298-2308. 

Asymmetric syntheses of natural products cyclization reactions using molybdenum and tungsten imido alkylidene complexes as efficient olefin-metathesis catalysts 03AG(E)4592. 

Molybdenum imido alkylidene complexes also have been employed for a variety of other catalytic reactions of interest to the organic or polymer chemist, among them selective cross-couplings of olefins, [99] polymerization of terminal alkynes, [100-102] step-growth polymerization of dienes, [103,104] and cyclopolymerization of 1,6-heptadiynes. [105-107]... 

In alkylidyne complexes that bear no ir-acid co-ligands, thermodynamic protonation occurs either on the alkylidyne face or at the metal, and steric considerations are invoked to explain the preference. Sub-stoichiometric quantities of H2O or HCl catalyse the tautomerisation of TpW( = C Bu)Cl(NHPh) to the corresponding imido alkylidene complex TpW( = CH Bu)Cl( = NR). The first step in the tautomerisation is, however, suggested to involve protonation of the amide group (Scheme 45) since weaker donors (i.e., those with electron-withdrawing... 

P and R have similar coordination stractures with an Mo-O-Si bond in terms of both their comparable NMR data and initial TOFs for olefin metathesis. The chemical bonding between the Si02 surface and the Mo imido alkylidene complex is suggested to prevent some deactivation pathways such as dimerization of reactive intermediates, resulting in longer Hfetime under metathesis reaction conditions [57]. The SiO -supported Mo imido alkylidene complex (P) was also efficient for alkane metathesis [58]. 

Bis(2,5-dimethylpyrrolide) imido alkylidene complexes are monomeric, flux-ional 18-electron species [13]. They were first employed as precursors to Mo(NR)(CHCMe2Ph)(diolate) complexes that contain the relatively electron-withdrawing binaphtholate (3,3-bis-(9-anthracenyl), 3,3 -bispentafluorophenyl, 3,3 -bis(3,5-bis(trifluoromethyl)phenyl), or biphenolate (3,3 -di-fBrt-butyl-5,5 -bistrifluoromethyl-6,6 -dimethyl-l,l -biphenyl-2,2 -diolate) ligands. In... 

The rates of interconversion of the syn and anti isomers were found to be one to two orders of magnitude faster for W MAP complexes than for Mo MAP complexes. Little is known about the rates of interconversion of syn-and anti-protons for MAP species [2c], although the rates of syn/anti interconversions in bisalkoxide Mo imido alkylidene complexes have been found to vary over approximately six orders of magnitude [57]. It is not surprising that the sterically demanding NArj jg52 ligand would destabilize the syn isomer for steric reasons and lead to mixtures that contain both syn and anti species, as observed. 

Oxo-alkyhdene complexes (e.g., W(0)(CHR)X2, where X is a chloride, alkoxide, etc.) are likely to be the catalysts in at least some of the classical olefin metathesis systems [65]. However, few tungsten oxo-alkylidene complexes have been isolated and characterized [66]. Of note, several have now been prepared that contain one or more terphenoxide ligands [41, 42]. Recent theoretical studies have focused on the subtle differences between imido alkylidene complexes and oxo-alkylidene complexes in olefin metathesis reactions [7f]. 

Molybdenum imido alkylidene complexes that contain bulky carboxylate ligands have been prepared and evaluated for the cyclopolymerizations of... 

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