Tungsten alkylidene complexes preparation

In retrospect it is not surprising that the niobium and tantalum alkylldene complexes we prepared are not good metathesis catalysts since these metals are not found in the "classical" olefin metathesis systems (2). Therefore, we set out to prepare some tungsten alkylidene complexes. The first successful reaction is that shown in equation 6 (L = PMe3 or PEt3) (11). These oxo... 

Vinyltin derivatives (92) have been prepared by the reaction of acyltin compounds with phosphonium ylides or with phosphonate carbanions. The tungsten complex (93) reacts with a wide range of aryl-substituted ylides (94), and the vinyl-substituted ylide (95), to give phosphorus to metal alkylidene transfer and hence provides a route to tungsten alkylidene complexes (96). ... 

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. 

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. 

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. 

A diamido tungsten(VI)-based catalyst was reported by Boncella and co-workers [72]. It was prepared from the dilithio salt of N,N -bistrimethylsilyl-protected o-phe-nylenediamine and W(NPh)Cl4(OEt2). The conversion of the intermediate bisalkyl complex into the corresponding tungsten alkylidene requires the addition of an ex-... 

The generally accepted mechanism for olefin cross-metathesis is outlined for the case of propene in Mechanism 14.4. The catalyst belongs to a class of organometallics known as a metallocarbene, carbene complex, or alkylidene complex. Its structure is characterized by a carbon-metal double bond. In olefin metathesis the metal is typically ruthenium (Ru), tungsten (W), or molybdenum (Mo). Transition-metal carbene complexes were first prepared by Ernst O. Fischer (Munich) who shared the 1973 Nobel Prize in Chemistry with Geoffrey Wilkinson. 

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]. 

O Donoghue, M. B. Schrock, R. R. LaPointe, A. M. Davis, W. M. Preparation of well-defined, metathetically active oxo alkylidene complexes of tungsten. Organometallics 1996, 15, 1334-1336. 

In the preparative section 3.2 devoted to metal-carbene complexes, it is shown how the a-elimination reaction from high oxidation state early-transition-metal-alkyl complexes is one of the general methods of synthesis of Schrock s Ta and Nb alkylidene complexes. The other direction, formation of an alkylidene from an alkylidyne complex, can also be a valuable route to metal alkylidenes. For instance, Schrock s arylamino-tungsten-carbynes can be isomerized to imido-tungsten-carbene by using a catalytic amount of NEts as a base. These compounds are precursors of olefin metathesis catalysts by substitution of the two Cl ligands by bulky alkoxides (dimethoxyethane then decoordinates for steric reasons), and this route was extended to Mo complexes ... 

In 1964, Fischer prepared and characterized unambiguously the first metal carbene complex 3 obtained by nucleophilic attack of phenyl lithium at tungsten hexacarbonyl followed by 0-alkylation. This was followed by Schrock s synthesis of a high oxidation state metal alkylidene complex 4 obtained by a-hydrogen abstraction from tris(neopentyl) tantalumfv) dichloride (Scheme 1.1)/ ... 

Although the tungsten initiators allow a variety of functionalized monomers to be studied, the analogous molybdenum complexes show an even greater tolerance of functional groups.525,526 The synthetic route developed for Mo(NAr)(CHR)(OR )2 allows for a wide variety of imido, alkoxide and alkylidene substituents to be prepared, and several (e.g., (210) and (211)) are commercially available527"530... 

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