Oxo alkylidene complexes

Scheme 35. Synthesis of tungsten imido or oxo alkylidene complexes (Tp = Tp, Tp X = C1, Br ...

Ab initio calculations by Rappe and Goddard show that the active catalysts for the metathesis reaction in the case of Mo, W, and Re compounds containing these metals in high oxidation states are oxo-alkylidene complexes. " The oxygen ligand stabilizes the intermediate metallacyclobutane compound. The addition of oxygen indeed increases the activity of many catalysts. According to Ivin, this increased activity is... 

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. 

The carbene mechanism of COER according to our hypothesis consists of forward (Z = O, X = Y = C) and backward (X = O, Y = Z = O) Wittig-like reactions. A transition metal-carbene complex reacts with a carbonyl compound generating an olefin and transition metal oxo-complex. Then the oxo-complex reacts with another olefin generating a new carbonyl compound and regenerating the transition metal-carbene complex. It is known that oxo-alkylidene complexes can be generated via oxidative addition reaction between some tungsten complexes with carbonyl compounds. 

J. R. Missert, and W. J. Youngs, Tungsten-oxo alkylidene complexes as olefin metathesis catalysts and the crystal structure of W(0)(CHCMe3)(PEt3)Cl2, J. Amer. Chem. Soc. 102 4515 (1980). 

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... 

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. 

On the basis of these studies and some calculations by Rappe and Goddard (19) it would seem incontrovertible that the oxo ligand prevents reduction of the metal and perhaps also enhances the rate of reforming an alkylidene complex from a metallacyclobutane complex. The next question was whether other strong n-donor ligands such as alkoxides could take over the oxo s function (lib). 

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... 

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. 

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. 

---