Metal alkylidene complexes catalysts

Metal alkyl cocatalysts, 20 153 Metal-alkylidene catalysts, 26 948 Metal-alkylidene complexes, 26 927 discovery of, 26 925 Metal amalgams, amines by reduction, 2 492 93... 

The expected intermediate for the metathesis reaction of a metal alkylidene complex and an alkene is a metallacyclobutane complex. Grubbs studied titanium complexes and he found that biscyclopentadienyl-titanium complexes are active as metathesis catalysts, the stable resting state of the catalyst is a titanacyclobutane, rather than a titanium alkylidene complex [15], A variety of metathesis reactions are catalysed by the complex shown in Figure 16.8, although the activity is moderate. Kinetic and labelling studies were used to demonstrate that this reaction proceeds through the carbene intermediate. 

There are no mechanistic details known from intermediates of copper, like we have seen in the studies on metathesis, where both metal alkylidene complexes and metallacyclobutanes that are active catalysts have been isolated and characterised. The copper catalyst must fulfil two roles, first it must decompose the diazo compound in the carbene and dinitrogen and secondly it must transfer the carbene fragment to an alkene. Copper carbene species, if involved, must be rather unstable, but yet in view of the enantioselective effect of the ligands on copper, clearly the carbene fragment must be coordinated to copper. It is generally believed that the copper carbene complex is rather a copper carbenoid complex, as the highly reactive species has reactivities very similar to free carbenes. It has not the character of a metal-alkylidene complex that we have encountered on the left-hand-side of the periodic table in metathesis (Chapter 16). Carbene-copper species have been observed in situ (in a neutral copper species containing an iminophosphanamide as the anion), but they are still very rare [9],... 

Nucleophilic metal alkylidene complexes are more useful for promoting the metathesis polymerisation of cycloolefins than electrophilic metal carbenes. For instance, Br2(Me3CCH20)2W=CHCMe3 is a moderately active catalyst [75,89] that can be further activated by the addition of Lewis acids such as GaBr3 to... 

The evidence for the proposed mechanism and reactions 7.11 to 7.13 come from a variety of observations. First of all cleavage of the alkenes only at the double bonds, that is, generation of species such as 7.38 and 7.40, is indicated by isotope-labeling studies. A mixture of but-2-ene and perdeuterated but-2-ene on exposure to metathesis catalysts shows that the product but-2-ene is duterated only at the 1,2 positions. Second, fully characterized metal -alkylidene complexes such as 7.43 and 7.44 have been shown to be active metathesis catalysts. 

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]. 

C-H bond activation of a ligand in which a C atom is located in a, p or y position vs. the metal involves a-bond metathesis in many cases, in particular for a and y-elimination. These aspects are dealt with in Chaps 3 (stoichiometric reactions leading to metal-alkylidene complexes section 5), 15 (Ziegler-Natta polymerization section 1) and 20 (silica-supported alkane metathesis catalysts section 5). 

Figure 2.1 Metal-alkylidene complexes as metathesis catalysts. Schrock s catalyst [Mo]-l, the first benzyl ene ruthenium complex [Ru]-IV, Grubbs-I catalyst (Cl2(CyjP)2Ru cHph) Grubbs-II catalyst...

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... 

In this chapter I will cover only well-defined or well-characterized compounds. Results will be included that have appeared since reviews in 1991 on alkylidene and metalacyclobutane complexes [41] and in 1993 on ring-opening metathesis polymerization [30], but an overview of prior results that are especially relevant to olefin metathesis in particular will also be included. (An excellent and comprehensive text also has been published recently [1].) The terms well-defined or well-characterized originally were meant to imply that the alkylidene complex is isolable and is essentially identical to that in a catalytic reaction except for the identity of the alkylidene. These terms have been watered down from time to time in the literature, even to the point where they are used to describe a catalyst that is formed from a well-characterized transition metal precursor complex, but whose identity actually is not known. In this article I... 

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). 

Metathesis reactions involving the metal-alkylidene catalyst component as the active species nonbulky monomers are more active than their bulky counterpart. To address this limitation, 5-norbomene monomer containing an unhindered or bulky 2-substituents were metathesized with favorable reaction kinetics using ruthenium-containing complexes illustrated below. 

These carbene (or alkylidene) complexes are used as either stoichiometric reagents or catalysts for various transformations which are different from those of free carbenes. Reactions involving the carbene complexes of W, Mo, Cr, Re, Ru, Rh, Pd, Ti and Zr are known. Carbene complexes undergo the following transformations (i) alkene metathesis (ii) alkene cyclopropanation (iii) carbonyl alkenation (iv) insertion to C—H, N—H and O—H bonds (v) ylide formation and (vi) dimerization. Their chemoselectivity depends mainly on the metal species and ligands, as discussed in the following sections. 

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