Metal carbene complexes detection

Intermediate molybdacyclobutane complexes have also been detected in the reactions of 7 with 21-24115. Only in the case of 21 is the ultimate product a long-chain polymer, but in all cases one may observe, at 0-60 °C, a clean first-order rearrangement of the initial metallacyclobutane complex to the first metal carbene adduct, consisting of an equilibrium mixture of syn and anti rotamers in the ratio 9 1 (see below). Except in the case of 21, the metal carbene complexes do not survive for very long. For 21, however, ROMP is propagated, and distinct H NMR signals are seen for the longer-chain metal carbene complexes in both syn and anti forms. 

The stability of metal alkylidene (carbene) complexes and the corresponding metallacycles can be dependent on various factors, but it is worth noting that the kind of metal, the metal oxidation state and the ligands surrounding the metal are considered to be of essential significance. Although stable metal carbene complexes are usually obtained from W and Mo compounds whereas metallacycles are obtained from Ti compounds, systems have been found in which both the metal alkylidene complex and its precursor metallacyclobutane can be detected at lowered temperature by NMR spectroscopy [45]. 

The observation that the metal carbene complex, (CO)5W = C(Ph)2 [22], catalyzed the polymerization of cyclic olefins to ring opened polymers containing the diphenylmethylene unit of the catalyst provided additional evidence that carbenes were involved in the catalytic cycle. The formation of the initiating metal carbenes in the classic systems that consist of transition metal halides and alkylating agents was proposed to involve metal alkylation followed by oc-hydrogen loss, Eq. (6). Methane and propene were detected in the early stages of these reactions [23]. 

The mechanism of transition metal-catalyzed nitrogen extrusion from diazo compounds is not known in detail, but there is general agreement that metal-carbene complexes are formed in this process (Scheme 3) [6,7,11,28,29,30,31, 32]. Attempts to detect these elusive species in catalytic reactions have not been... 

Polymerization of acetylenes by metathesis-type catalysts such as M0CI5 and WCle/PluSn was first observed in the 1970s (Woon 1974 Masuda 1974, 1976), but the nature of the chain carrier was then in some doubt. However, it was soon found (i) that metal carbene complexes would initiate the polymerization of MesCC=CH at 60°C (Katz 1980a) (ii) that end-groups derived from such initiators could be detected in polymers of PhC=CH (Kunzler 1988b) and (iii) that triblock copolymers could be made by successive addition of norbomene, acetylene and norbomene to such initiators (Schlund 1989). All types of acetylene can be polymerized in this way and the reactions proceed by a ROMP-type mechanism see Ch. 10. 

The metal carbene/metallacyclobutane mechanism of olefin metathesis, as outlined in Section 1.3, was first proposed by Herisson and Chauvin in 1971. By 1975 the evidence in its favour had become so compelling that the earlier pairwise mechanism had been totally discarded. From 1980 onwards well-defined carbene complexes of Ta, Mo, W, Re, and Ru were discovered which would act as initiators without the need for activation by heat, light, or cocatalyst. This in turn led to the spectroscopic detection of the propagating metal-carbene complexes in many systems, to the detection of the intermediate metallacyclobutane complexes in a few cases, and in one case to the detection of the metal-carbene-olefin complex that precedes the formation of the metallacyclobutane complex. In no individual case have all three intermediates been detected at most two have been observed, sometimes one, more often none. After 1980 metallacyclobutane complexes of Ti and Ta were found which would act as initiators at 60°C, but where the intermediate metal carbene complexes could not be detected. 

Stable metal carbene complexes, such as W[=C(OMe)Me](CO)5, were first prepared by Fischer, E.O. (1964). These 18-electron complexes can be activated as catalysts for the metathesis of pent-l-ene or the ROMP of cycloalkenes by the use of a cocatalyst, or by heat or UV irradiation see Table 2.1. For such complexes to become active as initiators of olefin metathesis it is necessary for a CO ligand to be displaced, allowing the substrate to enter the coordination shell and react with the metal carbene bond. For the ROMP of 1-methyl-rrans-cyclooctene initiated by W(=CPh2)(CO)5 at 50°C the Ph2C= end groups may be detected in the polymer by the UV absorption at 245 nm (Lee, S.J. 1976). 

In Section 3.6 we shall consider cases of initiation by metallacyclobutane complexes. In the present section we are concerned with the detection of metallacyclobutane complexes in systems initiated by metal carbene complexes. 

For the reactions of RC=CH with Me3CC=W(OCMc3)3, metathesis products can be detected in the early stages, but the metathesis reaction is rapidly overtaken by polymerization (Bray 1993 Mortreux 1995). This results from elimination of a hydrogen atom from the intermediate metallacyclobutadiene, leading to the formation of a metal carbene complex which then propagates the polymerization reaction Scheme 10.3 (also see McCullough 1983). 

It is interesting to note that the decrease in metal electron density that accompanies the change from five- to six-coordinate geometry does not have a detectable effect on the metal-carbene carbon bond length in these complexes. The metal-carbyne carbon bond in several osmium carbyne... 

Detection of propagating metal-carbene-olefin complexes. 1508... 

The metal protonated adducts (100) could not be detected directly because they are very acidic (p < ) This means they cannot be generated at the required pH because the thiolate ions are protonated under these conditions and the corresponding thiols are not reactive enough to add to the carbene complex. However there is kinetic evidence for 100. When 99 that has been generated at high pH is reacted with HCl, the pseudo-first-order rate constant for the conversion of 99 back to 98 shows a non-linear dependence on an+, as shown in Fig. 5 for some representative examples. This dependence is consistent with H -catalyzed... 

In the reaction of 2-pyridine-substituted imidazolium salts with Pd(dba)2 (Scheme 4), the formation of either cis or trans complexes having two carbenes per palladium center was observed [38]. No metal hydride was detected despite the fact that the ratio of metal to ligand was varied from 1 1 to 1 4. One of the proposed mechanisms for the formation of such complexes involves a double oxidative addition and elimination of H2 from a Pd(IV) center. The transient Pd-H species is then presumably too reactive toward the second molecule of the imidazolium salt and cannot be observed. A second possible mechanism involves two oxidative steps separated by a reductive elimination from the Pd(II) center. 

The general format of this chapter is the same as that used in previous years. The review is restricted to detectable complexes of Groups 8,9, and 10 which contain a metal-carbon o-bond, including carbenes and carbynes. Species postulated as intermediates (e.g. in catalytic cycles) have been in the main excluded, and coverage of metal-metal bonded complexes has been minimised. 

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