Metal-carbyne complexes Synthesis

The importance of transition metal carbene complexes (compounds with formal M=C bonds) and of transition metal carbyne complexes (compounds with formal M=C bonds) is now well appreciated. Carbene complexes are involved in olefin metathesis (7) and have many applications in organic synthesis (2), while carbyne complexes have similar relevance to... 

Following the synthesis of metal carbyne complexes, the first metalladiyne derivative was prepared by treatment of W =C(OEt)C=CPh (CO>5 with BX3 (X = C1, Br, I) (pentane, -45°C) to give rranj-W(=CC=CPh)(X)(CO)4 (334 Scheme 77) in good yields (30-60%). Subsequent reactions with NHMea give W sCCH=CPh(NMc2) (X)(CO)4 by addition to the C=C triple bond, the structure of which indicates a contribution from the vinylidene resonance form. ... 

The reversible [2+2] cycloaddition of metal alkylidyne or Fischer-type metal carbyne complexes remains the only general methodology for the synthesis of metallacyclobutadiene complexes. Recent literature revolves principally around the heavier group 6 metals and the investigation of intermediates in catalytic alkyne metathesis (Scheme 25 Equation 45) <1996CHEC-II(lb)887> (W <2005OM4684>, Mo <2003JOM56>). 

Transition metal carbyne complexes are described by the general formula L M=CR where the carbyne ligand (=CR) is bonded to the metal by a metal-carbon triple bond. Transition metal carbene complexes have found numerous applications in synthetic organic chemistry through a variety of carbene transfer and cycloaddition reactions [17]. In contrast, carbyne (L M=CR) and vinylidene (L M=C=CRR ) complexes have far fewer applications, in part because their overall chemistry is significantly less developed [18]. Addition reactions to transition metal vinylidene complexes will be discussed in Chapter 21. The first successful synthesis of a carbyne complex was reported by Fischer and co-workers in 1973 [Eq. (8) 19]. Subsequently, many other carbyne complexes have been synthesized by the classic route of Fischer or by new synthetic methods [20]. 

Complexes of nucleophilic carbenes are expected to react, like ylids, with electrophiles whereas complexes of electrophilic carbenes are expected to react, like carbocations, with nucleophiles and bases. All the complexes of terminal carbenes have in common the reactions with olefins, although their nature also varies. The principles of these reactions are detailed here, and application in catalysis and organic synthesis, are exposed in Parts IV and V respectively. Reactions of metal-carbene complexes leading to metal-carbyne complexes are mentioned in section 2. 

A novel synthesis of metal-carbyne complexes that involves the treatment of metal-acyl derivatives with (COX>2 in dichloromethane has teen desCTited.161 Deoxygenation of [Me4N][M(COR)(CO)5] (M = Cr, Mo, W) with oxalyl halides results in generation of p((CO)4M(CR)] (R = Me, Ph) in good yield,l6i Scheme 8.21 ... 

In practice this approach to the synthesis of compounds with dimetallacyclo-propane or -propene ring systems has been very successful. Complexes (1) - (12) are representative of those which have been readily prepared by reacting various low-valent metal compounds either with carbene-metal species, or with the carbyne complexes [w=CR( CO) 2(11-05 )] or [w=CR(Br)(CO) ]... 

The synthesis of cyclopropyl carbyne complexes follows the general Fischer synthesis of carbyne complexes from alkoxycarbene complexes typical of transition metals of group 6 (Cr, Mo, W). Thus, addition of cyclopropyllithium to chromium and tungsten hexacar-bonyl followed by alkylation of the acylmetallate intermediate with triethyloxonium fluo-roborate gave cyclopropyl ethoxycarbene complexes which, upon subsequent reaction with boron tribromide at -25 °C, afforded the corresponding /ra 5 -bromotetracarbonyl cyclopropylcarbyne complexes (equation 91). However, whereas the monotungsten... 

Abstraction reactions are elimination reactions in which the coordination number of the metal does not change. In general, they involve removal of a substituent from a ligand, often by the action of an external reagent, such as a Lewis acid. Two types of abstractions, a and 3 abstractions, are illustrated in Figure 14-12 they involve, respectively, removal of substituents from the a and P positions (with respect to the metal) of coordinating ligands. a-Abstraction has been encountered previously, in the synthesis of carbyne complexes discussed in Section 13-6-3. 

The chemistry of metal-carbon triple bonds has developed considerably during the late 1980s. The synthetic basis was broadened, the utility of high-valent metal alkylidynes in metathesis reactions was further developed and refined, and the potential of low-valent carbyne complexes for applications in organic synthesis has become more apparent. The discovery of novel iridium alkylidyne complexes indicates that the full range of metal-carbon triple bonds is not yet known. We can therefore expect that future work in this area of organometallic chemistry will lead to new discoveries with fundamental implications and practical applications. 

This chapter will focus on the nucleophilic addition reactions of transiton metal carbene and carbyne complexes with Grignard reagents. The synthesis and some general reactions of these carbene and carbyne complexes will be presented. A more detailed description of the chemistry of these complexes can be found in the literature [1]. This chapter, although not exhaustive, is descriptive of the prototypical nucleophilic addition reactions of metal-carbon (M-C) multiple bonds with Grignard reagents. 

The reactions of Fischer and Schrock carbyne complexes are of interest because they may act as intermediates in chemical synthesis. Typical reactivity of carbyne complexes with nucleophiles (i.e., alkyl lithium reagents and metal(I) alkoxides) are consistent with the electronics and molecular orbital calculations for these types of complexes [Eq. (9) 22]. The nucleophile adds to the carbyne-carbon, resulting in the formation of a carbene complex. The reaction of a Grignard reagent with a carbyne complex is expected to demonstrate similar reactivity. 

Utilization of the electron-rich metal-carbyne bond to function as a ir-ligand toward coordinatively unsaturated metal complexes has widely been employed by Stone and co-workers (226,227) and represents a versatile route for the synthesis of bimetallic and trimetallic complexes with metal-metal bonds. Bimetallic carbido-bridged complexes [M]=C=[M]... 

Ernst Otto Fischer (bom 1918) was a student of Walter Hieber at Technische Hochschule Miinchen where he received his Ph.D. degree in 1948. He elucidated the molecular stmcture of ferrocene shortly after this compound was discovered. Further highlights in his life s work were the synthesis of dibenzene-chromium (C6H5)2Cr in 1955, the discoveries of the first metal carbene (1967), and the first me-tal-carbyne complex (1971). In 1964 he succeeded Walter Hieber to the chair of inorganic chemistry at Technische Hochschule Miinchen from which he retired in 1985. He received the Nobel prize for chemistry jointly with Geoffrey Wilkinson (Imperial College London) in 1973. 

Nine years after Fischer s report of the first stable transition metal-carbene complex, the first report was made of the synthesis of complexes containing a transition metal-carbon triple bond fittingly this report was also made by the Fischer group.15 Carbyne complexes have metal-carbon triple bonds, and they are formally analogous to alkynes.16 Many carbyne complexes are now known examples of carbyne ligands include the following. 

In Chapter 8 we described how metal carbides (first mentioned in Section 6-1-4) could serve as precursors to carbyne complexes by way of electrophilic addition. Scheme 10.8 revisits a portion of Scheme 8.12, showing Os-carbide complex 72—with its nucleophilic Ccarbide atom—reacting with methyl triflate or tropylium ion to give alkylidynes 73 and 74, respectively. Comparable reactions occur with the corresponding Ru-carbide complex.87 This method may become more general after the synthesis of additional carbide complexes occurs. 

Other Lewis acids react with metal carbynes. Although the initial attack often occurs at Ccaibyne, the final product may be the result of subsequent reactions that may be difficult to rationalize mechanistically.99 Some of the problems at the end of Chapter 10 will depict more of the chemistry of carbyne complexes. Perhaps the most significant reaction of carbyne complexes is n bond metathesis, which has been alluded to in the discussion on procedures for metal-carbyne synthesis. We will see more consideration of this reaction in Chapter 11. 

The synthesis and reactivity of metal carbynes has been comprehensively reviewed. See M. A. Gallop and W. R. Roper, Adv. Organomet. Chem., 1986, 25, 121 H. P. Kim and R. J. Angelici, Adv. Organomet. Chem., 1987, 27, 51 and A. Mayr and H. Hoffmeister, Adv. Organomet. Chem., 1991,32, 227. More recent summaries of carbyne reactions appear annually in the journal Coordination Chemistry Reviews, see, for example, G. Jia, Coord. Chem. Rev., 2007, 251, 2167, for a recent review of the chemistry of Os carbyne complexes. 

Cyclopropyl-substituted transition-metal carbene and carbyne complexes, especially as cyclo-propylidene- or as cyclopropylcarbene complexes, have been used as synthetic building blocks with or without ring opening. Examples of cyclopropane synthesis via decomplexation are described in Section 1.A.5.2.6. 

The chemistry of metal carbene and carbyne complexes has undergone very rapid development since the sixties and olefin metathesis reactions now form part of a much larger family of [2 + 2] metathesis reactions between multiply bonded compounds. In Section 4.2 we shall briefly consider this wider context. Some examples will be given of the use of stoichiometric metathesis reactions of such complexes in organic synthesis (Dotz 1984 Schubert 1989). 

Nucleophiles react with carbyne complexes to promote (or trap) carbyne-carbonyl coupling products, attack the metal-carbon triple bond, or displace a substituent on the carbyne carbon. Complexes of the form Tp M( = CR)(CO)2 are coordinatively saturated and in the case of Tp = Tp, the metal also enjoys a substantial degree of steric protection. Accordingly, the reaction of these complexes with nucleophiles does not, in general, involve attack at the metal but rather at a coligand. Attempted synthesis of Tp W( = CMe)(CO)2 via reaction of MeLi with the... 

The first synthesis of a complex containing a metal-carbon triple bond was reported in 1973 [1]. Since then, numerous carbyne complexes have been prepared. In recent years, the study of the reactivity of these complexes has attracted considerable interest [2]. E.g. carbyne complexes have extensively been used as building blocks in the synthesis of transition metal clusters [3]. The coupling of carbyne ligands with CO or isocyanide ligands has also been studied in detail [4]. However, the number of reports on the use of carbyne complexes in synthetic organic chemistry is rather limit in contrast to carbene complexes which have found many applications in the synthesis of carbo- and heterocycles [5]. 

The activation of adequate small unsaturated-C molecules, typically isocyanides or alkyne-derived vinylidenes and alkynyls, towards 3-electrophilic attack by electron-rich group 6 (Mo or W) or 7 (Re) transition metal centres constitutes a convenient and established route for the synthesis of carbyne complexes, which has been discussed in this paper. 

Metal Carbenes and Metal Carbynes as Precursors for a Rational Synthesis of Carbido and Hydrocarbon Bridged Complexes... 

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