Carbynes

Carbynes M=CR also have extreme bonding formulations analogous to Fischer and Schrock carbenes, although the distinction is less marked 

FIGURE 11.3 Doublet and quartet forms of (a) a carbyne can be considered as the parents of the (b) Fischer and (c) Schrock carbyne complexes. 

Oxidation state assignments again depend on the carbyne type. For example, the Fischer carbyne, Br(CO)4W=CR, is considered as W(II), and the Schrock carbyne, Br3L2W=CR, as W(VI). Once again, we have ambiguity in intermediate cases. 

Fischer first prepared carbyne complexes (1973) by electrophilic abstraction of methoxide ion from a methoxy methyl carbene. 

If L is CO, then the halide ion (Cl, Br, or I) displaces the CO trans to the carbyne in the intermediate cationic complex, showing the high trans effect of the carbyne. On the other hand, if L is PMe3, then the cationic species is the final product. 

Carbenes can be heteroatom stabilized (Fischer) or not (Schrock). Fischer carbenes have an electrophilic carbene carbon and Schrock carbenes have a nucleophilic carbene carbon. 

Metal carbyne complexes MsCR also have Fischer and Schrock extreme bonding formulations, although the distinction i less marked than for carbenes. In one bonding model, the free carbyne can be considered as doublet for Fischer and quartet for Schrock forms (Fig. A doublet carbene is a 2e donor via its 

Another common model considers the Fischer type as CR bound to L M with weak back donation from two M(d ) orbitals. This leaves a net partial positive charge on carbon as in the Fischer caibene case. On this model, moving to very strong back donation converts the Fischer to the Schrock type with a net partial negative charge on carbon. 

Also reported were the reactions of [Br(OC)4W CPh] with [(Ph2As)aCH2] to afford [Br(OC)3LW=CPh], (Br(OC)2L2W 3 h] L=unidentate (Ph2As)2CH2 and [(Br(OQ2WC CPh)2L], and the synthesis of a half-ylide via a chromium carbyne [equation (29)].  

Fischer, W. Kleine, U. Schubert, and B. Neugebauer, /. Organometallic Chem., 

The chemistry of the remarkable neopentylidyne complexes has been extended for the first time to Group VIA elements [equation (30)]. The products are 

Physical and theoretical investigations on organotransition-metal compounds in 1975, and structures determined during 1976 have been reviewed. 

Complexes containing Metal-Carbon the Groups Iron, Cobalt, and Nickel 

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Carbutamide [339-43-5] Carbylamines Carbyne Carbynes Carcinogenicity Carcinogens... 

Butynediol is principally used in pickling and plating baths. Smak amounts are used in the manufacture of brominated derivatives, useful as flame retardants. Itwas formerly used in awkd oat herbicide, Carbyne (Barban), 4-chloro-2-butynyl-A/-(3-chlorophenyl)carbamate [101-27-9] C H Cl2N02 (77). 

Carbynes are a form of carbon with chains of carbon atoms formed from conjugated C(sp )=C(sp ) bonds (polyynes) ... 

Carbyne Chaoite a-carbyne P-earbyne carbon VI Carbolite I... 

The different forms of carbynes were assumed to be polytypes with different numbers of carbon atoms in the chains lying parallel to the hexagonal axis and different packing arrangements of the chains within the crystallite. Heimaim et al [23] proposed that the sizes of the unit cells were determined by the spacing between kinks in extended carbon chains, Fig. 3A. They were able to correlate the Cg value for the different carbyne forms with assumed numbers of carbon atoms, n (in the range n = 6 to 12), in the linear parts of the chains. 

Fig. 3. A, A kinked polyyne chain model for linear carbynes (after [23]) B, eyclo C-18 earbync [25].

X-ray diffraction peaks were rather broad with coherence lengths as low as 20 nm and this was attributed to rapid quenching. It was proposed that the carbon atoms are arranged in polyyne chains (n = 4) along the c-axis. The density of Carbolite (1.46 g-cm ) is lower than values for other carbynes and for diamond and graphite - hence the name - and this was attributed to a rapid quenching process. 

Molecular orbital calculations indicate that cyclo C-18 carbyne should be relatively stable and experimental evidence for cyclocarbynes has been found [25], Fig. 3B. Diederich et al [25] synthesised a precursor of cyclo C-18 and showed by laser flash heating and time-of flight mass spectrometry that a series of retro Diels-Alder reactions occurred leading to cyclo C-18 as the predominant fragmentation pattern. Diederich has also presented a fascinating review of possible cyclic all-carbon molecules and other carbon-rich nanometre-sized carbon networks that may be susceptible to synthesis using organic chemical techniques [26]. 

Despite many publications on carbynes, their existence has not been universally accepted and the literature has been characterised by conflicting claims and counter claims [e.g., 27-29]. This is particularly tme of meteoritic carbynes. An interesting account of die nature of elemental carbon in interstellar dust (including diamond, graphite and carbynes) was given by Pillinger [30]. Reitmeijer [31] has re-interpreted carbyne diffraction data and has concluded that carbynes could be stratified or mixed layer carbons with variable heteroelement content (H,0,N) rather than a pure carbon allotrope. 

In addition to questions over interpretation of diffraction data, diere are reservations about the stability of carbynes. Lagow et al [32] note that the condensation of the compound Li-C=C-Br to form carbon chains is potentially explosive. There is also the possibility of cross-linking between carbyne chains and the nature of the termination of the carbyne chains is unclear. Eastmond et al [33] showed that polyyne compounds of the type ... 

Chapter 1 contains a review of carbon materials, and emphasizes the stmeture and chemical bonding in the various forms of carbon, including the foui" allotropes diamond, graphite, carbynes, and the fullerenes. In addition, amorphous carbon and diamond fihns, carbon nanoparticles, and engineered carbons are discussed. The most recently discovered allotrope of carbon, i.e., the fullerenes, along with carbon nanotubes, are more fully discussed in Chapter 2, where their structure-property relations are reviewed in the context of advanced technologies for carbon based materials. The synthesis, structure, and properties of the fullerenes and... 

It has recently been reported that a molecule, claimed to contain a high concentration of conjugated alkyne units, can be prepared by electrochemical reduction of polytetrafluoroethylene (PTFE) [32,33]. The reduction is carried out using magnesium and stainless steel as anode and cathode respectively. The electrolyte solution contains THE (.30 cm ), LiCI (0.8 g) and FeCl2 (0.48 g). A 10 X 10 nm PTFE film, covered with solvent, is reduced to carbyne (10 V for 10 h)... 

The remaining classes oF monohapto organic ligands listed in Table 19.2 are carbene (=CR2), carbyne (=CR), and carbido (C). Stable carbene complexes were first reported in 1964 by E. O. Fischer and A, Maasbol. Initially they OMe... 

Carbyne complexes were first made In 1973 by the unexpected reaction of methoxycarbene... 

H. Fi.scher, C. Troll, and J. Schleu, in Transition Metal Carbyne Complexes, (F. R. Kreissl ed.), p.79. Kluwer Academic Publishers, Dodrecht, 1993. 

When methylene chloride was used as a solvent, it was found that 28 are obtained in minor amounts, while the dominating product is the -coordinated chloro-carbyne species [(> -Tp )Mo(CO)2(=CCl)], whose yield increases abruptly with substitution in the pyrazol-l-yl fragments (3-methyl-, 3,4,5-trimethyl-, and 3,5-dimethyl-4-chloro derivatives) [90AX(C)59,95JCS(D) 1709]. The tungsten analog can be prepared similarly. The chlorocarbyne molybdenum complex follows also from the reaction of the parent anion with triphenylsulfonium cation but conducted in dichloromethane. The bromo- and iodocarbyne derivatives are made similarly. 

Much current research is centering on polyynes—linear carbon chains of sp-hybridized carbon atoms. Polyynes with up to eight triple bonds have been detected in interstellar space, and evidence has been presented for the existence of carbyne, an allotrope of carbon consisting of repeating triple bonds in long chains of indefinite length. 

Several stable Group 6 metal-ketene complexes are known [14], and photo-driven insertion of CO into a tungsten-carbyne-carbon triple bond has been demonstrated [15]. In addition, thermal decomposition of the nonheteroatom-stabilized carbene complexes (CO)5M=CPh2 (M=Cr, W) produces diphenylke-tene [16]. Thus, the intermediacy of transient metal-ketene complexes in the photodriven reactions of Group 6 Fischer carbenes seems at least possible. 

An obvious drawback in RCM-based synthesis of unsaturated macrocyclic natural compounds is the lack of control over the newly formed double bond. The products formed are usually obtained as mixture of ( /Z)-isomers with the (E)-isomer dominating in most cases. The best solution for this problem might be a sequence of RCAM followed by (E)- or (Z)-selective partial reduction. Until now, alkyne metathesis has remained in the shadow of alkene-based metathesis reactions. One of the reasons maybe the lack of commercially available catalysts for this type of reaction. When alkyne metathesis as a new synthetic tool was reviewed in early 1999 [184], there existed only a single report disclosed by Fiirstner s laboratory [185] on the RCAM-based conversion of functionalized diynes to triple-bonded 12- to 28-membered macrocycles with the concomitant expulsion of 2-butyne (cf Fig. 3a). These reactions were catalyzed by Schrock s tungsten-carbyne complex G. Since then, Furstner and coworkers have achieved a series of natural product syntheses, which seem to establish RCAM followed by partial reduction to (Z)- or (E)-cycloalkenes as a useful macrocyclization alternative to RCM. As work up to early 2000, including the development of alternative alkyne metathesis catalysts, is competently covered in Fiirstner s excellent review [2a], we will concentrate here only on the most recent natural product syntheses, which were all achieved by Fiirstner s team. 

This is a carbyne. The intermediates CF and CCl were generated similarly from CHFBr2 and CHClBr2, respectively. 

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