Electrophiles carbyne complexes

If a given vinylidene complex is not sufficiently electrophilic, protonation at Cp can promote nucleophilic addition at C by intermediate formation of an electrophilic carbyne complex [89] (Figure 2.9, Section 2.1.8). 

Fig. 2.14. Conversion of electrophilic carbyne complexes into heteroatom-substituted carbene complexes.

Some carbyne complexes, in particular cationic ones with good Ji-accepting ligands, can react with nucleophiles to give carbene complexes [187,521]. Several reductions of carbyne complexes to carbene complexes by treatment with metal hydrides have been reported. Similarly, organolithium or other carbanionic reagents can react with electrophilic carbyne complexes to yield carbene complexes. Illustrative examples of both reactions are sketched in Figure 3.23. 

Closely related to the a-addition of nucleophiles is the P-deprotonation of electrophilic carbyne complexes. In many of the examples reported [143,530,531] the resulting vinylidene complexes could not be isolated but were generated in situ and either oxidized to yield stable carbene complexes [532] or used as intermediates for the preparation of other carbyne complexes [527]. Cationic carbyne complexes can be rather strong acids and undergo quick deprotonation to vinylidene complexes with weak bases [143]. An interesting example of the use of anionic vinylidene complexes as synthetic intermediates is sketched in Figure 3.24. 

In view of the similarities between the bonding models for carbene and carbyne complexes it is not surprising that similar patterns of reactivity should be observed for these compounds. Thus nucleophilic and electrophilic additions to the metal-carbon triple bond are anticipated under appropriate circumstances, and both orbital and electrostatic considerations will be expected to play a role. 

Formation of the thioacyl complex 109 may be contrasted with the preparation of analogous chalcoacyl compounds by electrophilic addition to the zero-valent carbyne complex 79 (see above). 

When imines are the nucleophiles used, the initially formed iminium intermediates can undergo intramolecular electrophilic alkylation of the other ligands (e.g. Entry 2, Table 2.10 see also [143]). In addition to this, carbyne complexes can also react with azides to give metallatriazoles [185,186] (Entry 6, Table 2.10). 

Additional methods for preparing non-heteroatom-substituted carbene complexes include nucleophilic or electrophilic additions to carbyne complexes (Section 3.1.4), electrophilic additions to alkenyl or alkynyl complexes (Section 3.1.5), and the isomerization of alkyne or cyclopropene complexes (Section 3.1.6). 

Electron-rich carbyne complexes can react at the carbyne carbon atom with electrophiles to yield carbene complexes. Numerous examples of such reactions, mostly protonations, have been reported [519]. Depending on the nucleophilicity of the carbyne complex, such reactions will occur more or less readily. The protonation of weakly nucleophilic carbyne complexes requires the use of strong acids, such as triflic [533], tetrafluoroboric [534] or hydrochloric acid [535,536]. More electron-rich carbyne complexes can, however, even react with phenols [537,538], water [393,539], amines [418,540,541], alkyl halides, or intramolecularly with arenes (cyclometallation, [542]) to yield the corresponding carbene complexes. A selection of illustrative examples is shown in Figure 3.25. 

Fig. 3.25. Generation of carbene complexes by addition of electrophiles to carbyne complexes [533,538,540,543],...

This carbyne was shown not to be the RCM active species. At —20 °C it rearranged spontaneously into the indenylidene complex XV with release of TfOH. This intramolecular transformation corresponds to the electrophilic ortho-substitution of one phenyl group by the electrophilic carbyne carbon of XIV. The carbene complex XV was identified as the species thermally formed in situ from the catalyst precursors Ia,b in the range 25-80 °C. 

Isocyanides are rather more reactive than CO at low-valent molybdenum centres and may react with nucleophiles, e.g. R NH2, to give carbene complexes,la e.g. equation (2), or with electrophiles to give carbyne complexes,1,4 e.g. (3). 

Electrophilic attack on //-vinylidene complexes can occur either on the methylene carbon, or at the metal-metal bond. With the manganese complexes (45, R = H or Me), protonation affords the//-carbyne complexes (46), which in the case of R = Me, exist in the stereoisomeric forms shown (57). Interconversion of the two forms is slow at room temperature ... 

The lability of the N2 ligand towards alkyl and aryl isocyanides has been demonstrated by equation (2) which occurs readily with both alkyl and aryl isocyanides.40 The resulting complexes are electron rich , the alkyl isocyanide derivatives ReCl(CNR)(dppe) (R = Me or Bul) undergoing electrophilic attack upon treatment with HBF4 to form the carbyne complexes trcms-[ReCl(CNHR)(dppe)2]BF4.41 Other noteworthy features of the series of complexes ReCl(CNR)(dppe)2 and ReCl(CNAr)(dppe)2, are the low value of v(CN), the presence of a bent M—CNR coordination mode, and their ease of oxidation (as measured by cyclic voltammetry) to the corresponding rhenium(II) monocations.40... 

Electrophilic attack at the (3-atom of CO, isocyanides and chalco-carbonyls has already been met as a route to carbyne complexes (Figure 3.19). This reaction may be extended to the P-carbon of vinylidenes, which are discussed below. The remaining routes to carbyne complexes at present lack generality however, selected examples are included in... 

Electrophilic attack at carbyne complexes may ultimately place the electrophile on either the metal or the (former) carbyne carbon, the two possibilities being related in principle by a-elimination/migratory insertion processes (Figure 5.39). The reactions of the osmium carbyne complex are suggestive of an analogy with alkynes. Each of these reactions (hydro-halogenation, chlorination, chalcogen addition, metal complexation see below) have parallels in the chemistry of alkynes. 

Carbyne complexes containing the Mn=CR moiety undergo an interesting variety of reactions. " Scheme 13 provides a sampler of these reactions. The electrophilic nature of the carbyne in the cationic Cp(CO)2Mn=CR" has been exploited in cycloadditions, metatheses, and ketene formation methodologies. Extended chains containing unsaturated carbon networks that are truncated by metal... 

The molecular orbital analysis of the nucleophilic addition at the carbyne C atom infers the orbital control of the reaction since the C atom undergoing attack is the most negative one in the carbyne complex. [2 + 2] cycloadditions of [ReCp(CO)2(CPh)]+ with MeN=C(Ph)H, t-BuN=0, and ArN=NAr (Ar = aryl) but not with aUcenes or aUcynes, give the metallacycles. These reactions are driven by the nucleophilic attack of the lone pairs of the N atom at the electrophilic carbyne carbon atom. These metallacycles are... 

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

The carbyne complexes [Mo(=CBu )(SAr)3] (SAr = TMT, TIPT) have been synthesized by adding 3 eq of Li[SAr] to [Mo(=CBu )Cl3(dme)] (dme = dimethoxyethane). The analogous W derivatives were made by a slightly modified route (32). The initial aim was to probe the acetylene metathesis catalytic properties of the complexes [M ( Bu KSArlg] (M = Mo, W SAr = TMT, TIPT). However, none of the complexes were active for metathesis, which was in contrast to the high activity of the analogous alkoxide compounds for metathesis. This was attributed to the stronger electron donation power of thiolate, which reduces the electrophilic nature of the metal center (32). 

A stepwise twofold electrophilic attack occurs on the carbyne complex 229 to first give the cationic t/ -carbene complex 230 and then the dica-tionic dithiatungstabicyclo[ 1.1,0]butene complex 231 156). A similar tf-carbene ligand forms in the reaction of the thiocarbyne complex [W(CO)2(CSMe) HB(pz)3)] (232) with Me2(MeS)S+(757). 

The ran -aminocarbyne(iodo)tungsten complexes 125 react with CF3SO3CH3 to give the inflate derivatives 126 and methyl iodide [Eq. (106)] 138). This reaction is a rare example of direct attack of electrophiles at the trans halide ligand of a carbyne complex. 

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