Carbyne complexes electrophilic addition

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). 

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). 

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). 

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. 

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

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. 

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... 

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. 

Schrock carbynes and Group 8 (M = Os, Ru) alkylidynes react with electrophiles, typically at Ccarbyne. Equation 10.57 shows the electrophilic addition of HC1 across the M=C of an Os carbyne complex in a manner reminiscent of Markovnikov addition of HC1 across an unsymmetrical C=C bond. The reaction presumably begins by attack of H+ at Ccarbyne, followed by ligand attachment of Cl. 97... 

Complex 3 has been converted to a number of carbynes, disubstituted vinylidenes, and related complexes via electrophilic addition to the carbon of the vinylidene. - Complex 3 has also been used to build heterodinuclear transition metal complexes. ... 

The pattern of chemical reactions observed for these compounds clearly sets them apart from "Fischer-type" carbyne complexes of GroupVI e.g., W(hCR)X(CO)4. Whereas the "Fischer-type" complexes typically react with nucleophiles at the carbyne carbon all of the reactions observed for the five coordinate mthenium and osmium complexes, including the cationic examples, are electrophilic additions to the MsC bond. The following sections deal individually with, protonation, addition of halides of the coinage metals, addition of chlorine and chalcogens, and finally an attempted nucleophilic addition where the nucleophile is directed to a remote site on the aryl ring of the carbyne substituent. 

In many cases, complexes containing bridging carbyne ligands are formed via electrophilic addition to the oxygen atom of coordinated carbonyl group or to the carbon atom of carbide clusters [reactions (3.71)-(3.83), Section 3.10]. 

Tungsten carbyne complexes are synthesised from alkynide complexes by double addition of electrophiles. Thus addition of FS03Me or Et30 to [W(CCBu )C0)5] leads to the vinylidene W(=C=CButR)(C0)5 (R = Me or Et). Addition of CF3S03H/Me4NI to this vinylidene or directly to the alkynide allows the high yield isolation of t ran -WI (CO ) [ C (CHBu R ) ]. ... 

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