Carbyne complexes electrophilic attack

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... 

Figure 1.12 suggests that for carbonyl complexes the HOMO is localized primarily on the metal centre, with only a modest contribution from oxygen orbitals. Thus by far the majority of reactions of metal carbonyls with electrophiles involve direct attack at the metal, with the carbonyl serving as a spectator ligand. If, however, the metal centre is (i) particularly electron rich and (ii) sterically shielded and the electrophile is hard (in the HSAB sense) and also sterically encumbered, then attack may occur at the oxygen. Thiocarbonyls (LM-CS) are stronger 71-acids than CO and the sulfur is both softer and more nucleophilic. Thus electrophilic attack at the sulfur of thiocarbonyls is more common if the metal centre is electron rich (vcs < 1200 cm-1). Similarly, coordinated isocyanides (CNR) are more prone to attack by electrophiles at nitrogen. This is noteworthy in the sense that free isocyanides are attacked by electrophiles at carbon (Figure 3.19). The resulting carbyne ligands will be discussed in Chapter 5.

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. 

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... 

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). 

CH2BU or p-MeCjH4]. The reaction is considered to be charge controlled and to proceed via electrophilic attack by Sg on the carbyne carbon (178). An additional complex (267, M = W, E = S, L = CO, R = Me) can be obtained from [W=CMe(Cp)(CO)2] and cyclohexenesulfide (179). With the osmium complex 268 (R = /7-tolyl), however, reaction with sulfur does not proceed beyond the t/ -thioacyl complex (269, E = S or Se). Seleno-and telluroacyl complexes result from analogous reactions (180). 

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. 

As with carbene complexes, metal carbynes display a range of reactivity with electrophiles and nucleophiles. Molecular orbital calculations show that even cationic Fischer carbyne complexes are polarized as M, +=C A neutral Fischer- and Schrock carbyne complexes have an even greater negative charge on Ccarbyne.93 If all reactions between carbyne complexes and other species were charge-controlled, we would predict that nucleophiles would always attack at the metal and electrophiles at Ccarbyne. As we should expect by now, the picture is more complicated in practice. 

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... 

These Fischer-type carbene complexes can be readily converted to Fischer-type carbyne complexes, and this material has been reviewed. " Methods to remove the a-alkoxy group include treatment with BBr3, oxide abstraction from acyl ligands, and electrophilic attack on the /3-carbon of... 

Just like the isoelectronic carbon monoxide, an isocyanide is an excellent ligand to metal ions. The chemistry of metal isocyanide complexes has been reviewed by Singleton and Oosthuizen. Only a few examples will be given here. Insertion of an isocyanide into a metal-carbon bond frequently occurs. It is not always clear whether the key step is electrophilic or nucleophilic attack on the coordinated isocyanide or whether the reaction is concerted. Insertion into metal-carbene and metal-carbyne complexes have been reviewed by Aumann. Coordination to the metal considerably affects the chemistry of the isocyanide. If the metal is electron-donating, as in nitrogenase-like centres, the coordinated isocyanide is apt to electrophilic attack at nitrogen cf. Section III. 

Examples of the electrophilic modification of a-carbons also include electrophilic attack at carbyne complexes. Calculations suggest that carbyne carbons bear a significant negative charge, but reports of electrophilic attack upon these carbons are rare. In Equation 12.40, protonation of CUMejPl WsCR gives a distorted alkylidene complex the C-H bond interacts with the coordinatively unsaturated metal in an "agostic" fashion. 

Electrophilic attack on the 3-carbon of alkynyl ligands (Equation 12.50) is common and is a route to vinylidene complexes introduced in Chapter 3. Examples of protonation and electrophilic alkylation of an anionic acetylide complex at the 3-carbon are shown in Equation 12.51. Attack of two protons on an anionic carbyne complex generates a new carbyne complex, as shown in Equation 12.52. ° This reaction, presumably, occurs by initial formation of a vinylidene complex. 

ABSTRACT. Alkynes, when activated by an electron-rich d Re, Mo or W phosphinic centre, undergo hydrogen shift reactions (to give, e.g., vinylidene species) or oxidatively add to the metal (forming alkynyl--hydrido or alkynyl complexes). These alkyne-derived products undergo 3-protonation to afford a variety of carbyne-fluoro or -chloro complexes, whereas aminocarbynes are obtained upon 3-electrophilic attack (e.g., by a protic or a Lewis acid) at isocyanides when ligating such metal sites. Mechanistic studies, by stopped-flow spectrophotometry, are also indicated. 

These reactions provided the first examples of electrophilic attack at an isocyanide ligand and constituted a novel route for carbyne complexes. 

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. 

ABSTRACT. Dicarbonyl(t 5-cyclopentadienyl)carbyne complexes of molybdenum and tungsten prove to be a valuable synthetic tool Reaction with phosphines provides substituted carbyne complexes and leads via an intramolecular CC-coupling to t 1- or Tj -ketenyl complexes respectively. Electrophiles attack the metal carbyne triple bond forming hetero- and acyclic carbene complexes, r 2-acyl compounds, T -ketene complexes and metalla-dithia-bicyclobutane cations. Dithio-carboxylates are formed in reaction of these dicarbonyl(Ti5 cyclo-pentadienyl)carbyne complexes with sulfur or cyclohexene sulfide. 

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