Vinylidene complexes protonation

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). 

Protonation of alkenyl complexes has been used [56,534,544,545] for generating cationic, electrophilic carbene complexes similar to those obtained by a-abstraction of alkoxide or other leaving groups from alkyl complexes (Section 3.1.2). Some representative examples are sketched in Figure 3.27. Similarly, electron-rich alkynyl complexes can react with electrophiles at the P-position to yield vinylidene complexes [144,546-551]. This approach is one of the most appropriate for the preparation of vinylidene complexes [128]. Figure 3.27 shows illustrative examples of such reactions. 

The alkenyl(amino)aUenylidene complex 41 is also prone to undergo electrophilic additions at the Cp atom of the cumulenic chain. Thus, treatment of 41 with HBF4 OEt2 led to the spectroscopically characterized dicationic vinylidene complex 65 (Fig. 10) [52, 53]. Related Cp-protonations of complexes 35 (Fig. 6) have also been described [49]. 

Ready addition of nucleophiles (Nu ) to metal-allenylidene complexes affords alkynyl derivatives. Subsequent protonation or alkylation, as described in Section 1.2.3 above, then gives the corresponding vinylidene complexes (Equation 1.8) ... 

Attempts to generate a hexapentaenylidene complex by protonation of ]Cp (NO) (PPh3)Re-C = C-C = C-C = C-C6H4Me-4] have failed. Protonation occurs at C2 to give the corresponding cationic vinylidene complex [36]. 

In 1979, Rudler ef al. reported another example of the presence of a vinylidene complex during the reaction of pentacarbonyl[methoxy(methyl)carbene] tungsten 5 with MeLi followed by acidification with TFA [4]. It was proposed that the vinylidene complex 7 was generated by deprotonation of the a-proton of the carbene complex followed by elimination of methoxide and reaction with the dimethylcarbene complex 8, the addition-elimination product of MeLi with the starting carbene complex, to give the dinuclear complex 6 (Scheme 5.2). 

In fact, the first isolation ofthe vinylidene pentacarbonyltungsten complexes was reported by Mayr et al. in 1984 [6]. The vinylidene complexes 16 were obtained by the alkylation of anionic pentacarbonyltungsten t-butylacetylide complex 15, obtained by the reaction of [Et4N ] [W(CO)5Cl ] with lithium acetylide, with FS03Me or [Et30 ] [Bp4 ]. Further protonation with CF3SO3H in the presence of Me4N P afforded a unique method for the preparation of carbyne complexes 17 (Scheme 5.4). 

Rapid development of this area followed the discovery of routes to these complexes, either by ready conversion of terminal alkynes to vinylidene complexes in reactions with manganese, rhenium, and the iron-group metal complexes (11-14) or by protonation or alkylation of some metal Recent work has demonstrated the importance of vinylidene complexes in the metabolism of some chlorinated hydrocarbons (DDT) using iron porphyrin-based enzymes (15). Interconversions of alkyne and vinylidene ligands occur readily on multimetal centers. Several reactions involving organometallic reagents may proceed via intermediate vinylidene complexes. 

Vinylidene complexes have been obtained from reactions between PhC=CEPh3 (E = Si, Ge, or Sn) and manganese complexes (11, 18) solvolysis of the C-E bond is followed by transfer of a proton from the solvent. The yields (E = Si, 0% Ge, 1% Sn, 15%) are inversely proportional to the stability of the intermediate tp-alkyne complex. 

Protonation or alkylation of several ethynyl-metal derivatives gives the corresponding vinylidene complexes in high yield (14,24). This is a convenient route to the disubstituted vinylidene complexes, as well as the parent compounds, which cannot be obtained from l-alkynes they are formed if MeOS02F or [R30]+ (R = Me, Et) is used as alkylating agent ... 

The formation of Mn(=C=CMe2XCO)2(f7-C5H5) by reaction of the -methyl propiolate complex with LiNPrj proceeds via an insoluble yellow anionic alkynylmanganese complex, which is protonated or methylated to give the vinylidene complexes (26) ... 

A similar sequence of reactions occurs with LiBu, although in this instance methylation or protonation of the intermediates formed after addition of either two or three equivalents of alkyllithium reagent, respectively, allow the corresponding acyl - or hydroxyalkyl-vinylidene complexes to be isolated (26) ... 

In the latter case, the red anionic vinylidene complex is formed at — 78°C quenching with DzO gives a 1 1 mixture of the mono- and dideuterocar-byne complexes, as a result of a primary kinetic isotope effect by which the anion selectively abstracts a proton from the monodeuterocarbyne complex before quenching is complete. Similar observations were made in the tungsten series (55). 

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 ... 

Acetylene-vinylidene rearrangements of silylacetylene-iron carbonyl complexes have been observed,537 while iron-acetylide hydride complexes of the type [Fe(H)(C=CR)(dmpe)2], where dmpe=l,2-bis(dimethylphosphino)ethane, have been found to react with anions to afford substituted alkenyl complexes. It has been proposed538 that a likely reaction course for this latter rearrangement involves initial protonation of the cr-bound acetylide ligand at the carbon (I to the metal centre to form a vinylidene complex. Metal-to-carbon hydride migration in this vinylidene complex with attack by the anion would then lead to the neutral complex (see Scheme 106). A detailed mechanistic investigation has been carried out539 on the novel metathetical... 

The vast majority of work exploring the reactivity of ruthenium viny-lidene complexes has focused on the attack of alcohols at the electrophilic a carbon of monosubstituted vinylidenes, resulting in the formation of ruthenium alkoxycarbene complexes. Bruce and co-workers have determined, for example, that the phenylvinylidene complex 80 is slowly transformed in refluxing MeOH to the methoxycarbene complex 82 in good yield (73,83). The mechanism for this reaction must involve initial attack of the alcohol at the electrophilic Ca to form a transient vinyl intermediate 81 which is rapidly protonated at the nucleophilic Cp, generating the product carbene 82 [Eq. (79)]. In contrast to monosubstituted vinylidene complexes, disubstituted vinylidene complexes are generally unreactive to nucleophiles even the relatively small dimethylvinylidene complex 83 shows no reaction with MeOH after 70 hours at reflux [Eq. (80)]. 

The formation of complexes 109 has been shown to proceed via a vinylidene ruthenium intermediate (112), which has been indirectly isolated by protonation of an acetylide-ruthenium complex (112). Arene ruthenium vinylidene complexes 113 appear to be much more reactive than their isoelectronic (C5H5)(R3P)2Ru=C=CHR+ complexes (63,66). 

Several groups have completed computational studies on the relative stabilities of osmium carbyne, carbene, and vinylidene species. DFT calculations on the relative thermodynamic stability of the possible products from the reaction of OsH3Cl(PTr3)2 with a vinyl ether CH2=CH(OR) showed that the carbyne was favored. Ab initio calculations indicate that the vinylidene complex [CpOs(=C=CHR)L]+ is more stable than the acetylide, CpOs(-C=CR)L, or acetylene, [CpOs() -HC=CR)L]+, complexes but it doesn t form from these complexes spontaneously. The unsaturated osmium center in [CpOsL]+ oxidatively adds terminal alkynes to give [CpOsH(-C=CR)L]+. Deprotonation of the metal followed by protonation of the acetylide ligand gives the vinylidene product. 

A range of vinylidene complexes [ReX(=C=CFlR)(dppe)2] (X = F, Cl) has been prepared by deprotonation of the corresponding carbynes [ReX(=C-CH2R)(dppe)2]+. The mechanism of the protonation of these complexes has shown that, depending on the reaction conditions, protonation occurs either directly at the vinylidene /3-carbon or at the metal, followed by migration to the referred /3-carbon. ... 

The mechanism proposed by Albertin et al. involves ptotonation of 62 to give the vinylidene complex 63, as confirmed by the highly deshielded signal at 6 382.8 ppm in the spectrum (Scheme 5). The vinyl phospho-nium complex 64 was formed by intramolecular attack of one phosphonite on of the vinylidene ligand. The proton-coupled and -decoupled NMR spectra of 17 allowed clear assignment of the characteristic carbon atoms in the molecule the carbonyl appeared as a multiplet at 8 207.7 ppm, the metallate C-Co at 8 168.4 ppm, and the two alkene carbons at 8 63.8 and 32.1 ppm. 

The synthesis of alkylidyne complexes by y -addition of electrophiles to vinylidene and acetylide ligands is now well established (5,6). Pombeiro and co-workers synthesized several new rhenium alkylidyne complexes by protonation of the electron-rich vinylidene complexes 13 [Eq. (18)] (55). The mechanism of formation of the benzylcarbyne complex 14 (R = Ph)... 

A rather significant result was reported by Hbhn and Werner the spectroscopic characterization of the first iridium alkylidyne complexes [Eq. (20)1 (61,62). Protonation of the iridium vinylidene complexes 17 was found to occur initially at the metal center to afford the vinylidene hydrido... 

Protonation of vinylidene and acetylide ligands was also found to be useful for the synthesis of high-oxidation state molybdenum alkylidyne complexes. Green and co-workers demonstrated protonation of the vinylidene complex 20 as shown in Eq. (21) (64). Selegue and co-workers... 

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