Complexes metal-carbyne, protonated

The photochemistry of metal carbyne complexes is in many ways similar to the photochemistry of metal carbene complexes, but the reactions have not been developed or become as synthetically useful as the photochemistry of metal carbene complexes. Among reported reactions are couplings with ancillary CO ligands to form ketenyl complexes, protonation of the carbyne carbon, insertions into C-H bonds, addition of the carbyne carbon to an alkyne to produce a cyclopropenyl complex, and electron-transfer reactions. ... 

These results suggest the presence of two competing pathways to products, which depend upon the location of protonation at the M=C carbyne bond. Charged controlled protonation at the carbyne carbon followed by nucleophilic attack of the CT leads to the butadiene complex. Frontier control of protonation results in attack at the metal center, leading ultimately to the hydride complex164. This has been verified by reaction of the... 

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 ... 

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 trigonal bipyramidal osmium carbyne complex 115 adds HCl across the metal-carbon triple bond to give the octahedral carbene complex 116 [Eq. (101)] (56). Protonation of 115 with aqueous HCIO4 gives the cationic... 

A related reactivity pattern is observed when the complexes 4c or 4d are protonated with HBF4 Et20, using the donor molecule NCMe as a solvent (29). In this instance, a dication results as the carbyne ligand is transformed into an rj1-acyl group in a stepwise process such as described earlier (Scheme 4). The coordination sphere of the metal is filled by two NCMe molecules, yielding the salts [ W (CO)( NCMe)2( 2-COCH2RXf7-... 

On the other hand, neutral Os alkylidyne 82 undergoes reaction with methanol to give carbene complex 83 (equation 10.56).96 It would appear that 82 undergoes reaction with nucleophilic methanol at 0 first, which is followed by proton transfer to Os. Such reactivity would be consistent with that associated with Fischer carbyne complexes, yet the metal center is more electron-rich than the group 6 metal complex reactant in equation 10.55. 

Reaction of the Mo Schrock carbyne complex in equation 10.58 with HBF4 results in protonation at Ccaibyne, followed by rearrangement to the thermodynamically more stable Mo-H complex. The BF4 ion is such a weakly coordinating ligand that substitution at the metal does not occur.98... 

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. 

The basicity at the 3-carbon is illustrated by the reactions in Equations 13.26 and 13.27. Reaction of the octahedral rhenium vinylidene in Equation 13.26 with HBF generates the cationic carbyne complex from addition of a proton to the basic 3-carbon. Because low-valent metals are often basic, addition of a proton to the vinylidene 3-carbon is likely to occur by a multi-step process initiated by protonation of the metal center. This initial protonation at the metal center would then be followed by migration of the proton to the 3-carbon. The reaction of acid with the iridium vinylidene in Equation 13.27 illustrates this mechanism. In this case, protonation first generates an iridium-hydride complex. The hydride in this complex tlien migrates to the p-carbon to generate an alkylidyne 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. 

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