Vinylidene from metal alkynyls

One of the first theoretical studies used to investigate this problem employed the MP2 approach (later found to be less reliable for transition-metal complexes than most DFT approaches) with simplified phosphine ligands, PHg, and an unsubstituted alkyne (entry 1, Table 3.1) [47]. This study demonstrated that conversion of the alkyne complex [RhCl(ti -HC=CH)(PH3)2] to the alkynyl hydride [Rh(-C=CH)C1H(PH3)2] may proceed via a transition state in which the ligand is bound as a C-H c-complex. The vinylidene complex, which is the global minimum on the PES, may be accessed by hydride migration from the alkynyl hydride, but not directly from the alkyne. An alternative bimolecular pathway (proceeding via transition state TS2 3D) for the conversion of the alkyl hydride to the vinylidene complex was also considered. The calculations... 

Already 20 years ago, Antonova et al. proposed a different mechanism, with a more active role of the transition metal fragment [3], The tautomerization takes place via an alkynyl(hydrido) metal intermediate, formed by oxidative addition of a coordinated terminal alkyne. Subsequent 1,3-shift of the hydride ligand from the metal to the P-carbon of the alkynyl gives the vinylidene complex (Figure 2, pathway b). 

Alkynes react readily with a variety of transition metal complexes under thermal or photochemical conditions to form the corresponding 7t-complexes. With terminal alkynes the corresponding 7t-complexes can undergo thermal or chemically-induced isomerization to vinylidene complexes [128,130,132,133,547,556-569]. With mononuclear rj -alkyne complexes two possible mechanisms for the isomerization to carbene complexes have been considered, namely (a) oxidative insertion of the metal into the terminal C-Fl bond to yield a hydrido alkynyl eomplex, followed by 1,3-hydrogen shift from the metal to Cn [570,571], or (b) eoneerted formation of the M-C bond and 1,2-shift of H to Cp [572]. 

Table 1.3 Some metal vinylidene complexes, L M=C=CRR, obtained from alkynyl-metal systems.

Scheme 4.5 shows several possible pathways from r -acetylene metal complexes RE to metal vinylidenes PR. In the first pathway (al + a2), metal vinylidenes PR can be obtained from an intermediate (INI) with a 1,2 hydrogen shift from C to Cp. The second pathway (bl + b2) is through an intermediate (IN2) with an r agostic interaction between the metal center and one C—H bond, which undergoes a 1,2 hydrogen shift to PR. The third pathway (bl + b3 + b4) also starts from IN2 but then goes into another intermediate, the hydrido-alkynyl IN3, which leads to PR with a 1,3 hydrogen shift from the metal center to Cp. 

Protonation of metal-acetylide complexes affords the corresponding vinylidene complexes e.g. 20 and 99, Figure 1.48). Proceeding from 20 to 99 leads to a lowering of (3 values, by a factor of five. As the vinylidene complexes can be easily deprotonated to give back to the alkynyl precursors, and this sequence can be repeated, these complex pairs can provide an interesting protically switchable NLO system. 

According to the results from isotope labeling experiments, the isomerization involves the initial attack of the proton from the acid to the Cp atom of the alkynyl ligand of 197. The subsequent migratory insertion of the resulting vinylidene into the metal-benzyl bond gives an alkenyl intermediate containing a coordinated trifluoroacetate anion, which evolves into 198 by trifluoroacetic acid elimination (Scheme 53). 

Movassaghi and Hill developed a ruthenium-catalyzed cycloisomerization of 3-azadienynes to the corresponding pyridines [11]. The alkynyl imines were produced from a variety of iV-vinyl and iV-aryl amides by amide activation and nucleophilic addition of copper(I) (trimethylsilyl) acetylide sequence reaction. Then by Ru-catalyzed protodesilylation and cycloisomerization, the desired pyridine derivatives were formed selectively in good to excellent yields (Scheme 2.7). For the reaction mechanism, C-silyl metal vinylidene was found to be the key intermediate. 

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