Vinylidene complex, determination

Beside the spectroscopic evidence, the type A configuration is confirmed also by an X-ray structure determination of the vinylidene complex [Ru(bdmpza)Cl(=C=CHTol)(PPh3)] (33b) (Fig. 23). 

Obviously, the first intermediates in the syntheses with terminal alkynols are the vinylidene complexes [Ru(bdmpza)Cl(=C= CH(CH2) +iOH)(PPhg)] (n = 1, 2), which then react further via an intramolecular addition of the alcohol functionality to the a-carbon (Scheme 22), although in none of our experiments we were able to observe or isolate any intermediate vinylidene complexes. The subsequent intramolecular ring closure provides the cyclic carbene complexes with a five-membered ring in case of the reaction with but-3-yn-l-ol and with a six-membered ring in case of pent-4-yn-l-ol. For both products type A and type B isomers 35a-I/35a-II and 35b-I/ 35b-II are observed (Scheme 22, Fig. 22). The molecular structure shows a type A isomer 35b-I with the carbene ligand and the triphenylphosphine ligand in the two trans positions to the pyrazoles and was obtained from an X-ray structure determination (Fig. 25). 

The number of known, isolated and characterized complexes depends strongly on the length of the chain and drastically decreases with the number of carbon atoms in the chain. A great number of vinylidene complexes of many metals, with different terminal substituents R and various co-ligands have been synthesized and the reactivity has been studied extensively. At present, the solid-state structure of more than 230 vinylidene complexes has been determined by X-ray structure analyses. The number of isolated allenylidene complexes is somewhat smaller. 

NMR data. Similar energy barriers were determined for a number of other vinylidene complexes in this study. 

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 reluctance of the carbyne carbon to react with nucleophiles is revealed by the reaction with LiEt3BH (see Scheme 6). Here the most electrophilic site is not the carbyne carbon but the ipara position of the aryl ring in the carbyne substituent Both ruthenium and osmium five coordinate, cationic, carbyne complexes undergo this reaction. The structure of a representative example, the osmium compound derived from the p-tolyl carbyne complex, has been determined by X-ray crystallography [16]. The unusual vinylidene complex reacts with HCl to produce a substituted benzyl derivative. The reaction may proceed through the intermediate a-vinyl complex depicted in Scheme 6 although there is also the possibility that the vinylidene compound is in equilibrium with the carbene tautomer as shown below. 

No coalescence of NMR signals was observed for the pseudotetrahedral alkylidene and vinylidene complexes [Re(C5H5)(NO)(PPh3)(=CHPh] and the related CH2 and C=CH2 complexes which sets lower limits for rotation barriers about the Re = C bonds of 18-19 kcal mol". " However, slow isomerizations of kinetically formed isomers of the CHPh and C=CMePh complexes to thermodynamic mixtures could be followed by NMR methods and this has allowed kinetic activation parameters for the ligand rotation to be determined in these cases. 

The first Mo-vinylidene complexes have been prepared by the unusual carbyne to carbene conversions illustrated in Scheme 18. The nature of (8) was confirmed by a crystal and molecular structure determination. ... 

Wakatsuki et al. (4) proposed vinyl complex, 5, and presented DFT results supporting isomerization to a vinylidene hydride as the rate determining step. Our results indicate that the rate determining step involves H-OH bond breaking and that protonation of a bound alkyne is the rate determining step in this... 

Another focus of this chapter is the alkynol cycloisomerization mediated by Group 6 metal complexes. Experimental and theoretical studies showed that both exo- and endo- cycloisomerization are feasible. The cycloisomerization involves not only alkyne-to-vinylidene tautomerization but alo proton transfer steps. Therefore, the theoretical studies demonstrated that the solvent effect played a crucial role in determining the regioselectivity of cycloisomerization products. [2 + 2] cycloaddition of the metal vinylidene C=C bond in a ruthenium complex with the C=C bond of a vinyl group, together with the implication in metathesis reactions, was discussed. In addition, [2 + 2] cycloaddition of titanocene vinylidene with different unsaturated molecules was also briefly discussed. 

In their metal complexes, bonding of either species to the metal atom is via a ligand - > metal a donor bond and a metal - >ligand n bond, enabling back donation of electron density to the n orbitals of the C-C multiple bond system to take place. Vinylidene is one of the best 7t-acceptors known, and is exceeded only by S02 and CS in this respect the relationship between phenylvinylidene and other common ligands has been determined (18) from the CO force constants exhibited by a series of Mn(CO)2(q-C5H5) complexes, which increase in this order ... 

Cp Ru [14] and TpRu [20] complexes have also been studied in depth. As represented in Scheme 2c, the catalytic alkyne dimerization proceeds via coordinatively unsaturated ruthenium alkynyl species. Either a direct alkyne insertion and/or previous vinylidene formation are feasible pathways that determine the selectivity. The head-to-tail dimer cannot be formed by the vinylidene mechanism, whereas the E or Z stereochemistry is controlled by the nature of the alkynyl-vinylidene coupling. 

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