A novel route to functionalized terminal alkynes through eta(1)-vinylideneto eta(2)-alkyne tautomerizations in indenyl-ruthenium(II) monosubstitutedvinylidene complexes: Synthetic and theoretical studies

Heating under reflux solutions of the monosubstituted vinylidene complex [R u{=C=C(H)Ph}(eta(5)-C9H7)(PPh3)(2)][PF6] (1) in nitriles yields the complex es [Ru(N=CR)(eta(5)-C9H7) (PPh3)(2)][PF6] (R = Me (2a), Et (2b), Ph (2c)) a nd phenylacetylene. The process proceeds via an initial eta(1)-vinylidene-e ta(2)-alkyne tautomerization followed by the displacement of the coordinate d pi-alkyne by the solvent. Vinylidene complexes [Ru{=C=C(H)R}(eta(5)-C9H7) (PPh3)(2)][PF6] (R = (eta(5)-C5H4)Fe(eta(5)-C5H5) (3), 4-NO2-C6H4 (4)) also react with acetonitrile to yield the nitrile derivative 2a and the corresp onding terminal alkynes HC=CR. Cationic alkenyl-vinylidene derivatives [Ru{ =C=C(H)CH=(CRR2)-R-1}(eta(5)-C9H7)(PPh3)(2)][BF4] (R-1 = R-2 = ph (7a), R-1 = H; R-2 = 4-OMe-C6H4 [(Z)-7b], 4-NO2-C6H4 [(E,Z)-7c], (eta(5)-C5H4)Fe(eta (5)-C5H5) [(E)-7d]) behave similarly. Thus, the treatment of 7a-d with acet onitrile at reflux results in the formation of complex 2a and the liberatio n of the corresponding terminal 1,3-enyne HC=CCH=(CRR2)-R-1 (8a-d). The for mation of the enynes 8b-d is stereoselective, giving rise to the E stereois omer. The allenylidene complex [Ru{=C=C=C(C13H20)} (eta(5)-C9H7)(PPh3)(2)][ PF6] (9), containing the bicyclic [3.3.1]non-2-en-9-ylidene moiety C13H20, reacts with NaC=CH in THF at -20 degrees C to yield the neutral sigma-alkyn yl derivative [Ru{C=CC(C=CH)C13H20}(eta(5)-C9H7)(PPh3)(2)] (10) in a regios elective manner. Protonation of 10 with HBF4. Et2O, in diethyl ether at -20 degrees C, affords the vinylidene complex [Ru{=C=C(H)C(C=CH)C13H20}(eta(5) -C9H7)(PPh3)(2)][BF4] (11), which can be easily demetalated by heating in r efluxing acetonitrile to give 2a and the unprecedented diyne (HC=C)(2)CC13H 20 (12). These demetalation processes allow the quantitative recovery of th e metal auxiliary as the labile complex 2a, which can be used as starting m aterial for further reactions. Ab initio molecular orbital calculations on the eta(1)-vinylidene to eta(2)-alkyne tautomerization have been performed. It is shown that the process proceeds through a 1,2[H] shift mechanism sho wing that the conversion requires an energy barrier of 29.9 kcal/ mel. This is a value low enough to be overcome under the experimental reaction condi tions allowing the formation of the labile eta(2)-alkyne complex and the su bsequent exchange of the coordinated alkyne by acetonitrile.