A Novel Route to Functionalized Terminal Alkynes through η 1 -Vinylidene to η 2 -Alkyne Tautomerizations in Indenyl−Ruthenium(II) Monosubstituted Vinylidene Complexes: Synthetic and Theoretical Studies

Heating under reflux solutions of the monosubstituted vinylidene complex [Ru{ C C(H)Ph}(η 5 -C 9 H 7 )(PPh 3 ) 2 ][PF 6 ] ( 1 ) in nitriles yields the complexes [Ru(N⋮CR)(η 5 -C 9 H 7 )(PPh 3 ) 2 ][PF 6 ] (R = Me ( 2a ), Et ( 2b ), Ph ( 2c )) and phenylacetylene. The process proceeds via an initial η 1 -vinylidene-η 2 -alkyne tautomerization followed by the displacement of the coordinated π-alkyne by the solvent. Vinylidene complexes [Ru{ C C(H)R}(η 5 -C 9 H 7 )(PPh 3 ) 2 ][PF 6 ] (R = (η 5 -C 5 H 4 )Fe(η 5 -C 5 H 5 ) ( 3 ), 4-NO 2 -C 6 H 4 ( 4 )) also react with acetonitrile to yield the nitrile derivative 2a and the corresponding terminal alkynes HC⋮CR. Cationic alkenyl−vinylidene derivatives [Ru{ C C(H)CH CR 1 R 2 }(η 5 -C 9 H 7 )(PPh 3 ) 2 ][BF 4 ] (R 1 = R 2 = Ph ( 7a ), R 1 = H; R 2 = 4-OMe-C 6 H 4 [( Z )- 7b ], 4-NO 2 -C 6 H 4 [( E, Z )- 7c ], (η 5 -C 5 H 4 )Fe(η 5 -C 5 H 5 ) [( E )- 7d ]) behave similarly. Thus, the treatment of 7a − d with acetonitrile at reflux results in the formation of complex 2a and the liberation of the corresponding terminal 1,3-enyne HC⋮CCH CR 1 R 2 ( 8a − d ). The formation of the enynes 8b − d is stereoselective, giving rise to the E stereoisomer. The allenylidene complex [Ru{ C C C(C 13 H 20 )}(η 5 -C 9 H 7 )(PPh 3 ) 2 ][PF 6 ] ( 9 ), containing the bicyclic [3.3.1]non-2-en-9-ylidene moiety C 13 H 20, reacts with NaC⋮CH in THF at −20 °C to yield the neutral σ-alkynyl derivative [Ru{C⋮CC(C⋮CH)C 13 H 20 }(η 5 -C 9 H 7 )(PPh 3 ) 2 ] ( 10 ) in a regioselective manner. Protonation of 10 with HBF 4 ·Et 2 O, in diethyl ether at −20 °C, affords the vinylidene complex [Ru{ C C(H)C(C⋮CH)C 13 H 20 }(η 5 -C 9 H 7 )(PPh 3 ) 2 ][BF 4 ] ( 11 ), which can be easily demetalated by heating in refluxing acetonitrile to give 2a and the unprecedented diyne (HC⋮C) 2 CC 13 H 20 ( 12 ). These demetalation processes allow the quantitative recovery of the metal auxiliary as the labile complex 2a, which can be used as starting material for further reactions. Ab initio molecular orbital calculations on the η 1 -vinylidene to η 2 -alkyne tautomerization have been performed. It is shown that the process proceeds through a 1,2-[H] shift mechanism showing that the conversion requires an energy barrier of 29.9 kcal/mol. This is a value low enough to be overcome under the experimental reaction conditions allowing the formation of the labile η 2 -alkyne complex and the subsequent exchange of the coordinated alkyne by acetonitrile.