Efficient Synthetic Routes to Terminal γ-Keto−Alkynes and Unsaturated Cyclic Carbene Complexes Based on Regio- and Diastereoselective Nucleophilic Additions of Enolates on Ruthenium(II) Indenyl Allenylidenes

Ruthenium(II) indenyl allenylidene complexes [Ru{ C C C(R 1 )Ph}(η 5 -C 9 H 7 )(PPh 3 ) 2 ][PF 6 ] (R 1 = Ph ( 1 ), H ( 2 )) regioselectively react with lithium enolates LiCH 2 COR 2 (R 2 = Ph, i Pr, Me, Fc, ( E )-CH CHPh) at the C γ atom to yield the neutral σ-alkynyl derivatives [Ru{C⋮CC(R 1 )Ph(CH 2 COR 2 )}(η 5 -C 9 H 7 )(PPh 3 ) 2 ] ( 3a − e, 4a − d ). Protonation of 3a − e and 4a − d with HBF 4 ·Et 2 O affords the cationic vinylidene derivatives [Ru{ C C(H)C(R 1 )Ph(CH 2 COR 2 )}(η 5 -C 9 H 7 )(PPh 3 ) 2 ][BF 4 ] ( 5a − e, 6a − d ), which can be easily demetalated, by treatment with acetonitrile, to yield the terminal γ-keto-substituted alkynes HC⋮CC(R 1 )Ph(CH 2 COR 2 ) ( 7a − e, 8a − d ) and the nitrile complex [Ru(N⋮CMe)(η 5 -C 9 H 7 )(PPh 3 ) 2 ][BF 4 ] ( 9 ). The addition of lithium enolates LiCH 2 COR (R = Ph, i Pr, Me) on the optically active allenylidene complex [Ru{ C C C(C 9 H 16 )}(η 5 -C 9 H 7 )(PPh 3 ) 2 ][PF 6 ] ( 12; C(C 9 H 16 ) = (1 R )-1,3,3-trimethylbicyclo[2.2.1]hept-2-ylidene) proceeds in a regio- and diastereoselective manner, affording σ-alkynyl derivatives [Ru{C⋮CC(C 9 H 16 )(CH 2 COR)}(η 5 -C 9 H 7 )(PPh 3 ) 2 ] ( 13a − c ). The X-ray crystal structure of 13b shows that these enolate additions take place on the less sterically congested exo face of the allenylidene chain. Chiral alkynes HC⋮CC(C 9 H 16 )(CH 2 COR) ( 15a − c ) have been also prepared from 13a − c via initial protonation with HBF 4 ·Et 2 O and subsequent treatment of the resulting vinylidenes 14a − c with acetonitrile. Cyclic alkenyl derivatives (R 1 = R 2 = Ph, R 3 = Ph ( 18a ), i Pr ( 18b ); R 1 R 2 = C 9 H 16, R 3 = Ph ( 20 )) have been obtained by treatment of dichloromethane solutions of σ-alkynyl complexes 3a, b and 13a with catalytic amounts of AlCl 3 at room temperature. Protonation of these species affords the cyclic carbenes ( 19a, b, 21 ).