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}(eta (5)-C9H7)(PPh3)][PF6] (R-1 = Ph (1), H (2)) regioselectively react with lithium enolates LiCH2COR2 (R-2 = Ph, Pr-i, Me, Fc, (E)-CH=CHPh) at the C gamma atom to yield the neutral sigma -alkynyl derivatives [Ru{C equivalent to CC(R-1)Ph(CH2COR2)}(eta (5)-C9H7)(PPh3)(2)] (3a-e, 4a-d). protonation of 3a-e and 4a-d with HBF4. Et2O affords the cationic vinylidene derivatives [Ru{=C=C(H)C(R-1)Ph(CH2COR2)}-(eta (5)-C9H7)(PPh3)(2)][BF4] (5a-e, 6a-d), which can be easily demetalated, by treatment with acetonitrile, to yield the terminal gamma -keto-substituted alkynes HC equivalent to CC(R-1)Ph(CH2COR2) (7a-e, 8a-d) and the nitrile complex [Ru(N equivalent to CMe)(eta (5)-C6H7)(PPh3)(2)][BF4] (9). The addition of lithium enolates LiCH2COR (R = Ph, Pr-i, Me) on the optically active allenylidene complex [Ru{=C=C=C(C9H16)}(eta (5)-C9H7)(PPh3)(2)] [PF6] (12; C(C9H16)= (1R)-1,3,3-trimethylbicyclo[2.2.1]hept-2-ylidene) proceeds in a regio- and diastereoselective manner, affording sigma -alkynyl derivatives [Ru{C equivalent to CC(C9H16)(CH2COR)}(eta (5)-C9H7)(PPh3)(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 equivalent to CC(C9H16)(CH2COR) (15a-c) have been also prepared from 13a-c via initial protonation with HBF4. Et2O and subsequent treatment of the resulting vinylidenes 14a-c with acetonitrile. Cyclic alkenyl derivatives [Ru{C=CHC(R-1)(R-2)CH=C(R-3)O}(eta (5)-C9H7)(PPh3)(2)] (R-1 = R-2 = ph, R-3 = ph (18a), Pr-i (18b); (RR2)-R-1 = C9H16, R-3 = Ph (20)) have been obtained by treatment of dichloromethane solutions of a-alkynyl complexes 3a,b and 13a with catalytic amounts of AlCl3 at room temperature. Protonation of these species affords the cyclic carbenes [Ru{=CCH2C(R-1)(R-2)CH=C(R-3)O}-(eta (5)-C9H7)(PPh3)(2)][BF4] (19a,b, 21).