In search of optically active γ-keto acetylenes via regioselective coupling of allenylidene groups and cyclic enolates

(Indenyl)ruthenium(II) allenylidene complexes [Ru{=C=C=C(R)Ph}(eta(5)-C9H7)(PPh3)(2)] [PF6] (R = Ph (1), H (2)) regioselectively react with enolates derived from cyclopentanone and cyclohexanone at the C, atom to yield the sigma-alkynyl derivatives [Ru{CdropCC(R)Ph(CHCOCH2 (CH2)(n)CH2)}(eta(5)-C9H7)(PPh3)(2)] (R = Ph, n = 1 (3a), 2 (3b); R = H, n = 1 (4a), 2 (4b)). Protonation of these species at C-beta of the alkynyl chain with HBF4 affords vinylidene complexes 5a,b and 6a,b, which can easily be demetalated with acetonitrile to yield the gamma-keto acetylenes HCdropCC(R)Ph(CHCOCH2(CH2)(n)CH2) (7a,b and 8a,b). Compounds 4a,b, 6a,b and 8a,b, derived from the monosubstituted allenylidene complex 2, have been obtained as nonseparable mixtures of two diastereoisomers. The optically active allenylidene [Ru{=C=C= C(C9H16)}(eta(5)-C9H7)(PPh3)(2)][PF6] (10) (C(C9H16) = (IR)-1,3,3-trimethylbicyclo[2.2.1]hept-2-ylidene) undergoes a selective exo addition of the cyclopentanone enolate to afford the sigma-alkynyl diastereoisomers [Ru{CdropCC(C9H16)(CHCOCH2CH2CH2)}(eta(5)-C9H7)(PPh3)(2)] (11a,b). Demetalation of 11a and 11b, via their corresponding vinylidenes, allows the preparation of optically pure terminal alkynes HCdropCC(C9H16)(CHCOCH2CH2CH2) (13a,b, respectively). The oxacycloalkenyl derivative [Ru{C=C(H)C(C9H16)C=C(CH2CH2CH2)O}(eta(5)-C9H7)(PPh3)(2)] (14) has been obtained by treatment of 11a or 11b with a catalytic amount of AlCl3. Protonation of 14 affords the corresponding cyclic carbene 15. The reactivity of allenylidene complexes 1, 2, and 10 toward lithium enolates derived from the optically active ketones (R)-(-)-carvone and (R)-(+)-pulegone has been also explored. For diphenylallenylidene 1 diastereoselective additions are observed, yielding the optically pure sigma-alkynyl complexes 16 and 19, respectively. While attempts to demetalate 19 failed, demetalation of 16 yields the optically pure y-keto alkyne HCdropCCPh(2)(C10H13O) (18) in excellent yield. The crystal structures of compounds 11b and 14 have been determined by X-ray diffraction.