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Diastereoselective Attack on Chiral-at-Metal Ruthenium Allenylidene Complexes To Give Alkynyl Complexes

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journal contribution
posted on 13.10.2014, 00:00 by Matthew J. Queensen, Nigam P. Rath, Eike B. Bauer
New chiral ruthenium­(II) allenylidene complexes were synthesized, and their reactivity with nucleophiles to give alkynyl complexes was investigated. The new allenylidene complex (RRu,Rax)-[Ru­(Ind)­(PPh3)­(6)­{CCC­(t-Bu)­(2-naphthyl)}]+PF6 was synthesized from the chloro precursor complex (RRu,Rax)-[RuCl­(Ind)­(PPh3)­(6)] and the racemic propargylic alcohol HCCC­(OH)­(t-Bu)­(2-naphthyl) and obtained in 96% yield, where (Rax)-6 is a chiral phosphoramidite and Ind an anionic indenyl ligand. The precursor and the allenylidene complex are chiral-at-metal, and the chiral information is completely transferred from the chloro precursor to the product allenylidene complex, both of which show the same absolute configuration, as demonstrated by X-ray diffraction. Together with the known allenylidene complex (RRu,Rax)-[Ru­(Ind)­(PPh3)­(6)­(CCCPh2)]+PF6, the attack of n-BuLi, MeLi, LiCCPh, and lithium 1-phenylethenolate nucleophiles on the allenylidene chain of (RRu,Rax)-[Ru­(Ind)­(PPh3)­(6)­{CCC­(t-Bu)­(2-naphthyl)}]+PF6 was investigated. The nucleophiles (Nu) reacted selectively with the gamma carbon of the allenylidene complexes to give the alkynyl complexes (RRu,Rax)-[Ru­(Ind)­(PPh3)­(6)­(CC–CPh2Nu)] and (RRu,Rax)-[Ru­(Ind)­(PPh3)­(6)­{CC–C­(t-Bu)­(2-naphthyl)­Nu}] in 40% to 96% isolated yields. In the case of (RRu,Rax)-[Ru­(Ind)­(PPh3)­(6)­{CC–C*­(t-Bu)­(2-naphthyl)­Nu}], the gamma carbon C* becomes stereogenic upon attack of the nucleophiles. As assessed by 31P­{1H} NMR, diastereodifferentiation took place, and the alkynyl complexes were isolated as diastereomeric mixtures with diastereomeric ratios between 60:40 and 84:16. The diastereodifferentiation originated only from the stereogenic metal center and the monodentate, chiral ligand. The study allows for investigation of stereoselective, nucleophilic attack of allenylidene complexes to give optically active, quaternary alkynes, which play a role in potential catalytic versions of nucleophilic substitution reactions of propargylic alcohols.