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

Abstract

New chiral ruthenium(II) allenylidene complexes were synthesized, and their reactivity with nucleophiles to give alkynyl complexes was investigated. The new allenylidene complex (R Ru ,R ax )-[Ru(Ind)(PPh 3 )(6){C CC(t-Bu)(2-naphthyl)}] + PF 6 − was synthesized from the chloro precursor complex (R Ru ,R ax )-[RuCl(Ind)(PPh 3 )( 6)] and the racemic propargylic alcohol HCCC(OH)(t-Bu)(2naphthyl) and obtained in 96% yield, where (R ax )-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 (R Ru ,R ax )-[Ru(Ind)(PPh 3 )(6)(CCCPh 2 )] + PF 6 − , the attack of n-BuLi, MeLi, LiCCPh, and lithium 1-phenylethenolate nucleophiles on the allenylidene chain of (R Ru ,R ax )-[Ru(Ind)(PPh 3 )-( 6){CCC(t-Bu)(2-naphthyl)}] + PF 6 − was investigated. The nucleophiles (Nu) reacted selectively with the gamma carbon of the allenylidene complexes to give the alkynyl complexes (R Ru ,R ax )-[Ru(Ind)(PPh 3 )(6)(CC−CPh 2 Nu)] and (R Ru ,R ax )-[Ru(Ind)(PPh 3 )(6){CC−C(t-Bu)(2-naphthyl)Nu}] in 40% to 96% isolated yields. In the case of (R Ru ,R ax )-[Ru(Ind)(PPh 3 )-(6){CC−C*(t-Bu)(2-naphthyl)Nu}], the gamma carbon C* becomes stereogenic upon attack of the nucleophiles. As assessed by 31 P{ 1 H} 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.

Nucleophilic Additions to Allenylidene Ruthenium Complexes

“…The regioselectivity of the nucleophilic addition reactions to the allenylidene chain depends on both electronic and steric factors of (i) the metallic fragment; (ii) the substituents on the allenylidene chain; and (iii) the nucleophile. Thus, anionic nucleophiles tend to react through the C γ atoms, leading to neutral alkynyl complexes. , In fact, stereoselective nucleophilic attack on the C γ atom has been used in the stoichiometric formation of optically active alkynyl complexes, and recently diastereoselective nucleophilic attacks on a chiral-at-metal ruthenium allenylidene complex have been reported to give alkynyl complexes in diasteromeric ratios up to 84:16 . However, these nucleophilic attacks are not always regioselective, and mixtures of complexes resulting from the attack to both C α and C γ can be obtained …”

“…5,6 In fact, stereoselective nucleophilic attack on the C γ atom has been used in the stoichiometric formation of optically active alkynyl complexes, 7 and recently diastereoselective nucleophilic attacks on a chiral-at-metal ruthenium allenylidene complex have been reported to give alkynyl complexes in diasteromeric ratios up to 84:16. 8 However, these nucleophilic attacks are not always regioselective, and mixtures of complexes resulting from the attack to both C α and C γ can be obtained. 9 When neutral nucleophiles containing acidic hydrogens such alcohols, amines, and thiols are used, vinylcarbene complexes, resulting from the initial addition of the nucleophile to the αcarbon and protonation of the β-carbon, are usually the reaction products.…”

The chemistry of the carbon-transition metal double and triple bond: Annual survey covering the year 2014

“…from racemic propargylic alcohols [1035]. The allenylidene complex 904 was employed as a propargylic substitution catalyst [1036].…”

Carbon-rich cyclopentadienyl ruthenium allenylidene complexes

“…14,[21][22][23][24][25][26] Transition metal allenylidene complexes turned out to be interesting and useful complexes and intermediates for a huge range of chemistry and catalytic processes. 23,[27][28][29] Especially, the building block properties of allenylidene complexes in organic syntheses as well as their catalytic properties, for example as catalysts or pre-catalysts for the olefin metathesis, [30][31][32][33][34][35][36][37][38] arouse interest. Other reports focus on the esterification of propargyl alcohols 39 or the ring opening polymerization.…”