Syntheses of Arylphosphonium Salts from Cyclotrimerization of Terminal Aryl Alknyes by a Ruthenium Pentadienyl Complex and Revisiting the Catalytic Dimerization

Abstract

The synthesis of polyaryl phosphonium salts by cyclotrimerization of aryl alkynes is induced by a stoichiometric amount of the ruthenium η 5 -pentadienyl complex (η 5 -C 5 H 7 )(PPh 3 ) 2 RuCl (1). With only 1 mol % quantity, complex 1 efficiently catalyzed the dimerization of aryl alkynes at room temperature to afford the corresponding (Z)-1,4-diarylbut-1-en-3-yne derivatives as the major products. ■ INTRODUCTIONResearch over several decades on transition-metal pentadienyl compounds has revealed significant differences in the steric and electronic properties between the η 5 -pentadienyl (C 5 H 7 ) and η 5 -cyclopentadienyl (C 5 H 5 or Cp) ligands in transition-metal compounds. 1 The two ligands are isoelectronic; however, the C 5 H 7 ligand (cone angle 180°), 2 which is both a better δ backbonding acceptor and a better σ donor than the Cp ligand, 1 is also sterically more demanding than the Cp ligand (cone angle 145°). 3 Calculations predict that η 5 to η 3 conversions in the C 5 H 7 ligand should be easier than that in a C 5 H 5 ligand. 1 These differences among C 5 H 7 , Cp, and indenyl (C 9 H 7 ) ligands are expected to influence various properties of their compounds. The potential ring slippage from η 5 to η 3 in the C 5 H 7 ligand that may possibly create novel catalytic reactivity in ruthenium metal complexes has been investigated. 4 Dimerization of a terminal aryl alkyne catalyzed by [Ru]-Cl (1, [Ru] = (η 5 -C 5 H 7 )(PPh 3 ) 2 Ru) concomitantly giving an unidentified brown powder was disclosed in 2007. 5 Aiming at understanding the chemistry of ruthenium pentadienyl species derived from alkyne, we explore the reactions of arylacetylene with 1. In this paper, we report a practical and switchable reaction system using either a stoichiometric or a catalytic quantity of 1, leading either to the synthesis of polyaryl phosphonium salts via cyclotrimerization or to dimerization of aryl alkynes under mild condition, respectively. ■ RESULTS AND DISCUSSIONTrimerization. Treatment of phenylacetylene (2a) with a stoichiometric quantity of 1 affords no ruthenium acetylide or vinylidene complexes which are generally formed from the reaction of the corresponding (η 5 -C 5 H 5 )(PPh 3 ) 2 RuCl complex. Instead, the reaction of 2a with 1 and KPF 6 , both in stoichiometric quantities, gives a brown powder as the major product. The previously reported catalytic dimerization of 2a by 1 without any additive 5 revealed a similar brown powder which displayed a singlet resonance at δ 25.5 in the 31 P NMR spectrum in low yield and was not fully characterized. Our product formed in the presence of KPF 6 shows a slightly different resonance at δ 23.0 and displays a set of peaks at δ −150 attributed to PF 6 − . The previously reported complex is believed to have a chloride counterion, instead of PF 6 − , thus showing slightly different 31 P NMR data. 5 The structure of our brown powder has been fully characterized by a single-crystal X-ray diffraction analysis as the polyaryl phosphonium salt 6a, resulting from addition of PPh 3 to...

Phosphonium salts and P-ylides

“…Structural properties of phosphonium molecules were also investigated by NMR in a paper by Aganova et al, who described two pyridoxine-substituted phosphonium salts. 5 Lin and Chen 6 reported that (Z 5 -C 5 H 7 )(PPh 3 ) 2 RuCl catalysed the dimerization of arylalkynes when used in amounts as low as 1 mol%. Surprisingly, stoichiometric amounts of the same ruthenium complex led to the formation of the cyclotrimerization product, yielding polyarylphosphonium salts.…”

Transition metal-mediated metathesis between P–C and M–C bonds: Beyond a side reaction

“…While investigating the catalytic dimerization reactivity of Ru-cyclopentadienyl complexes with alkynes, an unidentified brown powder was observed by Kirss in 2007, which was produced as a side product during the reaction but has not been fully characterized [102]. Later, Lin and coworkers noted the formation of the brown-colored polyaryl phosphonium salt resulting from the reductive elimination of the alkyne trimerization product with PPh3 (Scheme 39) [103]. A plausible reaction pathway was proposed to start with metal activation of the terminal alkynyl C−H bond, leading to formation of the acetylide complex (42).…”

Phosphonium salts and P-ylides

“…As an example, Müller, Rosenthal and co-workers reported the study of a chromium-based catalyst for the selective trimerization of ethylene. 37 6 À was used by Dridi, Mechria and Msaddek as an allylating agent for the synthesis of cationic Z 3 -methallylpalladium complexes. 40 A very easy procedure for the preparation of 2-carboxyethyltriphenyl phosphonium tribromide, was reported by Dey and Dhar.…”