An Unusual Example of Allyl-to-Alkynyl Migration in a Phenylacetylide-Bridged Heterobinuclear Complex of Rhodium and Iridium

Abstract

The reactivity of the alkynyl-bridged complex [RhIr(CO) 2 (µ 2 -η 1 :η 2 -C 2 Ph)(dppm) 2 ][X] (X ) BF 4 (1a), SO 3 CF 3 (1b); dppm ) Ph 2 PCH 2 PPh 2 ) with electrophiles has been demonstrated. Protic acids HX (X ) BF 4 , SO 3 CF 3 ) first yield the oxidative-addition products [RhIr(X)(CO) 2 -(µ-H)(µ-C 2 Ph)(dppm) 2 ][X], which under carbon monoxide result in displacement of the weakly coordinating BF 4or SO 3 CF 3anions and subsequent conversion to the vinylidene-bridged 5), Cl ( 6)), by coupling of the alkynyl and allyl groups at the β-position of the alkynyl moiety. NMR studies at low temperatures show coordination of allyl halide at Ir at -80 °C, followed by allyl halide loss and subsequent oxidative addition at -50 °C. The oxidative-addition intermediates, [RhIr( 9), Cl ( 10)), rearrange to the allylvinylidene products (5 and 6) at ambient temperature. Although halide removal from compounds 5 and 6, using AgBF 4 , does not result in destabilization of the allylvinylidene fragment, resulting instead in replacement of halide by fluoborate ion, the reaction of 1 with allyl halide in the presence of a silver salt does not lead to coupling of the allyl and alkynyl moieties, but gives [RhIr(ηAddition of halide ion to this η 3 -allyl complex at ambient temperature again leads to formation of 5 or 6. On the basis of these results a mechanism is proposed for the allyl/alkynyl coupling reaction.

C-C and C-O Coupling Reactions of Terminal Alkynes by a Water Soluble Organoiridium Electron-transfer Mediator in Thin Layer of Water on Gold Electrode

“…Terminal alkynes are known to coordinate toward metal center to give stable π-alkyne or σ-alkynyl compound [6][7][8][9][10]. In fact, it has been well-documented that metal vinylidene is obtained directly from the rearrangement of π-coordinated terminal alkynes [6][7][8][9][10][11][12][13][14][15][16][17][18][19]39,40]. It seems reasonable to include the direct attack of H 2 O on the carbon of the Ir-(HC≡CR) to produce ketones and on the α-carbon of the vinylidene moiety (Ir=C=CHR) to yield a hydroxyl carbene compounds followed by reductive elimination to give aldehydes.…”

“…The dimerization of alkynes catalyzed by a variety of transition metal compounds has been extensively studied and suggested the elaborated mechanism [11][12][13][14][15][16][17][18]. The key step of alkyne coupling reaction has been commonly proposed to involve the rearrangement of metal π-alkyne to metal vinylidene form [11][12][13][14][15][16][17][18].…”

“…This resulting anionic radical compound could produce highly reactive species which will be used to catalyze the activation of organic substrates such as alkynes. Metal-catalyzed hydration and oligomerization of alkynes provide important processes in organic chemistry to aldehydes, ketones, and conjugated olefin compounds since they are the carbon-oxygen [6][7][8][9][10] and -carbon [11][12][13][14][15][16][17][18][19] bond forming reactions which produce a variety of new inexpensive starting materials.…”

“…Among water-soluble ligands, PAr 3 (P(m-C 6 H 4 SO 3 Na) 3 ) is the most commonly used ligand to prepare water-soluble metal compounds [27][28][29][30][31][32]. Iridium-Cp * (C 5 Me 5 -) compounds have been investigated and have drawn much attention due to their diverse and interesting reactivity with alkynes [11][12][13][14][15][16][17][18][27][28][29][30][31][32][33][34][35][36][37].…”

“…Since it is well-known that terminal alkynes can be activated by organoiridium compounds [11][12][13][14][15][16][17][18], this voltammetric observation prompted us to look into details of reduced-metal catalyzed activation of alkynes in biphasic (H 2 O/CH 2 ClCH 2 Cl) solution. The biphasic…”