C–C coupling reactions in the coordination sphere of rhodium(i) and rhodium(iii): New routes for the di- and trimerization of terminal alkynes

Abstract

The alkynyl(vinylidene)rhodium(I) complexes trans-[Rh(CCR)(CCHR)(PiPr₃)₂] 2, 5, 6 react with CO by migratory insertion to give stereoselectively the butenynyl compounds trans-[Rh{η¹-(Z)-C(CHR)CCR}(CO)(PiPr₃)₂] (Z)-7–9, of which (Z)-7 (R = Ph) and (Z)-8 (R = tBu) rearrange upon heating or UV irradiation to the (E) isomers. Similarly, trans-[Rh{η¹-C(CH₂)CCPh}(CO)(PiPr₃)₂] 12 and trans-[Rh{η¹-(Z)-C(CHCO₂Me)CCR}(CO)(PiPr₃)₂] (Z)-15, (Z)-16 have been prepared. At room temperature, the corresponding “non-substituted” derivative trans-[Rh{η¹-C(CH₂)CCH}(CO)(PiPr₃)₂] 18 is in equilibrium with the butatrienyl isomer trans-[Rh(η¹-CHCCCH₂)(CO)(PiPr₃)₂] 19 that rearranges photochemically to the alkynyl complex trans-[Rh(CCCHCH₂)(CO)(PiPr₃)₂] 20. Reactions of (Z)-7, (E)-7, (Z)-8 and (E)-8 with carboxylic acids R′CO₂H (R′ = CH₃, CF₃) yield either the butenyne (Z)- and/or (E)-RCCCHCHR or a mixture of the butenyne and the isomeric butatriene, the ratio of which depends on both R and R′. Treatment of 2 (R = Ph) with HCl at −40 °C affords five-coordinate [RhCl(CCPh){(Z)-CHCHPh}(PiPr₃)₂] 23, which at room temperature reacts by C–C coupling to give trans-[RhCl{η²-(Z)-PhCCCHCHPh}(PiPr₃)₂] (Z)-21. The related compound trans-[RhCl(η²-HCCCHCH₂)(PiPr₃)₂] 27, prepared from trans-[Rh(CCH)(CCH₂)(PiPr₃)₂] 17 and HCl, rearranges to the vinylvinylidene isomer trans-[RhCl(CCHCHCH₂)(PiPr₃)₂] 28. While stepwise reaction of 2 with CF₃CO₂H yields, via alkynyl(vinyl)rhodium(III) intermediates (Z)-29 and (E)-29, the alkyne complexes trans-[Rh(κ¹-O₂CCF₃)(η²-PhCCCHCHPh)(PiPr₃)₂] (Z)-30 and (E)-30, from 2 and CH₃CO₂H the acetato derivative [Rh(κ²-O₂CCH₃)(PiPr₃)₂] 33 and (Z)-PhCCCHCHPh are obtained. From 6 (R = CO₂Me) and HCl or HCCCO₂Me the chelate complexes [RhX(CCCO₂Me){κ²(C,O)-CHCHC(OMe)O}(PiPr₃)₂] 34 (X = Cl) and 35 (X = CCCO₂Me) have been prepared. In contrast to the reactions of [Rh(κ²-O₂CCH₃)(CCE)(CHCHE)(PiPr₃)₂] 37 (E = CO₂Me) with chloride sources which give, via intramolecular C–C coupling, four-coordinate trans-[RhCl{η²-(E)-ECCCHCHE}(PiPr₃)₂] (E)-36, treatment of 37 with HCCE affords, via insertion of the alkyne into the rhodium–vinyl bond, six-coordinate [Rh(κ²-O₂CCH₃)(CCE){η¹-(E,E)-C(CHE)CHCHE}(PiPr₃)₂] 38. The latter reacts with MgCl₂ to yield trans-[RhCl{η²-(E,E)-ECCC(CHE)CHCHE}(PiPr₃)₂] 39, which, in the presence of CO, generates the substituted hexadienyne (E,E)-ECCC(CHE)CHCHE 40.