Reactions of σ-π alkenyl complexes [Fe 2(CO) 6(μ-alkenyl)(μ-PPh 2)] with bis(diphenylphosphino)methane (dppm); carbonyl substitution, migratory carbonyl insertion, phosphorus–carbon bond formation and cleavage, proton shift and alkenyl isomerisation reactions

Abstract

Products of the thermal reactions of μ-alkenyl complexes [Fe 2(CO) 6(μ-R 1CCHR 2)(μ-PPh 2)] ( 1) ( a, R 1=R 2=H; b, R 1=EtO, R 2=H; c, R 1=H, R 2=Ph; d, R 1=Ph, R 2=H; e, R 1=R 2=Ph; f, R 1=CMeCH 2, R 2=H), with bis(diphenylphosphino)methane (dppm) are substituent dependent. Complexes 1a– c afford simple substitution products trans-[Fe 2(CO) 4(μ-alkenyl)(μ-PPh 2)(μ-dppm)] ( 2a– c); while with 1d, e, α,β-unsaturated acyl complexes trans-[Fe 2(CO) 4{μ-OC–C(Ph)CH(R 2)}(μ-PPh 2)(μ-dppm)] ( 3d, e) are the major products formed via a migratory–insertion reaction. A minor product of the reaction with 1d is [Fe 2(CO) 5{μ-Ph 2PC(Ph)CH 2}(μ-dppm)] ( 4), the result of phosphorus–carbon bond formation. Reaction of 1f also leads to phosphorus–carbon bond formation together with a 1,4-proton shift giving the μ-alkylidene complex [Fe 2(CO) 4{μ-HC–C(Me)C(Me)PPh 2}(μ-dppm)] ( 5). Dppm addition to 1c has been followed in detail, allowing a complete reaction scheme to be developed. Initial carbonyl substitution affords the η 1-dppm complex [Fe 2(CO) 5(η 1-dppm)(μ-HCCHPh)(μ-PPh 2)] ( 6). This subsequently isomerises to trans-[Fe 2(CO) 4(μ-OC–CHCHPh)(μ-PPh 2)(μ-dppm)] ( 3c) which then readily loses CO to give the μ-alkenyl 2c. Loss of CO from isomeric 3d occurs only upon prolonged thermolysis and also affords 2c, a result of α,β-alkenyl isomerisation. Further, heating 4 also yields 2c after CO loss, phosphorus–carbon bond cleavage, and alkenyl isomerisation. While the β-substituted phenylethenyl complex 2c is stable to prolonged reflux in toluene, heating isomeric cis-[Fe 2(CO) 4(μ-PhCCH 2)(μ-PPh 2)(μ-dppm)] ( 7) results in formation of the 5-electron μ-acyl complex [Fe 2(CO) 3{μ-OC–C(Ph)CH 2}(μ-dppm)(μ-PPh 2)] ) ( 8); also prepared upon heating 4. Crystal structures have been carried out on 3e and 5.