Oxidative addition/reductive elimination of aldehydes and ketones at rhodium

Article abstract of DOI:10.1021/om00104a011

The trigonal-bipyramidal, TBP, monohydrides [(NP₃)RhH] and [(PP₃)RhH] undergo electrophilic attack by MeOSO₂CF₃ in THF to give CH₄ and the 16-electron fragments (NP₃)Rh⁺ and (PP₃)Rh⁺, respectively [NP₃ = N(CH₂CH₂PPh₂)₃; PP₃ = P(CH₂CH₂PPh₂)₃]. The (NP₃)Rh⁺ fragment inserts across sp² C-H bonds of aliphatic and aromatic aldehydes to yield stable Rh(III) cis hydrido acyl complexes of general formula [(NP₃)Rh(H)(COR)]⁺ (R = H, Me, Ph, 4-MeC₆H₄, 4-MeOC₆H₄, CH=CHCH=CO, trans-MeCH=CH, trans-PhCH=CH). These can be isolated as tetraphenylborate salts. Under the same conditions, the (PP₃)Rh⁺ fragment is unable either to directly activate aldehydes or to keep hydride (or alkyl) and acyl groups in mutually cis positions of the coordination sphere. As a matter of fact, protonation with HOSO₂CF₃ or alkylation with ROSO₂CF₃ (R = Me, Et) of the TBP σ-acyl complexes [(PP₃)Rh(COR)] (R = Me, Ph) promotes the reductive elimination of aldehydes or ketones, respectively. Reaction of the (PP₃)Rh⁺ fragment with HCHO in THF results in the decarbonylation of formaldehyde and formation of the TBP carbonyl [(PP₃)Rh(CO)]⁺. Both the NP₃Rh and the PP₃Rh systems react with acrolein to yield, regardless of the temperature, the η²-alkene derivatives [LRh{η²-CH₂=CH(CHO)}]⁺ (L = NP₃, PP₃), which exhibit fluxional behavior in solution. When propargylaldehyde, HC≡CCHO, is used, cis hydrido acetylide derivatives of formula [LRh(H)(C≡CCHO)]⁺ are obtained (L = NP₃, PP₃), thus indicating that the cleavage of the sp C-H bond prevails over that of the formyl sp² C-H bond. The (NP₃)Rh⁺ and (PP₃)Rh⁺ fragments react with o-(diphenylphosphino)benzaldehyde, PCHO, yielding cis hydrido acyl complexes, in which a phosphine donor of the tripodal ligands is replaced by the PCHO phosphorus atom.