117985-02-1Relevant articles and documents
Oxidative addition/reductive elimination of aldehydes and ketones at rhodium
Bianchini, Claudio,Meli, Andrea,Peruzzini, Maurizio,Ramirez, José A.,Vacca, Alberto,Vizza, Francesco,Zanobini, Fabrizio
, p. 337 - 345 (2008/10/08)
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.
Activation of 1-alkynes at 16-electron rhodium fragments. Some examples of thermodynamically favored rearrangements [M(π-HC≡CR)] → [M(H)(C≡CR)]
Bianchini, Claudio,Masi, Dante,Meli, Andrea,Peruzzini, Maurizio,Ramirez, José A.,Vacca, Alberto,Zanobini, Fabrizio
, p. 2179 - 2189 (2008/10/08)
The 16-electron fragments [(NP3)Rh]+ and [(PP3)Rh]+ react with 1-alkynes in THF at room temperature yielding Rh(III) cis hydride acetylide complexes of formula [(L)Rh(H)(C≡CR)]BPh4 [L = NP3, N-(CH2CH2PPh2)3; L = PP3, P(CH2CH2PPh2)3; R = H, n-C3H7, Ph, SiMe3, CH2OH, CHO, CO2H, CO2Et]. The crystal structure of the ethynyl derivative [(NP3)Rh(H)(C≡CH)]BPh4·1.5THF was determined by X-ray crystallography. The metal atom in the complex cation is octahedrally coordinated by the four donor atoms of NP3, a σ-bonded ethynyl group, and a hydride ligand. Crystal data: monoclinic, space group P21/n, a = 15.769 (3) A?, b = 32.458 (6) A?, c = 13.277 (6) A?; β = 105.21 (2)°, U = 6557 (1) A?3; Z = 4. The structure was solved by Patterson and Fourier techniques and refined to an R factor of 0.079 (Rw = 0.086) by using 3735 reflections with I > 3σ(I). Decreasing the temperature to -40°C does not change the nature of the products except for the reactions with HC≡CCO2Et. In this case, the π-alkyne adducts [(L)Rh-(π-HC≡CCO2Et)]BPh4 (L = NP3, PP3) are formed which are thermodynamically unstable in ambient temperature solutions to irreversibly give the corresponding hydride acetylide derivatives. The [Rh(π-HC≡CCO2Et)] → [Rh(H)(C≡CCO2Et)] rearrangement is preceded at lower temperature by a fluxional process on the 31P NMR time scale which involves the π-alkyne species. At low temperature, also propiolic acid, HC≡CCO2H, forms with the [(NP3)Rh]+ fragments a π-alkyne complex which converts, in ambient temperature solutions, into hydride acetylide and hydride carboxylate derivatives via two independent, almost equally favored pathways. Most of the hydride acetylide complexes of rhodium(III) react with excess NaBH4 in THF/ethanol yeilding Rh(I) σ-acetylides of formula [(L)Rh(C≡CR)] (R = n-C3H7, Ph, SiMe3, CH2OH, CO2Et, CHO). No interconversion between the NP3 hydride acetylide complexes and the parent vinylidenes [(NP3)Rh{C≡C(H)(R)}]BPh4 was observed.
