Bis (ditertiaryphosphine) complexes of rhodium, and catalytic asymmetric hydrogenation
Creators: name: Mahajan, Devinder
Publisher: name: University of British Columbia
Year: 2010

description: Rhodium(I)-bis(ditertiaryphosphine) complexes of the general formula Rh(P⁀P)₂Cl[P⁀P = Ph₂P(Ch₂)n PPh₂, n = 1-4, and (+)-diop (diop = 2,3-0 isopropylidene 2,3-dihydroxy-1,4 bis(diphenylphosphino)butane] have been prepared by treating [Rh(Ccyclooctene)C₂Cℓ]₂ with the appropriate ditertiaryphosphine. The n=1, and n=4 and diop species are five-coordinate in the solid state and in non-polar solvents, while the n=2 and 3 species contain ionic chloride. The cationic complexes Rh(P⁀P)₂ +X- were prepared from the Rh(P⁀P)₂ Cℓ species by adding AgX[X=SbF₆,PF₆,BF₄] . Reaction of the chloro complexes with borohydride has yielded the hydrides, H Rh(P⁀P)₂, for the n=2 and 3 diphosphines, and for (+)-diop. ¹H and ³¹P nmr, as well as visible spectral data, are presented: a solvent-dependent deshielding of ortho protons of the phenyl groups is observed in some of the complexes, and the ligand CH₂ protons are coupled to the rhodium in the Rh(Ph PCI^PPh^^ cation; the P atom in this bis(diphenylphosphino) ligand shows an unusual highfield shift on coordination to rhodium. Preliminary kinetic data for catalytic hydrogenation of methylene succinic0 acid or itaconic acid (IA) show that the cationic and hydrido complexes are more active than the corresponding chloro complexes, and that activity generally increases with increasing chain length of the diphosphine. The rhodium-bis(diop) complexes efficiently catalyze the asymmetric hydrogenation of a number of prochiral substrates, optical purities of &gt;90% being obtained in the hydrogenation of N-acylaminoacrylic acids. Steric factors at the olefinic bond, and coordination of the -NHCOR group through the "^C=0 moiety, seem important in determining the hydrogenation rates. The rates are slower in the more strongly coordinating DMA compared to n-butanol-toluene mixtures. The solvent medium has little effect on the +degree of asymmetric induction, when using thre Rh[(+)-diop]2^ or HRh[(+)-diop]2 complexes, but reversal of product configuration is observed when using the Rh[(+)-diop]^Cl complex in DMA or in n-butanol-toluene mixtures. An unusual increase in optical purity of the product with increasing temperature has been observed in the hydrogenation of IA. Detailed kinetic and spectroscopic studies on the hydrogenation of IA catalyzed by HRh[(+)-diop] are explained in terms of a mechanism involving the formation of a metal alkyl via coordination of the olefinic substrate, followed by reaction with H2 to yield the saturated product (S.P.) and regenerated catalyst (equations [l]-[3]). A monodentate diop(diop*) is invoked: HRh(diop)2 _ HRh(diop)(diop*) [1] HRh(diop) (diop*) + olefin k Rh(diop) (diop*) (alkyl) [2] Rh(diop)(diop*)(alkyl) + &gt; HRh(diop)(diop*) + S.P. [3] The initial hydride catalyst is slowly decomposed by protons of the acidic substrate to give Rh(diop)2+. To avoid this complication, a mechanistic study was carried out on the HRh(diop)2"Styrene-H2 system, which was found to proceed via the same mechanism as outlined in equations [l]-[3], A mechanistic study on the Rh(diop)2+BF^ -catalyzed hydrogenation of IA shows that the reaction proceeds mainly via the 'hydride' route: Rh(diop)2+ + H2 ^ Rh(diop)2H2+ [4] Rh(diop)2H2+ + IA v=^Rb(diop) (diop*) (H)2(IA)+ [5] Rh(diop) (diop*) (H)2(IA)+ &gt; Rh(diop)2+ + S.P. [6] A complete inhibition of the hydrogenation by small amounts of added diop(diop:Rh&gt;0.2) is tentatively attributed to formation of an inactive polymeric species: nRh(diop)2H2+ + diop &gt; [Rh(diop)(diop*)H2+]n [7] The forward step of Reaction [4] was studied in detail in the absence of olefinic substrate. Spectroscopic and kinetic data are best explained in terms of dihydride formation via the consecutive reactions outlined in equation [8]: Rh(diop)„+ s . •&gt; Rh(diop)(diop*)S+ ——• Rh(diop) (diop*) (H) „S+ [8] Rh(diop)2H2+ The dehydrogenation reaction was also studied. The reactions of [Rh(P~P)2]A complexes (A=C£,BF4) with C0,C&gt;2,H2 and HC&amp;(g) yield several new complexes. Thus the [Rh(P P)2XY] BF^ complexes rs rs r\ (P P = dpm,dpp;XY=CO, P P=dpm,dpe,dpp;XY=02, P P=dpp, (+)-diop: XY=H2, and rs P P=dpm,dpe,dpp;XY=HC&amp;) were isolated and characterized. The solution 31 structures were determined using especially variable temperature P nmr spectroscopy. The formally six-coordinate rhodium(III) dioxygen and the dihydrido complexes were assigned cis geometries, whereas the HC£ complexes were more fluxional and cis geometries could only be assigned with certainty to the dpp complex; for dpm and dpe complexes, the limiting spectra could not be achieved even at -60°C. For the five-coordinate rhodium(I) CO complexes, the dpp complex has been assigned a TBP structure but the dpm complex is fluxional even at -60°C. Some stopped-flow kinetic data are presented for the addition of CO, 02, and B.^ to the Rh(P P&gt;2 complexes. For the dpp system, the rate increased in the order CO&gt;H2&gt;02, although the reactions are not simple 1:1 single step additions, solvated species probably playing an important role (cf. equation [8]).