Allosteric effects in asymmetric hydrogenation catalysis? Asymmetric induction as a function of the substrate and the backbone flexibility of C1-symmetric diphosphines in rhodium-catalysed hydrogenations
作者:Angharad Baber、Johannes G. de Vries、A. Guy Orpen、Paul G. Pringle、Karl von der Luehe
DOI:10.1039/b607479k
日期:——
The new unsymmetrical, optically active ligands 1,2-C2H4(PPh2)(2′R,5′R-2′,5′-dimethylphospholanyl) (Laa) and 1,3-C3H6(PPh2)(2′R,5′R-2′,5′-dimethylphospholanyl) (Lbb) form complexes of the type [Rh(L)(cyclooctadiene)][BF4] where L = Laa (1a) or Lbb (1b), [PtCl2(L)] where L = Laa (2a) or Lbb (2b) and [PdCl2(L)] where L = Laa (3a) or Lbb (3b). The crystal structures of 2a and 2b show the chelate ligand backbones adopt δ-twist and flattened chair conformations respectively. Asymmetric hydrogenation of enamides and dehydroaminoesters using 1a and 1b as catalysts show that the ethylene-backboned diphosphine Laa gives a more efficient catalyst in terms of asymmetric induction than the propylene-backboned analogue Lbb. The greatest enantioselectivities were obtained with 1a and enamide substrates with ees up to 91%. Substrate-induced conformational changes in the Rh–diphosphine chelates are proposed to explain some of the ees observed in the hydrogenation of enamides.
新型不对称光学活性配体1,2-C2H4(PPh2)(2′R,5′R-2′,5′-二甲基膦烷) (Laa)和1,3-C3H6(PPh2)(2′R,5′R-2′,5′-二甲基膦烷) (Lbb)形成类型为[Rh(L)(环辛二烯)][BF4]的复合物,其中L = Laa (1a)或Lbb (1b),[PtCl2(L)],其中L = Laa (2a)或Lbb (2b),以及[PdCl2(L)],其中L = Laa (3a)或Lbb (3b)。2a和2b的晶体结构显示,螯合配体骨架分别采用δ扭转和扁平椅形构象。使用1a和1b作为催化剂对烯酰胺和脱氨基酸酯进行不对称氢化的实验表明,乙烯骨架的二膦配体Laa在不对称诱导方面比丙烯骨架的类似物Lbb更有效。使用1a和烯酰胺底物时,获得了最高的对映选择性,ee值高达91%。提出底物诱导的构象变化以解释在烯酰胺氢化过程中观察到的一些ee值。