A New Atropisomeric P,N Ligand for Rhodium-Catalyzed Asymmetric Hydroboration

Abstract

A new optically active and large dihedral angle atropisomeric P,N ligand, pyphos, which contains a tertiary phosphine and pyridine moiety, was prepared and resolved through diastereomeric complexation with chiral palladium amine complexes. The hexafluorophosphate salt of the diastereomers were found to be separable by fractional recyrstallization, while the corresponding chloride salt did not. [Rh(COD)pyphos]PF(6) complex was synthesized by metalation of pyphos with [Rh(COD)Cl](2) followed by anion exchange with NH(4)PF(6) in excellent yield, and the target rhodium complex was characterized by single-crystal X-ray crystallography. The chiral cationic rhodium complex was utilized in the enantioselective hydroboration of vinylarenes. Excellent regioselectivity and good enantioselectivity were observed, and the ee values were found to be dependent on the electronic properties of para-substituted styrenes.

The Development of Bidentate P,N Ligands for Asymmetric Catalysis

“…[172] To date only ligand 450 has been applied in asymmetric catalysis, Table 42, entries 18 ± 20. [173] As with QUINAP a strong dependence of enantioselectivity upon the electronic characteristics of the substrate was found. Interestingly the pyridyl unit of the ligand makes it possible to recycle 450 by a simple acidic extraction/neutralisation/organic extraction procedure.…”

The Development of Enantioselective Rhodium-Catalysed Hydroboration of Olefins

“…(R)-Pyphos (98) was applied in the enantioselective hydroboration of para-substituted vinylarenes and regioselectivities were excellent ( > 98 : 2 to 99 : 1) regardless of the substituent. [127] However, the reaction is only effective at low temperatures in contrast to other P,N ligands such as Quinap and the Quinazolinaps which induce high degrees of asymmetry, even at room temperature. Although the dihedral angle of (R)-Pyphos (98) (878) is much larger than those of QUINAP (82), PHENAP (87) or the Quinazolinaps 91 -96, (65 -678) comparable asymmetries were induced.…”

“…Hence the electron-rich substrate is more strongly influenced by the chiral environment which gives rise to enhanced enantioselectivity. [127] …”

In Quest of Factors That Control the Enantioselective Catalytic Markovnikov Hydroboration/Oxidation of Vinylarenes

“…[16] However, the nature of the chelating ligand is not the only parameter that affects both the regio-and enantioselectivity, the alkene and the temperature reaction also have a considerable effect. [17,18] The fact that small electronic and steric variations can lead to unpredictable differences in the asymmetric B-H addition to vinylarene substrates, in many cases leads to an experimentally frustrating "trial and error" sequence. In this context and based on a previous postulation, [19] we have recently used model systems from spectroscopic postulated key intermediates in the catalytic cycle, [20] to rationalise the role of the steric and electronic features of the ligands and substrates.…”