Development of an axially chiral sp3P/sp3NH/sp2N-combined linear tridentate ligand—fac-selective formation of Ru(II) complexes and application to ketone hydrogenation

Abstract

A newly developed chiral linear tridentate ligand, R-PN(H)N (R = H or Ph), possesses Ph2P and PyCH2NH groups at C(2) and C(2') positions of the 1,1'-binaphthyl skeleton without or with a C(3)-Ph substituent. The steric effect of C(3)-Ph and the electronic effect of the DMSO coligand realize the facial selective generation of fac-RuCl2(Ph-PN(H)N)(dmso) and fac-[Ru(H-PN(H)N)(dmso)3](BF4)2, respectively. Both an H-Ru-sp 3 N-H reaction site responsible for the donor-acceptor bifunctional catalyst (DACat) and a fence/plane chiral context were constructed by means of the following advantageous points: i) the sp 3 P, sp 3 N, and sp 2 N ligating atoms have different electronic properties; ii) DMSO trans to sp 3 N strongly coordinates to Ru and is fixed by a PyC(6)H-O=S hydrogen bond; and iii) the single NH function simplifies the DACat reaction site. The synergistic effect has led to success in the asymmetric hydrogenation of sterically demanding ketones. Structural characteristics of first-row transition metal complexes of R-PN(H)N have been also investigated.

Bifunctional Homogeneous Catalysts Based on Ruthenium, Rhodium and Iridium in Asymmetric Hydrogenation

“…Kitamura and co-workers developed a chiral ruthenium(II) complex based on an axially chiral PN(H)N ligand, called binan-Py-PPh2, and containing dimethylsulfoxides as additional ligands. [176][177] Once combined with a catalytic amount of base and an excess of dimethylsulfoxide, this complex catalysed effectively the challenging AH of functionalized and unfunctionalized tert-alkyl ketones, the resulting alcohols being obtained in high yields, with good to excellent enantioselectivities (Scheme 43). [176][177] Scheme 43.…”

“…[176][177] Once combined with a catalytic amount of base and an excess of dimethylsulfoxide, this complex catalysed effectively the challenging AH of functionalized and unfunctionalized tert-alkyl ketones, the resulting alcohols being obtained in high yields, with good to excellent enantioselectivities (Scheme 43). [176][177] Scheme 43. AH of sterically hindered ketones using a PN(H)N ligand-based ruthenium catalyst, by Kitamura and co-workers.…”

“…AH of sterically hindered ketones using a PN(H)N ligand-based ruthenium catalyst, by Kitamura and co-workers. [176][177] Kitamura and co-workers investigated the mechanism of these catalysed hydrogenations through structural analyses, NMR experiments including isotope labelings, kinetic and rate law studies as well as control experiments (Scheme 44). [177][178] At first, the linear and flexible tridentate ligand PN(H)N coordinated to ruthenium and formed selectively a fac-octahedral complex by reacting with DMSO.…”

Ruthenium(II) complexes bearing (NNN) ligand: catalytic evaluation of different solvent-mediated coordination modes

“…Considering the distinctive features of the complexes here proposed, we then evaluated their capability to be used as precatalysts in asymmetric transfer hydrogenation of different aryl ketones (Table 1). 29 12,30 The cooperation of the metal hydride and the amine nitrogen hydrogen functions in the ATH of ketones and imines with 2-propanol or triethylammonium formate as the hydrogen source, was a key discovery by Noyori group and this represented an outstanding example of outer-sphere mechanism. [31][32][33][34] This last one remains the generally accepted mechanism for hydrogen-transfer step in the case of complex A, in which only two nitrogen atoms resulted coordinated to metal center.…”

“…[8][9][10] On the other hand, there is a great interest in transition metal complexes with tridentate amine ligands, especially those containing a pyridine due to its unique electronic and steric properties. 8,[11][12][13][14][15][16][17] Some of them have good efficiency in the catalytic epoxidation reaction [18][19] and in transfer hydrogenation of aryl ketones. 20 Starting from this consideration here is reported the synthesis of a new tridentate ligand 1 (Figure 1) and the study of its different coordination modes to a ruthenium metal center.…”