Expanding the Scope of Atropisomeric Monodentate P‐Donor Ligands in Asymmetric Catalysis: Hydrogen‐Transfer Reduction of α,β‐Unsaturated Acid Derivatives by Rhodium/Ph‐binepine Catalysts

Abstract

Dedicated to Professor Giambattista Consiglio, a good friend and excellent chemist, for his remarkable contributions to the advancement of stereoselective catalysis A range of a,b-unsaturated acids and esters have been selectively reduced to the corresponding saturated acid derivatives by hydrogen transfer. As the reducing agent, formic acid was used in the presence of Rh I complexes formed with the powerful chiral ligand Ph-binepine (1), an axially chiral binaphthalenetype monodentate P-donor ligand. Very high stereoselectivities (up to 97% ee) were obtained in the case of itaconic acid (2a).Introduction. -The reduction of unsaturated functional groups by means of transition-metal-mediated hydrogen transfer from a suitable hydrogen donor such as i-PrOH or HCOOH has experienced increasing success over the last years [1]. This mild methodology represents a viable alternative to catalytic hydrogenation by molecular H 2 due to its operational simplicity and reduction of the risks associated with the use of an easily inflammable gas of high diffusibility. Significant advances in this area have been achieved in the asymmetric reduction of ketones and imines with catalysts of excellent activity/selectivity based on Ru and Rh complexes with monotosyl-substituted diamines or amino alcohols as chiral modifiers [1a].Less efforts have been devoted to the transfer hydrogenation of C=C bonds. Although this process is thermodynamically favored, even when alcohols are used as H-donors, only the reduction of conjugated C=C bonds is of practical significance, while simple alkenes and dienes are poorly reactive. A perusal of the literature shows that transfer hydrogenation of conjugated acid derivatives such as itaconic acid and a-(acetamido)cinnamic acid selectively takes place at the C=C bond, and stereoselectivities higher than 90% can be sometimes attained in this reaction when performed in the presence of Ru [2] or Rh complexes [3] with chiral bidentate diphosphines such as binap [2], bppm [3a -f], or deguphos [3g]. No example of the application of monodentate phosphines as chiral inducers in this reaction has been reported thus far, in spite of the excellent performances displayed by binaphthalene-templated monoA C H T U N G T R E N N U N G dentate P-ligands [4], such as phosphonites [5], phosphites [6], and phosphoramidites [7], in the Rh-catalyzed hydrogenation of a wide variety of substrates.

BINEPINES: chiral binaphthalene-core monophosphepine ligands for multipurpose asymmetric catalysis

“…Cationic [Rh(nbd)(5a) 2 ] + X À derivatives, where nbd is norbornadiene and X À is a noncoordinating anion (CF 3 SO 3 À ; BF 4 À ; PF 6 À ; SbF 6 ; BaRF À ) show in solution a NMR pattern consistent with the presence of two units of P-ligand coordinated to the metal and an overall C 2 -symmetry of the complex. 21 The same structural features can be extracted from the NMR spectra of neutral dichloro and cationic ditriflate Pt-complexes synthesized by reacting Pt(cod)Cl 2 (cod = 1,5-cycloctadiene) with two equivalents of 5a followed by treatment of the dichloro derivative with aqueous silver triflate. 22 Crystals suitable for X-ray analysis have been obtained for two metal complexes containing just one unit of (S)-Ph-BINEPINE 5a: the cyclopalladated complex with the (R)-phenethylamine derivative 24 6 and the cycloplatinated complex 25a 23 (Fig.…”

New Ruthenium Catalysts for Asymmetric Transfer Hydrogenation of Prochiral Ketones

“…[21] However, only ligand (S)-4 showed comparable enantioselectivity to PPh 3 of 95 % ee, while (S)-5 gave poor selectivity and conversion, due to a similar basicity to achiral alkylphosphines (Table 3, entry 16 and 17).…”

Synthesis and application of chiral monodentate phosphines in asymmetric hydrogenation

“…A more convincing approach to obtain enantiopure itaconic acid was recently reported by one of us (Scheme 35) [68]. Excellent enantioselectivities up to 97% ee were obtained when subjecting itaconic acid to a rhodium-catalyzed transfer hydrogenation under mild reaction conditions in the presence of well-defined cationic [Rh(nbd)(21a) 2 ] + complexes.…”