Rh(I) COMPLEXES CONTAINING FULLY ALKYLATED MONO- AND DIPHOSPHINE LIGANDS AS HIGHLY ACTIVE HYDROGENATION CATALYSTS FOR CARBONYL COMPOUNDS

Abstract

Our search for highly active hydrogenation catalysts for carbonyl compounds, starting with neutral Rh(I) hydride complexes, [RhH(PR3)n] (R=i-Pr, n=3; R=Cy, n=2), has led to the discovery of cationic Rh(I) complexes with fully alkylated diphosphine ligands, [Rh{(i-Pr)2P(CH2)nP(i-Pr )2}(NBD)]C104 (n=3,4). These compounds prove to be versatile and efficient for hydrogenation of a variety of carbonyl compounds, including aldehydes.A number of rhodium complexes, e.g., neutral rhodium(I) complexes of Wilkinson type, [RhXL3] (L=triarylphosphines) and cationic rhodium(I) complexes containing an aryl-substituted diphosphines, were found to be active catalysts for olefin hydrogenation.)) These metal complexes, however, are not very active for ketone hydrogenation.)) An enhancement of the activity of rhodium complex catalysts was observed on the addition of strong alkali, i.e., [RhCI(C8H12)(PPh3)]-NaBH4-KOH2a) or [RhCl2(bpy)2]Cl-NaOH.2b) In this paper we wish to report that the catalytic activity of rhodium(I) species for hydrogenation of carbonyl compounds can be markedly improved with electron-donating fully alkylated phosphines.Our principal strategy was to increase the electron density of the metal center by utilizing the electron-donating trialkylphosphines together with a hydrido ligand. Since [RhH{P(i-Pr),},]their activity was tested for several ketones (Table 1). The results were encouraging.[RhH(PCy3)2] was even active for PhCOPh which could not readily be hydrogenated by Osborn's system, [Rh-(PPhMe2)H2(S)2]+-H20 (S=solvent), under ambient conditions. 4) This is rather remarkable since the Rh(I) hydride containing triarylphosphine, e.g., [RhH(DBP)4] (DBP=5-phenyl-5H-dibenzophosphole) is known to be totally inactive as a hydrogenation catalyst for ketones.5) Table 1 also includes results of transfer hydrogenation of some ketones effected with [RhHhexanone is mainly (94%) trans-4-tert-butylcyclohexanol in contrast to the predominant formation of the cis-product effected by IrCl6 -P(OMe)3 in acidic medium.6) In the latter case an incipient protonation of the carbonyl oxygen atom may probably be involved, followed by the hydride transfer to the less hindered carbonyl face. The predominant formation of trans-alcohol in the present case suggests an extensive thermodynamic control which in turn implies a rapid reverse the overall catalytic rate with Rh(I) complexes of monodentate trialkylphosphines should have been impaired by rapid dehydrogenation of the product alcohol.

Heterogenized Wilkinson‐Type Catalyst for Transfer Hydrogenation of Carbonyl Compounds

“…[6] Later, RuCl 2 (PPh 3 ) 3 was used as hydrogen transfer catalyst, [7] and in 1991 it was reported that the addition of small amounts of base had a dramatic effect in the RuCl 2 (PPh 3 ) 3 -catalyzed transfer hydrogenation of ketones giving a rate enhancement of about 10 3 to 10 4 times. [8] Also, (phosphane)rhodium(I) complexes can catalyze the transfer hydrogenation of aldehydes and ketones by 2-propanol, [9] and it was found that base also accelerates these reactions, [10] but not to the same extent as the RuCl 2 (PPh 3 ) 3 -catalyzed transfer hydrogenations. [8] To make the Rh system more favorable for scale-up, we heterogenized Wilkinson's catalyst on an easily available solid support to become recyclable.…”

Bis(phospholane) Ligands Containing Chiral Backbones. Matching and Mismatching Effects in Enantioselective Hydrogenation of α-Keto Esters

“…In terms of hydrogenation rates, perusal of the literature indicates that a certain degree of backbone flexibility (larger available bite angle) within a diphosphine ligand is necessary to achieve good catalytic activity in keto group hydrogenation reactions. Thus, a three- or four-carbon diphosphine backbone tends to afford much more active catalysts relative to two-carbon-bridged systems . Also, the electronic character of the ligand appears important; more electron-rich (e.g., more highly alkylated) phosphines evoke higher rates in Rh-catalyzed hydrogenation of carbonyl groups. , On the basis of these two concepts, we recently developed rhodium catalysts based upon the achiral ligand 1,1‘-bis(diisopropylphosphino)ferrocene (DiPFc) 9a.…”

O–H Activation and Addition to Unsaturated Systems

“…6.17), the hydrogenation of CO to MeOH, hydrogen transfer reactions and alcohol homologation. However, the successful isolation of such complexes from the catalytic systems was very rare [32][33][34][35][36][37]. [38].…”