Asymmetric hydrogenation reactions with Rh and Ru complexes bearing phosphine–phosphites with an oxymethylene backbone

Abstract

Phosphine-phosphites 3a and 3b, derived from diphenylhydroxymethyl phosphine have been prepared. From these ligands [Rh(COD)(3a)]BF 4 (5a) and RuCl 2 (3b)[(S,S)-DPEN] (6b, DPEN = 1,2-diphenylethylenediamine) were synthesized and their structure determined by X-ray diffraction. Ligands 3 are characterized by a small bite angle of 83º. In addition, 5a led to an active catalyst for the hydrogenation of olefins, giving enantioselectivities up to 96 % ee. Likewise, compound 6b showed a good activity and enantioselectivity in the hydrogenation of N-1-phenyl ethylidene aniline and completed a reaction at S/C = 500 in 24 h with a 83 % ee.

Features and Application in Asymmetric Catalysis of Chiral Phosphine–Phosphite Ligands

“…For instance, for Pd(Cl) 2 (P‐OP) complexes, distances between 2.227 and 2.272 Å have been reported for the phosphine group (Table , entries 1–5), while the corresponding distances for the phosphite fragment are in the range between 2.187 and 2.214 Å 31–33. This trend is also observed in the case of [Rh(diolefin)(P‐OP)]BF 4 complexes (Table , entries 8–14), with ranges of 2.265–2.410 and 2.182–2.220 Å for the phosphine and phosphite fragments, respectively 34–38. A similar trend is observed in an analogous Ir complex (Table , entry 15) 27.…”

“…On the other hand, there is substantial information about the structure of cationic rhodium complexes of formula [Rh(diolefin)(P‐OP)] + (diolefin=1,5‐cyclooctadiene, 1,3‐norbornadiene). In this class of complexes, the Rh–C bonds corresponding to the olefin fragment trans to the phosphite ligand are generally longer than those trans to the phosphine (Table , entries 8–14) 34–38. Finally, for Ru(Cl) 2 (DPEN)( L7h ) (DPEN=1,2‐diphenylethan‐1,2‐diamine) ligands, no clear trend was observed for the Ru–N bond lengths 42…”

“…It is remarkable that very similar results were obtained with a catalyst containing L16b , characterized by an ( R )‐BINOL fragment, which suggested that a similar atropisomerization to that mentioned above operated for L16a ., Pizzano and co‐workers also observed high enantioselectivities in the hydrogenation of N‐aryl imines catalyzed by Ru complex Ru(Cl) 2 ( L7i )( S,S )‐DPEN, as characterized by a similar phosphite fragment and a chiral diamine 42. Notably, in the structure of related complex Ru(Cl) 2 ( L3c )( S,S )‐DPEN, the phosphite occupies a cis position to both nitrogen atoms, which produces a close proximity between Ph substituents of the diamine and the biphenyl moiety; thus favoring an R configuration of the latter (Figure ) 35…”

Metal-Catalyzed Asymmetric Hydrogenation of C═N Bonds

“…While using the optimal precatalyst Ir- L19 , N -arylimine derivatives were hydrogenated under mild conditions (i.e., room temperature and 4 atm of H 2 ) furnishing the chiral amines with full conversion and high ee (81–96%). In some cases, with electron-poor substrates, a higher pressure of H 2 was necessary to reach the full conversion of the starting substrates due to a lower reactivity and selectivity …”

“…In some cases, with electron-poor substrates, a higher pressure of H 2 was necessary to reach the full conversion of the starting substrates due to a lower reactivity and selectivity. 30 Interestingly, promising reactivity was observed while using the much simpler P(OMe) 3 monodentate phosphite ligand in combination with the chiral pybox (2,6-bis(oxazoline)pyridine) L21 ligand (Scheme 18). 31 This ligand combination allows the generation of the highly active Ru-complex, able to catalyze the highly enantioselective reduction of a small panel of N-aryl imines.…”