Rhodium-catalyzed hydrogenation BINAP

In contrast to the high enantioselectivity achieved for the Z-isomeric substrates, hydrogenation of the E-isomers usually proceeds with lower rates and afford products with diminished enantioselectivities [92]. The rhodium-catalyzed hydrogenation of the - and Z-isomers, with BINAP as a ligand in THE, affords products with opposite absolute configurations [16]. Remarkably, the DuPhos-Rh system provides excellent enantioselectivity for both isomeric substrates with the same absolute configuration, irrespective of the /Z-geometry (Eqs. 1 and 2). This result is particularly important for the construction of alkyl dehydroamino acid derivatives, which are difficult to prepare in enantiomericaUy pure form. 

Rate-determining step, hydroformylation, 163 Reactivity, enantiomers, 286 Recognition, enantiomers, 278 Reduction and oxidation, 5 Reductive coupling, dissolving metal, 288 Reductive elimination, 5, 111 Resolution. See Kinetic resolution Rhenium-carbene complexes, 288 Rhodium-catalyzed hydrogenation, 17, 352 amino acid synthesis, 18, 352 BINAP, 20... 

Modification of the electronic and steric properties of BINAP, BIPHEMP, and MeO-BI-PHEP led to the development of new efficient atropisomeric ligands. Although most of them are efficient for ruthenium-catalyzed asymmetric hydrogenation [3], Zhang et al. have recently reported an ortho-substituted BIPHEP ligand, o-Ph-HexaMeO-BIPHEP, for the rhodium-catalyzed asymmetric hydrogenation of cyclic enamides (Scheme 1.2) [31]. 

Tanaka et al. overcame this limitation by designing the enantioselective completely intramolecular double [2- -2-1-2] cycloaddition. The reaction of diphenylphosphinoyl-substituted hexayne 94, prepared from triyne 93 in two steps, in the presence of the cationic rhodium(I)/tol-BINAP catalyst furnished C2-symmetric axially chiral biaryl bisphosphine oxide 95 in moderate yield with high enantioselectivity (Scheme 9.32) [25]. Subsequent recrystallization andreduction of bisphosphine oxide 95 furnished the corresponding enantiopure bisphosphine 96, which could be used as an effective chiral ligand for rhodium-catalyzed asymmetric hydrogenation and cycloaddition [25]. 

Asymmetric Hydrogenation. The diene-free cationic rhodium complex of (R)-BINAP catalyzes the enantioselective hydrogenation of dehydroamino acids. a-(Benzoylamino)acrylic acid is hydrogenated at rt to afford (S)-lV-benzoylphenylalanine in 100% ee (eq 1). To obtain maximal stereoselectivity the reaction should be carried out under a low concentration of substrate (100% in 0.013 M vs. 62% in 0.15 M) and low initial hydrogen pressure (100% at 1 atm, but 71% at 50 atm). 

Asymmetric hydrogenation was boosted towards synthetic applications with the preparation of binap 15 by Noyori et al. [55] (Scheme 10). This diphosphine is a good ligand of rhodium, but it was some ruthenium/binap complexes which have found spectacular applications (from 1986 up to now) in asymmetric hydrogenation of many types of unsaturated substrates (C=C or C=0 double bonds). Some examples are listed in Scheme 10. Another important development generated by binap was the isomerization of allylamines into enamines catalyzed by cationic rhodium/binap complexes [57]. This reaction has been applied since 1985 in Japan at the Takasago Company for the synthesis of (-)-menthol (Scheme 10). 

In addition to rhodium phosphane complexes, ruthenium phosphane complexes have also been successfully applied as catalysis for enantioseleetive hydrogenation of 2-acylamino-2-alkenoic acids and esters1 71,72b 3, enol acetates 18 (R = i-Pr E = COOEt X = OCOCH3 98% ee with BINAP)137, and itaeonic acid138. The absolute configuration of the products from the ruthenium-catalyzed reactions shown below is opposite to that obtained with the corresponding rhodium catalysts. 

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