T. V. RajanBabu

Beyond Nature's Chiral Pool: Enantioselective Catalysis in Industry.

Enantioselective catalysts produce organic compounds in enantiomerically enriched form. They are highly efficient tools for the synthesis of biologically active materials, such as pharmaceuticals and crop-protection chemicals, in which enantiomeric purity can be critical. The design of chiral ligands is the key to developing new enantioselective catalysts. Three unusual families of ligands have been used to develop practical technology for enantioselective hydrocyanation of olefins, ring-opening of epoxides, and hydrogenation of various compounds.

Enantioselective desymmetrization of meso-aziridines with TMSN3 or TMSCN catalyzed by discrete yttrium complexes.

using a variety ofnucleophiles have been the subject of extensive research. Theless developed ring-opening reactions, those of meso-aziri-dines by carbon and nitrogen nucleophiles, give direct accessto enantiopure b-amino acids and 1,2-diamines—two classesof compounds which have broad chemical and pharmaceut-ical relevance. Li, Fernndez, and Jacobsen first reportedenantioselective ring-opening of meso-aziridines withTMSN

Asymmetric hydrovinylation of unactivated linear 1,3-dienes.

Monosubstituted acyclic (E)-1,3-dienes undergo efficient hydrovinylation giving (Z)-3-alkylhexa-1,4-dienes upon treatment with catalytic amounts of bidentate phosphine-CoCl(2) complexes {[P~P](CoCl(2))} and Me(3)Al in an atmosphere of ethylene. The regioselectivity of the reaction (i.e., 1,4- or 1,2-addition) depends on the nature of the ligand and temperature at which the reaction is carried out. Complexes derived from (RR)-DIOP and (SS)-BDDP at -45 degrees C give very high enantioselectivities for several prototypical 1,3-dienes. In sharp contrast to the corresponding Ni(II)-catalyzed hydrovinylation, 1-aryl-substituted 1,3-dienes give almost exclusively achiral linear 1,4-addition products, unless the 2-position is also substituted. Solid-state structures of the precatalysts are presented.

Regiodivergent Ring Opening of Chiral Aziridines

A bimetallic catalyst selects different ring-opening sites, depending on the chiral configuration of the substrate. Kinetic resolution of strained three-membered rings has proven broadly useful for the generation of enantiopure organic intermediates. Herein we demonstrate a complementary approach whereby a single catalyst transforms a racemic mixture of aziridines to a pair of regioisomeric products, each in good yield with exceptionally high enantioselectivity. Specifically, the dimeric yttrium salen catalyst accelerates the ring opening of aliphatic aziridines by trimethylsilylazide, inducing nucleophilic attack at the primary position of one enantiomer and the secondary position of the other. Both rate and selectivity are highly sensitive to the catalyst structure.

Electronic effects in asymmetric catalysis: Hydroformylation of olefins

Abstract Highly tunable carbohydrate vicinal diphosphinites are viable ligands for the Rh-catalyzed hydroformylation of olefins. Substitution of electron-withdrawing aryl groups at phosphorus in these diphosphinites increases the enantioselectivity of the hydroformylation process. Very high branched to linear ratios of product aldehydes (94%) were obtained. Thus far only moderate enantioselectivity (up to 72%) has been achieved.

Conformationally Driven Asymmetric Induction of a Catalytic Dendrimer

Natures catalysts promote the reactions necessary for life with extremely high specificity by folding into specific shapes capable of communicating remote structural information to an active site. Achieving this objective in synthetic systems has been hampered by the lack of information concerning how dynamic conformational chirality can influence the stereoselectivity of a catalytic process. Herein, we report the first illustration of a catalytic dendrimer that achieves high enantioselectivity by amplifying/propagating local chirality via a dynamically folded structure. Experimental evidence supports a chiral relay mechanism that propagates local terminal chirality of the dendron to the axial chirality of the biphenyl core through the helical secondary structure of the dendron.

Efficient, Selective, and Green: Catalyst Tuning for Highly Enantioselective Reactions of Ethylene

Fine tuning of the biaryl and amino moieties of Feringas phosphoramidite ligands yields structurally simpler, yet more efficient and selective, ligands for asymmetric hydrovinylation of vinylarenes and acylic 1,3-dienes. The enantioselectivities and yields observed in the formation of the 3-arylbutenes are among the highest for all asymmetric catalytic processes reported to date for the synthesis of intermediates for the widely used antiinflammatory 2-arylpropionic acids including naproxen, ibuprofen, fenoprofen, and flurbiprofen.

Ligand Tuning in Asymmetric Hydrovinylation of 1,3-Dienes. A Stereoselective Route to either Steroid-C20 (S) or -C20 (R) Derivatives

1,3-Dienes derived from steroidal D-ring C 17-ketones undergo Ni(II)-catalyzed hydrovinylation to give 1,2- or 1,4-addition of ethylene. Using finely tuned phosphoramidite ligands, it is possible to synthesize either the C 20 ( R)- or ( S)-derivatives without mutual contamination. The proportion of the 1,4-adduct, which is also formed stereoselectively, can be minimized by optimizing the reaction conditions. Because the two alkenes in the resultant dienes have differing steric demands for many potential reactions, and are ideally juxtaposed for further D-ring functionalization, these intermediates could be useful for the preparation of biologically important compounds such as vitamin D analogs and various antitumor steroidal glycosides.

Hydrovinylation and Related Reactions: New Protocols and Control Elements in Search of Greater Synthetic Efficiency and Selectivity

New reaction conditions and stereochemical control elements for heterodimerization between ethylene (or propylene) and functionalized vinyl arenes are highlighted (see equation). For example, an enantioselective version of the hydrovinylation reaction uses [{(allyl)NiBr}2], a noncoordinating counter anion, (bis-CF3−C6H3)4B−, and a hemilabile ligand such as MOP. Other applications include intramolecular cyclization of 1,6-dienes and heterodimerization of norbornene and ethylene.

Asymmetric hydrovinylation of vinylindoles. A facile route to cyclopenta[g]indole natural products (+)-cis-trikentrin A and (+)-cis-trikentrin B.

Vinylindoles undergo Ni(II)-catalyzed asymmetric hydrovinylation under very mild conditions (-78 °C, 1 atm ethylene, 4 mol % catalyst) to give the corresponding 2-but-3-enyl derivatives in excellent yields and enantioselectivities. Hydroboration of the alkene and oxidation to an acid, followed by Friedel-Crafts annulation, gives an indole-annulated cyclopentanone that is a suitable precursor for the syntheses of cis-trikentrins and all known herbindoles. For example, the cyclopentanone from 4-ethyl-7-vinylindole is converted into (+)-cis-trikentin A in four steps (Wittig reaction, alkene isomerization, diastereoselective hydrogenation, and nitrogen deprotection). The previous synthesis of this molecule from (S)-(-)-malic acid involved more than 20 steps and a preparative HPLC separation of diastereomeric intermediates.