Dramatic Ligand Effect in Catalytic Asymmetric Reductive Aldol Reaction of Allenic Esters to Ketones

Copper-catalyzed recycling of halogen activating groups via 1,3-halogen migration

A Cu(I)-catalyzed 1,3-halogen migration reaction effectively recycles an activating group by transferring bromine or iodine from a sp(2) to a benzylic carbon with concomitant borylation of the Ar-X bond. The resulting benzyl halide can be reacted in the same vessel under a variety of conditions to form an additional carbon-heteroatom bond. Cross-over experiments using an isotopically enriched bromide source support intramolecular transfer of Br. The reaction is postulated to proceed via a Markovnikov hydrocupration of the o-halostyrene, oxidative addition of the resulting Cu(I) complex into the Ar-X bond, reductive elimination of the new sp(3) C-X bond, and final borylation of an Ar-Cu(I) species to turn over the catalytic cycle.

Construction of contiguous tetrasubstituted chiral carbon stereocenters via direct catalytic asymmetric aldol and Mannich-type reactions

Catalytic asymmetric synthesis of unnatural amino acids with vicinal tetrasubstituted chiral carbon stereocenters is described. In the first part, direct catalytic asymmetric aldol reaction of simple non-activated ketone electrophiles with α-substituted α-isothiocyanato ester donors was realized. A Mg/Schiff base catalyst promoted the aldol reaction, and α-amino-β-hydroxy esters were obtained in up to 98% ee and 98:2 d.r. In the second part, the Mg/Schiff base catalyst and a Sr/Schiff base catalyst were utilized for stereodivergent direct asymmetric Mannich-type reaction of ketimines. The Mg/Schiff base catalyst gave syn-α,β-diamino esters, while the Sr/Schiff base catalyst produced anti-α,β-diamino esters in good to high enantioselectivity, up to 97% ee.

A straightforward synthesis of cyclobutenones via a tandem Michael addition/cyclization reaction of 2,3-allenoates with organozincs

An efficient method for synthesis of polysubstituted cyclobutenones, which are not readily available from traditional methods due to the intrinsic ring strain, is described. The reaction of 2,3-allenoates and organozinc reagents proceeds via a tandem Michael addition/cyclic 1,2-addition/elimination mechanism with the functional groups from the organozinc reagents being introduced to the 3-position of the cyclobutenone products regiospecifically in moderate to excellent yields. Application to the synthesis of stereodefined β,γ-unsaturated enones is demonstrated.

Catalytic enantioselective olefin metathesis in natural product synthesis. Chiral metal-based complexes that deliver high enantioselectivity and more

Chiral olefin metathesis catalysts enable chemists to access enantiomerically enriched small molecules with high efficiency; synthesis schemes involving such complexes can be substantially more concise than those that would involve enantiomerically pure substrates and achiral Mo alkylidenes or Ru-based carbenes. The scope of research towards design and development of chiral catalysts is not limited to discovery of complexes that are merely the chiral versions of the related achiral variants. A chiral olefin metathesis catalyst, in addition to furnishing products of high enantiomeric purity, can offer levels of efficiency, product selectivity and/or olefin stereoselectivity that are unavailable through the achiral variants. Such positive attributes of chiral catalysts (whether utilized in racemic or enantiomerically enriched form) should be considered as general, applicable to other classes of transformations.

Enantioselective copper-catalyzed reductive Michael cyclizations

In the presence of siloxanes as stoichiometric reductants, chiral copper-bisphosphine complexes catalyze highly enantioselective reductive Michael cyclizations of substrates containing two alpha,beta-unsaturated carbonyl moieties. The diastereochemical outcome of these reactions is dependent upon whether biaryl- or ferrocene-based chiral bisphosphines are employed.

Diastereo- and Enantioselective Reductive Aldol Addition of Vinyl Ketones via Catalytic Hydrogenation

An overview of studies on hydrogenative reductive aldol addition is presented. By simply hydrogenating enones in the presence of aldehydes at ambient temperature and pressure, aldol adducts are generated under neutral conditions in the absence of any stoichiometric byproducts. Using cationic rhodium complexes modified by tri(2-furyl)phosphine, highly syn-diastereoselective reductive aldol additions of vinyl ketones are achieved. Finally, using novel monodentate TADDOL-like phosphonite ligands, the first highly diastereo- and enantioselective reductive aldol couplings of vinyl ketones were devised. These studies, along with other works from our laboratory, demonstrate that organometallics arising transiently in the course of catalytic hydrogenation offer byproduct-free alternatives to preformed organometallic reagents employed in classical carbonyl addition processes.