/pdf/self-assembled-monolayers-of-thiolates-on-metals-as-a-form-1neb0hj3k4.pdf

Self-assembled monolayers of thiolates on metals as a form of nanotechnology.

About Supramolecular Assemblies of π-Conjugated Systems

Asymmetric enamine catalysis.

Most enzymatic transformations have a synthetic counterpart. Often though, the mechanisms by which natural and synthetic catalysts operate differ markedly. The catalytic asymmetric aldol reaction as a fundamental C-C bond forming reaction in chemistry and biology is an interesting case in this respect. Chemically, this reaction is dominated by approaches that utilize preformed enolate equivalents in combination with a chiral catalyst.1 Typically, a metal is involved in the reaction mechanism.1d Most enzymes, however, use a fundamentally different strategy and catalyze the direct aldolization of two unmodified carbonyl compounds. Class I aldolases utilize an enamine based mechanism,2 while Class II aldolases mediate this process by using a zinc cofactor.3 The development of aldolase antibodies that use an enamine mechanism and accept hydrophobic organic substrates has demonstrated the potential inherent in amine-catalyzed asymmetric aldol reactions.4 Recently, the first small-molecule asymmetric class II aldolase mimics have been described in the form of zinc, lanthanum, and barium complexes.5,6 However, amine-based asymmetric class I aldolase mimics have not been described in the literature.7 Here we report our finding that the amino acid proline is an effective asymmetric catalyst for the direct aldol reaction between unmodified acetone and a variety of aldehydes. Recently we developed broad scope aldolase antibodies that show very high enantioselectivities, have enzymatic rate accelerations, and use the enamine mechanism of class I aldolases.4 During the course of these studies, we found that one of our aldolase catalytic antibodies (Aldolase Antibody 38C2, Aldrich) is an efficient catalyst for enantiogroup-differentiating aldol cyclodehydrations of 2,6-heptanediones to give cyclohexenones, including the Wieland-Miescher ketone.8,9 These intramolecular reactions are also catalyzed by proline (Hajos-Eder-Sauer-Wiechert reaction)10 and it has been postulated that they proceed via an enamine mechanism.11 However, the proline-catalyzed direct intermolecular asymmetric aldol reaction has not been described. Further, there are no asymmetric small-molecule aldol catalysts that use an enamine mechanism.7 Based on our own results and Shibasaki’s work on lanthanum-based small-molecule aldol catalysts,4,6 we realized the great potential of catalysts for the direct asymmetric aldol reaction. We initially studied the reaction of acetone with 4-nitrobenzaldehyde. Reacting proline (30 mol %) in DMSO/acetone (4:1) with 4-nitrobenzaldehyde at room temperature for 4 h furnished aldol product (R)-1 in 68% yield and 76% ee (eq 1). This result

https://www.scripps.edu/barbas/pdf/List00JACS.pdf

Proline-Catalyzed Direct Asymmetric Aldol Reactions

In studying the evolution of organic chemistry and grasping its essence, one comes quickly to the conclusion that no other type of reaction plays as large a role in shaping this domain of science than carbon-carbon bond-forming reactions. The Grignard, Diels-Alder, and Wittig reactions are but three prominent examples of such processes, and are among those which have undeniably exercised decisive roles in the last century in the emergence of chemical synthesis as we know it today. In the last quarter of the 20th century, a new family of carbon-carbon bond-forming reactions based on transition-metal catalysts evolved as powerful tools in synthesis. Among them, the palladium-catalyzed cross-coupling reactions are the most prominent. In this Review, highlights of a number of selected syntheses are discussed. The examples chosen demonstrate the enormous power of these processes in the art of total synthesis and underscore their future potential in chemical synthesis.

Palladium-catalyzed cross-coupling reactions in total synthesis.

New Synthesis of 4-Acetoxy-2-azetidinones by Use of Electrochemical Oxidation.

Novel chiral spiro bis(isoxazoline) ligands 3 and 4 are synthesized from an optically active olefin. The Cu(II)-spiro bis(isoxazoline) complex prepared in situ from Cu(OTf)2 and 3 promoted the glyoxylate-ene reaction in high yield and moderate enantioselectivity (up to 70%).

Enantioselective Glyoxylate‐ene Reaction Using a Novel Spiro Bis(isoxazoline) Ligand in Copper Catalysis.

Pd(II)—SDP-Catalyzed Enantioselective 5-exo-dig Cyclization of γ-Alkynoic Acids: Application to the Synthesis of Functionalized Dihydrofuran-2(3H)-ones Containing a Chiral Quaternary Carbon Center.

The reaction of cinnamyl chloride with indole in the presence of a ruthenium catalyst possessing a planar-chiral cyclopentadienyl ligand resulted in the substitution at the 3-position of the indole...

Regio- and Enantioselective Allylation of Indole Catalyzed by a Planar-Chiral Cyclopentadienyl—Ruthenium Complex.

Several chiral Lewis acid catalysts were prepared from lanthanoid trifluoromethanesulfonate (triflate) and a chiral sulfonamide ligand, and were applied to the Mukaiyama aldol reaction. The reaction proceeded in a catalytic manner with moderate enantiomeric excess.

Hiroaki Sasai

Papers

New Synthesis of 4-Acetoxy-2-azetidinones by Use of Electrochemical Oxidation.

Enantioselective Glyoxylate‐ene Reaction Using a Novel Spiro Bis(isoxazoline) Ligand in Copper Catalysis.

Pd(II)—SDP-Catalyzed Enantioselective 5-exo-dig Cyclization of γ-Alkynoic Acids: Application to the Synthesis of Functionalized Dihydrofuran-2(3H)-ones Containing a Chiral Quaternary Carbon Center.

Regio- and Enantioselective Allylation of Indole Catalyzed by a Planar-Chiral Cyclopentadienyl—Ruthenium Complex.

Preparation of a New Asymmetric Lanthanoid Complex Catalyst for the Mukaiyama Aldol Reaction.