/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.

The first imidazoline-mediated highly enantioselective Friedel—Crafts-type reaction between aldimines and naphthols is reported.

C3‐Symmetric Chiral Trisimidazoline‐Catalyzed Friedel—Crafts (FC)‐Type Reaction.

Diastereoselective and enantioselective domino cyclization/cycloaddition reactions of alkenyl oximes were established using a Pd(II)–( R )-Tol-SDP complex and triflic acid. The present process gave polycyclic heterocycles with four chiral stereogenic centers in almost quantitative yields and high stereoselectivities (up to 70% ee, exo / endo  = 97/3).

Pd(II)-Catalyzed Diastereoselective and Enantioselective Domino Cyclization/Cycloaddition Reactions of Alkenyl Oximes for Polycyclic Heterocycles with Four Chiral Stereogenic Centers.

This study processes a facile and green approach for the Markovnikov-selective hydroamination of styrene with naphthylamine through irradiation with UV LED light (365 nm) via an electron donor–acceptor complexation between naphthylamines and oxygen in situ. This protocol showcases the synthetic potential for aerobic C–N bond formation without using a metal catalyst and photosensitizer. Three naphthylamines were examined and afforded desired C–N bond formation product in moderate yield.

https://www.mdpi.com/1420-3049/28/1/356/pdf?version=1672569304

Metal-Free Aerobic C–N Bond Formation of Styrene and Arylamines via Photoactivated Electron Donor–Acceptor Complexation

Rare earth-Li-BINOL complexes were used to catalyze nitroaldol reactions of α,α-difluoro adehydes with nitromethane in a good enantioselective manner. The optical yields of nitroaldol 2a depend on the size of rare earth metals, and an Sm-Li-BINOL complex gave the highest enantioselectivity. The enantiotopic face selection of α,α-difluoro aldehyde 1a was found to be different from that of nonfluorinated aldehydes employed in the usual catalytic asymmetric nitroaldol reactions.

Hiroaki Sasai

Papers

C3‐Symmetric Chiral Trisimidazoline‐Catalyzed Friedel—Crafts (FC)‐Type Reaction.

Pd(II)-Catalyzed Diastereoselective and Enantioselective Domino Cyclization/Cycloaddition Reactions of Alkenyl Oximes for Polycyclic Heterocycles with Four Chiral Stereogenic Centers.

Metal-Free Aerobic C–N Bond Formation of Styrene and Arylamines via Photoactivated Electron Donor–Acceptor Complexation

Catalytic Asymmetric Nitroaldol Reaction of α,α‐Difluoro Aldehydes Mediated by Rare Earth‐Lithium‐BINOL Complexes.

P‐Chirogenic Organocatalysts: Application to the Aza‐Morita—Baylis—Hillmann (Aza‐MBH) Reaction of Ketimines.