Asymmetric catalysis by chiral hydrogen-bond donors

A Powerful Chiral Counterion Strategy for Asymmetric Transition Metal Catalysis

Traditionally, transition metal-catalyzed enantioselective transformations rely on chiral ligands tightly bound to the metal to induce asymmetric product distributions. Here we report high enantioselectivities conferred by a chiral counterion in a metal-catalyzed reaction. Two different transformations catalyzed by cationic gold(I) complexes generated products in 90 to 99% enantiomeric excess with the use of chiral binaphthol-derived phosphate anions. Furthermore, we show that the chiral counterion can be combined additively with chiral ligands to enable an asymmetric transformation that cannot be achieved by either method alone. This concept of relaying chiral information via an ion pair should be applicable to a vast number of metal-mediated processes.

Recent Developments in Asymmetric Transfer Hydrogenation with Hantzsch Esters: A Biomimetic Approach

By utilizing Hantzsch esters as the hydrogen source, asymmetric transfer hydrogenation of C=C, C=N, and C=O is realized in the presence of an organocatalyst or a metal-ligand complex, thus affording versatile chiral building blocks in high yields with excellent enantioselectivities under mild conditions. A detailed discussion of recent findings and an assessment of this biomimetic approach are presented in this review.

Non-racemic atropisomeric (thio)ureas as neutral enantioselective anion receptors for amino-acid derivatives: origin of smaller Kass with thiourea than urea derivatives

The synthesis of a limited series of non-racemic atropisomeric 1-(2-(4-methyl-2-thioxothiazol-3(2H)-yl)phenyl)-3-(hetero)aryl-(thio)ureas is described. Using NMR titration experiments monitoring the shift of the two NH of the (thio)urea and the C-5 hydrogen of the heterocycle, the binding constants for some optically pure (thio)-ureas with the enantiomers of N-protected amino acid tetrabutylammonium salts were determined in CD3CN. The obtained enantioselectivities were modest. Contrary to what was expected on the basis of the NH acidity in thiourea versus urea group, the association constants were smaller with the thiourea than with the corresponding urea. X-ray data, DFT calculations, and NMR provided the explanation of that unexpected behavior: the urea presents a pre-organized (Z,Z) conformation suitable for a double hydrogen bond with the carboxylate anion, the thiourea presents a (Z,E) conformation, which must be reorganized in a constrained (Z,Z) conformation in the complex. An intramolecular hydrogen bond between one NH and the thiocarbonyl group of the heterocycle, which is present in the thiourea and absent in the urea, might also contribute to the smaller K(ass) for the thiourea. The possible implication of these observations in the field of bifunctional organocatalysis is briefly discussed.

Peptidic catalysts developed by combinatorial screening methods

In recent years, oligopeptides have been developed as efficient catalysts for a range of important organic reactions, including acylation, silylation, oxidation, ester hydrolysis and aldol reactions. With many peptidic catalysts, high yields and chiral induction can be achieved under mild reaction conditions. Discovery and optimization of these catalysts typically involves the testing of compound collections and is therefore strongly linked to advances in combinatorial screening methods. This review summarizes recent developments in the field of catalytically active short-chain peptides, highlighting the combinatorial techniques that have facilitated their discovery.

Enantioselectively Organocatalytic Michael Addition of Ketones to Alkylidene Malonates

An organocatalytic asymmetric Michael addition of ketones to alkylidene malonates has been developed. In the presence of 20 mol % of urea 1a or N-(pyrrolidin-2-ylmethyl)trifluoromethanesulfonamide 1j, the reactions of ketones with alkylidene malonates afford the desired Michael adducts in moderate to good yields with good to high enantioselectivities under mild conditions.

Enantioselective Organocatalysis Using SOMO Activation

The asymmetric α-addition of relatively nonpolar hydrocarbon substrates, such as allyl and aryl groups, to aldehydes and ketones remains a largely unsolved problem in organic synthesis, despite the wide potential utility of direct routes to such products. We reasoned that well-established chiral amine catalysis, which activates aldehydes toward electrophile addition by enamine formation, could be expanded to this important reaction class by applying a single-electron oxidant to create a transient radical species from the enamine. We demonstrated the concept of singly occupied molecular orbital (SOMO) activation with a highly selective α-allylation of aldehydes, and we here present preliminary results for enantioselective heteroarylations and cyclization/halogenation cascades.

Chiral 1,1'-binaphthyl-2,2'-diammonium salt catalysts for the enantioselective Diels-Alder reaction with alpha-acyloxyacroleins

[reaction: see text] A diammonium salt of chiral 1,1'-binaphthyl-2,2'-diamine and trifluoromethanesulfonimide (Tf(2)NH) shows excellent catalytic activity and enantioselectivity for the Diels-Alder reaction of alpha-acyloxyacroleins with cyclic dienes. For example, in the presence of 5 mol % of the ammonium catalyst, the Diels-Alder reaction of alpha-(cyclohexanecarbonyloxy)acrolein with cyclopentadiene proceeded in EtCN at -75 degrees C to give the adducts in 88% yield with 92% exo and 91% ee. This catalyst can be easily prepared in situ by mixing the commercially available chiral diamine and Tf(2)NH.

Asymmetric Organocatalytic Domino Reactions

The current status of organic synthesis is hampered by costly protecting-group strategies and lengthy purification procedures after each synthetic step. To circumvent these problems, the synthetic potential of multicomponent domino reactions has been utilized for the efficient and stereoselective construction of complex molecules from simple precursors in a single process. In particular, domino reactions mediated by organocatalysts are in a way biomimetic, as this principle is used very efficiently in the biosynthesis of complex natural products starting from simple precursors. In this Minireview, we discuss the current development of this fast-growing field.

Chemoselective Nucleophilic Fluorination Induced by Selective Solvation of the SN2 Transition State

Reaction of the fluoride ion with secondary alkyl halides leads to 90% of elimination reaction and only 10% of nucleophilic substitution in dipolar aprotic solvents. Adding water to the organic phase, the SN2 yield increases in the cost of decreased reactivity. Using ab initio calculations, we have shown that it is possible to increase the reaction rate and the selectivity toward the SN2 process through supramolecular organocatalysis. The catalytic concept is based on selective solvation of the transition state through two hydrogen bonds provided by the 1,4-benzenedimethanol. The two hydrogen bonds between the catalyst and the SN2 transition state favor this pathway while just one strong hydrogen bond between the catalyst and the fluoride ion leads to a lower stabilization of the nucleophile, resulting in a higher reaction rate. Our calculations predict that the substitution product increases to 40% yield because of the selective catalysis provided by the 1,4-benzenedimethanol.

A reductase-mimicking thiourea organocatalyst incorporating a covalently bound NADH analogue: efficient 1,2-diketone reduction with in situ prosthetic group generation and recycling

A new class of bifunctional organocatalyst promotes the chemoselective reduction of diketone electrophiles at catalytic loadings in the presence of an inorganic co-reductant.

Cupreines and cupreidines: an emerging class of bifunctional cinchona organocatalysts

In the steadily expanding field of organocatalysis, cinchona alkaloids play a prominent role. Until the late 1990s, bifunctional catalysts based on this scaffold relied exclusively on the C9-hydroxy group as the hydrogen-bond donor. Recently, new cinchona catalysts have been developed that feature a phenolic OH group in the C6' position-a structural feature that allows a diverse set of reactions to be catalyzed in a highly stereoselective fashion. This Minireview describes the scope and modes of action of this new class of asymmetric bifunctional organocatalysts.

Mechanism of hydrogen bond assisted cis-trans isomerization in the excited state of 2-naphthol

Stationary points on the ground and first two excited state potential energy surfaces of 2-naphthol and the hydrogen bonded complex 2-naphtholNH(3) have been located using an approximate method (time dependent density functional based tight-binding) based on time dependent density functional theory (TDDFT). For the obtained extrema, single point calculations at the full TDDFT level (PW916-311+G(**)) were performed for validation purposes. The energetical ordering of the cis and trans forms of 2-naphthol in the different electronic states is found to be in good agreement with available experimental data. Interestingly, also the optical spectrum is correctly predicted by TDDFT in contrast to calculations on unsubstituted acenes in the literature. Besides the stationary points, also the minimum energy paths connecting them are calculated using an adaptive nudged elastic band approach in order to estimate isomerization barriers. While the increase of barrier height going from the ground to the first excited state is already documented, the authors find as a new result a significant barrier reduction upon hydrogen bond formation in the S(2). The observed effect can contribute to the explanation of recent experiments on infrared laser induced cis-trans isomerization in 2-naphthol.

An enantioselective Brønsted acid catalyzed enamine Mannich reaction

An enantioselective Brønsted acid catalyzed Mannich reaction between acetophenone derived enamines and N-Boc imines has been developed. Simple diol (S)-H(8)-BINOL has been identified as the optimal catalyst, to afford versatile beta-amino aryl ketones in good yield and enantiomeric excess.