Jiuyuan Li

Asymmetric Supramolecular Primary Amine Catalysis in Aqueous Buffer: Connections of Selective Recognition and Asymmetric Catalysis

A new approach of asymmetric supramolecular catalysis has been developed by combining the supramolecular recognition of beta-cyclodextrin (beta-CD) and the superior property of a chiral primary amine catalyst. The resulted beta-CD enamine catalysts could effectively promote asymmetric direct aldol reactions with excellent enantioselectivity in an aqueous buffer solution (pH = 4.80). The identified optimal catalyst CD-1 shows interesting characteristics of supramolecular catalysis with selective recognition of aldol acceptors and donors. A detailed mechanistic investigation on such supramolecular catalysis was conducted with the aid of NMR, fluorescence, circular dichroism, and ESI-MS analysis. It is revealed that the reaction is initialized first by binding substrates into the cyclodextrin cavity via a synergistic action of hydrophobic interaction and noncovalent interaction with the CD-1 side chain. A rate-limiting enamine forming step is then involved which is followed by the product-generating C-C bond formation. A subsequent product release from the cavity completes the catalytic cycle. The possible connections between molecular recognition and asymmetric catalysis as well as their relevance to enamine catalysis in both natural enzymes and organocatalysts are discussed based on rational analysis.

Asymmetric SN1 α‐Alkylation of Cyclic Ketones Catalyzed by Functionalized Chiral Ionic Liquid (FCIL) Organocatalysts

Echoed with the sustainable chemistry concept, the developments of highly efficient and practical asymmetric catalysts have been and continue to be the fore of organic synthesis. Functionalized chiral ionic liquids (FCILs), which combine the green credential of ionic liquids and the catalytic principles of modern asymmetric catalysis, have recently appeared as such a type of asymmetric catalyst. Following the initial work of Howarth and Seddon; the applications of FCILs have moved beyond the use as chiral additives, chiral reagents, or chiral inducing reaction media, and now have reached the stage of asymmetric catalysts. However, the promising potentials of FCILs as asymmetric catalysts still remain largely underdeveloped at present and the current successes have been primarily confined to the reactions like Michael additions and aldol reactions. As such, in our continuing efforts in exploring this type of catal ACHTUNGTRENNUNGysis,[5d–f, n] we have sought new reaction development by taking advantage of the intrinsic properties of ionic liquids. Herein, we report the first FCIL-catalyzed enantioselective SN1 type alkylation of cyclic ketones, in which the ionic liquid moieties are found to be critical for both the catalytic activity and stereocontrol. The catalytic asymmetric direct a-alkylation reaction of carbonyl compounds has long been a daunting challenge in asymmetric catalysis and synthesis. Before the renaissance of organocatalysis, particularly the enamine-based amino ACHTUNGTRENNUNGcatACHTUNGTRENNUNGalysis, asymmetric phase-transfer catalysis was the only successful approach to this end and it was mostly used in the synthesis of a-amino acids through alkylation of glycine derivatives. In 2004, Vignola and List reported the first catalytic intramolecular nucleophilic a-alkylation of aldehydes by means of enamine activation. Later on, several cascade reactions including an intramolecular a-alkylation of aldehydes as the key step were also reported. However, the intermolecular a-alkylation of aldehydes or ketones are still a challenging task due to the depletion of catalytic activity through the N-alkylation of the aminocatalysts and various competing bypaths therewith. The past few years have witnessed notable breakthroughs in the development of asymmetric intermolecular a-alkylations of carbonyl compounds, specifically of aldehydes. Melchiorre and Cozzi have independently reported SN1 type aalkylation of aldehydes by enamine catalysis. This novel approach was based on alkyl donors endowed with suitable leaving groups (Scheme 1, I), which can generate a stabi-

Chiral Primary−Tertiary Diamine Catalysts Derived From Natural Amino Acids for syn-Aldol Reactions of Hydroxy Ketones

A series of primary-tertiary diamine catalysts were designed and synthesized from primary natural amino acids. Application of these new chiral catalysts in direct aldol reactions of alpha-hydroxyketones showed very good catalytic activity (up to 97% yield) and high syn selectivity (up to syn/ anti = 30:1, 99% ee).

Asymmetric Retro- and Transfer-Aldol Reactions Catalyzed by a Simple Chiral Primary Amine

One stone four birds: A single chiral primary amine was found to catalyze unprecedented asymmetric retro-aldol and transfer-aldol reactions, leading to four different chiral aldol adducts from one common chiral source with up to 99:1 d.r. and 99 % ee (see scheme).

Chiral Primary−Tertiary Diamine−Brønsted Acid Salt Catalyzed Syn-Selective Cross-Aldol Reaction of Aldehydes

Highly syn-selective cross-aldol reaction of aldehydes has remained a challenging subject in the field of aminocatalysis. To achieve this end, chiral primary amines have been explored and the primary-tertiary diamine-Brønsted acid salts are found to promote the cross-aldol reactions of aldehydes with high activity and syn selectivity. Among various vicinal diamines screened, l-phenylalanine derived 2a/TfOH conjugate is identified as the optimal catalyst, showing good catalytic activity (up to 97% yield) and high syn selectivities (syn/anti up to 24:1, 87% ee). The current catalysis works selectively with small aliphatic aldehydes donors such as propionaldehyde and isobutyraldehyde, but not with aliphatic aldehydes bearing larger beta-substitute (>Me). In addition, the use of 2a/TfOH conjugate has also enabled the first syn-selective cross-aldol reactions of glycoaldehyde donors.

Asymmetric Direct Aldol Reactions of Acetoacetals Catalyzed by a Simple Chiral Primary Amine

An asymmetric direct aldol reaction of acetoacetals is described. Under the catalysis of a simple chiral primary amine, the direct aldol reactions of acetoacetals occur exclusively on the gamma-position to give vinylogous-type aldol products with high diastereo- and enantioselectivity.

Tropospheric correction in VLBI phase-referencing using GPS data

Comparing the tropospheric zenith delays derived from VLBI and GPS data at VLBA stations collocated with GPS antenna, the systematic biases and standard deviations of the difference are both found to be at the level of a sub-centimeter. Based on this agreement, we used GPS data to correct the tropospheric effects in VLBI phase-referencing, resulting in close peak-to-noise ratios of images after tropospheric correction using GPS and VLBI data.