Seminars 2010

Title:
A Small Molecule with a Big Impact: Inspirations from Proline Catalysis
Speaker:Professor Benjamin List
Date:13 December 2010 
Time:11.00am to 12.30pm 
Venue:
NTU SPMS CBC Building Level 2, Conference Room 
Host:Assistant Professor Robin Chi Yonggui
Abstract:

Organocatalysis, the catalysis with low-molecular weight catalysts where a metal is not part of the catalytic principle, can be as efficient and selective as metal- or biocatalysis. Important discoveries in this area include novel Lewis based-catalyzed enantioselective processes and, more recently, BrØnsted acid organocatalysts that rival the efficiency of traditional metal-based asymmetric Lewis acid-catalysts. Contributions from our laboratories include several new amine- and amino acid-catalyzed asymmetric reactions. We have discovered the proline-catalyzed direct asymmetric intermolecular aldol reaction. Soon after this we have developed the enamine catalysis concept and introduced novel asymmetric Mannich-, Michael-, α-amination-, intramolecular enolexo-aldolization, and aldehyde α-alkylation reactions. In addition, collaborative efforts have provided a clearer mechanistic understanding of enamine catalysis and provided the basis for the design of new reactions and catalysts. Today, we begin to appreciate how proline catalysis and the thinking about how it may actually work have inspired several stimulating concepts for catalysis.

Our latest work deals with chiral anions in asymmetric catalysis. We have recently developed a powerful concept for asymmetric catalysis, asymmetric counteranion directed catalysis (ACDC). This concept is useful for organocatalysis, transition metal catalysis, and Lewis acid catalysis alike. 

 

Title:
Discovery and Exploitation of AZADOs: The Highly Active Organocatalysts for Alcohol Oxidation
Speaker:Professor Yoshiharu Iwabuchi
Date:8 December 2010 
Time:2.15pm to 3.45pm 
Venue:
NTU SPMS CBC Building Level 2, Conference Room 
Host:Assistant Professor Shunsuke Chiba
Abstract:

Stable organic nitroxyl radicals, as exemplified by 2,2,6,6-tetramethypiperidinyl-1-oxy (TEMPO: 1), find their indispensable utility in chemistry and biology. We have particularly been interested in their use as oxidation catalyst that allows facile and efficient conversion of alcohols to the corresponding carbonyl compounds with a safe bulk oxidant such as bleach.

The widely accepted mechanism of TEMPO-catalyzed oxidation of alcohols is shown in Scheme 1 where an oxoammonium ion plays a crucial role. Note that TEMPO exhibits remarkable chemoselectivity of preferring primary alcohols rather than secondary alcohols due to the inherent steric hindrance around the catalytic center. In this connection, TEMPO meanwhile hesitate oxidation of sterically crowded secondary alcohols, exposing a limitation of its synthetic applicability. We designed AZADO (azaadamantan N-oxyl) type nitroxyl radical by merging N-oxyl moiety into adamantane skeleton, with expectation of enhancing robustness and accessibility toward substrates. Thus, it was envisaged that the α-hydrogen of nitroxyl group within AZADO skeleton should resist to participating in the well-known isomerization to give a nitrone because of the Bredt’s rule and, therefore, should allow facile access of varieties of substrates to the active center, enabling them to proceed the subsequent oxidation events. We established a reliable synthetic route to 1-Me-AZADO (2) starting from 1-adamantanol. 1-Me-AZADO exhibit superb catalytic activity even for oxidation of sterically crowded secondary alcohols that TEMPO gave insufficient result.

In this seminar, I wish to describe about the versatility of AZABO-catalyzed alcohol oxidation

 

Title:
Palladium-Catalyzed Oxidative Functionalization of Alkenes
Speaker:Professor Liu Guosheng
Date:8 December 2010 
Time:11.00am to 12.30pm
Venue:
NTU SPMS CBC Building Level 2, Conference Room 
Host:Assistant Professor Robin Chi Yonggui
Abstract:

Reactions that selectively functionalize unactivated alkenes provide a direct route from feedstock chemicals to valuable synthetic molecules. Traditional alkene functionalization routes rely upon highly electrophilic reagents such as strong acids, boron hydrides, dihalides, or stoichiometric high oxidation-state metals to activate the weakly nucleophilic double bond. An attractive alternative involves alkene activation by a catalytic transition metal, a process that activates the double bond for attack by a broad range of nucleophiles. For examples, palladium(II)-catalyzed alkene functionalization has attracted fervent attention, and over the past half century efficient methods for the construction of C-O, C-N and C-C bonds have been developed.

In this presentation, two types Pd-catalyzed oxidative functionalizaiton will be discussed: the first part will focus on Pd-catalyzed allylic C–H bond oxidative functionalization with highly regioselectivity;1 the second part will describe the Pd-catalyzed difunctionalization of double bond, especial on the aminofluorination, in which two different pathways were applied to construct C-F bond.2

 

Title:
Tuning of surface structure and property with organic layers and its application in organic electronic devices 
Speaker:Professor Tao Yu-Tai
Date:7 December 2010 
Time:10.45am to 12.15pm 
Venue:
NTU SPMS CBC Building Level 2, Conference Room 
Host:Professor Loh Teck Peng
Abstract:

The surface of a solid substrate can be systematically modified by the adsorption of properly chosen organic molecules. In particular, the adsorption of a well-ordered monolayer can alter the wetting, polarity (dipole moment), chemical reactivity, lattice, and surface potential at the surface. These in turn affect the packing, orientation and morphology of organic film that is deposited on the surface. In this talk, we demonstrate the application of self-assembled monolayers (SAMs) in modulating the interfacial structure commonly involved in organic electronic devices, such as organic light-emitting diodes, organic field-effect transistors and organic memories.

The barrier for charge injection from the electrode to the organic layer can be varied systematically and continuously with SAMs/mixed SAMs. Implication to the charge balance in OLED devices will be examined. Effect of SAMs modification of dielectric layer on the morphology and charge mobility of OFET devices will be also be examined. Finally, SAMs were used to modulate the surface property and charge trapping capability of Au nanoparticles in the fabrication of transistor/memory devices. 

 

Title:
New Directions in Electrochemical Imaging: Probes, Modes and Combinations
Speaker:Professor Patrick Unwin
Date:6 December 2010 
Time:2.00pm to 3.30pm 
Venue:
NTU SPMS CBC Building Level 2, Conference Room 
Host:Nanyang Assistant Professoer Martin Pumera
Abstract:

Electrochemical imaging methods have thrived and expanded over the past two decades as a consequence of continued developments in instrumentation, theory and a diversity of applications. These developments have taken electrochemical imaging methods beyond the field of electrochemistry into chemistry in general and its borders with materials science, engineering and the life sciences. With a diversity of applications come opportunities to greatly enhance the capability of such methods, particularly with regard to multifunctional imaging and pushing these methods to the nanoscale. This contribution will provide an overview of work from Warwick in this area and highlight some recent and ongoing developments that are seeking to develop multifunctional electrochemical imaging through:

(i)the use of new types of probes, particularly new variants of the scanning micropipette contact method (SMCM)1 to allow high resolution investigations of electrode and mineral surfaces, resolving surface topography and activity unambiguously;
(ii)the development of completely new imaging modes, specifically intermittent contact- scanning electrochemical microscopy (IC-SECM)2 and scanning electrochemical cell microscopy (SECCM)3  that allow the activity, topography and properties of surfaces and interfaces to be resolved simply and effectively. These developments build on our earlier pioneering work with combined scanning electrochemical microscopy-atomic force microscopy (SECM-AFM)4
(iii)combinations of SECM with spectroscopic techniques, such as fluorescence confocal laser scanning microscopy (CLSM)5 to allow rapid (millisecond) visualization of diffusion and heterogeneous reactions, including the tracking of membrane transport.

These various methods will be illustrated with applications including: the elucidation of complex electrode activity, including novel carbon electrodes (nanotubes and conducting diamond) and alloys; probing the behavior of biomaterials; and mapping the properties of biomimetic systems and, ultimately, living cells.

We are grateful to the European Research Council, EPSRC and the Advantage West Midlands Science Cities programme for generous support.

 

Title:
A Journey through a Vanadium Trail: Some Retrospection
Speaker:Professor Muktimoy Chaudhury
Date:3rd December 2010 
Time:11.00am – 12.30pm
Venue:
NTU SPMS CBC Building Level 2, Meeting Room 
Host:Associate Professor Leong Weng Kee
Abstract:

This lecture consists of two parts. The first part deals with the chemistry of μ-oxo divanadium compounds containing [V2O3]n+ (n = 4, 3 and 2) cores. Compounds containing such cores have been widely studied in recent time because of there high thermodynamic stability and ease of synthesis. In literature at least thirty such compounds have been reported for which the crystal structures are known. All these compounds have symmetrical structures involving identical ligand molecules attached to both the vanadium centers. The purpose of the work presented in this part is to develop synthetic protocols to introduce ligand asymmetry in to the [V2O3]n+ cores and to study the influence of this asymmetry on the structural and spectroscopic behaviors of these compounds. The methodology developed has been extended to synthesize hetero-dinuclear compounds containing discrete V-O-M (M = Mn3+, Fe3+ and Cr3+) cores.

The second part centers around a single crystal-to-single crystal (SCSC) transformation observed with a binuclear alcoholato-vanadium(V) compound to an oligomeric aldehyde-coordinated vanadium(IV) product. This irreversible transformation is triggered by photo-induced generation of phenoxyl radical that initiates an internal redox process where vanadium(V) is reduced to vanadium(IV) at the expense of a two-electron ligand-based oxidation from an alcoholate to an aldehyde. Such solid-state SCSC reactions impose severe demands on the structural similarity between the reactant and the product and are consequently quite rare. Attempts will be made to establish the mechanism of this internal redox process based on extensive X-ray diffraction study, IR and EPR spectroscopy.

 

Title:
Asymmetric N-Phosphonyl and N-Phosphinyl Imine Chemistry  (Part I & II)
Speaker:Professor Guigen Li
Date:Part I : 29 November 2010 
 Part II: 1 December 2010 
Time:Part I: 11.00am to 12.30pm 
 Part II: 2.30pm to 4.00pm
Venue:
NTU SPMS CBC Building Level 2, Conference Room
Host:Part I: Associate Professor Li Tianhu
 Part II: Assistant Professor Robin Chi Yonggui
Abstract:

The asymmetric catalytic Strecker reaction of a series of achiral N-phosphonyl imines with metal cyanide sources that are non-toxic and non-volatile has been established. Metal species formed from BINOLs and amino alcohols derivatives have been proven to be effective catalysts to afford excellent enantioselectivities (up to >99%ee) and yields (up to 96%). N-phosphonyl group can be readily cleaved under mild conditions and enable purification of crude products simply by washing with hexane. The cleaved N,N-dialkyl diamine auxiliary can be recovered quantitatively via n-BuOH extraction. Extensive scope for both N-phosphonyl imines and catalysts was seen for this new catalytic system

 

Title:
Amide-based N,N’-dioxide and Bisguanidine Catalysts in Asymmetric Reactions
Speaker:Professor Feng Xiaoming
Date:26 November 2010 
Time:11.00am to 12.15pm 
Venue:
NTU SPMS CBC Building Level 2, Conference Room
Host:Professor Loh Teck Peng
Abstract:

Amides are pervasive in nature and technology as structural materials which could be easily obtained from natural amino acids. Based on the strategy of bifunctional activation, we introduced Lewis base functional groups, such as N-oxide and guanidine, into chiral amide group to construct new organocatalysts or ligands.

C2-symmetric N,N’-dioxide could serve as both organocatalysts in asymmetric cyanosilylation reactions, chlorination of β-keto esters with NCS, and chiral ligands. Powerful chiral N,N'-dioxide-metal complex catalysts have realized numbers of asymmetric transformations. Recently, the first highly efficient asymmetric Roskamp reaction of a-alkyl-a-diazoesters with aromatic aldehydes,1 and asymmetric bromoamination of chalcones2 have been discovered by N,N'-dioxide-Sc(III) complex. The desired enantioenriched products could be afforded with 0.01-0.05 mol% catalyst loading.

The molecular containing guainidine and amide group could also perform as efficient bifunctional organocatalysts. Asymmetric 1,4-addition of β-ketoesters to nitroolefins demonstrate high stereoselectivities and yields.3 An unexpected inverse-electron-demand Diels-Alder reaction of chalcones with azlactones has been realized by C2-symmetric bisguanidine catalyst.4 Chiral γ,δ-unsaturated δ-lactones containing a quaternary stereocenter were performed in excellent stereoselectivities with minor Michael adduct. 

 

Speaker:Professor Yao Zhu-Jun
Date:26 November 2010 
Time:9.45am to 11.00am 
Venue:
NTU SPMS CBC Building Level 2, Conference Room
Host:Nanyang Assistant Professor Steve Zhou
Abstract:

1. Chemical Biology Studies on Annonaceous Acetogenin Mimetics

Annonaceous acetogenins are a unique family of plant metabolites exhibiting remarkable cytotoxicity against a variety of tumor cells. Successful simplification of natural bullatacin (a representative acetogenin) has been achieved. Among these, AA005 was found to exhibit remarkable anti-cancer activities and present significant selectivity between human cancer and normal cells. A parallel synthetic strategy was then established and applied to the focused libraries. Mechanisms have also been studied using chemical biology principles, including acquirement of various biochemical tools by synthetic chemistry and subsequently applications to cell biology research. 

2. New Synthesis of Camptothecin Alkaloids

Camptothecin (CPT) is a pentacyclic alkaloid firstly isolated from the Chinese medicinal plant Camptotheca acumunata. Its potent anticancer activities and unique action mechanisms by inhibition of DNA topoisomerase I have made it an excellent lead of drug design in the past several decades. Continuous successful advances in medicinal chemistry of CPT have resulted in the discovery of a number of CPT derivatives with medicinal interests. The recent achievements in the chemical syntheses of CPT-family alkaloids will be reported utilizing our recently developed cascade multiple-bond formation methodologies. 

 

Title:
Platinum and Gold Catalyzed Cycloisomerization of Enynes, Allenynes and Allene Dienes
Speaker:Professor Max Malacria
Date:24 November 2010 
Time:11.00am to 12.30pm 
Venue:
NTU SPMS CBC Building Level 2, Conference Room
Host:Nanyang Professor Koichi Narasaka
Abstract:

We have recently shown that the unprecedented PtCl2-catalyzed cycloisomerization of allenyne systems is a highly versatile tool for obtaining products that cannot be attainable with other metals . Simple adjustment of the allene or alkyne substitution can direct the reactivity in a completely selective manner and give birth to important carbocyclic platforms : hydrindienes, cyclic vinylallenes, and trienes.

The gold catalysis has provided deeper insight into the mechanism of  the cycloisomerization of allenynes. Because no standard metallacyclic route could be envisaged, two unusual mechanistic pathways which take into account the presence of cationic intermediates have been proposed : a 1,5-proton shift for the Alder-ene products and a 1,5-hydride shift for the vinyl allenes. We also found evidence for an intriguing halide effect that completely alters the cycloisomerisation process.

Finally, we will present an expedient method for the synthesis of polycyclic compounds from propargyl acetates or vinyl allenes involving up to three Au(I)-catalyzed elemental steps : 3,3-rearrangement, metalla-Nazarov reaction, and electrophilic cyclopropanation.

These rearrangements proceed with chirality transfer and have been applied to the synthesis of a natural product.

 

Title:
Amino Acid-based Organocatalysts for Enantioselective Organic Transformations
Speaker:Associate Professor Lu Yixin
Date:22 November 2010 
Time:11.00am to 12.30pm 
Venue:
NTU SPMS CBC Building Level 2, Conference Room
Host:Assistant Professor Xing Bengang
Abstract:

Amino catalysis mediated by primary amino acids/amines has been intensively investigated recently and established as a powerful activation mode in asymmetric synthesis. In this context, our research group has developed and utilized an array of primary amino acid-based organocatalysts in enantioselective organic reactions in the past few years, and our contributions in this area will be presented. It is noteworthy that many of our reported reactions were better than those achieved with reactions promoted by pyrrolidine-based secondary amine catalysts.

To further expand the uses of primary amino acids in asymmetric catalysis, we recently started a research program towards the development of novel and versatile organocatalysts based on amino acid structures. We introduced a novel tryptophan-based bifunctional thiourea catalyst that was remarkably effective in promoting enantioselective Mannich reaction of a-fluoro-b-ketoesters, and the details of this study will be described in this talk. Our most recent examples of applying primary amino acid-based nucleophilic phosphine catalysts and trifunctional catalysts with amino acid scaffolds in asymmetric catalysis will also be discussed in this lecture, and the mechanistic insights to understand these catalytic processes will be presented. 

 

Title:
Metal Nanoparticle Catalysts for Organic Reactions under Chemically Green Conditions
Speaker:Professor Jaiwook Park
Date:15 November 2010 
Time:11.00am to 12.30pm 
Venue:
NTU SPMS CBC Building Level 2, Conference Room
Host:Nanyang Professor Kim Sunggak

 

Title:
Stereoselective Annulation of Carbo- and Heterocyclic Compounds via Gold
Catalysis: Focus on Internal Redox Reaction
Speaker:Associate Professor Shin Seunghoon
Date:27 October 2010 
Time:11.00am to 12.30pm 
Venue:
NTU SPMS CBC Building Level 2, Conference Room
Host:Assistant Professor Hong Soon Hyeok 
Abstract:
The advantage of using gold complexes for catalysis derives from their exceptional ability to activate alkyne, allene, and alkene toward nucleophilic attack. When they activate alkyne, the resulting intermediates somewhat resemble classical
cationic intermediates that are highly reactive and often follow a cascade cyclization pathway for a rapid assembly of useful organic compounds. In the context of 1,5-enyne cyclization, a comparative study between gold-catalyzed and iodonium-promoted reactions will be presented and the mechanistic similarities/differences will be discussed. On the other hand, preliminary results on the intramolecular addition of tertiary amines onto alkynes forming quaternary ammonium salts will be presented and the interesting finding of unusual acidity at alpha-C-H bond will be disclosed.

 

Second aspects that our group is focusing on, is redox-economical reactions displayed by N-O bond substrates. The readily available hydroxylamine derivatives function as atom-economical oxidant and gold-catalyzed redox reactions of these substrates generate a variety of highly reactive intermediates, such as carbene, azomethine ylide, imine and enolate for tandem annulations. For example, we have established a new synthetic equivalence between readily available alkynyl nitrones and 3-oxidopyridinium betains for inter- and intramolecular [3+2] cycloaddition. Also, further utilization of this reactivity revealed a redox-pinacol-Mannich-Michael cascade for rapid assembly of pharmaceutically interesting heterocycles as well as 3-pyrrolidinones and nitrones. Through these examples, I hope to demonstrate that hydroxylamine derivatives that are cheap and operationally safe can be appealing oxidant that shows a versatile reactivity through N-O bond cleavage.