DOCSLIB.ORG

The Combination of Lewis Acid with N-Heterocyclic Carbene (NHC) Catalysis

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
The Combination of Lewis Acid with N-Heterocyclic Carbene (NHC) Catalysis catalysts Review The Combination of Lewis Acid with N-Heterocyclic Carbene (NHC) Catalysis Qianfa Jia 1 , Yaqiong Li 1, Yinhe Lin 1 and Qiao Ren 2,* 1 Chongqing Key Laboratory of Inorganic Special Functional Materials, College of Chemistry and Chemical Engineering, Yangtze Normal University, Fuling, Chongqing 408100, China; [email protected] (Q.J.); [email protected] (Y.L.); [email protected] (Y.L.) 2 College of Pharmaceutical Science, Southwest University, Chongqing 400715, China * Correspondence: [email protected]; Tel.: +86-23-6825-1225 Received: 16 September 2019; Accepted: 14 October 2019; Published: 16 October 2019 Abstract: In the last ten years, the combination of Lewis acid with N-heterocyclic carbene (NHC) catalysis has emerged as a powerful strategy in a variety of important asymmetric synthesis, due to the ready availability of starting materials, operational simplicity and mild reaction conditions. Recent findings illustrate that Lewis acid could largely enhance the efficiency and enantioselectivity, reverse the diastereoselectivity, and even influence the pathway of the same reaction partners. Herein, this review aims to reveal the recent advances in NHC-Lewis acid synergistically promoted enantioselective reactions for the expeditious assembly of versatile biologically important chiral pharmaceuticals and natural products. Keywords: N-heterocyclic carbenes (NHC); Lewis acid; cooperative catalysis; asymmetric synthesis; umpolung 1. Introduction N-heterocyclic carbenes (NHCs) are roughly categorized into three sections in accordance with their properties and applications: (i) excellent ligands for transition metals; (ii) coordination to p-block elements and (iii) organocatalysts [1–7]. The first N-heterocyclic carbene stabilized by two bulky adamantyl substituents was isolated and characterized by the Arduengo group in 1991 [8], which opened up an intriguing class of organic compounds for investigation. So far, a variety of novel carbenes have been revealed and synthesized, for instance, the dominant thiazolium-, imidazolium- and triazolium-based carbenes [9–12]. As powerful and efficient organocatalysts, they have been employed successfully for the synthesis of highly complex molecular architectures [13–18]. However, the stereoselectivities and/or regioselectivities of assembled products were limited for the single mode of activation during NHC catalyzed processes. Inspired by the importance and advantages of the cooperative catalysis strategy [19–25], which could activate the starting materials simultaneously with satisfying enantio- and stereoselectivities, N-heterocyclic carbenes as an important class of Lewis bases can cooperate with various Lewis acids to enhance yield and reverse selectivity or regioselectivity [26]. This strategy has emerged as a powerful approach for the direct access to various carbocyclic and heterocyclic compounds [27–32]. The recent developments in NHC/Lewis acid cooperative catalysis for the synthesis of some important enantioenriched molecules will be discussed in this review. They are categorized into several sections according to the species of the Lewis acid, including LiCl, Mg(Ot-Bu)2, Sc(OTf)3, La(OTf)3, Ti(Oi-Pr)4, etc. Due to the unique umpolung capacity, NHCs are widely applied in a variety of asymmetric transformations, resulting in the construction of versatile active acyl anions, enolates, homoenolates and α,β-unsaturated acylazolium equivalents from the corresponding carbonyl Catalysts 2019, 9, 863; doi:10.3390/catal9100863 www.mdpi.com/journal/catalysts Catalysts 2019, 9, 863 2 of 23 Catalysts 2019, 9, x FOR PEER REVIEW 2 of 23 compoundscarbonyl compounds (Scheme1 )[(Scheme33–41], 1) while [33–41 Lewis], while acids Lewis as co-catalysts acids as co improve-catalysts the improve reactivities the reactivities or activate inactiveor activate electrophiles. inactive Theelectrophiles. related exciting The discoveriesrelated exciting involving discoveries NHCs/ Lewisinvolving acid cooperativeNHCs/Lewis catalysis acid arecooperative presented catalysis herein. are presented herein. SchemeScheme 1. 1. CarbonylCarbonyl compounds compounds and corresponding and corresponding N-heterocyclic carbeneN-heterocyclic (NHC)-bound carbene (NHC)-bound intermediates. intermediates. 2.2. Cooperative Cooperative NHCNHC/Mg/Mg CatalysisCatalysis ThoughThough NHCs NHCs have have been been proved proved to to be be good good ligands ligands for for many many transition transition metals metals on accounton account of their of strongtheir strong donor properties,donor properties the Scheidt, the Scheidt group reported group repo a pioneeringrted a pioneering investigation investigation and made and an importantmade an breakthrough,important breakthrough which defies, conventionalwhich defies wisdom conventional with respect wisdom to the potentialwith respect incompatibility to the potential of Lewis acidsincompatibility and bases. ofγ-lactam Lewis acids derivatives and bases. 2 with γ- highlactam enantioselectivities derivatives 2 with and high diastereoselectivities enantioselectivities wereand obtaineddiastereoselectivit by a formalies [3were+ 2] obtained cycloaddition by a formal of N-acyl [3 +hydrazones 2] cycloaddition 1 and cinnamaldehydeof N-acyl hydrazones (Scheme 1 and2). Thecinnamaldehyde results indicated (Scheme that the 2) employment. The results of Mg(Oindicatedt-Bu) 2thatcould the increase employment the yield of of Mg(γ-lactamOt-Bu) distinctly2 could fromincrease 31% the to yield 78% andof γ-lactam enhance distinctly the enantioselectivity from 31% to 78% slightly and enhance in the the presence enantioselectivity of 5 mol% slightly of NHC catalyst.in the presence Subsequently, of 5 mol% the substratesof NHC catalyst. scope was Subsequently, surveyed and the asubstrates broad range scope of functionalitieswas surveyed turnedand a outbroad to berange well of accommodated. functionalities turned In these out processes, to be we carbenell accommodated. precursor AIn wasthese deprotonated processes, carbene by the baseprecursor 1,5,7-triazabicyclo[4.4.0]dec-5-ene A was deprotonated by the (TBD)base 1,5,7 followed-triazabicycl by additiono[4.4.0 to]decα,β-5-unsaturated-ene (TBD) followed aldehyde by to produceaddition homoenolate to α,β-unsaturated equivalent aldehyde 4 by to raising produce the homoenolate HOMO energy. equivalent Simultaneously, 4 by raising magnesium the HOMO (II) di-energy.tert-butoxide Simultaneously, was selected magnesium as the optimal (II) di-tert Lewis-butoxide acid towas activate selectedN-acyl as thehydrazones optimal Lewis by lowering acid to theactivate LUMO N- energy.acyl hydrazones Further kineticby lowering studies the indicated LUMO energy. that the Further reaction kinetic initial-rate studies emerged indicated an that inverse the first-orderreaction initial relation-rate for emerge Mg (II)d an concentration inverse first- [order42]. relation for Mg (II) concentration [42]. TheThe Zhao Zhaogroup group reportedreported thethe pioneeringpioneering work on the kinetic resolution resolution of of tertiary tertiary alcohols alcohols 6 6 by by NHC-catalyzedNHC-catalyzed esterificationesterification underunder oxidativeoxidative conditionsconditions (Scheme(Scheme3 3).). TheThe presencepresence ofof Mg(OTf)Mg(OTf)22 andand NaBFNaBF44in in thisthis catalyticcatalytic systemsystem turnedturned out to be eefficientfficient to improve both selectivity selectivity and and reaction reaction rate. rate. AA broad broad range range of of substrates substrates was was investigated investigated and and displayed displayed the the practicability practicability of this of protocol.this protocol In most. In cases,most thecases, unreacted the unreacted starting tertiary starting alcohols tertiary 8were alcohols recovered 8 were in high recovered to excellent in high enantioselectivities. to excellent Theenantioselectivit proposed mechanismies. The proposed revealed mechanism that the Lewis revealed acid that Mg(OTf) the Lewis2 may acid activate Mg(OTf) the2 substratemay activate in a cooperativethe substrate way in to a benefitcooperative the presumable way to benefit attack ofthe tertiary presumable alcohol attack on acyl of azolium tertiaryintermediate alcohol on acyl from theazolium opposite intermediate side of the catalyst’sfrom the chiralopposite backbone. side of However, the catalyst the oxindole’s chiral structurebackbone. was However, proved tothe be criticaloxindole for structure this system was to proved work smoothly to be critical [43 ].for this system to work smoothly [43]. Catalysts 2019, 9, 863 3 of 23 CatalystsCatalysts 2019 2019, 9, ,9 x, xFOR FOR PEER PEER REVIEW REVIEW 33 of of 23 23 Scheme 2. NHC/Mg(Ot-Bu)2 strategy for the stereoselective synthesis of γ-lactams. SchemeScheme 2. NHC/Mg(ONHC/Mg(Ot-Bu)-Bu)2 sstrategytrategy for the s stereoselectivetereoselective synthesis of γ--lactams.lactams. SchemeScheme 3. 3.Kinetic Kinetic resolutionresolution ofof tertiarytertiary alcoholsalcohols by NHC and Lewis acid. Scheme 3. Kinetic resolution of tertiary alcohols by NHC and Lewis acid. Catalysts 2019, 9, 863 4 of 23 Catalysts 2019, 9, x FOR PEER REVIEW 4 of 23 InIn 2018,2018, WangWang andand co-workersco-workers discloseddisclosed aa [3[3+ + 3]3] atroposelectiveatroposelective annulationannulation ofof alkynylalkynyl acylacyl azoliumsazoliums 1515 withwith 1,3-dicarbonyls1,3-dicarbonyls 1212 (Scheme (Scheme4 ).4) 3,3. 3,3′0,5,5,5,5′0-tetratert-butyldiphenoquinone-tetratert-butyldiphenoquinone (DQ)(DQ) asas anan externalexternal oxidant,oxidant, acted
Load more

Recommended publications
  • Fischer Carbene Complexes in Organic Synthesis Ke Chen 1/31/2007
    Baran Group Meeting Fischer Carbene Complexes in Organic Synthesis Ke Chen 1/31/2007 Ernst Otto Fischer (1918 - ) Other Types of Stabilized Carbenes: German inorganic chemist. Born in Munich Schrock carbene, named after Richard R. Schrock, is nucleophilic on November 10, 1918. Studied at Munich at the carbene carbon atom in an unpaired triplet state. Technical University and spent his career there. Became director of the inorganic Comparision of Fisher Carbene and Schrock carbene: chemistry institute in 1964. In the 1960s, discovered a metal alkylidene and alkylidyne complexes, referred to as Fischer carbenes and Fischer carbynes. Shared the Nobel Prize in Chemistry with Geoffery Wilkinson in 1973, for the pioneering work on the chemistry of organometallic compounds. Schrock carbenes are found with: Representatives: high oxidation states Isolation of first transition-metal carbene complex: CH early transition metals Ti(IV), Ta(V) 2 non pi-acceptor ligands Cp2Ta CH N Me LiMe Me 2 2 non pi-donor substituents CH3 (CO) W CO (CO)5W 5 (CO)5W A.B. Charette J. Am. Chem. Soc. 2001, 123, 11829. OMe O E. O. Fischer, A. Maasbol, Angew. Chem. Int. Ed., 1964, 3, 580. Persistent carbenes, isolated as a crystalline solid by Anthony J. Arduengo in 1991, can exist in the singlet state or the triplet state. Representative Fischer Carbenes: W(CO) Cr(CO) 5 5 Fe(CO)4 Mn(CO)2(MeCp) Co(CO)3SnPh3 Me OMe Ph Ph Ph NEt2 Ph OTiCp2Cl Me OMe Foiled carbenes were defined as "systems where stabilization is Fischer carbenes are found with : obtained by the inception of the facile reaction which is foiled by the impossibility of attaining the final product geometry".
    [Show full text]
  • Recent Advances in Titanium Radical Redox Catalysis
    JOCSynopsis Cite This: J. Org. Chem. 2019, 84, 14369−14380 pubs.acs.org/joc Recent Advances in Titanium Radical Redox Catalysis Terry McCallum, Xiangyu Wu, and Song Lin* Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States ABSTRACT: New catalytic strategies that leverage single-electron redox events have provided chemists with useful tools for solving synthetic problems. In this context, Ti offers opportunities that are complementary to late transition metals for reaction discovery. Following foundational work on epoxide reductive functionalization, recent methodological advances have significantly expanded the repertoire of Ti radical chemistry. This Synopsis summarizes recent developments in the burgeoning area of Ti radical catalysis with a focus on innovative catalytic strategies such as radical redox-relay and dual catalysis. 1. INTRODUCTION a green chemistry perspective, the abundance and low toxicity of Ti make its complexes highly attractive as reagents and Radical-based chemistry has long been a cornerstone of 5 1 catalysts in organic synthesis. synthetic organic chemistry. The high reactivity of organic IV/III radicals has made possible myriad new reactions that cannot be A classic example of Ti -mediated reactivity is the reductive ring opening of epoxides. This process preferentially readily achieved using two-electron chemistry. However, the − high reactivity of organic radicals is a double-edged sword, as cleaves and functionalizes the more substituted C O bond, the selectivity of these fleeting intermediates can be difficult to providing complementary regioselectivity to Lewis acid control in the presence of multiple chemotypes. In addition, promoted epoxide reactions. The synthetic value of Ti redox catalysis has been highlighted by their many uses in total catalyst-controlled regio- and stereoselective reactions involv- 6−10 ing free-radical intermediates remain limited,2 and the synthesis (Scheme 1).
    [Show full text]
  • N-Heterocyclic Carbenes (Nhcs)
    Baran Lab N - H e t e r o c y c l i c C a r b e n e s ( N H C s ) K. J. Eastman An Introduction to N-heterocyclic Carbenes: How viable is resonance contributer B? Prior to 1960, a school of thought that carbenes were too reactive to be smaller isolated thwarted widespread efforts to investigate carbene chemistry. base ! N N N N R R R R Perhaps true for the majority of carbenes, this proved to be an inaccurate X- assessment of the N-heterocyclic carbenes. H longer Kirmse, W. Angerw. Chem. Int. Ed. 2004, 43, 1767-1769 In the early 1960's Wanzlick (Angew. Chem. Int. Ed. 1962, 1, 75-80) first investigated the reactivity and stability of N-heterocyclic carbenes. Attractive Features of NHCs as Ligands for transition metal catalysts: Shortly thereafter, Wanzlick (Angew. Chem. Int. Ed. 1968, 7, 141-142) NHCs are electron-rich, neutral "#donor ligands (evidenced by IR frequency of reported the first application of NHCs as ligands for metal complexes. CO/metal/NHC complexes). Surprisingly, the field of of NHCs as ligands in transition metal chemistry Electron donating ability of NHCs span a very narrow range when compared to remained dormant for 23 years. phosphine ligands In 1991, a report by Arduengo and co-workers (J. Am. Chem. Soc. 1991, Electronics can be altered by changing the nature of the azole ring: 113, 361-363) on the extraodinary stability, isolation and storablility of benzimidazole<imidazole<imidazoline (order of electron donating power). crystalline NHC IAd. NHC-metal complex stability: NaH, DMSO, MeOH N N N N + H2 + NaCl NHCs form very strong bonds with the majority of metals (stronger than Cl- phosphines!) H IAd N-heterocyclic carbenes are electronically (orbital overlap) and sterically (Me vs.
    [Show full text]
  • Asymmetric Lewis Acid Catalysis Directed by Octahedral Rhodium Centrochirality† Cite This: Chem
    Chemical Science View Article Online EDGE ARTICLE View Journal | View Issue Asymmetric Lewis acid catalysis directed by octahedral rhodium centrochirality† Cite this: Chem. Sci.,2015,6,1094 Chuanyong Wang,‡a Liang-An Chen,‡b Haohua Huo,a Xiaodong Shen,a Klaus Harms,a Lei Gongb and Eric Meggers*ab A rhodium-based asymmetric catalyst is introduced which derives its optical activity from octahedral centrochirality. Besides providing the exclusive source of chirality, the rhodium center serves as a Lewis acid by activating 2-acyl imidazoles through two point binding and enabling a very effective asymmetric induction mediated by the propeller-like C2-symmetrical ligand sphere. Applications to asymmetric Michael additions (electrophile activation) as well as asymmetric a-aminations (nucleophile activation) Received 9th October 2014 are disclosed, for which the rhodium catalyst is found to be overall superior to its iridium congener. Due Accepted 7th November 2014 to its straightforward proline-mediated synthesis, high catalytic activity (catalyst loadings down to 0.1 DOI: 10.1039/c4sc03101f mol%), and tolerance towards moisture and air, this novel class of chiral-at-rhodium catalysts will likely www.rsc.org/chemicalscience to become of widespread use as chiral Lewis acid catalysts for a large variety of asymmetric transformations. Creative Commons Attribution 3.0 Unported Licence. Lewis acids are capable of activating a large variety of carbon- now wish to report for the rst time that rhodium can also serve heteroatom and carbon–carbon bond forming reactions and as the combined source of centrochirality and Lewis acidity in chiral Lewis acids have therefore become indispensable tools substitutionally labile octahedral metal complexes.
    [Show full text]
  • An Investigation of N-Heterocyclic Carbene Carboxylates
    AN INVESTIGATION OF N-HETEROCYCLIC CARBENE CARBOXYLATES: INSIGHT INTO DECARBOXYLATION, A TRANSCARBOXYLATION REACTION, AND SYNTHESIS OF HYDROGEN BONDING PRECURSORS by Bret Ryan Van Ausdall A dissertation submitted to the faculty of The University of Utah in partial fulfillment of the requirement for the degree of Doctor of Philosophy Department of Chemistry The University of Utah May 2012 Copyright © Bret Ryan Van Ausdall 2012 All Rights Reserved The University of Utah Graduate School STATEMENT OF DISSERTATION APPROVAL The dissertation of Bret Ryan Van Ausdall has been approved by the following supervisory committee members: Janis Louie , Chair 7-8-2011 Date Approved Cynthia Burrows , Member 7-8-2011 Date Approved Thomas Richmond , Member 7-8-2011 Date Approved Matthew S. Sigman , Member 7-8-2011 Date Approved Debra Mascaro , Member 7-8-2011 Date Approved and by Henry S. White , Chair of the Department of Chemistry and by Charles A. Wight, Dean of The Graduate School. ABSTRACT A series of 1,3-disubstituted-2-imidazolium carboxylates, an adduct of CO2 and N-heterocyclic carbenes, was synthesized and characterized using single crystal X-ray, thermogravimetric, IR, and NMR analysis. The TGA analysis of the imidazolium carboxylates shows that as steric bulk on the N-substituent increases, the ability of the NHC-CO2 to decarboxylate increases. Single crystal X-ray analysis shows that the torsional angle of the carboxylate group and the C-CO2 bond length with respect to the imidazolium ring is dependent on the steric bulk of the N-substituent. Rotamers in the t unit cell of a single crystal of I BuPrCO2 (2f) indicate that the C-CO2 bond length increases as the N-substituents rotate toward the carboxylate moiety, which suggests that rotation of the N-substituents through the plane of the C-CO2 bond may be involved in the bond breaking event to release CO2.
    [Show full text]
  • The Chemistry of Carbene-Stabilized
    THE CHEMISTRY OF CARBENE-STABILIZED MAIN GROUP DIATOMIC ALLOTROPES by MARIHAM ABRAHAM (Under the Direction of Gregory H. Robinson) ABSTRACT The syntheses and molecular structures of carbene-stabilized arsenic derivatives of 1 1 i 1 1 AsCl3 (L :AsCl3 (1); L : = :C{N(2,6- Pr2C6H3)CH}2), and As2 (L :As–As:L (2)), are presented herein. The potassium graphite reduction of 1 afforded the carbene-stabilized diarsenic complex, 2. Notably, compound 2 is the first Lewis base stabilized diatomic molecule of the Group 13–15 elements, in the formal oxidation state of zero, in the fourth period or lower of the Periodic Table. Compound 2 contains one As–As σ-bond and two lone pairs of electrons on each arsenic atom. In an effort to study the chemistry of the electron-rich compound 2, it was combined with an electron-deficient Lewis acid, GaCl3. The addition of two equivalents of GaCl3 to 2 resulted in one-electron oxidation of 2 to 1 1 •+ – •+ – give [L :As As:L ] [GaCl4] (6 [GaCl4] ). Conversely, the addition of four equivalents of GaCl3 to 2 resulted in two- electron oxidation of 2 to give 1 1 2+ – 2+ – •+ [L :As=As:L ] [GaCl4 ]2 (6 [GaCl4 ]2). Strikingly, 6 represents the first arsenic radical to be structurally characterized in the solid state. The research project also explored the reactivity of carbene-stabilized disilicon, (L1:Si=Si:L1 (7)), with borane. The reaction of 7 with BH3·THF afforded two unique compounds: one containing a parent silylene (:SiH2) unit (8), and another containing a three-membered silylene ring (9).
    [Show full text]
  • Photochemical Generation of Carbenes and Ketenes from Phenanthrene-Based Precursors Part I: Dimethylalkylidene Part II: Diphenylketene
    Colby College Digital Commons @ Colby Honors Theses Student Research 2017 Photochemical Generation of Carbenes and Ketenes from Phenanthrene-based Precursors Part I: Dimethylalkylidene Part II: Diphenylketene Tarini S. Hardikar Student Tarini Hardikar Colby College Follow this and additional works at: https://digitalcommons.colby.edu/honorstheses Part of the Organic Chemistry Commons Colby College theses are protected by copyright. They may be viewed or downloaded from this site for the purposes of research and scholarship. Reproduction or distribution for commercial purposes is prohibited without written permission of the author. Recommended Citation Hardikar, Tarini S. and Hardikar, Tarini, "Photochemical Generation of Carbenes and Ketenes from Phenanthrene-based Precursors Part I: Dimethylalkylidene Part II: Diphenylketene" (2017). Honors Theses. Paper 948. https://digitalcommons.colby.edu/honorstheses/948 This Honors Thesis (Open Access) is brought to you for free and open access by the Student Research at Digital Commons @ Colby. It has been accepted for inclusion in Honors Theses by an authorized administrator of Digital Commons @ Colby. Photochemical Generation of Carbenes and Ketenes from Phenanthrene-based Precursors Part I: Dimethylalkylidene Part II: Diphenylketene TARINI HARDIKAR A Thesis Presented to the Department of Chemistry, Colby College, Waterville, ME In Partial Fulfillment of the Requirements for Graduation With Honors in Chemistry SUBMITTED MAY 2017 Photochemical Generation of Carbenes and Ketenes from Phenanthrene-based Precursors Part I: Dimethylalkylidene Part II: Diphenylketene TARINI HARDIKAR Approved: (Mentor: Dasan M. Thamattoor, Professor of Chemistry) (Reader: Rebecca R. Conry, Associate Professor of Chemistry) “NOW WE KNOW” - Dasan M. Thamattoor Vitae Tarini Shekhar Hardikar was born in Vadodara, Gujarat, India in 1996. She graduated from the S.N.
    [Show full text]
  • Carbene Metal Amide Photoemitters: Tailoring Conformationally Flexible Amides for Full Color Range Emissions Including White-Emi
    Chemical Science View Article Online EDGE ARTICLE View Journal | View Issue Carbene metal amide photoemitters: tailoring conformationally flexible amides for full color Cite this: Chem. Sci., 2020, 11,435 † All publication charges for this article range emissions including white-emitting OLED have been paid for by the Royal Society a b b of Chemistry Alexander S. Romanov, * Saul T. E. Jones, Qinying Gu, Patrick J. Conaghan, b Bluebell H. Drummond, b Jiale Feng, b Florian Chotard, a Leonardo Buizza, b Morgan Foley, b Mikko Linnolahti, *c Dan Credgington *b and Manfred Bochmann *a Conformationally flexible “Carbene–Metal–Amide” (CMA) complexes of copper and gold have been developed based on a combination of sterically hindered cyclic (alkyl)(amino)carbene (CAAC) and 6- and 7-ring heterocyclic amide ligands. These complexes show photoemissions across the visible spectrum with PL quantum yields of up to 89% in solution and 83% in host-guest films. Single crystal X-ray diffraction and photoluminescence (PL) studies combined with DFT calculations indicate the important Creative Commons Attribution 3.0 Unported Licence. role of ring structure and conformational flexibility of the amide ligands. Time-resolved PL shows efficient delayed emission with sub-microsecond to microsecond excited state lifetimes at room Received 12th September 2019 À temperature, with radiative rates exceeding 106 s 1. Yellow organic light-emitting diodes (OLEDs) based Accepted 12th November 2019 on a 7-ring gold amide were fabricated by thermal vapor deposition, while the sky-blue to warm-white DOI: 10.1039/c9sc04589a mechanochromic behavior of the gold phenothiazine-5,5-dioxide complex enabled fabrication of the rsc.li/chemical-science first CMA-based white light-emitting OLED.
    [Show full text]
  • Research in the Department of Homogeneous Catalysis
    Homogeneous Catalysis – Overview Research in the Department of Homogeneous Catalysis Researchers in our department continue focusing on the development of new catalysis concepts within the areas of organocatalysis and transition metal catalysis. We explore new catalysts, expand the substrate scope of certain catalytic reactions, apply asymmetric catalysis in natural product and pharmaceutical synthesis, and study mechanisms of homogenous catalytic reactions (B. List, K.-R. Pörschke, M. Klußmann, N. Maulide). Since leadership of the department of homogenous catalysis was taken over by Professor Benjamin List in 2005, it has grown significantly from ca. 15 members to currently > 50 members, including the groups of Professor K.-R. Pörschke, who has been a group leader at the institute since two decades now, Dr. M. Klußmann (group leader since 2007), and now also of Dr. N. Maulide, who has joined the department in 2009. The group of Professor List primarily advances enantioselective organocatalysis as a fundamental approach complementing the already more advanced fields of biocatalysis and transition metal catalysis. The List group has a profound interest in developing “new reactions”, designs and identifies new principles for the development of organocatalysts, expands the scope of already developed catalysts such as proline, uses organocatalysis in the synthesis of natural products and pharmaceuticals, and also investigates the mechanism by which organocatalysts activate their substrates. Furthermore, in 2005 the group has first conceptualized and then significantly advanced another approach to asymmetric catalysis, asymmetric counteranion directed catalysis (ACDC). Initially, merely an idea, this approach has progressed within the department but now also at other institutions around the globe, into a truly general strategy for asymmetric synthesis and has found utility in organocatalysis but also in transition metal catalysis and Lewis acid catalysis.
    [Show full text]
  • L. Lewis Acid Catalysis
    L. LEWIS ACID CATALYSIS Background Twenty amino acids is not enough. The breadth of chemistry handled by enzymes requires that additional chemical species be employed in catalysis. So-called cofactors are non-amino acid components of enzymes that may be either associated or bonded to proteins and contribute to rate acceleration. Roughly one-third of all enzymes studied to date employ metal ion cofactors. Metal cations can play one of two roles; they may act as Lewis acids or as redox reagents. As Lewis acids, they function in a role similar to general acids, stabilizing negative charge in the enzyme active site. This chapter will focus on Lewis acid catalysis in reactions that employ water as a nucleophile. Lewis acid catalysis can be observed to play a role in either (a) activating water as a nucleophile or (b) activating the electrophile for nucleophilic attack. Activation of Water as a Nucleophile As noted earlier, water is a poor nucleophile due to the build up of positive charge on the oxygen that accompanies attack upon an electrophile. General base catalysis overcomes that issue by promoting deprotonation of the water molecule as it attacks. Another approach is to stabilize the hydroxide ion within the enzyme active site. While the metal ion itself is a Lewis acid (electron pair acceptor), the hydrated metal ion is a Bronsted acid (a proton donor) according to Equation L.1. n+ (n-1)+ + M(H2O)6 ⇔ M(H2O)5(OH) + H Eq L.1 In this instance Lewis acidity and Bronsted acidity are linked. The capacity of the metal cation to stabilize negative charge from a hydroxide (hydroxo in inorganic-speak) ligand is a reflection of the Lewis acidity of the cation.
    [Show full text]
  • Bulky Aluminum Lewis Acids As Novel Efficient and Chemoselective
    Erschienen in: Synlett ; 1999 (1999), 5. - S. 584-586 https://dx.doi.org/10.1055/s-1999-2667 584 Bulky Aluminum Lewis Acids as Novel Efficient and Chemoselective Catalysts for the Allylation of Aldehydes Andreas Marx, Hisashi Yamamoto* Graduate School of Engineering, Nagoya University, CREST, Chikusa, Nagoya 464-8603, Japan Fax +81(52)789-3222; E-mail: [email protected] position of the bulky phenol ligands resulted in further ac- Abstract: Bulky aluminum Lewis acids such as MAD and MABR celeration of the rate. Thus, methylaluminum bis(4-bro- are introduced as new efficient catalysts in substoichiometric 8,9 amounts for the allylation of aldehydes with allyltributylstannane. mo-2,6-di-tert-butyl-phenoxide) (MABR) performed In addition, chemoselective allylation of a less hindered or aromatic the reaction between 1 and 2 to yield the desired product aldehyde was achieved with catalytic amounts of the carbonyl re- 3 in excellent yield (93%) within 1 h at -20 °C (Table 1, ceptor aluminum tris(2,6-diphenyl-phenoxide) (ATPH). Entry 5). It turned out that MABR is the reagent of choice Keywords: allylation, allyltributylstannane, Lewis acid catalysis, for the catalytic allylation of aldehydes with allyltributyl- MABR, ATPH stannane. It is noteworthy that aluminum tris(2,6-diphe- nylphenoxide) (ATPH)8,9 which is known to complex carbonyl functionalities very tightly due to its bowl-like The Lewis acid promoted allylation of aldehydes using shape11 is capable of promoting catalytic allylation of ben- allystannanes or their less reactive counterparts allylsi- zaldehyde as well (Table 1, Entry 6). Bulkiness of the phe- lanes is a versatile synthetic tool.1 Numerous Lewis acids noxide ligands in the organoaluminum Lewis acids seems ◊ to be a prerequisite for catalytic activity and efficiency.
    [Show full text]
  • On the Road to Carbene and Carbyne Complexes
    ON THE ROAD TO CARBENE AND CARBYNE COMPLEXES Nobel Lecture, 11 December 1973 by ERNST OTTO FISCHER Inorganic Chemistry Laboratory, Technical University, Munich, Federal Republic of Germany Translation from the German text INTRODUCTION In the year 1960, I had the honour of giving a talk at this university* about sandwich complexes on which we were working at that time. I think I do not have to repeat the results of those investigations today. I would like to talk instead about a field of research in which we have been intensely interested in recent years: namely, the field of carbene complexes and, more recently, carbyne complexes. If we substitute one of the hydrogen atoms in a hydrocarbon of the alkane type - for example, ethane - by a metal atom, which can of course bind many more ligands, we arrive at an organometallic compound in which the organic radical is bound to the metal atom by a σ-bond (Fig. la). The earliest compounds of this kind were prepared more than a hundred years ago; the first was cacodyl, prepared by R. Bunsen (1), and then zinc dialkyls were prepared by E. Frankland (2). Later V. Grignard was able to synthesise alkyl magnesium halides by treating magnesium with alkyl halides (3). Grignard was awarded the Nobel Prize in 1912 for this effort. We may further recall the organo-aluminium compounds (4) of K. Ziegler which form the basis for the low pressure polymerisation, for example of ethylene. Ziegler and G. Natta were together honoured with the Nobel Prize in 1963 for their work on organometallic compounds.
    [Show full text]