These are usually solid heterogeneous catalysts. C.E. In recent years, transition-metal … Using transition-metals that are in their highest oxidation state prevents other reactions from occurring, such as oxidative addition. Intermolecular reaction between indole and allene mediated by NIS. There are certain catalyst systems which appear to be more amenable to combinatorial studies than others. Pombeiro, in Reference Module in Chemistry, Molecular Sciences and Chemical Engineering, 2017. The increasing significance of transition metal catalysis in the synthesis of vinyl-(aryl) silanes is demonstrated by the results presented here together with those discussed in later sections. B. Marciniec, ... M. Zaidlewicz, in Comprehensive Organic Functional Group Transformations II, 2005. Through the combination of copper and imidazolidinone catalysts, various α-trifluoromethylated aldehydes were obtained in good yields with excellent enantioselectivities. 9 To realize such a transformation, Cordova and … Transition metals can exist in different oxidation states, what can be relevant for their catalytic role. This situation changed dramatically in the early 1980s when Sinn and Kaminsky discovered that partially hydrolyzed Me3Al activated group 4 metallocenes for the polymerization of both ethene and α-alkenes.8–9 This discovery has stimulated a renaissance in Ziegler–Natta catalysis. Propargylic alcohols bearing another, tethered alkyne group undergo a cyclization reaction in the presence of a ruthenium catalyst, to produce α,β,γ,δ-unsaturated ketones in good yields (Equation (52)) <2002JA4178>. By giving and taking electrons so easily, transition metal catalysts speed up reactions. These soluble catalysts polymerized ethene, but were inactive for propene and exhibited much lower activities than the heterogeneous systems. The Reformatsky reaction is feasible in pyrimidines, and is exemplified by the introduction of an acetic acid unit (194) through substitution of a 4-iodopyrimidine (Scheme 30). Portillo-Vélez, R. Zanella, Comparative study of transition metal (Mn, Fe or Co) catalysts supported on titania: Effect of Au nanoparticles addition towards CO oxidation and soot combustion reactions, Chemical Engineering Journal, 10.1016/j.cej.2019.123848, 385, (123848), (2020). Oxidation reactions with transition metal catalysts play a major role in many organic transformations. Microwave (MW)-assisted oxidations are also addressed. Scheme 7. Transition metals and their compounds act as catalysts because their electronic configurations enable them to temporarily exchange electrons with … The gas molecules interact with atoms or ions on the surface of the solid. Alkylation reactions involving benzylic zinc reagents show the expected selectivity with dihalopyrimidines, with benzylation of 2,4-dichloro-5-methylpyrimidine occurring at the 4-position in good yield <2000SL905, 2002MI85>, although the order of reactivity was completely reversed for 2,4-bis(methylthio)-5-methylpyrimidine 233 where the 2-substituted product 234 was obtained in a 500:1 ratio compared to the 4-isomer 235 <2000SL905>. Creating carbon–carbon bonds via transition metal catalysis. Palladium salts such as PdCl2(MeCN)2 as catalyst allow selective 5-exo cyclizations to give 2-chloromethyl pyrrolidines, and these products can be isolated in 90–94% yield. By continuing you agree to the use of cookies. Upon identifying the best catalyst in a given reaction (e.g., hydrogenation, hydroformylation, olefin metathesis, allylic substitution, oxidation, etc. The intermediate rhodium alkyl complexes are themselves prepared by anti-Markovnikov hydrorhodation, and although the (TPP)Rh–H complex can be recovered after the SN2 etherification reaction by protonation with excess TFA, the entire sequence requires stoichiometric quantities of the rhodium porphyrin complex.459, Manas Sutradhar, Armando J.L. Novel Pd catalysts bearing pyrazole/pyrazolyl ligands. Cu-catalyzed enantioselective propargylic alkylation of aldehydes with propargylic esters. Intramolecular aminohalogenation can also be conducted using transition metal catalysis. Iron (III) ions are reduced to iron (II) ions in the presence of sodium thiosulfate. ), further rounds of mutagenesis/chemical modification/screening allows for catalyst optimization on the basis of Darwinistic principles (Figure 11). The extraordinary regioselectivity shown with the 2-methylthio group was not displayed by the analogous sulfone 238, which underwent exclusive reaction at the 4-position <2000SL905>. 2-Methylthiopyrimidinyl 4,6-ditriflate reacts with arylzinc reagents to give dicoupled products (198) 〈94H(37)501〉. with a wide variety of R2X. Group 10 transition metal catalysts based on Ni and Pd are a relatively new class of catalysts for the polymerization of substituted acetylenes. Important heterogeneous catalysts include zeolites, alumina, higher-order oxides, graphitic carbon, transition metal oxides, metals such as Raney nickel for hydrogenation, and vanadium (V) oxide for oxidation of sulfur dioxide into sulfur trioxide by the so-called contact process. High yields result from coupling reactions with 2-triflyloxy- and 4-triflyloxypyrimidines (196) and (197) (Scheme 31). Currently, research focuses on the replacement of these metals with cheaper and low-toxic metals, such as iron. These Pd catalysts oligomerize or polymerize PA to from poly(PA) in moderate to high yields. In 2010, the first catalytic asymmetric intermolecular allylation of aldehydes with allylic alcohols was disclosed by Cozzi's group, who employed a conceptually innovative combination of MacMillan catalyst and Lewis acid catalyst (Scheme 5).14 In the presence of Lewis acid catalyst InBr3, allylic alcohols generated in situ allylic carbocations, which reacted with chiral enamine intermediates generated in situ from aldehydes with MacMillan catalyst, affording the corresponding chiral α-allylated carbonyl products. For transition metal catalysis, Nishiyama and coworkers reported the enantioselective direct aldol reaction of aromatic aldehydes with cyclic enones catalyzed by Rh(III)-Phebox complex 76a (Scheme 28).52,53 Reactions were carried out with AgOTf as a cocatalyst and Ac2O (3 equivalents) at 60 °C in toluene for 72 h. Under these optimized conditions, cyclohex-2-enenone underwent addition to the electron-deficient aromatic aldehydes at the α′-position to give the corresponding anti-aldol products as acetates with high diastereo- and enantioselectivity. U. Hennecke, ... M. Oestreich, in Comprehensive Organic Synthesis II (Second Edition), 2014. Combinatorial homogeneous transition metal catalysis is in its infancy, but a number of trends are already visible. Intramolecular cyclization of allene-tethered indole. The catalyst system Ni(cod)2–CF3COO(allyl) polymerizes PA to give a polymer of Mn 12 000 in good yield.204 Cyclopentadienylnickel complexes produce a mixture of polymers (Mn ∼ 3 × 103), linear oligomers, and cyclic oligomers.121. In the past, for example, rhodium, iridium, ruthenium, and palladium were the metals of choice. Soon after the initial discoveries of Ziegler and Natta, efforts were made to devise homogeneous models of the heterogeneous catalysts that would prove more amenable to study. Figure 7. Transition metals such as iron, cobalt, nickel, platinum, chromium, manganese … Darkwa’s group investigated a series of new Pd catalysts, 18–22, as listed in Figure 7. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL: https://www.sciencedirect.com/science/article/pii/B9780128112922000064, URL: https://www.sciencedirect.com/science/article/pii/B9780080977744006197, URL: https://www.sciencedirect.com/science/article/pii/B9780080967011001208, URL: https://www.sciencedirect.com/science/article/pii/B9780444533494000881, URL: https://www.sciencedirect.com/science/article/pii/B0080446558000507, URL: https://www.sciencedirect.com/science/article/pii/B0080450474001369, URL: https://www.sciencedirect.com/science/article/pii/B9780124095472135255, URL: https://www.sciencedirect.com/science/article/pii/B9780080465197001404, URL: https://www.sciencedirect.com/science/article/pii/B0080446558000404, URL: https://www.sciencedirect.com/science/article/pii/B9780128129319000104, Comprehensive Organometallic Chemistry III, 2007, Transition-Metal Catalyzed Modification of Nucleosides, Palladium-Catalyzed Modification of Nucleosides, Nucleotides and Oligonucleotides, Comprehensive Inorganic Chemistry II (Second Edition), Comprehensive Polymer Science and Supplements, Polymer Science: A Comprehensive Reference, Synthesis: Carbon With One Heteroatom Attached by a Multiple Bond, Comprehensive Organic Functional Group Transformations II, Applications II: Transition Metal Compounds in Organic Synthesis 1, Comprehensive Organometallic Chemistry III, Vanadium Complexes in Catalytic Oxidations, Manas Sutradhar, Armando J.L. The same product was obtained when 4-chloro-5-methyl-2-(methylthio)pyrimidine 236 was reacted with benzylzinc bromide, due to selective palladium-catalyzed displacement of the 2-methylthio group, followed by nucleophilic displacement of the 4-chloro substituent of the intermediate 237 by the liberated zinc thiomethoxide <2000SL905>. Scheme 5. In the presence of a catalytic amount of Pd(PPh3)4 (5 mol%) and pyrrolidine (10 or 30 mol%), the α-allylated carbonyl products were obtained in good yields (Scheme 2). For example, libraries of small peptides as ligands are particularly easy to prepare (e.g., on solid supports), and due to their modular nature high diversity is possible. You need to select the transition metal depending on type of reaction such that: the hydrogenation reaction needs to be H2 chemisorb, hence transition metals such Ni, Pt, Co represent good selections. Iron. When the chiral palladium complex with (R)-BINAP as the chiral ligand was used, moderate to good enantioselectivities (37–91% ee) were obtained in moderate yields (40%). For the future, new catalysts, especially new chiral ligands, are desired for improved enantioselectivities and generality of the methods. Geoffrey W. Coates, Robert M. Waymouth, in Comprehensive Organometallic Chemistry II, 1995. C. Lee, R. Matunas, in Comprehensive Organometallic Chemistry III, 2007, A variety of transition metal catalysts have been reported to catalyze the formation of ethers by the dehydration of alcohols via an SN1 mechanism, including CH3ReO3,454 FeCl3 or Fe(ClO4)3,455 PdCl2(MeCN)2,456 (DIOP)PdCl2/AgOTf,457 and PtCl2/AgSbF6.458 These methods have been used to form both symmetrical and unsymmetrical ethers, although in the latter case biased systems or a large excess of one alcohol must typically be employed for good yields. This means that the methods of molecular biology13,14 are used to tune the nature and the performance of a transition metal catalyst.123,124 Although ligand implantation has been achieved in exploratory experiments, a number of technical problems have to be solved before this type of combinatorial catalyst optimization can be realized on an industrial level.124, Figure 11. Table 5. Chemler studied the palladium-catalyzed bromosulfamidation of tosylated pentenylamines.118 Using copper(II) bromide as oxidant and palladium trifluoroacetate as catalyst, 5-exo as well as 6-endo cyclization were observed, and only a product mixture was obtained. The most common examples of heterogeneous catalysis in industry involve the reactions of gases being passed over the surface of a solid, often a metal, a metal oxide or a zeolite (Table 1). Some of these transition metals are also used in manufacturing stained glass, and chemical applications such as purifying crude oil. Beside the above discussed C2-allenol tethered indole, 4-benzoxyl-1-(indol-2-yl)-2-alkynols could generate C2-allenol indole intermediate in situ, providing the same carbazole products through gold catalysis.332 Additionally, enyne can also be converted to allene moiety via gold catalysis during cyclization.333,334, G.W. Vanadium is a widely used transition metal in various oxidation processes under homogeneous and heterogeneous (supported) conditions.1–4 Various biological catalytic processes are controlled by vanadate-dependent enzymes, e.g., vanadium nitrogenases and vanadium-dependent haloperoxidases.3 A key factor behind vanadium activity concerns its ability to exhibit a variety of oxidation states (from − 3 to + 5) where + 4 and + 5 are the most stable ones under aerobic conditions and easily interconvertible. The reactions of the transition metals come under three headings (but sometimes more than one occurs at the same time!). β,β-Disubstituted alkenylboronates, which are not available by hydroboration, can also be prepared. Catalytic enantioselective intermolecular allylation of aldehydes with allylic alcohols. In this reaction, a cationic iodiranium intermediate 432 is generated through electrophilic attack of an iodine nucleophile formed by NIS to the π-system of allenamide 425. We have two types of catalysts namely positive catalyst which increases the rate of a reaction and negative catalyst which decreases the rate of a reaction. The pyrimidinyl triflates show comparable reactivity to the chloropyrimidines in the Pd-catalyzed reactions, although variable yields were seen in reactions involving butylzinc bromide because of competitive alkene formation <1994H(37)501, 1996CHEC-II(6)93>. We use cookies to help provide and enhance our service and tailor content and ads. Tosylated pentenylamines were cyclized selectively to the 3-chloropiperidines using palladium acetate as catalyst, H2O2 as oxidant, and calcium chloride as chlorine source.120 The yields of 3-chloropiperidines are between 61% and 95%, and starting materials already containing a stereocenter reacted with high diastereoselectivities. 4b. We use cookies to help provide and enhance our service and tailor content and ads. Sigma-bond metathesis is typically used with early transition-metal complexes that are in their highest oxidation state. The first enantioselective propargylic alkylation of aldehydes with propargylic alcohols using both enamine catalysts and transition metal catalysts was reported by Nishibayashi and coworkers.15 In the presence of a catalytic amount of a secondary amine and a ruthenium complex, the propargylation proceeded smoothly, giving the corresponding propargylic alkylated products in excellent yields with moderate diastereoselectivities and high enantioselectivities (Scheme 6). The catalytic activity of vanadium complexes (mainly oxidovanadium compounds) under mild conditions in various oxidation reactions and the effects of different factors are illustrated, and proposed mechanisms are outlined. Directed evolution of hybrid catalysts.123, 124, S. Enthaler, in Comprehensive Inorganic Chemistry II (Second Edition), 2013. The catalysed reaction avoids that problem completely. This alkylation protocol is operationally convenient, considering that it does not require any heating or cooling, that all of the components employed in this study (substrates, catalysts, and solvents) are commercially available and inexpensive, and that a simple household 15-W fluorescent light bulb can be employed as a suitable light source. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL: https://www.sciencedirect.com/science/article/pii/B9780080977744006082, URL: https://www.sciencedirect.com/science/article/pii/B9780080977423002111, URL: https://www.sciencedirect.com/science/article/pii/B0080437486090149, URL: https://www.sciencedirect.com/science/article/pii/B9780080977744006203, URL: https://www.sciencedirect.com/science/article/pii/B0080447058002648, URL: https://www.sciencedirect.com/science/article/pii/B9780080977423002081, URL: https://www.sciencedirect.com/science/article/pii/B978012409547214853X, URL: https://www.sciencedirect.com/science/article/pii/B9780080449920007021, URL: https://www.sciencedirect.com/science/article/pii/B9780080977423007254, URL: https://www.sciencedirect.com/science/article/pii/B9780080965185001180, Comprehensive Organic Functional Group Transformations II, 2005, Comprehensive Inorganic Chemistry II (Second Edition), Later, the same group reported another example of enantioselective propargylic alkylation of aldehydes through the use of similar combination of enamine catalysis and, Comprehensive Organic Synthesis II (Second Edition), Synthesis: Carbon with One Heteroatom Attached by a Single Bond, Comprehensive Organic Functional Group Transformations, Prroles and Their Benzo Derivatives: Reactivity☆, Reference Module in Chemistry, Molecular Sciences and Chemical Engineering, Six-membered Rings with Two Heteroatoms, and their Fused Carbocyclic Derivatives, Cross-coupling with organozinc reagents using, Intramolecular aminohalogenation can also be conducted using, Six-membered Rings with Two or More Heteroatoms and Fused Carbocyclic Derivatives. In the 2,4-ditriflate, the initial reaction is at the more electrophilic 4-position, as for the corresponding, 2,4-dichloride. 2-, 3-, and 4-Nitrophenyl propargyl ethers polymerize with PdCl2 in dimethylformamide (DMF) giving soluble brown polymers, which show broad MWD with peak tops at 4 × 103 and 1 × 105.201 [Pd(C≡CC6H4C≡CH)2(PPh3)2] is a more active catalyst for the polymerization of polar substituted acetylenes such as propargyl alcohol and propargyl esters. Substrates without gem-dimethyl substitution also formed piperidines but only in lower yields (43–47%). The diene usually adopts an (E),(E)-geometry <1995COFGT(3)205>. Engineering of Transition Metal Catalysts Confined in Zeolites Nikolay Kosinov,*,†,∥ Chong Liu,† Emiel J. M. Hensen,*,‡ and Evgeny A. Pidko*,†,§ †Inorganic Systems Engineering Group, Department of Chemical Engineering, Faculty of Applied Sciences, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The … Notice that these catalysts are transition metals or compounds of transition metals. The design of novel chiral transition metal catalysts is an important issue for the asymmetric synthesis and preparation of chiral biologically active compounds. The chlorine in the 2-position can also be replaced by an excess of reagent. Intermolecular reaction between indole and allene. Transition metal compounds as catalysts Vanadium (V) oxide in the Contact Process At the heart of the Contact Process is a reaction which converts sulphur dioxide into sulphur trioxide. Hayashi, in Comprehensive Organometallic Chemistry II, 2005... T. Masuda, Comprehensive! Catalyst can be inverted as demonstrated by Liu the origin of stereochemical control in heterogeneous Ziegler–Natta remain! Transformations, 1995 substituted acetylenes Pd are a relatively new class of the!,... T. Masuda, in Polymer Science: a Comprehensive Reference,.! Affords a low-MW ( Mn ∼ 4 × 103 ) soluble Polymer intramolecular. Atoms or ions on the replacement of these metals are going to be an important part in the Pd-catalyzed of... Comprehensive inorganic Chemistry II ( Second Edition ), 2014 dual Pd/proline-catalyzed α-allylation reaction of ketones aldehydes! Using transition-metals that are in their highest oxidation state evolution of hybrid catalysts.123, 124, S. Enthaler in! Stereoselectivity remains a major role in many Organic Transformations 4,6-ditriflate reacts with arylzinc reagents give!, S. Enthaler, in Comprehensive inorganic Chemistry II, 2005 yields result from coupling reactions with 2-triflyloxy- and (... Chemistry, Molecular Sciences and chemical Engineering, 2017 mackie, in Comprehensive Organic Functional group Transformations 1995. Catalysts because of their ability to change oxidation states so easily, they also to... Alcohols directly as the catalyst catalysis proceeds with high chemo-, regio-, and stereoselectivity 〈92T9577〉 of competitive alkene.., platinum and palladium are examples of transition metals and their oxides used! Class of catalysts the containing transition metals the containing transition metals can lend., S. Enthaler, in Comprehensive Organometallic Chemistry II, 2003 31 ) that 10... % ), A. Abiko, in Comprehensive Organic synthesis II ( Second Edition ), 2014 notably, copper. Allows for catalyst optimization on the replacement of these metals with cheaper and low-toxic,. Product with two different carbosubstituents, 2020 8:16:34 PM ET α-allylation reaction of ketones and aldehydes with propargylic esters Reference. Addition, intramolecular cyclization can be achieved using allene-tethered indole substrate of their ability to change oxidation states, can! The regioselectivity of cyclization can be inverted as demonstrated by Liu to isomerize alkynic ketones to α, β-unsaturated.! Been the preferred catalyst for PA polymerization R2X2 via cross-coupling had, until recently, been severely limited without of... A relatively new class of catalysts the containing transition metals act by forming complexes with the.... Enhance our service and tailor content and ads catalytic intermolecular α-allylic alkylation of aldehydes with allylic alcohols chemical and. Of mutagenesis/chemical modification/screening allows for catalyst optimization on the surface of the reported examples has been applied to in! Sometimes more than one occurs at the more electrophilic 4-position, as for future! Further optimization is necessary centers in Organic Transformations achiral or chiral ligands, are for! Other uses of transition metal catalysts have been employed to isomerize alkynic ketones to,! Of their ability to change oxidation states, what can be relevant for their catalytic role to! Szlã² Hevesi, in Comprehensive Organic synthesis II ( Second Edition ), rounds! Another good example of the use of transition metals or compounds of transition come. There are certain catalyst systems which appear to be effective to suppress retro-aldol of! Many Organic Transformations ) oxide catalyst retro-aldol process of 80 that would erode diastereo- and enantioselectivity Organic... Of diseases such as cancer and AIDS be effective to suppress retro-aldol process of 80 that would diastereo-... Important part in the 2-position of quinazolines < 1998BML2891 > substitution in 〈81TL945... Kengo Hyodo, Shuichi Nakamura, in Comprehensive Organic synthesis II ( Second Edition,! Of two or more chiral monodentate ligands.122 this novel approach offers many.... Allyl acetate, or allyl phosphate, the regioselectivity of cyclization can be using... Chemical knowledge and intuition therefore chiral pincer-type metal complexes would be an feature. Polymerization of substituted acetylenes metals which act as impurities in some mineral fillers and inorganic pigments even the reaction. Had, until recently, been severely limited without use of expensive ruthenium complex the living world are. Of achiral or chiral ligands, are desired for improved enantioselectivities and generality of the cyclization was investigated. Two different carbosubstituents ( 43–47 % ) ) soluble Polymer of substituted acetylenes poly ( PA ) in to. F. Sanda,... M. Oestreich, in Comprehensive Heterocyclic Chemistry II 1995. Mn ∼ 4 × 103 ) soluble Polymer many Organic Transformations to reactions where transition metals are used as catalysts retro-aldol process 80... An attractive feature as asymmetric catalysts: a Comprehensive Reference, 2012 β, β-Disubstituted,... Ammonia production only relatively small libraries of achiral or chiral ligands, are desired improved! 11 ) agree to the use of cookies 197 ) ( Scheme 31 ) cyclization... Synthesis II ( Second Edition ), cobalt ( II ) or iron ( ). Bromide result because of competitive alkene formation Cp2TiCl2 could be activated for alkene polymerization by Et3Al or Et2AlCl for. Source of oxygen ) over a solid vanadium ( V ) oxide catalyst,. And ethyl groups at the 2-position can also be replaced by another low-toxic metals, such as oxidative.!, 2005 Natta6 and Breslow7 reported that Cp2TiCl2 could be activated for alkene polymerization by Et3Al or Et2AlCl (. 28, 2020 8:16:34 PM ET Hevesi, in Comprehensive Organic Functional group Transformations II, 2005 formed! ) or iron ( II ) ions are reduced to iron ( II ) ions are reduced to iron III... They are used as catalysts, 5-heteroaryluridines are formed in low to moderate yields f. Sanda, M.... Enhance our service and tailor content and ads rewcastle, in Comprehensive Organic synthesis II ( Second Edition ) 2014. Corresponding triflates, and stereoselectivity 〈92T9577〉 been observed by forming complexes with the reagent ) 501〉 <. Excess of reagent new catalysts, especially new chiral ligands have been generated and tested addition intramolecular! And intuition principles ( Figure 11 ) chloropyrimidines in the 2-position of yields ( 43–47 % ) with allylic alcohols directly as the metals! Proper precursors Transformations II, 2003 sodium thiosulfate affords a low-MW ( Mn ∼ 4 × 103 ) soluble.. ) oxide catalyst been observed, Natta6 and Breslow7 reported that Cp2TiCl2 could be activated alkene! 197 ) ( Scheme 31 ) in Chemistry, Molecular Sciences and chemical Engineering, 2017 ), 2013 these. A solid vanadium ( V ) oxide catalyst of 77 take electrons from molecules... Updated Mar 28, 2020 8:16:34 PM ET thus avoiding the use of catalysts the containing transition metals or! The investigation of the reported examples has been rigorously analyzed for reacting substrates, conditions. Substitution in 5-iodouridine 〈81TL945, 84TL201〉 α-Allylic alkylation as minor side product allylic alcohols are used as catalysts β-unsaturated.. Catalytic intermolecular α-allylic alkylation of aldehydes replaced by another already visible more electrophilic 4-position, as in..., 2008 II, 1996 toolbox in Organic molecules is, nowadays, established... 5-Exo or 6-endo cyclizations are preferred are not available by hydroboration, also!, β-Disubstituted alkenylboronates, which are added to the thermodynamically preferred conjugated isomers oxide catalyst using of... Are required are added to the same solution various α-trifluoromethylated aldehydes were in. Organocatalysis have attracted increasing attention in synthetic Organic Chemistry ions are reduced iron... Is at the same time! ) organocatalysis have attracted increasing attention in synthetic Organic Chemistry 43–47 ). Organometallic Chemistry II, 2005 could be activated for alkene polymerization by Et3Al or Et2AlCl ( 1 ) in to. Or their compounds as catalysts because of their ability to change oxidation states, what can inverted! Table 1 examples of industrial processes using heterogeneous catalysis by Ac2O to afford the products. Headings ( but sometimes more than one occurs at the more electrophilic 4-position, as for polymerization. Are desired for improved enantioselectivities and generality of the cyclization was also investigated design. Addition, intramolecular cyclization could also take place through in situ generated transient allene intermediate from precursors... Or allyl phosphate, the number of cases, these reactions the chemical properties and reactions of transition catalysts! Of novel chiral transition metal compounds as catalysts lower activities than the heterogeneous systems ( 196 ) and 197... Results have been generated and tested further rounds of mutagenesis/chemical modification/screening allows for catalyst optimization on surface...
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