Experimental results provide factual information on reactions. Although agreement between a rate law and the experimental data suggests that the proposed mechanism is valid, disagreement is enough to rule out the proposed mechanism. However, It should be noted that agreement between the predicted and measured kinetics is not always enough to assign a mechanism, unless the number of constants in the numerical rate formula is limited.
Sophisticated reaction rate laws can be considered, but such rate laws usually include a high number of constants, which makes it difficult to identify certain mechanisms. Users can select articles or chapters that meet their interests and gain access to the full content permanently in their personal online InfoSci-OnDemand Plus library. When ordering directly through IGI Global's Online Bookstore, receive the complimentary e-books for the first, second, and third editions with the purchase of the Encyclopedia of Information Science and Technology, Fourth Edition e-book.
Instant access upon order completion. Farag, Hamdy and Masahiro Kishida. Abstract Kinetic models were developed to account for the partial contributions of intermediates in complex parallel—consecutive reactions. The models allow precise estimation of the apparent rate constants of all steps in such a reaction network. The hydrodesulfurization HDS of dibenzothiophene DBT over CoMo-based alumina and carbon catalysts, and over an unsupported molybdenum sulfide catalyst, were investigated in a batch reactor and used to represent this type of reaction.
The HDS reactions proceeded through two parallel—consecutive reaction pathways, i. Francesca Cardona, Camilla Parmeggiani. Opportunities, Challenges, Benefits, and Risks Editor s: Zengshe Liu, George Kraus. Sustainable Preparation of Metal Nanoparticles: Methods and Applications Editor s: Rafael Luque, Rajender S Varma. Principles and Applications Natural Product Extraction: Principles and Applications Editor s: Ionic Liquids in the Biorefinery Concept: Challenges and Perspectives Editor s: Alternatives to Conventional Food Processing: Edition 2 Alternatives to Conventional Food Processing: Edition 2 Editor s: Sustainable Synthesis of Pharmaceuticals: A Pharmaceutical Perspective Editor s: Rakesh Kumar Sharma, Rakeshwar Bandichhor.
Heterogeneous Catalysis for Today's Challenges: Synthesis, Characterization and Applications Editor s: Moreover, an optical active compound could be generated during the reaction process since a chiral catalyst proline is used in the reactions.
However, enantioselectivity was not observed by chiral HPLC analysis, and 3-pentanone gives rise to a mixture of diastereoisomers. Following this synthetic strategy, a solution-phase parallel synthesis of 1,2-dihydroisoquinolines has been developed by Larock, providing a membered library for biological assays [ 58 ]. Iminium intermediate 28 , generated in situ from the aldimine 27 under silver triflate catalysis is the usual electrophilic intermediate, whereas the nucleophile, in this case, is the oxonium ylide The oxonium ylide 39 was prepared by a well-known procedure involving a rhodium carbenoid intermediate, generated in situ from the corresponding diazoacetate 37 under Rh 2 OAc 4 catalysis, and water or alcohols The scope of the reaction was thoroughly investigated.
Thus, methyl aryl diazoacetates and N -aryl aldimines, with electronically diverse metha or para -substituents on the aryl moieties, as well as ethyl 2-diazobutanoate gave good results, only nitro and ortho -substituted aryl derivatives were unreactive. Interestingly, two stereocenters are generated during the reactions.
However, the observed diastereoselectivities were poor, ranging from When tosylhydrazide 41 is used as G—NH 2 component, the silver promoted MCR can afford 2-amino-1,3-disubstituted-1,2-dihydroquinolines and, when the third component Nu bears the appropriate substituents, to polycyclic derivatives Scheme The reactions between 2-alkynylbenzaldehydes 25 and tosylhydrazide 41 afford the corresponding hydrazono derivatives 42 , which, in turn, yield the isoquinoliniumylamides 43 under silver triflate catalysis Scheme The three-component process is co-catalyzed by silver triflate and an N -heterocyclic carbene.
Scheme 24 [ 67 ]. Subsequently, methanol would be involved in the reaction via deprotonation and the nucleophilic addition to the carbonyl group to produce the desired 2-amino-1,2-dihydroisoquinoline Concurrently, the released N -heterocyclic carbene would re-enter the catalytic cycle. The avoidance of the use of IPr and the usage of 2 equiv of potassium hydroxide leads to a different reaction mechanism and allows for the synthesis of tricyclic H -pyrazolo[5,1- a ]isoquinoline 48 Scheme Synthesis of tricyclic H -pyrazolo[5,1- a ]isoquinoline Synthesis of tricyclic H -pyrazolo[5,1- a ]isoquinolines The synthesis of both 1,2-dihydroquinolines 47 and 48 takes advantage from the easy formation of the isoquinoliniumylamide 43 under silver triflate catalysis.
This intermediate can be trapped by other nucleophilic reagents enamines and carbanions or be involved in cycloaddition reactions affording tricyclic compounds by cascade processes. An unprecedented, co-catalyzed reaction involving enamines 51 as nucleophilic partners, also yields the H -pyrazolo[5,1- a ]isoquinoline nucleus 48 , in the presence of silver triflate and copper II chloride under air Scheme 27 [ 69 ]. However, with respect to the reaction reported in Scheme 26 , also affording isoquinolines of general formula 48 , a diverse arrangement of substituents can be achieved.
The proposed reaction mechanism takes into account the recent applications of an oxygen-copper catalytic system for the oxidation of aliphatic C—H bonds [ 70 ]. Thus, oxidation of the aliphatic C—H bond, alpha to the reacting amine 50 , resulted in the formation of nucleophilic enamine 51 , which is able to react with the isoquinoliniumylamide 43 , thereby affording a tricyclic intermediate, which by loss of the tosyl group and base-catalyzed aromatization yields the H -pyrazolo[5,1- a ]isoquinoline Finally, a new series of fully aromatic pyrazolo[5,1- a ]isoquinolines 53 , bearing an amino group in position 2 can be synthesized under silver triflate catalysis by the usual three-component reaction involving nitriles 52 as pro-nucleophiles Scheme 28 [ 71 ].
Starting from these results, a MC approach to 1- isoquinolinyl guanidines 55 was efficiently developed by a silver triflate-catalyzed three-component reaction of 2-alkynylbenzaldehydes 25 , tosylhydrazide 41 and carbodiimides 54 Scheme 29 [ 76 ]. Further intramolecular rearrangement yields the desired 1- isoquinolinyl guanidine Both these reactions are co-catalyzed, the former by silver triflate and copper bromide and the latter by silver triflate and nickel II perchlorate Scheme 30 and Scheme The overall process proceeds efficiently to generate the 2-amino- H -pyrazolo[5,1- a ]isoquinolines 58 in moderate to excellent yields under mild conditions and with good substrate tolerance.
Cycloaddition reactions of activated cyclopropanes with nitrones under Lewis acid catalysis have been previously described by Kerr and may proceed on the activated cyclopropane by a stepwise or concerted mechanism [ 81 ]. Similar mechanisms could be also operative in the reaction of ylidic species 43 for the synthesis of Good substrate tolerance and moderate to excellent yields are reported.
We focus on Mannich-type reactions characterized by the addition of a nucleophile to an imine.
Missen- Introduction To Chemical Reaction Engineering And Kinetics | Silvia Jy - theranchhands.com
In several MCRs with this type of reactivity, silver I salts and complexes have been used to activate either the nucleophile or the imine. Isocyanides have been found to be versatile reagents in heterocyclic synthesis [ 82 — 83 ]. The scope of these reactions could be extended to isocyanides with other substituents by using methanol as a solvent. The reaction occurs via a Mannich-type addition of the deprotonated isocyanide intermediate 64 to an in situ generated iminium salt, a subsequent intramolecular cyclization and proton shift results in dihydroimidazole 65 showing predominantly cis -arrangment around the C4—C5 bond.
Additionally, the use of sterically demanding amines results in lower yields. The reaction proceeds through the formation of iminium ion 67 [ 92 ]. The isocyanide carbon atom is sufficiently nucleophilic to attack iminium ion Subsequent deprotonation and cyclization yields oxazoles 68 Scheme Another cluster of silver-mediated Mannich-type reactions involves the enantioselective addition of siloxyfurans 70 to imines 69 vinylogous Mannich reaction, VM affording chiral butenolide derivatives 71 Scheme The reaction proceeds in the presence of amino acid-based chiral phosphine ligands and AgOAc via bidentate chelation of a properly substituted aldimine.
Chiral phosphine—silver I complexes are emerging as a valuable tool for carbon—carbon bond forming reactions. These catalysts are effective in promoting enantioselective allylations, aldol reactions, Mannich-type reactions, hetero Diels—Alder reactions, 1,3-dipolar cycloadditions and nitroso aldol reactions [ 93 ].
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The process was firstly accomplished with preformed aryl-substituted aldimines [ 94 ] and then developed as a MCR for less stable alkyl-substituted aldimines, which were prepared in situ from arylamines 72 and alkylaldehydes 73 to avoid decomposition [ 95 ]. Scheme 35 shows the general reaction outcome for both processes. The two main features of the reported three-component Ag-catalyzed process are i the mild reaction conditions and ii the high degree of diastereo- and enantioselectivity.
Moreover, the N -aryl group can be easily removed from the final compounds under oxidative conditions yielding the corresponding amino compounds. An OMe substituent is essential as a directing group for aryl-substituted aldimines. Thus, the Lewis acidic chiral complex may associate with the aldimine substrate through bidentate chelation Scheme The substrate is bound anti to the bulky amino acid substituent R and reacts with the siloxyfuran via endo -type addition.
Intramolecular silyl transfer, iPrOH mediated desilylation of the amide terminus, and protonation of the N—Ag bond delivers the final product and the catalyst. Such a pathway is not allowed for the siloxyfuran bearing a methyl group in position 3, which reacts by an exo addition. Alkyl-substituted aldimines can also participate in these reactions.
However, they must be generated in situ MCR. In the latter reactions, best results were obtained when arylamines 72 bear an o -thiomethyl and a p -methoxy substituent instead of a single o -methoxy substituent. The corresponding electron-rich aldimines are less electrophilic and subsequently more stable under the reported reaction conditions. Two more examples of enantioselective reactions involving silver catalysts have been recently reported.
However, the adopted method to induce chirality in the final products is rather dissimilar. Stereoselective three-component approach to pirrolidines 77 by means of a chiral auxiliary. On the one hand, as an electron withdrawing group, it decreases the nucleophilicity of the amine, thus avoiding the formation of detrimental Michael-type adducts with the alkene. Moreover, as a chiral auxiliary it promotes the cycloaddition governing the stereochemistry of the process. The chiral auxiliary can be removed at the end of the reaction.
Instead, an Ag I complex based on BINAP and AgSbF 6 was employed as a catalyst for the enantioselective 1,3-dipolar cycloaddition reaction of azomethine ylides and alkenes for the synthesis of pyrrolidines 81 and 82 Scheme 38 [ 97 ]. The reaction was developed mainly as a two-component reaction and only two examples of MC approaches have been included in the manuscript.
Silver and gold-catalyzed multicomponent reactions
The reported examples involve hetero aryl aldehydes 77 , methyl glycinate 78 and maleimide 79 or E -1,2-bis phenylsulfonyl ethylene 80 as electrophilic alkenes. Stereoselective three-component approach to pyrrolidines 81 and 82 by means of a chiral catalyst. The reported work is an extension of a previous paper dealing with the use of BINAP—AgClO 4 as a chiral catalyst in the same two-component reaction [ 98 ].
An interesting application of silver catalysis in the allene chemistry field has been recently proposed by Jia and co-workers [ 99 ]. Thus, they believed that new cycloaddition reactions could be accessed if isocyanide was employed as a nucleophile instead of phosphine.
This behavior originates from the involvement of the isocyanide in the cyclization step. Reactions involving organosilver reagents.
Information about organosilver compound chemistry with respect to the coordination chemistry of silver salts and complexes is scarce in the literature. This could be related to the lower stability of these compounds, increasing in the order C sp3 —Ag, C sp2 —Ag, C sp —Ag, compared to other organometallic compounds.
The majority of the screened literature discusses the use of organosilver compounds as reagents. A recent review on organosilver compounds by Pouwer and Williams exhaustively highlights all these aspects of silver chemistry [ ]. For example, functionalized propiolic acids can be selectively prepared by an AgI catalyzed carboxylation of terminal alkynes with CO 2 under ligand free conditions with the intermediacy of an organosilver compound, namely silver acetilide C sp —Ag [ ]. The direct carboxylation of active C—H bonds of hetero arenes [ ] and terminal alkynes [ ] with CO 2 in the presence of copper or gold-based catalysts has also been reported.
However, these latter transformations require expensive ligands and often harsh bases, whereas the silver-mediated process depends on a simple but efficient catalyst such as AgI and Cs 2 CO 3 as base. This feature has been clearly highlighted by Anastas who realized the multicomponent synthesis of regioisomeric arylnaphthalene lactones 89 and 90 from arylacetylenes 87 , carbon dioxide and 3-bromoarylpropynes 88 Scheme 40 [ ]. In the reaction sequence a 1,6-diyne was generated in situ and cyclized to afford the two possible regioisomeric compounds.
The level of regioselectivity can be enhanced by the tuning of electronic properties of the reactant species. The latter approach was successfully adopted for the preparation of dehydrodimethylconidendrin and dehydrodimethylretroconidendrin. In gold I and gold III -catalyzed reactions the metal acts as a carbophilic Lewis acid, facilitating nucleophilic addition to unsaturated systems. Moreover, also the oxophilic character of gold species has been highlighted by several authors. More recently, gold-promoted transformations involving higher oxidation states from Au I precatalysts have been achieved by the addition of a stoichiometric oxidant enabling two-electron redox cycles typically exhibited by other late transition metals.
With respect to Ag I -mediated MCRs, less information can be found in the literature about the corresponding gold-mediated processes. Thus, major research efforts have been directed to the development of tandem, sequential or cascade reactions and to the area of asymmetric transformations. As reported for silver, this part of the review is divided in sections relating to the nature of the activated functionalities.
Reactions involving the activation of carbon—carbon multiple bonds. The reactions take advantage from the high functional group tolerance and from the generally mild reaction conditions. These are the first examples of an intermolecular catalytic asymmetric synthesis of spiroacetals. Previously reported methodologies involved preformed substrates in intramolecular reactions [ — ]. Enantioselective synthesis of spiroacetals by Gong [ ]. The condensation reaction between glyoxylic acid 93 and aniline gives rise to imine 94 which, by double interaction with the gold phosphate, leads to an activated species.
Subsequent nucleophilic addition of 92 to 94 gives oxonium intermediate 95 , which provides the final product 96 upon cyclization regenerating the catalyst.
1. Introduction
Interestingly, in the first catalytic cycle the main role of the catalyst is played by its cationic part, the gold I ion, being responsible for the activation of the alkynol Meanwhile, in the second catalytic cycle, the main role is played by the anionic part of the catalyst, the phosphate, creating the appropriate chiral environment to produce the final enantioenriched product. The model proposed for the chiral phosphoric acid catalyzed reactions between glyoxylates and enecarbamates is reported in Scheme 41 see box.
The key feature is the formation of a double hydrogen-bonded complex in which only the si face is fully accessible for the enol ether attack to afford the final cyclization product As reported in Scheme 42 the method proposed by Gong and co-workers allows for the synthesis of aromatic spiroacetals The key step of the sequence is again the addition of an enol ether to an imine followed by an intramolecular cyclization reaction. The enol ether 98 is generated from ortho -alkynylbenzyl alcohol 97 under gold catalysis, and the imine from salicylaldehyde 99 and aniline.
The MC synthesis of bi- and tricyclic ketals and takes advantage from a mechanism involving the oxyauration of a carbon—carbon triple bond [ ].
Thus, starting from 4-acyl-1,6-diynes , H 2 O and alkanols, under AuCl 3 -catalysis, polyfunctionalized fused bicyclic ketals and bridged tricyclic ketals have been prepared with a high degree of regio- and diastereocontrol. Synthesis of polyfunctionalized fused bicyclic ketals and bridged tricyclic ketals The reaction course can be directed toward the formation of and by a fine-tuning of the reaction conditions.
Proposed reaction mechanism for the synthesis of ketals and Under the optimized reaction conditions mentioned above, a double oxyauration reaction leads to intermediate I. The addition of water then results in the formal hydration of I affording dicarbonyl compound II. The subsequent addition of alcohol and the hydrochloric acid release affords the intermediate auric complex III , from which cyclic ketals and are formed by the inter- or intramolecular addition of alcohol, respectively.
The proposed reaction mechanism also accounts for the high degree of diastereoselectivity, which can be rationalized by a series of intramolecular chiral inductions. The initial steps of the MCR encompass the Au I -catalyzed hydration of the alkyne to give the ketone and the conversion of the aldehyde to the corresponding acetal The Au I -catalyzed ionization of the acetal then provides the oxocarbenium ion , which is captured by the enol tautomer of ketone Scheme The authors reported a nice investigation of the involved reaction mechanism and carried out a catalytic screening devoted to the selection of the best catalytic system and optimal reaction conditions.
Newly reported examples of gold-catalyzed multicomponent reactions encompass the synthesis of nitrogen containing heterocycles, namely N -substituted 1,4-dihydropyridines [ ] and tetrahydrocarbazoles [ ]. The first example takes advantage of the ability of a cationic gold I catalyst to promote the formation of a new C—N bond through the hydroamination of a carbon—carbon triple bond.
The three-component reaction includes methanamine , activated alkynes and aldehydes as reactants, a cationic gold I complex generated in situ from triphenylphosphine gold chloride and silver triflate as a catalyst, and KHCO 3 as base.