Electrochemically induced chemical reactions: Kinetics of competition with electron transfer

Acid—base or electrophile—nucleophile chemical reactions can be induced by electrochemical means in the case where the reaction becomes faster at the +1e (or ?1e) level than at the starting level. This is typically the case for SRN1 aromatic nucleophilic substitution. When bond cleavage occurs at the +1e (or ?1e) level, a competing route may be opened by the electroactivity of the cleaved species being higher than that of the substrate. Electron transfer at the electrode (ECE) or in the solution (DISP) thus appears as a possible side-reaction decreasing the efficiency of the electrochemical inducement. The kinetics of this competition is investigated in the context of cyclic voltammetry. The kinetic characteristics are shown to be dramatically different in the ECE and the DISP cases providing an example of the operational significance of the distinction between these two modes of electron transfer. Diagnostic criteria and procedures for rate constant determination are discussed. An experimental illustration of the role of the various operational parameters substrate and nucleophile concentrations, sweep rate, is given. It involves the electrochemically catalyzed aromatic nucleophilic substitution of 2-chloroquinolin by benzenethiolate in liquid ammonia.     

Nitro compounds as alkylating reagents in friedel-crafts conditions : Reaction of 2-nitropropane with benzene

Friedel-Crafts reaction of benzylic or tertiary nitro compounds with benzene gave normal alkylation products. Similarly, 2-nitropropane (2-NP) gave cumene (1) which underwent further transformation affording a mixture whose behaviour with time was investigated : results were consistent with oxidative formation of the cumyl carbonium ion (7').     

A catalytic asymmetric total synthesis of (–)-perophoramidine

We report a catalytic asymmetric total synthesis of the ascidian natural product perophoramidine. The synthesis employs a molybdenum-catalyzed asymmetric allylic alkylation of an oxindole nucleophile and a monosubstituted allylic electrophile as a key asymmetric step. The enantioenriched oxindole product from this transformation contains vicinal quaternary and tertiary stereocenters, and is obtained in high yield along with high levels of regio-, diastereo-, and enantioselectivity. To install the second quaternary stereocenter in the target, the route utilizes a novel regio- and diastereoselective allylation of a cyclic imino ether to deliver an allylated imino ether product in near quantitative yield and with complete regio- and diastereocontrol. Oxidative cleavage and reductive amination are used as final steps to access the natural product.     

Evidence of reversibility in azo-coupling reactions between 1,3,5-tris(N,N-dialkylamino)benzenes and arenediazonium salts

The reactions between strongly electron-rich aromatic substrates (1,3,5-tris(N,N-dialkylamino)benzenes, neutral carbon super nucleophiles) and diazonium salts produce moderately stable sigma complexes (Wheland complexes). The reactivity of Wheland complexes with electrophiles (other diazonium salts, or 4,7-dinitrobenzofuroxan) produces exchange reactions in the electrophilic part: the better electrophile replaces the less powerful electrophile. In the same way, in Wheland complexes with the 1,3,5-tris(morpholinyl)benzene, the 1,3,5-tris(piperidinyl)benzene replaces the less powerful nucleophile 1,3,5-tris(morpholinyl)benzene. Evidence is reported here indicating that for the title system the reaction of the attack of the electrophilic reagent producing Wheland complexes is a reversible process. The final products of the diazo-coupling reactions undergo a further attack of some diazonium salts. From the final products of the double diazo-coupling reactions (diazo compounds), we collected evidence that is a clear instance of complete reversibility of the diazo-coupling reaction.     

纳米HZSM-5催化剂催化烃类转化反应

制备了纳米(20~50 nm)HZSM-5催化剂, 用XRF, TEM和NH3-TPD等手段对催化剂进行了表征. 以正辛烷及苯和正辛烷混合物的转化为模型反应, 研究了单烃和混合烃在纳米HZSM-5催化剂上的转化行为, 考察了反应条件对产物分布的影响. 结果表明, 纳米HZSM-5沸石催化剂具有很强的烃类转化能力, 烃类通过芳构化、 异构化和烷基化等反应转化为高辛烷值的异构烷烃和芳烃, 产物中异构烷烃(C4~C6)和芳烃的质量分数超过90%. 直链烷烃转化为芳烃以生成苯环为主, 混合烃转化为芳烃以苯和小分子烃的烷基化为主. 控制反应条件可抑制苯和C+9芳烃的生成. 产物分析结果表明, 烃类在纳米HZSM-5催化剂上的裂解、芳构化和异构化等遵循正碳离子机理.     

Probing the Electronic Demands of Transmetalation in the Palladium-Catalyzed Cross-Coupling of Arylsilanolates

The electronic characteristics of coupling partners in the transmetalation step for the cross-coupling reaction of arylsilanolates have been investigated. The ability to interrogate the transmetalation event by in situ preparation of the arylpalladium(II) silanolate intermediate has enabled a Hammett analysis for both the aryl electrophile and arylsilanolate to be conducted. These studies reveal that electron-donating groups on the silicon nucleophile and electron-withdrawing groups on the electrophile accelerate the transmetalation process.     

Density functional theory analysis of dimethylphosphate hydrolysis: effect of solvation and nucleophile variation

We model the hydrolytic cleavage of dimethylphosphate by hydroxide ion and water in the gas phase and in implicit water using density functional theory. In all cases the rate-determining step is the nucleophilic attack. The barrier for this nucleophilic attack in vacuum is much larger for the hydroxide than for the neutral nucleophile. However, in water the barriers are similar. The rate-determining step in the attack by the neutral nucleophile involves the concerted transfer of a proton from the water molecule to the phosphate ion and the formation of the P–O bond.     

Simple d-glucosamine-based phosphine-imine and phosphine-amine ligands in Pd-catalyzed asymmetric allylic alkylations

A new family of phosphine-imine and phosphine-amine ligands based on d-glucosamine were synthesized in order to probe previous asymmetric allylic alkylation results with those of disaccharide ligands of the same class. In most cases, good-to-excellent activities and enantioselectivies were observed with these ligands with ee’s reaching up to 87% in the Pd-catalyzed allylic alkylation reaction of racemic (E)-1,3-diphenyl-2-propenyl acetate with dimethyl malonate as the nucleophile.     

Quantum chemical studies of azoles

The results of theoretical search for model transition states of the electrophilic substitution reaction in 2H-tetrazole (1) without the preliminary formation of N-protonated azolium salts are presented for two routes that were previously suggested by the authors and thermodynamically investigated: A, the attack of molecule 1 by the nucleophile (HO^–(aq)) to form the anion to which the electrophile H₃O⁺(aq)) is added and B, the attack of molecule 1 by the same electrophile followed by the addition of the same nucleophile to the specifically solvated protonated species formed in the preceding reaction step. The calculations were performed using the DFT/B3LYP/6-31G(d) method and the scanning procedure of the potential energy surface (PES). Both steps of route A turned out to be nearly barrierless, while in route B only its first step is barrierless and the second one is conjugated with passing an activation barrier of ～45 kcal mol^–1 between non-interacting or weakly interacting reactants and electrophilic substitution products. Unlike the specifically solvated protonated species of 1H-tetrazole in an aqueous solution, a similar species of 2H-tetrazole does not form a prereaction complex with the attacking nucleophile (HO^–(aq)) and the five-membered ring is destroyed in fact in the nitrogen-containing reaction product formed after passing the activation barrier. The optimized structure of the transition state differs strongly from the nitrogen-containing structure of the reaction product with the destroyed ring, which was found by scanning of the PES.     

Synthesis of 5-arylpyrrolo[2,1-c][1,4]benzodiazepines under mild cyclodehydration conditions

The efficiency of a cyclodehydration reaction leading to benzodiazepinones is markedly improved by N-methylation of the amide link connecting the nucleophile and the electrophile, which is attributed to its favouring both the more reactive E-rotamer and the exit of the leaving group.     

Recent advances in oxidative C–C coupling reaction of amides with carbon nucleophiles

The C–C coupling reaction of N-electron withdrawing group (EWG) protected amides with coupling partners is one of the most important methods for C–C bond formation at the α-position of amides to directly give α-substituted amides. Of the four reactions, namely, the reaction via the generation of carbanion with an electrophile, that via the generation of carbon radical with a radical donor, that via the generation of iminium ion species with a nucleophile (oxidative coupling reaction), and that using a transition metal carbenoid, the oxidative coupling reaction presents a challenge although the reaction products are very useful for the transformation of a wide range of nitrogen-containing derivatives. In this review, recent developments in the oxidative coupling reaction of N-EWG protected amides with nucleophiles are summarized with focus on the reaction using a transition metal, the transition-metal-free reaction, the enantioselective reaction using a chiral catalysts, and the organocatalyzed oxidative coupling reaction.     

Trimethylchlorosilane-promoted aza-Mannich reaction of enecarbamates and aldimines

A trimethylchlorosilane-promoted aza-Mannich reaction is reported utilizing enecarbamates as the nucleophile and aromatic N-Boc aldimines as the electrophile. A variety of nucleophiles and electrophiles are tolerated by the reaction conditions, delivering the adduct products in excellent yields with high E-stereoselectivities.     

Palladium catalyzed allylic C-H alkylation: a mechanistic perspective

The atom-efficiency of one of the most widely used catalytic reactions for forging C-C bonds, the Tsuji-Trost reaction, is limited by the need of preoxidized reagents. This limitation can be overcome by utilization of the recently discovered palladium-catalyzed C-H activation, the allylic C-H alkylation reaction which is the topic of the current review. Particular emphasis is put on current mechanistic proposals for the three reaction types comprising the overall transformation: C-H activation, nucleophilic addition, and re-oxidation of the active catalyst. Recent advances in C-H bond activation are highlighted with emphasis on those leading to C-C bond formation, but where it was deemed necessary for the general understanding of the process closely related C-H oxidations and aminations are also included. It is found that C-H cleavage is most likely achieved by ligand participation which could involve an acetate ion coordinated to Pd. Several of the reported systems rely on benzoquinone for re-oxidation of the active catalyst. The scope for nucleophilic addition in allylic C-H alkylation is currently limited, due to demands on pK(a) of the nucleophile. This limitation could be due to the pH dependence of the benzoquinone/hydroquinone redox couple. Alternative methods for re-oxidation that does not rely on benzoquinone could be able to alleviate this limitation.     

Fluoride ion-catalyzed desilylative-defluorination for synthetic organic chemistry

Two types of fluoride ion-catalyzed desilylative-defluorination (intermolecular and intramolecular versions) have been reported. The intermolecular desilylative-defluorination consists of sequential reactions: generation of a nucleophile by fluoride ion-promoted desilylation, its reaction with a fluorinated electrophile, and simultaneous formation of a final product and regeneration of fluoride ion. Precursors of nucleophiles used in the catalytic reaction system involve disilane, silyl ethers, CF₃-TMS, and FSO₂CF₂CO₂TMS. The intramolecular version is quite useful for the preparation of gem-difluoroalkenes including α-substituted 2,2-difluorostyrenes, 2-trimethylsilyloxy-3,3-difluoroacrylate, tetrafluoroquinodimethane, which arise from 1,2-, 1,4- and 1,6-desilylative-defluorination, respectively. The potential advantages for both types of the desilylative-defluorinations are (1) effective preparation of base or nucleophile-sensitive products, (2) very mild and essentially neutral conditions, and (3) use of a catalytic amount of fluoride ion.     

Alkylation mechanism of benzene with 1-dodecene catalyzed by Et3NHCl-AlCl3

The isotope exchange method was employed to investigate the catalytic mechanism of ionic liquid in alkylation of benzenes with olefins. It is proposed that alkylation was induced by the Lewis acid AlCl₃ which attracted π electrons of 1-dodecene to shift toward 1-carbon, thus forming a carbonium ion. The carbonium ion further reacted with benzenes to form a complex. Due to unstabilit of the complex, a deuterated ring proton was transferred into an electronegative 1-carbon of the side chain to substitute for the AlCl₃, accordingly 2-phenyldodecane was generated.     

A novel and stereospecific synthesis of (+)-exo-brevicomin

A novel and stereospecific synthesis of (+)-exo-brevicomin is disclosed. The key step of the reaction sequence employs the sulfinyl moiety as an intramolecular nucleophile to functionalize an alkene π-complexed to a bromonium ion.     

Der A2+-Mechanismus der säurekatalytischen Hydrolyse von Epoxiden und anderen cyklischen Oxa- Verbindungen

It is suggested that the acid catalyzed hydrolyses of isobutene oxide and propene oxide proceed via rate-determining bimolecular reaction between a carbonium ion and solvent water (A2⁺ mechanism). The carbonium ion is formed by ring opening of the protonated epoxide, in a relatively fast preceding step. This mechanism is in agreement with all experimental facts known today. There is evidence for a similar mechanism in the acid catalyzed hydrolyses of cyclic acetals.     

Studies in trifluoromethaneslphonic acid—I: Protonation of aromatic derivatives

Solutions of aromatic compounds in trifluoromethanesulphonic acid have been investigated using ¹H-NMR and UV specstrocpy. In addition, conductivity measurements were made over a wide concentration range. At low concentrations ( < 0.001M) all derivatives were completely protonated. For 0.2-1.0M evidence was obtained for ion pairing in the cases of benzene and toluene. An intimate ion pair is postulated in which the partial charges on the arenium ion and on the sulphonate anion match up to maximise electrostatic interaction. The stability of these two species is discussed for a series of methylbenzenes.Exchange reactions are also discussed in terms of ion pairing. Preliminary results show that both trans alkylation and isomerisation reactions occur with great facility in this solvent. Dihydroanthreacene undergoes a disproportionation reaction in which it acts as a very powerful hydride donor.     

Activation and stabilization of aldimines by an ortho-trifluoromethyl substituent in direct vinylogous Mannich-type reactions

Lewis acid catalyzed direct vinylogous Mannich-type reaction with a weak nucleophile dienol, generated in situ by ring-opening and rearrangement of vinyloxiranes, could be demonstrated in excellent yields under mild conditions using benzylidene(4-methoxy-2-trifluoromethyl)aniline (MTMA) as an electrophile. The o-trifluoromethyl substituent can stabilize imines by a steric effect and activate by an electron-withdrawing effect. It proved to be an easily deprotectable protecting group for direct Mannich-type reactions.     

Structures and reaction mechanisms of cumene formation via benzene alkylation with propylene in a newly synthesized ITQ-24 zeolite: an embedded ONIOM study

The cumene formation via benzene alkylation with propylene on the new three-dimensional nanoporous catalyst, ITQ-24 zeolite, has been investigated by using the ONIOM2(B3LYP/6-31G(d,p):UFF) method. Both consecutive and associative reaction pathways are examined. The contributions of the short-range van der Waals interactions, which are explicitly included in the ONIOM2 model, and an additional long-range electrostatic potential from the extended zeolite framework to the energy profile are taken into consideration. It is found that benzene alkylation with propylene in the ITQ-24 zeolite prefers to occur through the consecutive reaction mechanism. The benzene alkylation step is the reaction rate-determining step with an estimated activation energy of 35.70 kcal/mol, comparable with an experimental report in beta-zeolite of 34.9 kcal/mol. The electrostatic potential from the extended zeolite framework shows a much more significant contribution to the transition state selectivity than the van der Waals interactions.