Reaction of essentially free benzyl cations with acetonitrile; synthesis of ethanimidic carboxylic anhydrides and unsymmetrical diacylamines

Benzyl cations were generated via the thermal decomposition of N-benzyl-N-nitrosopivalamide in acetonitrile and acetonitrile-water mixtures at 25 degrees C. The first-formed (primary) benzylating agent, the benzyl cation, was scavenged competitively by pivalate (trimethyl acetate) ion to yield benzyl pivalate, by acetonitrile to yield the corresponding N-benzylnitrilium ion, and by water (when present) to yield benzyl alcohol. The nitrilium ion underwent a cascade of reactions to yield an array of products whose identities and relative yields as a function of time were used to elucidate the mechanistic paths involved. Thus, the N-benzylnitrilium ion reacted with pivalate ion to yield the Z-isomer of N-benzylethanimidic pivalic anhydride, followed by its conversion into the E-isomer. This article appears to be the first to document compounds of this type. The E-isomer is labile under the reaction conditions, rearranging into N-acetyl-N-pivalylbenzylamine. In the presence of water as a diluent, a significant fraction of the nitrosoamide was hydrolyzed to benzyl alcohol; hydrolysis of the nitrilium ion yielded N-benzyl acetamide. The yield of hydrosylates was directly proportional to the mole fraction of water in the medium.     

An enantioselective total synthesis of (+)- and (-)-saudin. Determination of the absolute configuration.

A short efficient enantioselective synthesis of both (+)- and (-)-saudin, a naturally occurring hypoglycemic diterpene, is described. This synthesis establishes the absolute configuration of natural (-)-saudin for the first time. The key steps include the enantioselective construction of a dimethyl Hagemann's ester by an asymmetric Michael reaction and establishment of the key 1,3 disposed quaternary centers by means of a novel Ti(IV) promoted Claisen rearrangement. The assembly of the polycyclic ketal skeleton was likely under kinetic control proceeding via formation of the C1oxygen-C7 bond through an oxonium ion intermediate in the final stage.     

Intermolecular hydroamination of allenes with N-unsubstituted carbamates catalyzed by a gold(I) N-heterocyclic carbene complex

Reaction of 2,3-pentadienyl benzoate with benzyl carbamate catalyzed by a 1:1 mixture of (NHC)AuCl and AgOTf in dioxane at 23 degrees C for 5 h led to isolation of (E)-4-(benzyloxycarbonylamino)-2-pentenyl benzoate in 84% yield as a single regio- and diastereomer. Gold(I)-catalyzed hydroamination was effective for a number of N-unsubstituted carbamates and a range of substituted allenes.     

A ring-fusion/ring-fission mechanism for the metathesis reaction of macrocyclic formaldehyde acetals

Important insight has been obtained into the mechanism of the reversible acid-catalysed transacetalation of cyclophane formaldehyde acetals (formals) C(i) in CDCl(3), at 25 degrees C. The order of appearance of the lowest oligomers in the early stages of the equilibration reaction is fully consistent with ring-fusion/ring-fission processes in which oxonium ion intermediates undergo S(N)2 reactions, according to an acid-catalysed bimolecular (A2) mechanism. The alternative acid-catalysed monomolecular (A1) reaction path, based on "back-biting" processes of carbenium ions generated by S(N)1-type cleavage of oxonium ion intermediates, predicts sequences that are in marked contrast with experimental findings.     

Gas-phase activation and reaction dynamics of chiral ion-dipole complexes

A family of enantiomerically pure oxonium ions, that is O-protonated 1-aryl-1-methoxyethanes, has been generated in the gas phase by the (CH(3))(2)Cl(+) methylation of the corresponding 1-arylethanols. Some information on their reaction dynamics was obtained from a detailed kinetic study of their inversion of configuration and dissociation. The activation parameters of the inversion reaction are found to obey two different isokinetic relationships depending upon the nature and the position of the substituents in the oxonium ions. In contrast, the activation parameters of the dissociation reaction obey a single isokinetic relationship. The inversion and dissociation rate constants do not follow simple linear free-energy relationships. This complicated kinetic picture has been rationalized in terms of different activation dynamics in gaseous CH(3)Cl, which, in turn, determine the reaction dynamics of the oxonium ion. When the predominant activation of the oxonium ion involves resonant energy exchange from the 1015 cm(-1) CH(3) rocking mode of unperturbed CH(3)Cl, the inversion reaction proceeds through the dynamically most favored TS, characterized by the unassisted C(alpha)bond;O bond elongation. When, instead, the activation of the oxonium ions requires the formation of an intimate encounter complex with CH(3)Cl, the inversion reaction takes place via the energetically most favored TS, characterized by multiple coordination of the CH(3)OH moiety with the H(alpha) and H(ortho) atoms of the benzylic residue. The activation dynamics operating in the intimate encounter complex with CH(3)Cl is also responsible for the dissociation of most selected oxonium ions.     

Gas-phase Meerwein reaction of epoxides with protonated acetonitrile generated by atmospheric pressure ionizations

Ethylnitrilium ion can be generated by protonation of acetonitrile (when used as the LC-MS mobile phase) under the conditions of atmospheric pressure ionizations, including electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) as well as atmospheric pressure photoionization (APPI). Ethylnitrilium ion ( CH₃ - C o \mathop N⁺ HCH_3 - C \equiv \mathop N\limits^ + H and its canonical form CH₃ - \mathop C⁺ = NHCH_3 - \mathop C\limits^ + = NH) is shown to efficiently undergo the gas-phase Meerwein reaction with epoxides. This reaction proceeds by the initial formation of an oxonium ion followed by three-to-five-membered ring expansion via an intramolecular nucleophilic attack to yield the Meerwein reaction products. The density functional theory (DFT) calculations at the B3LYP/6-311 + G(d,p) level show that the gas-phase Meerwein reaction is thermodynamically favorable. Collision-induced dissociation (CID) of the Meerwein reaction products yields the net oxygen-by-nitrogen replacement of epoxides with a characteristic mass shift of 1 Da, providing evidence for the cyclic nature of the gas-phase Meerwein reaction products. The gas-phase Meerwein reaction offers a novel and fast LC-MS approach for the direct analysis of epoxides that might be of genotoxic concern during drug development. Understanding and utilizing this unique gas-phase ion/molecule reaction, the sensitivity and selectivity for quantitation of epoxides can be enhanced.     

The mass spectral behavior of β-bromoethyl benzoate and some of its ring substituted derivatives: Evidence for oxygen participation in the formation of [M ? Br]+

The expulsion of a bromine atom from the molecular ion of β-bromoethyl benzoate displays a kinetic behavior which resembles that of a rearrangement reaction. The two oxygen atoms in the resulting [M ? Br]⁺ ion become equivalent before or during the secondary decomposition of this ion, as shown by oxygen labeling. In addition, the primary ions generated from benzophenone ethylene ketal and acetophenone ethylene ketal by phenyl and methyl expulsion, respectively, undergo subsequent fragmentation in the same way as the [M ? Br]⁺ from β-bromoethyl benzoate. These results strongly indicate that the carbonyl-oxygen atom participates in the expulsion of bromine. The effect of ring substituents on the competing loss of bromine and McLafferty rearrangement is also discussed.     

Synthesis of bicyclic gamma-ylidenetetronates

The bicyclic gamma-ylidenetetronate motif found in several Stemona alkaloids was prepared in a stereoselective manner by addition of lithium methyl tetronate to an alkoxy oxonium ion formed from a lactone. The corresponding mixed alkyl ketal obtained was subjected to a Lewis acid-base-promoted dealkoxylation reaction to deliver the desired products.     

Preparation of d,l-Phenylalanine by Amidocarbonylation of Benzyl Chloride

The preparation of d,l-phenylalanine via amidocarbonylation of benzyl chloride with acetamide and CO/H(2) is described. The rate of the reaction is dependent upon the CO pressure below 250 bar, but independent of the hydrogen pressure. A reaction temperature of 100 degrees C gives optimum yields. A relatively large amount of the catalyst, Co(2)(CO)(8), is needed for complete conversion because of inhibition caused by hydrogen chloride which is formed during the reaction. Addition of NaHCO(3) removes HCl as insoluble NaCl, resulting in improved conversion and selectivity of the reaction. It also allows the use of a stoichiometric amount of acetamide, whereas a 2- to 3-fold excess of acetamide is needed for complete conversion of benzyl chloride without NaHCO(3). Amidocarbonylation of benzyl alcohol gave d,l-phenylalanine in only 8% yield.     

Transacetalization with gaseous carboxonium and carbosulfonium ions

Primary carboxonium (H2C=O+-R) and carbosulfonium (H2C=S+-R) ions (R = CH3, C2H5, Ph) and the prototype five-membered cyclic carboxonium ion are found to react in the gas phase with cyclic acetals and ketals by transacetalization to form the respective O-alkyl-1,3-dioxolanium and S-alkyl-1,3-oxathiolanium ions. The reaction, which competes mainly with proton transfer and hydride abstraction, initiates by O-alkylation and proceeds by ring opening and recyclization via intramolecular displacement of the carbonyl compound previously protected in its ketal form. As indicated by product ion mass spectra, and confirmed by competitive reactions, carbosulfonium ions are, by transacetalization, much more reactive than carboxonium ions. For acyclic secondary and tertiary carboxonium ions bearing acidic alpha-hydrogens, little or no transacetalization occurs and proton transfer dominates. This structurally related reactivity distinguishes primary from both secondary and tertiary ions, as exemplified for the two structural isomers H2C=O+-C2H5 and CH3C(H)=O+-CH3. The prototype five- and six-membered cyclic carboxonium ions react mainly by proton transfer and adduct formation, but the five-membered ring ion also reacts by transacetalization to a medium extent. Upon CID, the transacetalization products of the primary ions often dissociate by loss of formaldehyde, and a +44 u neutral gain/-30 u neutral loss MS3 scan is shown to efficiently detect reactive carboxonium and carbosulfonium ions. Transacetalization with either carboxonium or carbosulfonium ions provides a route to 1,3-oxathiolanes and analogs alkylated selectively either at the sulfur or oxygen atom.     

Proton transfer reactions of photogenerated cyanoaromatic-methylaromatic radical ion pairs2

The photochemical reactions of a number of cyanoaromatic (acceptor) and methylaromatic (donor) molecules have been investigated. These reactions can result in the formation of photosubstitution products or benzyl radical coupling products. A survey of our results and previously published data indicates that exergonic photostimulated electron transfer is a necessary but not sufficient condition for the observation of reaction products. The efficiency of proton transfer from the donor cation radical to the acceptor anion radical is determined by the kinetic acidity and basicity of the radical ion pair. Mechanistic evidence is presented which indicates that proton transfer requires diffusion apart and reencounter of the initially formed radical ion pair. Predominant radical pair combination is observed for anion radicals which yield electron-deficient free radicals upon protonation, whereas predominant cage escape and benzyl radical coupling is observed for anion radicals which yield electron-rich free radicals upon protonation.     

醇钠催化苯甲醛合成苯甲酸苄酯——介绍一个探索性有机化学实验

To cultivate students' innovation ability,we designed the synthetic method of benzyl benzoate as an organic chemistry laboratory experiment.Benzyl benzoate was directly synthesized from benzaldehyde in the presence of a sodium benzyloxide via the Tishchenko reaction.The experimental conditions are optimized with a benzaldehyde to sodium benzyloxide mole ratio of 33:1 and a 50-60℃ reaction temperature for 1 h.A yield up to 69% was achieved with the purity higher than 99%.The experimental results show that the process is in accordance with the requirements of organic chemistry laboratory teaching.     

Flash photolytic generation of o-quinone alpha-phenylmethide and o-quinone alpha-(p-anisyl)methide in aqueous solution and investigation of their reactions in that medium. Saturation of acid-catalyzed hydration.

o-quinone alpha-phenylmethide was generated as a short-lived transient species in aqueous solution by flash photolysis of o-hydroxy-alpha-phenylbenzyl alcohol, and its rate of decay was measured in HClO4 and NaOH solutions as well as in CH3CO2H, H2PO4-, and HCO3- buffers. These data show that hydration of this quinone methide back to its benzyl alcohol precursor occurs by acid-, base-, and uncatalyzed routes. The acid-catalyzed reaction gives the solvent isotope effect kH+/kD+ = 0.34, whose inverse nature indicates that this reaction occurs via rapid preequilibrium protonation of the quinone methide on its carbonyl oxygen atom followed by rate-determining capture of the ensuing carbocationic intermediate by water, a conclusion supported by the saturation of acid catalysis in concentrated HClO4 solution. o-quinone alpha-(p-anisyl)methide was also generated by flash photolysis of the corresponding benzyl alcohol and of the p-cyanophenol ether of this alcohol as well, and its rate of decay was measured in HClO4 and NaOH solutions and in HCO2H, CH3CO2H, HN3, CF3CH2NH3+, imidazolium ion, H2PO4-, (CH2OH)3CNH3+, (CH3)3CPO3H-, and HCO3- buffers. Acid-, base-, and uncatalyzed hydration reaction routes were again found, and solvent isotope effects as well as saturation of acid catalysis, this time in dilute HClO4, confirmed a preequilibrium mechanism for the acid-catalyzed reaction. Analysis of the buffer data gave buffer-base rate constants that did not conform to the Br?nsted relation, consistent with the expected nucleophilic nature of the buffer reactions.     

A concise method for the synthesis of 2-tetralone by titanium tetrachloride-promoted cyclization of 4-aryl-2-hydroxybutanal diethyl acetal

4-Aryl-2-hydroxybutanal diethyl acetal, prepared from the reaction of benzyl Grignard reagent and glycidaldehyde diethyl acetal, was treated with titanium tetrachloride to give 2-tetralone in good yield. This highly efficient transformation involves tandem oxonium formation, intramolecular Friedel-Crafts alkylation, deethoxylation, and tautomerization in the same flask.     

Enzyme-catalysed synthesis and reactions of benzene oxide/oxepine derivatives of methyl benzoates

A series of twelve benzoate esters was metabolised, by species of the Phellinus genus of wood-rotting fungi, to yield the corresponding benzyl alcohol derivatives and eight salicylates. The isolation of a stable oxepine metabolite, from methyl benzoate, allied to evidence of the migration and retention of a carbomethoxy group (the NIH Shift), during enzyme-catalysed ortho-hydroxylation of alkyl benzoates to form salicylates, is consistent with a mechanism involving an initial arene epoxidation step. This mechanism was confirmed by the isolation of a remarkably stable, optically active, substituted benzene oxide metabolite of methyl 2-(trifluoromethyl)benzoate, which slowly converted into the racemic form. The arene oxide was found to undergo a cycloaddition reaction with 4-phenyl-1,2,4-triazoline-3,5-dione to yield a crystalline cycloadduct whose structure and racemic nature was established by X-ray crystallography. The metabolite was also found to undergo some novel benzene oxide reactions, including epoxidation to give an anti-diepoxide, base-catalysed hydrolysis to form a trans-dihydrodiol and acid-catalysed aromatisation to yield a salicylate derivative via the NIH Shift of a carbomethoxy group.     

Photodecarbonylation of dibenzyl ketones and trapping of radical intermediates by copper(II) chloride in frozen aqueous solutions

This paper presents a quantitative and qualitative study of the Norrish type I reaction of dibenzyl ketone (DBK) and 4-methyldibenzyl ketone (MeDBK), producing the benzyl radicals and consequently recombination products, in frozen aqueous solutions over a broad temperature range (-80 to 20 degrees C). This work extends previous research on the cage effects in various constrained media to provide information about the dynamics and reactivity of the photochemically generated intermediates at the grain boundaries of ice matrix. As the temperature of aqueous solutions decreases, the solute concentrations become high at layers covering ice crystals, causing efficient molecular segregation. The cage effect experiments have shown that diffusion of the benzyl radicals within such reaction aggregates is still remarkably efficient at temperatures below -50 degrees C, independently of the initial ketone concentration in the range of 10(-6)-10(-4) mol L(-1). In addition, the study of trapping the benzyl radicals formed in situ by CuCl2 was used as a qualitative probe of heterogeneous bimolecular reactions in the frozen aqueous matrix and on its surface. Molecules of both solutes were found to be segregated from the ice phase to the same location and underwent chemical reactions within diffusion and intermediates lifetimes limits. Understanding the fundamental physicochemical processes in ice is unquestionably important in related environmental or cosmochemical investigations.     

Theoretical elucidation of a classic reaction: protonation of the quadruple bond of the octachlorodimolybdate(II,II) [Mo2Cl8]4- anion

The protonation reaction of the unbridged quadruple metal-metal bond of [Mo(2)Cl(8)](4-) anion producing the triply bonded hydride [Mo(2)(μ-H)(μ-Cl)(2)Cl(6)](3-) is studied by accurate Density Functional Theory computations. The reactant, product, stable intermediates, and transition states are located on the potential energy surface. The water solvent is explicitly included in the calculations. Full reaction profiles are calculated and compared to experimental data. The mechanism of the reaction is fully elucidated. This involves two steps. The first is a proton transfer from an oxonium ion to the quadruple bond, being rate determining. The second, involves the internal rearrangement of chlorine atoms and is much faster. Activation energies with a mean value of 19 kcal/mol are calculated, in excellent agreement with experimental values.     

Difluorocarbene-Induced Ring-Opening Difluoromethylation-Halogenation of Cyclic (Thio)Ethers with TMSCF2X (X=Br,Cl)**

The ring-opening difluoromethylation-halogenation of cyclic (thio)ethers is reported through a simple strategy relying on carbon-chalcogen bond activation with difluorocarbene. The reaction proceeds through in situ protonation of the previously little-known difluoromethylene oxonium or sulfonium ylide intermediate followed by ring-opening with halide ion to afford halogenated acyclic difluoromethyl (thio)ethers that can then be employed for further elaboration. TMSCF₂X (X=Br, Cl) are unique reagents to achieve this synthetic purpose, which serve as both the difluorocarbene source and the halide ion source.     

Asymmetric protonation of ketone enolates using chiral beta-hydroxyethers: acidity-tuned enantioselectivity

New chiral hydroxyethers 1a-f were prepared for asymmetric protonation of achiral enolates prepared from prochiral ketones. The enantioselectivity of protonation was highly dependent upon the acidity of the chiral alcohols, the highest enantioselectivity (90% ee) being achieved with 3,5-dichloro-substituted beta-hydroxyether 1c. A salt-free enolate generated from trimethylsilyl enol ether 4 provided product of the highest ee. Unlike other reagents, chloro-substituted alcohols provided almost consistent enantioselections throughout the reaction temperatures examined (-25 to -98 degrees C). Protonation of other aromatic ketones showed selectivity similar to that of 2-methyl-1-tetralone.     

Oxidation of vanadium(III) by hydrogen peroxide and the oxomonoperoxo vanadium(V) ion in acidic aqueous solutions: a kinetics and simulation study

The reaction between vanadium(III) and hydrogen peroxide in aqueous acidic solutions was investigated. The rate law shows first-order dependences on both vanadium(III) and hydrogen peroxide concentrations, with a rate constant, defined in terms of -d[H(2)O(2)]/dt, of 2.06 +/- 0.03 L mol(-)(1) s(-)(1) at 25 degrees C; the rate is independent of hydrogen ion concentration. The varying reaction stoichiometry, the appreciable evolution of dioxygen, the oxidation of 2-PrOH to acetone, and the inhibition of acetone formation by the hydroxyl radical scavengers, dimethyl sulfoxide and sodium benzoate, point to a Fenton mechanism as the predominant pathway in the reaction. Methyltrioxorhenium(VII) does not appear to catalyze this reaction. A second-order rate constant for the oxidation of V(3+) by OV(O(2))(+) was determined to be 11.3 +/- 0.3 L mol(-)(1) s(-)(1) at 25 degrees C. An overall reaction scheme consisting of over 20 reactions, in agreement with the experimental results and literature reports, was established by kinetic simulation studies.