An intramolecular benzyl rearrangement of 1-(N-benzyloxycarbonylamino)alkylphosphonate diesters under electrospray ionization conditions

The mass spectrometric behavior of eleven 1-(N-benzyloxycarbonyl(Cbz)amino)alkylphosphonate diesters was studied under positive ion electrospray ionization (ESI) conditions. Their fragmentation pathways are depicted and supported by tandem mass spectrometry. Besides the common eliminations of ether, benzyl alcohol, phosphite and an ether plus benzyl alcohol from molecular ions, the title compounds show a tendency to undergo an interesting intramolecular benzyl rearrangement to yield benzylphosphonate ions. The fragmentation patterns do not depend on the substituent attached to the alpha-carbon atom.     

Insertion of group ivb metal dihalides into iron—carbon sigma bonds

The insertion reaction of GeCl₂, SnCl₂ and SnBr₂ into the iron—carbon sigma bond in the compounds Cp(CO)₂FeR (R = methyl, ethyl, n-propyl, p-methoxybenzyl, benzyl and p-trifluoromethyl- benzyl), to yield complexes with the Group IV metal bound to iron, is a radical chain process. A small substituent effect is observed in all cases for the benzyl series, the reaction being favoured by increasing electron donating ability of the para substituent. No clear reactivity trend is noted for the other alkyl complexes.     

Synthesis,electrochemical and anti-microbial study of 2,5-diamino benzoquinones

Fifteen symmetric and unsymmetric 2,5-diamino benzoquinones have been synthesized and their electrochemical activity was studied by cyclic voltammetry. Presence of electron donating substituent like amino group found to shift the half wave potential towards more negative value which is more prominent in benzyl amino substituted amino benzoquinones (ABQs). All the compounds were checked for their anti-bacterial and anti-fungal activity by agar well diffusion technique. The more negative E_1/2 values of benzyl amino substituted ABQs make them more active towards various bacterial and fungal speciesas is evident from MIC values.     

A kinetic study of the mechanism of esterification of 1-arylethanols in trifluoroacetic acid

Aryl substituent effects upon the rate constants for the esterification of a series of 1-arylethanols in trifluoroacetic acid are in accordance with a reverse A_AL1 mechanism, for which the Hammett reaction constant ? = ?3.69 has been determined by correlation with σ⁺ substituent constants. The rates of reaction are ca. 50-fold faster than those for corresponding benzyl alcohols which bear an electron-donating aryl substituent and which are also believed to undergo esterification in trifluoroacetic acid by the reverse A_AL1 mechanism; the reverse A_AC2 mechanism applies to benzyl alcohols which bear an electron- withdrawing aryl substituent.     

Nucleophilic substitution reaction of benzyl benzenesulfonates with anilines in methanol-acetonitrile mixtures. Part 3

Kinetic studies on the nucleophilic substitution reaction of para-nitrobenzyl benzenesulfonates with anilines in methanol-acetonitrile mixtures are reported. It was found that the reaction proceeds via a dissociative S_N2 mechanism with a more product -like transition state compared to that for substrate with a less electron attracting substituent. A stronger electron withdrawing substituent in the benzyl moiety and in the leaving group is shown to enhance both bond making and bond breaking but the enhancement of bond making was found to be greater than that of bond breaking.     

Effect of substituents in the ring on the structure and electron density distribution in benzyl halides

The results of calculations for benzyl chlorides and benzyl bromides that contain substituents in the ring by different quantum-chemical methods are compared. The electron density on the benzyl carbon atom increases as the electron-withdrawing properties of substituents are enhanced due to the shift of the electron density from the benzyl halogen atom to the adjacent carbon atom. A topological analysis according to Bader confirmed the main reason for the change in the paramagnetic shielding of the benzyl¹³C nuclei. The results of calculations provide, for the first time, an explanation for the resonance upfield¹³C shift of this atom as the Hammett constant of a substituent increases in compounds of the series under consideration. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2111–2119, December, 1997.     

Chemiluminescence characteristics of N-(4-substituted benzyl)isoluminol

We synthesized N-(4-substituted benzyl)isoluminol which has 4-bromo-, 4-methyl-, 4-methoxy-, 4-nitrogroups. These compounds produced chemiluminescence by the reaction with the oxidizing agent, potassium hexacyanoferrate and hydrogen peroxide, in an alkaline medium. The chemiluminescence intensities of these compounds were 0.03-4.7 times that of isoluminol. We used Hamett substituent constants as a parameter for the electronic substituent effects. The relationship between the amino-H chemical shift value and the Hamett substituent constants showed a good linear correlation. The relationship between the chemiluminescence intensities and the Hamett substituent constants showed a good linear correlation. The relationship between the fluorescence intensities and the Hamett substituent constants also showed a good linear correlation. These results suggest that the change in the electron density around the amino group strongly influences the fluorescence intensities and corresponding chemiluminescence intensities of these derivatives.     

Synthesis of 8‐substituted tetrahydro‐γ‐carbolines

The Fischer reaction is applied to the synthesis of 8‐substituted tetrahydro‐γ‐carbolines with electron‐donating or electron‐withdrawing groups, using catalytic or thermal methods. The reaction conditions must be varied according to the nature of the N 1 substituent of the piperidone. The best results are observed when a releasing group is present on the arylhydrazine and a benzyl substituent on the nitrogen of piperidone. Formation of carbolines with a withdrawing substituent is observed in soft acidic conditions; in others, reaction ended at the hydrazone level or did not evolve.     

Substituent Effects on the Photocleavage of Benzyl-Sulfur Bonds. Observation of the "Meta Effect"

Benzyl phenyl sulfide has been used to investigate the photocleavage mechanism for benzyl-sulfur bonds. Four experiments have shown that the reaction goes through a radical intermediate. First, the photoproducts observed can all be justified by radical mechanisms. Second, the radical intermediate was trapped with a five hexenyl tether. Third, UV analysis of analogs for the 4-NO(2) derivative indicate no exciplex or electron transfer pathway. Fourth, no strong correlation is observed between sigma values and the quatum yields for loss of substituted benzyl phenyl sulide. The effect of oxygen on quantum yields is best observed after samples are thoroughly outgassed with consecutive freeze-pump-thaw cycles. It is shown that oxygen diminishes the substituent effect. Upon photolysis of the outgassed samples, the meta-substituted derivatives showed more significant variances than the para derivatives. The meta derivatives are most efficiently cleaved in the following order: 3-CN > 3-NO(2) > 3-CF(3) > 3-CH(3) > 3-OCH(3). These findings are justified by an increase in electron density of the radical ipso to the forming benzyl radical for the 3-OCH(3) derivative and a decrease in the electron density of the radical ipso to the forming benzyl radical for the 3-CN derivative.     

Substituent Effect on the Tautomerization of 1‐Arylazonaphthalen‐2‐ols by Mass Spectrometric Analysis

An electron‐ionization (EI) mass spectra of a series of 1‐arylazonaphthalen‐2‐ols was obtained for studying the substituent effect on the fragmentation. The correlation between the ratio, molecular ion and fragment ion, and Hammett’s constants is applied to examine the effect of the substituent on the fragmentation. The negative correction between the ratio, I_{molecular ion}/(I_171amu + I_143amu + I_115amu), and Hammett’s constants indicates an electron‐withdrawing group destabilized the molecular ion. An unusual long‐range hydrogen transfer demonstrates an important role in the fragmentation process     

Quantitative Description of Structural Effects on the Stability of Gold(I) Carbenes

The gas‐phase bond‐dissociation energies of a SO₂–imidazolylidene leaving group of three gold(I) benzyl imidazolium sulfone complexes are reported (E₀=46.6±1.7, 49.6±1.7, and 48.9±2.1 kcal mol^?1). Although these energies are similar to each other, they are reproducibly distinguishable. The energy‐resolved collision‐induced dissociation experiments of the three [L]–gold(I) (L=ligand) carbene precursor complexes were performed by using a modified tandem mass spectrometer. The measurements quantitatively describe the structural and electronic effects a p‐methoxy substituent on the benzyl fragment, and trans [NHC] and [P] gold ligands, have towards gold carbene formation. Evidence for the formation of the electrophilic gold carbene in solution was obtained through the stoichiometric and catalytic cyclopropanation of olefins under thermal conditions. The observed cyclopropane yields are dependent on the rate of gold carbene formation, which in turn is influenced by the ligand and substituent. The donation of electron density to the carbene carbon by the p‐methoxy benzyl substituent and [NHC] ligand stabilizes the gold carbene intermediate and lowers the dissociation barrier. Through the careful comparison of gas‐phase and solution chemistry, the results suggest that even gas‐phase leaving‐group bond‐dissociation energy differences of 2–3 kcal mol^?1 enormously affect the rate of gold carbene formation in solution, especially when there are competing reactions. The thermal decay of the gold carbene precursor complex was observed to follow first‐order kinetics, whereas cyclopropanation was found to follow pseudo‐first‐order kinetics. Density‐functional‐theory calculations at the M06‐L and BP86‐D3 levels of theory were used to confirm the observed gas‐phase reactivity and model the measured bond‐dissociation energies.     

Lutidyl Radical Photoelectron Spectra Reveal Additive Substituent Effects on Benzyl Derivatives’ Ionization Energy

Understanding how isomerism influences photoelectron spectra helps in the assignment and analysis of reactive mixtures, especially for heterocycles with numerous isomers. Threshold photoelectron spectra of lutidyl radical isomers, i. e., benzyl derivatives with a nitrogen heteroatom and a methyl substituent, are recorded using vacuum ultraviolet synchrotron radiation. The radicals are produced by flash pyrolysis from aminomethyl methylpyridine precursors. Experimental ionization energies are determined to be 7.54, 7.50, and 7.45 eV for 2,4-, 2,6- and 3,5-lutidyl, respectively, in excellent agreement with composite method calculations. Franck–Condon simulations aid the TPES assignment but are also shown to exhibit artifacts if large-amplitude motions, notably the methyl internal rotation are assumed to be active in the double harmonic approximation. Based on calculated adiabatic ionization energies (AIE) of benzyl, picolyl, and xylyl radicals, the N and CH₃ substituent effects are found to be additive, position-dependent and decrease in the para>ortho≳meta order in magnitude with the nitrogen heteroatom increasing and the methyl substituent decreasing the AIE. These effects are discussed in light of the charge distribution upon ionization. The additivity of the substituent effects also helps predict the influence of substituents on the binding energy of the unpaired electron in analogous radicals.     

Chemoselective S-benzylation of indoline-2-thiones using benzyl alcohols

Indoline-2-thiones were chemoselectively S-benzylated using a variety of benzyl alcohols by boron trifluoride etherate-catalyzed reactions. The aryl substituent effect on the reactivity of the benzyl alcohols toward S-benzylation is also discussed.     

Solid‐State Emissive Aroyl‐S,N‐Ketene Acetals with Tunable Aggregation‐Induced Emission Characteristics

N‐Benzyl aroyl‐S,N‐ketene acetals can be readily synthesized by condensation of aroyl chlorides and N‐benzyl 2‐methyl benzothiazolium salts in good to excellent yields, yielding a library of 35 chromophores with bright solid‐state emission and aggregation‐induced emission characteristics. Varying the substituent from electron‐donating to electron‐withdrawing enables the tuning of the solid‐state emission color from deep blue to red.     

Substituent chemical shift (SCS) correlations for the hydroxyl proton of substituted benzyl alcohols

The hydroxyl proton chemical shift in substituted benzyl alcohols may be used, either at infinite dilution in CCl₄, or in dilute solutions in DMSO, as a reliable indicator of substituent effects. The sensitivity of the hydroxyl proton to substituents is much greater than that of the methylene protons, and this is shown to be due to an increased dependence of the hydroxyl proton on the field effect of the substituent. The percent resonance contribution to the OH chemical shifts is identical in benzyl alcohols and phenols.     

Regioselective bond cleavage in the dissociative electron transfer to benzyl thiocyanates: the role of radical/ion pair formation

Important aspects of the electrochemical reduction of a series of substituted benzyl thiocyanates were investigated. A striking change in the reductive cleavage mechanism as a function of the substituent on the aryl ring of the benzyl thiocyanate was observed, and more importantly, a regioselective bond cleavage was encountered. A reductive alpha-cleavage (CH(2)-S bond) was seen for cyano and nitro-substituted benzyl thiocyanates leading to the formation of the corresponding nitro-substituted dibenzyls. With other substituents (CH(3)O, CH(3), H, Cl, and F), both the alpha (CH(2)-S) and the beta (S-CN) bonds could be cleaved as a result of an electrochemical reduction leading to the formation of the corresponding substituted monosulfides, disulfides, and toluenes. These final products are generated through either a protonation or a nucleophilic reaction of the two-electron reduction-produced anion on the parent molecule. The dissociative electron transfer theory and its extension to the formation/dissociation of radical anions, as well as its extension to the case of strong in-cage interactions between the produced fragments ("sticky" dissociative electron transfer (ET)), along with the theoretical calculation results helped rationalize (i) the observed change in the ET mechanism, (ii) the dissociation of the radical anion intermediates formed during the electrochemical reduction of the nitro-substituted benzyl thiocyanates, and more importantly (iii) the regioselective reductive bond cleavage.     

Investigation of steric and electronic effects in the copper-catalysed asymmetric oxidation of sulfides

Steric and electronic effects in the copper-catalysed asymmetric oxidation of aryl benzyl, aryl alkyl and alkyl benzyl sulfides have been investigated. The presence of an aryl group directly attached to the sulfur is essential to afford sulfoxides with high enantioselectivities, with up to 97% ee for 2-naphthyl benzyl sulfoxide, the highest enantioselectivity achieved to date for copper-catalysed asymmetric sulfoxidation. In contrast, the benzyl substituent can be replaced by sterically comparable groups with no effect on enantioselectivity. Copper-mediated oxidation of substituted aryl benzyl sulfides display modest steric and electronic effects resulting in comparable or lower enantioselectivities to those obtained with the unsubstituted benzyl phenyl sulfide.     

The effect of a 3-benzyl group on the coordination chemistry of homoscorpionate ligands

New homoscorpionate ligands containing a 3-benzyl substituent, hydrotris(3-benzyl-5-methylpyrazol-1-yl)borate, Tp(Bn,Me), and hydrotris(3-benzyl-4-phenylpyrazol-1-yl)borate, Tp(Bn,4Ph), have been synthesized, and the dynamic behavior of a number of metal complexes was studied by NMR. Structures of the complexes Tl[Tp(Bn,Me)], 1, Tl[Tp(Bn,4Ph)], 2, Co[Tp(Bn,Me)][Tp(Np)], 3, Mo[Tp(Bn,Me)](CO)(2)NO, 4, Co[Tp(Bn,4Ph)][Tp], 5, and Mo[Tp(Bn,Me)](CO)(2)(eta(3)-methallyl), 6, were determined by X-ray crystallography. In the Tp(Bn,Me) ligand, the benzyl group is freely rotating and provides less steric hindrance to the coordinated metal than a neopentyl group, but steric hindrance is increased in the Tp(Bn,4Ph) ligand, where the rotation of the benzyl substituent is restricted by the 4-phenyl substituent.     

Carbon-13 NMR study of substituted benzyl N,N-dimethylcarbamates

The ¹³C NMR chemical shifts of m- and p-substituted benzyl N,N-dimethylcarbamates were measured in CDCl₃. The meta and para ¹³C substituent chemical shifts were analysed by means of dual substituent parameter (DSP) equations. Good correlations were obtained, especially for the para-carbon substituent chemical shifts. The computed transmission coefficients, ρ_I and ρ_R, are consistent with the general features of the fitting parameters. It has been shown that no significant electron demand is imposed by the ? CH₂OCON(CH₃)₂ substituent.     

Stereoselective formation of (2-thenyl)-substituted alkenes from 2-chloromethylthiophene and lithium trialkylalkynylborates

The reactions of 2-chloromethylthiophene (an analog of a benzyl halide) with lithium trialkylalkynylborates gave, after protonation, E-alkenes with a (2-thenyl) substituent in good yields, and the reaction stereo-selectivity was different from that of the reaction of benzyl bromide with an alkynylborate.