Kinetics and mechanism of unimolecular heterolysis of cage-like compounds: XIX. Effect of the nucleofuge nature on the activation parameters of heterolysis of 1-halo-1-methylcyclohexanes in cyclohexane. Heterolysis rate ratio in aprotic and protic solvents

Heterolysis of 1-bromo-1-methylcyclohexane in cyclohexane (E1 reaction) involves solvation of the transition state (ΔS^≡ = ?81 J mol^?1K^?1), while heterolysis of 1-chloro-1-methylcyclohexane is characterized by desolvation of the transition state (ΔS^≡ = 92 J mol^?1K^?1). The probability for the formation of transition state (interaction between cationoid intermediate and solvent cavity) increases in the first case due to enhanced stability of the solvated intermediate, and in the second, due to reduction in its size. The bromide/chloride heterolysis rate ratio decreases as the ionizing power of aprotic solvent decreases and that of protic solvent increases.     

Nature of solvation effects and mechanism of heterolysis of <Emphasis Type="Italic">tert</Emphasis>-alkyl halides

Specificities of heterolysis of tert-alkyl halides in protic and aprotic solvents were analyzed. Values of log k ₂₅ for heterolysis of tert-butyl chloride, tert-butyl bromide, tert-butyl iodiede, 1-chloro-1-methylcyclopentane, 1-chloro-1-methylcyclohexane, 1-bromo-1-methylcyclopentane, 1-bromo-1-methylcyclohexane, 2-chloro-2-phenylpropane, 1-iodoadamantane, and 2-bromo-2-methyladamantane in 19 to 44 solvents, determined mostly by the verdazyl technique were collected. Correlation analysis of solvation effects was performed in terms of multiparameter equations based on the linear free energy relationship principle, as well as in the logk-E _T coordinates. The nature of solvation effects and mechanism of heterolysis of a covalent C-Hlg bond were discussed.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXXIII.1 Correlation Analysis of Solvation Effects in Monomolecular Heterolysis of 1-Bromo-1-methylcyclopentane, 1-Bromo-1-methylcyclohexane, and 2-Bromo-2-methyladamantane

The rate of heterolysis of 1-bromo-1-methylcyclopentane and 1-bromo-1-methylcyclohexane is determined by the equation v = k[RBr], mechanism E1. Comparative correlation analysis of solvation effects in heterolysis of these substrates and 2-brom-2-methyladamantane was performed.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXXII.1 Solvent Effects on Activation Parameters of Heterolysis of 1-Chloro-1-methylcyclopentane. Correlation Analysis of Solvation Effects

Kinetics of heterolysis of 1-chloro-1-methylcyclopentane in MeOH, BuOH, cyclohexane, i-PrOH, t-BuOH, tert-C₅H₁₁OH, -butyrolactone, MeCN, PhCN, PhNO₂, acetone, PhCOMe, cyclohexanone, and 1,2-dichloroethane at 25-50°C were studied by the verdazyl method. Correlation analysis of solvent effects on activation parameters of the reaction in 8 protic (additionally, AcOH and CF₃CH₂OH) and 8 aprotic solvents together and separately in either group of solvents was performed. In all the solvents studied, two H -S compensation effects were revealed.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXXI. Solvent Effect on the Rate of 1-Methyl-1-chlorocyclopentane Heterolysis. Correlation Analysis of Solvation Effects

Heterolysis of 1-methyl-1-chlorocyclopentane in protic and aprotic solvents occurs by the E1 mechanism. The reaction rate in aprotic solvents or in a set of protic and aprotic solvents is satisfactorily described by the parameters of the polarity and electrophilicity or ionizing power of the solvents. In protic solvents, the reaction rate grows with increasing polarity or ionizing power of the solvent and decreases with increasing nucleophilicity.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXXVII.1 Effect of Nucleofuge and Solvent of the Relative Rates of Heterolysis of 1-Halo-1-methylcyclopentanes and 1-Halo-1-methylcyclohexanes. Correlation Analysis of Solvation Effects

The effect of solvent ionizing ability on heterolysis rate enhances in the series 1-chloro-1-methylcyclohexane < 1-bromo-1-methylcyclohexane 1-chloro-1-methylcyclopentane < 1-bromo-1-methyl- cyclopentane. The lower sensitivity of cyclohexyl substrates compared with cyclopentyl is determined by conformational effects. Bromides are more sensitive to solvent effects than chlorides because of the stronger polarizability of the C-Br bond.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXXVI.1 Solvent Effect on the Activation Parameters of Heterolysis of 1-Methyl-1-chlorocyclohexane. Correlation Analysis of Solvation Effects in Heterolysis of 1-Methyl-1-chlorocyclohexane and 1-Methyl-1-chlorocyclopentane

The kinetics of heterolysis of 1-methyl-1-chlorocyclohexane in six protic and eight aprotic solvents at 25-50°C was studied by the verdazyl method; v = k[RCl], E1 mechanism. The correlation analysis of the solvent effects on the activation free energy G , enthalpy H , and entropy S of heterolysis of 1-methyl-1-chlorocyclohexane and 1-methyl-1-chlorocyclopentane was performed for the same sets of solvents.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXXV.1 Solvation Effects on the Activation Parameters of Heterolysis of 1-Bromo-1-methylcyclopentane and 1-Bromo-1-methylcyclohexane. Correlation Analysis of Solvation Effects

Kinetics of heterolysis of 1-bromo-1-methylcyclopentane and -cyclohexane in protic and aprotic solvents were studied. Correlation analysis of the effect of solvent parameters on G , H , and S was performed.     

Solvation and Conformational Effects in Heterolysis of 1-Methylcycloalkyl Halides

In the series of substrates 1-bromo-1-methylcyclopentane, 1-bromo-1-methylcyclohexane, 1-methyl-1-chlorocyclopentane, 1-methyl-1-chlorocyclohexane, the heterolysis rate in acetone at 25 °C is reduced by four orders of magnitude; v = k[RX], E1 mechanism. The decrease in reaction rate as we go from a cyclopentyl compound to a cyclohexyl compound is due to the decrease in entropy of activation as a result of rapid solvation of the transition state as the conformational barrier is overcome.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXX.1 Correlation Analysis of Solvation Effects in Heterolysis of tert-Butyl Chloride

Quantitative analysis of the effect of solvent parameters on the rate of heterolysis of tert-butyl chloride was performed; the reaction rate is fairly described by the polarity, polarizability, and electrophilicity parameters or by the ionizing ability parameter, while the nucleophilicity of the solvent has no rate effect. A negative effect of nucleophilic solvation was revealed in protic solvents.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXIX. Solvent Effects on the Activation Parameters of Heterolysis of tert-Butyl Chloride

The kinetics of heterolysis of t-BuCl in sulfolane, PhCN, PhNO₂, acetophenone, cyclohexanone, chloroform, and 1,2-dichloroethane at 30-50°C were studied by the verdazyl method. Quantitative analysis of the effect of solvent parameters on the G , H , S , and log k _{2 5} values for heterolysis of t-BuCl in a set of 15 protic and 16 aprotic solvents and separately in either group of solvents was performed. In the above set of solvents, three H -S compensation effects are observed, associated with jump changes in the potential energy of the reaction.     

Kinetics and Mechanism of Monomolecular Heterolysis of Cage-Like Compounds: XVIII. Solvent Effect on the Rate of Heterolysis of 3-Bromocyclohexene. Correlation Analysis of Solvation Effects

The kinetics of E1 dehydrobromination of 3-bromocyclohexene in 23 aprotic and 9 protic solvents were studied by the verdazyl technique. The reaction rate is described by the polarity, electrophilicity, and ionizing power parameters of the solvent. Nucleophilicity, polarizability, and cohesion parameters of the solvent do not affect the reaction rate. The effects of equilibrium and nonequilibrium solvation of the transition state are discussed.     

Kinetics and mechanism of monomolecular heterolysis of commercial organohalogen compounds: XLIII. Solvent effect on activation parameters of dehydrochlorination of 3-chloro-3-methylbut-1-ene. Correlation analysis of solvation effects

The influence of temperature on the rate of dehydrochlorination of 3-chloro-3-methylbut-1-ene in 17 aprotic and 13 protic solvents, ν = k[C₅H₉Cl], was studied by the verdazyl method. In aprotic solvents, the electrophilicity, ionizing power, and cohesion of solvents decrease ΔG ^≠ by increasing ΔS ^≠. The nucleophilicity and polarizability increase both ΔH ^≠ and ΔS ^≠ to equal extent and therefore do not affect ΔG ^≠. In protic solvents, the solvent nucleophilicity increases ΔH ^≠ to a greater extent than ΔS ^≠, and the overall effect of the nucleophilic solvation is small and negative.     

Studies directed toward the synthesis of conformationally restricted neurotransmitter analogs: The addition of N-hydroxyimides to ethyl propiolate

N-Hydroxyimides were found to add readily to ethyl propiolate to yield the imidooxyacrylates in both protic and aprotic solvents. The trans isomer only was formed in aprotic solvents while both isomers were formed in protic solvents.     

Kinetics and mechanism of monomolecular heterolysis of commercial organohalogen compounds: XXXVIII. Correlation analysis of solvent effects on the activation parameters of heterolysis of <Emphasis Type="Italic">t</Emphasis>-BuBr and <Emphasis Type="Italic">t</Emphasis>-BuI

Heterolysis of t-BuBr and t-BuI in aprotic solvents involves a H - S compensation effect. The G of t-BuBr heterolysis in aprotic solvents decreases with increasing solvent polarity and cohesion, whereas the respective value for t-BuI heterolysis decreases with increasing solvent polarity, nucleophilicity, and polarizability. In protic solvents, a negative effect of nucleophilic solvation is observed.Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 9, 2004, pp. 1476–1483.Original Russian Text Copyright © 2004 by Ponomarev, Zaliznyi, Dvorko.This revised version was published online in April 2005 with a corrected cover date.     

Reactions of dichloromaleimides with alcohols,phenols, and thiols

Thiols react with dichloromaleimides in the presence of base to give 2,3-bis[alkyl(aryl)-mercapto]maleimides. Alcohols and phenols in equivalent amounts give 2-alkyl(aryl)oxy-3-chloromaleimides. With two equivalents, phenols give 2,3-bis(aryloxy)maleimides, but alcohols give 2-chloro-3,3-dialkoxysuccinimides in protic solvents and dimeric compounds in aprotic solvents.     

Ultrafast Relaxation Dynamics of the Excited States of 1‐Amino‐ and 1‐(N,N‐Dimethylamino)‐fluoren‐9‐ones

The dynamics of the excited states of 1‐aminofluoren‐9‐one (1AF) and 1‐(N,N‐dimethylamino)‐fluoren‐9‐one (1DMAF) are investigated by using steady‐state absorption and fluorescence as well as subpicosecond time‐resolved absorption spectroscopic techniques. Following photoexcitation of 1AF, which exists in the intramolecular hydrogen‐bonded form in aprotic solvents, the excited‐state intramolecular proton‐transfer reaction is the only relaxation process observed in the excited singlet (S₁) state. However, in protic solvents, the intramolecular hydrogen bond is disrupted in the excited state and an intermolecular hydrogen bond is formed with the solvent leading to reorganization of the hydrogen‐bond network structure of the solvent. The latter takes place in the timescale of the process of solvation dynamics. In the case of 1DMAF, the main relaxation pathway for the locally excited singlet, S₁(LE), or S₁(ICT) state is the configurational relaxation, via nearly barrierless twisting of the dimethylamino group to form the twisted intramolecular charge‐transfer, S₁(TICT), state. A crossing between the excited‐state and ground‐state potential energy curves is responsible for the fast, radiationless deactivation and nonemissive character of the S₁(TICT) state in polar solvents, both aprotic and protic. However, in viscous but strong hydrogen‐bond‐donating solvents, such as ethylene glycol and glycerol, crossing between the potential energy surfaces for the ground electronic state and the hydrogen‐bonded complex formed between the S₁(TICT) state and the solvent is possibly avoided and the hydrogen‐bonded complex is weakly emissive.     

Spectral and photophysical properties of thioxanthone in protic and aprotic solvents: the role of hydrogen bonds in S1-thioxanthone deactivation.

Spectral and photophysical properties of thioxanthone (9H-thioxanthen-9-one, TX) were determined in a few protic solvents (H2O, D2O, hexafluoro-2-propanol) and compared with those in aprotic solvents. On the basis of the time-resolved and steady-state emission measurements and available literature data, it has been shown that the dominant S1-TX deactivation process in protic solvents is the formation of the S1-complex. The important modes of deactivation of the S1-complex are fluorescence (phiF approximately 0.4-0.5) and intersystem crossing to the T1 state. The S1-complex-->S0 internal conversion plays, at most, an insignificant role in S1-complex deactivation, which is evidenced by the absence of an isotope effect of protic solvents on the lifetime and quantum yield of fluorescence.     

Mechanistic study of the photochemical hydroxide ion release from 9-hydroxy-10-methyl-9-phenyl-9,10-dihydroacridine

The excited-state behavior of 9-hydroxy-10-methyl-9-phenyl-9,10-dihydroacridine and its derivative, 9-methoxy-10-methyl-9-phenyl-9,10-dihydroacridine (AcrOR, R = H, Me), was studied via femtosecond and nanosecond UV-vis transient absorption spectroscopy. The solvent effects on C-O bond cleavage were clearly identified: a fast heterolytic cleavage (τ = 108 ps) was observed in protic solvents, while intersystem crossing was observed in aprotic solvents. Fast heterolysis generates 10-methyl-9-phenylacridinium (Acr(+)) and (-)OH, which have a long recombination lifetime (no signal decay was observed within 100 μs). AcrOH exhibits the characteristic behavior needed for its utilization as a chromophore in the pOH jump experiment.     

Kinetics of the electrochemical oxidation of 1,1-bis-hydroperoxy-4-methylcyclohexane on platinum

The electrochemical synthesis of 3,12-dimethyl-7,8,15,16-tetraoxadispiro[5.2.5.2]hexadecane (1,2,4,5-tetraoxane) from 1,1-bis-hydroperoxy-4-methylcyclohexane on platinum electrode in a cell with separated and unseparated cathode and anode space in an aprotic solvent is conducted. The kinetics of electrochemical oxidation of 1,1-bis(hydroperoxy)-4-methylcyclohexane is studied. The current yield of the reaction is determined.