The regiochemistry of trapped radical anion-radical cation pairs from aryl substituted phthalimides

A series of 4-substituted N-methyl phthalimides was irradiated in the presence of 2,3-dimethyl-2-butene in methanol and the product distributions of the trapped radical anion-radical cation analyzed on the basis of substituent effects on the radical anion.     

在氯化亚锡存在下2,3-二甲基-2-丁烯与乙酐的催化酰化反应

一种新的催化剂,SnCl2·2H2O,已用于2,3-二甲基-2-丁烯与乙酐的酰化反应来制备高产率的3,3,4-三甲基-4-戊烯-2-酮 (TMP). 发现SnCl2·2H2O是室温下2,3-二甲基-2-丁烯与乙酐进行酰化反应的有效催化剂. 考察了催化剂用量、乙酐用量、反应时间和溶剂的加入等因素对该酰化反应的影响. 发现当催化剂/2,3-二甲基-2-丁烯摩尔比在0.30/1和 0.60/1之间、反应时间为2 h时,所得酰化产品的产率最高. 该酰化反应进行到完全所需的时间取决于催化剂用量和乙酐用量. 在需要加入溶剂的情况下,二氯甲烷或氯仿是一种合适的溶剂.     

Aryl cations from aromatic halides. Photogeneration and reactivity of 4-hydroxy(methoxy)phenyl cation

The photochemistry of 4-chlorophenol (1) and 4-chloroanisole (2) has been examined in a range of solvents and found to lead mainly to reductive dehalogenation, through a homolytic path in cyclohexane and a heterolytic path in alcohols. Heterolysis of 1 and 2 in methanol and 2,2,2-trifluoroethanol offers a convenient access to triplet 4-hydroxy- and 4-methoxyphenyl cations. These add to pi nucleophiles, viz., 2,3-dimethyl-2-butene, cyclohexene, and benzene, giving the arylated products in medium to good yields. Wagner-Meerwein hydride and alkyl migration are evidence for the cationic mechanism of the addition to alkenes. Arylation (with no rearrangement) was obtained to some extent also in nonprotic polar solvents such as MeCN and ethyl acetate, reasonably via an exciplex and with efficiency proportional to the nucleophilicity of the trap (2,3-dimethyl-2-butene > cyclohexene > benzene).     

Isomerism in jet-cooled 1-cyanonaphthalene complexes

The effect of complexation on the first electronic transition of 1-cyanonaphthalene has been studied for complexes with aprotic polar solvents (acetonitrile, diethylether) or olefins (2-methyl-2-butene, 2,3-dimethyl-2-butene). In both cases, isomerism has been evidenced. With polar solvents, two strong absorption bands appear in the 0 ₀ ⁰ region, one being shifted to the red and the other one to the blue side of the bare cyanonaphthalene transition. These two bands have been assigned to two isomeric forms involving specific interactions between the solvent and the cyanonaphthalene molecule. These isomers have been shown to interconvert at higher vibronic energies. With the 2,3-dimethyl-2-butene a broad absorption band leading to a red shifted fluorescence has been observed and assigned to an exciplex. With the 2-methyl-2-butene, one observes the superposition of narrow and broad bands in the excitation spectrum. These two systems have been assigned to two isomeric forms, one of them crossing to an exciplex in the excited state. At higher vibronic energy, emission from the two conformers is observed.     

Solvent dependence of singlet oxygen / substrate interactions in ene-reactions, (4+2)- and (2+2)-cycloaddition reactions

Product formation of singlet oxygen reactions with simple olefins occurring as ene-reactions, (4+2)- and (2+2)-cycloaddition reactions is independent on solvent polarity. Thus, 2,3-dimethyl-2-butene (1) and 2-methy]-2-butene (3), 1,3-cyclohexadiene (6), and benzvalene (8) yield allylic hydroperoxides (2) and(4) (54%) + (5) (46%), endoperoxide (7), and dioxetane (9), respectively. The rates of the ene-reactions and (4+2)-cycloaddition reactions are only slightly dependent, those of the (2+2)-cycloaddition reaction, however,are clearly dependent on solvent polarity. “Physical” quenching of singlet oxygen by the olefins is negligible, but substantial by the sensitizer tetraphenylporphin (TPP) in chlorinated solvents.     

Synthesis and Polymerization of Some New Bis-Triazolinediones: A Stability Study of 4-Substituted Triazolinediones

Three new bis-triazolinediones, 3,3′-dimethyl-4,4′-bis-[3,5-dioxo-1,2,4-triazoline-4-yl]biphenyl, t-1,4-bis-[3,5-dioxo-1,2,4-triazoline-4-yl]methyl cyclohexane, and 4,4′-bis-[3,5-dioxo-1,2,4-triazoline-4-yl] phenyl ether, were synthesized from their corresponding bis-amines or bis-isocyanates. The compounds were identified by their quantitative ene reaction with 2,3-dimethyl-2-butene. The high degree of reactivity of the triazoline moiety makes solvent selection for reaction media rather difficult. This fact prompted a study of rates of reaction with a variety of polar and nonpolar solvents, including halogenated aliphatics, aromatics, tetrahydrofuran (THF), and N,N-dimethylformamide (DMF). The compounds exhibited reasonable stability in the halogenated solvents, as well as in the aliphatic and aromatic hydrocarbons, but they underwent reaction with THF and DMF. The structure of the reaction product of N-phenyl-1,2,4-triazoline-3,5-dione in DMF solution was determined, and a mechanism for product formation was proposed. Two of the bis- triazolinediones were polymerized via a base-catalyzed condensation mechanism which eliminates N₂ from the triazolinedione ring.     

A comparison of hydrogen bonding solvent effects on the singlet oxygen reactions of allyl and vinyl sulfides, sulfoxides, and sulfones

The singlet oxygen (¹Δ_g) photooxidations of 2-methyl-3-phenylthio-2-butene (1a), 1-[(4-nitrophenyl)thio]-2,3-dimethyl-2-butene (2c), 2-methyl-3-phenylsulfinyl-2-butene (3), 2-methyl-3-phenylsulfonyl-2-butene (6), and 1-[(4-nitrophenyl)sulfonyl]-2,3-dimethyl-2-butene (7c) were conducted in the following deuterated solvents: acetonitrile, benzene, chloroform, methanol, or methanol/water mixture. In each case the ene allylic hydroperoxide products and/or the [2+2] cycloaddition products were quantified and inspected for possible hydrogen bonding induced differences in product selectivity and regiochemistry. After comparison to literature values for related substrates, the results indicate that only photooxidations of vinyl sulfides are susceptible to hydrogen bonding solvent effects.     

Intramolecular photoarylation of alkenes by phenyl cations

Acetone-sensitized irradiation of various o-chlorophenyl allyl ethers in polar solvents led to either (dihydro)benzofurans or chromanes. The reaction appeared to involve photoheterolysis of the aryl-Cl bond followed by phenyl cation addition onto the tethered double bond either in 5-exo or 6-endo modes. The adduct cation gave the end products by deprotonation; addition of chloride anion or of the solvent, depending on the structure; and the conditions used. Preference for the 5-exo mode increased in passing from medium polarity (methylene chloride, ethyl acetate) to high polarity solvents (aqueous acetonitrile, methanol, 2,2,2-trifluoroethanol), for which this was often the exclusive path. The same compounds underwent photohomolysis when irradiated in cyclohexane, and radical cyclization was one of the process occurring. Substitution of a methylene group for the ether oxygen atom made 6-endo cyclization by far the main path in a related o-chlorophenylbutene. Again, the selectivity was higher in polar protic solvents. The results are discussed in terms of in cage ion pair versus free phenyl cation reactions.     

Kinetics of radical heterolysis reactions forming alkene radical cations

Rate constants for heterolytic fragmentation of beta-(ester)alkyl radicals were determined by a combination of direct laser flash photolysis studies and indirect kinetic studies. The 1,1-dimethyl-2-mesyloxyhexyl radical (4a) fragments in acetonitrile at ambient temperature with a rate constant of k(het) > 5 x 10(9) s(-1) to give the radical cation from 2-methyl-2-heptene (6), which reacts with acetonitrile with a pseudo-first-order rate constant of k = 1 x 10(6) s(-1) and is trapped by methanol in acetonitrile in a reversible reaction. The 1,1-dimethyl-2-(diphenylphosphatoxy)hexyl radical (4b) heterolyzes in acetonitrile to give radical cation 6 in an ion pair with a rate constant of k(het) = 4 x 10(6) s(-1), and the ion pair collapses with a rate constant of k < or = 1 x 10(9) s(-1). Rate constants for heterolysis of the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(diphenylphosphatoxy)ethyl radical (5a) and the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(trifluoroacetoxy)ethyl radical (5b) were measured in various solvents, and an Arrhenius function for reaction of 5a in THF was determined (log k = 11.16-5.39/2.3RT in kcal/mol). The cyclopropyl reporter group imparts a 35-fold acceleration in the rate of heterolysis of 5a in comparison to 4b. The combined results were used to generate a predictive scale for heterolysis reactions of alkyl radicals containing beta-mesyloxy, beta-diphenylphosphatoxy, and beta-trifluoroacetoxy groups as a function of solvent polarity as determined on the E(T)(30) solvent polarity scale.     

Nucleophile-solvent isotope effects between methanol isotopomers during the interception of aziridinium imide-‘like’ closed intermediates

Deuterated methanol isotopomers were found to compete efficiently with normal methanol during the interception of an intermediate with structural characteristics of the aziridinium imide, formed in the reaction of N-phenyltriazolinedione with simple alkenes such as 2-methylpropene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene. In general, a (trideuterio)methyl-group bearing methanol was found to add at the tertiary carbon atom of the intermediate more efficiently with regard to hydrogen isotopomeric methanol, and this result is explained in terms of the nucleophile-solvent isotope effect in an S_N2-‘like’ transition state of solvent addition to the intermediate.     

Mechanism of Exciplex Decay: The Quantum Yields and the Rate Constants of Triplet Formation from 9-Cyanophenanthrene Exciplexes

The quantum yields of the formation of triplet states from the exciplexes of 9-cyanophenanthrene with 1,2,3- and 1,3,5-trimethoxybenzenes in solvents of different polarity and the rate constants of intersystem crossing in these exciplexes were measured. In the solvents of weak and intermediate polarity, the exciplexes decay predominantly via internal conversion and intersystem crossing, whereas the decay in polar solvents occurs additionally via dissociation into radical ions.     

Substituent effects on the ionization reaction of beta-mesylate phenethyl radicals

A series of beta-methanesulfonate phenethyl radicals bearing a range of electron donating and withdrawing aromatic substituents were generated and studied in a variety of solvent mixtures using nanosecond laser flash photolysis. Rate constants for the formation of the corresponding styrene radical cation via heterolytic loss of the beta-mesylate leaving group were measured using time-resolved absorption spectroscopy. The ionization reaction was investigated in a variety of solvents and solvent mixtures including 1,1,1,3,3,3-hexafluoro-2-propanol, 2,2,2-trifluoroethanol, acetonitrile, methanol and water. The influence of substituent electronic effect and solvent polarity on the kinetics of the beta-heterolysis reaction are discussed and assessed using the sigma+ Hammett parameter and Y(OMs) values, respectively. The small magnitude of m calculated for the formation of the 4-methoxystyrene radical cation by ionization of the mesylate group (m = 0.33) in aqueous methanol mixtures is compared to values obtained for the formation of the same radical cation via loss of chloride and bromide where m = 0.56 and m = 0.45, respectively.     

在H3PO4存在下2,3-二甲基-2-丁烯与乙酐的酰化反应

在H3PO4存在下进行了2，3－二甲基－2－丁烯与乙酐的酰化反应，结果表明，H3PO4是室温下催化2，3－二甲基－2－丁烯与乙酐酰化反应的有效催化剂，在H3PO4存在下，酰化产物的收率主要取决于：（a)H3PO4的用量，（b)乙酐的用量；（c)反应温度和（d)反应时间，在适宜反应条件下，所得3，3，4－三甲基－4－戊烯－2－酮（TMP）约为99％，另外还发现，反应体系中加入少量乙酸，不会明显减少酰化产物的收率，乙酐的纯度，反应物的加入顺序对该反应几乎没有任何影响，在有和没有溶剂CH2Cl2的条件下所得酰化产物的收率是可比的。     

Photoinduced, ionic Meerwein arylation of olefins

Irradiation of 4-chloroaniline or of its N,N-dimethyl derivative in polar solvents generates the corresponding triplet phenyl cations. These are trapped by alkenes yielding arylated products in medium to good yields. B3LYP calculations show that the triplet cation slides with negligible activation energy to a bonded adduct with ethylene, whereas it forms only a marginally stabilized CT complex with water (chosen as a representative sigma nucleophile). The structure of the final products depends on the preferred path from the adduct cation with the alkene. In the case of aryl olefins, this deprotonates to stilbene derivatives, while, from 2,3-dimethyl-2-butene and allytrimethylsilane, allylanilines are obtained by elimination of an electrofugal group in gamma. In the case of mono- and disubstituted alkenes the cation adds chloride rather than eliminating and beta-chloroalkylanilines are obtained. The regio- and sterochemistry of the addition across the alkene are best understood with a phenonium ion structure for the adduct. The nucleophile entering in beta can be varied under conditions in which the adduct cation is trapped more efficiently than the starting phenyl cation. Thus, beta-methoxyalkylanilines are formed when the irradiation is carried out in methanol. beta-Iodoalkylanilines are obtained in acetonitrile containing iodide and unsubstituted alkylanilines in the presence of sodium borohydride. A case of intramolecular nucleophilic trapping is found with 4-pentenoic acid. The reaction is a wide-scope ionic analogue of the radicalic Meerwin arylation of olefins.     

Photoionization versus photoheterolysis of all-trans-retinol. The effects of solvent and laser radiation intensity

The time-resolved formation of the retinyl carbocation from all-trans-retinol and all-trans-retinol acetate was studied by use of picosecond flash photolysis. From both precursors, the retinyl cation is produced by heterolytic C-O bond cleavage in solvents of medium polarity (acetonitrile, tetrahydrofuran, propanol with Reichardt polarity parameter ET(N) approximately 0.5) and high polarity (EtOH, MeOH, TFE, HFIP, ET(N) > 0.6) during the laser pulse (< or =5 ps) where its lifetime is >10 ns. The absorption maximum of the cation at early times (t < 100 ps) is at lambda = 590-600 nm; it shifts to shorter wavelengths (Deltalambda = 5-10 nm) within 1-10 ns. This spectral shift is suggested to be due to contact ion pair --> solvent-separated ion pair --> free-ion transformation. The quantum yield of cation formation phi(cat) is independent of excitation wavelength (213, 266 or 355 nm). Photoheterolysis proceeds via a one-quantum process. In chlorinated solvents, i.e. n-BuCl, 1,2-dichloroethane, chloroform or CCl(4), formation of the retinol radical cation (which is characterized by a peak at 610 nm and further absorption maxima at approximately 840 and approximately 940 nm) by intermolecular electron transfer to the solvent molecules was detected. The radical cation lifetime in all these solvents is 1.5-2 ns, except for CCl(4) where it is 0.25 ns. The formation of the radical cation or cation was not detected in the low polarity solvents: cyclohexane, hexane, dioxane and p-xylene. However, in solvents of medium and high polarity, at high radiation intensities the radical cation may form in addition to the cation (as a result of two-quantum ionization). DFT calculations confirm our experimental results. The rate of retinol S(1) depopulation (k = 0.3-1 x 10(9) s(-1)) is almost independent of the solvent polarity in the range from cyclohexane to methanol. In highly polar solvents (ET(N) > 0.9) the rate increases to (0.5-5) x 10(10) s(-1).     

Photoaddition Reactions of 5-Fluoro-4,4-dimethyl-2-cyclopentenone

The photoaddition of 5-fluoro-4,4-dimethyl-2-cyclopentenone ( 4 ) to 2,3-dimethyl-2-butene leads specifically (in cyclohexane) and selectively (in acetonitrile) to the formation of the oxetanes 16 . The title compound is compared in its behaviour to the analogous 6-fluoro-4,4-dimethyl-2-cyclohexenone ( 1 ) and both α'-fluoro-4,4-dimethyl-2-cycloalkenones in turn are compared to the corresponding 2-cycloalkenones ( 6 and 3 ) and 4,4-dimethyl-2-cycloalkenones ( 5 and 2 ). The quantum yield for the addition of these enones to 2,3-dimethyl-2-butene and to cyclopentene are discussed.     

用气相色谱法研究乙酸乙烯酯不对称氢甲酰化反应中溶剂的影响

用毛细管气相色谱、色－质谱和旋光色散及圆二色性谱仪等方法对乙酸乙烯酯在非极性和极性溶剂中的不对称氢甲酰化反应产物进行分离和鉴定。实验结果表明,在非极性溶剂中反应的收率、选择性、光学收率ｅ．ｅ．值（ｅｎａｎｔｉｏｍｅｒｉｃｅｘｃｅｓｓ）均比极性溶剂中的结果为好。由此探讨了不对称氢甲酰化反应中溶剂的影响。     

Photon‐Quantitative 6π‐Electrocyclization of a Diarylbenzo[b]thiophene in Polar Medium

The high reactivity of 6π‐electrocyclization in polar solvents has remained one of the important challenges for diarylethenes because of the emergence of a twisted intramolecular charge transfer (TICT) state at the excited state in such polar media, which usually quenches the photocyclization reaction. Herein we report on the preparation and highly efficient photocyclization of 2,3‐diarylbenzo[b]thiophenes with nonsymmetric side‐aryl units in a polar solvent. While the dithiazolylbenzo[b]thiophene showed a suppressed quantum yield of 6π‐electrocyclization of 54 % in methanol, the replacement of a thiazole unit with a thiophene ring led to a photon‐quantitative 6π‐cyclization reaction. The nonsymmetrical modification into the side‐aryl units was considered to enhance the CH/π interactions between side‐aryl units to support a photoreactive conformation in methanol. The stabilization of the photochromic reactive conformation is expected to suppress the formation of the TICT state at the excited state, leading to highly efficient photoreactivity.     

C2 molecule: formation from bromoacetylene and reactions with cyclohexene or 2,3-dimethyl-2-butene

The C₂ molecule (1,2-ethynediyl) has been prepared by dehydrohalogenation of 1,2-dibromoethylene with an excess of potassium tert-butoxide in 2,3-dimethyl-2-butene as the solvent and the reagent. The major products of this reaction were 2,3-dimethylbut-3-en-2-ol and dibromoacetylene.     

Mechanisms of the reactions of chloromethanes with trimethylstannylsodium. Synthesis of tetrakis[trimethylstannyl]methane.

The reactions of CCl₄, CHCl₃, and CH₂Cl₂ with trimethylstannylsodium in tetraglyme have been examined. Tetrakis[trimethylst?annyl]methane can be obtained by reaction with CCl₄ in yields of about 40%. Among other products which have been characterized in varying degree are the following: trichloromethyltrimethylstannane, bis[trimethylstannyl]dichloromethane, tris[trimethylstannyl]methane, and hexamethyldistannane, The first step in the reaction leads to the formation of the trichloromethyl anion, and dichlorocarbene has been trapped with 2,3- dimethyl-2-butene. Dichlorobis[trimethylstannyl]methane reacts with Me₃SnNa to form tetrakis[trimethylstannyl]methane in 85% yield. When the reaction was carried out in the presence of $\text{t}$-butyl alcohol the only product formed was bis[trimethylstannyl]methane, indicating carbanionic intermediates. Chloroform yields tetrakis[trimethylstannyl]methane and bis[trimethylstannyl]methane, but no detectable tris[trimethylstannylmethane. Trapping experiemtns with 3-pentanol and 2,3-dimethyl-2-butene indicate the intermediacy of dichloromethyl anion and of dichlorocarbene. Methylene chloride yields bis[trimethylstannyl]methane. No intermediates could be trapped by added alcohol.