Theoretical study of nucleophilic substitution at two-coordinate sulfur

A series of nucleophilic substitution reactions involving simple species (chloride, phosphide, methoxide, hydroxide, and amide) as nucleophile and leaving group in methylsulfenyl derivatives were examined at B3LYP/aug-cc-pVDZ. The reactions involving hydroxide and amide correspond to deprotonation and not substitution. The substitution reactions follow an addition-elimination pathway, possessing a triple-well potential energy surface. The intermediate along this pathway is of trigonal bipyramid geometry with the nucleophile and leaving group occupying apical positions.     

Theoretical study of nucleophilic substitution at the disulfide bridge of cyclo-l-cystine

The reaction of cyclo-l-cystine with thiolate is examined at the B3LYP/6-31+G level. The two isomers of cyclo-l-cystine differ in their dihedral angle about the disulfide bond; the M isomer (with dihedral angle of -90.1 degrees) is found to be slightly lower in energy. The nucleophilic substitution reaction at sulfur follows the addition-elimination mechanism, exemplified by the hypercoordinate sulfur intermediate on the reaction surface. The reaction is exergonic (DeltaG = -6.16 kcal mol(-1)), and both the entrance and exit transition state lie below the reactant energies.     

Theoretical study of the mechanism of nucleophilic substitution in cyclobutane derivatives

The mechanism of nucleophilic substitution in cyclobutane derivatives has been studied by means of ab initio molecular orbital computations on a model system. The results are in favour of a reaction which proceeds with inversion of configuration, in agreement with experimental data.     

Theoretical study of nucleophilic substitution at sulfur in sulfinyl derivatives

A series of gas-phase nucleophilic substitution reactions at sulfur of methanesulfinyl derivatives by small anions (chloride, cyanide, hydroxide, methoxide, amide, and phosphide, identical to the leaving group in each case) were examined by Hartree-Fock, MP2, and DFT computations. In most cases, substitution was found to follow an addition-elimination mechanism, resulting in a triple-well potential energy surface with small barriers of activation on either side of the central, tetracoordinate-sulfur minimum. The geometries of the central minima, as in the analogous methanesulfenyl cases, are unsymmetrical trigonal bipyramidal, with the nucleophiles and leaving groups occupying apical positions and the sulfur lone pair an equatorial position. The apparent exception, cyanide, may undergo an S(N)2 displacement.     

Theoretical study on the mechanisms of the nucleophilic substitution reactions of hydrosulfide ion and halomethanes

Three species involved in the nucleophilic substitution reaction of hydrosulfide ion and halomethanes are investigated by ab initio calculations. Geometries for stationary structures along the reaction paths are fully performed with the second‐order Møller–Plesset perturbation approximation with the cc‐pVDZ basis set. The monomer geometries determined by the MP2 method match the experimental results very well. Single point energy calculations are carried out at the coupled cluster with perturbative triple excitations CCSD (T) theory with aug‐cc‐pVDZ basis set. Halomethanes have three conformers here, which lead to the three product channels, HSCH₃ + F^?1, HSCH₃ + Cl^?1, and HSCH₃ + Br^?1. The investigation encompasses the six complexes formed among three channels, respectively. By selecting the six complexes as the model, we investigate the binding energy, topological property of the electron charge density and their Laplacian in detail theoretically. Electrostatic density potential maps of halomethanes are generated for the determination of attractive interaction sites. It is proved that the similar misshaped electron clouds of the three halogen atoms result in the similar properties of the carbon‐halogen bonds, and reveals that the product ion‐dipole complexes interactions are predominantly electrostatic in nature. The calculated results predict the binding energy of the most stable complex in six complexes is ?47.06 kcal/mol at the MP2 level of theory. The second channel has the lowest energy barrier, which is ?3.63 kcal/mol at the CCSD (T) levels of theory, is expected to be the most important pathway. It occurs via C? Cl cleavage accompanied by C? S bond formation. The other two channels have higher energy barriers and are expected to have smaller rates. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Theoretical study of the nucleophilic 5-endo-trigonal cyclization of 1,1-difluoro-1-alkenes

The nucleophilic 5-endo-trigonal cyclization of 1,1-difluoro-1-alkenes has been studied at the B3LYP/6-31+G(d) level in an Onsager continuum model for DMF. The reaction takes an addition-elimination path. Both the transition-state structures and the IRC analyses suggest the delocalization of the negative charge to highly electronegative two fluorine atoms during the addition reaction is the origin of the high reactivity of 1,1-difluoro-1-alkenes. Judging from the activation energies, both dichloro and dibromo counterparts are much less reactive for 5-endo-trigonal cyclization. In these substrates, the cyclization reaction is promoted by chlorine or bromine atom with their good leaving-group ability, and the addition of oxyanion to the pi-bond occurs along with the simultaneous elimination of halogen atom. The study on the cyclizations of beta-monofluoro-o-hydroxystyrenes and beta-bromo-beta-fluoro counterparts shows that one fluorine atom is not enough to delocalize the negative charge in the addition step.     

人类谷胱甘肽S-转移酶M1a-1a催化的亲核芳香取代反应的理论研究

利用分子力学和量子力学方法研究人类谷胱甘肽S-转移酶M1a-1a催化谷胱甘肽对1-氯-2,4二硝基苯(CDNB)的亲核芳香取代反应的细节.所获得的反应路径显示反应仅经历一个过渡态且能垒很低.电荷布居分析证明电子从谷胱甘肽基团流向二硝基苯,验证了反应的发生.计算结果表明活性位点3个残基(Tyr6,His107和Tyr115)参与了催化反应,尤其是His107,它在反应后期通过与产物形成氢键从而加速了Cl的释放.结果支持了Patskovsky等人提出的机理,并有助于其他谷胱甘肽S-转移酶的研究.     

Stereochemical studies on the nucleophilic substitution in the reaction of allyl phosphates with organoaluminium reagents

The reaction of cis- or trans-5-isopropenyl-2-methyl-2-cyclohexenyl diethyl phosphate (I) with Me₂AlX (X = OPh, SPh, NHPh) in hexane results in substitution of the -O-PO(OEt)₂ group with X under predominant inversion. The solvent effects on the stereochemistry in these reactions have been disclosed.     

Lewis base-catalyzed double nucleophilic substitution reaction of N-tosylaziridinofullerene with thioureas or guanidines

Lewis base-catalyzed double nucleophilic substitution reaction of N-tosylaziridinofullerene with thioureas or guanidines affords the fullerothiazolidin-2-imine or fulleroimidazolidin-2-imine derivatives, respectively. In the case of unsymmetrical thioureas connecting an alkyl and an aryl group on each of the nitrogen atom, the transformation exhibits excellent chemoselectivity with only the aryl substituted nitrogen atom bonding to C₆₀. The generated tri-4-methoxyphenyl substituted fulleroimidazolidin-2-imine reacts with CS₂ smoothly to generate di-4-methoxyphenyl fulleroimidazolidin-2-thione.     

One-pot process in phosphonium-iodonium ylides: nucleophilic substitution and the Wittig reaction

The nucleophilic substitution in mixed phosphonium-iodonium ylides was investigated. The iodonium group is replaced by such S-containing nucleophiles as the thiocyanate anion or thiourea, as well as by halide ions. The structure of the reaction product with the thiocyanate ion was established by X-ray diffraction. A one-pot process involving the nucleophilic substitution and the Wittig reaction was developed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 391–395, February, 2008.     

Competitive reaction pathways in the nucleophilic substitution reactions of aryl benzenesulfonates with benzylamines in acetonitrile

The reactions of aryl benzenesulfonates (YC6H4SO2OC6H4Z) with benzylamines (XC6H4CH2NH2) in acetonitrile at 65.0 degrees C have been studied. The reactions proceed competitively by S-O (kS-O) and C-O (kC-O) bond scission, but the former provides the major reaction pathway. On the basis of analyses of the Hammett and Br?nsted coefficients together with the cross-interaction constants rho(XY), rho(YZ), and rho(XZ), stepwise mechanisms are proposed in which the S-O bond cleavage proceeds by rate-limiting formation of a trigonal-bipyramidal pentacoordinate (TBP-5C) intermediate, whereas the C-O bond scission takes place by rate-limiting expulsion of the sulfonate anion (YC6H4SO3-) from a Meisenheimer-type complex.     

Elimination-addition mechanism for nucleophilic substitution reaction of cyclohexenyl iodonium salts and regioselectivity of nucleophilic addition to the cyclohexyne intermediate

The reaction of 4-substituted cyclohex-1-enyl(phenyl)iodonium tetrafluoroborate with tetrabutylammonium acetate gives both the ipso and cine acetate-substitution products in aprotic solvents. The isomeric 5-substituted iodonium salt also gives the same mixture of the isomeric acetate products. The reaction is best explained by an elimination-addition mechanism with 4-substituted cyclohexyne as a common intermediate. The cyclohexyne formation was confirmed by deuterium labeling and trapping to lead to [4 + 2] cycloadducts and a platinum-cyclohexyne complex. Cyclohexyne can also be generated in the presence of some other mild bases such as fluoride ion, alkoxides, and amines, though amines are less effective bases for the elimination. Kinetic deuterium isotope effects show that the anionic bases induce the E2 elimination (k(H)/k(D) > 2), while the amines allow formation of a cyclohexenyl cation in chloroform to lead to E1 as well as S(N)1 reactions (k(H)/k(D) approximately 1). Bases are much less effective in methanol, and methoxide was the only base to efficiently afford the cyclohexyne intermediate. Nucleophiles react with the cyclohexyne to give regioisomeric products in the ratio dependent on the ring substituent. The observed regioselectivity of nucleophilic addition to substituted cyclohexynes is rationalized from calculated LUMO populations, which are governed by the bond angles at the acetylenic carbons: The less deformed carbon has a higher LUMO population and is preferentially attacked by the nucleophile.     

N-heterocyclic carbene catalyzed nucleophilic substitution reaction for construction of benzopyrones and benzofuranones

N-Heterocyclic carbene as an efficient organic catalyst was employed to catalyze an intramolecular nucleophilic substitution reaction. When R(2) was a phenyl group, the cyclization process underwent isomerization, leading to generation of benzofuranone.     

A nucleophilic substitution reaction performed in different types of self-assembly structures

A nucleophilic substitution reaction between 4-tert-butylbenzyl bromide and potassium iodide has been performed in oil-in-water microemulsions based on various C12Em surfactants, i.e., dodecyl ethoxylate with m number of oxyethylene units. The reaction kinetics was compared with the kinetics of reactions performed in other self-assembly structures based on very similar surfactants and in homogeneous liquids. The reaction was fastest in the micellar system, intermediate in rate in the microemulsions, and most sluggish in the liquid crystalline phase. Reaction in a Winsor I system, i.e., a two-phase system comprising an oil-in-water microemulsion in equilibrium with excess oil, was equally fast as reaction in a one-phase microemulsion. The reactions in microemulsion were surprisingly fast compared to reaction in homogeneous, protic liquids such as methanol and ethanol. The rate was independent of the microstructure of the microemulsion; however, the rate was very dependent on the type of surfactant used. When the C12Em surfactant was replaced by a sugar-based surfactant, octyl glucoside, the reaction was much more sluggish. The high reactivity in microemulsions based on C12Em surfactants is belived to be due to a favorable microenvironment in the reaction zone. The reaction is likely to occur within the surfactant palisade layer, where the water activity is relatively low and where the attacking species, the iodide ion, is poorly hydrated and, hence, more nucleophlic than in a protic solvent such as water or methanol. Sugar surfactants become more hydrated than alcohol ethoxylates and the lower reactivity in the microemulsion based on the sugar surfactant is probably due to a higher water activity in the reaction zone.     

Investigation of the reaction order for nucleophilic substitution of dialkyl methylphosphonates by alkoxides

The rate of nucleophilic substitution at the phosphorus centre of dialkyl methylphosphonates by methoxide and ethoxide has been studied to investigate the possible involvement of hexacoordinated phosphorus species in this reaction. For alkoxide concentrations less than ca. 1.5 M the rate increases with the square of alkoxide concentration. However, consideration of the activity of the alkoxides, represented by an appropriate acidity function, reveals that only one equivalent of alkoxide is involved in the rate-determining step. Thus, there is no requirement to invoke the intermediacy of a hexacoordinated species in the reaction pathway. © John Wiley & Sons, Inc.     

Formal nucleophilic substitution of bromocyclopropanes with azoles

A highly diastereoselective protocol for the formal nucleophilic substitution of 2-bromocyclopropylcarboxamides with azoles is described. A wide range of azoles, including pyrroles, indoles, benzimidazoles, pyrazoles, and benzotriazoles, can be efficiently employed as pronucleophiles in this transformation, providing expeditious access to N-cyclopropyl heterocycles.