Sorption characteristics ofN-alkylimidazoles under conditions of capillary chromatography

The retention indices forN-n-alkyl (C₁-C₅) substituted imidazoles with Me, Et, Pr, and Bu groups in different positions of the cycle were determined. Two capillary columns with OV-101/KF and PEG-40M/KF were used. The two nitrogen atoms of the imidazole molecule were shown to have different effects on the contributions of the alkyl groups to the retention indices. The maximum and minimum contributions are observed for the substituents at the 5 and 4 positions of the imidazole cycle, respectively. An increase in a size of the C _m H_2m+1 substituent attached to the N(1) atom has a notable effect only on the contributions of the alkyl groups at the 2 and 5 positions. The retention indices values for a homologous series with ann-alkyl group attached to the N or C(2) atom were described by a universal type equation. The obtained equations can be used for predicting the retention indices of new homologs and identification ofN-alkylimidazoles in complex mixtures.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 309–313, February, 1995.     

Stationary phase effects in reversed-phase chromatography II. Substituent selectivities for retention on various hydrocarbonaceous bonded phases

Summary Linear free-energy relationships are manifest in the linear dependence of the logarithmic retention factor on the carbon number for homologous series in reversedphase chromatography and allow the comparison of the free-energy increments of substituents for retention of a given elute by different stationary phases. Three methods were used for such comparison. In the first two, methylene group selectivities obtained from retention data obtained with homologous series on different hydrocarbonanceous bonded phases were compared by using different approaches. In the third method the energetics of retention of the end groups such as phenyl-, bromo- and disulfide-moieties of homologous series were examined. The results show that at fixed mobile phase composition and temperature the free energy increments due to methylene groups upon retention by alkyl-silica stationary phases are identical. Only hydrocarbonaceous bonded phases with short (C₁–C₂) alkyl chains or functional groups of unusual shape show consistent differences from octyl- or octadecyl-silicas. The results broaden the theoretical basis for the development of quantitative structure-retention relationships to be used for the prediction or interpretation of retention data in reversed-phase chromatography.Part I: ref. [23]Dedicated to Professor István Halász for his six tieth birthday.     

Synthesis of halogen-free amino-functionalized polymethyl methacrylate by atom transfer radical polymerization (ATRP)

In order to obtain amino-terminated polymethyl methacrylate (PMMA-NH₂) free of halogen we used the atom transfer radical polymerization (ATRP) to polymerize methyl methacrylate (MMA) in presence of an initiator containing an alkyl bromide unit and a protected amine functional group. The use of CuBr / N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) as co-catalyst system results in a polymer free of halogen due to hydrogen transfer from PMDETA to the growing polymer chain. However, side reactions occur affecting the typically “living” character of the ATRP. The measured molecular weights are consistently higher than the theoretical ones and the molecular weight distributions are relatively broad.     

Aggregation of Halogen Atoms in Crystalline Isomers

A comparison of halogen (Hal) atom aggregation patterns was made for 26 groups of crystalline C _m H _n Hal _p isomers (position isomers with different locations of halogen atoms and/or alkyl/phenyl groups and stereoisomers were primarily compared). A total of 58 substances with low halogen contents are considered. It is found that crystals of some isomers with similar molecular structures have fully identical Hal aggregates. In most cases, however, the Halaggregation patterns turn out to be only partly similar; i.e., isomers have either the same type of aggregates (finite or infinite in one, two, or three dimensions) or close local characteristics, such as k(Hal) (coordination number of halogen atoms with respect to neighboring halogen atoms) and r (mean distance to k nearest neighbors). Different positions of methyl groups in isomer molecules have no effect on the characteristics of Hal aggregates in the examined substances, whereas different positions of ethyl and phenyl groups lead to significant changes in the Halaggregation pattern.     

Carbon-13 NMR spectra of polybromoalkanes and polychlorobromoalkanes. Structural increments of halogens in polyhalogenated groups

The ¹³C NMR spectra of 48 polychlorobromoalkanes have been studied. Unlike the ¹³C signals of chlorine-containing groups (38–105 ppm), those of bromine-containing fragments, with the exception of CBr₂ (60–70 ppm), appear in a rather narrow range (25–50 ppm) and are shifted to higher field in relation to similar chlorine-containing groups. The spin–spin coupling constants in similar bromine- and chlorine-containing groups practically coinciEN. Calculation of the chemical shifts for the polyhaloalkanes under study according to the additivity scheme, as previously observed for polychloroalkanes, renders values which are in considerable discord with experimental values (up to –32 ppm for CBr₃). These discrepancies may be compensated for by corrections for the binary interaction of halogen atoms by grouping the halogen-containing fragments according to the geminal, vicinal, 1,3-, 1,3,5- and 1,2,3-arrangement of halogen atoms, and by introducing an increment for the position of the halogen at the secondary atom. It is established that as compared to 1-monohaloalkanes: (a) in the case of the geminal arrangement of halogen atoms the α- and γ-effects diminish (Δ α from –3.2 to –8 ppm; Δγ = 2.6 ppm), while the β-effect increases slightly (from 0 to 1.2 ppm); (b) in the case of a vicinal arrangement both the α- and β-effects diminish (by about –3.5 ppm) and the γ-effect remains constant, as if the vicinal system of the halogens was topologically insulated; (c) for the 1,3- and 1,3,5-arrangement of halogens their mutual influence is weak (about –0.5 ppm for each halogen atom in the α- and γ-positions); (d) the 1,2,3 system (serial arrangement of halogen atoms) is the sum of two vicinal fragments and hardly deviates from the additivity scheme; (e) the arrangement of a halogen at the secondary C atom enhances the α-effect (Δα = 2.8 and 1.0 for methyl and methylene, respectively, in the case of Cl, and 3.5 and 3.7 ppm in the case of Br); the variation of the β-effect has a different sign in relation to CH₃ and CH₂ groups (+1.2 and –1.7 for Cl, and +2.5 and –1.0 for Br). More distant effects of halogens (δ and ?) were not considered. The determined increments (Δα, Δβ and Δγ) for the α-, β- and γ-effects of chlorine and bromine atoms allow the prediction of the 13^C chemical shifts in polyhaloalkanes with an accuracy up to ±1.5 ppm. Some deviations of up to ±5 ppm may be connected with the influence of a three particle interaction of halogen atoms, which was taken into account only in the case of a geminal arrangement of halogen atoms.     

Effect of the alkyl chain length of the bonded stationary phase on solute retention in reversed-phase high-performance liquid chromatography

Summary A comparison of the chromatographic retention characteristics of nonpolar bonded stationary phases was investigated. The results show that the interactions between the solutes and the stationary phases having C₂, C₈ and C₁₈ alkyl groups are almost similar in the range of the mobile phase investigated. This interaction is considered as the solvophobic effect.     

Synthesis and oxidation of sulfides containing an isocyanurate fragment

The reaction of 1-(ω-haloalkyl)-3,5-dimethylisocyanurates with sodium salt of methyl thioglycolate afforded the corresponding 1-[ω-(methoxycarbonylmethylthio)alkyl]-3,5-dimethylisocyanurates, the oxidation of which with the system 34% aqueous H₂O₂-0.2 M NaHCO₃-Mn^II leads to the corresponding sulfones. The oxidation of these compounds with 34% aq. H₂O₂ in Ac₂O gives alternative products, viz., sulfones, sulfoxides, or α-acyloxy sulfides, depending on the temperature and number of methylene groups between the isocyanurate fragment and the sulfur atom.     

Retention of 1,ω-bis-(dialkylamino)-polysulphides and related compounds in reversed-phase HPLC

Summary The reaction of secondary amines R₂NH with disulfur dichloride S₂Cl₂ in dichloro methane yields mixtures of bis-dialkylamino polysulphides R₂N–S_n–NR₂ with sulphur chain lengths varying from n_s=2 to 14. The S chains are believed to be non-branched. The compounds form homologous series in each of which sulphur atoms give a constant contribution to retention. The sulphur retention index depends slightly on the carbon number in the terminal alkyl groups R. The presence of nitrogen lowers retention. The index increment of N is ca. –160 units, which is distinctly less negative than values found in alkylamines.     

Regular/abnormal variation in the strength and nature of the halogen bond between H2Te and the dihalogens: Prominent effect of methyl substituents

The halogen-bonded complexes between H₂Te/Me₂Te and the dihalogen molecules XY (XY = F₂, Cl₂, Br₂, I₂, ClF, ClBr, BrF, BrCl, BrI, IF, ICl, IBr) have been studied to investigate the dependence of its strength and nature on the halogen donor X and its adjoining atom Y, as well as the methyl groups in the electron donor. The interaction energy varies between −1.7 and − 43.5 kcal/mol, indicating that the Te atom in H₂Te/Me₂Te has a strong affinity for the dihalogen molecules. For the H₂Te-XY complex, the halogen bond is stronger for the heavier halogen donor X atom and the strong electron-withdrawing group Y. However, for Me₂Te-XY, the halogen bond is stronger for the lighter halogen donor X atom. The H₂Te/Me₂Te-F₂ complex has the largest interaction energy, although the σ-hole on F₂ is the smallest in magnitude. In most of the complexes, the electrostatic and polarization contributions to the binding strength are similar in magnitude. However, for H₂Te/Me₂Te-F₂, the polarization contribution is much larger than the electrostatic contribution, with a significant contribution from charge transfer.     

Investigation of interaction in C60 embedded complexes (X@C60) (X = alkali or halogen) at a series of radial positions by Buckingham potential function

Two typical series of C₆₀ embedded complexes (X@C₆₀) (X = Li, Na, K, Rb, Cs; F, Cl, Br, I) have been chosen to study as prototypes, in which the Buckingham potential (exp-6-1) function was applied to calculating the interactions of the atom pairs. The potential parameters are obtained from related crystals by the simulations using molecular mechanics methods. To utilize the symmetry of the potential field in C₆₀, the calculation is carried out along five typical radial directions. The computational results show that the interaction between the embedded atom and the C₆₀ cage is not purely electrostatic. The repulsive energy, E_rep, accounts for from 0.2% to 6.6% (for the alkali series), and from 1.5% to 58% (for the halogen series); the dispersive energy E_dis accounts for from 1.2% to 6.5% (for the alkali series), and from 2.2% to 42% (for the halogen series); and the electrostatic energy, E_es, accounts for 99% to 87% (for the alkali series) and from 96% to 0% (for the halogen series) when the embedded atom is put at the center of the cage. E_rep reaches up to 8% ∼ 35% (alkali), and 16% ∼ 704% (halogen); E_dis up to 4% ∼ 16% (alkali) and 7% ∼ 26% (halogen); and E_es falls down to about 88% ∼ 49% (alkali), and 96% ∼ 0% (halogen), when the embedded atom deviates 1.8 A from the cage center. The total interactions, E_inter, are all attractive for X (X = Li, Na, K, Rb, Cs; F. Cl, Br), but repulsive for the I atom. It is shown that the potential field in the C₆₀ cage has nearly spherical symmetry in an area with a radius of 1.8 Å around the cage center. The same kinds of interactions for the atoms in the two individual series are compared, and some variation rules are obtained. For (Li@C₆₀), the minimum energy equilibrium point deviates from the center by about 0.5 Å. © 1996 by John Wiley & Sons, Inc.     

The mechanism of 1,4 alkyl group migration in hypervalent halonium ylides: the stereochemical course

Rhodium(II)-acetate-catalyzed decomposition of either 1,3-cyclohexanedione phenyliodonium ylide or 5,5-dimethyl-1,3-cyclohexanedione phenyliodonium ylide in the presence of alkyl halides yields the corresponding 3-alkoxy-2-halocyclohex-2-enones via a 1,4 alkyl group migration shown to be concerted and intramolecular. In the case of (S)-alpha-phenethyl chloride, the rearrangement proceeds with essentially 88.6% retention of configuration. Theoretical calculations at the B3LYP/6-31G level reveal an activation energy of 5.4 kcal/mol for the process. A Claisen-like rearrangement occurs in the case where allylic halides, such as dimethylallyl or methallyl chorides, are used. The mechanistic pathway proposed for these processes involves addition of the halogen atom of the alkyl or allyl halide to the rhodium carbenoid from the iodonium ylide to yield a halonium intermediate that undergoes halogen to oxygen group migration. Aryl halides, such as chloro-, bromo-, iodo-, and fluorobenzene, behave differently under the same reaction conditions, yielding the product of electrophilic aromatic substitution, namely, the 2-(4-halophenyl) 1,3-cyclohexanedione.     

Chlorine atom migration in the fragmentation of α-mesyl sulfonyl chlorides

The mass spectra of alkanesulfonyl chlorides can be rationalized by loss of a chlorine atom from the molecular ion, followed by loss of SO₂ with concomitant alkyl cation formation. The mass spectra of two α-mesyl sulfonyl chlorides exhibit a new fragmentation process in which the chlorine atom initially attached to sulfur migrates to the α-carbon atom with loss of SO₂, thereby resulting in retention of chlorine in the alkyl cation.     

Reversible solvent and temperature induced “monomeric-like” —“Polymeric-like” transitions in alkyl bonded silica

Summary The study of behaviour of homologous series (and more precisely, the plots of methylene selectivity versus the carbon number, n, of the alkyl chain of the homologue) on monomeric and polymeric phases in liquid chromatography enabled us to characterize two kinds of retention mechanism. These mechanisms are not independent; changes in the temperature or mobile phase composition allow passage from one to the other. The first, for which the methylene selectivity versus n decreases with a discontinuity, defines a retention mechanism where solutes are inserted between the grafted chains. It occurs for monomeric phases at ambient temperature, for polymeric phases, at high temperature or with a strong solvent like THF, CH₂Cl₂, AcMe or CHCl₃. The second gives a plot of methylene selectivity versus n with a maximum. It is consistent with a partition mechanism with total immersion of the solute in the bed of the stationary phase. It occurs for a polymeric phase at ambient temperature or for a monomeric one at low temperature.     

Determination of the activation rate constants of alkyl halide initiators for atom transfer radical polymerization

The second-order activation rate constants k_A for low-molar-mass alkyl halides catalyzed by cuprous halide complexes Cu(I)X/2L (X = Cl or Br; L = 4,4′-diheptyl-2,2′-bipyridine) were determined by the nitroxide capping method along with ¹H NMR. The k_A for 1-phenylethyl bromide, a typical initiator for atom transfer radical polymerization (ATRP), with the Cu(I)Br complex was found to be close to the known value of the k_A for a polystyryl bromide, being large enough for the initiation to be completed at an early stage of polymerization. It was also found that k_A strongly depends on the kind of halogen and the steric factor of the alkyl halide in question.     

RADIATION CHEMISTRY OF AMINOALKYITHIOIS

Abstract

Radiation chemistry of aminoalkylthiols and some derivatives (aminoalkyl-S-thiophosphates and aminoalkyl-S-thiosulfates among which are most effective radioprotectors) in aqueous solutions has been studied in order to establish the relation between the structure of these compounds and their radiation-chemical properties. Increase in the number of methylene groups between the sulfur atom and nitrogen atom in the aminothiol molecule from 2 to 4 and substitution of the amino group hydrogene by an alkyl radical has been found to be without appreciable effect both on the “spectra” of radiolisis products and rate constants for interaction of H, e^? _aq and OH with aminothiols     

Synthesis of functionalised β-lactones via silylcarbocyclisation/desilylation reactions of propargyl alcohols

Functionalised β-lactones were prepared starting from propargyl alcohols by means of an efficient rhodium-catalysed silylcarbocyclisation reaction. This process took place in high yields and complete stereoselectivity using DBU as the base, a sterically hindered alcohol and/or hydrosilane. Otherwise, a silylformylation competitive reaction occurred affording 3-hydroxy-2-[(aryldimethylsilyl)methylene]alkanals with complete regioselectivity. Indeed, while α-(aryldimethylsilylmethylene)-β-lactones were generated exclusively starting from an alkynol characterised by two alkyl groups on the propargyl carbon atom, significant amounts of the aldehydes were observed for secondary alcohols. In these cases, the use of ortho-substituted arylsilanes improved the chemoselectivity towards the α-methylene-β-lactones. Such molecules were then successfully converted into α-methylaryl-β-lactones by a fluoride-induced rearrangement of the aryl group that migrates from silicon to carbon with retention of configuration.     

Properties of halogen atoms for representing intermolecular electrostatic interactions related to halogen bonding and their substituent effects

The electrostatic properties of halogen atoms are studied theoretically in relation to their ability of halogen bonding, which is an attractive intermolecular interaction of a covalently bonded halogen atom with a negatively charged atom of a neighboring molecule. The electric quadrupole (of electronic origin) with a positive zz component Θ_zz of a covalently bonded halogen atom, where the z axis is taken along the covalent bond involving the halogen atom, is mainly responsible for the attractive electrostatic interaction with a negatively charged atom. This positive Θ_zz is an intrinsic property of halogen atoms with the p_x²p_y²p_z configuration of the valence electronic shell, as shown by ab initio molecular orbital calculations for isolated halogen atoms with this electronic configuration, and increases in the order of F < Cl < Br < I, in parallel with the known general sequence of the strength of halogen bonding. For halogen‐containing aromatic compounds, the substituent effects on the electrostatic properties are also studied. It is shown that the magnitude of Θ_zz and the electric field originating from it are rather insensitive to the substituent effect, whereas the electric field originating from atomic partial charges has a large substituent effect. The latter electric field tends to partially cancel the former. The extent of this partial cancellation is reduced in the order of Cl < Br < I and is also reducible by proper substitution on or within the six‐membered ring of halobenzene. Perspectives on the development of potential function parameters applicable to halogen‐bonding systems are also briefly discussed. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010     

Halogen cleavage reactions of cis-(threo-PhCHDCHD)Mn(CO)4PEt3

Halogen cleavage reactions of $\text{cis}$-($\text{threo}$-PhCHDCHD)Mn(CO)₄, PEt₃ ($\text{3}$) are studied in detail and are found to proceed either predominantly with retention or inversion, or non-stereospecifically, depending on reaction conditions. Reaction mechanisms involving S_E2 (inversion), S_E2 (retention) and S_E (oxidative) processes are proposed, demonstrating that halogen cleavage reactions of $\text{3}$ straddle the mechanistic borderline between electrophilic substitution and oxidation processes of transition metal alkyl compounds.     

Theoretical study on the substituent effect of halogen atom at different position of 7-azaindole-water derivatives: relative stability and excited-state proton-transfer mechanism

We have theoretically investigated the substituted effect on the first excited-state proton-transfer process of nX7AI-H₂O (n?=?2~6, X?=?F, Cl, Br) complex at the TD-M06-2X/6-31?+?G(d, p) level. Here X is the substituted halogen atom, and n value denotes the substituted position of X, such as C₂, C₃, C₄, C₅, or C₆. For the substituted 7-azaindole clusters, 6X7AI-H₂O molecule is the most stable structure in water. The replacement of halogen atom X does not affect the characters of the HOMO and LUMO, but influence the S₀?→?S₁ adiabatic transition energies of nX7AI-H₂O (n?=?2~6, X?=?F, Cl, Br). Our calculated results show that the double proton transfer occurs in a concerted but asynchronous protolysis pathway no matter which H atom is replaced by halogen atom. The halogen substitution changes the structural parameters evidently and leads to amply the asynchronousity during the proton-transfer process. The ESPT barrier height increases or decreases due to the halogen atom and substituted position.     

Polynuclear branched tetrazole systems. 3. Acidity of α,ω-ditetrazol-5-ylalkanes

α,ω-Ditetrazol-5-ylalkanes with 1 to 5 methylene groups in the alkyl fragment display the properties of dibasic heterocyclic NH acids with pK_a values lying in the range 3.4–6.1. The pK_a values of these compounds are in a linear dependence on the values of the dielectric permeability of the medium, on the chemical shifts of the signals of the endocyclic carbon atom, and of the carbon atom of the α-methylene group in the ¹³C NMR spectra. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 535–541, April. 2006.