Substituent effects in addition of iodine thiocyanate to alkenes

The plots of logarithms of relative rates of ISCN addition to alkenes versus alkene IPs and versus alkene HOMO energies reveal that the alkene relative reactivity depends upon both electronic and steric effects of the substituents. Steric effects are related not only to the degree of substitution on the CC bond but also to the relative position, size, and branching of alkyl substituents.     

Substituent effects on the kinetics of acid-catalyzed hydrolysis of methyl cellulose

The kinetics of the hydrolysis of methyl cellulose (MC, DS 1.27 and 1.95) was studied by a two-step procedure, comprising partial hydrolysis in 1 M TFA in water and water/acetone at 120 °C for various time periods, labeling of generated reducing ends by reductive amination, complete depolymerization by methanolysis followed by trimethylsilylation, and gas chromatographic analysis of the two sets of partially O-methylated glucose derivatives. Rate constants of MCs were all in the order of 10^?4 s^?1. In aqueous TFA, overall rate of hydrolysis of the MC with lower DS was faster than of the MC with higher DS. When substituting half of the water by acetone, reaction was slowed down while selectivity regarding different O-methyl glucosyl residues increased. Compared to the parent glucosyl unit methylation at O-2 and at O-6 decreased rate of hydrolysis, while 3-O-methyl favored it especially in the early stage of the conversion of the macromolecules. Beside slight differences between the two MCs and reaction conditions, rate constants k _i (i = position of methyl) followed the order k ₃₆ ≈ k ₃ > k ₀ ≈ k ₂₃ > k ₆ > k ₂ ≥ k ₂₃₆ > k ₂₆. For the higher substituted MC2 an initial slow phase with more pronounced differences of k _i, followed by a faster less selective period was observed. Regioselectivity of hydrolysis with respect to methyl positions was expressed as standard deviation of k _i and was between 16 and 46% depending on MC and conditions. Findings are discussed with respect to electronic effects, solvent-effect, H-bonding pattern and solution state.     

Water-controlled nitro-oximation of alkenes under catalyst-free conditions

A convenient and precise nitration and oximation of alkenes with tert-butyl nitrite has been reported, yielding α-nitro ketoximes in satisfactory yields with broad substrate generality and excellent stereoselectivity under mild conditions. Experiments indicate that tert-butyl nitrite serves as both NO and NO₂ sources and water plays a key role in this difunctionalization reaction.     

Substituent effects in endocyclic cleavage-recyclization anomerization reaction of pyranosides

Pyranosides with 2,3-trans carbamate or 2,3-trans carbonate groups are anomerized under mild acidic conditions via endocyclic cleavage reaction. In order to understand the nature of the anomerization reaction via the endocyclic cleavage-recyclization process, the substituent effects at various positions were investigated.     

Melamine trisulfonic acid-catalyzed N-formylation of amines under solvent-free conditions

A highly convenient method for N-formylation of amines via treatment by formic acid in the presence of melamine trisulfonic acid as a catalyst has been developed. This method showed improvements over previous reports in terms of yield, reaction time and chemoselectivity.     

Brönsted acid-catalyzed synthesis of diarylmethanes under non-genotoxic conditions

Triflic acid and triflimide were found to efficiently catalyze the formation of a wide diversity of diarylmethanes from the non-genotoxic benzylic acetates and electron-rich arenes or heteroarenes. The reaction worked best with acetates capable of generating a stabilized benzylic cationic species. In most cases, the reactions were conveniently run in the absence of solvent under mild conditions.     

Substituent effects in the reaction of phenols with polyacrylonitrile radicals

Substituent effects in the reactivities of phenols towards polyacrylonitrile radicals have been studied in terms of Swain and Luptons' field (F_k) and resonance (R_k) components of the substituent parameters and the unique positional weighting factors (f_j and r_j) proposed by Williams and Norrington, with the aid of the following equation:

$\text{P}{}_{\mathrm{i}}\text{= x}{}_{\mathrm{i}}\text{?}{}_{\mathrm{i}}\text{F}{}_{\mathrm{k}}\text{+ ß}{}_{\mathrm{i}}\text{r}{}_{\mathrm{j}}\text{K}{}_{\mathrm{i}}\text{+ e}{}_{\mathrm{i}}\text{+}\text{P}\text{i}\text{o}$

P_i′s are the rate parameters, P_i⁰ being that for a standard reference compound. Two types of rate parameters have been employed—one, suggested by Simonyi, Tüdös and Pospisil (β) and another, suggested by us, (K), which is obtained from a plot of [Monomer]/(rate of polymerisation) vs [Phenol]. The correlations have been found to be quite satisfactory with both β and K. An attempt has been made to ascertain the nature of the transition state from the reaction parameters α_i and β_i; a dipolar transition state is suggested. The anomalous kinetic behaviour of hydroxy phenols has been discussed.     

Effect of reaction conditions on the hydration of isoprene

Effect of conditions (partial pressure of isoprene, temperature, catalyst concentration, medium) on the rate and selectivity of reactions occurring in interaction of isoprene with water in the presence of orthophosphoric acid as a catalyst was studied at elevated temperatures and pressures in the liquid phase. The reaction orders with respect to isoprene were determined for reactions in which 3-methylbutanone-2,2-methylbutanal, and 2,2-dimethylpropanal are formed. The applicability of various acidity characteristics for describing the rates of the reactions under study are considered.     

Substituent effects on the stereochemistry in the [2 + 2] photocycloaddition reaction of homobenzoquinone derivative with variously substituted alkenes and alkynes

Irradiation of a homobenzoquinone derivative with variously substituted alkenes and alkynes gave the [2 + 2] photocycloadducts, tricyclic diones, almost quantitatively as a mixture of regio- and stereoisomers. The preferred regioisomer for all reactions is attributed to the more stable 1,4-biradical intermediate (major addition mode), and the minor isomer is attributed to the less stable biradical (minor addition mode). A radical trapping experiment using benzeneselenol proved the generation of these two regioisomeric biradicals, reflecting the regioselectivity in selenol-free photoreaction. Both biradicals tended to preferentially yield the endo-isomer for the alkenes with smaller substituents such as ethoxy, cyano, and acetoxy groups, but the exo-isomer for the alkenes with larger substituents such as phenyl, carbazolyl, and tert-butyl groups. The logarithmic exo/endo ratios were well correlated with a combination of Taft's steric factor E(s) and the energy gain (DeltaE') associated with the orbital interactions between the spin centers of 1,4-biradicals. These results were interpreted in terms of Griesbeck's SOC mechanism as well as the possible bond rotation around the armed radical chain. Therefore, it is concluded that a balance of repulsive steric hindrance and the attractive FMO interaction determines the stereochemical course of the [2 + 2] photoaddition of homobenzoquinone derivative with variously substituted alkenes.     

Substituent effects in the reaction between acetophenones and polyacrylonitrile radicals

The reactivity of substituted acetophenones towards polyacrylonitrile radical has been correlated with Swain and Luptons field and resonance parameters inclusive of the unique positional weighting factors suggested by Williams and Norrington. From the sign and magnitude of the reaction dependent regression constants, the transition state has been found to be essentially ‘non-polar’ in character.     

Substituent effects in the reaction between benzaldehydes and polyacrylonitrile radicals

The effects of substituents on the reaction of benzaldehydes with polyacrylonitrile radicals have been studied in terms of the multiparameter equations based on Swain and Lupton's Field and Resonance (F and R) components with the unique positional weighting factors f and r developed by Williams and Norrington. From the nature of the reaction constants, a polar transition state has been postulated.     

Platinum-catalyzed reductive coupling of activated alkenes under hydrogenation conditions

The Pt complex generated from PtCl₂, PR₃, and SnCl₂ catalyzes the reductive coupling of activated alkenes under environmentally benign hydrogenation conditions. Various bis-enones participated in the intramolecular cyclization, forming the desired cyclization products in moderate to good yield. Intermolecular reductive coupling of the enone and the aldehyde provided the coupling product in good yield. This methodology illustrates the first use of platinum complexes in hydrogen-mediated couplings of activated alkenes.     

Permanganate oxidation of alkenes. Substituent and solvent effects. Difficulties with MP2 calculations

The permanganate oxidation of alkenes has been studied both experimentally and computationally. Transition state structures were located for the reaction of permanganate ion with a variety of monosubstituted alkenes at the B3LYP/6-311++G** level. Although the calculated activation energy for the reaction with ethene was reasonable, the calculated effect of substituents, based on the energies of the reactants, was much larger than that experimentally found. This was shown to be due to the formation of an intermediate charge-dipole complex which led to the transition state. Reaction field calculations found the complex to disappear in a high dielectric constant medium, and the range of activation energies for the reaction in solution became quite small. MP2 calculations were carried out in order to have a comparison with the DFT results. MP2-MP4 gave unusual results for calculations on permanganate ion as well as chromate ion and iron tetraoxide. They also gave markedly unreasonable results for the activation energy of the reaction of permanganate with ethane. CCSD/6-311++G** calculations gave satisfactory results for permanganate ion and chromate ion. At this level of theory, the reaction of permanganate with ethene was found to have a very early transition state, when the bond lengths of the reactants just began to change. The reaction was calculated to be very exothermic (-69 kcal/mol), and this was confirmed via calorimetry. The rates of permanganate oxidation of allyl alcohol and acrylonitrile were determined, and they had similar reactivities. The kinetics and the products of the reaction of permanganate with crotonate ion were examined in some detail.     

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.     

Lewis acid-catalyzed, copper(II)-mediated synthesis of heteroaryl thioethers under base-free conditions

A Lewis acid (Ag(I), Ni(II), or Fe(II)) catalyzed, Cu(II)-mediated thiolation reaction between heteroarenes and thiols was achieved with good yield under base-free conditions. DMSO could serve as an effective methylthiolation reagent for the synthesis of heterocyclic methyl thioethers.     

Substituent effects in cyclohexenes

Summary The effects of the C¹-CH₃ group in the¹³C spectra of trans-4,5-dimethylcyclohex-4-ene-1,2-dicarboxylic and syn-txans-⁴-octalin-1,2-dicarboxylic acid and their diesters show the existence in solution of a considerable proportion of the trans-a,a forms of these compounds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1310–1312, June, 1980.     

Mizoroki-Heck type arylation of alkenes using aroyl chlorides under base-free conditions

The Mizoroki-Heck type arylation of alkenes with aroyl chlorides accompanied by decarbonylation efficiently proceeds in the presence of a palladium catalyst system, PdCl₂(PhCN)₂/(PhCH₂)Bu₃NCl, even without adding any base. The products can be isolated by a very simple procedure.     

Substituent stereochemistry effects on diastereoselective methylation reaction of 4-chloroadamantan-2-ones

π-Facial diastereoselectivity effects of the substituent in 4-position on the nucleophilic addition of substituted adamantan-2-ones were observed for the methylation reaction of 4-chloroadamantan-2-ones. NMR study revealed that when chlorine atom is in axial stereochemical position, exclusively anti-addition occurs, whereas selective preference for syn-addition was observed with stereochemical equatorial position for chloro substituent. The success of this strategy can be attributed to the important role that CeCl₃ plays in increasing the nucleophilicity and decreasing the basicity of the methylorganometallic reagent.     

A highly efficient method for the hydroaminomethylation of long-chain alkenes under aqueous, biphasic conditions

The use of salts of secondary and primary amines with different inorganic and organic acids in hydroaminomethylation enables the quantitative conversion of 1-octene with high selectivity for saturated amines. We propose that a cationic rhodium species is formed under the acidic conditions which catalyses the hydrogenation of the enamine or imine formed subsequently. Thus the use of acids and amine salts enables the hydroaminomethylation of long-chain alkenes under aqueous, biphasic conditions with quantitative conversions and short reaction times.