Reactions of carbenes with anions

CHBrCl₂, CHCl₂I and CHBr₂I are prepared by a simple method based on the reaction of dichloro or dibromocarbene with the corresponding alkali metal halogenides under phase transfer conditions.     

Photochemical desulfurization of thiols and disulfides

A visible light-induced desulfurization process for thiols and disulfides using triethylphosphite and triethylborane is reported. The reaction can be effected on a range of organic molecules having either primary, secondary or tertiary thiol groups and disulfides without the need of protecting groups. Thus, after treating l-cystine 7, l-cystine dimethylester 8, thioctic amide 9 and glutathione disulfide 10, first with tributylphosphine, later with triethylborane/triethylphosphite under irradiation in a one-pot reaction, the corresponding desulfurized compounds l-Ala, l-Ala, 1-octanamide and γ-l-Glu-l-Ala-Gly, respectively, are prepared in high yields with retention of configuration.     

Indo study of some cyclic carbenes

INDO calculations, with geometry optimization, have been performed on the cyclic carbenes cyclopropenylidene (I) cyclopropylidene (II), and cyclopentadienylidene (III). It is found that all three systems have a singlet ground state and, due to aromaticity, the triplet-singlet energy separations for I and III are particularly large. In addition, for each system, the electronic structure and its influences on geometry and reactivity are discussed.     

Reactions of thiols and organic disulfides with iron and its oxides

The influence of O₂ and H₂O on the formation of iron dithiolates (IDTs) by the reactions of thiols and organic disulfides with the iron powder was studied under the static (without mechanical activation of the reaction) and dynamic (with mechanical activation of the reaction) conditions. Under the static conditions in the absence of O₂ or H₂O, only BnSH and PhSH give IDTs, whereas DdSH (Dd is dodecyl) produces IDTs in the presence of O₂ or H₂O. For the mechanical activation of the reactions of organic disulfides with the iron powder, the formation of IDTs is possible if the reaction mixture contains H₂O. On the interaction with iron oxides excess thiols are oxidized to organic disulfides and also afford IDTs. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1383–1386, August, 2006.     

Synthesis of new cyclic dialkoxy disulfides

Five new cyclic dialkoxy disulfides have been synthesized and fully characterized. An X-ray structure was obtained for the 2,3-furandimethylene dialkoxy disulfide.     

Reactions of carbenes with bicyclobutanes and quadricyclane : Cycloadditions with two σ bonds

Dicarbomethoxycarbene and dichlorocarbene add to 1,2,2-trimethylbicyclo[1.1.0]butanc in such a fashion as to suggest a concerted addition in which the central and side bonds are cleaved simultaneously. MNDO calculations support such a pathway and suggest that endo attack on the bicyclobutane is preferred to exo. Reaction of the same two carbenes with quadricyclane gives substituted derivatives of the exo-tricyclo[3.2.1.0^2,4]oct-6-ene system. Although these products rearrange further under the conditions of reaction and/or analysis, they can be shown to be primary products. It is suggested that quadricyclane reacts with carbenes in a concerted fashion.     

Oxygen ylides - I. : Reactions of carbenes with oxetane

The ylides generated from carbenes (:CH₂, :CHCO₂Et, :CHPh) and oxetane in the presence of methanol undergo Stevens rearrangement and protonation competitively, yielding tetrahydrofurans and 1,3-dialkoxycyclopropanes as major products.     

Deoxygenation and desulfurization of oxiranes and thiiranes by carbenes: a theoretical study

The potential-energy surfaces for the abstraction reactions of carbenes with oxirane and thiirane have been characterized in detail by using density functional theory (B3LYP/6-31G*), which include zero-point corrections. Six carbene species: dimethylcarbene, cyclobutylidene, cyclohexylidene, phenylchlorocarbene, methoxyphenylcarbene, and dimethoxycarbene have been chosen in this work is model reactants. All the interactions involve the initial formation of a loose donor-acceptor complex followed by a heteroatom shift via a two-center transition state. The complexation energies, activation barriers, and enthalpies of the reactions were used comparatively to determine the relative carbenic reactivity, as well as the influence of substituents on the reaction potential-energy surface. As a result, our theoretical investigations indicate that, irrespective of deoxygenation and desulfurization, the relative carbenic reactivity decreases in the order: cyclobutylidene > dimethylcarbene approximately equals cyclohexylidene > phenylchlorocarbene > methoxyphenylcarbene > dimethoxycarbene. Namely, the alkyl-substituted carbene abstractions are much more favorable than those of the pi-donor-substituted carbenes. Moreover, for a given carbene, while both deoxygenation and desulfurization are facile processes, the deoxygenation reaction is more exothermic but less kinetically favorable. Furthermore, a configuration-mixing model based on the work of Pross and Shaik is used to rationalize the computational results. The results obtained are in good agreement with available experimental observations, and allow a number of predictions to be made.     

Transformation of mononuclear aryl thiocarbamates to cyclic disulfides

Thermolysis of the melt of octakis(O-thiocarbamoyl)tetra(C-phenethyl)resorcinarene in a microwave reactor afforded a cavitand, the upper rim of which was formed due to the formation of disulfide bridges between the neighboring benzene rings of the resorcinarene framework. The reaction of 2,2’-bis(carbamoylthio)-1,1’-methylenedinaphthalene with LiAlH₄ gave a tricyclic derivative, in which the naphthalene rings were bound by methylene and disulfide bridges. The structure of 2,2’-dithiinedinaphthylmethane was confirmed by single crystal X-ray diffraction analysis of this compound.     

Synthesis of cyclic N‐nitrourethanes by the simultaneous oxidative desulfurization and nitration of cyclic thiourethanes

The reaction of five‐ and six‐membered cyclic thiourethanes with acetyl nitrate results in a vigorous reaction that generates copious amounts of red–brown nitrogen oxide fumes and produces the corresponding cyclic N‐nitrourethanes in high yields (>95%). The overall yield of the cyclic N‐nitrourethanes starting from an aminoalcohol using the “thiourethane” route is superior to the conventional route going through the cyclic urethane. J. Heterocyclic Chem., (2011).     

Synthesis of symmetrical disulfides by reaction of fluorine-containing thiiranes with cyclic amines

Regioselective opening of the thiirane ring in fluorine-containing thioglycidyl ethers and [(perfluorobutyl) methyl]thiirane by the action of cyclic amines afforded 1,2-aminothiols which were oxidized in situ to symmetrical disulfides. The rate of formation of the latter depended on the amine basicity. According to the NMR data, the resulting disulfides were mixtures of erythro and threo diastereoisomers.     

High yield preparation of cyclic disulfides using alkyltin thiolates

Cyclic disulfides with ring sizes ranging from 5 to 10 are formed by iodination or bromi nation of alkyltin thiolates without the need of high dilution.     

Reactions of carbenes in solidified organic molecules at low temperature

Studies on reactions of carbenes in reactive organic glasses at low temperatures clearly reveal that solution results and liquid phase mechanistic rules cannot be readily extrapolated to matrix conditions. Thus, the usual course of reaction of a carbene with an alkene in solution results in the formation of a cyclopropane for both the singlet and triplet states although a one-step addition possible for singlet carbene produces the cyclopropane stereospecifically and a stepwise pathway with the triplet state affords two possible stereoisomers of the cyclopropane. In a sharp contrast, the formal insertion products into the allylic C-H bonds of alkenes are produced at the expense of the cyclopropane when carbene is generated in alkene matrix at low temperature. Similar results are obtained in the reaction with alcohols, where the C-H insertion products are formed in low temperature alcoholic matrices at the expense of the O-H insertion products which are predominant products in the reaction with alcoholic solution at ambient temperature. The ¹³C labelling experiments as well as deuterium kinetic isotope effects suggest that these C-H insertion products are most probably produced from the triplet carbene, not from the singlet, by abstraction of H atom from the matrix followed by the recombination of the resulting radical pairs. Kinetic studies using ESR and laser flash photolysis techniques demonstrate that the mechanism of a H-atom transfer reaction changes from a completely classical process in a soft warm glass to a completely quantum mechanical tunneling process in a cold hard glass. Thus, as the reaction temperature is lowered, the classical reaction rate decreases, and eventually becomes much slower than decay by hydrogen atom tunneling. The members of the radical pairs which usually diffuse apart in a fluid solution are not able to diffuse apart owing to the limited diffusibility within a rigid matrix and therefore recombine with high efficiency to give the CH “insertion” products. A rather surprising and intriguing difference between the C-H insertion undergone by singlet carbenes in fluid solution at ambient temperatures and one by triplet carbenes in matrix at low temperature is noted. Thus, a marked increase in the primary and secondary C-H insertion over the tertiary is observed in the matrix reaction indicating that triplet carbenes tend to abstract H from less crowded C-H bonds. This is interpreted to indicate that the distance between carbenic center and tunneling H becomes important in H atom tunneling process. More surprisingly, the C-H insertion by triplet carbene by the abstraction-recombination mechanism in a rigid matrix proceeds with retention of the configuration, suggesting that the solid state prevents motion of the radicals to the extent that does not allow racemization to occur. Reactions with heteroatom substrates such as ethers, amines, alkyl halides and ketones are also subject to the matrix effects and the C-H insertion products increase at the expense of singlet carbene reaction products resulting from the interaction with the heteroatoms. Stereoselectivities of cyclopropanation to styrenes are also shown to be affected by the matrix effects. t-Butyl alcohol matrix is shown to be unreactive toward carbenes and thus can be used as a “solvent” in matrix carbene reactions presumably due to a large inert guest cavity provided by bulky tertiary alcohol which binds a molecular aggregate inside it. H atom tunneling in the matrix is also shown to compete with very efficient intramolecular migration of hydrogen to the carbenic center. Migration aptitude as well as stereochemistry are also found to be subject to the matrix effects.     

Cobalt carbonyl catalyzed reactions of disulfides: Carbonylation to thioesters and desulfurization to sulfides

aromatic and benzylic disulfides react with carbon monoxide and a catalytic amount of cobalt carbonyl to give thioesters and carbonyl sulfide, while the presence of t-butyl peroxide results in the formation of sulfides in high yields.     

Controlled radical polymerization of styrene in the presence of cyclic 1,2‐disulfides

The thermal polymerization of styrene (St) in the presence of cyclic 1,2‐disulfides at 120 °C was investigated. In the polymerization of St in the presence of 1,2‐dithiane (DT), that is, six‐member cyclic 1,2‐disulfide, the polymer yields and molecular weights increased with the reaction time. The linear relation between the polymer yields and molecular weights was observed, and the line passed through an original point. The molecular weight distributions of the polymers remained almost constant but were not narrow. For this polymerization with a living nature, we proposed the following mechanism: the propagating St radical reacted with thiyl radicals derived from DT, leading to the formation of dormant species, and the formed C S bond of the dormant was dissociated again to give the propagating polystyryl radical and thiyl radical. Similar results were obtained in the thermal polymerization of St at 120 °C in the presence of 1,2‐dithiacycloheptane, that is, seven‐member cyclic 1,2‐disulfide. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 145–151, 2001