Reductive desulfurization of 3-cyano-2-methylthiopyridines under the action of Raney nickel

The action of Raney nickel on substituted 3-cyano-2-methylthiopyridines was studied. Under conditions of catalytic hydrogenation, the reaction yields a mixture containing the aminosulfide resulting from reduction of the nitrile group with retention of the methylthio group, the nitrile resulting from elimination of the methylthio group, and the amine resulting from both reduction of the nitrile group and elimination of the methylthio group. Treatment of 3-cyano-2-methylthiopyridines with a large amount of Raney nickel under desulfurization conditions induces simultaneous elimination of the methylthio group and reduction of the nitrile group to the aminomethyl group. When reductive desulfurization is carried out in methanol or THF, primary amines are formed, while the reactions in isopropyl or ethyl alcohol give secondary or tertiary amines, which are formed upon alkylation of the amino group with alcohols. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2497–2500, November, 2005.     

Electron impact induced water elimination from hydroxyamides. 1—N-(2′-hydroxyethyl)piperidone-2 and N-3′-hydroxypropyl piperidone-2

The main process of the electron impact induced water elimination of the title compounds takes a reaction course comprising several individual steps. A characteristic neighbouring group participation of the carbonyl function is involved, onto which a hydrogen is transferred in a first rate-determining reaction step. This rearranged hydrogen is finally lost together with the hydroxyl group. The reaction of N-(2′-hydroxyethyl)piperidone follows what seems to be a [1,1] elimination whereas the H₂O elimination from N-(3′-hydroxypropyl)piperidone represents a formal [1,2] elimination.     

Functionalized cyclobutanes via Heck cyclization

Heck-type 4-exo-trig cyclization of linear 2-enol triflate-1,5-hexadienes provides functionalized methylene cyclobutanes. Intramolecular palladium coordination can initiate beta-hydride elimination leading to 1,2-dimethylene cyclobutane derivatives, which are obtained with high selectivity if substrates having a geminal diphenyl group at Calpha are used. In parallel, formal 5-endo-trig cyclization and beta-hydride elimination form 1-methylene cyclopent-2-en derivatives.     

Side-chain reactions in π-chromium tricarbonyl-complexed arenes IV. Solvent and deuterium isotope effects in E2 reactions of some 2-phenylethyl and 1-phenyl-2-propyl derivatives

The rates and products of elimination with sodium ethoxide in ethanol, and potassium t-butoxide in t-butanol, of chromium tricarbonyl-complexed (2-phenylethyl bromides), (2-phenylethyl tosylates), (1-phenyl-2-propyl bromides), and (1-phenyl-2-propyl tosylates) were determined. Complexation increases the rate of elimination, the size of the effect depending on leaving group, solvent/base system and, to a minor extent structure. The Cr(CO)₃ group increases the olefin percentage when substitution competes with elimination, whereas the relative proportions of the olefin isomers obtained from secondary substrates are practically unchanged whether the ring is complexed or not. Deuterium isotope effects have been measured for the elimination from uncomplexed, complexed and 2-(p-nitrophenyl)ethyl compounds. The magnitude of the isotope effect does not vary appreciably with substituents.     

An annulation method for the synthesis of alkyl-substituted 6-carbomethoxy-2-pyridones

A protocol for the synthesis of 5-alkyl and 3,5-dialkyl-6-carbomethoxy-2-pyridones was devised. Key steps include a Mannich reaction, acylation of a tosylamine, and a PPh₃/TiCl₄-promoted intramolecular Reformatsky-type reaction with a thioester as the electrophile. The latter process typically afforded a vinylogous thiocarbamate via elimination of water rather than the Dieckmann-type product which would have resulted from elimination of the thiol. However, the Dieckmann-type ketone product was obtained in one instance. Subsequent elimination of the tosyl group and desulfurization completed the pyridone synthesis.     

Syntheses and Reactions of 1′,2′-Unsaturated 2′,3′-Seconucleoside Analogues

The 1′,2′-unsaturated 2′,3′-secoadenosine and 2′,3′-secouridine analogues were synthesized by the regioselective elimination of the corresponding 2′,3′-ditosylates, 2 and 18 , respectively, under basic conditions. The observed regioselectivity may be explained by the higher acidity and, hence, preferential elimination of the anomeric H–C(1′) in comparison to H? C(4′). The retained (tol-4-yl)sulfonyloxy group at C(3′) of 3 allowed the preparation of the 3′-azido, 3′-chloro, and 3′-hydroxy derivatives 5–7 by nucleophilic substitution. ZnBr₂ in dry CH₂Cl₂ was found to be successful in the removal (85%) of the trityl group without any cleavage of the acid-sensitive, ketene-derived N,O-ketal function. In the uridine series, base-promoted regioselective elimination (→ 19 ), nucleophilic displacement of the tosyl group by azide (→ 20 ), and debenzylation of the protected N(3)-imide function gave 1′,2′-unsaturated 5′-O-trityl-3′-azido-secouridine derivative 21 . The same compound was also obtained by the elimination performed on 2,2′-anhydro-3′-azido-3′-azido-3′-deoxy-5′-O-2′,3′-secouridine ( 22 ) that reacted with KO(t-Bu) under opening of the oxazole ring and double-bond formation at C(1′).     

Fragmentation reactions of the enolate ions of 2-pentanone

The reaction of [OH]^? with 2-pentanone produces two enolate ions, [CH₃CH₂CH₂COCH₂]^? and [CH₃COCHCH₂CH₃]^?, by proton abstraction from C(1) and C(3), respectively. Using deuterium isotopic labelling the fragmentation reactions of each enolate have been delineated for collisional activation at both high (8 keV) and low (5–100 eV) collisional energies. The primary enolate ion fragments mainly by elimination of ethene. Two mechanisms operate: elimination of C(4) and C(5) with hydrogen migration from C(5), and elimination of C(3) and C(4) with migration of the C(5) methyl group. Minor fragmentation of the primary enolate also occurs by elimination of propane and elimination of C₂H₅; the latter reaction involves specifically the terminal ethyl group. The secondary enolate ion fragments mainly by loss of H₂ and by elimination of CH₄; for the latter reaction four different pathways are operative. Minor elimination of ethene also is observed involving migration of a C(5) hydrogen to C(3) and elimination of C(4) and C(5) as ethene.     

Reduction of 2-amino-3- and -5-nitropyridine derivatives with hydrazine hydrate

The reactions of 2-amino-3-nitropyridine and 2-amino-5-nitropyridine with hydrazine hydrate resulted in elimination of the amino group and reduction of the nitro group with formation of 3-aminopyridine. A probable reaction mechanism involves addition of hydrazine hydrate at the N-C² bond, followed by elimination of ammonia and reduction of the nitro group to amino. 2-Amino-4-methyl-3-nitropyridine and 2-amino-5-methyl-3-nitropyridine reacted with hydrazine hydrate in a similar way.     

Arylation of styrene derivatives using aryliron complexes [CpFe(CO)2Ar] revealed by density functional theory calculations: Fe(II)-assisted group exchange through Fe-C bond cleavage and Fe-X bond formation

The detailed mechanism for arylation of styrene or its α-CF₃ substituted analog using aryliron complex [CpFe(CO)₂Ar] was studied using density functional theory calculations. Results of calculations show that the arylation mechanism mainly involves three steps: (1) a ligand exchange process between a CO and styrene or its derivative; (2) migration of Ar group from Fe to β-C of styrene; (3) β-H (or β-F) elimination and dissociation of the stilbene derivative from the CpFeHCO (or CpFeFCO) moiety. Both of Steps (2) and (3) experience a similar four-memberred cyclic transition state. The d_π-p_π interaction stabilizes the CC π coordinated complexes and the agostic interaction plays an important role in stabilizing intermediates and promoting elimination of the β-H (or β-F if available). For arylation of the α-CF3 substituted styrene, our calculations clarified that the dissociation of ethylene derivatives to give PF (product for β-F elimination) is kinetically and thermodynamically more favorable than to give PH (product for β-H elimination), which is the determined step for the selectivity of the final products.     

Understanding E2 versus SN2 Competition under Acidic and Basic Conditions

Our purpose is to understand the mechanism through which pH affects the competition between base-induced elimination and substitution. To this end, we have quantum chemically investigated the competition between elimination and substitution pathways in H₂O+C₂H₅OH₂⁺ and OH⁻+C₂H₅OH, that is, two related model systems that represent, in a generic manner, the same reaction under acidic and basic conditions, respectively. We find that substitution is favored in the acidic case while elimination prevails under basic conditions. Activation-strain analyses of the reaction profiles reveal that the switch in preferred reactivity from substitution to elimination, if one goes from acidic to basic catalysis, is related to (1) the higher basicity of the deprotonated base, and (2) the change in character of the substrates LUMO from C^β−H bonding in C₂H₅OH₂⁺ to C^β−H antibonding in C₂H₅OH.     

Electron impact induced water elimination from hydroxyamides. 2—N-acetyl- and N-benzoyl-4a-hydroxydecahydroquinoline and N-Methyl-4a-hydroxy-2-oxodecahydroquinoline

Electron impact induced water elimination from the metastable molecular ions of N-acetyl- and N-benzoyl-4a-hydroxydecahydroquinoline mainly follows a formal [1,2] elimination. The initiating and ratedetermining step in the reaction is the hydrogen rearrangement from C-8a onto the carbonyl group. The transferred hydrogen is subsequently lost, together with the hydroxyl group. The almost complete absence of H₂O loss from both diastereomers of N-methyl-4a-hydroxy-2-oxodecahydroquinoline confirms that the reaction can only proceed when the carbonyl group is able to function as ‘hydrogen carrier’ by occupying positions in the vicinity of both a hydrogen and the hydroxyl function.     

Umsetzungen von Metall- und Metalloidverbindungen mit mehrfunktionellen Molekülen, 2. Mitt.: Reaktionsprodukte von Aminobenzonitrilen mit Halogenboranen

Both 4-aminobenzonitrile and 3-aminobenzonitrile react with halogenoboranes to yield mainly the corresponding amine-halogenoboranes and further products derived from the latter by elimination of hydrogen halide. In contrast, reaction between halogenoboranes and 2-aminobenzonitrile leads to insertion of the nitrile group into one of the B-halogen bonds. Derivatives of the 1.3.2-diazaboranaphthalene ring result from reactions of the insertion products with the amino group with or without elimination of hydrogen halides.     

Unimolecular reactions including ClF interchange of vibrationally excited CF2ClCHFCH2CH3 and CF2ClCHFCD2CD3

Vibrationally excited CF(2)ClCHFC(2)H(5)(CF(2)ClCHFC(2)D(5)) molecules were prepared in the gas phase at 300 K with approximately 93 kcal mol(-1) of energy by recombination of CF(2)ClCHF and C(2)H(5) or C(2)D(5) radicals. Three unimolecular reactions were observed. 1,2-ClF interchange converts CF(2)ClCHFC(2)H(5)(CF(2)ClCHFC(2)D(5)) into CF(3)CHClC(2)H(5)(CF(3)CHClC(2)D(5)), and subsequent 2,3-ClH (ClD) elimination gives CF(3)CH=CHCH(3) (CF(3)CH=CDCD(3)). 2,3-FH(FD) elimination gives cis- and trans-CF(2)ClCH=CHCH(3) (CF(2)ClCH=CDCD(3)), and 1,2-ClH elimination gives CF(2)=CFCH(2)CH(3) (CF(2)=CFCD(2)CD(3)). The experimental rate constants for CF(2)ClCHFC(2)H(5) (CF(2)ClCHFC(2)D(5)) were 1.3 x 10(4) (0.63 x 10(4)) s(-1) for 1,2-FCl interchange and 2.1 x 10(4) (0.61 x 10(4)) s(-1) with a trans/cis ratio of 3.7 for 2,3-FH(FD) elimination. The 1,2-ClH process was the least important with a branching fraction of only 0.08 +/- 0.04. The rate constants for 2,3-ClH (ClD) elimination from CF(3)CHClC(2)H(5) (CF(3)CHClC(2)D(5)) were 1.8 x 10(6) (0.49 x 10(6)) s(-1) with a trans/cis ratio of 2.4. Density functional theory was used to compute vibrational frequencies and structures needed to obtain rate constants from RRKM theory. Matching theoretical and experimental rate constants provides estimates of the threshold energies, E0, for the three reaction pathways; 1,2-FCl interchange has the lowest E0. The unimolecular reactions of CF(2)ClCHFC(2)H(5) are compared to those of CF(2)ClCHFCH(3). Both of these systems are compared to CH(3)CHFC(2)H(5) to illustrate the influence of a CF(2)Cl group on the E0 for FH elimination.     

Theoretical study of bimolecular elimination (E2) reactions. Possibility ofsyn E2 elimination in the series of 2-R-2-R'-1-halocyclopropanes

Reactions of the methoxide ion with substituted halocyclopropanes, which result in E2 elimination, have been studied by the semiempirical quantum-chemical AM1 method. The transition states corresponding totrans andcis routes have been localized. The energetic predominance of thetrans route over thecis route is reduced by 2.6 kcal mol^–1 on going from 1-chloropropane to chlorocyclopropane because of the features of cyclopropane geometry. It has been demonstrated that, in the gas phase,cis elimination may predominate overtrans elimination for a particular stereoisomer of 2-cyano-2-methyl-1-halocyclopropanes due to weakening of orbital interactions and Coulomb repulsion between the cyano group and the MeO^– anion in thetrans E2 transition state.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 620–623, April, 1995.     

Efficient preparation of 4-methoxy-5,6-dihydro-2H-pyran

We report the efficient synthesis of 4-methoxy-5,6-dihydro-2H-pyran (MDHP) via the TiCl₄ driven elimination of MeOH from 4,4-dimethoxytetrahydropyran. The previous difficulty of preparing MDHP restricted the wider use of 4-methoxytetrahydropyran-4-yl (MTHP) acyclic acetals, which have desirable protecting group properties when compared to more commonly used MOM- and THP-acetals. The behaviour of the elimination on related acetals is also examined.     

Mechanistic Studies of the Radical S‐Adenosylmethionine Enzyme DesII with TDP‐D‐Fucose

DesII is a radical S‐adenosylmethionine (SAM) enzyme that catalyzes the C4‐deamination of TDP‐4‐amino‐4,6‐dideoxyglucose through a C3 radical intermediate. However, if the C4 amino group is replaced with a hydroxy group (to give TDP‐quinovose), the hydroxy group at C3 is oxidized to a ketone with no C4‐dehydration. It is hypothesized that hyperconjugation between the C4 C? N/O bond and the partially filled p orbital at C3 of the radical intermediate modulates the degree to which elimination competes with dehydrogenation. To investigate this hypothesis, the reaction of DesII with the C4‐epimer of TDP‐quinovose (TDP‐fucose) was examined. The reaction primarily results in the formation of TDP‐6‐deoxygulose and likely regeneration of TDP‐fucose. The remainder of the substrate radical partitions roughly equally between C3‐dehydrogenation and C4‐dehydration. Thus, changing the stereochemistry at C4 permits a more balanced competition between elimination and dehydrogenation.     

Cyclisation des amides d'acides phénylthio-1 glyoxaliques avec les dérives carbonylés

The carbonyl-containing compounds react with the amides of phenyl-1-thioglyoxalic acids ( 3 ), and if methyl iodide is present, cause cyclisation into 4-methylthio-5-phenyl-2,5-dihydrooxazol-5-ols ( 4 ). The ketal hydroxyl group of the latter can be methylated. When a hydrogen atom exists in the 2-position (cyclisation with an aldehyde), its elimination provides an oxazole 7 .     

Dimerization of alkynes promoted by a pincer-ligated iridium complex. C-C reductive elimination inhibited by steric crowding

The pincer-ligated species (PCP)Ir (PCP = kappa3-C6H3-2,6-(CH2PtBu2)2) is found to promote dimerization of phenylacetylene to give the enyne complex (PCP)Ir(trans-1,4-phenyl-but-3-ene-1-yne). The mechanism of this reaction is found to proceed through three steps: (i) addition of the alkynyl C-H bond to iridium, (ii) insertion of a second phenylacetylene molecule into the resulting Ir-H bond, and (iii) vinyl-acetylide reductive elimination. Each of these steps has been investigated, by both experimental and computational (DFT) methods, to yield unexpected conclusions of general interest. (i) The product of alkynyl C-H addition, (PCP)Ir(CCPh)(H) (3), has been isolated and, in accord with experimental observations, is calculated to be 29 kcal/mol more stable than the analogous product of benzene C-H addition. (ii) Insertion of a second PhCCH molecule into the Ir-H bond of 3 proceeds rapidly, but with a 1,2-orientation. This orientation gives (PCP)Ir(CCPh)(CPh=CH2) (4) which would yield the 1,3-diphenyl-enyne if it were to undergo C-C elimination; however, the insertion is reversible, which represents the first example, to our knowledge, of simple beta-H elimination from a vinyl group to give a terminal hydride. The 2,1-insertion product (PCP)Ir(CCPh)(CH=CHPh) (6) forms more slowly, but unlike the 1,2 insertion product it undergoes C-C elimination to give the observed enyne. (iii) The failure of 4 to undergo C-C elimination is found to be general for (PCP)Ir(CCPh)(vinyl) complexes in which the vinyl group has an alpha-substituent. Thus, although C-C elimination relieves crowding, the reaction is inhibited by increased crowding. Density-functional theory (DFT) calculations support this surprising conclusion and offer a clear explanation. Alkynyl-vinyl bond formation in the C-C elimination transition state involves the vinyl group pi-system; this requires that the vinyl group must rotate (around the Ir-C bond) by ca. 90 degrees to achieve an appropriate orientation. This rotation is severely inhibited by steric crowding, particularly when the vinyl group bears an alpha-substituent.     

THE SECONDARY REACTION IN THE CONDENSATION OF 1-METHYL-2-METHYLTHIO-IMIDAZOLINE WITH ACTIVE METHYLENE COMPOUNDS

Mono-substituted heterocyclic ketene aminals are formed by the reactionof 1-methyl-2-methylthio-imidazoline with active methylene compoundscontaininq an acetyl or benzoyl group by the elimination of both amethylthio and an acyl group.This is resulted by the secondary reactioof the produced methanethiol to attack the more active carbonyl group.     

Catalytic coupling of haloolefins with anilides

A strategy in which C-H activation reactions promoted by Pd(II) have been combined with beta-heteroatom elimination to create a catalytic cycle achieving the arylation of haloacrylates is reported. The catalytic cycle can be subdivided into four parts: (1) C-H activation; (2) the functionalization step, migratory insertion of the olefin into a metal-carbon bond; (3) beta-heteroatom elimination; and (4) exchange of metal halide (if X = halogen) for a less coordinating anion. In this catalytic cycle, the oxidation state of the metal does not change, and an oxidant is not required. The method is more functional group tolerant compared with the existing alkene-arene coupling methods based on electrophilic C-H activation.