Allylic Hydrogen Atoms Research Articles

Nicotinylidene derivatives for the structural elucidation of glycerol mono-ethers and mono-esters by gas chromatography/mass spectrometry

Nicotinylidene and methylnicotinylidene derivatives of representative glycerol ethers and glycerol esters were prepared by their reaction with pyridine-3-carboxaldehyde and 3-acetyl pyridine respectively. Each compound gave two sets of diastereoisomers which could be separated by gas-liquid chromatography. The extent of formation of the derivatives was nearly quantitative and chromatographic peak shape was good. The compounds producing both peaks gave similar spectra but with differences that were characteristic of the isomer. The derivatives fragmented in an analogous manner to that reported earlier for nicotinate derivatives of alcohols and picolinyl derivatives of carboxylic acids, with the production of a series of diagnostic ions formed by radical-induced cleavage of the chain following random hydrogen abstraction. Branched chain compounds, exemplified by glycerol ethers, gave spectra in which one of the ions diagnostic of the branch-point was missing. Unsaturated compounds, exemplified by the glycerol esters, gave characteristic mass spectra in which separation between ions representing formal cleavage at each side of the double bond enabled the position of this bond to be determined. In addition the spectra of unsaturated compounds contained two abundant ions whose formation could be rationalized by abstraction of the allylic hydrogen atoms. The nicotinylidene derivatives gave more diagnostic spectra than the methylnicotinylidene derivatives, whose spectra were complicated by the appearance of ions produced by loss of the methyl and pyridyl groups from the derivative. Both derivatives were superior to the nicotinate derivatives of these dihydroxy compounds in that only one pyridine residue was introduced on derivatization; this gave a lower molecular weight increment and allowed compounds with longer chains to be examined.(ABSTRACT TRUNCATED AT 250 WORDS)

Stereoselectivity in the Ene reaction of syn- and anti-1-(4-tert-butylcyclohexylidene)-4-tert-butylcyclohexane with singlet oxygen, nitrosyl hydride, nitrosoformaldehyde, 4-phenyl-1,2,4-triazol-3,5-dione, diethyl azodicarboxylate and methyl propiolate

The stereochemistries of the ene reactions of syn- and anti-1-(4-tert-butylcyclohexylidene)-4-tert-butylcyclohexane (I and II) with a series of enophiles XY (singlet oxygen, nitrosyl hydride, nitroso-formaldehyde, 4-phenyl-1,2,4-triazol-3,5-dione, diethyl azodicarboxylate and methyl propiolate) to give, by axial or equatorial attack as shown, the allylic adducts III and IV, have been investigated. [graphic omitted]None of the reactions is stereospecific, the products being as follows (ene, enophile. III:IV): I, 1O2, 60–68:40–32; II1O2, 33–50:67–50; I, HNO, 10:90; II HNO, 11:89; I, HCONO, 25:75; II, HCONO, 50:50; I, [graphic omitted]O, 17:83; II, [graphic omitted]O, 20:80; I, EtOCONNCO2Et, 77:23; II, EtOCONNCO2Et, 27:73; I, HCCCO2Me, 53:47; II, HCCCO2Me, 67:33.The allylic hydroperoxides which are formed by the reaction of singlet oxygen react further with triplet oxygen to give allylic dihydroperoxides. The ene reactions of methyl propiolate are accompanied by the formation of cyclobutenes via cycloaddition.The stereoselectivities of the ene reactions preclude a concerted suprafacial attack of the eno-philes, but they support the model of the formation of an intermediate CC/XY adduct which can undergo conformational change before the allylic hydrogen atom is transferred.

Some aspects of stabilization and degradation of nigerian natural rubber. I. Photostabilization of natural rubber with bound di‐tert‐butyl‐4‐hydroxybenzyl thioglycollate (dbhbt)

Abstract3,5‐Di‐tert‐butyl‐4‐hydroxybenzyl thioglycollate (DBHBT) has been reacted with Nigerian Natural rubber latex at 65 ± 1°C using 4,4‐azobiscyanovaleric (AZBN) acid‐ferrous sulfate redox system to obtain a binding efficiency of 13%. The bound DBHBT was investigated as a UV stabilizer in unprocessed and unvulcanized rubber using spectroscopic method. With respect to the CO index, the masterbatch diluted to 56% gave the optimum activity and higher concentrations showed significant pro‐oxidant activity. The CC bonds in particular, showed greater stability which is a function of DBHBT concentration. The methylenic and the allylic hydrogen atoms, the most susceptible, and probably the sites of the primary attack, were not protected by DBHBT.

Thermisch initiierte Addition von Alkanen an Alkene. V. Addition von Cyclohexan an Oct‐1‐en in einer freien Radikalkettenreaktion unter überkritischen Bedingungen

Thermal Addition of Alkanes to Alkenes. V. Addition of Cyclohexane to 1‐Octene in a Free Radical Chain Reaction at Supercritical Fluid ConditionsAlkanes can be added to alkenes in a thermally initiated free radical chain reaction (“ane reaction”). Kinetic and mechanistic studies on the addition of cyclohexane (1) to 1‐octene (2) (molar ratio = 100:1) at 330−450°C and pressures of up to 300 bar have been performed in a high pressure ‐ high temperature flow apparatus (“HP‐HT” apparatus). The free radical chain is initiated by comproportionation of1and2to give alkyl radicals6and7. Addition of cyclohexyl radical6to alkene2and competing abstraction of an allylic hydrogen atom from2give radicals7aand8, respectively. Subsequent stabilization of the radicals7aand8by hydrogen abstraction from1yields the addition product cyclohexyloctane (3) and isomerized octenes (4), respectively. The relative rate of addition and isomerization can be controlled by pressure and temperature. The regioselectivity of the addition reaction was also measured.

Ene reactions of methyl trifluoropyruvate

Reactions of methyl trifluoropyruvate (I) with alkenes containing an allylic hydrogen atom have been studied. Reactions of (I) with terminal alkenes proceed slowly at 20°C and rapidly at 100°C. Similar reactions of (I) with cycloalkenes and allyl iodide occur above 100°C. It is found that all these reactions yield methyl esters of α-oxy-α-trifluoromethyl-γ,δ- unsaturated acids. Catalysis of these reactions with SnCl4 reduces reaction times and temperatures to -40-20°C. With terminal alkenes, the use of catalyst does not change the direction of the reaction. With cycloalkenes, methyl esters of α-oxy-α-trifluoromethyl-β, γ-unsaturated acids are obtained. Both allyl methyl ether and allylbenzene react with (I) yielding substituted tetrahydrofurans (II-III) as main products. ▪The reactions of (I) with terminal acetylenes have been studied. 1-Hexyn gives the allene derivative. The reaction of (I) with propargyl methyl ether yields a mixture of dihydrofuran (IY) and oxetane (Y) ▪The obtained products are useful as synthons for the synthesis of lactones lactams, substituted dioxans and morpholines.

Rates and mechanisms of reactions of nitrogen dioxide with alkenes in solution

Rates of reaction of alkenes with nitrogen dioxide have been determined in aprotic solvents. The reactions are first order in the alkene. At high concentrations of NO/sub 2/N/sub 2/O/sub 4/, a kinetic dependence that is first order in N/sub 2/O/sub 4/ or second order in NO/sub 2/ is observed. As the concentration of NO/sub 2//N/sub 2/O/sub 4/ approaches zero, the kinetic order in NO/sub 2/ approaches unity. These data indicate the presence of at least two distinct reaction pathways: one involving the diamagnetic dimer, N/sub 2/O/sub 4/, in an addition mechanism and the other(s) involving the paramagnetic monomer NO/sub 2/. The reaction of the monomer proceeds, at least in part, through abstraction of allylic hydrogen atoms.

Rearrangements of Free Radicals, XII1) ESR Spectroscopic Study of the Ring Opening of the Homobenzvalenyl Radical

AbstractAbstraction of an allylic hydrogen atom in homobenzvalene (4) either in solution by photolytically generated tert‐butoxyl radicals or in an adamantane matrix by X‐rays produces the homobenzvalenyl radical (5), which thermally rearranges to the tropylium radical (7). In solution the activation energy for the rate determining step of the reaction sequence was determined to be 13.4 ± 0.5 kcal/mol.

Interaction between dipotassium hexahalogeno-osmates K 2OsX 6 (X = Cl, Br) and olefins: metal hydride formation by allylic hydrogen atom abstraction and catalytic isomerization of olefins

Dipotassium hexahalogeno-osmates, K 2OsX 6 (X = Cl, Br) dissolved in non-aqueous inert organic solvents (benzene, o-dichlorobenzene, bromo-benzene) by addition of dicyclohexyl-18-crown-6 ether (C 20H 36O 6) are catalytic precursors for the multiple double bond shift in olefins. Studies on the interaction between K 2OgX 6 and several olefins (1-heptene, 1-heptene-3d 2, 1-pentene, cis-stilbene, 3,3-dimethyl-l-butene) support an osmium hydride species, probably [OsHX 5] =, as the true catalyst, which affords the olefin isomerization through successive 1,2-addition-elimination steps to the CC double bond. Such an alleged osmium hydride species is formed by allylic hydrogen atom abstraction from the olefin with formation of the corresponding conjugate diolefin.

Photooxidation of Organosulfur Compounds

Abstract Considerable attention has focused on the oxidation of sulfur compounds in connection with the study of phot odynamic action with biological substrates. Irradiation of alkyl sulfides in the presence of a photosensitizer and oxygen gives the corresponding sulfoxides in approximately quantitative yields after consumption of one half mole of oxygen per mole of sulfide. These reactions presumably involve an intermediate such as a peroxy sulfoxide or possibly a cyclic peroxide which is capable of converting another sulfide molecule to the corresponding sulfoxide or rearranging more slowly to the isomeric sulfone. Clean conversions to sulfoxides were observed upon direct photolysis of sulfides in air. In this case an excited charge transfer complex mechanism between oxygen as an electron acceptor and the sulfide as an electron donor has been proposed. A number of biologically important molecules containing disulfide bond are known to be succeptible to photodynamic action. Photosensitized oxygenation of the simplest acyclic disulfides has been reported to give the corresponding thiolsulfinates in good yield. The molecular oxygen reacts with thiones very efficiently to yield the corresponding carbonyl compounds, and singlet oxygen may be involved as the reactive species. Several mechanistic studies of the photooxygenation of vinyl sulfides show oxidative cleavage of the carbon-carbon double bond to take place via the corresponding 1,2-dioxetane intermediate. On the other hand, tri- and tetrasubstituted thioethylenes give products due to oxidative cleavage of the C-S bond. The oxidation of vinyl sulfides possessing an activated double bond and allylic hydrogen atoms affords both 1,2-dioxetane mode products and ene reaction mode products. The dioxetane mode is favored exclusively in protic solvents and slightly favored over the ene mode in aprotic solvents. The photooxidation of vinyl sulfides sensitized by 9,10-dicyanoanthracene has also been studied, and may proceed by electron transfer from the sulfur atom to the sensitizer which again transfers its electron to oxygen to give a superoxide anion radical. Photosensitized oxygenation of heterocyclic compounds containing sulfur atoms gives products derived from an intermediate endoperoxide. Some sulfur ylides have been oxidized with singlet oxygen to give carbonyl oxide intermediates which can transfer an oxygen atom to an acceptor.

The mechanisms of dimethylbutene isomerization over alumina

Reactions of dimethylbutenes over pure and chlorided γ-alumina have been studied in the presence of selective poisons and/or with various host molecules for deuterium label. The extent of H 2S poisoning has been used to determine the relative roles of π-allylic species and alkyi carbonium ions in the isomerization of the 2,3-dimethylbutenes on alumina at 273 K. The absence of poisoning by H 2S of skeletal isomerization of 3,3-dimethylbut-l-ene at 352 K is evidence that H2S does not inhibit reaction through carbonium ions whereas it is known to poison activity involving π-allylic species. The isomerization of 3,3-dimethylbut-l-ene in the presence of suitably labeled alkenes showed that vinylic and allylic hydrogen atoms can equilibrate with Brønsted-acid-type hydrogen on alumina at about 273 K. But adsorbed vinyl species have been found to retard skeletal isomerization, suggesting identity, or at least close proximity, of the sites for Brønsted acid reactions and vinylic CH exchange. Catalyst chloriding enhanced isomerization but retarded exchange of CH bonds with D. The influence of activation temperature and of impurities is discussed for reactions of dimethylbutenes over alumina.

Isomerization of 3-carene over MgO and CaO

The isomerization of 3-carene was studied over MgO, and CaO. The main product was 2-carene which resulted from a double bond migration within a six-membered ring. The catalytic activity and selectivity varied with starting materials and pretreatment temperature of the catalysts. Two kinds of magnesium oxides were prepared. From magnesium hydroxide, MgO(I) was prepared and MgO(II) from magnesium carbonate hydroxide. Both MgO(I) and MgO(II) became active when pretreated at 400 °C, and attained their maximum activities when pretreated at 900 and 500 °C, respectively. Active sites on both kinds of MgO were strongly poisoned by carbon dioxide and butylamine but were only slightly poisoned by triethylamine. The activities of CaO(I), which was prepared from calcium hydroxide, and CaO(II), prepared from calcium carbonate, appeared when pretreated at 300 and 600 °C, respectively, and reached their maximum activities when pretreated at 600 and 1000 °C, respectively. Over MgO(I) and MgO(II) pretreated at 500 °C, 3-carene and 2-carene exchanged their allylic hydrogen atoms in the six-membered ring more easily than the allylic hydrogen atoms in the methyl group. Dehydrogenation of 3-carene to m- and p-cymenes occurred over CaO(II) pretreated at 600–800 °C but it hardly took place over CaO(I), MgO(I), and MgO(II) pretreated at any temperatures.

Synthese de quelques complexes tricarbonyldiene fer

Tricarbonyliron—diene complexes starting from α- or β-non-saturated alcohols and of various Diels—Alder addition products, viz. 4-vinylcyclohexene and the adduct from the dimethyl ester of acetylene dicarboxylic acid and 1-( N, N-diethylamino)-1,3-butadiene, have been synthesized. Higher yields are obtained when the starting alcohols have no allylic hydrogen atoms. The formation of complexes from non-conjugated dienes is accompanied by shifts of the most reactive double bonds.

Copper-catalysed reactions of t-butyl perbenzoate with cis- and trans-p-Menth-2-ene and other cyclic olefins

Each of the olefins cis-p-menth-2-ene (23), its trans-isomer (24), 3-methylcyclohexene (10) and 4- methylcyclohexene (ll), when treated with t-butyl perbenzoate and cupric octanoate catalyst in benzene, affords a mixture of allylic benzoates which can be analysed by gas chromatography after reduction with lithium aluminium hydride. In most cases the reaction proceeds with a high degree of stereo- and regio-selectivity, the major products being those containing a disubstituted double bond and bearing a benzoate group in a trans relationship to an alkyl substituent. Evidence is presented in support of a free-radical chain mechanism involving preferential abstraction of allylic hydrogen atoms occupying quasi-axial positions, followed by ligand transfer from cupric carboxylate via a cyclic transition state. When catalysed by the bipyridyl complex of cupric octanoate the reaction is less selective and appears to proceed by a different mechanism.

Mass spectrometry of heterocyclic compounds VIII–electron‐impact‐induced fragmentation of 1,2‐diphenyl‐pyrazolidine‐3,5‐dione and some 4‐substituted derivatives

AbstractThe mass spectra of 1,2‐diphenyl‐pyrazolidine‐3,5‐dione and twenty‐one 4‐substituted derivatives are reported. Their fragmentation patterns have been studied by deuterium labelling, exact mass measurements, metastable studies by the defocusing technique and low energy spectra. Hydrogen rearrangements from the 4‐position of the heterocycle and/or from the ß‐position of the 4‐substituent groups, lead to the main primary fragment ions [C12H11N2]+ (m/e 183) as shown by the metastables. The 4,4‐d2 derivative shows an appreciable isotope effect even for molecular ions decomposing in the ion source. By comparison with the metastable abundances of competitive reactions, the molecular ions (m/e 252) of the 4‐unsubstituted compound appear to be structurally different from the corresponding m/e 252 fragment ions formed from 4‐derivatives by the loss of 4‐substituent with H rearrangement. If only vinylic or aromatic hydrogen atoms are present, primary cleavage of the heterocyclic ring occurs with loss of OH·, C3O2 and C3HO2. Important rearrangements leading to elimination of C6H6N and C6H7N are typical for unsaturated substituents on position four having allylic hydrogen atoms. Fragment ions, identical to molecular ions of some compounds discussed here, are obtained by electron‐impact and/or thermal decompostion of some complex compounds containing more than one 1,2‐diphenyl‐pyrazolidine‐3,5‐dione system. The [C6H5N2]+ (m/e 105) and [C6H5]+ (m/e 77) ions are common fragments of all the title compounds. Any hydrogen scrambling reactions between phenyl and heterocycle or 4‐substituent groups can be excluded.

The mechanism of isomerization and exchange of olefins over metal catalysts: IV. Reactions of some C 5, C 6 and C 7 alkenes in the presence of perdeuteropropene on iron films

The exchange and isomerization of pent-1-ene, cis-pentene, cis-hept-2-ene, trans-hept-3-ene, 2-methylbut-2-ene and 2,3-dimethylbut-2-ene have been investigated on iron films in the presence of a large excess of perdeuteropropene. Further evidence has been given for the occurrence of a direct intramolecular hydrogen shift in double bond migration, and, in cis-trans isomerization, for a direct process without the formation or the breaking of any CH bond. The classical Horiuti-Polanyi mechanism seems to be restricted to cis-trans isomerization and involves only two consecutive steps. The pronounced maxima for d 8, d 5 and d 4 deutero-isomers in the distributions of but-1-ene, pent-1-ene and 3-methylbut-1-ene obtained from the corresponding alk-2-enes show that the isomerization mechanisms involve a dissociative adsorption at a vinylic carbon atom, accompanied by a complete exchange of all the adjacent primary allylic hydrogen atoms. It is suggested that interconversions between σ-π-binuclear surface complexes are responsible for this process.

ChemInform Abstract: REACTION OF SINGLET OXYGEN WITH 2‐METHYLNORBORN‐2‐ENE, 2‐METHYLIDENENORBORNANE, AND THEIR 7,7‐DIMETHYL DERIVATIVES, THE TRANSITION STATE GEOMETRY FOR HYDROPEROXIDATION

AbstractDie Bestrahlung des Norbornens (Ia) in Acetonitril in Gegenwart von Sauerstoff und Methylenblau als Sensibilisator und nachfolgende Reduktion mit NaBH4 liefert hauptsächlich das exo‐Norbornanol (IIa) und daneben sein endo‐Isomeres (IIIa).

On stereochemical preference in the S E2? reaction

Attempts are described to detect stereochemical preference during an SE2′ reaction in a synthetic model system. 7β-Deuterio-4,5-secocholest-5-en-4-al ethylene acetal (20d) cyclised with Lewis acid to give 7-deuterio-4α-(2-hydroxyethoxy)-5β-chloest-6-ene (23c) with complete retention of deuterium. By contrast, 7α-deuterio-4,5-secocholest-5-en-4-yl p-bromobenzenesulphonate (29c) cyclised in 2,2,2-trifluoroethanol to 5β-cholest-6-ene (31) with total loss of deuterium. In both cases there is a syn-relationship between the newly formed C–C bond and the allylic hydrogen atom that is lost. The implications of these findings for the SE2′ reaction are discussed.

Reaction of Singlet Oxygen with 2‐Methylnorborn‐2‐ene, 2‐Methylidenenorbornane, and their 7,7‐Dimethyl Derivatives. The transition state geometry for hydroperoxidation

AbstractThe dye‐sensitized photo‐oxygenation of 2‐methylnorbonr‐2‐ene (3), 2‐methylidene‐norbornane (4) and their 7,7‐dimethyl derivatives (5 and 6) has been studied. In all cases allylically rearranged hydroperoxides were formed, except that 4 also gave a little norbornanone (presumably from the dioxetane) and 5 gave some endo‐3,7,7‐trimethylnorbornan‐2‐one as a secondary photo‐product. It was found that the exo/endo attack ratios by singlet oxygen on 3 and 5 are 66 and 0.19. By exploiting the C (3) monodeuteriated derivatives, 4 and 6 showed ratios of 28 and 0.67.Rates of reactivity of the olefins 3 and 4 were compared to methylidenecyclopentane, 1‐methylcyclopentene and 1‐methylcyclohexene as monocyclic standards. Additionally, comparative rates between the 7,7‐dimethyl olefins and their parents were measured. When further comparison was made of the rate ratios partitioned for exo and endo attack, it was seen that oxygen experienced a 500‐ to 1000‐fold rate retardation on approach to the endo side of 3 compared to that for its monocyclic analogue. Exo rates between the parent norbornene 3 and its 7,7‐dimethyl derivative 5 showed a 250‐fold decrease. Although four times smaller than the difference reported for epoxidation, the evidence clearly pointed to a one‐step cyclic process as the rate determining step for photo‐oxygenation. The steric evidence, taken with the low values found for the intermolecular isotope effects of 1.14 ± 0.01 and 1.02 ± 0.01 observed for exo and endo‐3‐deuterio‐2‐methylidene‐norbornanes, permits the deduction that the transition state is largely dipolar. In the early stages of the addition bonding between one end of the oxygen molecule and the terminal vinyl carbon is advanced. At the same time positive charge is dispersed by hyperconjugation between the central carbon atom and the allylic carbon‐hydrogen bond. At a later stage the anionic oxygen atom abstracts the loosened allylic hydrogen atom to create the hydroperoxide. No evidence for the formation of a discrete perepoxide intermediate was obtained.

Linear free energy relationships in heterogeneous catalysis: XI. Deep oxidation of lower olefins on nickel oxide catalyst

Linear free energy relationship studies were made on the rates of deep oxidation of some lower olefins over a nickel oxide catalyst. Since the trend of reactivities of olefins seems to be determined by the type of allylic hydrogens and their number for each olefin, quite similar treatments as previously reported on the dehydrogenation of cyclohexanes may be applied to this case. That is, the overall rate of deep oxidation of olefin R at temperature T, υ(R, T), can be represented as the weighted sum of the characteristic rates for allylic hydrogen atoms as v( R,T)= ∑ m w( R,m)·v(m,T) where w(R, m) means the statistical factor, i.e., the number of mth allylic hydrogen (primary, secondary, or tertiary) for olefin R and υ( m, T), the rate of the mth hydrogen at temperature T. The logarithm of υ( m, T) is demonstrated to have a linear relationship with delocalizability, D r R( H), a quantum chemical reactivity index for hydrogen abstraction, as log υ(m,T) = log υ(0,T) + γ(T) · ΔD r R ( H m) 2.3RT where υ(0, T) is the rate of hypothetical hydrogen with delocalizability of 1.00 and γ( T) is a proportional constant. Furthermore, γ( T) is proved to be practically independent of temperature. This means that the preexponential factor of the mth hydrogen, υ( m,∞), is independent of the type of allylic hydrogen but its activation energy, E A( m) , decreases proportionately with D r R(H m ). Finally, the overall rates can be expressed as follows: v( R,T)= ∑ m w( R,m)·v(0,∞)· exp{−[E A(0)−λ D·ΔD r R( H m)]/ RT} where υ(0,∞) and E A(0) are the preexponential factor and the activation energy, respectively, for hypothetical hydrogen defined above and γ( T) is rewritten as γ D. Finally, the rate of deep oxidation of any olefin at any temperature can be predicted according to the above equation using the delocalizability inherent to allylic hydrogen and three parameters characteristic of the type of catalyst, i.e., υ(0,∞), E A(0) and γ D. This treatment is compatible with the tentative mechanism that the deep oxidation of olefins over nickel oxide involves an abstraction of allylic hydrogen as a rate determining step.

ESR study on the photosensitized decomposition of polyethylene

AbstractSeveral aromatic compounds were added to low‐density polyethylene to determine their effects on the photodegradation of polyethylene. It was shown that some aromatic compounds indeed sensitize the hydrogen abstraction of the allylic hydrogen of unsaturated groups contained in polyethylene as an improper bond. The mechanism of photosensitization was investigated by an ESR method. It was found that the higher photoexcited triplet state of an aromatic molecule produced by the biphotonic ultraviolet light absorption transfers its excess energy to the unsaturated bond to excite it, and the excited unsaturated group releases its allylic hydrogen atom, giving an allylic radical.