Showing papers in "Journal of The Chemical Society B: Physical Organic in 1971"

Polysaccharide conformation. Part VI. Computer model-building for linear and branched pyranoglycans. Correlations with biological function. Preliminary assessment of inter-residue forces in aqueous solution. Further interpretation of optical rotation in terms of chain conformation

Abstract: Homopolymers of glucopyranose, galactopyranose, mannopyranose, xylopyranose, and arabinopyranose, with various positions and configurations of linkage, have been compared by model-building in the computer in an attempt to formulate simple rules for conformational analysis. Regular conformations are very restricted by steric forces alone, and each polymer has one of four characteristic shapes: Type A—extended and ribbon-like, Type B—flexible and helical, Type C—rigid and crumpled, or Type D—very flexible indeed but, on the average, rather extended. Chain branching leads to increased steric restriction for many examples which involve two secondary positions.Correlations exist, but cannot always be explained, between the chain Type and the biological function: skeletal polysaccharides are usually of Type A, reserve and network polysaccharides are often of Type B, whereas loosely jointed polysaccharides have linkages of Type D, and chains of Type C are unnatural and rare.More evidence is given to support the method set forth in Part V for the interpretation of optical rotations of carbohydrate polymers in terms of chain conformations. When used with model-building calculations, this method shows that the flexibility of the glycosidic system is determined by the equatorial substituents on both residues which are next to the glycosidic oxygen; libratory freedom increases with the number of hydrogen atoms in these positions. For an equatorial–equatorial linkage, A → B, the conformation about the glycosidic bond, C(1)–O, is controlled by the steric bulk of neighbouring substituents on ring A, and the O–C conformation is similarly controlled by neighbouring substituents on ring B. Polar and hydrogen bonding influences seem to be small for most equatorial–equatorial linkages in aqueous solution. For α-linked disaccharides, however, the exoanomeric effect seems to dominate the conformation about C(1)–O, and to override steric considerations which are suggested by model-building calculations. In α-1,6- and β-1,6-linkages, the conformations about C(1)–O and C(5′)–C(6′) resemble those in corresponding methyl glycosides, and the conformation about O–C(6′) is, on the average, anti.

The correlation of solvent effects on physical and chemical properties

Abstract: A comprehensive study of the existing empirical solvent parameters has been made covering spectroscopic, rate, and equilibrium data. Various multiparameter equations have been tested by use of linear combinations of existing empirical solvent parameters. The most generally successful treatment combines Dimroth's E values with dielectric and index of refraction functions. The results of these correlations are discussed with respect to the various solvent–solute interaction mechanisms.

Polysaccharide conformation. Part VII. Model building computations for α-1,4 galacturonan and the kinking function of L-rhamnose residues in pectic substances

Abstract: D-Galacturonic acid residues in α-1,4 linked chains are shown, by 1H n.m.r. spectroscopy of oligomers in solution and by mathematical model-building to have the Reeves C1 conformation. The probable conformation of the polymer in the solid state is also derived. Calculations show that multiple-helix formation is impossible for α-1,4-galacturonan; therefore, despite physical similarities, the gel properties of pectins and pectates cannot be explained in the same terms as for agar and carrageenans. There is, however, a strong analogy with the other biological-network polysaccharides in that anomalous units are interspersed to form ‘kinks’ which interrupt any tendency to an ordered chain conformation; these units are L-rhamnose in pectic substances and correspond to galactose 6-sulphate in carrageenans and to L-iduronic or D-glacuronic acid in mucopolysaccharides.

Crystal structure of the C form of stearic acid

Abstract: The crystal structure of the C form of stearic acid has been determined from X-ray powder diffraction patterns, The crystals are monoclinic, space group P21/a, with unit-cell dimensions: a= 9·36, b= 4·95, c= 50·7 A. β= 128 °15′ and Z= 4. The crystal structure has been determined by trial-and-error methods to R 0·086. The structural parameters are compared with those of similar compounds, in particular the same acid in the polymorphous B form.

A theoretical study of the Edward–Lemieux effect (the anomeric effect). The stereochemical requirements of adjacent electron pairs and polar bonds

Abstract: Many data, collected from the literature, support the postulation of two general rules which describe the static and dynamic results of having in a molecule or generating in an intermediate adjacent electron pairs and/or polar bonds. In disagreement with current chemical intuition, structures which contain the maximum number of gauche-interactions between lone electron pairs or polar bonds represent energy minima and, frequently, the lowest minima. The stereochemical implications of the phenomenon are discussed, along with possible physical explanations. It is suggested that partitioning of the total energy of the system into attractive-dominant and repulsive-dominant interactions provides the most helpful framework for the construction of a physical picture of the phenomenon. The total energy and its components can be obtained by ab initio molecular quantum mechanical calculations.The only apparent exception to the phenomenon that has been found corresponds to a polar bond adjacent to two lone pairs (as in the case of two hetero-atoms attached to the same carbon atom). This exception, here termed the Edward–Lemieux effect, has been examined theoretically by an ab initio(Hartree–Fock) calculation using fluoromethanol as a model compound. The calculation has reproduced the Edward–Lemieux effect; the stable conformation has the C–F bond trans to one ‘electron pair’ and gauche to another, and the conformation in which the C–F bond bisects the ‘electron pairs’ is the energy maximum. The partitioning of the total energy into its components of attraction and repulsion and comparison of the results with other systems for which barriers to internal rotation have been obtained by ab initio methods reveals a similarity between fluoromethanol, hydrazine, hydroxylamine, and hydrogen peroxide. An interpretation of this result is provided in which an early suggestion by Lemieux and Chu is supported and the concept of ‘rabbit-ears’ is not.

Dehydration mechanisms in the thermal decomposition of gaseous formic acid

Abstract: The homogeneous thermal decomposition of formic acid has been studied in flow and static systems between 457 and 780°C. Dehydration is of the second order below 600°C, k= 108·44 exp(–132,500/RT) l mol–1 s–1, but the Arrhenius plot then becomes curved, straightening out again above 670°C to give k= 1012·39 exp(–253,000/RT) s–1. The order in this region is 1·5 and dehydration here appears to be a unimolecular reaction in the fall-off region. Decarboxylation is of the first order throughout the temperature range, with k= 109·47 exp(–203,000/RT) s–1.

A modification to the ferrioxalate actinometer

Abstract: When a solution of potassium ferrioxalate containing 0·02% of o-phenanthroline monohydrate and acetate buffer (final pH 3·7) is irradiated with 2537 and 3650 A light, the absorbance at 5100 A due to the formation of the ferrous o-phenanthroline complex has been found to increase linearly with dose. The suitability of this system as a more convenient chemical actinometer is discussed here.

Effect of 4-substitution on the thermodynamics of hydration of phenol and the phenoxide anion

Abstract: Enthalpies, entropies, and free energies of sublimation of phenol, 4-bromophenol, 4-formylphenol, and 4-nitrophenol have been evaluated from measurements of the vapour pressures of the solid phenols as a function of temperature. Heats of solution of the phenols in water have been determined calorimetrically and together with solubility data allow calculation of the entropies of solution. Enthalpies, free energies, and entropies of hydration are deduced and are discussed in relation to the effect of 4-substituents on the thermodynamics of ionization of phenol in water. Enthalpies of solution and solvation of the four phenols in methanol have also been measured.

The effect of alkyl substitution on ionisation potential

Abstract: The first ionisation potentials of series of aliphatic aldehydes, ketones, alcohols, iodides, and ethers have been measured by photoelectron spectroscopy. the effect of alkyl substitution on ionisation potential is defined in terms of two parameters; an alkyl group substituent parameter and a homologous series parameter. The same treatment is applied to other homologous series for which photoionisation values of ionisation potential are available. The substituent parameters correlate well with Taft's σ* values.

Comments on the mechanism of one-electron oxidation of phenols: a fresh interpretation of oxidative coupling reactions of plant phenols

Abstract: Fieser's critical oxidation potentials can be used to predict the relative ease of 1-electron abstraction from phenols, but the equation for Ecrit is pH dependent. For further oxidation to cations (ArO)+, E+crit is not pH dependent and so there should be an equipotential pH below which ArO· is metastable with respect to (ArO)+ and (ArO)–. Oxidative C–O–C coupling and aryloxy-polymer formation is ascribed to reactions of ArO· radicals and many C–C coupling reactions to electrophilic attack on phenol molecules by (ArO)+ cations.It is suggested that in plants those biochemical oxidations of phenols which lead to Aryl–O–Aryl structures probably involve the one-electron oxidation of aryloxy-anions to ArO· radicals which then dimerise, but that those oxidations which lead to aryl–aryl coupled products involve the electrophilic attack on phenol molecules by (ArO)+ cations. The diversity of the reaction paths which have now been established for biological oxidations of phenols in plants may be due to the involvement of two factors, (a) the oxidation potential of the reagent or active enzyme and (b) the pH of the reacting mixture, in the selection of the preferred oxidation route.

Nuclear magnetic resonance spectra of glycolaldehyde

Abstract: N.m.r. spectra of solutions of glycolaldehyde in dimethyl sulphoxide and in water (both fully deuteriated solvents) are reported. Assignments of resonances and coupling constants are made. In dimethyl sulphoxide solution glycolaldehyde exists as a symmetrical dimer (2,5-dihydroxy-1,4-dioxan) which is slowly converted into a mixture with the monomer and an unsymmetrical dimer. In aqueous solutions an equilibrium is rapidly established involving the two dimeric forms, the monomer, and the hydrated gem-diol form of the monomer. Approximate proportions of these forms in the two solvents are given.

The acid-catalysed decarboxylation of phosphonoformic acid

Abstract: Rate measurements on the title reaction define a new mechanism for decarboxylation in which carbon dioxide is lost as (HO)3C+. The other product, phosphorous acid, is formed by the tautomerism mechanism of electrophilic substitution at phosphorus.

Homolytic organometallic reactions. Part II. The kinetics and rate constants for the autoxidation of organoboron compounds in solution

Abstract: The kinetics of the autoxidation of a series of n-, s-, and t-butylboron compounds in iso-octane at 30° have been investigated. The reactions of tri-n- and tri-s-butylboranes (and of triphenylborane) were normally controlled by the rate of diffusion of oxygen into the solution but proceeded at measurable rates in the presence of pyridine which reduced the concentration of the boranes by complexation.In every case, the kinetics were compatible with a homolytic chain reaction involving the following steps.[graphic omitted]In the early stages of the reaction, initiation is by the direct reaction between borane and oxygen, but, as the reaction proceeds, the unimolecular decomposition of the alkylperoxyborane, or its bimolecular reaction with alkylborane become more important.Values for the ‘oxidizability’ of the different boranes at the same rates of initiation [Kp/(2Kt)1/2], and of the rate constants for the bimolecular homolytic substitution by the alkylperoxy radicals at the boron centre (Kp) are listed. These latter processes are influenced by counterbalancing electronic and steric effects in the alkyl groups, and by the presence of oxygen ligands on the boron, but in suitable cases they are considerably faster than the corresponding reaction at hydrogen, which is the corresponding propagation step in the autoxidation of a hydrocarbon.

Stereospecific electronegative effects. Part I. The 19F nuclear magnetic resonance spectra of deoxyfluoro-D-glucopyranoses

Abstract: A detailed study has been made of the 19F n.m.r. spectra of the anomeric pairs of all five deoxyfluoro-D-glucopyranoses and of 2-deoxy-2-fluoro-D-mannopyranose in solution in deuterium oxide. The values of the geminal and vicinal 19F–1H spin–spin coupling constants, together with the 1H n.m.r. parameters and observed equilibrium anomer concentrations, have been used to establish the configurations and conformational properties of these molecules. The small vicinal 1H–19F coupling constants found for 2-deoxy-2-fluoro- and 4-deoxy-4-fluoro-D-glucose have been rationalised in terms of the electronegativity of the ring oxygen atom and an empirical expression has been found which enables gauche 1H–19F coupling constants to be predicted. The large value of the fluorine coupling to H-5 in 6-deoxy-6-fluoro-D-glucose has been used to show the existence of a favoured rotational isomer in which F-6 is antiparallel to H-5. Vicinal 19F–1H coupling is shown to have a stereochemical dependence upon electronegative substituents, and the 19F chemical shifts and chemical shift differences between paris of anomeric isomers may also be explained in terms of a stereochemical dependence upon electronegative substituents elsewhere in the molecule.

Hydrolysis of phosphinic esters: general-base catalysis by imidazole

Abstract: Imidazole catalyses the hydrolysis of aryl diphenylphosphinates (I). At 25° the Hammett-σ-ρ value for the bimolecular rate constant (kim) is 2·88 and at 55° is 2·64; hydroxide-ion catalysis (kOH) at 25° has a smaller ρ value 1·55. Ratio of phosphate dianion catalysis (kHPO4=) to kim is ca. 5 for 4-nitrophenyl diphenylphosphinate; the deuterium oxide solvent isotope effect on kim is 2·15 and no departure from 1:1 stoicheiometry was observed in the imidazole-catalysed hydrolysis of the 4-nitrophenyl ester. These results indicate that imidazole acts as a general-base catalyst. Nucleophilic catalysis is observed with imidazole-catalysed hydrolysis of 4-nitrophenyl dimethyl-phosphate, diphenylphosphate and di-4-nitrophenylphosphate (kimH/kimD= 1·03, 1·02, 1·53 respectively). Steric crowding of the transition state renders nucleophilic catalysis inefficient for the diphenylphosphinate esters.A diagnostic tool not previously employed to distinguish general-base from nucleophilic catalysis by imidazole is discussed. The good Hammett-σ plot for imidazole catalysis in diphenylphosphinate ester hydrolysis argues for a general-base mechanism and for a stepwise rather than a concerted displacement.

Crystal structures of complexes between alkali-metal salts and cyclic polyethers. Part II. Complex formed from sodium bromide and 2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxacyclo-octadeca-2,11-diene (‘di-benzo-18-crown-6’)

Abstract: Sodium bromide reacts in methanol with the cyclic ether dibenzo-18-crown-6(L) to yield crystals of formula NaBrL,2H2O. Three-dimensional X-ray crystal structure analysis has shown that there are two kinds of sodium complex in the crystal, (A) a complexed cation, and (B) a complexed ion-pair. In both, the sodium ions have a hexagonal bipyramidal environment, with six oxygen atoms from L in the equatorial plane; the apices are occupied by water molecules at 2·31 and 2·27 A in (A), while one apex is occupied by a water molecule at 2·35 A, and the other by a bromide ion at 2·82 A in (B). Hydrogen bonding involving the unco-ordinated molecule of water and the unco-ordinated bromide ion, as well as the co-ordinated water molecules and bromide ion, results in a chain parallel to the b axis of the crystal with the two complexes alternately along it. Both molecules of cyclic ether have normal bond lengths and angles and the same conformation as found previously in the (Rb0·55, Na0·45)NCS complex. Sodium–oxygen (ether) distances average 2·705 A(2·64–2·84 A) in (A), and 2·72 A(2·54–2·89 A) in (B).Two formula units, C20H24O6, NaBr, 2H2O, comprise the asymmetric unit in a monoclinic unit cell with a= 10·558, b= 14·036, c= 30·677 A, β= 91·3°, and space group P21/c. Intensities were measured with a four-circle diffractometer. Non-hydrogen atoms were located from Patterson and Fourier syntheses and their parameters were refined by full-matrix least-squares methods to a final R of 0·083 for 2880 observations.

Anodic oxidation of amines. Part II. Electrochemical dealkylation of aliphatic tertiary amines

Abstract: NN-Dimethyl tertiary amines undergo oxidative dealkylation on anodic oxidation in aqueous alkaline solution at glassy-carbon electrode, and the major products are secondary amines and the appropriate aldehydes. On prolonged electrolysis the secondary amines produced undergo partial oxidative dealkylation to give primary amines and the appropriate aldehydes. The relative amount of dealkylation in unsymmetrical amines is predominantly governed by the acidity and the number of α-protons, but is also affected by the ease of oxidation of the radical R12NCHR2. Decarboxylation of the cation radical Me2N + CH2CO2– is involved in the anodic oxidation of NN-dimethylglycine.

Substitution at saturated carbon. Part VIII. Solvent effects on the free energy of trimethylamine, the nitrobenzyl chlorides, and the trimethylamine–nitrobenzyl chloride transition states

Abstract: Standard free energies of transfer from ethyl benzoate to 17 other solvents of trimethylamine, p-nitrobenzyl chloride, and the trimethylamine–p-nitrobenzyl chloride transition state have been calculated (at 298 K). The transition state is stabilised by polar solvents and is destabilised by non-polar solvents; dipolar aprotic solvents are much more effective in stabilising the transition state than are the aliphatic alcohols. Similar calculations have been carried out for the substitution of o-, m- and p-nitrobenzyl chlorides by trimethylamine for 6 solvents (at 303 K). A comparison of solvent effects on the free energy of the trimethylamine–p-nitrobenzyl chloride transition state, the t-butyl chloride solvolysis transition state, and the Et4N+l– ion pair indicates that the solvolysis transition state markedly resembles an ion pair in behaviour, but that the transition state for substitution of p-nitrobenzyl chloride by trimethylamine is very much less polar.

Rotational isomerism. Part XI. The nuclear magnetic resonance spectra and rotational isomerism of 1,2-difluoro- and 1,1,2-trifluoroethane

Abstract: The 1H and 19F spectra of 1,2-difluoroethane (I) and 1,1,2-trifluoroethane (II) are given and analysed.The solvent dependence of the coupling constants enables the rotamer energies and coupling constants to be obtained. In this process, the intrinsic solvent dependence of the vicinal HF and FF couplings (given by a control study of 1,1,1,2-tetrafluoroethane) was explicitly considered.The rotamer energies ΔE(Eg–Et) are for (I), ΔEv– 0·6, ΔE1– 2·6 and for (II)ΔEv 1·4, ΔE1 0·0 kcal/mol.The couplings in the rotamers of (I), (II), and 1,1,2,2-tetrafluoroethane are considered in detail. Increasing fluorine substitution gives a progressive but very nonlinear decrease in JtHH and JtHF which is much larger, for the latter, if the fluorine is introduced on the CH end of the CH·CF fragment.In (I), JgHH varies from 0 to 6·2 Hz depending on the orientation of the F nuclei, but increasing fluorine substitution evens out these effects to give JgHH 1·5—2·0 Hz. J t FF Is very dependent on the fluorine substitution, varying from –30 to 0 Hz but in contrast JgFF shows no dependence at all and is relatively constant at –13 to –5 Hz.

Crystal and molecular structure of 8,12-diethyl-2,3,7,13,17,18-hexamethylcorrole

Abstract: The crystal structure of 8,12-diethyl-2,3,7,13,17,18-hexamethylcorrole has been elucidated by X-ray crystallographic analysis and the geometry of the macrocyclic ring system has been found to be not strictly planar. The distortions are probably caused by short N–N contacts brought on by the direct C(1)–C(19) link. The ‘ruffling’ of the pyrrole rings is similar to that found in some substituted porphyrins, although individual ring geometries have been altered by the inner crowding. Crystallographic evidence suggests that the three inner hydrogen atoms are statistically distributed among the four nitrogen atoms. Packing is in the ‘parallel disc’ mode and is specific for the molecular distortion.

A comparison of sensitivities to substituent effects of five-membered heteroaromatic rings in gas phase ionization

Abstract: The molecular ionization potentials of a number of substituted furans, thiophens, selenophens, and pyrroles have been determined by the electron-impact technique. The ρ value for this reaction is more negative than that for the most selective electrophilic substitutions. The sensitivity to substituent effects appears to vary in the reverse order to that of the ‘ground-state aromaticity’ of the rings (furan > pyrrole > thiophen > benzene).

Radical anions of pyridine derivatives

Abstract: Pyridine radical anion, generated by alkali-metal reduction of pyridine shows an absorption maximum around 340 nm, the actual position varying with the counter-cation. The change in the spectra of the pyridine radical anion with time is due to formation of radical anions and dianions of 4,4′-, and 2,2′-bipyridyls. Alkylpyridines with substituents in the 3- or 4-position form stable radical anions which do not undergo dimerization. Electron-withdrawing groups produce a bathochromic shift of the absorption band of the pyridine radical anion.

Autoxidation of ketones and esters in basic solution

Abstract: The autoxidation of ketones and esters in aprotic solvents containing strong bases such as alkoxides was investigated extensively. Attention was paid to the autoxidation of other weakly acidic substrates such as nitriles, Schiff bases, and phenylhydrazones. The primary products are the α-hydroperoxides, which can be isolated in high yields when the oxidation is carried out at low temperatures, thus preventing their decomposition.The first step of the reaction is ionization of the substrate to yield a resonance-stabilized anion, which subsequently reacts with oxygen.A kinetic study showed that in some cases the oxidation is of the first order in anion and in oxygen, whereas in other cases (e.g., aliphatic esters) the ionization is the rate-determining step. Both oxidation and ionization show low activation energies (<10 kcal mol–1) in aprotic systems. Arguments are advanced for a non-radical mechanism involving interaction of anion and O2 to yield the hydroperoxide anion in one step. By considering the energies of the different steps involved it is shown that the autoxidation is favoured by the substrate being a weak acid as well as having a low C–H bond strength.

Intermediates in the decomposition of aliphatic diazo-compounds. Part VIII. The mechanism of ether formation from diarylmethylenes and alcohols

Abstract: The thermal decomposition of diphenyldiazomethane and its 4,4′-dichloro-, 4,4′-dimethoxy-, and 4,4′-dimethyl analogues at 85 °C in acetonitrile containing methyl and t-butyl alcohols (ca. 1M) has been studied. Kinetic studies indicate that the major reaction product, the alkyl diarylmethyl ether, is formed by attack of intermediate diarylmethylene on the alcohol. The relative reactivities of methyl and t-butyl alcohols towards the series of diarylmethylenes have been determined: methyl alcohol is the more reactive in all cases, but the difference in reactivity between the two alcohols decreases with increasing electron release by the 4-substituents. By examining the radioactivity of the product ethers formed by reaction of diarylmethylenes with hydroxyl-tritiated alcohols, substantial kinetic isotope effects on ether formation have been detected. The isotope effects are in all cases greater for formation of t-butyl ethers than for methyl ethers. The compatibility of these observations with three possible mechanisms of ether formation has been assessed, and it is concluded that the most likely route is that in which the carbene attacks the oxygen atom of the alcohol to form an ylid which is subsequently transformed by a prototropic rearrangement into the ether.

Spectroscopic studies of keto–enol equilibria. Part XIII. 15N-substituted imines

Abstract: Schiff bases have been synthesized from [15N]ammonia and 1-hydroxy-2-acetonaphthone, 2-hydroxy-1-naphthaldehyde, 5-methyl- and 4,5-dimethyl-acetophenones, 2-hydroxybenzophenone, benzoylacetone and 2-acetyl-5,5-dimethylcyclohexane-1,3-dione. The 1H n.m.r. and u.v. data permit assignment of tautomeric equilibria. The results indicate that the percentage of amide form is usually greater in these ammonia adducts than in the analogous methylamine or aniline derivatives. Proton–proton spin-coupling measurements provide important information concerning the electronic delocalization in the aromatic rings.

Rearrangements of pinane derivatives. Part I. Products of acid catalysed hydration of α-pinene and β-pinene

Abstract: The acid-catalysed hydration of α-pinene and β-pinene has been shown to proceed through a common intermediate stage, probably involving two equilibrating ions. These ions can eliminate to give α-pinene, or undergo ring expansion to bornane and fenchane derivatives, or ring opening to p-methane derivatives. The formation and subsequent rearrangements of the products have been followed.

Mechanistic aspects of the Strecker aminonitrile synthesis

Abstract: Asymmetric α-aminonitriles could not be prepared by the Strecker procedure catalysed by optically active acids or bases. Benzylideneaniline reacted readily and quantitatively with hydrogen cyanide to give α-cyanobenzylaniline, while mandelonitrile reacted very slowly with aniline, indicating that benzylideneaniline or a Schiff base may be an intermediate for the Strecker synthesis. The addition of hydrogen cyanide to benzylideneaniline in acetonitrile–methanol at 25 °C follows a third-order rate equation: –d[Ar1CHNAr2]/dt=k[Ar1CHNAr2][HCN][MeOH]. The rate is higher in protic dipolar solvents than in aprotic non-polar solvents. An electron-releasing group on either of the rings increases the rate with Hammett ρ values of –1·11 in the benzylidene and –1·39 in the aniline ring. A mechanism involving methanol-catalysed addition of hydrogen cyanide to benzylideneaniline is postulated and discussed.

Homolytic organometallic reactions. Part III. An electron spin resonance study of homolytic t-butoxydealkylation of organometallic compounds. Rate constants for the reaction at boron

Abstract: When di-t-butyl peroxide is photolysed in the presence of a variety of organometallic compounds (RM) in an e.s.r. cavity, the alkoxyl radicals which are formed induce an SH2 reaction at the metal centre, and the e.s.r. spectrum of the displaced alkyl radical R· can be observed [equation (i)]. The rates of these SH2 reactions for a variety of trialkyl- [graphic omitted] boranes and trialkylboroxines have been studied by causing the organoboranes to compete with cyclopentane for reaction with the t-butoxyl radicals [equation (ii)], and monitoring the relative concentrations of the alkyl and cyclopentyl radicals by e.s.r. The reactions are very fast (ki= 105–107 M–1 s–1) and are characterised by low activation energies (0–5 kcal mol–1), and A factors generally in the range 107–108.

The mechanism of oxidation of α-glycols by periodic acid. Part X. The oxidation of pinacol, and a general discussion of the stability of periodate esters and their role in the mechanism of oxidation

Abstract: The kinetics of oxidation of pinacol by periodate have been studied at 0° and 25° over the range pH 0–13. The kinetics are second order except at 0° and pH >11. Under the latter conditions, with excess pinacol, the kinetics are those of consecutive first-order reactions, showing that an intermediate is being formed in appreciable concentration. Several features of the kinetics resemble closely those observed for the formation of the cyclic ester from propane-1,2-diol and periodate. It is concluded that the oxidations of pinacol and propane-1,2-diol by periodate proceed via a mechanism which is probably common to all oxidations of 1,2-diols by periodate, viz. the formation of a periodate monoester, followed by ring-closure to a cyclic ester, then decomposition of a monoanion of the cyclic ester to the reaction products. The stages of the mechanism are discussed in detail, and it is shown that the different types of pH dependence observed in the oxidation of 1,2-diols by periodate can be readily accommodated by this mechanism. The stability of the periodate esters is discussed, and it is concluded that the general order of stability of esters containing six-co-ordinated iodine is: cyclic triester > cyclic diester (5-membered ring) > cyclic diester (6-membered ring) > monoester. A comparison of the acid strengths of H5IO6 and cyclic periodate esters suggests that the periodiate di- and tri-anions are appreciably dehydrated in aqueous solution to HIO52– and IO53–.

Topochemistry. Part XXXI. Formation of cyclo-octa-1,5-cis,cis-dienes from 1,4-disubstituted s-trans-butadienes in the solid state. A contribution to the problem of C4-versus C8-cyclodimerisation

Abstract: Solid penta-1,3-diene-1-carboxylic acid (4), penta-1,3-diene-1-carboxamide (5), buta-1,3-diene-1,4-dicarbonitrile (6), styrylacrylic acid (10), its methyl ester (46), and amide (49)(all trans,trans-configurated), all photodimerise on irradiation (λ > 290 nm), to divinylcyclobutane (C4) derivatives. The structures of the fully characterised photoproducts from (4), (5), (6), and (49) and the light-stability of (N-phenyl)styrylacrylamide (53), are predictable from the known or postulated packing arrangements of their monomers.Cyclo-octa-1,5-trans,trans-dienes, although topochemically and symmetry-allowed from monomers which crystallise with parallel butadiene chains [(4), (5), (6), and possibly (10)], are not observed. Although the formation of C8-cyclodimers cannot be rigorously excluded our results neither support nor require their presence.The (all-ax)-cyclo-octa-1,5-cis,cis-diene (C8) derivatives formed during irradiation of (4), (5), and (10) are not primary photoproducts but arise from thermal (Cope) rearrangements of photochemically produced cis-1,2-divinylcyclobutanes.