Showing papers in "Journal of The Chemical Society-perkin Transactions 1 in 1986"
TL;DR: Details are given of a novel technique for solid-phase peptide synthesis under continuous flow conditions that is illustrated by preparation of a pentadecapeptide sequence related to the repeating unit of adenovirus tail fibre.
Abstract: Details are given of a novel technique for solid-phase peptide synthesis under continuous flow conditions. The resin support is a poly(dimethylacrylamide) gel polymerised within the pores of rigid macroporous kieselguhr particles. Manual and semiautomatic (microprocessor controlled) instrumentation is described for the use of this resin in synthesis. The method is illustrated by preparation of a pentadecapeptide sequence related to the repeating unit of adenovirus tail fibre. The value of continuous spectrometric monitoring of solid-phase synthesis is discussed.
171 citations
TL;DR: In this paper, various derivatives of calix[4]arene have been synthesized and their ion-binding properties investigated by solvent extraction, ion-transport through liquid membranes, and u.v. spectroscopy.
Abstract: Various derivatives (ethoxycarbonylmethyl, carboxymethyl, and ethoxyethyl, etc.) of calix[4]arene, calix[6]arene, and calix[8]arene have been synthesized and their ion-binding properties investigated by solvent extraction, ion-transport through liquid membranes, and u.v. spectroscopy. The extraction of various metal ions (as picrate salts) from the aqueous phase into dichloromethane by the calixarene derivatives showed that the ethoxycarbonylmethyl derivatives exhibited markedly increased extraction efficiency compared with their unsubstituted phenol analogues, and the efficiency was in general comparable to those of crown ethers. The most notable feature was the selectivities exhibited by their ethoxycarbonylmethyl derivatives; calix[4]arene for Na+, calix[6]arene for Cs+, and calix[8]arene for K+. A liquid membrane study with a water–chloroform–water system also indicated that the ion-transport efficiencies exhibited by calixarenes were found to be in accordance with their extraction abilities, confirming the strong participation of the ester carbonyl oxygen in this function. However, the ester derivative of calix[4]arene was an exception in that inefficient carrier activity was observed in spite of high extraction efficiency. The pronounced Cs+ selectivity exhibited by the calix[6]arene ester was further confirmed by the changes in the u.v. spectrum of the ester from 277 and 270 nm to 275 and 267 nm upon interaction with Cs+ ion in methanol.
152 citations
TL;DR: In this paper, the tautomerism of thiohydroxamic acids has been examined by physical methods and it is confirmed that the esters used are derivatives of the thione tautomers.
Abstract: Carbon radicals derived from the esters of several types of thiohydroxamic acids have been trapped in a number of different ways. Particular attention has been paid to carbon–carbon bond formation by addition to suitably activated ethylenic linkages. Carboxylic acids can be conveniently converted into homo- and bishomo-acids by free radical chemistry based on thiohydroxamic esters. In a coda the tautomerism of the thiohydroxamic acids have been examined by physical methods. It is confirmed that the esters used are derivatives of the thione tautomer.
123 citations
TL;DR: In this paper, the structure elucidation of ring-expanded semisynthetic erythromycins has been established on the basis of their analytical and spectral data and acid-catalysed degradation into the aglycones.
Abstract: The synthesis of 10-dihydro-10-deoxo-11-azaerythromycin A (11) by the Beckmann rearrangement of erythromycin A oxime (2) and reduction of the imino ether so obtained (5) is described. The structure elucidation of the new ring-expanded semisynthetic erythromycins (5) and (11) has been established on the basis of their analytical and spectral data and acid-catalysed degradation into the aglycones (7) and (13), respectively. Finally, the complete structure of ring-expanded erythronolides (7) and (13) has been determined by X-ray crystallography.
123 citations
TL;DR: The absolute rate constants for the reaction of NO32˙ (formed by one-electron reduction of nitrate) with oxygen, benzoquinone, and methyl viologen have been determined by pulse radiolysis as discussed by the authors.
Abstract: The absolute rate constants for the reaction of NO32–˙(formed by one-electron reduction of nitrate) with oxygen, benzoquinone, and methyl viologen have been determined by pulse radiolysis. Experiments have shown that such reactions can occur in competition with the hydrolysis reaction leading to NO2˙ and that the formation of the latter can be catalysed by the presence of hydrogen or ammonium ions. Absolute rate constants for the oxidation of the phenothiazine derivative metiazinic acid, of 2,2′-azinobis-(3-ethyl-2,3-dihydrobenzothiazole-6-sulphonate), of ascorbate, and of dihydroxyfumarate by NO2˙ have also been measured.
111 citations
TL;DR: The postulated binding functions for the active site of Angiotensin Converting Enzyme (A.C.E), derived in an earlier study, have made possible the design of improved inhibitors as mentioned in this paper.
Abstract: The postulated binding functions for the active site of Angiotensin Converting Enzyme (A.C.E.), derived in an earlier study, have made possible the design of improved inhibitors. Consequently, (1S,9S)-9-[(1S)-(ethoxycarbonyl)-3-phenylpropylamino]octahydro-10-oxo-6H-pyridazo[1,2-a][1,2]diazepine-1-carboxylic acid (Cilazapril), and related compounds, have been synthesized. They are very active inhibitors of A.C.E. and are highly potent antihypertensives in vivo.
86 citations
TL;DR: The dipeptide analogue of gramicidin S exhibited equal antibacterial activity, and a closely similar c.d. spectrum, to that of GS, indicating that there are two β-turns in the biologically active conformation of GS.
Abstract: In order to demonstrate the usefulness of a bicyclic dipeptide derivative designed to simulate the backbone conformation of a β-turn (type II′), the dipeptide (3S, 6S, 9R)-2-oxo-3-amino-7-thia-1-azabicyclo[4.3.0]nonane-9-carboxylic acid has been incorporated into gramicidin S (GS) at the β-turns in place of the D-Phe-Pro sequences.The GS analogue exhibited equal antibacterial activity, and a closely similar c.d. spectrum, to that of GS, indicating that there are two β-turns in the biologically active conformation of GS. The method seems applicable to more flexible peptides of biological interest.
78 citations
TL;DR: In this paper, the structures of 1-phenylsulphonylpyrrole, 4,5,6,7-tetrahydro-1-pnsulphonylindole, and 1,2,3,4,9-PNS-carbazole have been determined by X-ray crystallography.
Abstract: The structures of 1-phenylsulphonylpyrrole, 1-phenylsulphonylindole, 4,5,6,7-tetrahydro-1-phenylsulphonylindole, and 1,2,3,4-tetrahydro-9-phenylsulphonylcarbazole have been determined by X-ray crystallography. The heterocyclic nitrogen is planar in some and pyramidal in others. A preferred geometry for the PhSO2N unit emerged from the structures; it is compared with available data for sulphonamides.
77 citations
TL;DR: In this paper, the substitution of the hydroxy group by fluorine proceeds via an SN2′ rearrangement to give the γ-fluoroalk-1-enylphosphonates exclusively.
Abstract: α-Fluorobenzylphosphonate esters are conveniently made by treating α-hydroxybenzylphosphonate esters with diethylaminosulphur trifluoride. The reaction is not subject to steric impedance and is extended to the α-fluorination of an α-hydroxybenzylphosphine oxide. For α-hydroxyallyl and α-hydroxycinnamylphosphonates, the replacement of the hydroxy group by fluorine proceeds via an SN2′ rearrangement to give the γ-fluoroalk-1-enylphosphonates exclusively. Dehydration rather than substitution occurs in the case of alcohols of secondary alkylphosphonates.
77 citations
TL;DR: In this article, a new compound named emestrin (1), C27H22H2O10S2, was isolated from the mycelial extract of Emericella striata.
Abstract: Together with ergosterol, sterigmatocystin, and emericellin, a new compound designated emestrin (1), C27H22H2O10S2, was isolated from the mycelial extract of Emericella striata. Violaceic acid (6) was also isolated from the culture filtrate. The structure of emestrin (1), including the absolute configuration, was determined on the basis of a spectroscopic investigation of some derivatives and X-ray crystallography of emestrin methanol solvate. Emestrin is a new macrocyclic epidithiodioxopiperazine, derived from two molecules of phenylalanine and one molecule of benzoic acid, and has strong antifungal activity.
75 citations
TL;DR: In this article, a base-line separation of keto and enol tautomers of β-dicarbonyl compounds has been achieved on silica gel packings with a mobile phase composed of hexane and propan-1-ol containing a small amount of acetic acid.
Abstract: The keto–enol tautomerism of some β-dicarbonyl compounds, including ethyl acetoacetate (1), acetylacetone (2), benzoylacetone (3), dibenzoylmethane (4), and 1,1,1-trifluoro-3-(2-thenoyl)acetone (5), has been investigated in various solvents at 25 °C, by using low-temperature (–20 to –50 °C) high-performance liquid chromatography (h.p.l.c.) for analytical measurements. Base-line separation of keto and enol tautomers of these compounds has been achieved on silica gel packings with a mobile phase composed of hexane and propan-1-ol containing a small amount of acetic acid. The ratios of keto and enol tautomers in different solvents have been determined; the percentages of enol tautomers have been found to be higher in polar solvents, in accord with n.m.r. data previously reported. In all solvents the proportions of enol tautomers of acetylacetone derivatives have been found to increase in the following order; (2) < (3) < (4); this has been attributed to the fact that enol tautomers of phenyl-substituted acetylacetones have been stabilized, owing to the formation of extended conjugated systems. For the ester (1), the presence of a very small amount of a third tautomer (0.1%) has been found, probably an unconjugated enol tautomer.
TL;DR: The Gif system as discussed by the authors is based on the idea of an iron catalyst which is reduced by electron transfer and oxidized by molecular oxygen simultaneously in the presence of a source of protons.
Abstract: Various systems for the selective oxidation of saturated hydrocarbons have been developed. These are based on the idea of an iron catalyst which is reduced by electron transfer and oxidized by molecular oxygen simultaneously in the presence of a source of protons. Four modifications of this system (the Gif system) have been devised of which the best (Gif IV) consists of an iron catalyst with metallic zinc as the reductant, acetic acid as the proton source and pyridine as the solvent. At room temperature, using oxygen or air saturated hydrocarbons are oxidized selectively to ketones in isolated yields superior to those reported for comparable model systems.
TL;DR: In this article, the preferred conformations of the pyranosyl radicals are discussed in terms of a stabilizing interaction of the singly occupied p-orbital with the σ*-LUMO of the adjacent β-C-OR bond.
Abstract: Carbohydrate free radicals wer regiospecifically generated at C-1 of acylated and alkylated pyranosyl derivatives in non-aqueous solution and their conformations were deduced from the hyperfine splittings of their e.s.r. spectra. The preferred conformations of the pyranosyl radicals are discussed in terms of a stabilizing interaction of the singly occupied p-orbital with the σ*-LUMO of the adjacent β-C–OR bond. From the α-13C coupling constant of the tetra-acetylglucosyl radical it is concluded that pyranosyl radicals are of π-type.
TL;DR: In this article, a thermosolvatochromic comparison method for measuring polarity and basicity parameters of solvents, which takes advantage of variations in solvent properties with temperature (0-105 °C), is proposed.
Abstract: A thermosolvatochromic comparison method for measuring polarity and basicity parameters of solvents, which takes advantage of variations in solvent properties with temperature (0–105 °C), is proposed. The effects of the vibrational structure of the absorption bands on (i) the reference (homomorphism) line of non-hydrogen-bond donor and non-hydrogen-bond acceptor solvents and the temperature line of hydrogen-bond acceptor solvents, and thus on (ii) the precision and sensitivity of the method, are analysed for a range of indicators. 4-Nitroanisole and 4-nitrophenol are recommended as reference polarity and basicity indicators, respectively. The reference (homomorphism) line is to be taken from data for gas-state, perfluorinated hydrocarbons, alkanes, CCl4 and/or C2Cl4, 1,2-dichloroethane, and mono- and poly-halogenated benzenes.
TL;DR: In this article, a low valent titanium-mediated crossed coupling of substituted benzophenones of the type 4-XPhCOPh, where X = MeO, ClCH2CH2O, BrCH 2CH2HO, CF3C6H4O, HO and Me2NCH2H2O with propiophenone affords the corresponding but-1-enes in high yield with a marked preponderance of the Z-isomer in most cases.
Abstract: Low valent titanium-mediated crossed coupling of substituted benzophenones of the type 4-XPhCOPh, where X = MeO, ClCH2CH2O, BrCH2CH2O, CF3C6H4O, HO and Me2NCH2CH2O, with propiophenone affords the corresponding but-1-enes in high yield with a marked preponderance of the Z-isomer in most cases. Remarkably, when X = OH the Z:E ratio is 9:1. The value of this method is illustrated by simple syntheses of (Z)-1-{4-[2-(N,N-dimethylamino)ethoxy]phenyl}-1,2-diphenylbut-1-ene (Tamoxifen) a drug widely used in the treatment of oestrogen-dependent breast tumours.
TL;DR: In this article, the reaction of a range of 1,1,2-trihalogenocyclopropanes (halogen = bromine, chlorine) with methyl-lithium in ether at -90-20 °C leads to 1,2 dehalogenation to the corresponding 1-halogencycle.
Abstract: Reaction of a range of 1,1,2-trihalogenocyclopropanes (halogen = bromine, chlorine) with methyl-lithium in ether at –90–20 °C leads to 1,2-dehalogenation to the corresponding 1-halogenocyclopropene. The halogenocyclopropenes are readily lithiated by lithium–halogen exchange with a second equivalent of methyl-lithium to give 1-lithiocyclopropenes, which are in turn trapped by electrophiles; an exception to this is compound (21; X = Cl), which leads to 3-methylbuta-1,2-dienylidene (31) by initial lithium–hydrogen exchange and loss of lithium chloride. The cyclopropenes (5; R = Me, X = Br or Cl) and (21; X = Br or Cl) decompose even at temperatures around ambient, leading either to enynes (6; R = Me) or halogenoalkynes (18; X = Br or Cl). A 12C-labelling study indicates that C-1 of the cyclopropene (21; X = Cl) becomes C-2 of the alkyne (18; X = Cl) on rearrangement.
TL;DR: In this article, it was shown that 13C n.m.r. chemical shifts of the anomeric and aglycone carbon atoms can be correlated with one of the glycoside torsion angles (ψ) which are involved in determining the conformation of oligosaccharides.
Abstract: It is shown that 13C n.m.r. chemical shifts of the anomeric and aglycone carbon atoms can be correlated with one of the glycoside torsion angles (ψ) which are involved in determining the conformation of oligosaccharides. The correlation is shown to be valid for oligosaccharides having an α-D-glucopyranosyl or α-D-galactopyranosyl unit at the non-reducing end of the molecule. Calculation of the minimum-energy conformation of the oligosaccharides is made on the basis of hard-spheres exo-anomeric (HSEA) effect calculations.
TL;DR: In this paper, the mass spectra of all the 5-arylmethylenehydantoins were determined by considering their 1H and 13C n.m.n.r., and u.v.
Abstract: Both (Z)- and (E)-5-arylmethylene-1-methylhydantoins have been obtained directly in substantial proportions from condensations of 1-methylhydantoin with aromatic aldehydes. However, the products of similar condensations of hydantoin or 3-methylhydantoins consist almost entirely of the Z-isomers. The configurations of all the 5-arylmethylenehydantoins prepared can be unambiguously assigned by consideration of their 1H and 13C n.m.r., i.r., and u.v. spectra. The mass spectra of the isomers show similar fragmentation patterns.
TL;DR: In this article, several aromatic sesquiterpenoids displaying antifungal properties have been isolated from the liverwort Herberta adunca together with a mother hydrocarbon, ent-herbertane, and their structures and absolute configurations were determined on the basis of extensive degradation reactions and spectroscopic evidence.
Abstract: Several aromatic sesquiterpenoids displaying antifungal properties have been isolated from the liverwort Herberta adunca together with a mother hydrocarbon with a novel irregular sesquiterpene skeleton, ent-herbertane, and their structures and absolute configurations have been determined on the basis of extensive degradation reactions and spectroscopic evidence. The biological activity is also described.
TL;DR: A new method for the preparation of γ-butyrolactones is described in which the key step is the highly stereoselective radical cyclization of bromoacetals to 2-alkoxytetrahydrofurans in the presence of polymeric or low-molecular weight organotin species.
Abstract: A new method for the preparation of γ-butyrolactones is described in which the key step is the highly stereoselective radical cyclization of bromoacetals to 2-alkoxytetrahydrofurans in the presence of polymeric or low-molecular weight organotin species. 2-Alkoxytetrahydrofurans can be easily converted into γ-butyrolactones by Jones oxidation.
TL;DR: In this paper, the synthesis of melanin concentrating hormone using a continuous-flow solid-phase method was described, where a composite polydimethylacrylamide-macroporous kieselguhr support was used permitting low pressure flow operation.
Abstract: Synthesis of the heptadecapeptide melanin concentrating hormone using a novel continuous-flow solid-phase method is described. A composite polydimethylacrylamide–macroporous kieselguhr support was used permitting low pressure flow operation. Fluorenylmethoxycarbonylamino acid anhydrides were used throughout for acylation reactions. The choice of compatible side-chain protecting groups is discussed. The synthetic product showed high melanin-concentrating properties in vitro.
TL;DR: In this paper, the dual fluorescence observed in dilute acid solutions and the similarity of the fluorescence excitation spectra to the absorption spectra indicate the existence of several excited states of benzimidazoles.
Abstract: Electronic absorption and fluorescence spectra of benzimidazole and six of its derivatives in different solvents and at various pH values have been studied. The dual fluorescence observed in dilute acid solutions and the similarity of the fluorescence excitation spectra to the absorption spectra indicate the existence of several excited states of benzimidazoles. The emitting transition of neutral benzimidazoles is πâ†�π*; in the case of the monocations the short-wavelength fluorescence band is from the πâ†�π* transition and the long-wavelength band arises from charge transfer. The pKa values for various prototropic reactions in aqueous solutions in the S0 and S1 states have been calculated, and the proton-induced fluorescence quenching of the monocations is discussed.
TL;DR: The structure and stereochemistry of Stemphyltoxins I-IV (1) and IV (4) from the fungus Stemphylium botryosum var. Lactucum have been elucidated, mainly on the basis of n.m.r.
Abstract: The structure and stereochemistry of Stemphyltoxins I–IV (1)–(4), four epoxy derivatives of reduced perylenequinones isolated from the fungus Stemphylium botryosum var. Lactucum, have been elucidated, mainly on the basis of n.m.r. evidence. A further metabolite, stemphyperylenol (5), is a hexahydro-1,4,7,10-tetrahydroxyperylene-3,9-quinone, a structure which suggests an unusual head-to-tail biosynthetic coupling of two pentaketide units.
TL;DR: In this article, the authors compared the loading and drying conditions of alkali metal fluride (alumina) supported reagents, by reaction studies and a series of analytical techniques.
Abstract: Reagent loading and drying conditions have been compared for alkali metal fluride (MF)–alumina supported reagents, by reaction studies and a series of analytical techniques. The reaction studies indicate that whereas a combination of low MF loadings and highly efficient drying procedures give the best reagents for catalytic reactions, somewhat higher loadings and more moderate drying conditions give the best reagents for non-catalytic reactions. The analytical studies enable some rationalisation of the reaction results as well as providing important clues concerning the nature of the active-basic species. The formation of OH– on the reagent surface cannot be entirely responsible for the high basicity of the reagents and it seems that co-ordinately unsaturated F– ions on the reagent surface play an important role in the behaviour of the reagents. High MF loadings and high drying temperatures result in significant structural damage to the reagents with resulting loss in activity.
TL;DR: In this article, two types of descriptor are proposed to deal with steric effects of substituents on reaction rates, shape is characterised by the ratio (G) of the substituent's surface area to its volume.
Abstract: To deal with steric effects of substituents on reaction rates, two types of descriptor are proposed. Shape is characterised by the ratio (G) of the substituent's surface area to its volume. Bulk is characterised by the volume (Va) of the portion of the substituent that is within 0.3 nm of the reaction centre. For alkyl substituents, rate constants for several series of reactions (including the classical series, acid hydrolysis of the esters RCO2Et) correlate fairly well with Va and G. Taft's steric parameters (Es) correlate less well. Values of Es for hetero-substituents are found to be contaminated by non-steric factors. It is possible to construe whether the contaminant acts to hinder or to enhance reaction.
TL;DR: In this article, the use of the 2-(trimethylsilyl)ethoxycarbonyl (TEOC) group for the protection of the indole system, and the pivaloyloxymethyl (POM) group was also described.
Abstract: Treatment of pyrrole (1a), 2,5-dimethylpyrrole (1b), indole (3a), 2-methylindole (3b), 3-methylindole (3c), 2,3-dimethylindole (3d), 1,2,3,4-tetrahydro-9H-carbazole (5a), 5,6,7,8,9,10-hexahydro-cyclohept[b]indole (5b), and 6,7,8,9,10,11-hexahydro-5H-cyclo-oct[b]indole (5c) with sodium hydride in tetrahydrofuran, followed by phenyl t-butyl carbonate (8) gives the corresponding N-t-butoxycarbonyl (t-BOC) derivatives [(2a), (2b), (4a), (4b), (4c), (4d), (6a), (6b), and (6c), respectively] in satisfactory yields. By using chlorodimethyl-t-butylsilane instead of (8), N-dimethyl-t- butylsilyl derivatives [(10a), (10b), (11a), (11b), (11c), (11d), (12a), and (12b)] are obtained, also in satisfactory yields. The use of the 2-(trimethylsilyl)ethoxycarbonyl (TEOC) group for the N-protection of the indole system, and the pivaloyloxymethyl (POM) group for the N-protection of the pyrrole and indole systems is also described.
TL;DR: In this article, the kinetic analysis of the esterolysis of p-nitrophenyl picolinate in neutral aqueous solutions of Cu2+ and Zn2+ in the presence of co-micelles made up of cetyltrimethylammonium bromide and either 6-(myristoylamido) methylpyridine-2-carboxylic acid (1) or 6-(mylmethyl-2)-hydroxymethyl pyridine (2) is reported.
Abstract: The kinetic analysis of the esterolysis of p-nitrophenyl picolinate in neutral aqueous solutions of Cu2+ and Zn2+ in the presence of co-micelles made up of cetyltrimethylammonium bromide and either 6-(myristoylamido) methylpyridine-2-carboxylic acid (1) or 6-(myristoylamido)methyl-2-hydroxymethyl-pyridine (2) is reported. The results strongly indicate the formation of reactive micellar ternary complexes composed of metal ion, (1) or (2), and the substrate. The higher reactivity observed with co-micelles of (2) than that with those of (1) suggests that the metal ion in the ternary complex with (2) acts as a template for an intracomplex nucleophilic attack of the activated hydroxy function of (2) on the carbonyl of p-nitrophenyl picolinate.
TL;DR: The photosensitized formation of NAD(P)H by enzyme-catalysed processes has been accomplished and the products exhibit high optical purity and the enzymes and the coenzymes show high turnover numbers and stability.
Abstract: The photosensitized formation of NAD(P)H by enzyme-catalysed processes has been accomplished. With Ru(bpy)2+3 as sensitizer, methyl viologen, MV2+ as primary electron acceptor, and (NH4)3EDTA or 2-mercaptoethanol, NADPH is formed in the presence of ferredoxin NADP+-reductase as enzyme catalyst. Zinc(II) meso-tetramethylpyridiniumporphyrin, ZnTMPyP4+ is used as sensitizer for the photoinduced production of NADH using the same components and lipoamide dehydrogenase as enzyme catalyst. The photoinduced NAD(P)H regeneration systems have been coupled to secondary enzyme-catalysed processes such as the reduction of butan-2-one to butan-2-ol, pyruvic acid to lactic acid, acetoacetic acid to β-hydroxybutyric acid, as well as to the reductive amination of pyruvic acid to alanine and of α-oxoglutaric acid to glutamic acid. The products exhibit high optical purity and the enzymes and the coenzymes show high turnover numbers and stability.
TL;DR: In this paper, it was shown that the solvent greatly assists delocalization of the negative charge in benzyl carbanions and that substituent-induced variations of the para-monitor in the anions are linearly related to pKa variations of precursor acids.
Abstract: 1 H, 13C, and 19F n.m.r. data of benzyl carbanions PhCH–X (VI) and p-FC6H4CH–X have been found to be appropriate monitors for probing the structural reorganization undergone by the precursor carbon acids PhCH2X upon deprotonation. Even in the highly dissociating Me2SO as solvent benzyl carbanions exhibit ion-pairing phenomena: these have been proved by the effect exerted by the [2.2.1] cryptand on 13C shifts, and by changing the countercation. 13C Parameters of benzyl anions (VI) provide experimental access to negative π charge density maps through equation (1). It is shown that the solvent greatly assists delocalization of the negative charge in these systems. The substituent-induced variations of the para-monitor in the anions are linearly related to pKa variations of the precursor acids. Carbanions appear to belong to two different classes depending on the mechanism with which the charge stabilizes the system. Thus, in enolate and nitronate anions the stabilization is provided by extensive charge transfer from the carbanion carbon, in contrast to carbanions activated by the third-row elements (P,S) and by the cyano group. Substituent electron demands qx(the amount of charge transferred to the activating substituent X) govern the charge distribution in the anions in related ways, the acidity of the precursor acids PhCH2X, and the sensitivities of these latter to remote aryl substitution.
TL;DR: In this article, the authors employed the complementary techniques of pulse-radiolysis and e.s.r. spectroscopy in a kinetic study of the dehydration of a variety of α,β-dihydroxyalkyl radicals into the appropriate carbonyl-conjugated radicals [CR2R3C(O)R1].
Abstract: The complementary techniques of pulse-radiolysis and e.s.r. spectroscopy have been employed in a kinetic study of the dehydration of a variety of α,β-dihydroxyalkyl radicals [˙CR1(OH)CR2R3OH] into the appropriate carbonyl-conjugated radicals [˙CR2R3C(O)R1]. The overall rates of proton-catalysed dehydration, as revealed by steady-state (e.s.r.) and time-resolved (pulse-radiolysis) experiments, indicate the importance of the electronic effects of substituents (contrast values of 1.2 × 109 and 9.8 × 108 dm3 mol–1 s–1 for the radicals from cyclohexane-1,2-diol and butane-2,3-diol, respectively, with that for the radicals from erythritol of 4.2 × 106 dm3 mol–1 s–1). Time-resolved experiments allow information to be obtained about the generation of the protonated species [˙CR1(OH)CR2R3OH2+] and for the loss of water from this intermediate.For ˙CR(OH)CMe2OH (R = H and Me) evidence is obtained for a rapid uncatalysed dehydration reaction, (k 1–2 × 104 s–1), which is believed to be assisted by the steric effect of the gem-dimethyl group as well as the polarity of the solvent. For the latter substrate, the reaction is characterized by a strongly negative activation entropy (–93 J mol–1 K–1).α,β-Dihydroxyalkyl radicals are shown to reduce methyl viologen with rate constants that depend strongly on the nature of the substituents; there is a correlation between the reducing power of the radical and its rate of dehydration.