Browsing by Author "Johnstone, Robert A. W."
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- 3-( E )-But-2-enoxy-1,2-benzisothiazole 1,1-dioxide: unusual C—O—C ether bond lengths and reactivityPublication . Barkley, J. V.; Cristiano, Maria Lurdes Santos; Johnstone, Robert A. W.; Loureiro, Rui M. S.Ethers such as the title compound, C~HIINO3S, (1), rearrange thermally to give N-allyl isomers, (2), in high yield. The X-ray structure determination of the title ether shows a central C--O--C linkage which has one very short (notional) C--O single bond and one exceptionally long single C--O bond. The thermal migration of allyl from the O to the N atom involves the breaking of one of the ether bonds in (1) and a shortening of the other as it becomes a formal carbonyl group in the product (2). The rearrangement is thus considerably assisted by the ground-state structure of the starting ether, in which the bond to be broken is already stretched and the one that is to form a carbonyl group is already a substantial partial double bond.
- 3-Hydroxy-2,6-dinitroacetophenone: an unusual substitution pattern resulting from nitration of 3-hydroxyacetophenonePublication . Cristiano, Maria Lurdes Santos; Johnstone, Robert A. W.; Pratt, M. J.Nitration of 3-hydroxyacetophenone gives 2,6-dinitro-3-hydroxyacetophenone, C8H6N206, in which the nitro groups have entered the sterically least favourable positions in the aromatic nucleus. None of the expected substitution in the 4-position was observed. The two nitro groups flanking the carbonyl side chain are different in that one is in the plane of the aryl ring but the other is twisted well out of the plane.
- Bond energy/eond order relationships for N-O linkages and a quantitative measure of ionicity: the rôle of nitro groups in hydrogen bondingPublication . Johnstone, Robert A. W.; Loureiro, Rui M. S.; Labat, G.; Cristiano, Maria Lurdes SantosThe nitro group is active in metabolic systems and can be found as an integral part of a number of useful curative drugs and many toxic substances. The basis for much of this activity is not fully understood. It is not necessarily caused directly by through-bond electronic effects but may also be due to direct H-bonding to nitro or to indirect interference by the nitro group with existing H-bonding. An unusual effect of a nitro substituent on kinetic results from urethane addition/elimination reactions (Scheme 1) has been ascribed to some form of self-association, which was neither specified nor quantified. To investigate self-association phenomena caused by a nitro group, a bond energy/bond order formula for N–O bonds has been developed and then used to interpret relative amounts of covalent and ionic contributions to total N–O bond energy. Calculated bond energies were then used to obtain enthalpies of formation for H-bonds to nitro groups in crystals and in solution. Similar results from solution data reveal that direct H-bonding to nitro is much weaker than in crystals, unless intramolecular H-bonding can occur. The results revealed that the 'self-association' effects observed for nitro substituents in urethanes (Scheme 1) were not caused by nitro participating directly in intermolecular bonding to NH of another urethane but by an indirect intramolecular action of the nitro group on pre-existing normal NH–O amide/amide type H-bonding.
- Electronic effects on C-O-C ether bonds in 3-aryloxy derivatives of benzisothiazole 1,1-dioxides: rapid ethanolysis of 3-(4-nitrophenoxy)-1,2-benzisothiazole 1,1-dioxide, (1), to give 3-ethoxy-1,2-benzisothiazole 1,1-dioxide, (2)Publication . Brigas, A. F.; Gonçalves, P. M.; Johnstone, Robert A. W.The bond lengths in the central C-O-C ether linkage of title compound (1), C13H8N2O5S, are comparable with those found in earlier work on similar compounds. However, (1) was found to undergo very easy solvolysis with ethanol to give (2), C9H9NO3S, for which a structure was also determined, but 3-(4-methoxyphenoxy)-1,2-benzisothiazole 1,1-dioxide, (3), did not hydrolyse under the same conditions. If ground-state structures are important for solvolysis, these results suggest that there should be a difference in the corresponding C-O bond lengths for the ethers (1) and (3). Such differences are not observed. The results can be rationalized by supposing that transition-state energies for ethanolysis are more important factors than those of the ground state.
- Intramolecularity of the thermal rearrangement of allyloxytetrazoles to N-allyltetrazolones†Publication . Cristiano, Maria Lurdes Santos; Johnstone, Robert A. W.The intramolecularity of the thermal rearrangement of 1-aryl-5-allyloxy-1H-tetrazoles 1 to 1-aryl-4-allyl-1,4-dihydrotetrazol-5-ones 2 has been investigated through cross-over studies: the results support the hypothesis for a concerted sigmatropic rearrangement occurring through a highly polar transition state, in which a partially positively charged allyl group migrates from oxygen to nitrogen, without leaving the solvent cage.
- Investigations into the mechanism of action of nitrobenzene as a mild dehydrogenating agent under acid-catalysed conditionsPublication . Cristiano, Maria Lurdes Santos; Gago, David J. P.; Gonsalves, Antonio M. d'A. Rocha; Johnstone, Robert A. W.; McCarron, Moya; Varejão, Jorge M. T. B.Protonated nitrobenzene can be used to dehydrogenate a range of hydrocarbons, which already possess at least one double bond. Kinetic and spectroscopic results, together with known electrode potentials, yield approximate limits within which protonated nitrobenzenes can be expected to effect dehydrogenation of hydroaromatic compounds. A high yielding synthesis of benzo[ j ]fluoranthene is described.
- A kinetic investigation of the thermal rearrangement of allyloxytetrazoles to N-allyltetrazolonesPublication . Cristiano, Maria Lurdes Santos; Johnstone, Robert A. W.The mechanism of the thermal rearrangement of 1-aryl-5-allyloxytetrazoles 1 to give 1-aryl-4-allyltetrazolones 2 in very high yield has been investigated through kinetic studies in one polar and one less polar solvent. The results suggest mainly a concerted [3,3] sigmatropic process, in which a partially positively charged allyl group migrates from oxygen to nitrogen, similar to the polar transition state found in the Claisen rearrangement.
- Metal-assisted reactions. Part 25. Heterogeneous and homogeneous catalytic transfer hydrogenolysis of allyloxytetrazoles to yield alkenes or alkanesPublication . Cristiano, Maria Lurdes Santos; Johnstone, Robert A. W.; Price, Peter J.Transfer hydrogenolysis of 5-allyloxy- 1-phenyltetrazoles using either a heterogeneous or a homogeneous palladium catalyst and a hydrogen donor leads to cleavage of the allyloxy C-O bond to yield an alkane or an alkene and 1-phenyltetrazolone, depending on the catalyst used.
- Photoactive materials applicable to imaging systemsPublication . Cristiano, Maria Lurdes Santos; Johnstone, Robert A. W.; Pratt, M. J.; Wade, M. J.Organic photoprecursors of amines are provided for use in photosensitive imaging systems, ?nding particular application in the preparation of lithographic printing plates. Said photoprecursors generate free amines on exposure to long Wavelength UV or visible radiation, have high solubility in organic solvents, and include photolabile 2-nitrobenZyl functional groups. Methods for the synthesis of the photo precursors are described, and the use of the said photopre cursors for the production of printing plates using both positive Working and negative Working techniques are dis cussed.
- Structural effects on sigmatropic shifts in heteroaromatic allyl ethersElectronic supplementary information (ESI) available: selected crystal data for compound 7. See http://www.rsc.org/suppdata/p1/b1/b102674g/Publication . Araujo, Nuna C. P.; Barroca, Pedro M. M.; Bickley, Jamie F.; Brigas, Amadeu; Cristiano, Maria Lurdes Santos; Johnstone, Robert A. W.; Loureiro, Rui M. S.; Pena, Paula C. A.In contrast to the known thermal, exclusively [3,3], O- to N- rearrangement of allyl groups in phenyltetrazoles (1, Scheme 1), the comparable migration of the allyl group in pseudosaccharyl ethers (3; Scheme 2) has been shown to proceed through both [1,3]- and [3,3]-mechanisms, 4, 5; for the pseudosaccharyl derivative of the natural product myrtenol (6; Scheme 3) only the product 7 of a [1,3]-shift has been observed; crystallographic data and theoretical calculations provide an explanation of this ease of [1,3]-isomerization and the observed selectivity as being due to conformational constraints and electronic factors.