Browsing by Author "Loureiro, Rui M. S."
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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.
- 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.
- Heteroaromatic benzyl ethers as intermediates for palladium-catalysed transfer hydrogenolysis of benzyl alcoholsPublication . Araujo, Nuna C. P.; Brigas, Amadeu; Cristiano, Maria Lurdes Santos; Frija, L.; Guimarães, Emanuel M. O.; Loureiro, Rui M. S.A chemoselective and easy to carry procedure for the hydrogenolysis of benzyl alcohols is described. Benzyl alcohols are readily converted into tetrazolyl and benzisothiazolyl ethers that can be catalytically hydrogenolysed to toluenes over palladium-on-charcoal using hydrogen donors. The effect of electronwithdrawing heteroaromatic substituents is interpreted on the basis of chrystallographic structure determinations and molecular orbital calculations.
- Palladium-catalysed reduction of heteroaromatic naphthyl ethers: structural effects on reactivityPublication . Frija, L.; Cristiano, Maria Lurdes Santos; Guimarães, Emanuel M. O.; Martins, Nelson C.; Loureiro, Rui M. S.; Bickley, Jamie F.Tetrazolyl and benzisothiazolyl naphthylmethylic ethers 3 and 4(a–e) are stable crystalline compounds that can be synthesised in high yields by reaction of the corresponding naphthyl methanols (1a–e) with the derivatizing agents 2a and 2b. Experimental conditions for palladium-catalysed hydrogenolysis of ethers 3, 4, with a hydrogen donor and with molecular hydrogen, were investigated. Analysis of the structure and reactivity indicates that naphthylmethylic ethers 3 and 4 are structurally similar to the corresponding benzyloxyderivatives around the ether bond but exhibit different reactivity. Structural analysis for these compounds is based on crystallographic structure determinations, for 5-(2-naphthylmethoxy)-1-phenyltetrazole (3a) and 3-(2-naphthylmethoxy)-1,2-benzisothiazole 1,1-dioxide (4a), and molecular orbital DFT(B3LYP)/6-311G(d) calculations, for all ethers. It can be concluded from this investigation that 5-chloro-1-phenyltetrazole 2a acts as a better derivatizing agent for naphthyl methanols than 3-chloro-1,2-benzisothiazole-11-dioxide 2b, this contrasting with what has been observed with phenols, allylic and benzylic alcohols.
- 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.
- Synthesis and structure of novel benzisothiazole-tetrazolyl derivatives for potential application as nitrogen ligandsPublication . Frija, L.; Fausto, R.; Loureiro, Rui M. S.; Cristiano, Maria Lurdes SantosThe synthesis and characterization of novel benzisothiazole-tetrazolyl derivatives differing on the spacer-group used for the linkage of two heterocycles are described. The application of these compounds as nitrogen ligands for coordination with transition metals was also explored, leading to the preparation of 3-[1-(2-1H-tetrazol-5-yl)ethoxy]-1,2-benzisothiazole 1,1-dioxide-Mn(II) and N-(1,1-dioxo-1,2-benzisothiazol-3-yl)-amine-1H-tetrazole-Mn(II) complexes.
- Thermal rearrangement of 3-allyloxy-1,2-benzisothiazole 1,1-dioxides: an unusual inversion of products of sigmatropic [3,3]-shift to give the [1,3]-IsomersPublication . Cristiano, Maria Lurdes Santos; Brigas, Amadeu; Johnstone, Robert A. W.; Loureiro, Rui M. S.; Pena, Paula C. A.3-Allyloxy-1,2-benzisothiazole 1,1-dioxides isomerize thermally to give [3,3]- and [1,3]-products 6 of sigmatropic shift, of which the former reacts further to give solely the [1,3]-isomer.