Eirian H. Curzon

Linkage of p-coumaroyl and feruloyl groups to cell-wall polysaccharides of barley straw

Abstract Treatment of cell walls of barley straw with Oxyporus “cellulase” (a mixture of polysaccharide hydrolases) released compounds containing p-coumaroyl and feruloyl groups bound to carbohydrates, two of which were identified as O-[5-O-(trans-p-coumaroyl)-α- l -arabinofuranosyl]-(1→3)-O-β- d -xylopyranosyl-(1→4)- d -xylopyranose (PAXX) and O-[5-O-(trans-feruloyl)-α- l -arabinofuranosyl]-(1→3)-O-β- d -xylopyranosyl-(1→4)- d -xylopyranose (FAXX).

Two-dimensional silicon-29 NMR studies of aqueous silicate solutions

Abstract Two-dimensional NMR techniques have been used to study the structures of species present in an alkaline aqueous solution of potassium silicate enriched in 29 Si. Both J -resolved and shift-correlated silicon-29 spectra are reported, together with projections and cross sections. The value of combining information from these different techniques is emphasized.

Rearrangements of (E,Z)-4-acetyl[17O]oxyhepta-2,5-diene catalysed by Pd0 and PdII: Mechanistic deductions from observations by 17O NMR spectroscopy

Abstract ( E,Z )-4-Acetyl[ 17 O]oxyhepta-2,5-diene [MeCH E =CHCH( 17 OCOMe)CH Z =CHMe] has been synthesised and rearranged under Pd 0 - and Pd II -catalysis. The distribution of 17 O in the products of these rearrangements has been determined by 17 O NMR spectroscopy. With Pd 0 catalyst the product is a 1:1 mixture of MeCH E =CHCH E =CHCH( 17 OCOMe)Me and MeCH E =CHCH E =CHCH(OC 17 OMe)Me formed from an intermediate Pd-coordinated pentadienyl species and 17 O-acetate. With Pd II catalyst the product is MeCH Z =CHCH E =CHCH(OC 17 OMe)Me formed via an intermediate acetoxonium ion.

Studies of pendant-arm macrocyclic ligands. Part 4. Two penta-aza macrocycles based on 1-(2′-dimethylaminoethyl)-1,5,9,13-tetra-azacyclohexadecane and its complexes with bivalent metal ions

Two new pendant-arm sixteen-membered ring penta-aza macrocycles, 1-(2′-dimethylaminoethyl)-1,5,9,13-tetra-azacyclohexadecane (L1) and 1-(2′-dimethylaminoethyl)-5,9,13-trimethyl-1,5,9,13-tetra-azacyclohexadecane (L2) have been prepared, and the complexes of L1 with Ni2+, Cu2+, Co2+, Zn2+, Cd2+, and Hg2+ investigated. The ions Ni2+, Cu2+, and Co2+ form high-spin five-co-ordinate complexes. Carbon-13 n.m.r. spectroscopy shows [Zn(L1)][NO3]2 to be a 1 : 1 mixture of two species, one symmetric and the other asymmetric, and [Cd(L1)][NO3]2 is analogous but with evidence of a small amount of a third species. Attempts to prepare [Hg(L1)]2+ have led to its immediate reduction to metallic mercury.

Pyramidal inversion in configurational isomers of tetracarbonyl[1,1,2,2-tetrakis(methylthio)ethane]chromium(0): a two-dimensional nuclear magnetic resonance exchange study

Pyramidal inversion of the co-ordinated sulphur atoms in the cis and trans configurational isomers of [Cr(CO)4{(MeS)2CHCH(SMe)2}] has been examined by n.m.r. spectroscopy. The two dimensional NOESY method was used to identify individual invertomers of each isomer at low temperature. One-dimensional band-shape analyses performed at higher temperatures yielded ΔG‡ values in the ranges 45–50 and 50–54 kJ mol–1 for the trans and cis isomers respectively. No evidence for cis–trans interconversion was obtained, probably because of the limited thermal stability of the complex.

Preparation and spectroscopic study of functionally substituted cyclopentadienides of thallium(I), potassium, rhodium(I), and iridium(I)

A 13C and 1H n.m.r. study has been carried out on a series of monosubstituted cyclopentadienide compounds of type MI(C5H4X)[M = K or Tl; X = Cl, COMe, CO2Me, CHO, Ph, COCO2Et, or C(CN)C(CN)2]. The different patterns observed for the ring nuclei reflect both the Lewis-acid character of the metal and the electronic effect of the substituent. Fluxional behaviour is observed for K(C5H4COMe). Pseudo-first-order rate constants have been evaluated for hydrogen–deuterium exchange in several of the potassium compounds. The thallium(I) compounds generally exhibit greater synthetic utility than their potassium analogues and several have been used in the synthesis of new [M(η5-C5H4X)(η2-C2H4)2](M = Rh or Ir) complexes. Activation barriers for alkene rotation and mass spectral fragmentation patterns are discussed for these compounds. The 1H n.m.r. spectra found for the cyclopentadienyl ring indicate that the M–ring bonding is not fully delocalised. Analysis of the products resulting from reaction between methyl chloroformate and cyclopentadienide ion confirms the strongly electrophilic character of the former reagent.

Conformational analysis of 2- and 3-phosphoglyceric acids by 1H and 13C nuclear magnetic resonance spectroscopy

The staggered rotamer populations about the C(2)–C(3) and C–O (phosphate) bonds of 2- and 3-phosphoglyceric acids (2PPG and 3PPG) have been estimated as a function of pH from 1H–1H, 1H–31P, 13C–1H, and 13C–31P vicinal coupling constants. For 2PPG in the pH range 5.0–10.8, between four and five conformations of the molecule may be significantly populated ( 10%). For 3PPG at low pH that conformer with the planar trans-arrangement of the C(2)–H and O–P bonds predominates and is possibly stabilised by intramolecular hydrogen bonding. At higher pH (9.8) there is an increasingly important contribution from that conformer with the C(2)–CO2– and O–PO32– bonds in a planar trans-arrangement, and this conformer may be stabilised by a combination of intramolecular hydrogen bonding and the electrostatic interaction.

Metal complexes of the new tetra-aza macrocyclic ligand 1,5,9,13-tetramethyl-1,5,9,13-tetra-azacyclohexadecane; carbon-13 nuclear magnetic resonance study of the complexes of Cd2+, Hg2+, and Pb2+ and X-ray crystal structure of the cadmium complex

The new 16-membered tetra-aza macrocyclic ligand 1,5,9,13-tetramethyl-1,5,9,13-tetra-azacyclohexadecane (L4) has been prepared, and the metal complexes of Cd2+ and Hg2+ isolated and characterised. The Pb2+ complex was also investigated by 13C n.m.r. spectroscopy in nitromethane solution. Carbon-13 spectra of the Cd2+ and Hg2+ complexes revealed the presence of two isomers in methanol solution, including the complex with the unusual ‘three up and one down’ set of nitrogen configurations, together with the more usual ‘four up’ arrangement of the N-methyl groups. A crystal structure of the Cd2+ complex establishes the presence of these two structures in the solid phase. The crystals contain [CdL4(NO3)]2[Cd(NO3)4] and are monoclinic, space group C2/c, with a= 53.360(14), b= 10.248(6), c= 31.938(7)A, β= 119.51 (2)°R= 0.063 for 4 920 diffractometer collected reflections with I/σ(I) 3.0. The cations have pseudo-trigonal-bipyramidal geometry, with bidentate nitrate groups occupying one equatorial position. They show disordered ligand conformations comprising a ‘three up and one down’ and two types of ‘all four up’ arrangements of the N-methyl groups. The anions are dodecahedral, with bidentate nitrate groups. Cd–N (axial) is 2.41 A(average) and Cd–N(equatorial) is 2.37 A(average), while Cd–O is the range 2.27–2.60 A, with the anions showing greater asymmetry.

Heavy-metal macrocyclic complexes: crystal and molecular structures of complexes of 1,5,9,13-tetra-azacyclohexadecane with Cd(NO3)2, HgCl2, PbCl2, and Pb(NO3)2

Heavy-metal complexes of the sixteen-membered macrocycle, 1,5,9,13-tetra-azacyclohexadecane (L3), of formulae [Cd(L3)(NO3)(H2O)]NO3·H2O, [Hg(L3)Cl]2[HgCl4], [Pb(L3)Cl]Cl, and [Pb(L3)][NO3]2 have been prepared. Their structures were examined by 13C and 15N n.m.r. spectroscopy, and determined by X-ray crystallography which reveals a macrocycle cavity size which is most suitable for Hg2+co-ordination. In each compound all four macrocyclic N–H groups point to the same side of the macro-cyclic plane. The other ligands occupy trans positions for Cd2+(NO3– and H2O) and Hg2+(Cl–), but are in cis positions for the larger Pb2+. In [Pb(L3)Cl]+ the chloride ligand and the lone pair occupy adjacent positions on one side of the Pb2+ ion. With [Pb(L3)][NO3]2 the five independent molecules include one with two symmetrically bidentate NO3– groups, two with one bidentate and one unidentate, and two Pb atoms linked by two NO3– groups. Bond distance averages are Cd–N 2.334(6), Hg–N 2.38(3), Pb–N 2.54(1), Cd–O 2.413(8)–2.832(8), Hg–Cl 2.64(2)–2.83(2), Pb–Cl 2.98(1), and Pb–O 2.74(3)–3.17(4)A.

Saturation transfer in n.m.r. via low-amplitude spining sidebands

Major artefacts are shown to arise in n.m.r. saturation transfer and nuclear Overhauser difference spectroscopy experiments through the agency of low-amplitude spinning sidebands.