HF Solvolysis Research Articles

Ennobling an Old Molecule: Thiazyl Trifluoride (N≡SF3), a Versatile Synthon for Xe–N Bond Formation

The fields of sulfur-nitrogen-fluorine chemistry and noble-gas chemistry have been significantly extended by the syntheses and characterizations of four new Xe-N-bonded cations derived from N≡SF(3). The adduct-cation, F(3)S≡NXeF(+), has provided the entry point to a significant chemistry through HF solvolysis of the coordinated N≡SF(3) ligand and HF-catalyzed and solid-state rearrangements of F(3)S≡NXeF(+). The HF solvolyses of [F(3)S≡NXeF][AsF(6)] in anhydrous HF (aHF) and aHF/BrF(5) solutions yield the F(4)S═NXe(+) cation, which likely arises from an HF-catalyzed mechanism. The F(4)S═NXe(+) cation, in turn, undergoes HF displacement to form F(4)S═NH(2)(+) and XeF(2), as well as HF addition to the S═N bond to form F(5)SN(H)Xe(+). Both cations undergo further solvolyses in aHF to form the F(5)SNH(3)(+) cation. The F(4)S═NXe(+) and F(4)S═NH(2)(+) cations were characterized by NMR spectroscopy and single-crystal X-ray diffraction and exhibit high barriers to rotation about their S═N double bonds. They are the first cations known to contain the F(4)S═N- group and significantly extend the chemistry of this ligand. The solid-state rearrangement of [F(3)S≡NXeF][AsF(6)] at 22 °C has yielded [F(4)S═NXe][AsF(6)], which was characterized by Raman spectroscopy, providing the first examples of xenon bonded to an imido nitrogen and of the F(4)S═N- group bonded to a noble-gas element. The rearrangement of [F(3)S≡NXeF][AsF(6)] in a N≡SF(3) solution at 0 °C also yielded [F(4)S═NXe−N≡SF(3)][AsF(6)], which represents a rare example of a N−Xe−N linkage and the first to be characterized by X-ray crystallography. Solvolysis of N≡SF(3) in aHF was previously shown to give the primary amine F(5)SNH(2), whereas solvolysis in the superacid medium, AsF(5)/aHF, results in amine protonation to give [F(5)SNH(3)][AsF(6)]. Complete structural characterizations were not available for either species. Isolation of F(5)SNH(2)·nHF from the reaction of N≡SF(3) with HF has provided a structural characterization of F(5)SNH(2) by Raman spectroscopy. Crystal growth by sublimation of F(5)SNH(2)·nHF at -30 to -40 °C has resulted in the X-ray crystal structure of F(5)SNH(2)·2[F(5)SNH(3)][HF(2)]·4HF and structural characterizations of F(5)SNH(2) and F(5)SNH(3)(+). The redox decomposition of [F(4)S═NXe−N≡SF(3)][AsF(6)] in N≡SF(3) at 0 °C generated Xe, cis-N(2)F(2), and [F(3)S(N≡SF(3))(2)][AsF(6)].

Solid-State and Solution Rearrangements of F3S≡NXeF+Leading to the F4S═NXe+Cation; Syntheses, HF Solvolyses, and Structural Characterizations of [F4S═NXe][AsF6] and [F4S═NH2][AsF6

The salt, [F(4)S=NXe][AsF(6)], has been synthesized by the solid-state rearrangement of [F(3)S[triple bond]NXeF][AsF(6)] and by HF-catalyzed rearrangement of [F(3)S[triple bond]NXeF][AsF(6)] in anhydrous HF (aHF) and HF/BrF(5) solvents. The F(4)S=NXe(+) cation undergoes HF solvolysis to form F(4)S=NH(2)(+), XeF(2), and the recently reported F(5)SN(H)Xe(+) cation. Both [F(4)S=NXe][AsF(6)] and [F(4)S=NH(2)][AsF(6)] have been characterized by (129)Xe and (19)F NMR spectroscopy in aHF and HF/BrF(5) solvents and by single-crystal X-ray diffraction. The [F(4)S=NXe][AsF(6)] salt was also characterized by Raman spectroscopy. The Xe-N bond of F(4)S=NXe(+) is among the shortest Xe-N bonds presently known (2.084(3) A), and the cation interacts with the AsF(6)(-) anion by means of a Xe---F-As bridge in which the Xe---F distance (2.618(2) A) is significantly less than the sum of the Xe and F van der Waals radii. Both F(4)S=NXe(+) and F(4)S=NH(2)(+) exhibit trigonal bipyramidal geometries about sulfur, with nitrogen in the equatorial plane and the nitrogen substituents coplanar with the axial fluorine ligands of sulfur. The F(4)S=NH(2)(+) cation is isoelectronic with F(4)S=CH(2) and, like F(4)S=CH(2), has a high barrier to rotation about the S=N double bond and to pseudorotation of the trigonal bipyramidal F(4)S=N- moiety. The solution and solid-state rearrangements of F(3)S[triple bond]NXeF(+) to F(4)S=NXe(+) are proposed to result from attack at sulfur by fluoride ion arising from HF in solution and from the AsF(6)(-) anion in the solid state. Quantum-chemical calculations were employed to calculate the gas-phase geometries, charges, bond orders, valencies, and vibrational frequencies of F(4)S=NXe(+) and F(4)S=NH(2)(+). The F(4)S=NXe(+) cation provides the first example of xenon bonded to an imido-nitrogen, and together with the F(4)S=NH(2)(+) cation are presently the only cations known to contain the F(4)S=N-group. Both cations are intermediates in the HF solvolysis pathways of F(3)S[triple bond]NXeF(+) which lead to F(5)SN(H)Xe(+) and F(5)SNH(3)(+), and significantly extend the chemistry of the F(4)S=N-group.

F5SN(H)Xe+; a Rare Example of Xenon Bonded to sp3-Hybridized Nitrogen; Synthesis and Structural Characterization of [F5SN(H)Xe][AsF6

The salt [F5SN(H)Xe][AsF6] has been synthesized by the reaction of [F5SNH3][AsF6] with XeF2 in anhydrous HF (aHF) and BrF5 solvents and by solvolysis of [F3S triple bond NXeF][AsF6] in aHF. Both F5SN(H)Xe(+) and F5SNH3(+) have been characterized by (129)Xe, (19)F, and (1)H NMR spectroscopy in aHF (-20 degrees C) and BrF5 (supercooled to -70 degrees C). The yellow [F5SN(H)Xe][AsF6] salt was crystallized from aHF at -20 degrees C and characterized by Raman spectroscopy at -45 degrees C and by single-crystal X-ray diffraction at -173 degrees C. The Xe-N bond length (2.069(4) A) of the F5SN(H)Xe(+) cation is among the shortest Xe-N bonds presently known. The cation interacts with the AsF6(-) anion by means of a Xe---F-As bridge in which the Xe---F distance (2.634(3) A) is significantly less than the sum of the Xe and F van der Waals radii (3.63 A) and the AsF6(-) anion is significantly distorted from Oh symmetry. The (19)F and (129)Xe NMR spectra established that the [F5SN(H)Xe][AsF6] ion pair is dissociated in aHF and BrF5 solvents. The F5SN(H)Xe(+) cation decomposes by HF solvolysis to F5SNH3(+) and XeF2, followed by solvolysis of F5SNH3(+) to SF6 and NH4(+). A minor decomposition channel leads to small quantities of F5SNF2. The colorless salt, [F5SNH3][AsF6], was synthesized by the HF solvolysis of F3S triple bond NAsF5 and was crystallized from aHF at -35 degrees C. The salt was characterized by Raman spectroscopy at -160 degrees C, and its unit cell parameters were determined by low-temperature X-ray diffraction. Electronic structure calculations using MP2 and DFT methods were used to calculate the gas-phase geometries, charges, bond orders, and valencies as well as the vibrational frequencies of F 5SNH3(+) and F5SN(H)Xe(+) and to aid in the assignment of their experimental vibrational frequencies. In addition to F5TeN(H)Xe(+), the F5SN(H)Xe(+) cation provides the only other example of xenon bonded to an sp (3)-hybridized nitrogen center that has been synthesized and structurally characterized. These cations represent the strongest Xe-N bonds that are presently known.

Structural characterisation and enzymic modification of the exopolysaccharide produced by Lactococcus lactis subsp. cremoris B891

Lactococcus lactis subsp. cremoris B891 grown on whey permeate produced an exopolysaccharide containing d-Gal and d-Glc in a molar ratio of 2:3. The polysaccharide was partially O-acetylated. By means of HF solvolysis, O-deacetylation, enzymic modification, sugar linkage analysis and 1D/2D NMR studies the exopolysaccharide was shown to be composed of repeating units with the following structure:

Structure determination of an exopolysaccharide from an alkaliphilic bacterium closely related to Bacillus spp.

An exopolysaccharide obtained from an alkaliphilic bacterium closely related to Bacillus spp. was found to contain D-galactopyranuronic acid (GalpA), 2,4-diacetamido-2,4,6-trideoxy-D-glucopyranose (QuipNAc4NAc), 2-acetamido-2-deoxy D-mannopyranuronic acid (ManpNAcA) and one uncommon unit of D-galactopyranuronic acid with the carboxyl group amide-linked to glycine [GalpA(Gly)]. The polysaccharide was studied by one-dimensional and two-dimensional 1H-NMR and 13C-NMR spectroscopy both on native polysaccharide and on monosaccharides and oligosaccharides obtained from methanolysis and from anhydrous HF solvolysis. The following linear structure of the repeating unit was established: -->3)-alpha-D-GalpA(Gly)-(1-->4)-beta-D-ManpNAcA-(1-->4)-alp ha-D-Galp A-(1-->3)-alpha-D-QuipNAc4NAc-(1-->. A preliminary phylogenetic assignment for the bacterium is also reported.

Structural studies of the O-antigenic polysaccharide from Escherichia coli O167.

The structure of the O-antigenic polysaccharide from Escherichia coli O167:H5 has been investigated. Sugar and methylation analyses, fast-atom-bombardment mass spectrometry and 1H- and 13C-NMR spectroscopy were the main methods used. The structure of the repeating unit of the polysaccharide was found to be: [formula in text]. Oligosaccharide derivatives of the polysaccharide were obtained by HF solvolysis and by a Smith degradation. Furthermore, base treatment of the polysaccharide led to a degraded polymeric material. For the methylated polysaccharide the amide linkage between alanine and the galacturonic acid residue was reductively cleaved with LiBD4 in ethanol, to give, among other things, a 3-O-methyl galactose derivative.

Mouse gastric mucin: cloning and chromosomal localization.

Mucins protect gastric epithelium by maintaining a favourable pH gradient and preventing autodigestion. The purpose of this study was to clone a mouse gastric mucin which would provide a foundation for analysis of mucin gene regulation. Mucin was purified from the glandular portion of gastric specimens and deglycosylated by HF solvolysis. Antibodies against native and deglycosylated mouse gastric mucin (MGM) were raised in chickens. Screening of a mouse stomach cDNA library with the anti-(deglycosylated MGM) antibody yielded partial clones containing a 48 bp tandem repeat and 768 bp of non-repetitive sequence. The 16-amino-acid tandem repeat has a consensus sequence of QTSSPNTGKTSTISTT with 25% serine and 38% threonine. The MGM tandem repeat sequence bears no similarity to previously identified mucins. The MGM non-repetitive region shares sequence similarity with human MUC5AC and, to a lesser extent, human MUC2 and rat intestinal mucin. Northern blot analysis reveals a polydisperse message beginning at 13.5 kb in mouse stomach with no expression in oesophagus, trachea, small intestine, large intestine, caecum, lung or kidney. Immunoreactivity of antibodies against deglycosylated MGM and against a synthetic MGM tandem repeat peptide was restricted to superficial mucous cells, antral glands and Brunner's glands in the pyloric-duodenal region. DNA analysis shows that MGM recognizes mouse and rat DNA but not hamster, rabbit or human DNA. The MGM gene maps to a site on mouse chromosome 7 homologous to the location of a human secretory mucin gene cluster on human chromosome 11p15. Due to sequence similarity and predominant expression in the stomach, the MGM gene may be considered a MUC5AC homologue and named Muc5ac.

A multichambered apparatus for HF solvolysis experiments: reaction of cellulose HF solvolysis products with acetic acid and acetic anhydride

Solvolysis of polysaccharides in anhydrous HF has not gained wide acceptance as an analytical tool despite superior reaction characteristics as compared to conventional hydrolytic agents. This reluctance seems primarily related to difficulties in handling HF. However, these difficulties are substantially reduced by an inexpensive, fully-enclosed, many-chambered reactor suitable for use in HF experiments as described in this article. The reactor has the advantages of multiple independent reaction vessels, means to precisely measure liquid HF, means to add measured volumes of reagents directly to the closed reaction vessels during the course of the solvolysis, and provision for rapid temperature changes and control. A series of experiments exploring the direct transition from HF solvolysis to acetolysis conditions by the addition of acetic acid and acetic anhydride, as an alternative to previously published solvolysis termination schemes, is also described.

Determination of the pattern of methyl esterification in pectin. Distribution of contiguous nonesterified residues

A method is described for determining the distribution of contiguous nonesterified galacturonic acid residues within pectins. First, the esterified galacturonic acids are converted to galactose by reduction with sodium borohydride, then the glycosidic linkages of the resulting galactose residues are cleaved selectively by liquid HF solvolysis. Separation and quantitation of the resulting galacturonic acid containing oligomers reveals the proportion of each stretch of contiguous nonesterified galacturonic acid residues in the original pectin. The distribution of nonesterified (GalA in a pectin fraction obtained from cotton suspension culture cell walls with ≈50% esterification appears to be far from random.

Solvolysis of lignocellulose by anhydrous hydrogen fluoride vapor: Yield comparisons from four cellulosic substrates

Water soluble sugar yields and total product recoveries from the vapor-phase HF solvolysis of four ligno-cellulose substrates were compared: Bigtooth Aspen wood, prehydrolyzed Bigtooth Aspen wood, filter paper, and a synthetic lignocellulose composite substrate containing 23·3% wt lignin. The lignocellulose composite substrates were prepared by solvent-impregnating milled wood lignin onto filter paper. Single wafers of each substrate were contacted with an 100% HF vapor stream at 30°C and 1·0 atm and reacted to completion. After mild posthydrolysis of water-soluble products, glucose yields averaged 84% of theoretical for wood (87% including glucose trapped in solid fraction) and 93–94% of theoretical for prehydrolyzed wood, filter paper, and the lignocellulosic composite substrate. Total measured product recoveries averged 85% for wood vs 93–94% for the other three substrates. Reactions of HF vapor with the lignocellulosic composite substrate indicated that glucosyl fluoride from solvolysis of the cellulose fraction did not condense with lignin to form a water-insoluble adduct. Lowered glucose and xylose recoveries from wood may be due to uncertainties in composition of wood and possible reactions of HF with hemicellulose/lignin fractions.

A microapparatus for liquid hydrogen fluoride solvolysis: Sugar and amino sugar composition of Erysiphe graminis and Triticum aestivum cell walls

The assembly and use of a simple and safe apparatus for HF solvolysis of microgram amounts of cell walls, polysaccharides, or glycoproteins are described. Using this apparatus the cell wall composition of Erysiphe graminis was compared with that of its wheat host. The HF solvolysis combined with TFA posthydrolysis considerably increased sugar yields compared with TFA hydrolysis alone, due mainly to increased yields of glucose from wheat, and glucosamine from Erysiphe, corresponding to cellulose and chitin, respectively. A potentially useful method for determining amounts of fungal hyphae in plant tissue is also provided.

Chemical deglycosylation of carcinoembryonic antigen for amino acid sequence studies

Two chemical methods are compared for the deglycosylation of the highly glycosylated glycoprotein, carcinoembryonic antigen (CEA). Solvolysis of CEA in anhydrous HF removed all of the carbohydrate except for N-glycosidically linked GlcNAc. CEA deglycosylated by HF solvolysis has a single polypeptide chain, which has been sequenced in high yield through the first 30 amino terminal residues. It retained 15% of its original antigenic activity in a radioimmunoassay and was able to inhibit up to 85% of the binding of intact CEA to anti-CEA. CEA deglycosylated by mild methanolysis had 3% carbohydrate remaining and was concomitantly split into at least 10 peptides, which appear to be amenable to further sequence studies.