The conformational behaviour of some dihydro-1,4-thiazines—III

Oxidation of 1, 3, 5, 13 and 14 by either m-chloroperbenzoic acid or sodium periodate gave single sulphoxides, which are assigned the R configuration. The minor sulphoxide formed by sodium periodate oxidation of 4 and the major sulphoxide obtained in the corresponding oxidation of 6 are assigned the S configuration. The conformational properties of the above sulphoxides have been examined by the NMR method; the derivatives show an overwhelming preference to adopt the conformation with the oxide function in the axial environment.     

The acid-catalysed oligomerisation of p-benzoquinone

p-Benzoquinone is transformed by mineral acids into a series of complex dibenzofuran derivatives. The phenolic trimer 5a is the dominant crystalline species. Small amounts of a helical tetrameric phenol (11) and a non-phenolic, cyclic tetramer the furo-[8]-circulene (6a) are also formed together with compounds of higher molecular weight. The furan rings are not generated by dehydration of 2,2'-dihydroxybiphenyl elements. A mechanism involving quinone hemiketals is suggested.     

Synthesis and conformational behaviour of tetra-anthranilides

The tetra-anthranilide derivatives ($\text{5}$)–($\text{7}$) have been synthesised. X-Ray crystallography shows that $\text{N}$,$\text{N}$′,$\text{N}$″-trimethyltetra-anthranilide ($\text{5}$) adopts a conformation with three $\text{trans}$ - and one $\text{cis}$-amide linkages in the solid state. Dynamic ¹H n.m.r. spectroscopy indicates that $\text{N}$-benzyl-$\text{N}$′,$\text{N}$″,$\text{N}$′″-trimethyltetra-anthranilide ($\text{7}$) exists as an interconverting ternary mixture of chiral non-planar conformational diastereoisomers in solution.     

Thermal behaviour of some phosphorus/chromium polyoxoanions

The thermal degradation of K₂HPCr₂O₁₀, Na₃PCr₃O₁₃ · 3H₂O, K₃PCr₄O₁₆ and (NH₄)₃PCr₄O₁₆ has been investigated by TG and DTA methods. The main decomposition process for the three firstly mentioned compounds occurs at around 400°C and is associated with the rupture of P-O-Cr bonds. The ammonium salt shows a different behaviour and decomposes explosively at 230°C. In the case of K₂HPCr₂O₁₀ a new polyoxoanion of formula K₄P₂Cr₄O₁₉ is generated as an intermediate during the thermal degradation. Detailed stoichiometries, sustained by the TG/DTA results as well as by the chemical and infrared spectroscopic analyses of the solid residues could be given. It is shown that, in all cases, CrPO₄ is one of the decomposition products.     

Triplet halocarbene chemistry: p-nitrophenylchlorocarbene and p-nitrophenylbromocarbene

Reactions of p-nitrophenylchlorocarbene with cumene and of p-nitrophenylbromocarbene with toluene afford C-H abstraction-recombination products that suggest the involvement of triplet arylhalocarbenes.     

pp Homoconjugation in organic radicals

ESR data for various radicals of type R₂?CR₂X, when R is H or alkyl, and X is either Cl, Br or I, or PR₂ and AsR₂, are discussed in terms of pp homoconjugation. For the β-halogen radicals, the small size of the anisotropic coupling provides some evidence for this effect, but for the β-PR₂ and β-AsR₂ radicals, the greatly enhanced isotropic coupling to ³¹P or ⁷⁵As compared with those for the corresponding cation-radicals, R₂?CR₂PR⁺₃ or R₂?CR₂AsR⁺₃, is taken as good evidence for homoconjugation.     

The micro-determination of gold with p-dimethyl-aminobenzylidenerhodanine

Gold can be determined in the range 4–14 μg within 2% by a simple extraction of chlorauric acid in 0.12 N hydrochloric acid with a solution of p-dimethylaminobenzylidenerhodanine in iso-amyl acetate A 1:1 complex is formed with maximum absorption at 515 mμ.     

The determination of cystine with p-aminodimethylaniline

The considerable background colour in previous methods for the absorptiometric determination of cystine by means of p-aminodimethylaniline has been almost eliminated by solvent extraction of the required coloured compound. Alkali treatment of the extract intensifies the colour. The improved test is two and a half times more sensitive than previous tests based on the same reaction and the colour produced has a more sharply defined absorption peak. Cystine (plus cysteine) in protein hydrolysates may be determined with an error of less than 1%.     

The thermal behaviour of some rhodium complexes

The thermal behaviour of several complexes of rhodium has been investigated. Complexes containing nitrogen ligands readily decompose to the oxide, Rh₂O_3^? Complexes with phosphorus and arsenic ligands decompose to the same oxide of rhodium, although difficulty is encountered in removing all the phosphorus and arsenic. The suitability of the decomposition to Rh₂O₃ as an analytical technique for rhodium is discussed.     

The conformational behaviour and P-selectin inhibition of fluorine-containing sialyl LeX glycomimetics

A combination of experimental J/NOE NMR data with molecular mechanics and dynamics calculations has been used to examine the conformational behaviour and assign the configuration of synthetically prepared epimeric 3-carboxymethyl-O-Gal-(1-->1)-alpha-Man-fluoro-C-glycosides. It is shown that the population distributions around the glycosidic linkages strongly depend on the configuration at the fluorinated carbon of the pseudoacetal residue. It is also shown that these compounds resemble the inhibition ability of sialyl LeX towards P-selectin.     

Autoxidation of p-hydroxydiphenylamine

Autoxidation of p-hydroxydiphenylamine (HDPA) gives N-phenyl-p-benzoquinoneimine and hydrogen peroxide as primary products over a wide range of pH and the previously reported formation of N-(4-hydroxyphenyl)-p-benzoquinoneimine in basic media is in error. An induction period which is independent of substrate concentration but decreases with increase in pH is observed. The rate of oxidation is first order with respect to oxygen and half order with respect to HDPA. Oxygen is used in the system at the rate comparable to its rate of dissolution. On the basis of various kinetic parameters, a mechanism involving electron transfer to oxygen from HDPA anion and subsequent formation of HO₂ radical is postulated. The agreement of the experimental results with the derived rate expression is shown to support the mechanism.Singlet oxygen, produced in situ by methylene blue sensitization, leads to oxidation but with different products indicating that this species is not involved in the autoxidation.     

Thermal decomposition kinetics of p-toluenesulphonamide and some of its n-halogeno and n-metal derivatives

The TG, DTG and DTA traces of p-toluene sulphonamide, chloramine-T, dichloramine-T, dibromamine-T and the Ag and Hg derivatives of p-toluene sulphonamide are given and interpreted. The kinetic parameters (energies and entropies of activation and pre-exponential factors) for the main decomposition stages have been calculated.     

The crystal structures of m-trifluoromethylphenylsilatranone and p-fluorophenylsilatranone

The crystal structures of two silatranone derivatives are reported. The close N → Si approach (2.106(3) Å in m-trifluoromethylphenyl-, and 2.129(3) Å in p-fluorophenyl-silatranone) indicates strong dative acceptor bonds. For various silatrane derivatives and inverse relation has been revealed between the mean group electronegativity of the substituent R attached to silicon and the N → Si dative bond distance. In both structures there are long (1.72 Å) SiO bonds in the SiOCO moiety. The m-trifluoromethylphenyl derivative contains a disordered CF₃ group.     

The photolysis of poly(p-hydroxystyrene)

The photochemistry of poly(p-hydroxystyrene) (PPHS) has been investigated, thin films being exposed to 254 nm radiation under high vacuum at 298K. H₂ is the principal volatile product, but small amounts of H₂O are also formed. The polymer becomes rapidly coloured, but unlike other styrene polymers, which remain yellow, PPHS undergoes a progressive colour change from yellow through orange to brown. Infrared spectral data indicate the depletion of OH group concentration, and a concomitant formation of carbonyl, possibly quinonoid species. Crosslinking is rapid and no chain scission was detected. It is likely that the polymeric phenoxy radicals formed by photolysis or by subsequent H-abstraction contribute to the formation of both quinonoid species and crosslinks. The mechanism of the photolysis is discussed.     

The determination of conformational effects in some fluorobenzenes by a combined mo/coupling constant treatment

The correlation of J_meta^FF in some 1-substituted-2,6-difluorobenzenes ( 1 ) with the bond order matrix of the fluorine 2p(π) orbitals provides information on the orientation of the substituents and the position of the counter-ion in the anilinium salt. The calculated charge densities in the analogous anilinium fluoride ion pair are compared with those of the free ion and neutral molecule.     

Metalations utilizing aryllithiums; ortho-functionalization of p-bromoanisole (pBrA)

An ortho-metalation protocol has been developed, which permits the survival of a bromine substituent in p-bromoanisole. Eight derivatives of the generated ortho-lithiated intermediate have been prepared. A neglected metalation concept is being explored here; one which proposes that minimizing the pK_a difference between the aryl substrate and the conjugate acid of the metalating agent will lead to a regiospecific and selective metalation process.