Ligand exchange and solvolytic reactions of99mTcBr 6 2− in non-aqueous media

Prior work had documented that^99mTcCl ₆ ^2– could undergo ready ligand exchange reaction under non-aqueous condition. We now wish to report the ligand exchange reaction of bromine in^99mTcBr ₆ ^2– in non-aqueous solvents using 8-hydroxyquinoline (oxine) as the displacing ligand. Analysis of the products obtained by paper chromatography, HPLC and electrophoresis suggest that a 12 Tcoxine complex appears to be the most stable of the complexes formed, probably^99mTc(oxine)₂ Br₂. However, displacement of bromine by polar solvents (both protic and aprotic) can also occur, both on^99mTcBr ₆ ^2– and in the above complex as a consequence of solvolytic reactions. Other Tc-oxine complexes can also be formed upon ligand exchange, but they appear to be stable only under aprotic, non solvolytic conditions. These studies again document that hexahalotechnetate complexes exhibit ligand exchange reactions under non-aqueous conditions, that they allow the ready synthesis of novel technetium complexes, but that because of their high reactivity the effect of competing reactions must be considered.     

The roles of hetero atoms in solvolytic reactions—II: Transannular participation by neighbouring sulphur atom

Esters of sulphur-containing 5- and 6-membered heterocycles, tetrahydro-3- and 4-thiopyranol and their 3- and 4-methyl, phenyl derivatives, and tetrahydro-2-thiophenemethanol, were synthesized. The solvolysis of β-thioesters in 80% aqueous acetone resulted in strong transannular S-participation and the products of solvolysis substantiated formation of bicyclic episulphonium ion intermediates. Such participation is considered to be present even in a tertiary system. On the other hand, γ-thioesters were solvolysed without any transannular S-participation,     

Magnetic fields and radical reactions: recent developments and their role in nature

Recombination of pairs of radicals is exceptional in being affected by magnetic fields. The mechanism has been known for some thirty years, but recently new applications have appeared and research has been extended to very high fields (up to 30 Tesla). Claims that low electromagnetic fields damage health have led to extensive medical, chemical and physical research: no firm evidence of hazards has emerged; on the other hand, migrating birds orient themselves in the earth's field (50 microT): radical pairs may provide the mechanism.     

The nature of the delocalized cations in azulenic bacteriorhodopsin analogs.

Depending on the size and shape of their azulenic chromophores, azulenic bacteriorhodopsin (bR) pigment analogs can exist as either an initial pigment P1, a more red-shifted final pigment P2 or an equilibrium mixture of both. The absorption spectra of red-shifted bR analogs exhibit characteristic narrow-band shapes similar to charge fully delocalized cyanine-like dyes. Therefore, all such red-shifted pigments are believed to be highly delocalized, bond-equalized carbocations. We have determined structural requirements that facilitate their formation. To describe fully the red-shift potentials of these retinal analogs, we have introduced a new parameter-percent red-shift (PRS). A large PRS value not only reflects the extent of red-shift, but is also suggestive of extensive delocalization of the positive charge. Relevance of these findings in consideration of the possibility of forming stable O-intermediates is presented. The postulated resonance hybrid-like structures for different cations of the positively charged protonated Schiff base chromophores are in fact structurally distinct species, equilibrating in response to local perturbations within the supramolecular protein environment.     

Rotational equilibria in 1,2,6-trisubstituted pyridinium cations and reactions of 2-Isopropylpyrylium cations

2-Isopropyl-6-phenyl- and 2,6-diisopropyl-pyridiniums with bulky 1-substituents show temperature-variable NMR spectra which are interpreted in terms of restricted rotation. 2-Isopropyl-4,6-diphenylpyrylium can be deprotonated at the isopropyl group to give an anhydro base which forms new pyryliums with electrophiles.     

Formation and reactions of aryldiazomethane radical cations

Dicyanoanthracene-sensitized photolysis of aryldiazomethanes affords stilbenes with cis/trans ratios of ～3 via free radical cations; anodic oxidation gives similar results but via radical cations adsorbed on electrode surface.     

Vinyl cations 33 : 1-alkene-3-yne-2-yl triflates,precursors of triple bond stabilized vinyl cations - synthesis and first solvolytic studies.

The α-alkynyl vinyl triflates ($\text{3}$) are synthesized and the solvolyses reactions studied in different solvents. The solvolysis products and kinetic data indicate, that the hitherto unknown triple bond stabilized vinyl cation $\text{2a}$ ? $\text{2b}$ is formed as an intermediate.     

Free radicals in the reactions of organic cations

By using the ESR and CIDNP methods the formation of free radicals in the reaction of nucleophilic and electrophilic substitution is proved. The role of free radicals in the mechanism of these reactions is discussed.     

Layering in cinnamate structures: The role of cations and anion substituents

Four materials have been used to investigate the effect of the cation and ring substituent on crystal packing. In potassium 3-bromo-trans-cinnamate (1), potassium 3-chloro-trans-cinnamate (2) and sodium 3-chloro-trans-cinnamate (3), the cations are accommodated in continuous two-dimensional layers formed by the interaction between cations and anions. The CC double bonds in 1 and 2 are parallel with optimal geometry for photochemical reaction, whereas 3 is disordered. The identity of the halogen substituent influences the structure through halogen-halogen contacts. In the structure of calcium di-trans-cinnamate (4), calcium-anion coordination leads to the formation of ribbons rather than two-dimensional layers.     

The roles of hetero atoms in solvolytic reactions—I: Preferential effect by nitrogen to facilitate carbonyl-oxygen cleavage of esters

3- and 4-Piperidinyl and 2-pyrrolidinylmethyl halides and p-nitrobenzoates were prepared. In the solvolysis, the remarkable difference in the effect of nitrogen on the two kinds of the leaving groups was observed. Halides underwent the solvolysis of S_N1 mechanism, whereas p-nitrobenzoates resulted in the preferential OCO cleavage. The results of IR absorption of ester-carbonyl group and kinetic deuterium isotope effect suggested that in the solvolysis of the esters, an initial solvation may be important for the cleavage of a OCO bond.     

The nature and role of pigments of marine invertebrates

Marine animals, especially those from tropical waters, are often brilliantly coloured, and bright colouration is widespread in both sessile and non-sessile invertebrates. These spectacular natural colours are common in species inhabiting shallow waters, and appear not only in animals exposed to bright light, but also in those living in dark areas where colours are visible only with artificial illumination. Marine organisms also show variation in colour with depth and geographical location, and display great variety in colour patterning. These colour characteristics are the result of several different processes, and serve various purposes - the distribution and function of pigments seems to vary between invertebrate groups. In addition to playing an important role in how marine organisms interact, pigments may be involved in physiological processes. Although nitrogenous pigments predominate, marine organisms contain pigments belonging to all the major structural classes of natural products, as well as some that are unique to the marine environment. This review discusses the nature and significance of such pigments, the chemical and biological processes involved, the factors responsible for and affecting bright colourations, as well as their evolution and speculation as to their function.     

N-heterocyclic phosphenium cations: syntheses and cycloaddition reactions

A series of trifluoromethanesulfonate (OTf) salts of N-heterocyclic phospheniums (NHP) bearing phenyl (1a), para-methoxyphenyl (1b), 2,6-diisopropylphenyl (1c) and mesityl (1d) substituents is reported. The compounds are made by a modification to a literature procedure that improves the overall yields for and by 15 and 23%, respectively. Two unwanted side-products in the synthesis of , the diammonium salt, [(2,6-iPr-C6H3)N(H)2CH2CH2N(H)2(2,6-iPr-C6H3)]Cl2 (4) and the bisphosphine (2,6-iPr-C6H3)N(PCl2)CH2CH2N(PCl2)(2,6-iPr-C6H3) (5), are crystallographically characterized, as is the intermediate cyclic chlorophosphine, C1PN(4-OMe-C6H4)CH2CH2N(4-OMe-C6H4) (3b). The phenyl-substituted NHP is fully characterized, including by X-ray crystallography, for the first time; this compound contains a short P-O contact of 2.1850(14) A. Cycloaddition reactions of with 2,3-dimethyl-1,3-butadiene give the expected spirocyclic phospholeniums, 7,8-dimethyl-1,4-diaryl-1,4-diaza-5-phopshoniaspiro[4.4]non-7-ene, as their OTf salts (6a-d), while reactions with N,N'-dimesityl-1,4-diaza-1,3-butadiene give, except in the case of , which is too bulky to react, the aza analogues, 1,4-dimesityl-6,9-diaryl-1,4,6,9-tetraaza-5-phosphoniaspiro[4.4]non-2-ene (7a, 7b and 7d). The sterically congested is in thermal equilibrium with and free diazadiene, and undergoes a substitution reaction with 2,3-dimethyl-1,3-butadiene to give .     

Ring contraction and other reactions of azidobenzene-and azidotoluene-tricarbonylmanganese cations

Azidobenzene- and azidotoluene-tricarbonylmanganese cations are reduced to the corresponding aniline and toluiden complexes not ony by lithium aluminium hydride but also by azide ion in polar solvents. Direct substitution of the azido group by methoxy occurs with sodium methoxide in methanol. Above 130°C nitrogen is lost from the azido compounds and ring contraction occurs yielding the corresponding (cyanocyclopentadienyl)tricarbonylmanganese complexes.     

Redox reactions between organic halo-compounds and ferrous cations in isopropanol

Summary Halo-compounds of which the half-wave reduction potential is equal to or less negative than that of carbon tetrachloride, are reduced by tetrachloroferrate(II) anions. The bimolecular rate constants measured at 250 in isopropanol are 2 × 10^–5 M^–1 s^–1 and 1 × 10^–4 M^–1 s^–1 respectively for CCl₄ and CCl₃CO₂Me.