C. Richard Hall

Phosphorus stereochemistry : Mechanistic implications of the observed stereochemistry of bond forming and breaking processes at phosphorus in some 5- and 6-membered cyclic phosphorus esters

Abstract The stereochemistry of endocyclic and exocyclic bond forming and breaking processes in 5- and 6-membered cyclic phosphorus esters is summarised and comparisons are made with analogous reactions in acyclic phosphorus esters. The factors that determine which bonds are broken and whether reactions occur with inversion or retention of configuration at phosphorus are complex and usually have more obvious effects for reactions in cyclic than in acyclic phosphorus esters ; in particular conformational effects may be important. The stereochemistry of migration of phosphorus ester groups across 1,3-diols is also described. It is suggested that nucleophilic substitutions at phosphorus are inherently stereospecific in the sense that trigonal bipyramidal reaction intermediates break down either directly or following a single Berry Pseudorotation or Turnstile rotation process. Multiple Turnstile rotations which would lead to racemisation, and which apparently do occur in stable phosphoranes, are insignificant for reactions involving trigonal bipyramidal intermediates.

An examination of how exposure to humid air can result in changes in the adsorption properties of activated carbons

Abstract Dynamic water adsorption isotherms and methanol and hexane breakthrough curves have been measured for samples of ASC, BPL and 208A activated carbons under conditions appropriate to their use in vapor filters. The differences in performance between control and samples that had previously been exposed to air at RH 80% and 22°C is discussed in terms of surface oxidation. Comparison of the breakthrough of methanol vapor through samples of dry charcoal is a convenient way of monitoring surface changes.

1,3,2-Oxazaphospholidines from (–)-ephedrine. Intermediates for the stereospecific synthesis of optically active dialkyl alkylphosphonothioates and -selenoates, trialkyl phosphoro-thioates and -selenoates, dialkyl methylphosphonates, and trialkyl phosphates

The configurations of the cis- and trans-isomers of 2-substituted 1,3,2-oxazaphospholidin(e)-2-ones, -2-thiones, and 2-selones derived from (–)-ephedrine have been established by spectroscopic and chemical methods. Displacements of the exocyclic 2-substituents occur with retention of configuration at phosphorus. With sodium alkoxides, the 1,3,2-oxazaphospholidine ring is opened by P–N rather than P–O bond cleavage, and inversion of configuration at phosphorus is observed. Under basic conditions the 2-methylamino-1-phenylpropyl phosphates derived from (–)-ephedrine rearrange to afford aziridines and phosphoric acid derivatives; in the cases of the sulphur- and selenium-containing derivatives optically active phosphorus thioacids and phosphorus selenoacids are formed and can be isolated as SMe and SeMe derivatives. These latter derivatives are converted into O-alkyl derivatives on treatment with alcohols in the presence of alkoxides or bromine or silver nitrate, and the stereoselectivities of these reactions have been determined. The optical purities of the title compounds have been measured by an n.m.r. method using chiral shift reagents.

The preparation and properties of some activated carbons modified by treatment with phosgene or chlorine

Abstract Treatment of coal or coconut shell activated carbons with either phosgene or chlorine at 180°C followed by washing with methanol or water results in chlorinated carbons with very similar pore structures to their precursors. Water adsorption experiments show that the modified materials are relatively Hydrophobic, presumably as a result of replacement of oxygen-containing surface groups by chlorine. Adsorption of the model hydrophobic vapour chloropicrin from humid air is usually better for the modified carbons which, unlike the controls, do not appear to be subject to degradation in performance on ageing in a humid atmosphere. The stability of the modified carbons is probably a consequence of the chlorination of those sites on the control carbons that are prone to ready oxidation or hydroxylation.

Differences between some biological properties of enantiomers of alkyl S-alkyl methylphosphonothioates.

BASELT, R. C. & CASARETT, L. J. (1972). Biochem. Pharmac., 21, 2705-2712. BECKETT, A. H., MITCHARD, M. & SHIHAB, A . A. (1971). J. Pharm. Pharmac., 23, 347-353. BECKETT, A. H. , VAUGHAN, D. P. & ESSIEN, E. E. (1972). Ibid., 24, 244. DAVIS, C. M. & FENIMORE, D. C. (1975). J . Chromat., 104, 193-195. MISRA, A. L., MULE, S. J . , BLOCK, R. & VADLAMANI, . L. (1973). J. Pharmac. exp. Ther., 185, 287-299. MISRA, A. L., BLOCH, R., VADLAMANI, . L. & MULL, S. J. (1974). Ibid., 188, 3 4 4 4 . SULLIVAN, H. R . & DUE, S. L. (1973). J . medl Chem., 16, 909-913. SULLIVAN, H. R., DUE, S. L. & MCMAHON, R. E. (1973). J. Pharm. Pharmac., 25, 1009-1010. Clin. Pharmac. Ther., 17, 258-266.

The preparation and properties of some chlorinated activated carbons, part II. Further observations

Abstract The adsorptive properties of chlorinated coal or nutshell carbons depend on the chlorination temperature. Chlorination at about 180°C, followed by extraction with water or methanol, yields carbons that have similar adsorption properties and are relatively hydrophobic compared to the controls. Under these conditions, stable surface carbon-chlorine complexes are formed, and the function of extraction is solely to displace strongly physically adsorbed chlorine. At lower chlorination temperatures, the hydrophobicity of the carbon surface is dependent on the chemical used for the extraction step; compared to the controls, the carbon surface is relatively hydrophilic after extraction with water and relatively hydrophobic after extraction with methanol. it is probable that at chlorination temperatures below about 180°C, weak carbon-chlorine complexes form, which chemically react during extraction. Chlorination at low temperatures may therefore offer a way of introducing different functional groups onto carbon surfaces. Unlike the controls, none of the treated carbons degrade (age) during storage in high humidity air. The suitability of carbons chlorinated at about 180°C for vapour filtration in situations where ageing effects degrade adsorption performance is confirmed from breakthrough measurements using chloropicrin, toluene, and carbon tetrachloride. Such carbons also afford some resistance to oxidation with the pollutant NO2.

ASYMMETRIC TETRACOORDINATE PENTACOVALENT ORGANOPHOSPHORUS COMPOUNDS: THEIR RESOLUTION AND DETERMINATION OF ENANTIOMERIC PURITY

Abstract During the last twenty to thirty years there has been considerable interest in the preparation of completely resolved enantiomers of tetracoordinate pentacovalent organophosphorus compounds. This has arisen for three main reasons:

Ring opening of 1,3,2-oxazaphospholidine-2-thiones with alkyl lithiums; synthesis of chiral phosphinothioic acids

Abstract Chiral phosphinothioic acids are prepared by a novel synthesis involving an unexpected P  O bond cleavage with retention of configuration at phosphorus.

The preparation and some reactions of 1,3,2-thiazaphospholidine-2-ones

Abstract 1,3,2-Thiazaphospholidine-2-ones have been prepared by rearrangement of the corresponding 1,3,2-oxazaphospholidine-2-thiones. The stereochemical course of ring opening reactions has been investigated.

Stereochemistry of some reactions between alkyl S-methyl methylphosphonothioates and chiral alkoxides

Abstract With R-(+) ethyl (or methyl) S-methyl methylphosphonothioate and (+)-pinacolyl alkoxide competitive and highly stereoselective displacements of O-alkyl and S-methyl occur, both reactions being with inversion of configuration. With the enantiomeric S-(-) ethyl (and methyl) S-methyl methylphosphonothioates and (+)-pinacolyl alkoxide the reactions, although still competitive, are no longer stereoselective. In contrast similar reactions with the sodium salt of (-)-menthol, (which might be considered to be the mirror image of (+)-pinacolyl alkoxide) occur highly stereoselectively with the S-(-) but not with R-(+) enantiomers. The displacement of O-alkyl from alkyl S-methyl methyl-phosphonothioates by ethoxide, pinacolyl alkoxide and menthyl alkoxide is not observed when methoxide is the nucleophile; in this case only displacement of S-alkyl group occurs.