C. Barry Thomas

The reactions of alkynes, cyclopropanes, and benzene derivatives with gold(III)

The reaction of gold(III) compounds with alkynes, arenes, and cyclopropanes have been studied. With the first two, the products can be accounted for in terms of electrophilic attack by the metal on the unsaturated centre. The reaction with alkynes bears a close resemblance to the mercury(II)-catalysed hydration but leads to higher conversions. Aromatic compounds react to give chloroarenes, the isomer distribution being consistent with initial metallation of the ring followed by replacement of the dichlorogold group by a chlorine atom. However, isolation of the intermediate arygold compounds did not prove possible. Cyclopropanes were not oxidised to the expected 1,3-adducts; instead 1,2-adducts resulting from initial gold(III)-catalysed isomerisation to an alkene were obtained.

Gold(III) as an electrophilic oxidant of alkenes

The products of oxidation of a variety of olefins by tetrachloroauric acid in methanol are reported. They are satisfactorily accounted for by a scheme in which gold(III) acts as an electrophile to give an organometallic adduct; this breaks down by heterolysis of the C–Au bond, accompanied by competition between rearrangement of a neighbouring substituent and uptake of a nucleophile. In all cases a close similarity was observed with the products obtained from oxidations by thallium(III) and lead(IV); where divergences occur they can be accounted for in terms of either the different ligands about the metal atom or the different leaving-group abilities of the metal substituents.

A comparative study of the oxidation of oct-1-ene by mercury(II), thallium(III), and lead(IV) acetates in methanol

A comparative study has been made of the oxidation by mercury(II), thallium(III), and lead(IV) acetates of a monosubstituted aliphatic olefin (oct-1-ene) in methanol. Reaction proceeds in two stages. In the first step, electrophilic addition occurs to give an organometallic adduct in which a methoxy-group is incorporated at C-2. There is no evidence for concerted addition of the two substituents. The adduct from mercury(II) is essentially stable under the reaction conditions but those from thallium(III) and lead(IV) decompose in three competing ways: (i) an acetate group is transferred from the metal atom to C-1 in an SNi process; (ii) a carbonium ion is generated on heterolysis of the carbon–metal bond and this undergoes competitive hydride shift and nucleophilic attack; (iii) anchimeric assistance to heterolysis is provided by the neighbouring methoxy-substituent which preferentially migrates to C-1 on collapse of the resultant ion-pair. The relative importance of these three processes is governed by the leaving-group capacity of the metal substituent and by the nature of the ligands on the metal.

Oxidation of oct-1-ene and trans-oct-4-ene by lead(IV), thallium(III), and mercury(II) acetates

The comparative study of the oxidation of oct-1-ene by lead(IV), thallium(III), and mercury(II) acetates in methanol has been extended to other solvents and to the oxidation of an internal olefin, trans-oct-4-ene. The general rules developed for the methanol system are found to be applicable to these solvents. In acetic acid, incorporation of an acetoxy-substituent on C-2 of oct-1-ene followed by decomposition of the organometallic adduct via an acetoxonium ion gives a high yield of hydroxy-acetates. This yield is boosted in lead(IV) oxidations by the addition of water to the system. The adduct from mercury(II) oxidation is stable but reduction of it with borohydride does not appear to offer a clean means of adding a carboxylic acid across a double bond: substantial reduction of the ester function occurs. There appear to be severe steric constraints to the oxidation of trans-oct-4-ene by initial electrophilic addition of the metal acetates but when this does occur there is a stereoselective preference for anti-addition. However, the major products are allylic. It is suggested that, in the case of lead(IV) oxidations, these arise by a homolytic process; indeed there is no evidence of heterolytic attack of this oxidant on oct-4-ene.

Reactions of palladium(II) with organic compounds. Part II. Oxidation of some benzenoid compounds in trifluoroacetic acid

Benzene, some monosubstituted benzenes, and di- and poly-methylated benzenes are readily oxidised by palladium(II) in trifluoroacetic acid. Benzene and most monosubstituted analogues give mainly or solely biaryls, as do o-xylene, m-xylene, and hemimellitene (1,2,3-trimethylbenzene). Most of the possible isomers were found in each case but the predominant ones are those in which one of the aryl groups is bonded to the second through the position most susceptible to electrophilic attack. Toluene, ethylbenzene, p-xylene, and pseudocumene (1,2,4-trimethylbenzene) give, in addition, diarylmethanes, and from mesitylene and durene (1,2,4,5-tetramethylbenzene) these latter compounds were the only products detected. Diarylmethane formation, in most cases, was of necessity selective but, even in those instances where more than one such compound might have been expected, only the anticipated product of electrophilic arylmethylation was observed. Mechanisms which account for the formation of the two types of product are discussed.

Neighbouring-group effects in the oxidation of olefins by mercury(II), thallium(III), and lead(IV) acetates

The participation of neighbouring groups during oxidation of olefins by mercury(II), thallium(III), and lead(IV) acetates has been investigated. With the examples chosen lead(IV) reacts exclusively at the neighbouring group (aryl or phenolic), there being no evidence for electrophilic attack on the double bond. Similarly, mercury(II) is capable of substituting an aromatic ring rather than reacting at an olefinic site; however, the latter reaction can be made to predominate in a more polar medium. The initial ion formed during thallium(III) oxidation is less selective towards nucleophilic attack than the corresponding mercury(II) species, and this is attributed to greater interaction between the metal atom and the positive centre in a mercurio- than in a thallio-cation. Whereas the organomercury adducts are stable to the reaction conditions, in thallium(III) oxidations suitable substituents can trap carbocation-like species during both the formation and the destruction of the adducts.

Selective nitration of aromatic substrates: reaction of nitrogen dioxide with arylthallium(III) compounds

A method of selectively nitrating aromatic substrates is described in which nitrogen dioxide displaces a thallium(III) substituent electrophilically. High yields of the para-isomer were obtained from alkylbenzenes and a number of other substrates containing ortho–para-directing groups. Similar specificity was observed with xylenes but the process proved only partially successful when applied to mesitylene where, it is postulated, steric interaction between the methyl groups and the bulky thallium substituent inhibits the reaction. Thermodynamic control leading to meta-isomers proved impossible, a low yield of nitroarenes being obtained, the bulk of which originates by direct attack of nitrogen dioxide on regenerated aromatic substrate. Compounds with side-chains which have been reported to direct the incoming thallium substituent to an ortho-site also failed to react and an explanation for this is offered.

Nitration of benzenoid compounds by palladium salts and by nitrogen dioxide

The nitration of benzenoid compounds by palladium nitrate and by a mixture of palladium acetate and sodium nitrite has been studied under a variety of conditions. The latter system yields nitrobenzene semi-catalytically in chloroacetic acid at 100°(ca. 1200% in an oxygen atmosphere), with relatively little by-product (ester) formation. The use of nitrogen dioxide in place of sodium nitrite results in a fully catalytic system. Observations which bear on the mechanisms are discussed. Aromatic nitration has also been effected in good yield by passing nitrogen dioxide into a solution of the aromatic compound in trifluoroacetic acid. In contrast to the reactions involving palladium(II), this is almost certainly an SE2 process involving the nitronium ion.

The mechanism of the rearrangement of ?-acyloxyalkyl radicals

Determination of the distribution of products from decarbonylation of 2-formyl-1,1-dimethylethyl heptanoate (6d) and other esters of 3-hydroxy-3-methylbutanal, and from the reaction of tributylstannane with 2-bromo-1,1-dimethylethyl benzoate (11b), 2-bromo-1-methyl-1-phenylethyl acetate (11c), and similar bromo-esters, has enabled the rates of rearrangement of some β-acyloxyalkyl radicals to be evaluated. The kinetic data together with the results of reactions using 18O labelled substrates indicate that 1,2-acyloxy-transfer proceeds via a concerted mechanism involving a five-membered cyclic transition state. The rate of rearrangement of the 2-acetoxy-2-phenylpropyl radical (12c) by 1,2-aryl migration has also been determined.

Reactions of palladium(II) with organic compounds. Part IV. Oxidation of some substituted stilbenes

The product distribution and the relative rates of reaction of some substituted with palladium(II) chloride in aqueous 1,2-dimethoxyethane are consistent with there being small but significant carbocation character in the transition state of the oxypalladation step.