The pressure dependence of rates of homolytic fission of metal–ligand bonds in aqueous solution

The volume of activation for the exclusively homolytic decomposition of protonated 4-pyridylmethylchromium(III) ion in aqueous HClO4 at 63.4 °C is +19 cm3 mol−1, with negligible dependence on pressure up to 350 MPa at least. The origins of the strongly positive volumes of activation that characterize homolysis of complex cations in aqueous solution are examined.

Molecular rearrangements. Part XX. Thermolysis of carboxylic acid esters

Thermolysis of phenyl benzoate on heating under reflux for ca. 10 days in a nitrogen atmosphere gives phenol, o- and p-hydroxy biphenyl, biphenyl, and carbon monoxide. Phenyl phenylacetate under the same conditions gives toluene, bibenzyl, stilbene, phenol, o- and p-benzylphenol, 9-phenylxanthene, water, and carbon monoxide. Thermolysis of benzyl benzoate is accompanied by decarboxylation and the formation of toluene, diphenylmethane, bibenzyl, stilbene, and biphenyl. Thermolysis of benzyl phenylacetate proceeds smoothly, affording carbon dioxide, toluene, bibenzyl, and stilbene. It was concluded that thermolyses of esters proceeds through a free radical mechanism involving homolytic fission of either the O—CO or O–alkyl bond with subsequent decomposition of the radicals so formed.

Reactions of cyclohexenyl halides with tributylstannane. Stereoelectronic effects on SH2 reactions at halogen

Allylic halogenation of t-butylcyclohexene (2a) with t-butyl hypochlorite or N-bromosuccinimide affords mixtures containing mainly trans-5-t-butyl-3-chlorocyclohexene (2b) and the corresponding trans bromo compound (2c) respectively. The copper-catalysed reaction of (2a) with t-butyl perbenzoate shows similar but less pronounced stereoselectivity. Halogen-atom abstraction by tributyltin radicals, generated from tributylstannane, proceeds more rapidly from the trans halides (2b) and (2c) than from their cis epimers (3b) and (3c). These results suggest that homolytic fission or formation of allylic C-halogen bonds is sensitive to stereoelectronic effects.