Facile oxidative homocoupling of benzenes to the respective biphenyls is effected under moderate conditions, using air or oxygen in the presence of catalytic PdCl2 and AcOH/AcONa as solvent. The fast regeneration of the active Pd 2+ species is accomplished by combining several oxygen‐binding catalysts, such as Zr(IV), Mn(II), and Co(II) acetates. In this way, it is possible to increase the content of active oxygen in solution so that the rate of catalyst regeneration is faster than the rate of aggregation of the „spent catalyst”︁, Pd(0), to palladium black. The effects of various process parameters are studied and some mechanistic implications are discussed.

Bis(benzene)chromium [(η6-C6H6)2Cr] (BBC) is shown to readily trap radical species R (R = H, D, isobutyronitrile (IBN)) to form complex paramagnetic adducts whose structure has been identified in frozen solution by ENDOR (electron nuclear double resonance spectroscopy). It is shown that radical attack yields the 17 valence electron species (η6-C6H6)(η5-C6H6R)Cr where R assumes the endo position of the η5-ring with respect to the metal in a dynamic interannular migration mediated by the central metal atom. It is further shown that PhSiH3 (Ph = phenyl) is an efficient source of hydrogen atoms in the presence of BBC as well as other conventional nitroxide spin traps.

A new route for the partial displacement of OH groups of p-tert-butylcalixarene via spirodienol derivatives is described. NaBH(4) reduction of the bis(spirodienone) calixarene derivatives 2a-2c afforded the corresponding bis(spirodienols) 3a-3c in stereospecific fashion. (1)H NMR NOESY spectroscopy indicated that in the case of 2a, the reaction proceeds by attack at the exo face of the two carbonyls (the face located anti to the spiro C-O bond). The spirodienols readily revert to p-tert-butylcalix[4]arene when heated. The reaction of 3a with the deoxofluorinating agent DAST (Et(2)NSF(3)) afforded a mixture of extraannular substituted calixarenes possessing one or two fluoro-substituted dehydroxylated rings. The bisfluorinated calixarene 6a adopts in the crystal a conformation (1,3-alternate) similar to that adopted in solution by the di-dehydroxylated calixarene 6b. An experiment conducted with a selectively deuterated spirodienol derivative indicated that the deoxofluorination reaction involves regiospecific nucleophilic attack at the gamma position of the pentadienol subunit.

The formal dehydration of two vicinal phenol moieties of p-tert-butylcalix[6]arene was achieved in two steps by mild oxidation of the calixarene followed by treatment of the resulting monospirodienone derivative (9c) with an ionic hydrogenation mixture (Et(3)SiH/CF(3)COOH). Reaction of 9c yielded the unsubstituted xanthenocalix[6]arene 11d, while treatment of the monospirodienone derivative of a spherand-type calixarene (13) with Et(3)SiH/CF(3)COOH afforded the dibenzofuran derivative 15. The formation of the latter product indicates that, at least for 13, the rings forming the Ar--O--Ar bond in the product are not those connected by the spiro bond in the starting material. Methylation of the phenolic hydroxyl groups of 11d with methyl p-toluenesulfonate/K(2)CO(3) or dimethyl sulfate/base afforded its dimethyl and tetramethyl ether derivatives. The parent xanthone calix[6]arene derivative 17b was prepared by O-methylation of the phenol groups followed by CrO(3) oxidation of the xanthene methylene group and deprotection of the OH groups. McMurry coupling of calixanthone 17a afforded the dixanthylene 18. Calixarenes 11d and 15 (which possess a xanthene and dibenzofuran group, respectively) were structurally characterized by X-ray crystallography.