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The ipso-nitration of calix[6]arene-based molecular receptors is a important synthetic pathway for the elaboration of more sophisticated systems. This reaction has been studied for a variety of capped calixarenes, and a general trend for the regioselective nitration of three aromatic units out of six in moderate to high yield has been observed. This selectivity is, in part, attributed to the electronic connection between the protonated cap at the small rim and the reactive sites at the large rim. In addition, this work highlights the fact that subtle conformational properties can drastically influence the outcome of this reaction. [ABSTRACT FROM AUTHOR]

Two molecules containing two phenylphosphaalkene moieties linked by an anthracene (1) or by a naphthalene (2) ring have been synthesized and their crystal structures have been determined. While electrochemical measurements show that these two systems are easily reduced, EPR spectra indicate that, at room temperature, the electronic structures of the two reduction compounds 1−and 2−are quite different. In 1−, in good accordance with DFT predictions, the unpaired electron is delocalized on the full molecule while in 2−it is confined on a single phosphaalkene moiety. This difference is attributed to the short distance between the two phenylphosphaalkene groups in 2−which hinders their reorientation after addition of an electron. The role of this motion is consistent with the fact that two additional paramagnetic species are detected at 145 K: the dianion 22−characterized by a rather small exchange coupling constant and the radical monoanion 2*−resulting from the formation of a one electron P–P bond. These two species are probably reaction intermediates which can lead to the formation of biphosphane. [ABSTRACT FROM AUTHOR]

Abstract: Two bis-triarylphosphines [5,17-dibromo-11,23-bis(diphenylphosphino)-25,26,27,28-tetrapropoxycalix[4]arene (1) and 5,17-bis(diphenylphosphino)-25,26,27,28-tetrapropoxy-calix[4]arene (2)] were assessed in metal-catalysed cross-coupling reactions. Effective Heck couplings between aryl bromides and styrene were achieved when operating in DMF at 110°C with 2mol% of Pd(OAc)2 in the presence of Cs2CO3; the highest TOF values (ca. 50) being observed with activated aryl bromides. The coupling reaction could be extended to aryl chlorides (conversions of ca. 20% within 24h) with both ligands, provided the reaction was carried out with NaOAc in a DMF/water mixture. The two phosphines were also found to be efficient in Suzuki cross-coupling of aryl halides with phenyl boronic acid, the turnovers being somewhat lower than those observed in Heck couplings. In Kumada–Corriu cross-coupling of aryl halides with phenylmagnesium bromide, remarkable activities were reached in the presence of palladium or nickel. TONs up to 800mol(converted PhCl)·mol(Ni)−1 were observed for the conversion of chlorobenzene into biphenyl. Unlike the mono-triaryl phosphine PPh3, ligands 1 and 2 both promote palladium-catalysed Suzuki and Kumada–Corriu couplings involving chlorobenzene. [Copyright &y& Elsevier]