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A new general synthetic access to carboxylated quaterpyridines (qpy), of interest as ligands for panchromatic dye-sensitized solar cell organometallic sensitizers, is presented. The strategic step is a Suzuki-Miyaura cross-coupling reaction, which has allowed the preparation of a number of representative unsubstituted and alkyl and (hetero)aromatic substituted qpys. To bypass the poor inherent stability of 2-pyridylboronic acid derivatives, we successfully applied N-methyliminodiacetic acid (MIDA) boronates as key reagents, obtaining the qpy ligands in good yields up to (quasi)gram quantities. The structural, spectroscopic (NMR and UV-vis), electrochemical, and electronic characteristics of the qpy have been experimentally and computationally (DFT) investigated. The easy access to the bis-thiocyanato Ru(II) complex of the parent species of the qpy series, through an efficient route which bypasses the use of Sephadex column chromatography, is shown. The bis-thiocyanato Ru(II) complex has been spectroscopically (NMR and UV-vis), electrochemically, and computationally investigated, relating its properties to those of previously reported Ru(II)-qpy complexes.

An efficient chemoselective differentiation of vicinal phenolic and benzylic alcohols on optically active binaphthyl skeletons has been achieved via TBS protection of the less sterically hindered, external 3,3' benzylic positions. Further manipulation of functionalities either in the 2,2' or 3,3' positions is then easily achieved in high yields via the use of BOC protecting groups, functioning orthogonally to TBS.

The ring inversion barrier for thianthrene tetraoxide was determined by making use of the variable temperature 13C NMR spectra of the 2,7-diisopropyl derivative (DeltaG(double dagger) = 6.5 kcal mol(-1)). The barrier is lower than that measured for a trans thianthrene dioxide derivative (DeltaG(double dagger) = 9.35 kcal mol(-1)). These results agree well with ab initio theoretical predictions.

Low-temperature and NOE NMR spectra of four of the title compounds indicate that they adopt a synclinal (sc) conformation, in agreement with the prediction of ab initio computations. In the case of the most-hindered derivative (compound 4), the conformation is syn-periplanar (sp), as is also shown by X-ray diffraction. Such stereolabile sp- or sc-atropisomers exist as two conformational enantiomers: the corresponding enantiomerization barriers, covering the range 6.6 to 9.7 kcal mol(-1), could be measured for all the examined compounds. In two cases (compounds 3 and 5), the minor antiperiplanar (ap) atropisomer has been also observed, and the sc to ap interconversion barrier measured (11.7 and 11.9 kcal mol(-1), respectively). In addition, restricted rotation of the isopropyl and tert-butyl substituents has been detected, and the corresponding barriers have been determined.

Carbinols of the Ar-C(OH)R(2) type, Ar being o-isopropylphenyl, exist as stereolabile syn-clinal (sc) and anti-periplanar (ap) atropisomers when R = Me, Et, i-Pr. In the latter compound, the major atropisomer also comprises two enantiomeric forms that interchange with a barrier of 6.4 kcal mol(-1). X-ray diffraction, NOE experiments, and ab initio calculations indicate that the sc-atropisomer is the more stable form when R = Me, i-Pr, t-Bu but is the less stable one when R = Et. NMR spectra at variable temperature allowed the determination of the barriers for the interconversion of the sc- into the ap-atropisomers (DeltaG++ = 7.6, 8.8, and 13.5 kcal mol(-1) for Me, Et, i-Pr, respectively). When R is a tert-butyl group, the two atropisomers are configurationally stable: the ap-atropisomer is obtained as the kinetic controlled compound, which can be transformed into the thermodynamically more stable sc-atropisomer with a free energy of activation of 29.3 kcal mol(-1). Both atropisomers exhibit restricted rotation of the tert-butyl moiety, the corresponding DeltaG++ values being 9.4 and 8.8 kcal mol(-1) for the sc- and ap-atropisomer, respectively.

The barriers for interconverting the conformational enantiomers (stereolabile atropisomers) of pyridine-substituted adamantane derivatives have been determined by dynamic 13C NMR spectroscopy. The trend of these values parallels that anticipated by MM calculations. In at least one case, the computed structure was found to agree with that obtained by single-crystal X-ray diffraction. In addition, it has been possible to achieve a physical separation of a pair of these stereolabile atropisomers at -60 degrees C by means of the enantioselective cryogenic HPLC technique., (Copyright 2004 American Chemical Society)

The low-temperature NMR spectra of 1,4-bis(mesitoyl)durene, 1, and of 1,4-bis(mesitylethenyl)durene, 2, reveal the presence of syn and anti rotamers at the equilibrium, their relative proportions depending on the dielectric constant of the solvent. In solution the more stable rotamer of 1 is the anti whereas, in the case of 2, the more stable is the syn. Depending on the crystallization solvent employed the more (anti) and the less stable (syn) rotamers were both observed (X-ray diffraction) in the solid state of 1. On the other hand, only the less stable rotamer (anti) was found to be present in the solid state of 2. As shown by MM calculations, the syn-to-anti interconversion occurs via a correlated process (cogwheel pathway) involving the mesityl-C and durene-C bond rotations: the dynamic NMR technique yields an experimental barrier of 8.2 kcal mol(-)(1) for 1 and 13.1 kcal mol(-)(1) for 2. In the case of derivative 2 a second barrier, due to a second type of correlated rotation process (torsion), was also determined (8.6 kcal mol(-)(1)). As a consequence of the restriction of this second torsional motion the anti rotamer of 2 displays two distinguishable NMR spectra at -133 degrees C, corresponding to a pair of conformers with different symmetry (anti C(i)() and anti C(2)).