Unusual approach to 3-aryl-2-aminopyridines through a radical mechanism: synthesis and theoretical rationale from quantum mechanical calculations.

Tris(trimethylsilyl)silane and azobis(cyclohexanenitrile) promoted the easy intramolecular arylation of aryl bromopyridine carbamates through a radical [1,6] ipso substitution process. These substrates showed a preference for this type of reaction over the alternative [1,7] addition. The results were rationalized by making use of quantum mechanical calculations and computer graphics.

Trapping unstable terminal M-O multiple bonds of monocyclopentadienyl niobium and tantalum complexes with Lewis acids.

Hydrolysis of [NbCp'Cl(4)] (Cp' = η(5)-C(5)H(4)SiMe(3)) with the water adduct H(2)O·B(C(6)F(5))(3) afforded the oxo-borane compound [NbCp'Cl(2){O·B(C(6)F(5))(3)}] (2a). This compound reacted with [MgBz(2)(THF)(2)] giving [NbCp'Bz(2){O·B(C(6)F(5))(3)}] (2b), whereas [NbCp'Me(2){O·B(C(6)F(5))(3)}] (2c) was obtained from the reaction of [NbCp'Me(4)] with H(2)O·B(C(6)F(5))(3). Addition of Al(C(6)F(5))(3) to solutions containing the oxo-borane compounds [MCp(R)X(2){O·B(C(6)F(5))(3)}] (M = Ta, Cp(R) = η(5)-C(5)Me(5) (Cp*), X = Cl 1a, Bz 1b, Me 1c; M = Nb, Cp(R) = Cp', X = Cl 2a) afforded the oxo-alane complexes [MCp(R)X(2){O·Al(C(6)F(5))(3)}] (M = Ta, Cp(R) = Cp*, X = Cl 3a, Bz 3b, Me 3c; M = Nb, Cp(R) = Cp', X = Cl 4a), releasing B(C(6)F(5))(3). Compound 3a was also obtained by addition of Al(C(6)F(5))(3) to the dinuclear µ-oxo compound [TaCp*Cl(2)(µ-O)](2), meanwhile addition of the water adduct H(2)O·Al(C(6)F(5))(3) to [TaCp*Me(4)] gave complex 3c. The structure of 2a and 3a was obtained by X-ray diffraction studies. Density functional theory (DFT) calculations were carried out to further understand these types of oxo compounds.

Measure of hemodynamic patterns by thoracic electrical bioimpedance in normal pregnancy and in preeclampsia.

OBJECTIVE: To compare the differences in the hemodynamics between normal pregnancy and preeclampsia, using thoracic electrical bioimpedance. STUDY DESIGN: We compared heart rate, end-diastolic volume, systolic volume, cardiac output, ejection fraction and peripheral vascular resistances in 18 healthy pregnant women with 15 with preeclamptic women at the following intervals: third trimester, 48 h post-partum, 2 and 6 months post-partum. We took the measurements by thoracic electrical bioimpedance. Statistical analysis was performed by means of Wilcoxon rank-sum test and p < 0.05 was considered statistically significant. RESULTS: The heart rate was lower in the preeclampsia group during the third trimester and the systolic volume was also lower at 48 h post-partum; this implies a lower cardiac output in women with preeclampsia during pregnancy and in the immediate puerperium. The systemic vascular resistances were higher in preeclampsia in the third trimester and at 48 h post-partum. At 2 and 6 months post-partum, the hemodynamic situation had equalized in both groups. CONCLUSIONS: Preeclampsia is a situation of low cardiac output and high peripheral resistances compared with a normal pregnancy.

Intramolecular triplet-triplet energy transfer in oxa- and aza-di-pi-methane photosensitized systems.

Two different mechanisms are proposed for the intramolecular triplet-triplet energy transfer in oxa- and aza-di-pi-methane systems, one thermally induced and the other photoinduced. These mechanisms involve a key structure that corresponds to an avoided crossing. The triplet sensitized photochemistry of these compounds, focusing attention in the di-pi-methane rearrangement, is discussed in light of these proposed mechanisms.

Triplet versus singlet photoreaction mechanism in the barrelene di-pi-methane rearrangement.

A detailed study of the potential energy surfaces involved in the di-pi-methane rearrangement (singlet and triplet states) reveals the factors that modulate the mechanisms (pathways) and reactivity in strained di-pi-methane systems such as bicyclo[2.2.2]octa-2,5,7-triene (barrelene). [structure: see text]