RESUMEN
A synthetic approach to a set of three inherently chiral [n]cyclophanes, [n](1,6)pyrenophanes (29a-c, n = 8-10) was investigated. Progress toward 29a was thwarted by the failure of the key dithiacyclophane-forming reaction. For the next higher homologue, the synthesis was completed, but the desired [9](1,6)pyrenophane (29b) could only be partially separated from an isomeric pyrenophane, [9](1,8)pyrenophane (28b), and an unidentified byproduct. Work aimed at the synthesis of the next higher homologue resulted in the isolation of a 7:4 mixture of [10](1,8)pyrenophane (28c) and [10](1,6)pyrenophane (29c), which could not be separated by column chromatography or crystallization. However, single-crystal X-ray structures of 28c and 29c were obtained after manual separation of two crystals with different morphologies from the same batch of crystals obtained from the 7:4 mixture of 28c and 29c. The pyrene system of 29c was found to have a gentle end-to-end bend as well as a significant longitudinal twist. Short intermolecular C(sp(3))-H···π contacts (2.64 to 2.76 Å) between H-atoms on the bridge and the centroids of three of the four six-membered rings of the pyrene system of a neighboring pyrenophane of like chirality give rise to the formation of single enantiomer columns. From a DNMR study of the mixture of 28c and 29c, the bridge in [10](1,8)pyrenophane (28c) was found to undergo a conformational flip from one side of the pyrene system to the other with ΔG() = 14.9 ± 0.2 kcal/mol. A two-stage preparative HPLC protocol was subsequently developed for the separation of 28c and 29c (Chiralpak AD-H column) and then the enantiomers of 29c (Chiralcel OJ-H column). This enabled the measurement of their optical rotations and CD spectra.
RESUMEN
The development of a palladium-catalyzed decarboxylative coupling reaction of arene carboxylates with olefinic substrates is described. The optimized procedure for decarboxylative palladation employs Pd(O2CCF3)2 as catalyst (0.2 equiv) in the presence of Ag2CO3 (3 equiv) in the solvent 5% DMSO-DMF and proceeds at temperatures of 80-120 degrees C with a wide range of arene carboxylates and alkenes as substrates. The process is proposed to proceed by an initial Ar-SE reaction involving ipso attack of an electrophilic Pd(II) intermediate on an arene carboxylate to form an arylpalladium(II) species with loss of carbon dioxide. This intermediate is then proposed to react with an olefinic substrate by steps common to the Heck coupling process. Reoxidation of the liberated Pd(0) in situ is proposed to establish the catalytic cycle.
RESUMEN
A series of 1,n-dioxa[n](2,7)pyrenophanes (n = 7-12) with increasingly nonplanar pyrene moieties was synthesized by a 9-10 step sequence starting from 5-hydroxyisophthalic acid. The crystal structure of each member of this series was determined crystallographically. Several spectroscopic properties were found to vary with the extent of the nonplanarity of the pyrene unit. The way in which the distortion from planarity of the pyrene system influences its pi-electron delocalization was investigated by using two quantitative descriptors of aromaticity based on geometry (HOMA) and magnetism (magnetic susceptibility and NICS). Both methods suggest that the aromaticity of the pyrene moiety is diminished only slightly upon increasing the bend angle theta from 0 degrees to 109.2 degrees.
RESUMEN
The two-electron reduction of strained pyrenes ([7](2,7)pyrenophanes) with lithium metal leads to the formation of a new sigma-bond as a means to "escape" strained antiaromaticity.
RESUMEN
The reduction of a series of [n](2,7)pyrenophanes (n = 7-10) with lithium or potassium metal shows that the strain in the system, controlled by the length of the tether, determines the nature of the reduction products. The reduction of [7](2,7)pyrenophane (2) and [2]metacyclo[2](2,7)pyrenophane (3) leads to reductive dimerization followed by novel intramolecular sigma-bond formation as a means of escaping strained anti-aromaticity. [8](2,7)Pyrenophane (4) affords only reductive dimerization, and no two-electron reduction is observed. The reduction of [9](2,7) pyrenophane (5) and [10](2,7)pyrenophane (6) leads to reductive dimerization, followed by the formation of a dianionic anti-aromatic species, which eventually cleaves the solvent, THF-d(8). The similarity between the reduction of the latter systems and the reduction of pyrene (1) is discussed.