V. Boekelheide

Intrusion of substituents into the cavity of aromatic π-electron clouds

The bridged [14]annulenes are exceptional models for testing theories and providing quantitative data. Thus, the dihydropyrene system demonstrates that diatropic ring current effects on chemical shifts are of the same magnitude for carbon-13 as for hydrogen for the same position in space relative to the aromatic π-electron cloud. The continuing need for additional samples of bridged [14]annulenes has led to the development of new synthetic methods. These, in turn, have led to syntheses of bridged [18]annulenes, tetra-bridged cyclophanes, and substituted [2.2]metaparacyclophanes. The latter substances provide opportunity for quantitative measurement of the interaction of substituents with aromatic π-electron clouds.

Syntheses of [2.2](2,6,2′,7′)naphthalenophane and [2.2](2,6,2′,7′)naphthalenophane-1, 11-diene and a study of their conformational flipping

Abstract Syntheses of [2.2](2,6,2′,7′)naphthalenophane and its corresponding diene have been accomplished by standard procedures from 2,13-dithia[3.3](2,6,2′,7′)naphthalenophane. The transition state for conformational flipping of [2/2](2,6,2′,7′)naphthalenophane requires the two naphthalene rings to become perpendicular to each other with insertion of the 1- and 8-hydrogens of one naphthalene ring into the cavity of the π-electron cloud of the other naphthalene ring. The kinetic parameters for the conformational flipping process have been measured by a selective pulse Fourier transform NMR technique.

The synthesis and electrochemical behavior of bis (η6-hexamethylbenzene)(η6,η6-triple-layered [2.2]paracyclophane)diruthenium(II,II) tetrakis(tetrafluoroborate) and its related mixed-valence ion

Abstract The synthesis and characterization of (η6-hexamethylbenzene)(η6-triple-layered[2.2]paracyclophane)ruthenium(II) bis(tetrafluoroborate), (4) and bis(η6-hexamethylbenzene)(η6,η6-triple-layered][2.2]paracyclophane)diruthenium(II,II) tetrakis(tetrafluoroborate) (6) together with their corresponding ruthenium(0) derivatives (5 and 7) are described. Cyclic voltammetry shows 4 to be reduced by a two-electron, reversible wave, E 1 2 (SCE) −0.816±0.008 V, and 6 to undergo reduction by two overlapping, reversible two-electron waves, E1 1 2 (SCE) - 0.796 ± 0.005 V. The comproportionation constant, Kc, for the equilibrium between 6, 7, and the corresponding RuIIRu0 mixed-valence ion (11 is 2.1 X 10133. Although this large value of Kc indicates significant interaction between the two ruthenium atoms of 11, the extent of interaction is appreciably smaller than that found for 2, the previous example of a mixed-valence ion in this series.

The Raman spectra of the cylophanes

Abstract Raman spectra have been obtained for the entire series of [2 n ]-cyclophanes and along with the i.r. spectra, several band assignments have been proposed. A correlation of the frequencies of the in-plane ring modes of the cyclophane with those of the identically substituted methylated benzene has been made. The CH 2 stretching bands have been found to move to higher frequency as the degree of steric hindrance increased. Tentative assignments have been proposed for the aromatic CH out-of-plane wag, and a possible origin for the distorted benzene band is discussed.

Methylated multibridged [2n]cyclophanes. All alternate synthesis of [26](1,2,3,4,5,6) cyclophane (superphane)

Abstract A sequence of formylation followed by a carbene insertion reaction has led to the stepwise introduction of additional ethano bridges into 4,5,7,8- tetramethyl [22](1,4)cyclophane ( 1 ), providing syntheses of 5,7,8-trimethyl- [23](l,2,4)cyclophane ( 6 ), a mixture of 5,8-dimethyl[24(1,2,4,5)cyclophane ( 10 ) and 5,7-dimethyl[24(1,2,3,5)cyclophane ( 11 , and-4-methyl[25](1,2,3,4,5)-cyclophane ( 14 ). This route to 14 completes a formal eight-step synthesis of [26](1,2,3,4,5,6)cyclophane ( 15 , superphane) with an overall yield of 17%. A Birch reduction of 6 readily gave 12,15-dihydro-5,7,8-trimethyl[23](1,2,4)-cyclophane ( 7 ) in 85% yield.

2,7,15,16-tetramethyl-trans-15,16-dihydropyrene☆

Abstract The synthesis of 2,7,15,16-tetramethyl- trans -15,16-dihydropyrene has been accomplished by two different routes. The compound is of interest in providing a better understanding of the ESR spectra of 15,16-dihydropyrenes.

Electronic spectra and triplet state properties of superphane

The emission and the zero-field splitting parameters D and E of the superphane 1 and the [25](1,2,3,4,5)-cyclophane 2 are investigated. The large number of six and five methylene bridges, respectively, leads to a more rigid molecular frame and to a smaller transanular distance as compared to other [2.2]cyclophanes. The experimental results exhibit characteristics features which are due to these unusual properties; they are discussed and compared with semi-empirical Π-electron calculations.

The radical anion of [24](1,2,4,5)cyclophane

The radical anion of [24](1,2,4,5)cyclophane (I) has been investigated by e.s.r. and e.n.d.o.r. spectroscopy under a variety of conditions. In dimethylformamide (counterion Et4N+) and in 1,2-dimethoxyethane–hexamethylphosphoric triamide (3 : 1)(counterion K+) a ‘free’ or loosely associated radical anion (If–˙) is formed (effective symmetry D2h), whereas in tetrahydrofuran and 2-methyltetrahydrofuran (counterion K+ or Cs+) a tightly ion paired species (Ip–˙) is present (symmetry C2v; aKca. 0.1 or aCsca. 0.6–0.8 mT). In 1,2-dimethoxyethane (counterion K+), at low temperatures, (If–˙) and (Ip–˙) give rise concurrently to distinct e.s.r. and e.n.d.o.r. spectra; the inter-conversion (If˙+)+ K+⇌(Ip˙–)K+ must thus be slow on the hyperfine time-scale. The concentration ratio [(Ip–˙)]/[(If–˙)] in 1,2-dimethoxyethane is ca. 5 at 178 K and increases upon raising the temperature.