Arenes, twisted

The reaction looks like a simple Friedel-Crafts alkylation, but there is a twist — the leaving group is not on the C which becomes attached to the ring. After formation of the C7 carbocation, a 1,2-hydride shift occurs to give a C6 carbocation. The 1,2-hydride shift is energetically uphill, but the 2° carbocation is then trapped rapidly by the arene to give a 6-6 ring system. 

The electronic effect of perfluoroalkyl substituents on the absorption spectra of arenes is relatively small (e.g., Xmax 3-perfluorooctylthiophe-ne = 229 nm 3-octylthiophene = 235 nm). Thus, the anomalously low absorption maximum of 504 is the effect of twisting around the backbone. It is apparent that the difference in size of the side chains is sufficiently large to cause twisting of the conjugated backbone of 504 due to steric interactions between the perfluoroalkyl substituents and the adjacent repeat unit. 504 exhibits green fluorescence (Xmax = 512 nm) in solution with a maximum blue shifted by 58 nm relative to POT (570 nm). Accordingly, 504 shows a Stokes shift of ca. 1.4 eV (186 nm) compared to only 0.6 eV (126 nm) for POT. 

When collecting, it s a good idea to grasp and twist only the stem (not the cap). All species should be separated in wax paper or paper bags (not baggies—mushrooms must breathe or they rapidly spoil), and care should be taken to see that they aren t crushed. 

In an interesting twist on the solvent free reaction, Petrukhina and coworkers co-sublimed a volatile metal complex, [Rh2(CF3C02)4], and arene molecules including paracyclophanes to yield organometallic coordination polymers. Moderate yields of crystalline products were obtained (35-70%),... 

A tentative structure for 44, incorporating one chemical defect and possessing exchange coupling of a symmetric spin trimer, was proposed.105 It was speculated that the annelation of calix[4]arene-based macrocycles leads to substantial strain, especially in the center macrocycle.105 Such strain could have two consequences (1) increased basicity of the arylmethyl carbanion sites, which would facilitate deuterium abstraction from the deuterated solvent, and (2) increased out-of-plane twisting of the 7r-system, which would weaken the exchange coupling.105... 

A self-assembled homooxacalix[3]arene-based dimeric capsule constructed by a Pd11-pyridine interaction which shows a novel chiral twisting motion in response to guest inclusion, A. Ikeda, H. Udzu, Z. Zhong, S. Shinkai, S. Sakamoto and K. Yamaguchi, J. Am. Chem. Soc., 2001, 123, 3872. 

The strain energies in the twisted polyarenes have been determined by comparison of heats of combustion with those of the corresponding nonstrained isomers. In the case of 4,5-dimethylphenanthrene, the comparison was with that of 2,7-dimethylphenanthrene [15]. The bathochromic and hyperchromic shifts of the UV spectra of twisted arenes have been used to detect the presence of twisted distortion [16]. 

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