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Rates of the reactions of various enol ethers with potassium permanganate and osmium tetraoxide in aqueous solution and carbon tetrachloride, respectively, have been measured at 30 "C. The reactions of some alkenes and acrylates were also examined for the sake of comparison. The osmium reaction was found to be electrophilic, while the permanganate reaction was accelerated by both electron-donating and -attracting groups. &Alkyl substitution enhanced the reactivity of vinyl ether while an a-methyl group exerted little effect in both reactions. The trans isomers were found to be more reactive than the cis counterparts. The results can be rationalized by assuming a transition state resembling intermediate cyclic esters.

The molecular conformations of 9,Y-bianthryl (l), its 10,lO'-diphenyl derivative (2), 9-p-tolylanthracene (3), and 9,lO-diphenylanthracene (4) have been investigated by X-ray diffraction. The dihedral angle between the two aromatic ring systems in crystalline 1, in which the asymmetric unit consists of two discrete molecules, was found to be 74.65 (4) and 81.76 (5)O, respectively. In 2, the two anthracene moieties are at an angle of 87.42 (4)O, and the phenyl rings are twisted out of the plane of the adjacent anthracene by 73.2 (1) and 78.1 (1)O. The corresponding torsion angle between the two aromatic ring systems in 3 is 79.59 (8)'. In centrosymmetric 9,lO-diphenylanthracene (4), the phenyl rings deviate from coplanarity with the anthracene ring system by 67.63 (5)O. Separating the two anthracene r-system of 9,Y-bianthryl by a methylene group increases steric congestion, as was established by X-ray diffraction analysis of di-9-anthrylmethane (5). Its molecular geometry is characterized by a 2-fold axis of symmetry, and the two anthracene systems are in an orthogonal arrangement. Intramolecular hydrogen-hydrogen distances in 5 are as short as 1.93 (3) A, and *-orbital interaction was found to be t~pologically facilitated by a contact distance of 2.601 (2) A between the 9-and 9'-carbon atoms. By contrast, in 9-(9-anthrylmethylidene)-9,lO-dhydroanthracene (6), which formally derives from 5 by a 1,5-hydrogen shift, the closest intramolecular hydrogen-hydrogen contact is 2.28 (6) A. Relief of steric strain derives from folding of the dihydroanthracene moiety by 39.0 ( 2 ) O . IntroductionThe molecular geometry of 9-arylanthracenes is of photochemical and photophysical interest because coplanarity of the anthracene and the aryl substituent ~s p t e m s is precluded due to intramolecular repulsions involving the hydrogen atoms in the 1/8 positions of the anthracene. Consequently, r-orbital conjugation will be impaired, and deviations from molecular planarity are borne out in electronic absorption and emission spectra. In particular, the fluorescence spectra of nonplanar conjugated bichro-

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The excited-state properties of a series of 1-substituted and 1,1-disubstituted 2-(9-anthryl)alkenes have been investigated in terms of their fluorescence spectra and photochemical isomerizations. Large Stokes shifts of up to 10000 cm"1 between absorption and emission maxima are attributed to geometrical differences between the ground state and the emitting excited state. The [cis]: [trans] ratio of the photostationary state was found to depend markedly on the nature of the 1-substituent. Photochemical isomerization of 1,3-and l,5-bis(9anthryl)-substituted trans olefins results in the formation of intramolecular [4 + 4it] or [4 + 2x] cycloaddition products. The cycloaddition reactions of carbonyl-substituted anthracenes are suggested to involve the triplet excited state.