Two series of compounds, 4,4-diarylcyclohexenones and 9-substituted dibenzobarrelenes, were synthesized and their photochemistry was investigated in the crystalline state and in solution medium. The differences in photoreactivity and product selectivity of these compounds between the solution and solid states have been determined, and possible structure-reactivity correlations are discussed based on X-ray crystallographic data for some of the substances. The photorearrangement of 4,4-diarylcyclohexenones was found to be affected moderately by the crystal lattice. The migratory aptitude of the aryl groups for 4-phenyl-4-arylcyclohexenones was shown to be controlled to a lesser extent by the electronic effect in the solid state compared to solution. Irradiation of 4,4-diphenyl-6-methyl-cyclohexenone in the crystalline phase gave the same photoproducts and a similar product distribution as that in the solution phase. In this case, conformational analysis reveals that both pseudo-axial and pseudo-equatorial phenyl groups of the enone moiety are capable of migrating. The di-π-methane rearrangement of a number of 9-substituted dibenzobarrelene derivatives gave two regioisomeric dibenzo-semibullvalene products upon photolysis in solution and in the solid state. In general, the effect of the crystalline environment on the reaction regioselectivity is found to be small. Resolved chiral substituents (handles) at the 9-position of the dibenzobarrelene moiety were used to force the compounds to crystallize in chiral space groups. Asymmetric inductions via photolysis of the chiral crystals varied from small to moderate depending on the nature of the chiral substituent. It was found that, in some cases, asymmetric induction in the solid state is significantly higher than in solution, but is lower in one instance. In addition, the chiral crystalline environment has different effects on the asymmetric inductions in a dual pathway di-π-methane rearrangement. The di-π-methane photorearrangement of 9,12-bridged dibenzobarrelene lactones was found to proceed very efficiently in solution and in the solid state. The unusual regioselectivity observed was rationalized in terms of an intramolecular steric effect rather than an electronic stabilizing effect. An absolute asymmetric synthesis with high optical yield was achieved by photolyses of chiral crystals of the achiral compound, methyl 3,5-dihydro-3-oxo-1H-5,9b[1',2']benzeno-naphtho[1′,2′-c]furan-4-carboxylate. Photochemical [2+2] cycloaddition of this substrate to 1,3-dienes was found to be mechanistically interesting, involving an excited triplet state of 1,3-diene which is formed by energy transfer. An unusual excited singlet state rearrangement of 9-chloromethyl substituted dibenzobarrelene diester was discovered. X-ray crystallographic analyses revealed that an intermolecular chlorine atom transfer is involved in the formation of unusual dibenzopentalene photoproducts. Finally, a novel solid state photochromism was found, and a radical species was proposed as the intermediate responsible for the observed photochromic phenomenon. / Science, Faculty of / Chemistry, Department of / Graduate

The solid state and solution phase photochemistry of three bridgehead-substituted dibenzobarrelene-11,12-diesters and a 2,3-naphthobarrelene diester derivative were investigated. These compounds were expected to undergo the di-π-methane rearrangement via their triplet excited states, and a rearrangement to a cyclooctatetraene (COT) derivative via their singlet excited states. All compounds investigated underwent smooth photoreactions in the solid state to produce the same products as observed in the corresponding solution phase photolyses. One exception to this was a bridgehead dimethyl- substituted dibenzobarrelene diester. In this case an unusual photo-product which was characterized as a dibenzopentalene derivative, was obtained in the solid state along with lesser amounts of the normal-solution products. The mechanism proposed for the formation of this product involves a 1,4-biradical intermediate which undergoes a novel double 1,2-ester migration. It was recognized that this biradical intermediate could also undergo fragmentation to produce a cyclooctatetraene (COT) derivative which differs in its symmetry from that of the COT expected based on the mechanism proposed by H. E. Zimmerman for similar transformations observed in the cases of benzo- and naphthobarrelenes. Thus, there are two structures possible for the COT formed which cannot be distinguished based on their spectral properties. For this reason, single crystal X-ray diffraction analysis of the COTs formed in each case was performed. Of the four COT structures determined by X-ray crystallography, three COTs had structures that were consistent with the fragmentation mechanism, while one had a structure consistent with the Zimmerman mechanism. In light of the possible unusual photorearrangements observed, it was thought desirable to establish the molecular structures of all photo-products obtained. To this end, crystal and molecular structures of 11 photoproducts were determined. Also, in an attempt to establish structure-reactivity relationships, crystal and molecular structures of four starting materials were determined. A bridgehead chloromethyl-substituted dibenzobarrelene diester was also found to produce dibenzopenatalene-like photoproducts in the solid state or in chloroform solution; these photoproducts were also characterized based mainly on X-ray crystallography. These results add to the generality of the unusual photobehavior of some dibenzobarrelene derivatives. In the solution phase photolysis of a bridgehead dichloro-substituted dibenzobarrelene diester, a novel cyclic peroxide product was obtained. This was rationalized as being derived from photolysis of the primary di-π-methane photoproduct followed by trapping of the resulting 1,3-biradical by traces of molecular oxygen present in the reaction mixture. Thus, in the present study it was found that bridgehead substituted dibenzobarrelene derivatives undergo di-π-methane rearrangement via their T₁ states as expected, but that their S₁ states may undergo unusual rearrangements to produce cyclooctatetraene derivatives with unexpected structures, and dibenzopentalene derivatives in some cases. / Science, Faculty of / Chemistry, Department of / Graduate