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Solution and solid state photochemistry of some bridgehead substituted dibenzobarrelene diesters : x-ray crystallography of starting materials and photoproducts Pokkuluri, Phani Raj


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.

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