5-Nonanone/urea complex

In a 5-nonanone/urea complex whose molar ratio suggests that the channels of the host are nearly filled with guest molecules [294], it was found that the time required to effect photoconversion is similar to that required in solution consistent with 5-A diameter channels (see Figure 1) attenuating only slightly the motions necessary for the initial y-hydrogen abstraction, the total quantum efficiency in the complex does not appear to be reduced appreciably. However, the photoproduct ratios clearly indicate that the fate of the hydroxy-1,4-biradicals is controlled by the urea channels. Only Norrish II photoproducts could be isolated, but they did not include the cis cyclobutanol (Eq. 12). 

The photodecomposition of alkanones included within urea complexes has been reported by Casal et al. Urea, when crystallized in the hexagonal lattice, possesses long channels of approximately 5 A diameter into which guest molecules are incorporated. For 5-nonanone the possible reactions which may occur are shown in Scheme 6.15. In solution photodecomposition occurs principally via the Norrish Type n reaction, leading to 2-hexanone, propylene, and two isomeric cyclobutanols. The crystalline hexagonal urea complex gave, on the other hand, essentially only one cyclobutanol isomer — the cis. The formation of the cis isomer requires less stringent rotational requirements and is, therefore, the preferred product in the constrained channel environment. 

Cyclobutanol formation is not usually an efficient process for simple aliphatic ketones. It has, however, been shown12 that irradiation of the urea inclusion complex of 5-nonanone is more effective, providing l-butyl-2-methylcyclobutanol in 40% yield, with the balance of the ketone undergoing photochemical fragmentation. The cyclobutanol product is a 97 3 cisjtrans mixture. In the absence of urea, photolysis proceeds to give the cyclobutanol in 24% yield, as a 60 40 cisjtrans mixture. Photocyclization has also been improved by inclusion in zeolites13. 

Nonanone is added to a concentrated solution of urea in methanol, which leads to crystal formation of the urea inclusion complex. After irradiation (313 nm line from a mercury lamp, isolated with an interference filter similar results are obtained with a Rayonet RPR-3000 lamp), the residue is taken up in methanol, then purified by VPC yield 40% d.r. (cisjtrans) 97 3. 

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