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Photochemical asymmetric synthesis and novel photoreactions in the solid state Kang, Ting

Abstract

The photochemistry of several cis- and trans-9-decalyl aryl ketones was studied in solution and the solid-state. Norrish/Yang type II photocyclization was found to be the major process in both media, and the endo-aryl cyclobutanols were obtained exclusively. No type II photoelimination products were observed. The reactions in the solid state were regiospecific and only one of two possible cyclobutanols was formed. The results were rationalized by means of crystal structure-reactivity correlation studies. The crystal structures of six starting ketones showed that the disposition of the carbonyl chromophore favored the abstraction of only one y-hydrogen in each case, resulting in only one cyclobutanol. The poor orbital overlap calculated from these structures eliminated the possibility of photocleavage. The least motion character of the solid-state reaction suppressed the formation of the exo-cyclobutanols, leading to very high diastereoselectivity. In both the cis- and trans- systems, some novel products were obtained. The rarely seen Phydrogen abstraction products were observed in the cis-ketones, presumably initiated by favorable stereoelectronic factors. However, the product formed in the solid state was different from those in solution and could not be obtained in the latter medium, reflecting a "latent reactivity". Unprecedented decarbonylation products were formed in the case of the transketones with a possible mechanism proposed. The "ionic chiral auxiliary" approach was applied to both the cis- and fraws-ketones and poor to excellent enantiomeric excess (ee) values were observed. From the crystal structures of three salts, it was found that the presence of a single conformation of the salt in the unit cell resulted in high ee's, and that "conformational enantiomerism", i.e., the co-existence of both enantiomeric conformations of the anion portion of the salt in the same unit cell, led to poor ee's. Two novel solid-state photochemical reactions were investigated. The first involved the formation of oxetanes from photolysis of crystals of 1-phenylcyclopentyl aryl ketones, while solution-state reactions of the same compounds afforded only typical Norrish type I products. Oxetane formation was rationalized by type I cleavage followed by hydrogen abstraction and a subsequent Paternd-Blichi reaction. Asymmetric induction in this reaction through formation of chiral salts gave poor to fair ee's. The extension of this reaction to acyclic as well as other cyclic systems met with failure. The second reaction was concerned with photochemical 1,3-phenyl migration in the ccphenyl substituted aromatic ketones. Although the reaction has been studied before in solution, the solid-state version was never investigated. The crystals of the starting ketones showed very high selectivity, yielding the migration products exclusively, while the solution reactions afforded both migration and type I cleavage products.

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