Hydrogen abstraction lattice

The exact mechanism of lattice oxygen incorporation and second hydrogen abstraction, and the precise sequence of elementary events is still a subject of speculation. Several authors assume that two distinct active sites are involved in the acrolein formation. The first, presumably a cation, participates in the formation of the initial allyl complex, while the second, which may contain a different cation and reactive oxygen anions, is the place where further hydrogen abstraction and oxygen incorporation take place. 

When salt crystals of the aryl 1-phenylcyclopenty 1 ketone carboxylic acid 40 with chiral amines such as (+ )-bomylamine or (—)-1-phenylethylamine were irradiated, the optically active exo- and endo-oxetanes 41 or 42 were formed in low to moderate enantiomeric excesses (Scheme 10) [57]. The formation of the oxetanes is believed to occur through Norrish type 1 cleavage and hydrogen abstraction, producing an alkene and an aldehyde, followed by a Paterno-Buchi reaction within the crystal lattice cage. In contrast, solution photolysis of 40 in acetonitrile afforded product 43 as the only isolable product via a typical Norrish type I a-cleavage followed by radical coupling. 

The a and P phases have a favorable balance of these two sites necessary to effect the rate-determining first hydrogen abstraction. The multicomponent system possesses the greatest number of active surface sites having the proper structure and composition and a solid state structure with the ability to rapidly reconstitute these surface sites with bulk lattice oxygen. 

The direct or sensitized irradiation (A > 330 nm) of the enone (157) in solution affords the [2 + 2] cycloadduct (158). When the irradiation is carried out using the same wavelength but with the compound in the crystalline phase the main product (75%) obtained in (159) formed by a hydrogen-abstraction path. Some of the [2 + 2]adduct (158, 25%) is also obtained. The change in the reaction from solution to crystal phase is the result of (157) being the preferred conformation in the crystal, a prediction which was verified by an X-ray structure. Scheffer has reviewed the influence of crystal lattice control on the outcome of such photochemical reactions. 

---