Y-Hydrogen abstraction

A mechanism has been proposed recently by O Neal and Blumstein for the gas-phase ozone-olefin reaction. This mechanism postulates that molozonide-biradical equilibrium is reached fast and postulates a competition between a-, 8-, and y-hydrogen abstraction reactions and the classical mechanism proposed by Criegee for the liquid-phase reaction. The main features of the Criegee mechanism (Figure 3-9) are the formation, from the initial molozonide, of the major carbonyl products and a second biradical intermediate, the zwitterion. The decomposition pathways of the zwitterion comprise unimolecular re-... 

Sakamoto et al. provided the first example of hydrogen abstraction by thiocarbonyl sulfur in the solid state [37,38]. Irradiation of AT-diphenylacetyl-AT-iso-propylthioaroylamides 6 in the solid state gave thioketones 7 (P-hydrogen abstraction products by thiocarbonyl sulfur), mercaptoazetidin-2-ones (y-hy-drogen abstraction products by thiocarbonyl sulfur), and aroylthioamides (y-hydrogen abstraction products by thiocarbonyl sulfur followed by cleavage). 

Irradiation of the powdered crystals of 6a-d with a high-pressure mercury lamp under argon at 0 °C showed a different photochemical behavior from that in benzene solution. Contrary to the photochemical results in solution media in which azetidin-2-ones 8 (y-hydrogen abstraction products) were formed as major products, thioketones 7 (P-hydrogen abstraction products) were obtained as major components in the solid-state photoreactions (Scheme 7). The... 

Figure 41. Distance and orientational parameters associated with the efficiency of y-hydrogen abstraction by a carbonyl oxygen atom. (Copied with permission [282a].)...

Bredt s rule. In this way, 1-adamantyl p-methoxyacetophenone 86a was forced to yield only cyclobutanols 87a and 88a as photoproducts [281]. Whereas (benzene) solution phase irradiations of 86a yielded a 2.6 ratio of 87a/88a, the solid state photoproduct ratio, 0.5, favors the more sterically hindered cyclobutanol. X-Ray diffraction studies predict a chair-like y-hydrogen abstraction pathway for 86a (in contrast to the boat-like transition states of 82) in which the C=0-Ha distance is 2.67 A. Other abstractable hydrogens (Hb) are at least 0.3 A farther from the carbonyl oxygen (Scheme 44). If i-BR has a conformation which mimics that of the ketone, its least motion pathway favors formation of the more sterically hindered cyclobutanol 88a. 

In fact, the rates of y-hydrogen abstraction in 97 must be attenuated severely in the restrictive channels of zeolites. Even in Na-X and Na-Y, where virtually no photoproduct selectivity from valerophenone is observed, its triplet state half-life is 1 /is (vs. 5 ns in fluid isotropic solvents). The narrower channels of Na-ZSM-5 increase the triplet half-life to 15 /is ... 

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). 

Scheme 78 Photochemical [3-lactam synthesis by y-hydrogen abstraction by a thiocarbonyl group...

In 2008, Sakamoto and coworkers have reported the synthesis of optically active (3-lactams via photochemical intramolecular y-hydrogen abstraction reaction of thioimides [177]. This reaction provides the first example of a chiral-memory effect for the photochemical y-hydrogen abstraction reaction of thiocarbonyl or carbonyl compounds, and a useful synthetic methodology for preparing optically active (5-lactams (Scheme 78). 

The E-anomeric hydrogen of compound 74 is too far away to permit abstraction and the Norrish II process results from the y-hydrogen abstraction of the aglycone chain. In this way, O-vinylglycosides 75 can be formed [74] (Scheme 39). 

The photo-cycloaddition of ethylene to 3-alkoxy chromones such as 315 has been applied to the synthesis of marine sesquiterpene filiformin and congeners (Scheme 43) <1996JOC4391>. Tandem [2+2] 7i-photo-cycloaddi-tion and y-hydrogen abstraction provided tetracyclic intermediate 316 which was converted to terpene 317 by subsequent oxetane ring reduction and acid-catalyzed rearrangement. 

The direct irradiation of 4-propanoyl-6-methylpyrimidine 109 in tert-butyl alcohol containing 5% benzene (>340 nm) leads to cyclopropanol 110 in 98% yield (Scheme 4.52) [75]. This reaction proceeds through y-hydrogen abstraction by N (3), but not by N(l) or by O. 

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