Hydrogen photochemical addition

Scheme 52 provides a sample of the types of reactions that have recently been conducted under a variety of different conditions. Example (1) illustrates the classical conditions,163 while example (2) shows that modified initiators that permit low temperature reactions may prove beneficial.29 Finally, example (3) illustrates that the photochemical addition of alcohols (and also acetals) to enones in the presence of benzophenone is a very useful preparative procedure.164 Here, the carbinyl hydrogen is abstracted by an excited state benzophenone molecule rather than a peroxy radical.163... 

Bromoandrost-16-ene adds hydrogen bromide under irradiation to give 16, 17j8-dibromoandrostane. A photochemical addition of trifluoroiodo-methane on to a A -olefin is mentioned on p. 258. 

Arene oxides show the characteristic reactions of epoxides (isomerization to ketones, reductions to alcohols, nucleophilic additions, deoxygenations) and olefins or conjugated dienes (catalytic hydrogenation, photochemical isomerization, cycloaddition, epoxidation, metal complexation). Where a spontaneous, rapid equilibration between the arene oxide and oxepin forms exists, reactivity typical of a conjugated triene is also found. 

The chromone (52) undergoes photochemical addition of ethene. The primary product from this cycloaddition, presumed to be (53), is photo-chemically reactive and is converted into (54) and (55). The former of these is a key intermediate in a synthetic strategy to tricothecene analogues. Both (54) and (55) arise via the Norrish Type II reactivity of (53). Thus hydrogen abstraction from the methoxy substituent by the excited carbonyl group results in a 1,4-biradical that either ring closes to (54) or fragments with the loss of methanal to yield the enol of (55). 

The methyl radical is not very discriminating, and shows none of the electrophilic character of most of the other radicals discussed earlier in this chapter. To what extent steric effects enter into the determination of the rate is hard to decide, but they probably are not pronounced, since the relative reactivity series is not greatly different from that of hydrogen atom additions to the same series of olefins. The relative rates are given in Table 50. It should be noted that these are derived from the absolute rates, and are not the results of competition reactions. Accordingly, there may be some substantial errors in the individual values. A very large number of miscellaneous olefinic compounds have been studied and the rate parameters for the addition are given in Table 51. In many of these cases, the methyl radicals were produced by photochemical decarbonyla-tion of acetaldehyde. After the methyl radical adds to the olefin, the new... 

The photoenolization of the quinone (286) can be carried by irradiation at 313 or 365 nm in acid solution. The steady state irradiation has identified the product as the unstable hydroxylated compound (287) which is formed via the enol (288). The presence of this intermediate was detected in a laser flash study of the reaction. The quinones (289) undergo cyclization when irradiated with visible light.The mechanism by which the compounds (289) are transformed into the derivatives (290) involves the production of an excited state that is either a zwitterion or a biradical. After the transfer of a hydrogen the intermediate (291) is formed. It is within this species that cyclization occurs to give the final products. (2+2)-Cyclo-adducts such as (292) and oxetanes can be obtained by the photochemical addition of quinones to homobenzvalene. Interest in the photo-SET in quinone systems has led to the synthesis of the pyropheophytin substituted naphthoquinone dyads (293). A pulse radiolysis study of vitamin K in solution has been reported. 

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