Spin-center-shift

Mechanistic remarks relating to the regioselectivity may be subdivided in three parts. The first deals with concepts within the framework of the classic Norrish-Yang reaction, i.e. homolytic hydrogen transfer is the basis of the reaction. In the second part the cyclization reactions based on photoinduced electron transfer (PET) will be discussed and in the third part the spin-center-shift approach is elucidated. 

The Norrish-Yang reaction, with the spin-center-shift extension, facilitates access to a variety of cyclic compounds. This will be discussed here, with examples of the synthesis of three- to six-membered rings synthesis of macro- and bicydic compounds and the photochemistry of imides will not be covered. The examples especially demonstrate the capabilities of the reaction with regard to stereoselectivity. 

The other way to prepare cyclopropanes is based on the spin-center-shift. Thus, aryl-alkyl ketones, 26, bearing a leaving group X adjacent to the excited carbonyl group undergo a smooth cyclization to benzoylcyclopropanes 27 in moderate to good yields (Scheme 6b) [4a, b]. The reaction tolerates a variety of functional groups, often proceeds stereoselectively, and can be extended to the preparation of bicyclo[n.l.O]alkane derivatives. 

Another example of the application of the spin-center-shift concept is depicted in Scheme 8b. On irradiation of /3-ketoamides 43, hydrogen abstraction occurs from the d-position, followed by smooth elimination of methanesulfonic acid. In contrast with the applications of spin-center-shift mentioned above, the enolate radical moiety of biradicals 44 reacts solely as oxygen radicals 44b to give the oxa-zinones 45 [20],... 

By y-Hydrogen Abstraction and Subsequent Spin Center Shift... 

The idea of a spin center shift as an extension of the synthetic scope of the Norrish-Yang reaction was already discussed twice in this chapter. A third example applies to the preparation of six-membered rings. Thus, by treating (3-ketoamides 42 (Sec. 3.4.2.1, Ih) with hypervalent iodine(III) reagents the oc-methanesulfonyloxy-(3-ketoamides 59 are obtained in excellent yields. On irradiation, the 1,5-biradicals 60 are formed, which are converted into the biradicals 61 by elimination of methanesulfonic acid. 

---