ABSTRACT
The photochemistry of ketones is mainly dominated by two types of reactions, both of them are connected with the partial radical character of the oxygen atom in the n-p*-excited state. The first one includes the attack of this oxygen atom onto a C-C double bond, is called Paterno`-Bu¨chi reaction and will be discussed in the following chapter. The second one, comprising the predominantly intramolecular abstraction of a hydrogen atom, was already introduced in the preceding chapter, which dealt with the abstraction of g-hydrogen atoms. This reaction is above all connected with the names Norrish and Yang. It was Norrish and his coworker Appleyard nearly seventy years ago [1], who discovered that ketones undergo a cleavage reaction upon irradiation. Nowadays, this process is called the Norrish Type II cleavage. Even though they assumed a concerted mechanism, they certainly didn’t suspect anything about the second kind of products, which are much more interesting for preparative chemists. In 1958 only Yang and Yang recognized the formation of cyclobutanes besides the Norrish Type II cleavage products [2]. This discovery was the basis of a very versatile and valuable ring closure reaction that should therefore be named the ‘‘Norrish-Yang reaction’’.
