Norrish Type 11 cleavage

In contrast to the observed photochemistry of adamantyltris(trimethylsilyl)silane which efficiently yields the appropriate silene compound, similar photolysis of the germanium analogue provided no evidence for the production of germene13. However, photolysis of the germanium compound in CCI4 did result in a Norrish type 1 cleavage of... 

A homolytic cleavage of a bond associated with an atom or group that is bonded to a specified group. An example of an alpha-cleavage would be a Norrish type 1 cleavage. 

When applied to ketones, this is called Norrish Type 1 cleavage or often just Type I cleavage, In a secondary process, the acyl radical R —CO can then lose CO to give R radicals, Another example of a category 1 process is cleavage of CI2 to give two Cl atoms. Other bonds that are easily cleaved by photolysis are the O—O bonds of peroxy compounds and the C—N bonds of aliphatic azo compounds R—N=N—R 36 The latter is an important source of radicals R since the other product is the very stable N2. 

Photolysis of a,/i-unsaturated acylpolysilanes of the general structure (Me3Si)3-SiCOCR=CR2 failed to yield significant amounts of the isomeric siladienes, and gave instead products derived from the Norrish type 1 cleavage of the acylsilane into the radicals (Me3Si)3Si and COCR=CR, 124. 

The Norrish type 1 cleavage of cyclic ketones has been found to be strongly influenced by the presence of a silyl group ji to the carbonyl group, where the acyl radical intermediate was believed to be stabilized by the fi effect184. For example, as shown in equation 41 with the ketone 349, Ci— C2 cleavage led to the diradical 350, and thence to the products 351... 

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. 

Muthuramu and Ramamurthy have reported the first example of Norrish Type 1 cleavage in a thione, or more specifically a dithiolactone. 

Pig. 3. Photochemistry of Benzoid Ethers (Norrish Type 1 cleavage)... 

Another important photochemical reaction of both aliphatic and aromatic ketones is the fragmentation reaction. Unconjugated ketones on photoexcitation undergo a-cleavage followed by decarbonylation and subsequent reactions of alkyl radicals to give product(s). All these processes are collectively known as Norrish type-1 cleavage reactions [5]. These reactions take place both in gaseous and liquid phases. 

Studies of the photolysis of carbonyl-containing polymers (See e.g. Guillet, 1985.) have revealed two main mechanisms of degradation. The first, the Norrish Type 1 cleavage, involves direct scission of the bond a to the excited carbonyl, with the production of two radicals. 

Phosphine oxides acyl and biacylphosphine oxides are photoinitiators which, if used alone or in combination with other photoinitiators, absorb well into the visible part of the spectrum and are useful to cure highly pigmented systems. Irradiation leads to highly reactive phosphinoyl and phosphinyl radicals generated by a Norrish type-1 cleavage. 

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