Vinyl groups photolysis

The 3.2.0 product is presumably derived from the other possible 1,4-biradical, 1,4-perinaphthadiyl resulting from bond formation at the termini of the vinyl groups. Interestingly, this species has been generated by photolysis of the appropriate azo compound at 77 K and gives the 3.2.0 material (Scheme 14.6). ... 

Re2(CO)gL2 complexes. One exception is PMe which reacts with both (122) and (124) to give products of dipolar addition to the jji-acetylide and -vinyl groups respectively. Both the U-acetylide and n-vinyl structures undergo a fluxional process which interchanges the a and tt bonds between the Re atoms. Photolysis of (124) in the presence of ethylene results in formal C-H insertion to give (125), though the. mechanism Involves... 

The quantum yields of the Norrish t5q>e I and type II reactions have been measured for a variety of aliphatic ketones (6, 87) the type I process seems to be far less important than type II. As already seen (Section II.2.2.2.), also in the photooxidation of polymer systems containing carbonyl groups the Norrish type II process is considered the main cause of chain scission (82). This assumption is further supported by several photolysis studies (80, 83, 84) of polyethylene, where the IR investigations indicate the formation of vinyl groups in the oxidized polymers. 

Photolysis of (A, A -diethyldithiocarbamyl) methyl styrene leads to the initiating benzyl radical with the inactive diethyldithiocarbamate radical. This benzyl radical can add to the vinyl group of a second molecule of iniferter to produce a dimer. The dimer corresponds to an AB2 monomer with one polymerizable vinyl group and two initiating/propagating sites. By repeating these elementary reactions, this self-condensing vinyl polymerization system proceeds to form hyper-branched polymers (Scheme 3.10). 

The Nazarov cyclization of vinyl aryl ketones involves a disruption of the aromaticity, and therefore, the activation barrier is significantly higher than that of the divinyl ketones. Not surprisingly, the Lewis acid-catalyzed protocols [30] resulted only in decomposition to the enone derived from 46,47, and CO. Pleasingly, however, photolysis [31] readily delivered the desired annulation product 48 in 60 % yield. The photo-Nazarov cyclization reaction of aryl vinyl ketones was first reported by Smith and Agosta. Subsequent mechanistic studies by Leitich and Schaffner revealed the reaction mechanism to be a thermal electrocyclization induced by photolytic enone isomerization. The mildness of these reaction conditions and the selective activation of the enone functional group were key to the success of this reaction. 

Photolysis of this polymer gives radicals on which side chains can be formed, giving graft polymerization 122, 123, 153). Similarly the polymerization of styrene (152) or vinyl acetate (157) in the presence of bromotrichloromethane gives telomers carrying terminal bromine atoms and trichloromethyl groups. By ultraviolet irradiation (3500 A) in the presence of methyl methacrylate the carbon-bromine links are broken and block copolymers are formed. The telomerization of acrylonitrile and acrylic acid with bromoform is based on the same technique the end groups of both polyacrylonitrile and polyacrylic acid were photolyzed in the presence of acrylamide and afforded polyacrylamide blocks linked to polyacrylonitrile or polyacrylic acid blocks (164, 165). 

In certain reactions, introduction of a bulkier silyl group in substrates may decrease the stereoselectivity. For example, photolysis of a mixture of benzaldehyde and silyl vinyl ethers 31 affords a mixture of diastereomeric [2 + 2] cycloadducts 32 and 33... 

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