Reactions involving Cyanoisopropyl Radicals

Cyanoisopropyl radicals (15) undergo unsymmetrical C-N coupling in preference to C-C coupling.11 1 The preferential formation of the ketenimine is a reflection of the importance of polar and steric influences.1 1 However, the ketenimine is itself thermally unstable and a source of 15, thus the predominant isolated product is often from C-C coupling. 

The self reaction of primary alkyl radicals gives mainly combination.118 For primary alkyl radicals [CH3(CH2)nCH2 ], kjkxa is reported to lie in the range 0.12-0.14, apparently independent of chain length (n=0-3).1,8,119 

A substantial number of studies give information on kJkK for polymerizations of S (5.2.2.2.1) and MMA (5.2,2.2.2). There has been less work oil other systems. One of the main problems in assessing kjk lies with assessing the importance of other termination mechanisms (i.e. transfer to initiator, solvent, etc., primary radical termination). 

Techniques applied in assessing the relative importance of disproportionation and combination include  

Hensley ei al.,n reported the only direct experimental observation of head-to-head linkages in PS by 2D INADEQUATE NMR on, 3C-enriched PS. The method did not enable these groups to be quantified with sufficient precision for evaluation of W tc- Zammit el a/.130 studied chain distribution of low molecular weight PS prepared with AIBN initiator by MALDI-TOF. Separate distributions of chains formed by combination and disproportionation were observed. They estimated kJkK. at 90 °C to be 0.057. 

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Table 5.4 Values of kjk for Reactions involving Cyanoisopropyl Radicals...

Primary radical termination may involve combination or disproportionation with the propagating radical, it is often assumed that small radicals give mainly combination even though direct evidence for this is lacking. Both pathways are observed for reaction of cyanoisopropyl radicals with PS (Scheme 3.14) (Section... 

Johns and Willing have reported an interesting example of polycyclization involving addition to a cyano group (Scheme 155). The reaction of the diallyl amine 444 with AIBN gives the ketone 448. This would result from addition of the cyanoisopropyl radical to 444 followed by Cy5 cyclization of 445 in a Cy5/Cy6 case to the pyrrolidine radical 446. This radical would cyclize to 447 by Cy6 intramolecular addition to the cyano group in a Cy6/Cy7 case and, finally, the intermediate imine would be hydrolyzed to the ketone 448. 

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