Mechanistic determine tion

In the alkylative cyclization of the 1,6-enyne 372 with vinyl bromide, formation of both the five-membered ring 373 by exn mode carbopalladation and isomerization of the double bonds and the six-membered ring 374 by endo mode carbopalladation are observed[269]. Their ratio depends on the catalytic species. Also, the cyclization of the 1,6-enyne 375 with /i-bromostyrene (376) affords the endo product 377. The exo mode cyclization is commonly observed in many cases, and there are two possible mechanistic explanations for that observed in these examples. One is direct endo mode carbopalladation. The other is the exo mode carbopalladation to give 378 followed by cyclopropana-tion to form 379, and the subsequent cyclopropylcarbinyl-homoallyl rearrangement affords the six-membered ring 380. Careful determination of the E or Z structure of the double bond in the cyclized product 380 is crucial for the mechanistic discussion. 

The identification of process-related impurities can ultimately lead to elucidation of their mechanistic pathways. The first step in this process entails determination of structures for the impurities using tools such as NMR or LC-MS. With an elucidated structure, one can then attempt to propose a mechanism for its formation and to identify the synthetic steps that lead to its formation. This information can then be utilized to manipulate synthetic conditions so as to minimize its formation or develop efficient recrystalfiza-tion procedures to reduce impurity level while stiU maintaining acceptable yield. 

This article approaches the field of radiation chemistry of polymers firom a mechanistic viewpoint, and does not describe in detail the growing list of apphca-tions of high energy radiation in poljuner science. The basic principles of radiation chemistry are described, and subsequently the interaction of radiation with specific classes of polymers. The mechanisms of reaction are described by reference to specific materials. Furthermore, the means by which these mechanisms have been determined, ie, the methods of analysis, are described in detail. Finally, an attempt is made to draw overall conclusions about the radiation sensitivity of the constituent nnits of synthetic polymers. 

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