Neighboring Group Participation

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Neighboring group participation is a term which encompasses all intra­ molecular reactions and all reactions which involve nonelectrostatic through-space interactions between groups within the same molecule. The term was invented in 1942 by Saul Winstein, whose many contributions to the growth and maturing of the subject are unequaled. Although the inventor of the term, Winstein was not the first worker to study neighboring group participation. An examination of Beilstein will show that many intramolecular reactions were known to the synthetic organic chemist weIl before the turn ofthe century, and as early as 1891 W. P. Evans, working at Giessen, described a kinetic investigation of the base-promoted cycliza­ ti on of ethylene chlorohydrins to ethylene oxides-an important intra­ molecular reaction. He was followed some twenty years later by Freundlich, whose va1uab1e studies on participation by the amino group began to appear in 1911. Freundlich was later joined by Salomon, who by the mid-thirties had developed a reasonable understanding of the efficiency of the neigh­ boring amino group in acyclic systems. In the late twenties to mid-thirties the subject began to expand with the work of Bennett on participation by thioether groups, Nilsson and Smith on neighboring hydroxyl, and Caldin and Wolfenden on neighboring carboxylate, and with discussions of the dependence of cyclization rates on ring size by Ruzicka, Salomon, and Bennett.