Chemical and biological studies on 1,2-dihydro-s-triazines. XIX. A nuclear magnetic resonance investigation of hindered internal rotation in 1-aryl derivatives

Substantial barriers to internal rotation about aryl CN bonds have been detected by pmr methods in a series of 2-methyl- and 2,2-dimethyl-substituted 4,6-diamino-1-aryl-1,2-dihydro-s-triazines. In most of the 2,2-dimethyl compounds the barriers are too high to be measured by the pmr lineshape method. When the blocking groups on the aryl moieties are large the barriers may be measured in the 2-methyl series by following the equilibration of diastereomeric rotamers. When the blocking groups on the hetero rings are sufficiently small, rate constants may be measured by pmr lineshape analysis. A substantial barrier to rotation has been measured in the absence of bulky blocking groups on the aryl ring.