Leaving groups steric hindrance deactivation

Tetraamination of 2,4,5,6-tetrafluoropyrimidine with dibutyl-amine involves the high reactivity of fluorine as a leaving group rather than activation by the 2,4,6-fluorines. The latter cannot account for the reactivity of the 5-fluorine since the 2,4,6-substituents undoubtedly all react first. Apparently, deactivation by the three dibutylamino groups so introduced (cf. 174) is diminished by steric hindrance to the necessary co-planarity with the ring. 

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