A novel isocyanate reaction - The formation and structure of unexpected cycloadducts

Phenyl isocyanate reacts with six-membered cyclic formamidines, e.g, with s ubstituted 3,4-dihydroquinazolines 1, at ambient temperature in an expected route to give the 1,3,5-triazines 4 by [2+2+2] cycloaddition. However, at elevated temperatures the unexpected cycloadducts 6, 7 and 8 are formed fro m these substrates in a novel isocyanate reaction occurring both at tho C-H bond of the formamidine moiety and at the C-H bond of the adjacent N-methy lene group. Azomethine ylides 9 are generated as key intermediates in this new reaction. The kinetically controlled [3+2] cycloadduct 6 is formed by t rapping 9 with isocyanate, whereas regioisomer 7 is the thermodynamically c ontrolled cycloadduct. Dimerization to 8 is preferred by sterically less hi ndered azomethine ylides 9 under thermodynamic control. One X-ray structure analysis is given for each type of the cycloadducts obtained.