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A systematic investigation on the metal-free, Cope-type hydroamination reactivity of hydrazides and analogues is reported. Optimization of the hydrazide structure resulted in more facile intramolecular reactivity and enabled intermolecular reactions of alkenes, thus providing a direct approach to polysubstituted hydrazides.
The ubiquity of nitrogen heterocycles in biologically active molecules challenges synthetic chemists to develop a variety of tools for their construction. While developing metal-free hydroamination reactions of hydrazine derivatives, it was discovered that carbazates and semicarbazides can also lead to alkene aminocarbonylation products if nitrogen-substituted isocyanates (N-isocyanates) are formed in situ as reactive intermediates. At first this reaction required high temperatures (150-200 °C), and issues included competing hydroamination and N-isocyanate dimerization pathways. Herein, improved conditions for concerted intramolecular alkene aminocarbonylation with N-isocyanates are reported. The use of βN-benzyl carbazate precursors allows the effective minimization of N-isocyanate dimerization. Diminished dimerization leads to higher yields of alkene aminocarbonylation products, to reactivity at lower temperatures, and to an improved scope for a reaction sequence involving alkene aminocarbonylation followed by 1,2-migration of the benzyl group. Furthermore, fine-tuning of the blocking (masking) group on the N-isocyanate precursor, and reaction conditions relying on base catalysis for N-isocyanate formation from simpler precursors resulted in room temperature reactivity, consequently minimizing the competing hydroamination pathway. Collectively, this work highlights that controlled reactivity of aminoisocyanates is possible, and provides a broadly applicable alkene aminocarbonylation approach to heterocycles possessing the β-aminocarbonyl motif.
Metal-free, intermolecular hydroaminations are performed upon heating aryl acetylenes and MeNHNH(2) at 140 degrees C, with preferential formation of the linear, "anti-Markovnikov" hydrazones.
Ring closure reactions O 0130Hydrazides as Tunable Reagents for Alkene Hydroamination and Aminocarbonylation. -N-Benzoyl hydrazides undergo Cope-type intramolecular hydroaminations to give five-, six-, and seven-membered N-benzoylamino-N-heterocycles. Analogous N-Boc-and N-carbamoyl hydrazides under microwave irradiation are intramolecularly aminocarbonylated to give bicyclic pyrazoles. -(ROVEDA, J.-G.; CLAVETTE, C.; HUNT, A. D.; GORELSKY, S. I.; WHIPP, C. J.; BEAUCHEMIN*, A. M.; J. Am. Chem. Soc. 131 (2009) 25, 8740-8741; Dep. Chem., Univ. Ottawa, Ottawa, Ont. K1N 6N5, Can.; Eng.) -Klein 48-040
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