Abstract
<jats:p> An iron-catalyzed deoxygenation reaction of in situ generated 1,2-oxazines was developed to enable the construction of polysubstituted pyrroles from nitrosoarenes and dienes. The reaction sequence is mild and mechanically simple to perform requiring no solvent swap or isolation of the oxazine intermediate. The scope is broad enabling access to mono-, di-, tri-, or tetrasubstituted pyrroles. Mechanistic investigations indicated that the reaction proceeds via non-radical intermediates, and kinetic analysis of the reaction revealed a first-order rate dependence in catalyst-, nitroarene-, and silane concentrations, and an inverse kinetic order in acetate was observed. The difference in rates between Ph <jats:sub>2</jats:sub> SiH <jats:sub>2</jats:sub> and Ph <jats:sub>2</jats:sub> SiD <jats:sub>2</jats:sub> was found to be 1.36 ± 0.11, and the reaction rates of a series of nitrosoarenes exhibited a linear correlation versus Hammett σp-values exhibiting a ρ-value of 0.26. The temperature dependence of the reaction rate was examined, and ΔH <jats:sup>‡</jats:sup> = 13.8 kcal•mol <jats:sup>–1</jats:sup> and ΔS <jats:sup>‡</jats:sup> = –37.5 cal•mol <jats:sup>–1</jats:sup> •K <jats:sup>–1</jats:sup> activation parameters were observed. These data are consistent with a mechanism where the turnover-limiting step is cleavage of the N–O bond, which occurs through an ordered transition state with little bond breaking. While the activation parameters are consistent with an inner-sphere reaction of an iron hydride, these data do not rule out an outer-sphere mechanism. These mechanistic experiments suggested that the iron hydride catalytic intermediate reacts with the water, and the addition of 4 Å molecular sieves allowed the concentration of silane to be reduced to 0.36 equivalents without attenuation of the yield. </jats:p>