Isolation and Reactivity of an Iron Azametallacyclobutene Complex

Abstract

A variety of C–N bond-forming methods are enabled by the [2 + 2] cycloaddition reaction of a transition metal imide complex and an alkyne substrate to generate an azametallacyclobutene intermediate. This type of reactivity has been primarily limited to early transition metals like zirconium and titanium. Herein, we describe the preparation of an iron azametallacyclobutene complex by [2 + 2] cycloaddition of a β-diketiminate iron imide complex and an internal alkyne, 1-phenyl-1-propyne. The metallacycle reacts further upon exposure to a terminal alkyne, phenylacetylene, by a proposed protonation pathway that is distinct from the chemistry of its group 4 congeners and is in line with formation of an azametallacyclobutene intermediate in iron-catalyzed alkyne carboamination. The iron azametallacyclobutene complex also undergoes migratory insertion of aldehyde and nitrile substrates to the metal–nitrogen bond, in contrast to the exclusive metal–carbon insertion that has been observed for zirconium and titanium analogs.

Two-Step, One-Pot Pyrrole Synthesis by Iron-Catalyzed ­Carboamination/Copper-Mediated Cyclization

“…24 Indeed, we demonstrated that the β-alkynyl enamine product of mesityl azide (MesN 3 where Mes = 2,4,6-trimethylphenyl) and phenylacetylene (Ar = phenyl) 1a undergoes cyclization in the presence of a copper(I) species to give the desired pyrrole 2a in 48% yield over two steps (Equation 1). 22…”

“…Our previous work demonstrated that electron-donating and electron-neutral substituents are well tolerated in iron-catalyzed alkyne carboamination, while electron-withdrawing substituents are detrimental to the process. 22 However, copper-mediated cyclization appears to favor substrates that are more electron-poor. 25 These opposing electronic effects track with observations made during our evaluation of the arylacetylene scope using our optimized conditions for two-step/one-pot pyrrole synthesis (Scheme 2 ).…”

“…12,21 We recently reported an iron-catalyzed carboamination process 22 that likely proceeds by a previously unknown iron azametallacyclobutene intermediate. 23 The reaction is completely regioselective and exclusively forms the (Z)-stereoisomer of the -alkynyl enamine products (1, Equation 1). Given this opportune geometry, we envisioned a route for pyrrole synthesis from 1 via subsequent copper(I)-mediated cyclization.…”

“…The underlying reactivity involved in iron-catalyzed carboamination to form -alkynyl enamines is the focus of ongoing research in our laboratory. 23 In the context of pyrrole synthesis, we were also interested in the mechanism of copper-mediated cyclization of the carboamination products. We gained insight about this process through isotopic labelling experiments.…”

Carbene-Like Reactivity in an Iron Azametallacyclobutene Complex: Insights from Electronic Structure

“…14 An azametallacyclobutene complex (X = NR) has the potential for electron delocalization throughout the ring system akin to a metallacyclobutadiene through involvement of electrons on nitrogen. We recently reported the isolation and characterization of the first monometallic iron azametallacyclobutene complex (1; Scheme 1b) 15 that was discovered during our investigation into the mechanism of iron-catalyzed alkyne carboamination. 16 Details from the molecular structure of 1 hinted at a delocalized π system within the metallacycle (vide infra).…”