Synthesis and electronic structure of bis(imino)pyridine iron metallacyclic intermediates in iron-catalyzed cyclization reactions.

The bis(imino)pyridine iron dinitrogen compound, ((iPr(TB))PDI)Fe(N2)2 ((iPr(TB))PDI = 2,6-(2,6-(i)Pr2-C6H3-N═C-(CH2)3)2(C5H1N)) is an effective precatalyst for the [2π + 2π] cycloaddition of diallyl amines as well as the hydrogenative cyclization of N-tosylated enynes and diynes. Addition of stoichiometric quantities of amino-substituted enyne and diyne substrates to ((iPr(TB))PDI)Fe(N2)2 resulted in isolation of catalytically competent bis(imino)pyridine iron metallacycle intermediates. A combination of magnetochemistry, X-ray diffraction, and Mössbauer spectroscopic and computational studies established S = 1 iron compounds that are best described as intermediate-spin iron(III) (SFe = 3/2) antiferromagnetically coupled to a chelate radical anion (SPDI = 1/2). Catalytically competent bis(imino)pyridine iron diene and metallacycles relevant to the [2π + 2π] cycloaddition were also isolated and structurally characterized. The combined magnetic, structural, spectroscopic, and computational data support an Fe(I)-Fe(III) catalytic cycle where the bis(imino)pyridine chelate remains in its one-electron reduced radical anion form. These studies revise a previous mechanistic proposal involving exclusively ferrous intermediates and highlight the importance of the redox-active bis(imino)pyridine chelate for enabling catalytic cyclization chemistry with iron.

Mechanistic investigation of the nickel-catalyzed Suzuki reaction of N,O-acetals: evidence for boronic acid assisted oxidative addition and an iminium activation pathway.

The mechanism of a recently reported Suzuki coupling reaction of quinoline-derived allylic N,O-acetals has been studied using a combination of structural, stereochemical, and kinetic isotope effect experiments. The data indicate that C-O activation is facilitated by Lewis acid assistance from the boronic acid coupling partner and an ionic S(N)1-like mechanism accounts for oxidative addition. In this context, we demonstrate the first direct observation of oxidative addition to a quinolinium salt. Notably, this mechanism is distinct from the more commonly described S(N)2(')-type oxidative addition of low-valent transition metals to most allylic electrophiles.

Oxidative addition of carbon-carbon bonds with a redox-active bis(imino)pyridine iron complex.

Addition of biphenylene to the bis(imino)pyridine iron dinitrogen complexes, ((iPr)PDI)Fe(N(2))(2) and [((Me)PDI)Fe(N(2))](2)(µ(2)-N(2)) ((R)PDI = 2,6-(2,6-R(2)-C(6)H(3)-N═CMe)(2)C(5)H(3)N; R = Me, (i)Pr), resulted in oxidative addition of a C-C bond at ambient temperature to yield the corresponding iron biphenyl compounds, ((R)PDI)Fe(biphenyl). The molecular structures of the resulting bis(imino)pyridine iron metallacycles were established by X-ray diffraction and revealed idealized square pyramidal geometries. The electronic structures of the compounds were studied by Mössbauer spectroscopy, NMR spectroscopy, magnetochemistry, and X-ray absorption and X-ray emission spectroscopies. The experimental data, in combination with broken-symmetry density functional theory calculations, established spin crossover (low to intermediate spin) ferric compounds antiferromagnetically coupled to bis(imino)pyridine radical anions. Thus, the overall oxidation reaction involves cooperative electron loss from both the iron center and the redox-active bis(imino)pyridine ligand.

Iron-catalyzed, hydrogen-mediated reductive cyclization of 1,6-enynes and diynes: evidence for bis(imino)pyridine ligand participation.

The bis(imino)pyridine iron dinitrogen complex (((i)Pr)PDI)Fe(N(2))(2) catalyzes the hydrogen-mediated reductive cyclization of enynes and diynes with turnover frequencies comparable to those of established precious metal catalysts. Amino, oxygenated, and carbon-based substrates are readily cyclized to the corresponding hetero- and carbocycles with 5 mol % iron and 4 atm H(2) at 23 degrees C. Stoichiometric reactions between selected substrates and the iron compound under a N(2) atmosphere established transfer dehydrogenation from an isopropyl aryl substituent to either the enyne or diyne substrate. In situ monitoring of the catalytic reaction by (1)H NMR spectroscopy coupled with deuterium labeling experiments established rapid cyclization followed by turnover-limiting hydrogenation.