Guo-Qiang Chen

Practical iron-catalyzed atom/group transfer and insertion reactions

Iron-catalyzed reactions are receiving a surge of interest owing to the natural abundance and biocompatibility of Fe and the urge to develop practically useful sustainable catalysis for fine chemical industries. This article is a brief account of our studies on the C–O and C–N bond formation reactions catalyzed by Fe complexes supported by oligopyridine, macrocyclic tetraaza, and fluorinated porphyrin ligands. The working principle is the in situ generation of reactive Fe=O and Fe=NR intermediates supported by these oxidatively robust N-donor ligands for oxygen atom/nitrogen group transfer and insertion reactions. The catalytic reactions include C–H bond oxidation of saturated hydrocarbons (up to 87 % yield), epoxidation of alkenes (up to 96 % yield), cis-dihydroxylation of alkenes (up to 99 % yield), epoxidation–isomerization (E–I) reaction of aryl alkenes (up to 94 % yield), amination of C–H bonds (up to 95 % yield), aziridination of alkenes (up to 95 % yield), sulfimidation of sulfides (up to 96 % yield), and amide formation from aldehydes (up to 89 % yield). Many of these catalytic reactions feature high regio- and diastereoselectivity and/or high product yields and substrate conversions, and recyclability of the catalyst, demonstrating the applicability of Fe-catalyzed oxidative organic transformation reactions in practical organic synthesis.

Selective oxidation of terminal aryl and aliphatic alkenes to aldehydes catalyzed by iron(III) porphyrins with triflate as a counter anion.

[Fe(Por)CF(3)SO(3)] (Por = porphyrin dianion) can efficiently catalyze selective oxidation of terminal aryl alkenes and aliphatic alkenes to aldehydes in good to high yields under mild conditions.

Formation of Thermally Robust Frustrated Lewis Pairs by Electrocyclic Ring Closure Reactions.

The phosphorus/boron-substituted hexatriene systems 6 undergo thermally induced electrocyclic ring closure to yield the cyclohexadiene-derived P/B frustrated Lewis pairs (FLPs) 7. Subsequent TEMPO oxidation gives the phenylene-bridged FLPs 8. Both systems activate dihydrogen and the thermally robust FLPs undergo carbon-carbon coupling reactions at a mesityl group upon treatment with dimethyl acetylenedicarboxylate at elevated temperatures.

Bifunctional Behavior of Unsaturated Intramolecular Phosphane–Borane Frustrated Lewis Pairs Derived from Uncatalyzed 1,4‐Hydrophosphination of a Dienylborane

Three unsaturated C4 -bridged phospane/borane frustrated Lewis pairs (P/B FLPs) are prepared by uncatalyzed hydrophosphination of a dienylborane. The systems are bifunctional. Consequently, two examples undergo clean hydroboration reactions with HB(C6F5)2 to yield B/B/P systems. The 1,4-P/B system (C6F5)2B-CH2CH=CMeCH2PMes2 reacts with benzaldehyde initially by allylborane addition, followed by internal P/B FLP addition to the pendant C=C double bond, to yield a bicyclic product. The corresponding reaction of (C6F5)2B-CH2CH=CMeCH2PtBu2 stops at the allylborane/benzaldehyde addition product. The related system (C6F5)2B-CH2CH=CMeCH2PPh2 shows a similar bifunctional reaction pattern, whereby allylborane addition to benzaldehyde is combined with P/B addition to a second aldehyde equivalent to form the eight-membered heterocyclic 1:2 addition product.

Phospha-Claisen Type Reactions at Frustrated Lewis Pair Frameworks

The C4-bridged unsaturated phosphane/borane frustrated Lewis pairs (P/B FLPs) 4 undergo borane induced phosphane addition to a variety of acetylenic esters or ketones to generate heterocyclic 10-membered intermediates that contain pairs of allyl phosphonium/allenic enolate functionalities. These subsequently undergo phospha-Claisen type rearrangement reactions to give the respective substituted phosphanyl pentadiene products. In two exceptional cases subsequent reactions leading to anomalous phospha-Claisen products were found. One example involved cyclopropane ring formation, and the other carbon-carbon bond activation. Potential mechanistic schemes leading to these products are discussed. Essential examples were characterized by X-ray diffraction.