meso-Tetraphenylporphyrin Iron Chloride Catalyzed Selective Oxidative Cross-Coupling of Phenols

Abstract

A novel catalytic system for oxidative cross-coupling of readily oxidized phenols with poor nucleophilic phenolic partners based on an iron meso-tetraphenylporphyrin chloride (Fe[TPP]Cl) complex in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) was developed. The unique chemoselectivity of this reaction is attributed to the coupling between a liberated phenoxyl radical with an iron-ligated phenolic coupling partner. The conditions are scalable for preparing a long list of unsymmetrical biphenols assembled from a less reactive phenolic unit substituted with alkyl or halide groups.

Catalytic Aerobic Phenol Homo‐ and Cross‐Coupling Reactions with Copper Complexes Bearing Redox‐Active Guanidine Ligands

“…This resultc an be explained by ac ompetitive inhibition of the catalystd ue to the coordination of HFIP to the catalyst. [20,30,31] Subsequently,w et ested the redox-activel igand 1 2 + + ,w hich was also used for phenol coupling [60] in aP CET reaction. In addition to the higher costs of HFIP,t he dramatic decrease in the reactionr ate led us explore an ovel concept that is not restricted to the use of this solvent.…”

“…In contrast,t he cross-coupling reaction is favored in ar adicalanion coupling mechanism of phenols with ac omplementary relationship (N B > N A ), leading to ad esired high cross-coupling chemoselectivity.O nt he other hand, phenols with an on-complementary relationship (N A > N B )g enerally display al ower cross-coupling chemoselectivity,e ven for ar adical-anion coupling mechanism. [31] This work relied on tert-BuOOH as the oxidizing reagent and HFIP as the solvent. [22,23] Kozlowski et al studied the aerobic cross-coupling of phenolsw ith ac omplementary relationship with different catalysts (namely,C r-salen complexes).…”

Transition Metal‐Catalyzed Directing‐Group‐Assisted C−H Activation of Phenols

“…[4] Oxidative functionalization of phenold erivatives, in contrast, alwaysr equiresp henols substituted by electron-donating groups, as well as blocking groups, to ensure ah igh degree of site selectivity. [5] Transition-metal-catalyzed selective CÀHa ctivation of phenol derivatives provides an alternative for the synthesis of functionalized phenols. In comparison to the above reactions, the directed CÀHa ctivationso fp henols generally have higher regio-and/orm onoselectivity.I na ddition, CÀHa ctivation at all positions aroundt he phenol ring (ortho, meta,a nd para)c an be achieved through directing-group and catalystc ontrol.…”

Selective Formation of 4,4′‐Biphenols by Anodic Dehydrogenative Cross‐ and Homo‐Coupling Reaction

“…An alternative synthetic approachw as published by Pappo and co-workers.T oa ccess the para-biphenols a meso-tetraphenylporphyrin iron chloride catalysta nd peroxidesa st he stoichiometrico xidant was employed. [12] Using peroxides as reagent is involving potential safety risks at least on technical-scale operations. [13] Moreover, these strategies create additional waste which has to be disposed upon synthesis.…”

“…[18] The highest yield of 77 %w as obtained by using the electron rich 2,6-dimethoxyphenol as component A and 2-methyl-5-(1-methylethyl)phenol as component B (Scheme 2, 10). This underlines that as terically demanding substituent in position 5i sa blet op revent over-oxidationt o the corresponding diphenoquinone and therefore increases the yield of the desired 4,4'-biphenol.W hen switching the isopropyls ubstituent in position 5t oamethyl group, three more derivatives in good yields up to 62 %a re accessible (Scheme 2, molecules [11][12][13]. Installingafluoro group in position 5, there is ad rop in yield and the desired cross-coupling product 16 was isolated in only 10 %y ield.…”