Recyclable Bismuth Complex Catalyzed 1,6-Conjugate Addition of Various Nucleophiles to para -Quinone Methides: Expedient Access to Unsymmetrical Diaryl- and Triarylmethanes

Csp2-H functionalization of phenols: an effective access route to valuable materials via Csp2-C bond formation

In the vast majority of top-selling pharmaceutical and industrial products, phenolic structural motifs are highly prevalent. Non-functionalized simple phenols serve as building blocks in the synthesis of value-added chemicals. It is worth mentioning that lignin, being the largest renewable biomass source of aromatic building blocks in nature, mainly consists of phenolic units, which enable the production of structurally diverse phenols. Given their remarkable applicability in the chemical value chain, many efforts have been devoted to increasing the molecular complexity of the phenolic scaffold. Among the key techniques, direct functionalization of Csp2-H is a powerful tool, enabling the construction of new Csp2-C bonds in an economical and atomic manner. Herein we present and summarize the large plethora of direct Csp2-H functionalization methods that enables scaffold diversification of simple, unprotected phenols, leading to the formation of new Csp2-C bonds. In this review article, we intend to summarize the contributions that appeared in the literature mainly in the last 5 years dealing with the functionalization of unprotected phenols, both catalytic and non-catalytic. Our goal is to highlight the key findings and the ongoing challenges in the stimulating and growing research dedicated to the development of new protocols for the valorization of phenols.

Synthesis of diarylmethyl thioethers via a DABCO-catalyzed 1,6-conjugate addition reaction of p-quinone methides with organosulfur reagents

A rapid and simple method was developed for the synthesis of diarylmethyl thioethers via a DABCO-catalyzed 1,6-conjugate addition reaction of para-quinone methides (p-QMs) with organosulfur reagents. A series of diarylmethyl thioethers were synthesized at 13-85% yields by this method. After that, the antibacterial activities of synthesized diarylmethyl thioethers and their derivatives were evaluated. The MIC range (μg mL^-1) against Staphylococcus aureus ATCC 25923 and clinically isolated methicillin-resistant S. aureus was 8-128 and 64-128, respectively.

Divergent oxidative dearomatization coupling reactions to construct polycyclic cyclohexadienones

Highly selective divergent oxidative dearomatization coupling reactions, in which the chemoselectivity is controlled by catalysts and bases, are reported herein. Three different kinds of polycyclic cyclohexadienones are produced from the same reactants (41 examples, 85-99% yield). Our method marks a novel copper- and palladium-catalyzed C-H oxidative dearomatization of phenolic derivatives.

Enantioselective decarboxylative Mannich reaction of β-keto acids with C-alkynyl N-Boc N,O-acetals: access to chiral β-keto propargylamines

The chiral keto-substituted propargylamines are an essential class of multifunctional compounds in the field of organic and pharmaceutical synthesis and have attracted considerable attention, but the related synthetic approaches remain limited. Therefore, a concise and efficient method for the enantioselective synthesis of β-keto propargylamines via chiral phosphoric acid-catalyzed asymmetric Mannich reaction between β-keto acids and C-alkynyl N-Boc N,O-acetals as easily available C-alkynyl imine precursors has been demonstrated here, affording a broad scope of β-keto N-Boc-propargylamines in high yields (up to 97%) with generally high enantioselectivities (up to 97 : 3 er).

Nucleophilic Reactivities of Thiophenolates

The nucleophilic reactivities of substituted thiophenolates were determined by following the kinetics of their reactions with a series of quinone methides (reference electrophiles) in DMSO at 20 °C. The experimentally determined second-order rate constants were analyzed according to the Mayr-Patz equation log k = s_N(N + E) to derive the nucleophile-specific reactivity parameters N and s_N for ten thiophenolate ions.

Construction of fully substituted carbon centers containing a heteroatom and a CF3 group via in situ generated p-quinone methides

1,6-Conjugate additions of in situ generated δ-CF3-δ-substituted p-quinone methides have been achieved with a variety of heteronucleophiles under mild conditions, which led to facile and practical construction of fully substituted carbon centers including a heteroatom and a CF3 group. In particular, it was revealed that some amines themselves worked for efficient cleavage of the TBS protective group, and addition of a catalytic amount of an appropriate Brønsted acid was found to sometimes improve the progress of the desired process.

Metal-Free, Acid/Phosphine-Induced Regioselective Thiolation of p-Quinone Methides with Sodium Aryl/Alkyl Sulfinates

A simple and efficient method for the regioselective thiolation of p-quinone methides with sodium aryl/alkyl sulfinates has been established using an acid/phosphine-induced radical route under transition-metal-free conditions. A broad range of sodium aryl/alkyl sulfinates and p-quinone methides (p-QMs) are compatible for the reaction, giving the expected products with good to excellent yields. Control experiments were also performed to gain insights into the generation mechanism of thiyl radicals and hydrogen-atom transfer process. This protocol provides a safe and feasible way for the formation of carbon-sulfur bonds.

Silver(i)-catalyzed novel ipso-cycloaddition and retro-Friedel–Crafts reaction of ortho-hydroxyphenyl-substituted para-quinone methides

The Lewis acid AgPF6 was found to catalyze novel [2 + 2] ipso-cycloaddition and retro-Friedel–Crafts reaction of o-hydroxyphenyl-substituted p-quinone methides in the presence of H2O, affording the polysubstituted triarylmethanes in up to 97% yields.