Tohru Kamitanaka

Coupling of Quinone Monoacetals Promoted by Sandwiched Brønsted Acids: Synthesis of Oxygenated Biaryls†

addition to the carbonyl group, conjugated addition to the a,b-unsaturated carbonyl moieties, and others. In contrast to the established addition chemistry of quinone monoacetals regarding the reactivity of the a,b-unsaturated carbonyl group, methods for utilizing the allyl acetal functionality are quite limited for the reactions with nucleophiles. In fact, reactions regarding the introduction of nucleophiles to the allylic position of the acetal units (which also corresponds to the a position of the a,b-unsaturated carbonyl) have rarely been reported, 5] thus significant advances in substitution chemistry are possible. Herein, we describe a general protocol for the introduction of nucleophiles to quinone monoacetals by substitution utilizing the unusual protons in polyanions, namely, sandwiched Brønsted acids, as activators. The strategy can provide an attractive new route to the valuable oxygenated biaryl compounds 3 [Eq. (1)].

Efficient synthesis of oxygenated terphenyls and other oligomers: sequential arylation reactions through phenol oxidation-rearomatization.

One by one: starting from simple phenols, a diverse series of oxygenated terphenyl compounds can be prepared in a concise and practical manner using sequential arylation reactions involving phenol oxidation/rearomatization and quinone monoacetal intermediates. Many of the terphenyl products can be used for preparing well-defined oligomers and, furthermore, contain valuable functional groups that can be transformed for further diversification.

[3+2] coupling of quinone monoacetals by combined acid-hydrogen bond donor.

The expeditious and efficient [3+2] coupling approach of quinone monoacetals 1 with alkene nucleophiles 2 by the action of an activated Brønsted acid in the presence of a hydrogen bond donor perfluorinated alcohol has been achieved. With the optimized combined acid, the reaction could proceed under mild conditions by only mixing the two reactants to afford the cycloadducts 3 in a short time (within 10 min) with good to quantitative yields.

An excellent dual recycling strategy for the hypervalent iodine/nitroxyl radical mediated selective oxidation of alcohols to aldehydes and ketones

Using a recyclable hypervalent iodine reagent 1, the authors have constructed versatile and green methods for the hypervalent iodine and nitroxyl radical-mediated selective oxidation of alcohols to aldehydes and ketones. The recyclable reagent 1 having a unique tetraphenyladamantane structure exhibited almost the same reactivity as the ordinary reagent, phenyliodine diacetate (PIDA), in the hypervalent iodine(III)/2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)-mediated oxidation. For recycling, the reagent 1 could be nearly quantitatively recovered as the reduced tetraiodide 2 by filtration after reaction completion, utilizing the insolubility of the formed 2 in a polar solvent, specifically, methanol. Based on the confirmed reactivity and excellent recycling operation of the reagent 1, the methodology has been further extended to a greener dual recycling strategy. By combining the recyclable iodine reagent 1 and silica-supported TEPMO catalyst, a variety of alcohols were effectively oxidized to the desired aldehydes, and the two types of used reagent and catalyst, the iodine 1 and immobilized TEMPO 5, could be separately recovered by an easy workup, and both repeatedly used without any loss of their original activities for at least four cycles.

Brønsted Acid-Controlled [3 + 2] Coupling Reaction of Quinone Monoacetals with Alkene Nucleophiles: A Catalytic System of Perfluorinated Acids and Hydrogen Bond Donor for the Construction of Benzofurans

We have developed an efficient Brønsted acid-controlled strategy for the [3 + 2] coupling reaction of quinone monoacetals (QMAs) with nucleophilic alkenes, which is triggered by the particular use of a specific acid promoter, perfluorinated acid, and a solvent, fluoroalcohol. This new coupling reaction smoothly proceeded with high regiospecificity in regard with QMAs for introducing π-nucleophiles to only the carbon α to the carbonyl group, thereby providing diverse dihydrobenzofurans and derivatives with high yields, up to quantitative, under mild conditions in short reaction times. The choice of Brønsted acid enabled us to avoid hydrolysis of the QMAs, which gives quinones, and the formation of discrete cationic species from the QMAs. Notably, further investigations in this study with regard to the acid have led to the findings that the originally stoichiometrically used acid could be reduced to a catalytic amount of 5 mol % loading or less and that the stoichiometry of the alkenes could be significantly improved down to only 1.2 equiv. The facts that only a minimal loading (5 mol %) of perfluoroterephthalic acid is required, readily available substrates can be used, and the regioselectivity can be controlled by the acid used make this coupling reaction very fascinating from a practical viewpoint.

Efficient Coupling Reaction of Quinone Monoacetal with Phenols Leading to Phenol Biaryls.

A simple and efficient synthesis of phenol biaryls by the cross-couplings of quinone monoacetals (QMAs) and phenols is reported. The Brønsted acid catalytic system in 1,1,1,3,3,3-hexafluoro-2-propanol was found to be particularly efficient for this transformation. This reaction can be extended to the synthesis of various phenol biaryls, including sterically hindered biaryls, with yields ranging from 58 to 90 % under mild reaction conditions and in a highly regiospecific manner.