The catalytic potential of copper(I)-exchanged zeolites was evaluated in the Ullmann-type synthesis of diaryl ethers. Among four investigated zeolites (i.e., USY, MOR, β, and ZSM5), Cu(I)-USY was the best catalyst and proved efficient under ligand-free conditions in toluene at 120 °C. Cu(I)-USY was also easy to recover and was recyclable up to five times without significant loss of activity.
The copper(I)-doped zeolite Cu-USY proved to be a versatile, efficient, and recyclable catalyst for various Ullmann-type coupling reactions. Easy to prepare and cheap, this catalytic material enables the arylation and heteroarylation of diverse O-, N-, S-, and C-nucleophiles under ligand-free conditions while exhibiting large functional group compatibility. The facility of this catalyst to promote C-O bond formation was further demonstrated with the total synthesis of 3-methylobovatol, a naturally occurring diaryl ether of biological relevance. From a mechanistic viewpoint, two competitive pathways depending on the nature of the nucleophile and consistent with the obtained results have been proposed.
A concise synthesis of two scyphostatin analogues is achieved from readily available ortho-substituted phenols. Key features include an asymmetric and biomimetic hydroxylative phenol dearomatization (HPD) reaction promoted by a chiral salen-type bis(λ-iodane) reagent, followed by an in situ regio- and diastereocontrolled epoxidation.
Two symmetric bis-flavylium dications were readily synthesized and further evaluated for their multistate profile. Both systems were shown fully stochastic and behave like a single compound suggesting that the two flavylium moieties do not communicate via the bridge linking them. Global pKa values of ca. 4 regarding the acid-base reaction between flavylium cation and quinoidal base were calculated by stopped flow. It was further demonstrated that the equilibrium between AH + -AH + and indistinguishable flavylium-quinoidal base isomers AH + -A (A-AH + ) can be calculated by subtracting 0.3 pH units to the observed acid-base constant. On the other hand, the equilibrium between both flavylium-quinoidal base isomers and the bis-quinoidal base A-A is obtained by adding 0.3 pH units. Moreover, both systems do not have cis-trans isomerization barrier and the rate constants of interconversion between flavylium cation and quinoidal base as a function of pH are fitted with a mono-exponential and follow a bell-shaped curve. The systems proved also to be photochromic and from the fitting of the bell-shaped curve, flash photolysis measurements and quantum yields, it is possible to calculate all rate and equilibrium constants and construct a global energy-level diagrams. It was also proved that the rate constant to form both isomers AH + -Ct from the bis-flavylium upon a pH jump from 1 to less acidic solutions is twice of the observed value and the formation of the bis-trans-chalcone from the both isomers AH + -Ct is equal to the observed rate constant. An energy-level diagram of all the multistate species was constructed from the equilibrium constants.
Dedicated to the memory of Prof. J. F. Normant, a brilliant scientist and a wonderful person, and in recognition of his outstanding achievements in organometallic chemistry.
Hypervalent Iodine-Mediated Oxygenative Phenol Dearomatization Reactions. -Hypervalent iodine compounds are useful reagents in the dearomatization of phenols and in the chemo-and regioselective demethylation of 2-methoxyphenols into catechols. In the latter reactions the use of stabilized IBX affords better results than the use of IBX itself. -(POUYSEGU, L.; SYLLA, T.; GARNIER, T.; ROJAS, L. B.; CHARRIS, J.; DEFFIEUX, D.; QUIDEAU*, S.; Tetrahedron 66 (2010) 31, 5908-5917,
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