Reductive Coupling of Aromatic Aldehydes Promoted by an Aqueous TiCl₃/tBuOOH System in Alcoholic Cosolvents

Abstract

The tert‐butoxyl radical, generated by the aqueous TiIII/TBHP system, abstracts an H atom from alcoholic cosolvents (EtOH, iPrOH), leading to α‐hydroxyalkyl radicals thatreduce aromatic aldehydes to the corresponding 1,2‐diols. The reactivities observed are explained by resonance stabilization of the α‐hydroxybenzyl radicals formed in the electron‐transfer (ET) process. Good Hammett‐type correlations are obtained. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Visible Light Induced Reduction and Pinacol Coupling of Aldehydes and Ketones Catalyzed by Core/Shell Quantum Dots

“…The crude product was chromatographed on silica (petroleum ether/EtOAc) to give a white solid, 83% (74.3 mg). dl- 7k : 1 H NMR (400 MHz, DMSO) δ 7.85 (d, J = 8.4 Hz, 4H), 7.38 (d, J = 8.3 Hz, 4H), 5.52 (dd, J = 3.2, 1.5 Hz, 2H), 4.68 (d, J = 4.3 Hz, 2H), 2.53 (s, 6H).…”

New domino radical synthesis of aminoalcohols promoted by TiCl4–Zn/t-BuOOH system: selective hydroxyalkylation of amines in alcohol or in cyclic ether cosolvents

“…16 We ascribed this behavior to the lower acidity of the reaction medium under the new conditions, while it is well known that redox potential of ketyls increases on decreasing the pH. 17 To verify this hypothesis and with the aim of improving the efficiency of the new protocol, we added a small amount of acetic acid (AcOH) to the reaction mixture and succeeded in increasing the yield up to 95%, even higher than that reported with the TiCl 3 /t-BuOOH system (entry 3).…”

New Advances in Titanium-Mediated Free Radical Reactions

“…In fact, in this case the ketyl radical formed behaves as a strong reducing agent [ 41 ] and competes with Ti(III) in the reduction of t -BuOOH, leading to the formation of the corresponding aldehyde. The latter, when in the presence of an aniline, promotes the formation of an aldimine, which is prone to nucleophilic attack by a second ketyl radical.…”

“…More recently we have unexpectedly found that, despite the non-anhydrous conditions, the aqueous TiCl 3 / t -BuOOH redox system promotes the hydrodimerization of aromatic aldehydes, when used in combination with an alcoholic co-solvent like ethanol [ 41 ].…”