Bu₄NI‐Catalyzed CO Bond Formation by Using a Cross‐Dehydrogenative Coupling (CDC) Reaction

Abstract

The cleavage and functionalization of CÀH bonds is of fundamental interest for both academia and industry. Generally, the transformation relies on transition metals, [1] which are involved in four major approaches: 1) electrophilic activation of the C À H bond by a high-valent transition metal; 2) oxidative addition to the C À H bond by low-valent transition metals; 3) C À H bond activation by s-bond metathesis, and 4) insertion of a metal carbenoid/nitrenoid into the CÀ H bond. After extensive studies, transition-metal-catalyzed CÀH activation has arisen as an excellent synthetic method to build complex structures because it reduces prefunctionalization while improving atom economy and energy efficiency. However, the use of expensive metal catalysts and the problems involved in removing the residual metals from the final products, which is usually a difficult and tedious process, limits the practical applications of this strategy. The discovery of an efficient CÀH transformation that does not require a metal catalyst would be of great value. This strategy would eliminate the requirement to remove traces of metal from the final products and solve the problem of disposal of the metal catalyst from the reaction mixtures. Recently, several groups disclosed a variety of novel C À C bond formations by using CÀH activation under transition-metalfree conditions. [2,3] The cross-dehydrogenative coupling (CDC) reaction, beyond traditional cross-couplings, has been the object of increasing interest over the last ten years. However, transition-metal catalysts, such as iron and copper salts, were usually required to promote this transformation. [4,5]

Copper Catalyzed C−H Activation

“…The reaction of cinnamic acid with 1,4‐dioxane gave (E)‐2‐styryl‐1,4‐dioxane in 55 % yield (Scheme , 6). In contrast, benzoic acid underwent cross‐dehydrogenative coupling with 1,4‐dioxane providing 1,4‐dioxan‐2‐yl benzoate in 91 % yield (Scheme , 8) …”

Development of new methods in modern selective organic synthesis: preparation of functionalized molecules with atomic precision

“…164 The coupling was carried out with low-molecular-mass ethers taken in a 20-fold excess with respect to the acid. It is assumed that the tert-butoxyl radical generated in the Bu n 4 NI7Bu t OOH system detaches a hydrogen atom from the a-position of the ether; the C-radical thus formed is oxidized to the carbocation, which reacts with the carboxylate anion to give the coupling product.…”

Oxidation and Reduction

“…The transformation involved a cross-dehydrogenative coupling (CDC) reaction of the C−H bond. 236 The 3,4-epoxy syn-alcohols (83), (84), and (85) …”

“…The asymmetric transfer hydrogenation of ketones, catalysed by Ru(II)-complex with the unsymmetrical, vicinal diamine-based ligand(236), in HCO 2 H-Et 3 N azeotrope, occurred readily to form alcohols with up to 85-98% ee. It was noted that the presence of an aromatic substituent on the carbon bearing the −NH 2 /NHTs group and a bulky alkyl substituent on the other side are important for the effectiveness of the ligand.…”