Heterogeneous Palladium‐Catalyzed Synthesis of Aromatic Ethers by Solvent‐Free Dehydrogenative Aromatization: Mechanism, Scope, and Limitations Under Aerobic and Non‐Aerobic Conditions

Abstract

Starting from cyclohexanone derivatives and alcohols, both non‐aromatic precursors, aryl ethers could be synthesized in good yields and with good selectivities in the presence of a catalytic amount of Pd/C, in one step, without added solvent, in a reaction vessel open to air. For less reactive substrates, the addition of 1‐octene in a closed system under non‐aerobic conditions improved the conversion. In addition, the catalyst could be recycled several times with no decrease in the yield of the aryl ether. The process was also used with tetralone derivatives and polyols. Several reactions were performed to propose a mechanism for this transformation. The formation of an enol ether followed by a dehydrogenation reaction seem to be the key steps of this reaction.

Palladium Nanoparticles Supported on Fibrous‐Structured Silica Nanospheres (KCC‐1): An Efficient and Selective Catalyst for the Transfer Hydrogenation of Alkenes

“…We further explored the hydrogenation of a cyclic enone, such as 2‐cyclohexen‐1‐one (Table 4, entries 16, and 17); this reaction afforded a mixture of cyclohexanone and phenol. The formation of phenol can be attributed to the aerobic dehydrogenation of the cyclohexenone moiety and this reaction has been reported to be promoted by molecular Pd complexes28 or by Pd/C;29 the involvement of Pd nanoparticles in this process has been proposed 30. Interestingly, cyclohexanone is also formed in the absence of formic acid, with phenol being the main product in this case.…”

Synthesis of 1‐Naphthol‐based Unsymmetrical Triarylmethanes: Heck‐type Desulfitative Reaction of Arylsulfonyl Chlorides with Tetralone‐derived Chalcones

“…Based on the observed products and in concurrence with mechanism proposed in the literature for the desulfitative coupling/conjugate addition of arylsulfonyl chloride with enones, [6] a plausible reaction pathway for the formation of 1‐naphthol‐based unsymmetrical triarylmethanes 7 in the Pd‐catalyzed Heck‐type desulfitative reaction/conjugate addition of arylsulfonyl chlorides 6 with tetralone‐derived chalcones 5 is proposed (Scheme 5). In concurrence with the proposed mechanism for the reactions involving the Pd‐catalyzed aromatization of tetralones into 1‐naphthols, [7a–f] it is proposed that after the initial Heck coupling step with the palladium species ( III ), [6] the formation of the O ‐Pd enolate 10 a in equilibrium with C ‐Pd enolate 10 b occurs. The intermediate 10 b undergoes the β ‐hydride elimination to afford the unsaturated α ‐tetralone intermediates 10 c or 10 d , which then affords the 1‐naphthol derivative 7 via the tautomerization process [7a] .…”

“… Proposed mechanism for the formation of 1‐naphthol‐based unsymmetrical triarylmethanes 7 from 5 [6c,7a–f,14–16] …”

Catalytic dehydrogenative aromatization: an alternative route to functionalized arenes

“…19,20 Chao-Jun Li Shortly afterwards, Lemaire et al presented a complementary carbon-supported palladium system for the synthesis of aryl ethers (Scheme 6). 21,22 Interestingly, cyclohexanones have a higher level of activity than cyclohexenones under the present system and, although requiring an elevated temperature to achieve complete conversion, tetralones were also suitable partners. It is also worth noting that this process was also efficient with 2-methyltetrahydrothiophen-3-one to give the corresponding aryl ether in good yields.…”