Palladium‐Catalyzed Hydrolytic Cleavage of Aromatic C−O Bonds

Abstract

Metallic palladium surfaces are highly selective in promoting the reductive hydrolysis of aromatic ethers in aqueous phase at relatively mild temperatures and pressures of H 2 .Atquantitative conversions,the selectivity to hydrolysis products of PhOR ethers was observed to range from 50 % (R = Ph) to greater than 90 %( R= n-C 4 H 9 ,c yclohexyl, and PhCH 2 CH 2 ). By analysis of the evolution of products with and without incorporation of H 2 18 O, the pathway was concluded to be initiated by palladium metal catalyzedp artial hydrogenation of the phenyl group to an enol ether.W ater then rapidly adds to the enol ether to form ah emiacetal, which then undergoes elimination to cyclohexanone and phenol/alkanol products.Ar emarkable feature of the reaction is that the stronger PhÀObond is cleaved rather than the weaker aliphatic O À Rb ond.

Palladium‐Catalyzed Reductive Insertion of Alcohols into Aryl Ether Bonds

“…[3c, 8a, 9] In order to develop quantitative alcoholysis, we report here a mechanistic study of the reductive transetherification of diphenyl ether and phenyl cyclohexyl ether, as examples of diaryl ethers, as well as aryl-alkyl ethers. Building upon our previous findings [5] on reductive hydrolysis of aryl ethers (e.g., PhOPh and PhOR) suggests that reduction in methanol should occur as in Scheme 1 to produce methoxycyclohexane) and ROH, or when R = Ph, phenol which ultimately is converted to methoxycyclohexane and cyclohexanol. In the following, we first analyze in detail the reactions of ethers in methanol.…”

“…We reported recently the mechanism of selective hydrolytic cleavage of the arene-oxygen bond in aromatic ethers. [5] Initiated by partial hydrogenation of the arene ring to an enol ether, the aromatic C-O bond is cleaved in this mechanism by addition of water. Water rapidly reacts with the enol ether to form a hemi-ketal, which undergoes elimination to cyclohexanone and phenol/alkanol (Eq.…”

“…1). [5][6] (Eq. 1) The question arises now, whether alcohols can also cleave such ether bonds and can be used for transetherification.…”

“…The initial selectivity to reductive methanolysis is 18 % for diphenyl ether and 73 % for cyclohexyl phenyl ether, lower than the selectivities for reductive hydrolysis in aqueous phase (49 % and 87 % for diphenyl ether and cyclohexyl phenyl ether, respectively). [5] For diphenyl ether, the difference in selectivities of the two solvents, water and methanol, is attributed to the different rates of solvolysis of 1-phenoxycyclohexene; e.g., in the absence of catalyst, 72 % of 1-phenoxycyclohexene is converted in 10 min in water, [5] compared to 66 % in 120 min in methanol (Figure 2). Thus, when 1-phenoxycyclohexene is formed from partial hydrogenation of diphenyl ether and desorbed from the metal surface, it reacts rapidly with water, whereas in methanol it readsorbs on the metal and is hydrogenated.…”

“…Hydrogenation of diphenyl ether accounts for 80% C-selectivity in methanol, as compared to 49% in aqueous phase. [5] For the conversion of the primary product aryl ether, the initial products from Pd-mediated reductive methanolysis are methoxycyclohexene and cyclohexanol (1:1), the former being rapidly hydrogenated to methoxycyclohexane.…”