Efficient Aerobic Oxidation of Acetals to Esters Catalyzed byN-Hydroxy phthalimide (NHPI) and Co(II) under Mild Conditions

Abstract

Efficient oxidation of a variety of structurally diverse acetals, including open-chain acetals, 1,3-dioxanes, 1,3-dioxalanes, with molecular oxygen using catalytic amounts of N-hydroxyphthalimide (NHPI) in the presence of Co(OAc) 2 as co-catalyst was investigated.

An improved protocol for aerobic oxidation of acetals to esters catalyzed by N-hydroxy phthalimide (NHPI) and lipophilic Co(II) complexes

“…Under these circumstances, 2a was almost entirely converted (more than 99% by GC) into 3a within 2 h giving 93% isolated yield ( Table 2, 2a, entry 1). This result is noteworthy, because similar transformation using a catalytic system consisting NHPI (20 mol%) and 1a (1 mol%) in CH 3 CN afforded the corresponding ester 3 after much longer (20 h) reaction time, although the loading of the catalyst was twice as the NHPI/1b system [18].…”

“…Very recently, we have developed a novel method for oxidation of a variety of structurally diverse acetals with molecular oxygen using NHPI combined with Co(OAc) 2 under mild Scheme 1. reaction conditions [18]. Although, this system was successfully applied to a range of benzylic and aliphatic acetals, the reaction time is prohibitively long and the loading of the catalysts required to advance the reaction to high conversion were high.…”

Cobalt(II)/N,N′,N′′‐Trihydroxyisocyanuric Acid Catalyzed Aerobic Oxidative Esterification and Amidation of Aldehydes

“…As evidence of the second, Karimi and co-workersr eported that the oxidative esterification did not occur with the use of paranitro-substituted acetals but proceeded smoothly in the presence of para-methoxy-substitutedo nes catalyzed by ac omparable system of Co II /NHPI at ambient temperature. [18] In this study,w ef ound such am anner of electronic effects to be altered under acidic (Scheme 3A)o rh eated (Scheme3B) conditions, albeit the precise mechanism, taking thesef actors into account,i sd ifficult to elucidate.N evertheless, electron-deficient aldehydes were preferentially convertedi nto esters under the updated conditions, such as at ah igh temperature of 120 8Cint he presence of Ts OH (2 mol %, Scheme 2).…”

“…In relation to this topic, Karimi and Rajabi successfully transformed structurally different acetals into the corresponding esters through an aerobic oxidation protocol by using ac ombination of Co II and N-hydroxyphthalimide (NHPI) as the catalyst. [18] We envisioned the formationo fe ster/amide from an aldehyde could be attained under catalysis by ac ombination of Co II and an itroxyl radical precursor (-NOH)u sing O 2 as the terminal oxidantt hrough ah emiacetal/hemiaminal route. However,c hallengess till remain in the searchf or ap ropers ystem that employs am ore reactive nitroxyl radical, which:( i) keeps the rate of addition of the weakly nucleophilic alkanol/alkoxy group towards the aldehyde higher than that of the aldehyde oxidation [19] and (ii)maintainsp roper reactivity of the nitroxyl radicalt owards the a-C(sp 3 )ÀHa djacent to the O/N atom of the hemiacetal/hemiaminal intermediate.…”

Aerobic oxidation of 4-tert-butyltoluene over cobalt and manganese supported hexagonal mesoporous silicas as heterogeneous catalysts

“…In the last decade, a new derivative, N-hydroxyphthalimide (NHPI), has been reported as an effective catalyst for C H activation by hydrogen abstraction in many oxidation reactions [14][15][16][17] including the oxidation of toluene derivatives [18,19]. However, a high selectivity is even harder to obtain because the corresponding products are usually more reactive so that it is difficult to stop at the corresponding alcohols or aldehydes [15,20].…”