N-Hydroxyphthalimide/Cobalt(II) Catalyzed Low Temperature Benzylic Oxidation Using Molecular Oxygen
Introduction
The selective oxidation of organic substrates using dioxygen (molecular oxygen) as the ultimate oxidant is a very important synthetic and industrial goal. With respect to its use in the manufacture of organic chemicals, catalytic oxidation with dioxygen traditionally holds a very prominent place in the petrochemical industry, where it is by far the most important technology for the upgrading of hydrocarbons.1 The dominant position of dioxygen as the oxidant for bulk chemical oxy-functionalizations is due to the fact that it is the only economically and environmentally feasible oxidant for large scale processing. In contrast, even though dioxygen is ideal from an economic and environmental point of view and the need for the development of sustainable processes steadily increases, the fine-chemical business is still heavily dependent on stoichiometric oxidants, such as permanganate and dichromate. One of the reasons for the lack of fine-chemical applications of catalytic oxy-functionalizations is that the high molecular complexity of fine-chemical substrates is usually not compatible with the rather forcing reaction conditions (high temperature and pressure) required for traditional oxidations with dioxygen. These processes are generally not very selective and accordingly only suitable for the oxidation of relatively simple substrates. Another problem associated with traditional dioxygen oxidations which hampers its application in fine-chemical manufacture is the low conversion that is usually required in order to obtain acceptable selectivities. Such low conversion processes are generally best carried out in dedicated, continuous plants commonly found in the bulk industry, but they are rarely compatible with the batch production facilities that are common in the fine-chemical industry. Thus, within the fine-chemical industry there is a need for selective, high conversion dioxygen based oxy-functionalizations that can be operated under mild conditions.
Recently, an efficient catalytic method for the low-temperature oxygenation of organic substrates with dioxygen was developed by Ishii et al.2 using N-hydroxyphthalimide (2-hydroxy-1H-isoindole-1,3-dione, NHPI, 1a) as a catalyst and a metal salt as co-catalyst. With this system, organic compounds containing sufficiently reactive C–H bonds, e.g. (cyclic) alkanes,3 sulfoxides,4 alkylbenzenes5 and aromatic compounds containing benzylic functions2., 6. have been oxidized at moderate temperatures up to high conversion.
The cobalt salt/NHPI system of Ishii resembles the classical Co/Br− catalyzed oxidation of hydrocarbons1 in the sense that under oxidative conditions, NHPI is converted into its corresponding radical phthalimide-N-oxyl (PINO 2), which, like bromine atoms formed from bromide in the Co/Br system, is able to abstract hydrogen from C–H bonds, thus propagating the radical oxidation chain (Fig. 1). However, in contrast to bromide catalysis, N-hydroxyphthalimide-based oxidation catalysis offers the possibility to tune the catalyst performance by modifying NHPI via introduction of substituents on the aryl ring. We have investigated this possibility and report our findings here.
In addition to studying variations of the NHPI catalyst, we wished to study different substrates in order to investigate the scope and limitations of the oxidation procedure. We started our research by studying the oxidation of substrates that are challenging for industrial purposes such as benzylic oxidation of phenylacetic esters and heteroaromatic compounds. Around 1950 the preparation of phenylglyoxalic acid by oxidation of methyl phenylacetate with air and with the aid of a cobalt(II) salt, at 110°C was reported in the Russian literature. After 36 h 86% of the glyoxylate was obtained.7 Only very recently was NHPI in combination with dioxygen used to oxidize ethyl phenylacetate to ethyl phenylglyoxylate (ethyl 2-oxophenylacetate).8 With n-Bu4NBr as additive, moderate conversions (65%) were achieved. With bis(acetylacetonato)cobalt(II) (Co(acac)2), a low conversion was reported. Only one other example in which an arylacetate is oxidized using molecular oxygen is known in the literature, viz. in one of the steps of the synthesis of (±)-clavizepine.9 To oxidize ethyl 2-[2(benzyloxy)phenoxy]-4,5-dimethoxyphenylacetate, lithium diisopropylamine (LDA) and molecular oxygen were used in combination with hexamethylphosphoric triamide (HMPA) in THF to obtain the corresponding glyoxylate in 63% yield, with concomitant formation of 14% of the hydroxyketone. Finally, oxidations of arylacetic esters to arylglyoxylic esters using hydroperoxides instead of molecular oxygen in combination with for example a vanadium catalyst have been reported in the literature.10 The yields of these oxidations vary from 24% (p-nitro substituted) to 88% (p-methoxy substituted) glyoxylate.
In this paper, the oxidation of a variety of substrates catalyzed by NHPI and cobalt(II) is described, followed by a study in which different substituted NHPI catalysts are tested. We end with a discussion of the reaction mechanism.
Section snippets
Oxidation of arylacetic esters
We investigated the aerobic oxidation of a range of arylacetic esters, using NHPI/cobalt(II) acetate (Co(OAc)2·4H2O) as the catalytic system. The results are presented in Table 1. It can be seen from this table that the benzylic position of arylacetic esters can be oxidized very efficiently. Remarkably, the conversion and selectivity towards glyoxylate exceed those obtained with the NHPI/Co(acac)2 aided system used by Matsunaka et al.8 It is important to note that, while the reaction time is
Discussion
A mechanism for the catalytic oxidation of toluene with NHPI, CoII and O2 has been proposed by the group of Ishii.5., 6. It involves the formation of the phthalimide-N-oxyl radical 2, and the subsequent abstraction by this radical of a hydrogen atom from the substrate as the initiating steps. After this, the substrate radical R
reacts rapidly with O2, forming an alkylperoxy radical. Two molecules of the latter rearrange to the ketone (K) and alcohol (A); the well-known Russell termination (
General
All chemicals were purchased from Acros Chimica or Aldrich and were used as received. NMR spectra were taken on a Bruker AM-300 (300 MHz) or Bruker ACF-200 (200 MHz) spectrometer, chemical shifts are given in ppm with TMS as internal standard. FT-IR measurements in KBr tablets were performed on a Perkin–Elmer 298 spectrometer. Elemental analyses were measured on a Carlo Erba Instruments CHNSO 1108. Mass spectra (EI) were taken on a Fisons VG7070E instrument.
Syntheses
The substituted N-hydroxyphthalimides 1a
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2022, Comprehensive Organometallic Chemistry IV: Volume 1-15