Benzylic alcohols, oxidation ionic liquid

Seddon, K.R. Stark, A. (2002) Selective Catalytic Oxidation of Benzyl Alcohol and Alkylbenzenes in Ionic Liquids. Green Chemistry, 4(2), 119-123. 

The application of ionic liquids as a reaction medium for the copper-catalyzed aerobic oxidation of primary alcohols was reported recently by various groups, in attempts to recycle the relatively expensive oxidant TEMPO [150,151]. A TEMPO/CuCl-based system was employed using [bmim]PF6 (bmim = l-butyl-3-methylimodazolium) as the ionic liquid. At 65 °C a variety of allylic, benzylic, aliphatic primary and secondary alcohols were converted to the respective aldehydes or ketones, with good selectiv-ities [150]. A three-component catalytic system comprised of Cu(C104)2, dimethylaminopyridine (DMAP) and acetamido-TEMPO in the ionic liquid [bmpy]Pp6 (bmpy = l-butyl-4-methylpyridinium) was also applied for the oxidation of benzylic and allylic alcohols as well as selected primary alcohols. Possible recycling of the catalyst system for up to five runs was demonstrated, albeit with significant loss of activity and yields. No reactivity was observed with 1-phenylethanol and cyclohexanol [151]. 

A new ionic liquid, l-butyl-3-methylimidazolium tribromide can act as an oxidizing agent to convert alcohols to aldehydes and ketones.338 In the case of benzyl alcohols and diols, [Bmim][Br3] combines oxidizing and brominating properties in a one-pot synthesis of /3-bromoethyl esters. 

Odourless and non-volatile organosulfur compounds grafted to an imidazolium ionic liquid scaffold has been synthesized. The sulfoxides have been used for an efficient oxidation of primary allylic and benzylic alcohols into aldehydes and secondary alcohols to ketones under Swern oxidation conditions and the corresponding sulfides can be recovered and recycled.110... 

An efficient and convenient methodology for the aerobic oxidation of alcohols catalysed by sol-gel trapped perruthenate and promoted by an encapsulated ionic liquid in supercritical carbon dioxide solution has been reported. The reaction is highly selective and useful for substrates otherwise difficult to oxidize.263 A four-component system consisting of acetamido-TEMPO-Cu(C104)2-TMDP-DABCO has been developed for aerobic alcohol oxidation at room temperature. The catalytic system shows excellent selectivity towards the oxidation of benzylic and allylic alcohols and is not deactivated by heteroatom-containing (S, N) compounds. The use of DMSO as the reaction medium allows the catalysts to be recycled and reused for three runs with no significant loss of catalytic activity.264... 

Benzylic alcohols are oxidized to aldehydes by using the TEMP0-HBr-H202 system in ionic liquid [bmim]PF6. The electron-deficient and electron-neutral benzylic alcohols were oxidized in good to excellent yields, whereas the electron-rich benzylic alcohols failed to afford the desired product due to side-reactions.193... 

Ruthenium compounds have been extensively studied as catalysts for the aerobic oxidation of alcohols [142]. They operate under mild conditions and offer possibilities for both homogeneous and heterogeneous catalysts. The activity of common ruthenium precursors such as RuCl2PPh3, can be increased by the use of ionic liquids as solvents (Fig. 4.58). Tetramethylammoniumhydroxide and aliquat 336 (tricaprylylmethylammonium chloride) were used as solvent and rapid conversion of benzyl alcohol was observed [145]. Moreover the tetra-methylammonium hydroxide/RuCl2(PPh3)3 could be reused after extraction of the product. 

Much effort has been devoted to finding synthetically useful methods for the palladium-catalyzed aerobic oxidation of alcohols. For a detailed overview the reader is referred to several excellent reviews [163]. The first synthetically useful system was reported in 1998, when Peterson and Larock showed that simple Pd(OAc)2 in combination with NaHC03 as a base in DMSO as solvent catalyzed the aerobic oxidation of primary and secondary allylic and benzylic alcohols to the corresponding aldehydes and ketones, respectively, in fairly good yields [164, 165]. Recently, it was shown that replacing the non-green DMSO by an ionic liquid (imidazole-type) resulted in a three times higher activity of the Pd-catalyst [166]. 

The use of Cu in combination with TEMPO also affords an attractive catalyst [200, 201]. The original system however operates in DMF as solvent and is only active for activated alcohols. Knochel et al. [202] showed that CuBr.Me2S with perfluoroalkyl substituted bipyridine as the ligand and TEMPO as cocatalyst was capable of oxidizing a large variety of primary and secondary alcohols in a fluorous biphasic system of chlorobenzene and perfluorooctane (see Fig. 4.69). In the second example Ansari and Gree [203] showed that the combination of CuCl and TEMPO can be used as a catalyst in l-butyl-3-methylimidazolium hexafluorophosphate, an ionic liquid, as the solvent. However in this case turnover frequencies were still rather low even for benzylic alcohol (around 1.3 h 1). 

The aerobic oxidation of benzylic and allylic alcohols to their corresponding carbonyl compounds in [C4Ciim][PF6] with TEMPO-CuCl (TEMPO = 2,2,6,6-tetramethylpiperidinyl-l-oxy) as catalyst was found to proceed at higher rates than that of aliphatic alcohols, which is in agreement with results in classical solvents.[75] After product extraction with diethyl ether, the ionic liquid was washed with water and dried at 70°C, prior to the next run. In that manner catalyst activity remained relatively stable for 8 cycles. 

Using this method, oxidation of benzylic alcohols with electron donor substituents takes place faster than benzylic alcohols with electron-withdrawing groups. Also, reaction of aliphatic alcohols occurred with lower yields and longer reaction times. Therefore, chemoselectivity with this system was observed. Ionic liquid is recyclable in this method and can be reused for four times without loss of activity. 

Scheme 14.24 Oxidation of benzylic alcohols in hexaalkylguanidinum-based ionic liquid...

Industrially performed catalytic oxidation reactions often suffer from drawbacks such as poor conversion and selectivity due to overoxidation, corrosive reaction media, lack of solvent and catalyst recycling, and negative environmental impact due to evaporation of the solvents. In order to provide a methodology that addresses these problems, ionic liquids have been investigated as reaction media. For example, the aerobic oxidation of benzyl alcohol and alkylbenzene to benzaldehyde and benzoic acids was performed in l-butyl-2,3-dimethylimidazolium tetrafluoroborate ([C4dmim][BF ]) using palladium and cobalt complexes respectively [34, 35]. 

In 2004, Kumar and coworkers reported that KMnO in the ionic liquid [bmim] [BF ] can be used as an efficient reagent for the chemoselective oxidation of ben-zylic alcohols in the presence of aliphatic ones (Scheme 14.51) [53]. They also showed that the oxidation of benzyl alcohols using recycled KMnO and [bmim] [BF ] gave benzaldehyde with excellent yields even after five runs. 

Reactions of ammonium hexanitratocerate(IV) with organic substrates other than benzyl alcohol have also been examined, and 1,4-hydroquinone was quantitatively transformed into 1,4-quinone. Anisole and naphthalene can be nitrated. For the cerium-mediated oxidation reactions in ionic liquids, high reaction temperature is beneficial because of the formation of smaller amounts of by-products. 

Jiang N, Ragauskas AJ (2005) TEMPO-catalyzed oxidation of benzylic alcohols to aldehydes with the HjO /HBr/ionic liquid [bmim][PFJ system. Tetrahedron Lett 46 3323-3326... 

Lee BS, Lee JC (2009) Oxidation of benzylic alcohols with iodine and lithium carbonate in ionic liquid. Bull Korean Chem Soc 30 3107-3108... 

Xie Y, Zhang Z, Hu S, Song J, Li W, Han B (2008) Aerobic oxidation of benzyl alcohol in supercritical CO catalyzed by perruthenate immobilized on polymer supported ionic liquid. Green Chem 10 278-282... 

Kumar A, Jain N, Chauhan SMS (2004) Oxidation of benzylic alcohols to carbonyl compounds with potassium permanganate in ionic liquids. Synth Commun 34 2835-2842... 

Hajipour AR, Rafiee F, Ruoho AE (2007) Eacile and selective oxidation of benzylic alcohols to their corresponding carbonyl compounds with sodium nitrate in the presence of Brpnsted acidic ionic liquids. Synlett 7 1118-1120... 

Lee JC, Lee JY, Lee JM (2005) Efficient procedure for oxidation of benzylic alcohols to carbonyl compounds by N-bromosuccinimide in ionic liquid. Synth Commun 35 1911-1914... 

Chang SU, Cho JH, Lee JC (2008) Efficient oxidation of benzylic alcohols to aldehydes and ketones in ionic liquid using N-chlorosuccinimide/AlClj-6HjO. Bull Korean Chem Soc 29 27-28... 

Xie H, Zhang S, Duan H (2004) An ionic liquid based on a cyclic guanidinium cation is an efficient medium for the selective oxidation of benzyl alcohols. Tetrahedron Lett 45 2013-2015... 

One of the main objectives of using ionic liquid as reaction medium is the possibility to recycle the catalyst. The catalytic solution could indeed be reused eight times for the oxidation of benzyl alcohol to benzaldehyde, with only a slight loss of activity. In addition, the recovered ionic liquid solvent could be recovered and reused for new oxidations of other substrates. 

These publications demonstrate that there is no limitation on the applicability of the ionic liquid methodology concerning the sort of alcohol. Nearly all oxidations in ionic liquids given in Table 5.3-7 were tested with primary, secondary, allyhc, benzylic alcohols and phenols with good results. Ruthenium with its high number of accessible oxidation states is a very suitable metal for oxidation reactions in ionic liquids (Table 5.3 -7, entries 1-3). As an additional benefit it can be used in the form of its permthenate salt. Thus immobilization and possibly stabilization of the metal is feasible. It should be mentioned that these oxidation reactions have been reported... 

As well as these epoxidation reactions, several oxidation reactions have also been reported in ionic liquids, including the oxidation of aromatic aldehydes to acids, of alcohols to aldehydes or ketones, of styrene to acetophenone or styrene carbonate and of benzyl alcohols. " Ionic liquids exhibit great stability toward oxidants, as has been demonstrated in electrochemical studies, leading to an ever-growing number of publications. 

The oxidation of alcohols to carbonyl compounds is a fundamental reaction that has synthetic and chemical importance. Using chromium-based catalysts, researchers have developed several catalysts that have impacted alcohol oxidation reactions. Recently, homogeneous catalysts have had problems with catalyst/product separation and suffer from poor catalyst recyclability. Therefore, the quest for a resolution to this problem has led researchers to scaffold salen complexes onto a silica-based material such as MCM-41. Zhou et al. used an ion-exchangeable, layered polysiloxane support to immobili.se their sulfonato-(salen)Cr(m) complex. They reacted benzyl alcohol, cyclo-hexanol and -hexanol with hydrogen peroxide as oxidant in an ionic liquid at 40 °C. Several ionic liquids were investigated [BMImX (BMIm = 1-n-butyl-3-methylimidazolium X =PF6, BF4, NOs")] and compared for each substrate. 

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