(Hypo)Iodite Catalysis for Enantioselective Oxidative Cyclisations

Scientific article - Peer reviewed CATALySIS Kazuaki Ishihara In situ-generated chiral quaternary ammonium (hypo)iodite catalysis for enantioselective oxidative cyclisations MUHAMMET UyANIK, KAZUAKI ISHIHARA* *Corresponding author Nagoya University, Furo-cho, Chikusa, Nagoya, 464-8603, Japan of ketones with moderate enantioselectivities (up to 39% ee) KEyWORDS: Hypervalent iodine, (hypo)iodite, quaternary using Koser-type chiral l3-iodane that was generated in situ ammonium salt, asymmetric oxidation, cycloetherification. from iodoarene 1 and p-toluenesulfonic acid with m-CPBA (6). ABSTRACT: Very recently, we developed the first enantioselective Kita and co-workers reported the intramolecular enantioselective oxidative cycloetherification of ketophenols to 2-acyl- oxidative dearomatisation of 1-naphthol derivatives to 2,3-dihydrobenzofuran derivatives catalysed by in situ- spirolactones with high enantioselectivities (up to 86% ee) 3 generated chiral quaternary ammonium (hypo)iodite with using a chiral l -iodane reagent, which has a conformationally hydrogen peroxide or tert-butyl hydroperoxide (TBHP) as an rigid 1,1-spiroindanone backbone (7). They also succeeded in 3 environmentally benign ideal oxidant. The optically active the catalytic use of chiral l -iodane (30 mol % based on 2-acyl 2,3-dihydrobenzofuran skeleton is a key structure in iodine) that was generated in situ from the corresponding several medicinally and biologically active compounds. iodoarene 2 in the presence of acetic acid, although the enantioselectivity was reduced to 69% ee (7). Recently, we reported the use of conformationally flexible C2-symmetric INTRODUCTION chiral iodoarene 3 as a more effective precatalyst for the enantioselective Kita oxidative spirolactonisation reaction (8). Over the past two decades, hypervalent iodine compounds have been the focus of great attention as environmentally benign oxidation reagents in place of rare or toxic heavy metal oxidants (1). However, the stoichiometric use of hypervalent iodine compounds has been limited because of their potentially shock-sensitive explosiveness and/or poor solubility in common organic solvents (1). Thus, the development of hypervalent iodine-catalysed oxidation reactions with co-oxidants is needed (2). In particular, the development of chiral hypervalent iodine- catalysed enantioselective oxidative coupling reactions is one of the most challenging areas in asymmetric organocatalysis. We describe here the results of studies on the enantioselective oxidative cycloetherification of ketophenols to 2-acyl-2,3- dihydrobenzofuran derivatives catalysed by in situ-generated C2-symmetric chiral binaphthyl-based quaternary ammonium (hypo)iodite with hydrogen peroxide or tert-butyl hydroperoxide (TBHP) (3). Several medicinally and biologically active compounds possess a 2,3-dihydrobenzofuran skeleton that contains a chiral carbon at the 2-position (4). While, several methods have been reported for the construction of these, mainly, 2-alkenyl-derivatives (5), there have been no reports of their successful asymmetric organocatalysis. hYPERVALENT IODINE-CATALYSED ENANTIOSELECTIVE OXIDATIVE TRANSFORMATIONS wITh m-CPBA To the best of our knowledge, there are only four examples of in situ-generated chiral aryl-l3- or aryl- l5-iodane catalysis with meta-chloroperbenzoic acid (m-CPBA) as a co-oxidant (Figure 1). Figure 1. Chiral hypervalent iodine-catalysed enantioselective oxidations Wirth and co-workers reported the asymmetric oxysulfonylation with m-CPBA. 18 chimica oggi/Chemistry Today - vol 29 n 1 January/February 2011 chimica oggi/Chemistry Today - vol 29 n 1 January/February 2011 CATALySIS or TBHP, an unprecedented enantioselective oxidative cycloetherification could be possible (Figure 2) (10). After several screenings, we found that a chiral spirobis(binaphthyl)- type ammonium cation, which had been developed as a counter cation of chiral phase-transfer catalysts by Maruoka and co-workers at Kyoto University, was suitable for asymmetric catalysis of the present reaction (Figure 3) (11). Figure 2. In situ generation of (hypo)iodite species, and possible catalytic cycle for enantioselective oxidative cycloetherification. A broader range of substrates and higher enantioselectivities of up to 92% ee were achieved using the l3-iodane catalyst generated in situ from 3 (8). Quideau and co-workers reported the asymmetric hydroxylative dearomatisation of 2-methyl-1- naphthol with modest enantioselectivities (up to 29% ee) using binaphthyl-based chiral l5-iodane generated in situ from 4 (9). Thus, in general, the enantioselectivity has been modest, except for our example. Additionally, these chiral hypervalent iodine catalysts were prepared in situ from the corresponding iodoarenes and m-CPBA, and meta-chlorobenzoic acid (m-CBA) was generated as waste (Figure 1). ChIRAL qUATERNARY AMMONIUM (hYPO)IODITE-CATALYSED ENANTIOSELECTIVE OXIDATIVE CYCLOEThERIFICATION To the best our knowledge, there have been no examples of chiral salt catalysts of inorganic iodine-derived oxoacids such as hypoiodous acid (IOH, I(I)), iodous acid (O=IOH, I(III)), iodic acid ((O=)2IOH, I(V)), and periodic acid ((O=)3IOH, I(VII)). We envisioned that if chiral quaternary ammonium salts of iodine- derived oxoacids could be generated in situ from the corresponding ammonium iodides with a milder and atom- Figure 3. Enantioselective oxidative cycloetherification of 5. economically ideal co-oxidant such as hydrogen peroxide a) After a single recrystallization. b) With 1 mol% of 7. chimica oggi/Chemistry Today - vol 29 n 1 January/February 2011 19 CATALySIS Additionally, we found that a 1-phenyl-1H-imidazol-2-yl moiety intermediate (9) (Figure 4). Notably, 9 and its analogues may (Z) at the 1-position of the substrates (5) was effective for be useful as synthetic intermediates for biologically active inducing high enantioselectivity (Figure 3). compounds, such as PPARa agonists, which were discovered Ammonium iodide 7 bearing bulky and electron-deficient at Merck Laboratories (4g). Other optically active 2-acyl-2,3- substituents (Ar = 3,5-[3,5-(CF3)2C6H3]C6H3) at the 3,3'-positions dihydrofurans 6 could also be key synthetic intermediates for gave the best results. To explore the generality and substrate- natural products (4, 5). scope of the present enantioselective oxidative cyclisation, After several control experiments, we proposed a catalytic several ketophenol derivatives 5 were examined as substrates cycle involving chiral quaternary ammonium hypoiodite + – + – under optimised conditions (Figure 3; Method A: 7 (10 mol%), ([R4N] [IO] ) or iodite ([R4N] [IO2] ), which should be 30 wt% hydrogen peroxide (1.1–2 equiv) in diethyl ether/water generated in situ from ammonium iodide (R4NI) and a (5/1, v/v) at room temperature; Method B: 7 (10 mol%), TBHP (2 co-oxidant (Figure 2) (3). Hypoiodite(I) reagents have been equiv) in diethyl ether at room temperature) (Figure 3). In most used for iodo-functionalisation in synthetic organic chemistry cases, high enantioselectivities were observed using either (iodination of alkenes or arenes; iodoacetoxylation, method A or B. Notably, the oxidation of 2-methyl-substituted azidoiodination and aziridination of alkenes, iodolactonisation substrate 5l gave 6l bearing a tetrasubstituted stereogenic of unsaturated carboxylic acid, etc.) (14). Notably, no center with 96% ee. Additionally, the pre-catalyst loading of 7 iodinated products were detected even in the oxidation of could be reduced to 1 mol% without reducing the chemical electron-rich phenol 5f under our catalytic conditions. yield or enantioselectivity. yamada et al. reported that potassium hypoiodite (IOK) was Products 6 are very useful chiral intermediates for further generated in situ from I2 and 2 equivalents of KOH (15). While synthetic elaboration (12). For instance, 6a was efficiently the oxidation of 3-(2-hydroxyphenyl)-1-phenylpropan-1-one transformed into the known (R)-ethyl ester (8) (4a, b) without a (phenyl ketone analogue of 5a) with I2 in the presence of 2 + – reduction in the optically purity, which could be a synthetic equiv of tetrabutylammonium hydroxide ([Bu4N] [OH] ) gave intermediate for natural products such as tremetone (4a, 5c), the desired (2,3-dihydrobenzofuran-2-yl)(phenyl)methanone fomannoxin (4c), and anodendroic acid (4c) (Figure 4). In in 91% yield, the oxidation of 5a gave a messy mixture under similar manner, 6l was also transformed efficiently into the the same conditions (3). Thus, we could not rule out the known (R)-methyl ketone (10) (13) through methyl ester possibility that a hypoiodite species was the actual oxidant, however, we also could not rule out the possibility of disproportionation from hypoiodite(+I) to iodide(–I) and iodite(+III) under these conditions. Unfortunately, little is known about the use of iodite in synthetic organic chemistry because of its instability. Thus, we could not unambiguously determine whether hypoiodite or iodite exactly was the actual oxidant under our conditions. There are several possibilities for the reaction mechanism (Figure 5). We speculated that enolate intermediate A or B might be generated at the initial stage of the reaction. However, the position of the iodine(I or III) and the ammonium cation, and the E/Z-selectivity of enolate anion, especially for 5l, are not clear for the intermediate A or B. This is the enantiodiscrimination step. If A is a real active intermediate, the product is obtained from A through cycloaddition of enolate to the oxygen atom of

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