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Sodium Iodidecatalyzed Direct Alkoxylation of Ketones With COMMUNICATIONS DOI: 10.1002/adsc.201500006 Sodium Iodide-Catalyzed Direct a-Alkoxylation of Ketones with Alcohols via Oxidation of a-Iodo Ketone Intermediates Cuiju Zhu,a Yuanfei Zhang,a Huaiqing Zhao,a Shijun Huang,a Min Zhang,a,* and Weiping Sua,* a State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Yangqiao West Road 155, Fuzhou, Fujian 350002, Peoples Republic of China E-mail: [email protected] or [email protected] Received: January 5, 2015; Published online: February 4, 2015 Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500006. Abstract: The direct a-alkoxylation of ketones with for the a-alkoxylation of carbonyl compounds is alcohols via a sodium iodide-catalyzed oxidative highly desirable. cross-coupling has been developed. This protocol Herein, we described an efficient method for the enables a range of alkyl aryl ketones to cross- construction of a-alkoxy ketones via direct oxidative couple with an array of alcohols in synthetically coupling of ketones with a broad range of alkyl and useful yields. The mechanistic studies provided solid benzyl alcohols as coupling partners using low-cost evidence supporting that an a-iodo ketone was sodium iodide as the catalyst. In principle, two cou- a key reaction intermediate, being converted into pling partners of the direct a-alkoxylation of ketones, an a-alkoxylated ketone via further oxidation to namely, the enol and the alcohol, are both nucleo- a hypervalent iodine species rather than a common philes and therefore electronically mismatched. This nucleophilic substitution, and was generated from kind of cross-coupling between two nucleophiles is the ketone starting material via a radical intermedi- conventionally realized through umpolung of one ate. These new mechanism insights should have an coupling component.[6] In this regard, MacMillan and effect on the design of iodide-catalyzed oxidative co-workers[6a] have recently achieved the Cu-catalyzed cross-coupling reactions between nucleophiles. direct a-amination of carbonyl compounds, and Loh [6b] et al. have discovered the I2-catalyzed a-amination Keywords: alcohols; a-alkoxylation; CÀO coupling; of aldehydes. These two reactions were proposed to hypervalent iodine; ketones proceed via a-halogenation of the carbonyl and sub- sequent nucleophilic substitution of the halide func- tionality by an amine. Metal-catalyzed oxidative cross-coupling has evolved into a powerful tool for Carbonyl compounds with a-alkoxy substituents have found numerous applications either as building blocks or synthetic intermediates,[1] and are present in many pharmaceuticals and biologically active compounds (Scheme 1).[2] Their prevalence has promoted synthet- ic organic chemists to seek efficient methods for the construction of this kind of molecular framework. Traditionally, the installation of alkoxy substituents at the carbonyl a-position can be accomplished through multistep processes via synthetic intermediates such as silyl enol ethers,[3] enol acetates[4] or a-diazo ke- tones.[5] These methods suffer from the inconvenient Scheme 1. Three examples illustrating the importance of a- multiple oxidation procedures or rigorous control of alkoxy ketones: a) (+)-(aR)-a-methoxy-2’,4’-dihydroxydihy- the reaction conditions. Moreover, the substrate scope drochalcone, a natural product;[2a] b) 2-(benzyloxy)-1,2-di- of these methods is limited as the introduction of phenylethanone, a small molecule that can inhibit apolipo- a long-chain alkoxy group remains underdeveloped. protein E production;[2b] c) 2-methoxy-1,2-diphenyletha- Accordingly, a general and straightforward method none, a photopolymerization initiator.[2c] Adv. Synth. Catal. 2015, 357, 331 – 338 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 331 Cuiju Zhu et al. COMMUNICATIONS forging new chemical bonds linking two nucleophilic Table 1. Optimization studies for the catalytic a-alkoxylation components.[7] For example, the amine-assisted Cu- of ketones.[a] catalyzed oxidative cross-coupling of aldehydes with alkenylboronic acids has been established for the enantioselective a-alkenylation of aldehydes.[8] On the other hand, hypervalent iodine compounds have proved to be versatile reagents for diverse oxi- dative transformations[9] including a-functionaliza- [10] Entry [I] Oxidant Additive Yield tions of carbonyl compounds. On the basis of the [b] advances in using hypervalent iodine reagents for or- (mol%) (equiv.) [%] ganic syntheses, aryl iodide- or iodide salt-catalyzed 1Bu4NI (20) TBHP TsOH (0.4) 0 oxidative transformations have recently been devel- 2Bu4NI (20) K2S2O8 TsOH (0.4) 0 oped, in which hypervalent iodine intermediates are 3Bu4NI (20) H2O2 TsOH (0.4) 0 [11] in-situ generated in catalytic processes. For in- 4Bu4NI (20) m-CPBA TsOH (0.4) 0 stance, Ochiai and Ishihara independently reported 5Bu4NI (20) m-CPBA BF3·Et2O (0.4) 0 aryl iodide- and Bu NI-catalyzed a-oxyacetylations of 6Bu4NI (20) m-CPBA NsOH (0.4) 36 4 7BuNI (20) m-CPBA NsOH (1.0) 70 ketones, respectively.[11c,f] Although a number of 4 8BuNI (20) m-CPBA NsOH (1.5) 85 methods have been established for the a-hydroxyl- 4 [12] 9 NaI (20) m-CPBA NsOH (1.5) 90 (86) ation of carbonyl compounds, the catalytic direct a- 10 KI (20) m-CPBA NsOH (1.5) 83 alkoxylation of carbonyl compounds with alcohols re- 11 LiI (20) m-CPBA NsOH (1.5) 70 mains a great challenge because of the undesired 12 PhI (20) m-CPBA NsOH (1.5) 60 Baeyer–Villiger oxidation of ketones to esters,[13] the 13 – m-CPBA NsOH (1.5) 0 oxidative consumption of alcohols,[14] the weak nucle- 14 NaI (100) – NsOH (1.5) 0 ophilicity of alcohols and the 1, 2-addition of alcohols [a] Reaction conditions: 1a (0.2 mmol), 2a (0.5 mL), iodide to the carbonyl C=O bond. An illustrative example catalyst, oxidant (2.5 equiv.), additive, CH3CN (2.0 mL), reported by Cheng and co-workers is the Bu4NI-cata- 808C, 24 h. TBHP =tert-butyl hydroperoxide, NsOH= lyzed reaction of ketones with benzylic alcohols using para-nitrobenzenesulfonic acid. TBHP as the oxidant that produced a-acyloxy ke- [b] GC yield using dodecane as an internal standard (values tones rather than a-alkoxy ketones.[15] Very recently, in parentheses refers to the isolated yield). Jiao and co-workers described the Cu-catalyzed aero- bic oxidative esterification of ketones with alcohols, further highlighting the complications of oxidative cross-coupling between ketones and alcohols.[16] To of 1a and activate 1a towards oxidation at the a-posi- date, the only example closely related to the direct a- tion with varyious oxidants such as TBHP, K2S2O8, alkoxylation of carbonyl compounds was reported by H2O2, m-CPBA, but the reaction did not give any de- Ishihara who achieved a quaternary ammonium sired product (entries 1–5). To our delight, employing iodide-catalyzed intramolecular a-phenolation of ke- 2.5 equiv. of m-CPBA as the oxidant, in combination tones.[17] To the best of our knowledge, our findings with 40 mol% para-nitrobenzenesulfonic acid represent the first example of a catalytic direct a-al- (NsOH) as the additive, led to formation of the de- koxylation of carbonyl compounds. Importantly, the sired product 3a in 36% yield (entry 6). We speculat- mechanistic studies provide evidence supporting ed that increasing the amount of acid would push for- a new reaction pathway for iodide-catalyzed oxidative ward the enolization of the ketones, thus favoring ac- cross-couplings between nucleophiles that involves an tivation of the ketones. As expected, an 85% yield a-iodo ketone as intermediate. The coupling of an a- was obtained after adding 1.5 equiv. of NsOH to the iodo ketone with a weak nucleophile such as alcohol reaction system (entry 8). Inorganic iodide salts such proceeds via further oxidation to a hypervalent iodine as NaI, KI and LiI also proved to be effective cata- intermediate rather than the usual nucleophilic substi- lysts and led to comparable conversions, among which tution. This mechanistic insight should be helpful for NaI gave the best result (entries 9–11). Notably, the design of iodide-catalyzed oxidative cross-cou- Baeyer–Villiger oxidation of ketones was not ob- pling reactions. served under the optimized conditions, probably be- Initially, we screened a variety of reaction parame- cause the a-position of the enol from the acid-pro- ters using the reaction of propiophenone 1a with an moted ketone tautomerization is much more reactive excess amount of methanol 2a as a model system than other positions. However, accompanying product (Table 1). To the model reaction carried out in 3a, a small amount of a-iodo ketone (e.g., 2-iodo-1- CH3CN at 808C in the presence of 20 mol% Bu4NI as phenylpropan-1-one 5) was always formed in the a catalyst, a variety of Lewis acids[18] such as TsOH, model reaction. When PhI was used as the catalyst in BF3·Et2O were introduced to promote the enolization place of NaI, the yield of 3a dropped to 60% 332 asc.wiley-vch.de 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Synth. Catal. 2015, 357, 331 – 338 COMMUNICATIONS Sodium Iodide-Catalyzed Direct a-Alkoxylation of Ketones with Alcohols Scheme 2. Scope of the ketone coupling partner. Reaction conditons: 1 (0.2 mmol), 2a (0.5 mL), NaI (20 mol%), NsOH (1.5 equiv.), m-CPBA (2.5 equiv.), CH3CN (2 mL); isolated yields. (entry 12). Control experiments confirmed that both cohols and benzyl alcohols could be directly synthe- iodide salt and oxidant were necessary for the reac- sized by this method (4c–4k), which are not readily tion to occur (entries 13 and 14). accessible using conventional methods because of the With the optimized conditions in hand, we first ex- complicated reactivity of the long-chain alcohols.
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