Anchoring of a Copper(II)–Schiff Base Complex onto Silica-Coated Ferrite Nanoparticles: A Magnetically Separable Catalyst for Oxidative C–O Coupling by Direct C(sp²)–H and C(sp³)–H Bond Activation

Abstract

A novel catalyst consisting of a Schiff base–copper complex on surface-modified silica-coated ferrite nanoparticles was been prepared and used for oxidative C–O cross-coupling reactions of 1,3-dicarbonyl compounds with formamides for the synthesis of enol carbamates. The new catalyst has also been used for esterification of alkylbenzenes with cyclic ethers.

In recent years, the heterogenization of homogeneous catalysts has attracted immense attention.[1] Although many homogeneous catalysts demonstrate high selectivity and can be used to control the stereochemistry and regiochemistry of the reaction products, it can be difficult to separate soluble catalysts from the reaction products.[2] Recovery and reusability of catalysts are highly desirable from both economic and environmental standpoints. As a result, several methods have been explored for the recovery of homogeneous catalysts. These include the use of organic and/or inorganic supports for heterogenized homogeneous catalysts.[3] In this regard, magnetic nanoparticles have a prominent rule because of their good stability and ease of separation by using a magnetic field.[4] The surface of the magnetic nanoparticles is usually coated with a layer of silica to prevent aggregation. Because of the importance of magnetic catalysts, the synthesis and characterization of these types of catalyst are of interest, and there have been many reports dealing with this area.[5]

In the past decade, transition-metal-catalyzed C−H functionalization has been developed as an attractive and powerful strategy, because this method permits more-efficient synthesis of functionalized molecules by minimizing the number of synthetic steps.[6] Although various transition-metal catalyst systems have been developed for C–H activation reactions,[7] copper remains one of the most important metal catalysts for this purpose because of the ability of the copper atom to act as a single-electron and/or a two-electron mediator and as a Lewis acid. Moreover, copper can act as either an electrophile or nucleophile, depending upon its oxidation state and the reaction conditions. Although copper-mediated transformations have been overshadowed by developments in palladium chemistry, many novel copper-catalyzed processes have recently appeared because of the low cost and ready availability of copper compounds in comparison to palladium or other precious metals, such as ruthenium, rhodium, and gold.[8] Oxidative C–O cross-coupling is one of the most important methods for the synthesis of various oxygen-bearing organic compound, such as ethers, esters, oxazoles, or carbamates.[9]

In a continuation of our work on heterogeneous catalysts and C–O coupling reactions,[3c] [8f] [10] we examined the synthesis of a copper–Schiff base complex covalently supported on magnetic nano particles, and its application in oxidative C–O reactions for the synthesis of esters and carbamates.

Initially, silica-coated ferrite nanoparticles (Fe₃O₄@SiO₂) were prepared from iron(III) chloride, iron(II) chloride, and tetraethyl orthosilicate, according to a reported procedure.[11] A copper (II)–Schiff base complex was prepared by the reported procedure[12] then treated with the silica-coated ferrite nanoparticles in methanol to give the new copper catalyst (Scheme [1]).

The new magnetic catalyst was characterized by X-ray diffraction, Fourier-transform IR spectroscopy, scanning electron microscopy, vibrating sample magnetometry (VSM), and inductively coupled plasma techniques (ICP) (see Supplementary Information). In the IR spectrum, absorption bands at 2925, 2852, 1664, and 1097 cm^–1, related to symmetric and asymmetric vibrations of CH₂, C=N, and Si–O–Si groups, clearly indicated the incorporation of the Schiff base complex onto the surface of the silica-coated ferrite nanoparticles. VSM analysis showed that the magnetic saturation value of the copper-containing catalyst was less than that of the silica-coated ferrite nanoparticles; this can be explained by a shielding effect of the functional copper complex on the magnetic nanoparticles. Finally, elemental analysis indicated a loading of 1.1 mmol g^–1.

After characterizing the catalyst, we decided to examine its activity in oxidative C–O coupling reactions for the synthesis of enol carbamates[13] and esters.[14]

First, we examined the oxidative coupling of methyl ­acetoacetate with N,N-dimethylformamide in the presence of a subequivalent amount of the catalyst with tert-butyl hydroperoxide as an oxidant. To optimize the reaction conditions, we investigated various parameters, such as the amount of catalyst, the temperature, and the nature and quantity of the oxidant. Optimal results were obtained for the reaction in the presence of 10 mg catalyst (containing 0.011 mmol copper) and tert-butyl hydroperoxide (1.5 equiv) at 80 °C for 20 minutes, which gave a 92% yield of methyl (2Z)-3-{[(dimethylamino)carbonyl]oxy}but-2-enoate (2a).[15] In addition, the pure silica-coated nanoparticles were tested for catalytic activity in this reaction, and no product was obtained under these conditions.

With the optimized reaction conditions in hand, we extended our study to various 1,3-dicarbonyl compounds. The reactions were generally clean, and gave the desired products 2a–g in good to excellent yields (Table [1]).

Table 1 Synthesis of Enol Carbamate Derivatives

Entry	R1	R2	Product	Yield (%)
1	Me	OMe	2a	92
2	Me	OEt	2b	90
3	Me	OBn	2c	85
4	Me	Me	2d	78
5	Ph	OEt	2e	76
6	Pr	OEt	2f	62
7	Me	OAll	2g	75

A plausible mechanism for the oxidative C–O coupling reaction between 1,3-dicarbonyl compounds and N,N-dimethylformamide has been proposed in the literature.[13]

Having successfully synthesized the enol carbamates 2a–g, we decided to evaluate the effect of this new catalyst system in an oxidative esterification reaction between al­kylbenzenes and cyclic ethers. For this purpose, our initial studies focused on determining the optimal conditions for the reaction of toluene with 1,4-dioxane. The best results were obtained in the presence of 15 mg of the catalyst (containing 0.016 mmol of copper) and 1.3 equivalent of tert-butyl hydroperoxide at 80 °C for 10 hours, which gave an 83% yield of the ester 5a.[16] When the reaction was attempted in the absence of the catalyst and/or the oxidizing agent, the desired product was not obtained. To explore the generality of the reaction, we extended our study to various al­kylbenzenes 3 and cyclic ethers 4 (Table [2]).

This protocol tolerates a variety of alkylbenzenes containing electron-withdrawing or electron-donating substituents. The electron-donating groups increased the yield of the reaction, because the electron-rich compounds are more easily oxidized than the electron-deficient compounds.

A plausible mechanism for the oxidative esterification reaction between alkylbenzenes and cyclic ethers has been proposed in the literature.[14g]

Next, we examined the reusability of the catalyst in both reactions. After completion of a first reaction by either protocol, the catalyst was recovered by using an external magnet, washed with ethanol and diethyl ether, and oven-dried at 100 °C overnight. The recovered catalyst could be used at least five times without significant loss in yield. To examine whether there was any leaching of the copper complex, we removed the catalyst by using a magnet 10 minutes after the beginning of the reaction, and we observed that the reaction did not reach completion, even after 24 hours. This clearly confirmed that the active species does not leach into the solvent. Furthermore, ICP analysis showed that there was no copper in solution

Table 2 Synthesis of Benzoate Derivatives

Entry	R	X	Product	Yield (%)
1	H	O	5a	83
2	Me	O	5b	85
3	Cl	O	5c	55
4	OMe	O	5d	90
5	Br	O	5e	64
6	H	CH2	5f	49
7	Me	CH2	5g	51
8	Cl	CH2	5h	38

In summary, we have developed a novel magnetic catalyst functionalized with a Schiff base–copper complex. The new catalyst was characterized by a range of physical techniques and its catalytic activity was successfully demonstrated in oxidative C(sp²)–O and C(sp³)–O coupling reactions. The recyclability of the catalyst, ease of manipulation, shorter reaction times in comparison with other reported catalysts, and good yields are the main advantages of this protocol.

• References and Notes

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• 4e Rossi LM, Silva FP, Vono LL. R, Kiyohara PK, Duarte EL, Itri R, Landers R, Machado G. Green Chem. 2007; 9: 379
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• 5a Wang S, Zhang Z, Liu B, Li J. Catal. Sci. Technol. 2013; 3: 2104
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• 5b Stevens PD, Li G, Fan J, Yen M, Gao Y. Chem. Commun. 2005; 4435
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• 6a Yamaguchi J, Yamaguchi AD, Itami K. Angew. Chem. Int. Ed. 2012; 51: 8960
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• 6b Xie J, Pan C, Abdukader A, Zhu C. Chem. Soc. Rev. 2014; 43: 5245
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• 6d Zheng C, You SL. RSC Adv. 2014; 4: 6173
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• 6e He Y.-P, Zhang C, Fan M, Wu Z, Ma D. Org. Lett. 2015; 17: 496
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• 6f Li B, Wang S.-Q, Liu B, Shi B.-F. Org. Lett. 2015; 17: 1200
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• 6g Wu X, Zhao Y, Ge H. J. Am. Chem. Soc. 2015; 137: 4924
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• 7a Kalyani D, Deprez NR, Desai LV, Sanford MS. J. Am. Chem. Soc. 2005; 127: 7330
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• 7b Wei C, Li C.-J. J. Am. Chem. Soc. 2003; 125: 9584
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• 7c Baggett AW, Vasiliu M, Li B. Dixon D. A., Liu S. Y. 2015; 137: 5536
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• 7d Dateer RB, Chang S. J. Am. Chem. Soc. 2015; 137: 4908
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• 7e Li M, Yang Y, Zhou D, Wan D, You J. Org. Lett. 2015; 17: 2546
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• 8a Klapars A, Antilla JC, Huang X, Buchwald SL. J. Am. Chem. Soc. 2001; 123: 7727
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• 8b Klapars A, Huang X, Buchwald SL. J. Am. Chem. Soc. 2002; 124: 7421
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• 8c Sequeira FC, Turnpenny BW, Chemler SR. Angew. Chem. Int. Ed. 2010; 49: 6365
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• 8d Xiong T, Li Y, Bi X, Lv Y, Zhang Q. Angew. Chem. Int. Ed. 2011; 50: 7140
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• 8e Neumann JJ, Suri M, Glorius F. Angew. Chem. Int. Ed. 2010; 49: 7790
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• 8f Alizadeh A, Ghanbaripour R. Synlett 2014; 25: 2777
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• 9a Ke Q, Zhang B, Hu B, Jin Y, Lu G. Chem. Commun. 2015; 51: 1012
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• 9b Wang CY, Song RJ, Wei WT, Fan JH, Li JH. Chem. Commun. 2015; 51: 2361
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• 9c Sarkar R, Mukhopadhyay C. Tetrahedron Lett. 2015; 56: 3872
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• 9d Shang X.-J, Liu Z.-Q. Tetrahedron Lett. 2015; 56: 482
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• 9e Xu W, Nachtsheim BJ. Org. Lett. 2015; 17: 1585
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• 10 Ghanbaripour R, Mohammadpoor-Baltork I, Moghadam M, Khosropour AR, Tangestaninejad S, Mirkhani V. J. Iran. Chem. Soc. 2012; 9: 791
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• 11 Liu ZL, Ding ZH, Yao KL, Tao J, Du GH, Lu QH, Wang X, Gong FL, Chen X. J. Magn. Magn. Mater. 2003; 265: 98
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• 12 Lim MH, Stein A. Chem. Mater. 1999; 11: 3285
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• 13a Kumar GS, Maheswari CU, Kumar RA, Kantam ML, Reddy KR. Angew. Chem. Int. Ed. 2011; 50: 11748
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• 13b Saberi D, Heydari A. Tetrahedron Lett. 2013; 54: 4178
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• 13c Azizi K, Karimi M, Heydari A. Tetrahedron Lett. 2015; 56: 812
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• 14a Yoo W.-J, Li C.-J. Tetrahedron Lett. 2007; 48: 1033
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• 14b Rout SK, Guin S, Chara KK, Banerjee A, Patel BK. Org. Lett. 2012; 14: 3982
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• 14c Yoo W.-J, Li C.-J. J. Org. Chem. 2006; 71: 6266
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• 14d Xu X, Ding W, Lin Y, Song Q. Org. Lett. 2015; 17: 516
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• 14e Li W, Duan Z, Jiang R, Lei A. Org. Lett. 2015; 17: 1397
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• 14f Zhou J, Jin C, Li X, Su W. RSC Adv. 2015; 5: 7232
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• 14g Rout SK, Guin S, Ali W, Gogoi A, Patel BK. Org. Lett. 2014; 16: 3086
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• 15 Enol Carbamates 2a–g; General Procedure A 70% aq solution of TBHP (0.193 g, 1.5 equiv) was added dropwise to a mixture of the appropriate 1,3-dicarbonyl compound 1 (1 mmol), the magnetic catalyst (10 mg), and DMF (2 mL). The mixture was heated to 80 °C, stirred for 20 min, and then cooled to r.t. The catalyst was removed by using an external magnet, and the residue was extracted with EtOAc (3 × 10 mL), dried (MgSO₄), filtered, and concentrated under vacuum. The crude product was purified by column chromatography. Allyl (2Z)-3-{[(Dimethylamino)carbonyl]oxy}but-2-enoate (2g) Yellow oil; yield: 160 mg (75%); IR (KBr): 1725, 1669, 1382, 1320, 1271, 1211, 1145, 1035, 750 cm⁻¹. ¹H NMR (400 MHz, CDCl₃): δ = 2.06 (s, 3 H, CH₃), 2.97 (s, 3 H, CH₃N), 3.02 (s, 3 H, CH₃N), 4.57 (dd, ³ J _HH = 5.6 Hz, ⁴ J _HH = 1.2 Hz, 2 H, CH₂O), 5.18–5.34 (m, 2 H, CH₂=CH), 5.58 (s, 1 H, CHCO₂), 5.82–5.97 (m, 1 H, CH₂=CH). ¹³C NMR (100 MHz, CDCl₃) δ = 22.21, 36.59, 64.62, 107.61, 118.03, 132.31, 152.99, 161.33, 163.66.
• 16 Aryl Esters 5a–5h; General Procedure In a round-bottomed flask, a mixture of alkylbenzene 3 (1 mmol), magnetic catalyst (15 mg), and 70% aq TBHP (0.167 g, 1.3 equiv) in 1,4-dioxane (2 mL) was stirred at 80 °C for 10 h, then cooled to r.t. The catalyst was removed by using an external magnet and the residue was extracted with EtOAc (2 × 10 mL). The extracts were washed with 5% aq NaHCO₃ (15 mL), dried (MgSO₄), filtered, and concentrated under vacuum. The crude product was purified by column chromatography. 1,4-Dioxan-2-yl Benzoate (5a) Pale yellow oil; yield: 156 mg (75%); IR (KBr): 3071, 2970, 2861, 1718, 1529, 1455, 1402, 1276, 1014, 911, 879, 715 cm⁻¹. ¹H NMR (400 MHz, CDCl₃) δ 3.64−3.69 (m, 1 H), 3.80−3.83 (m, 2 H), 3.85−3.89 (m, 2 H), 4.16−4.23 (m, 1 H), 6.08 (t, ³ J _HH = 2.1 Hz, 1 H), 7.42−7.46 (m, 2 H), 7.55−7.57 (m, 1 H), 8.09−8.11 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃) δ = 61.9, 66.4, 68.1, 90.0, 128.6, 129.9, 130.2, 133.56, 165.4.

• References and Notes

• 1 Minakata S, Komatsu M. Chem. Rev. 2009; 109: 711
  CrossrefSearch in Google Scholar
• 2a Hagen J. Industrial Catalysis: A Practical Approach. Wiley-VCH; Weinheim: 1999
  Search in Google Scholar
• 2b Cole-Hamilton DJ. Science 2003; 299: 1702
  Search in Google Scholar
• 3a Zhang Y, Riduana SN. Chem. Soc. Rev. 2012; 41: 2083
  Search in Google Scholar
• 3b Du X, Sun Y, Tan B, Teng Q, Yao X, Su C, Wang W. Chem. Commun. 2010; 46: 970
  CrossrefSearch in Google Scholar
• 3c Ghanbaripour R, Mohammadpoor-Baltork I, Moghadam M, Khosropour AR, Tangestaninejad S, Mirkhani V. Polyhedron 2012; 31: 721
  CrossrefSearch in Google Scholar
• 4a Reddy LH, Arias JL, Nicolas J, Couvreur P. Chem. Rev. 2012; 112: 5818
  CrossrefSearch in Google Scholar
• 4b Panella B, Vargas A, Baiker A. J. Catal. 2009; 261: 88
  CrossrefSearch in Google Scholar
• 4c Shin J, Kim H, Lee IS. Chem. Commun. 2008; 5553
• 4d Abu-Reziq R, Alper H, Wang D, Post ML. J. Am. Chem. Soc. 2006; 128: 5279
  CrossrefSearch in Google Scholar
• 4e Rossi LM, Silva FP, Vono LL. R, Kiyohara PK, Duarte EL, Itri R, Landers R, Machado G. Green Chem. 2007; 9: 379
  CrossrefSearch in Google Scholar
• 5a Wang S, Zhang Z, Liu B, Li J. Catal. Sci. Technol. 2013; 3: 2104
  CrossrefSearch in Google Scholar
• 5b Stevens PD, Li G, Fan J, Yen M, Gao Y. Chem. Commun. 2005; 4435
• 5c Laurent S, Forge D, Port M, Roch A, Robic C, Vander Elst L, Muller RN. Chem. Rev. 2008; 108: 2064
  CrossrefSearch in Google Scholar
• 6a Yamaguchi J, Yamaguchi AD, Itami K. Angew. Chem. Int. Ed. 2012; 51: 8960
  CrossrefPubMedSearch in Google Scholar
• 6b Xie J, Pan C, Abdukader A, Zhu C. Chem. Soc. Rev. 2014; 43: 5245
  CrossrefSearch in Google Scholar
• 6c Haines BE, Musaev DG. ACS Catal. 2015; 5: 830
  CrossrefSearch in Google Scholar
• 6d Zheng C, You SL. RSC Adv. 2014; 4: 6173
  Search in Google Scholar
• 6e He Y.-P, Zhang C, Fan M, Wu Z, Ma D. Org. Lett. 2015; 17: 496
  CrossrefSearch in Google Scholar
• 6f Li B, Wang S.-Q, Liu B, Shi B.-F. Org. Lett. 2015; 17: 1200
  CrossrefSearch in Google Scholar
• 6g Wu X, Zhao Y, Ge H. J. Am. Chem. Soc. 2015; 137: 4924
  CrossrefPubMedSearch in Google Scholar
• 7a Kalyani D, Deprez NR, Desai LV, Sanford MS. J. Am. Chem. Soc. 2005; 127: 7330
  CrossrefPubMedSearch in Google Scholar
• 7b Wei C, Li C.-J. J. Am. Chem. Soc. 2003; 125: 9584
  CrossrefSearch in Google Scholar
• 7c Baggett AW, Vasiliu M, Li B. Dixon D. A., Liu S. Y. 2015; 137: 5536
  Search in Google Scholar
• 7d Dateer RB, Chang S. J. Am. Chem. Soc. 2015; 137: 4908
  CrossrefSearch in Google Scholar
• 7e Li M, Yang Y, Zhou D, Wan D, You J. Org. Lett. 2015; 17: 2546
  CrossrefPubMedSearch in Google Scholar
• 8a Klapars A, Antilla JC, Huang X, Buchwald SL. J. Am. Chem. Soc. 2001; 123: 7727
  CrossrefPubMedSearch in Google Scholar
• 8b Klapars A, Huang X, Buchwald SL. J. Am. Chem. Soc. 2002; 124: 7421
  CrossrefPubMedSearch in Google Scholar
• 8c Sequeira FC, Turnpenny BW, Chemler SR. Angew. Chem. Int. Ed. 2010; 49: 6365
  CrossrefSearch in Google Scholar
• 8d Xiong T, Li Y, Bi X, Lv Y, Zhang Q. Angew. Chem. Int. Ed. 2011; 50: 7140
  CrossrefSearch in Google Scholar
• 8e Neumann JJ, Suri M, Glorius F. Angew. Chem. Int. Ed. 2010; 49: 7790
  CrossrefPubMedSearch in Google Scholar
• 8f Alizadeh A, Ghanbaripour R. Synlett 2014; 25: 2777
  Thieme ConnectSearch in Google Scholar
• 9a Ke Q, Zhang B, Hu B, Jin Y, Lu G. Chem. Commun. 2015; 51: 1012
  CrossrefSearch in Google Scholar
• 9b Wang CY, Song RJ, Wei WT, Fan JH, Li JH. Chem. Commun. 2015; 51: 2361
  CrossrefSearch in Google Scholar
• 9c Sarkar R, Mukhopadhyay C. Tetrahedron Lett. 2015; 56: 3872
  CrossrefSearch in Google Scholar
• 9d Shang X.-J, Liu Z.-Q. Tetrahedron Lett. 2015; 56: 482
  CrossrefSearch in Google Scholar
• 9e Xu W, Nachtsheim BJ. Org. Lett. 2015; 17: 1585
  CrossrefSearch in Google Scholar
• 10 Ghanbaripour R, Mohammadpoor-Baltork I, Moghadam M, Khosropour AR, Tangestaninejad S, Mirkhani V. J. Iran. Chem. Soc. 2012; 9: 791
  CrossrefSearch in Google Scholar
• 11 Liu ZL, Ding ZH, Yao KL, Tao J, Du GH, Lu QH, Wang X, Gong FL, Chen X. J. Magn. Magn. Mater. 2003; 265: 98
  CrossrefSearch in Google Scholar
• 12 Lim MH, Stein A. Chem. Mater. 1999; 11: 3285
  CrossrefSearch in Google Scholar
• 13a Kumar GS, Maheswari CU, Kumar RA, Kantam ML, Reddy KR. Angew. Chem. Int. Ed. 2011; 50: 11748
  CrossrefSearch in Google Scholar
• 13b Saberi D, Heydari A. Tetrahedron Lett. 2013; 54: 4178
  CrossrefSearch in Google Scholar
• 13c Azizi K, Karimi M, Heydari A. Tetrahedron Lett. 2015; 56: 812
  CrossrefSearch in Google Scholar
• 14a Yoo W.-J, Li C.-J. Tetrahedron Lett. 2007; 48: 1033
  CrossrefSearch in Google Scholar
• 14b Rout SK, Guin S, Chara KK, Banerjee A, Patel BK. Org. Lett. 2012; 14: 3982
  CrossrefSearch in Google Scholar
• 14c Yoo W.-J, Li C.-J. J. Org. Chem. 2006; 71: 6266
  CrossrefSearch in Google Scholar
• 14d Xu X, Ding W, Lin Y, Song Q. Org. Lett. 2015; 17: 516
  CrossrefSearch in Google Scholar
• 14e Li W, Duan Z, Jiang R, Lei A. Org. Lett. 2015; 17: 1397
  CrossrefPubMedSearch in Google Scholar
• 14f Zhou J, Jin C, Li X, Su W. RSC Adv. 2015; 5: 7232
  CrossrefSearch in Google Scholar
• 14g Rout SK, Guin S, Ali W, Gogoi A, Patel BK. Org. Lett. 2014; 16: 3086
  CrossrefSearch in Google Scholar
• 15 Enol Carbamates 2a–g; General Procedure A 70% aq solution of TBHP (0.193 g, 1.5 equiv) was added dropwise to a mixture of the appropriate 1,3-dicarbonyl compound 1 (1 mmol), the magnetic catalyst (10 mg), and DMF (2 mL). The mixture was heated to 80 °C, stirred for 20 min, and then cooled to r.t. The catalyst was removed by using an external magnet, and the residue was extracted with EtOAc (3 × 10 mL), dried (MgSO₄), filtered, and concentrated under vacuum. The crude product was purified by column chromatography. Allyl (2Z)-3-{[(Dimethylamino)carbonyl]oxy}but-2-enoate (2g) Yellow oil; yield: 160 mg (75%); IR (KBr): 1725, 1669, 1382, 1320, 1271, 1211, 1145, 1035, 750 cm⁻¹. ¹H NMR (400 MHz, CDCl₃): δ = 2.06 (s, 3 H, CH₃), 2.97 (s, 3 H, CH₃N), 3.02 (s, 3 H, CH₃N), 4.57 (dd, ³ J _HH = 5.6 Hz, ⁴ J _HH = 1.2 Hz, 2 H, CH₂O), 5.18–5.34 (m, 2 H, CH₂=CH), 5.58 (s, 1 H, CHCO₂), 5.82–5.97 (m, 1 H, CH₂=CH). ¹³C NMR (100 MHz, CDCl₃) δ = 22.21, 36.59, 64.62, 107.61, 118.03, 132.31, 152.99, 161.33, 163.66.
• 16 Aryl Esters 5a–5h; General Procedure In a round-bottomed flask, a mixture of alkylbenzene 3 (1 mmol), magnetic catalyst (15 mg), and 70% aq TBHP (0.167 g, 1.3 equiv) in 1,4-dioxane (2 mL) was stirred at 80 °C for 10 h, then cooled to r.t. The catalyst was removed by using an external magnet and the residue was extracted with EtOAc (2 × 10 mL). The extracts were washed with 5% aq NaHCO₃ (15 mL), dried (MgSO₄), filtered, and concentrated under vacuum. The crude product was purified by column chromatography. 1,4-Dioxan-2-yl Benzoate (5a) Pale yellow oil; yield: 156 mg (75%); IR (KBr): 3071, 2970, 2861, 1718, 1529, 1455, 1402, 1276, 1014, 911, 879, 715 cm⁻¹. ¹H NMR (400 MHz, CDCl₃) δ 3.64−3.69 (m, 1 H), 3.80−3.83 (m, 2 H), 3.85−3.89 (m, 2 H), 4.16−4.23 (m, 1 H), 6.08 (t, ³ J _HH = 2.1 Hz, 1 H), 7.42−7.46 (m, 2 H), 7.55−7.57 (m, 1 H), 8.09−8.11 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃) δ = 61.9, 66.4, 68.1, 90.0, 128.6, 129.9, 130.2, 133.56, 165.4.

• Supplementary Material