Acceptorless dehydrogenative lactonization of diols by Pt-loaded SnO₂ catalysts

Abstract

We report herein a new heterogeneous catalytic system for dehydrogenative lactonization of various diols under solvent-free and acceptor-free conditions using 1 mol% of Pt-loaded SnO₂, providing the first successful example of acceptorless lactonization of 1,6-hexanediol to ε-caprolactone by a heterogeneous catalyst.

---

Lactones are important compounds in organic synthesis and industrial production of solvents, flavors, and pharmaceutical compounds. Dehydrogenative^1–8 and oxidative^9–20 lactonizations of diols are potentially useful methods for the synthesis of lactones. A number of homogeneous and heterogeneous catalytic systems have been reported to be effective for the oxidative lactonization of diols by oxidants, such as carbonyl compounds,^9–14 oxygen^15–19 and hydrogen peroxide.²⁰ In view of atom-efficiency, a more preferable methodology is lactonization of diols in the absence of an oxidant (acceptor) via an acceptorless dehydrogenative coupling strategy.²¹ Homogeneous Ru^1,2,4 and Ir^3,5 catalysts were reported to be effective for this reaction, but these systems suffer from difficulties in catalyst/product separation and catalyst reuse except for a Ir-catalyzed system recently reported by Fujita et al.⁵ Although heterogeneous Cu catalysts are effective for the gas phase dehydrogenative lactonization of 1,4-butanediol,^6,7 these systems suffer from limited scope. An exceptional example of the heterogeneous catalytic system for acceptorless lactonization of various diols is reported by Kaneda et al.⁸ using 7.1 mol% of Cu-loaded hydrotalcite. During our continuing studies in heterogeneous Pt catalysts for acceptorless dehydrogenation of alcohols,²² dehydrogenative coupling of o-aminobenzamide with alcohols²³ and acceptorless self-coupling of primary alcohols to esters,²⁴ we found a new heterogeneous catalyst for the acceptorless lactonization; the solvent-free and acceptor-free lactonization of various diols was effectively catalyzed by 1 mol% of Pt-loaded SnO₂ (Pt/SnO₂).

As listed in Table 1, we tested lactonization of 1,2-benzenedimethanol 1a under solvent-free conditions in N₂ at 180 °C for 36 h as a model reaction using the catalysts containing 1 mol% (0.01 mmol) of transition metals (Pt, Ir, Re, Pd, Rh, Ru, Ag, Ni, Cu, Co). Among various transition metal-loaded SnO₂ (entries 1–10), Pt/SnO₂ (entry 1) showed the highest yield (90%) of the corresponding lactone, phthalide 2a. GCMS and TLC analyses of the reaction mixture showed no indication of byproducts such as products of intermolecular reactions such as oligomers. The lactonization of 1a did not occur in the presence of 39 mg of SnO₂ itself (entry 11). Comparison of various Pt catalysts on different support materials (SnO₂, Al₂O₃, Nb₂O₅, C, SiO₂, TiO₂, ZrO₂, MgO, HBEA zeolite) shows that SnO₂ is the best support material (entries 1, 12–19).

Table 1 Lactonization of 1,2-benzenedimethanol (1a) by various catalysts

Entry	Catalyst	Conv. (%)	Yield (%)
a Catalyst amount was 39 mg.
1	Pt/SnO2	100	90
2	Ir/SnO2	15	9
3	Ni/SnO2	15	7
4	Pd/SnO2	10	6
5	Co/SnO2	10	4
6	Rh/SnO2	5	3
7	Cu/SnO2	5	1
8	Ru/SnO2	4	1
9	Re/SnO2	3	1
10	Ag/SnO2	2	0
11^a	SnO2	0	0
12	Pt/Al2O3	9	3
13	Pt/Nb2O5	7	2
14	Pt/C	6	1
15	Pt/SiO2	2	0
16	Pt/TiO2	2	0
17	Pt/ZrO2	2	0
18	Pt/MgO	5	0
19	Pt/HBEA	2	0

---

After the reaction with Pt/SnO₂ for 5 h, we carried out quantification of the H₂ evolved in the gas phase. The yields of gas phase H₂ (35%) and the lactone 2a (35%) were identical to each other (eqn (1)), indicating that H₂ was evolved quantitatively during the dehydrogenative lactonization. The reaction under N₂ flow did not result in higher yield than the yield under the standard closed system.

Table 2 shows the effect of reaction conditions on the catalytic activity of Pt/SnO₂ for the standard lactonization of 1a. The reaction under the solvent-free conditions gave higher yield (90%) than the yields (62–80%) in the presence of 1 mL solvents (o-xylene, diglyme, mesitylene, n-dodecane). In the solvent-free condition, the reaction under 1 atm N₂ gave higher yield than that under 1 atm O₂ (51%). Under O₂, the metallic Pt surface can be partly covered with oxygen atoms, which inhibit the activation of the diol and lower the yield. In the solvent-free condition under N₂, the reaction at lower temperature (150 °C) resulted in low yield of 2a (55%). Consequently, the standard conditions was found to be the solvent-free conditions under 1 atm N₂ at 180 °C for 36 h, which gave full conversion of 1a and 90% yield of 2a. After completion of the reaction, 2-propanol (10 mL) was added to the reaction mixture and catalyst was separated by centrifugation. The catalyst was then washed by acetone three times, followed by centrifugation and drying in oven (under air) at 90 °C for 3 h, followed by H₂-reduction at 150 °C for 0.5 h. The recovered catalyst was reused three times, but the yield of 2a gradually decreased with increase in the cycle (Fig. 1). ICP-AES analysis of the solution after the first cycle showed that the content of Pt in the solution (1.6 ppm, 0.02% of Pt in the catalyst used) and that of Sn (3.0 ppm, 0.002% of Sn in the catalyst used) were quite low.

Table 2 Lactonization of 1a by Pt/SnO₂^a

Solvent	Gas	Conv. (%)	Yield (%)
a Conditions: 1 mol% Pt/SnO2 (0.01 mmol Pt), 1 mmol 1a, 0 or 1 mL solvent, 180 °C, 36 h. Yield was determined by GC.
No solvent	N2	100	90
o-Xylene	N2	80	72
Diglyme	N2	92	80
Mesitylene	N2	85	76
n-Dodecane	N2	70	62
No solvent	O2	67	51
No solvent (150 °C)	N2	69	55

---

Fig. 1 Reuse of Pt/SnO₂ for lactonization of 1a. Conditions are shown in Table 1.

To study the substrate scope of the present catalytic system, the reactions of various diols were conducted (Table 3). Benzylic diols (entries 1–4), linear (entries 5–7) and non-linear (entries 8–11) aliphatic diols were converted to the corresponding lactones in moderate to high yields of 66–90%. GC charts of the reaction mixture for entries 8–10 showed four to five small peaks due to byproducts with high boiling points, but the amount of the byproducts were too small for their identification. From a viewpoint of biorefinery, the lactonization of 1,6-hexanediol (1,6-HD) into ε-caprolactone is of particular importance, because 1,6-HD can be produced by multistep hydrogenation of lignocellulosic biomass such as 5-hydroxymethyl-2-furfural (HMF)^12,13 and furfural²⁵ and ε-caprolactone can be converted to ε-caprolactam as an important bulk chemical.^12,13 Heeres and co-workers^12,13 reported the first example of the oxidative lactonization of 1,6-HD to ε-caprolactone using methyl isobutyl ketone (MIBK) as oxidant and a Ru complex as homogeneous catalyst, but the system suffers from drawbacks such as a need of excess amount of MIBK and difficulty in catalyst/product separation. Obviously, acceptorless lactonization of 1,6-HD to caprolactone by a heterogeneous catalyst is preferred from an industrial viewpoint. Our result (entry 7) gave the first example of acceptorless lactonization of 1,6-HD by a heterogeneous catalyst, giving ε-caprolactone in high yield (86%). After the reactions, followed by adding 2-propanol (10 mL) to the mixture, Pt/SnO₂ was removed from the mixture and the lactones were isolated by column chromatography, resulting in high isolated yield (81%) of ε-caprolactone.

Table 3 Dehydrogenative lactonization of various diols catalyzed by Pt/SnO₂

Entry	Diol	Product	Conv. (%)	Yield^a (%)
a GC yields. Isolated yields are in parentheses.
1			100	90 (87)
2			93	85 (81)
3			74	66
4			96	83 (78)
5			100	80 (76)
6			100	85 (82)
7			93	86 (81)
8			97	75
9			96	75
10			98	70
11			91	83

---

Recently, we studied the dehydrogenative esterification of primary alcohols to esters by Pt/SnO₂.²⁴ On the basis of the results of model reactions and the IR result that the benzaldehyde adsorbed on SnO₂ gave the C=O stretching band at lower wavenumber than those on γ-Al₂O₃ and SiO₂, we proposed a possible pathway via dehydrogenation alcohol to adsorbed aldehyde species (on Sn⁴⁺ Lewis acid site) which undergo nucleophilic attack of another alcohol to give hemiacetal followed by its dehydrogenation to the ester. Considering that the similarity between this intermolecular esterification and the lactonization (intermolecular esterification), it is suggested that Sn⁴⁺ Lewis acid site is responsible for the high activity of Pt/SnO₂ for the present lactonization system.

In conclusion, we developed a new heterogeneous catalyst, Pt/SnO₂, for the acceptorless dehydrogenative lactonization of diols under solvent-free conditions. The method was effective for the synthesis of various lactones. We gave the first example of the catalytic acceptorless lactonization of 1,6-HD to ε-caprolactone as an intermediate of ε-caprolactam, the monomer for nylon-6. Considering recent advances in the catalytic synthesis of 1,6-HD from biomass-derived furfurals^12,13,25 this method can provide a key process in sustainable production of nylon-6 from renewable resources.

Experimental

Commercially available organic compounds (from Tokyo Chemical Industry or Kanto Chemical) were used without further purification. The GC (Shimadzu GC-2014) and GCMS (Shimadzu GCMS-QP2010) analyses were carried out with Ultra ALLOY capillary column UA⁺-1 (Frontier Laboratories Ltd.) using N₂ and He as the carrier gas.

SnO₂ was prepared by calcination of H₂SnO₃ (Kojundo Chemical Laboratory Co., Ltd.) at 500 °C for 3 h. Nb₂O₅ was prepared by calcination of niobic acid (CBMM) at 500 °C for 3 h. γ-Al₂O₃ was prepared by calcination of γ-AlOOH (Catapal B Alumina purchased from Sasol) at 900 °C for 3 h. ZrO₂ was prepared by calcination (500 °C for 3 h) of ZrO₂·nH₂O prepared by hydrolysis of zirconium oxynitrate 2-hydrate in water by aqueous NH₄OH solution, followed by filtration of precipitate, washing with water three times, and drying at 100 °C for 12 h. SiO₂ (Q-10, 300 m² g⁻¹) was supplied from Fuji Silysia Chemical Ltd. HBEA zeolite (JRC-Z-HB25, SiO₂/Al₂O₃ = 25 ± 5), MgO (JRC-MGO-3), TiO₂ (JRC-TIO-4) were supplied from Catalysis Society of Japan. Active carbon (296 m² g⁻¹) was purchased from Kishida Chemical.

Precursor of Pt/SnO₂ was prepared by impregnation method; a mixture of SnO₂ and an aqueous HNO₃ solution of Pt(NH₃)₂(NO₃)₂ (Furuya Metal Co, Ltd.) was evaporated at 50 °C, followed by drying at 90 °C for 12 h. Before each catalytic experiment, the Pt/SnO₂ catalyst (containing 5 wt% of Pt) was prepared by in situ pre-reduction of the precursor in a pyrex tube under a flow of H₂ (20 cm³ min⁻¹) at 150 °C for 0.5 h. Other supported Pt catalysts (containing 5 wt% of Pt) were prepared by the same method. SnO₂-supported metal catalysts, M/SnO₂ (M = Pt, Ir, Re, Pd, Rh, Ru, Ag, Ni, Cu, Co) with metal loading of 5 wt% were prepared by the impregnation method in the similar manner as Pt/SnO₂ using aqueous solution of metal nitrates (for Ni, Cu, Co, Ag), RuCl₃, IrCl₃·nH₂O or NH₄ReO₄ or aqueous HNO₃ solution of Pd(NH₃)₂(NO₃)₂ (Kojima Chemicals Co, Ltd.).

Pt/SnO₂ was used as a standard catalyst. After the pre-reduction at 150 °C, we carried out catalytic tests using a batch-type reactor without exposing the catalyst to air as follows. n-Dodecane (0.2 mmol) was injected to the pre-reduced catalyst inside the reactor (cylindrical glass tube, 18 cm³) through a septum inlet, followed by adding diols (1 mmol) under ambient conditions and by filling with 1 atm N₂. Then, the mixture was stirred and heated at 180 °C for 36 h. Conversion and yields of products were determined by GC using n-dodecane as an internal standard. The products were identified by GC-MS equipped with the same column as GC analyses as well as ¹H NMR and ¹³C NMR analyses of the isolated products. The analysis of the gas phase product (H₂) was carried out by the mass spectrometer (BELMASS). For the catalytic tests in entries 1, 4, 5 and 7 of Table 2, we determined isolated yields of lactones as follows. After the reaction, the catalyst was removed by filtration and the reaction mixture was concentrated under vacuum evaporator to remove the volatile compounds. Then, the lactones were isolated by column chromatography using silica gel 60 (spherical, 63–210 μm, Kanto Chemical Co. Ltd.) with hexane/ethylacetate (98–80/2–20) as the eluting solvent, followed by analyses by ¹H NMR, ¹³C NMR and GCMS.

Acknowledgements

This work was supported by Grant-in-Aids for Scientific Research B (26289299) from MEXT (Japan), a MEXT program “Elements Strategy Initiative to Form Core Research Center” and a Grant-in-Aid for Scientific Research on Innovative Areas “Nano Informatics” (25106010) from JSPS.

Notes and references

1. S. Murahashi, K. Ito, T. Naota and Y. Maeda, Tetrahedron Lett., 1981, 22, 5327–5330.
2. J. Zhao and J. F. Hartwig, Organometallics, 2005, 24, 2441–2446.
3. S. Musa, I. Shaposhnikov, S. Cohen and D. Gelman, Angew. Chem., Int. Ed., 2011, 50, 3533–3537.
4. J. Zhang, E. Balaraman, G. Leitus and D. Milstein, Organometallics, 2011, 30, 5716–5724.
5. K. Fujita, W. Ito and R. Yamaguchi, ChemCatChem, 2014, 6, 109–112.
6. N. Ichikawa, S. Sato, R. Takahashi, T. Sodesawa and K. Inui, J. Mol. Catal. A: Chem., 2004, 212, 197–203.
7. Y. L. Zhu, J. Yang, G. Q. Dong, H. Y. Zheng, H. H. Zhang, H. W. Xiang and Y. W. Li, Appl. Catal., B, 2005, 57, 183–190.
8. Y. Mikami, K. Ebata, T. Mitsudome, T. Mizugaki, K. Jitsukawa and K. Kaneda, Heterocycles, 2010, 80, 855–861.
9. Y. Ishii, K. Osakada, T. Ikariya, M. Saburi and S. Yoshikawa, Tetrahedron Lett., 1983, 24, 2677–2680.
10. T. Suzuki, K. Morita, M. Tsuchida and K. Hiroi, Org. Lett., 2002, 4, 2361–2363.
11. M. Ito, A. Osaku, A. Shiibashi and T. Ikariya, Org. Lett., 2007, 9, 1821–1824.
12. T. Buntara, S. Noel, P. H. Phua, I. M. Cabrera, J. G. Vries and H. J. Heeres, Angew. Chem., Int. Ed., 2011, 50, 7083–7087.
13. C. M. Nicklaus, P. H. Phua, T. Buntara, S. Noel, H. J. Heeres and J. G. de Vries, Adv. Synth. Catal., 2013, 355, 2839–2844.
14. C. Aellig, F. Jenny, D. Scholz, P. Wolf, I. Giovinazzo, F. Kollhoff and I. Hermans, Catal. Sci. Technol., 2014, 4, 2326–2331.
15. T. Nishimura, T. Onoue, K. Ohe and S. Uemura, J. Org. Chem., 1999, 64, 6750–6755.
16. Y. Endo and J.-E. Bäckvall, Chem.–Eur. J., 2011, 17, 12596–12601.
17. J. Huang, W. L. Dai and K. N. Fan, J. Phys. Chem. C, 2008, 112, 16110–16117.
18. T. Mitsudome, A. Noujima, T. Mizugaki, K. Jitsukawa and K. Kaneda, Green Chem., 2009, 11, 793–797.
19. J. Zheng, J. Huang, X. Li, W.-L. Dai and K. Fan, RSC Adv., 2012, 2, 3801–3809.
20. Y. Ishii, T. Yoshida, K. Yamawaki and M. Ogawa, J. Org. Chem., 1988, 53, 5549–5552.
21. C. Gunanathan and D. Milstein, Science, 2013, 249–260.
22. K. Kon, S. M. A. H. Siddiki and K. Shimizu, J. Catal., 2013, 304, 63–71.
23. S. M. A. H. Siddiki, K. Kon, A. S. Touchy and K. Shimizu, Catal. Sci. Technol., 2014, 4, 1716–1719.
24. S. K. Moromi, S. M. A. H. Siddiki, M. A. Ali, K. Kon and K. Shimizu, Catal. Sci. Technol., 2014, 4, 3631–3635.
25. K. Y. Chen, S. Koso, T. Kubota, Y. Nakagawa and K. Tomishige, ChemCatChem, 2010, 2, 547–555.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra03337c

---