Cationic Platinum-Catalyzed Etherification by Intra- and Intermolecular Dehydration of Alcohols

Abstract

Catalytic etherification of diols proceeds to give various cyclic ethers by use of cationic platinum salt, which is in situ prepared from PtCl₂ and AgSbF₆. Etherification of benzylic alcohols is also possible by intermolecular dehydration. Both of intra- and ­intermolecular etherifications smoothly proceed even under an ­atmosphere of air.

Key words

platinum - ethers - dehydration - diols - alcohols

References

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9

All of the 2,n-disubstituted cyclic ethers were obtained as a mixture of dl and meso isomers (ca. 1:1).

10

Typical Experimental Procedure for Cyclic Ethers from Diols (Table 2): PtCl₂ (1.1 mg, 0.004 mmol) was placed in a flask and a 1,2-dichloroethane solution (2.0 mL) of a diol (0.20 mmol, 0.10 M) was added. To the resulting mixture was added AgSbF₆ (3.6 mg, 0.010 mmol) and the mixture was stirred at the temperature cited in Table [2] for 0.5-8 h. After excluding the solvent to the volume of ca. 0.5 mL under reduced pressure, the obtained mixture was purified by column chromatography (hexane-EtOAc) using silica gel to give a pure cyclic ether.

11

2-Methyl-2-(3-phenylpropyl)tetrahydrofuran: IR (neat): 1496, 1454, 1057, 748, 700 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 1.16 (s, 3 H), 1.51-1.74 (m, 6 H), 1.82-1.95 (m, 2 H), 2.59-2.66 (m, 2 H), 3.74-3.85 (m, 2 H), 7.17-7.29 (m, 5 H). ¹³C NMR (100 MHz, CDCl₃): δ = 25.8, 26.1, 26.7, 36.5, 36.7, 40.9, 67.1, 82.5, 125.6, 128.1, 128.3, 142.5. HRMS (FAB): m/z calcd for C₁₄H₂₀O [M⁺]: 204.1514. Found: 204.1508.

12

Typical Experimental Procedure for Acyclic Ethers (Table 3): PtCl₂ (3.2 mg, 0.012 mmol) was placed in a flask and a 1,2-dichloroethane solution (4.5 mL) of an alcohol (0.30 mmol) and allyl alcohol (0.60 mmol) was added. To the resulting mixture was added AgSbF₆ (10.3 mg, 0.030 mmol) and the mixture was stirred at the temperature cited in Table [3] for 1-2 h. The following procedure is the same as in ref. 10.

13

Complete racemization of a chiral alcohol indicates that the present etherification proceeded via S_N1 pathway (Scheme [1] ).