Gold-Catalyzed, S_N1-Type Reaction of Alcohols to Afford Ethers and Cbz-Protected Amines

Abstract

The use of a gold-catalyzed, microwave protocol to activate alcohols through an intermolecular, S_N1-type reaction to directly form unsymmetrical ethers and N-benzyloxy carbamate- (Cbz) protected amines is reported. Results have shown this reaction to be highly reproducible and tolerant of moisture, and moderate to high product yields (53–99%) were obtained. Significantly, the intermolecular catalytic amination of alcohols to directly afford Cbz-protected amines is heretofore unprecedented using gold catalysis.

Key words

gold - catalysis - ethers - amination - substitution

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• 37 S_N1 Reaction; Typical Procedure for 20b: A stock solution was prepared in an oven-dried, 4-mL vial equipped with a septum cap; the stock contained the carbocation-forming alcohol 10 (0.375 mmol, 1.0 equiv), the nucleophilic alcohol 7b (1.88 mmol, 5.0 equiv), and undecane (internal standard for GC; 0.075 mmol, 0.2 equiv), which were dissolved in dichloroethane (2.4 mL) until homogeneous. In a glove box, a second solution was prepared in an oven-dried, foil-covered, 0.5–2.0 mL microwave vial by first measuring out AuClPPh₃ (6.2 mg, 0.0125 mmol, 5 mol%) and AgSbF₆ (4.3 mg, 0.0125 mmol, 5 mol%). The vial was sealed with a crimp-top cap equipped with a septum, then it was removed from the glovebox and attached to a needle connected to a nitrogen line. Dichloroethane (0.2 mL) was added into the microwave vial by using a syringe, and the resulting mixture was stirred for 10 min to initiate the catalyst. Two-thirds of the stock solution was then added to the microwave vial. The nitrogen line was removed, and the reaction vial was placed in a microwave reactor and heated to 50 °C for 90 min. The reaction was directly purified by flash chromatography on silica gel. After purification of the product, its GC response factor was determined and retroactively used to calculate the reaction yields from the corresponding GC data; GC yields (97%) were correlated with the isolated yield (48 mg, 94%) obtained from duplicate reactions. IR (Na plate): 2949, 2869, 1119, 1080 cm^–1; ¹H NMR (500 MHz, CDCl₃, 20 °C): δ = 7.43–7.41 (m, 1 H), 7.21–7.17 (m, 2 H), 7.12–7.09 (m, 1 H), 4.41 (t, J = 5.1 Hz, 1 H), 3.45 (dd, J = 8.7, 8.8 Hz, 1 H), 3.29 (dd, J = 8.7, 8.7 Hz, 1 H), 2.89–2.82 (m, 1 H), 2.77–2.70 (m, 1 H), 2.07–1.89 (m, 4 H), 1.79–1.72 (m, 1 H), 0.99 (d, J = 6.8 Hz, 3 H), 0.98 (d, J = 6.8 Hz, 3 H). ¹³C NMR (125 MHz, CDCl₃, 20 °C): δ = 137.4, 137.3, 129.1, 128.8, 127.2, 125.7, 75.8, 75.6, 29.2, 28.9, 28.1, 19.6, 19.1. HRMS (EI⁺): m/z [M + H – H₂] calcd for C₁₄H₁₉O: 203.1436; found: 203.1447.