Iron-Catalyzed Reductive Dehydroxylation of Benzylic Alcohols Using Polymethylhydrosiloxane (PMHS)

Abstract

The combination of FeCl₃ and PMHS is an efficient reducing system for the selective dehydroxylation of secondary benzylic alcohols, even in the presence of carbonyls, under very mild conditions.

reductive dehydroxylation - benzylic alcohol - siloxanes - FeCl₃

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20

General procedure for the reductive dehydroxylation of benzylic alcohols 4: Anhydrous FeCl₃ (2.4 mg, 0.015 mmol, 5 mol% equiv) was carefully weighed and stirred in 1,2-dichloroethane (2 mL) for 5 min. PMHS (0.05 mL, 0.9 mmol, 3.0 equiv) was then added to the prepared catalyst solution, followed by benzylic alcohol 4 (0.3 mmol, 1.0 equiv) and stirred at either r.t. or heated to the respective temperature. The residual crude product was concentrated in vacuo and purified by flash chromatography to afford the desired product 5. ^¹H NMR and ^¹³C NMR data of previously unknown compounds: 4-heptyl-1,2-dimethoxybenzene (5d). ^¹H NMR (400 MHz, CDCl₃): δ = 7.26-6.70 (m, 3 H), 3.87 (s, 3 H), 3.85 (s, 3 H), 2.54 (t, J = 7.8 Hz, 2 H), 1.60-0.86 (m, 13 H); ^¹³C NMR (100 MHz, CDCl₃): δ = 148.7, 147.0, 135.6, 120.1, 111.8, 111.2, 55.9, 55.8 35.6, 31.8, 31.7, 29.3, 29.2, 22.7, 14.1; HRMS (ESI): m/z [M + 1] calcd for C₁₅H₂₅O₂: 237.1855; found: 237.1850. 1-Bromo-4-heptyl-benzene (5k):^{¹¹ ¹}H NMR (400 MHz, CDCl₃): δ = 7.37 (d, J = 8.3 Hz, 2 H), 7.03 (d, J = 8.3 Hz, 2 H), 2.54 (t, J = 7.8 Hz, 2 H), 1.63-1.53 (m, 2 H), 1.36-1.19 (m, 8 H), 0.88 (t, J = 6.8 Hz, 3 H); ^¹³C NMR (100 MHz, CDCl₃): δ = 141.8, 131.2, 130.1, 119.2, 35.3, 31.8, 31.3, 29.1, 29.1, 22.6, 14.1; HRMS (ESI): m/z [M + 1] calcd for C₁₃H₂₀Br: 255.0748; found: 255.0757.