Reduction of Esters to Ethers Utilizing the Powerful Lewis Acid BF₂OTf·OEt₂

Abstract

The direct reduction of esters to their corresponding ethers has been achieved using the Lewis acid BF₂OTf·OEt₂ generated via anionic redistribution between TMSOTf and BF₃·OEt₂ with triethylsilane acting as the reducing agent. Isolated yields of up to 71% have been obtained with the corresponding alcohol as the only side product.

Key words

boron - esters - ethers - Lewis acid - reductions

References

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13

NMR analysis of the mixture showed the regeneration of BF₃·OEt₂ as well as peaks tentatively assigned to BF(OTf)₂·OEt₂, and B(OTf)₃·OEt₂: ¹H NMR showed two new etherate peaks downfield of BF₂OTf·OEt₂ [δ = 4.74 (q, J = 7.0 Hz, 4 H), 1.65 (t, J = 7.0 Hz, 6 H), and 4.61 (q, J = 6.9 Hz, 4 H), 1.60 (t, J = 6.9 Hz, 6 H)]. ¹¹B NMR showed two new singlets upfield of the BF₂OTf·OEt₂ (δ = -1.54, -2.28). ¹⁹F NMR showed two new singlets in the triflate region (δ = -75.47, -7.40).

14

The ¹⁹F NMR of a reaction run under conditions A (see footnote a, Table [2] ) showed the presence of a peak at -178 ppm suggesting the formation of triethylsilyl fluoride, which is likely responsible for the formation of the triethylsilyl ether product. The signal at -178 ppm was absent under conditions B.