Issue 5, 2019

Reductive α-borylation of α,β-unsaturated esters using NHC–BH3 activated by I2 as a metal-free route to α-boryl esters

Abstract

Useful α-boryl esters can be synthesized in one step from α,β-unsaturated esters using just a simple to access NHC–BH3 (NHC = N-heterocyclic carbene) and catalytic I2. The scope of this reductive α-borylation methodology is excellent and includes a range of alkyl, aryl substituted and cyclic and acyclic α,β-unsaturated esters. Mechanistic studies involving reductive borylation of a cyclic α,β-unsaturated ester with NHC–BD3/I2 indicated that concerted hydroboration of the alkene moiety in the α,β-unsaturated ester proceeds instead of a stepwise process involving initial 1,4-hydroboration; this is in contrast to the recently reported reductive α-silylation. The BH2(NHC) unit can be transformed into electrophilic BX2(NHC) moieties (X = halide) and the ester moiety can be reduced to the alcohol with the borane unit remaining intact to form β-boryl alcohols. The use of a chiral auxiliary, 8-phenylmenthyl ester, also enables effective stereo-control of the newly formed C–B bond. Combined two step ester reduction/borane oxidation forms diols, including excellent e.e. (97%) for the formation of S-3-phenylpropane-1,2-diol. This work represents a simple transition metal free route to form bench stable α-boryl esters from inexpensive starting materials.

Graphical abstract: Reductive α-borylation of α,β-unsaturated esters using NHC–BH3 activated by I2 as a metal-free route to α-boryl esters

Supplementary files

Article information

Article type
Edge Article
Submitted
27 Sep 2018
Accepted
17 Nov 2018
First published
19 Nov 2018
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY license

Chem. Sci., 2019,10, 1434-1441

Reductive α-borylation of α,β-unsaturated esters using NHC–BH3 activated by I2 as a metal-free route to α-boryl esters

J. E. Radcliffe, V. Fasano, R. W. Adams, P. You and M. J. Ingleson, Chem. Sci., 2019, 10, 1434 DOI: 10.1039/C8SC04305A

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

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