Reductive α-borylation of α,β-unsaturated esters using NHC-BH 3 activated by I 2 as a metal-free route to α-boryl esters

James E. Radcliffe, Valerio Fasano, Ralph W. Adams, Peiran You, Michael J. Ingleson*

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)

7 Citations (Scopus)

Abstract

Useful α-boryl esters can be synthesized in one step from α,β-unsaturated esters using just a simple to access NHC-BH 3 (NHC = N-heterocyclic carbene) and catalytic I 2 . 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-BD 3 /I 2 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 BH 2 (NHC) unit can be transformed into electrophilic BX 2 (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.

Original languageEnglish
Pages (from-to)1434-1441
Number of pages8
JournalChemical Science
Volume10
Issue number5
Early online date19 Nov 2018
DOIs
Publication statusPublished - 7 Feb 2019

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