Abstract
While radical additions to π-bonds are well established, additions to σ-bonds are far less explored. We have found that electron deficient radicals derived from alkyl iodides under visible light irradiation add to the central strained bond of bicyclobutyl (BCB)-boronate complexes and lead to 1,3-alkyl disubstituted cyclobutyl boronic esters in high yields, with full stereospecificity and high levels of stereoselectivity. Novel cyclobutyl-substituted structures, including peptide and steroid boronic ester derivatives can be accessed. Additionally, although the use of electron-rich alkyl iodides as radical precursors was found to be ineffective, an alternative route involving alkylsulfonylation of the BCB-boronate followed by reductive desulfonylation provided access to simple alkyl substituted cyclobutane products.
Highlights
Whilst radical additions to bonds are well established, additions to -bonds are far less explored
We show that radicals can add to the central σ-bond of a strained BCB-boronate complexes, thereby providing a stereocontrolled synthesis of 1,3-dialkyl substituted cyclobutyl boronic esters 6
We and others recently showed that electrophilic radicals readily add to π-bonds of unsaturated boronate complexes 1 (Scheme 1a).[7]
Summary
Whilst radical additions to bonds are well established, additions to -bonds are far less explored. We show that radicals can add to the central σ-bond of a strained BCB-boronate complexes, thereby providing a stereocontrolled synthesis of 1,3-dialkyl substituted cyclobutyl boronic esters 6.
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