Visible‐Light‐Driven Strain‐Increase Ring Contraction Allows the Synthesis of Cyclobutyl Boronic Esters

Abstract

There are a limited number of ring‐contraction methodologies which convert readily available five‐membered rings into strained four‐membered rings. Here we report a photo‐induced radical‐mediated ring contraction of five‐membered‐ring alkenyl boronate complexes into cyclobutanes. The process involves the addition of an electrophilic radical to the electron‐rich alkenyl boronate complex, leading to an α‐boryl radical. Upon one‐electron oxidation, ring‐contractive 1,2‐metalate rearrangement occurs to give a cyclobutyl boronic ester. A range of radical precursors and vinyl boronates can be employed, and chiral cyclobutanes can be accessed with high levels of stereocontrol. The process was extended to the preparation of benzofused cyclobutenes and the versatility of the boronic ester was demonstrated by conversion to other functional groups.

Gold-Catalyzed Synthesis of Small Rings

“…95−97 The use of highly polarized cycloaddition partners is a general theme in this context, but metal-catalyzed [2 + 2] cycloadditions of unbiased alkenes and alkynes have also been achieved. 98,99 Non-cycloaddition strategies for the synthesis of 4-membered rings include ring expansion, 100−103 ring contraction, 104,105 strain release opening, 106,107 hydroalkylation, 108 or hydroacylation reactions 109 among others. 110,111 1.3.…”

“…The most classical approach for the synthesis of cyclobutanes is the photoexcitation of alkenes, which can engage in concerted [2 + 2] cycloaddition pathways, thermally forbidden by symmetry (Scheme C). − These transformations have found a broad range of applications in organic synthesis over the years. − Apart from direct photoexcitation pathways, cyclobutanes have also been assembled via formal [2 + 2] cycloadditions with the aid of photoredox , or Lewis acid catalysis. − The use of highly polarized cycloaddition partners is a general theme in this context, but metal-catalyzed [2 + 2] cycloadditions of unbiased alkenes and alkynes have also been achieved. , Non-cycloaddition strategies for the synthesis of 4-membered rings include ring expansion, − ring contraction, , strain release opening, , hydroalkylation, or hydroacylation reactions among others. , …”

Recent Advances in the Construction of Fluorinated Organoboron Compounds

“…Upon one-electron oxidation, ring-contractive 1,2-metalate rearrangement occurs to provide a cyclobutyl boronic ester ( Scheme 17 (VII)). 120 …”

A Modular Access to 1,2‐ and 1,3‐Disubstituted Cyclobutylboronic Esters by Consecutive Radical Additions

“…Thus, Liu et al reported in 2019 an efficient method to access 2-aminocyclobutylboronic esters by photoisomerization of 1,2-azaborine. [11] The Aggarwal group, another prominent contributor to this field, recently described various new methods: by metalation of a cyclobutane bearing a leaving group (E), [12] by strain release from bicyclobutanes in a radical [13] and ionic [14] fashion (F), as well as by harnessing the power of 1,2-metallate shifts in a strain increase [15] and a ringexpansion strategy (G). [16] Finally, Hall et al performed enantioselective conjugate borylation on substituted cyclobutenones and cyclobutene-1-carboxylates to access a wide variety of enantiopure cyclobutyl boronic esters (H).…”