Catalysis with Diboron(4)/Pyridine: Application to the Broad-Scope [3 + 2] Cycloaddition of Cyclopropanes and Alkenes

Abstract

In contrast to the extensive but non-recyclable use of tetraalkoxydiboron(4) compounds as stoichiometric reagents in diverse reactions, this article reports an atom-economical reaction using a commercial diboron(4) as the catalyst. The key to success was designing a catalytic cycle for radical [3 + 2] cycloaddition involving a pyridine cocatalyst to generate from the diboron(4) catalyst and reversibly mediate the transfer of boronyl radicals. In comparison with known [3 + 2] cycloaddition with transition metal-based catalysts, the current reaction features not only metal-free conditions, inexpensive and stable catalysts, and simple operation but also remarkably broadened substrate scope. In particular, previously unusable cyclopropyl ketones without an activating group and/or alkenes with 1,2-disubstitution and 1,1,2-trisubstitution patterns were successfully used for the first time. Consequently, challenging cyclopentane compounds with various levels of substitution (65 examples, 57 new products, up to six substituents at all five ring atoms) were readily prepared in generally high to excellent yield and diastereoselectivity. The reaction was also successfully applied in concise formal synthesis of an anti-obesity drug and building natural product-like complex bridged or spirocyclic compounds. Mechanistic experiments and computational investigation support the proposed radical relay catalysis featuring a pyridine-assisted boronyl radical catalyst. Overall, this work demonstrates the first approach to use tetraalkoxydiboron(4) compounds as catalysts and may lead to the development of new, green, and efficient transition metal-like boron-catalyzed organic reactions.

Anion Migrated Ring Opening and Rearrangement in Anionic Polymerization Induced C7 and C8 Polymerizations

“…Thus, new polymerization mechanisms and synthetic strategies should be developed over time to advance the field of polymer synthesis. The high ring strain of cyclopropyl and cyclobutyl groups is the key factor in the versatility of the structure of molecules. − Further investigations of whether the available olefin monomers with cyclopropyl or cyclobutyl groups are useful to prepare unprecedented polymer structures through a new polymerization mechanism are worthwhile. To date, cyclopropenes and their derivatives have been well-employed not only in ROMP (C3 polymerization) − but also in living/controlled vinyl addition polymerization (C2 polymerization). , Additionally, based on our previous work, 1-cyclopropylvinylbenzene (CPVB) and 1-cyclobutylvinylbenzene (CBVB), bearing cyclopropyl and cyclobutyl groups, respectively, undergo combination nucleophilic addition and ring-opening reactions. , During each propagation step, the carbanion is generated during the nucleophilic addition between living species and the double bond.…”

Diboron(4)‐Catalyzed Remote [3+2] Cycloaddition of Cyclopropanes via Dearomative/Rearomative Radical Transmission through Pyridine

“…Very recently we reported an example where suitable combination of a tetraalkoxydiboron( 4) with an electron-poor pyridine could serve as efficient catalyst for [3+2] cycloaddition of cyclopropyl ketones with alkenes. [14] In the current work, using just diboron(4) as the catalyst, a few issues have to be addressed for the proposed catalysis to succeed. First, can the pyridine substrate be used to generate boronyl radical?…”

Photocatalyzed [3sigma + 2sigma] and [3sigma + 2pi] cycloadditions for the synthesis of bicyclo[3.1.1]heptanes and cyclopentenes.

“…[1][2][3][4][5][6] In addition to activated Donor -Acceptor systems, which have been widely studied in the past decades, 3 less reactive acceptor-only cyclopropanes could also be recently functionalized through the reductive activation of a carbonyl group on the ring (Scheme 1A). [7][8][9][10][11][12][13][14][15] Due to the high redox potential of most carbonyl-substituted cyclopropanes, 7 strong reductants were often required such as Sm, 8,9 the combination of Ti and Mn 10 or more recently boron and pyridine derivatives. 11 Photoredox catalysis could be also applied, yet the coordination with a Lewis acidic transition metal is crucial to facilitate the reduction as reported by Yoon [12][13][14] and Meggers.…”

“…[7][8][9][10][11][12][13][14][15] Due to the high redox potential of most carbonyl-substituted cyclopropanes, 7 strong reductants were often required such as Sm, 8,9 the combination of Ti and Mn 10 or more recently boron and pyridine derivatives. 11 Photoredox catalysis could be also applied, yet the coordination with a Lewis acidic transition metal is crucial to facilitate the reduction as reported by Yoon [12][13][14] and Meggers. 15 As an alternative to reductive approaches, energy transfer catalysis allows the direct activation of substrates into a biradical excited state under mild conditions.…”

“…8,10 To challenge our approach, we chose phenyl acetylene (3a) as a radical trap under visible light irradiation due to small number of reports on this type of transformation 7,8,15 compared to olefin partners. [9][10][11][12][13][14][15] Based on previous study of Brown and co-workers on carbonyl-substituted BCBs, 33 thioxanthone (PC1) was chosen as energy transfer catalyst for the reaction. However, no cycloaddition product was obtained with PC1 (entry 1).…”