Ambient-Temperature Cobalt-Catalyzed Cycloaddition Strategies to Aromatic Boronic Esters

Abstract

The room-temperature cobalt-catalyzed [4 + 2] cycloaddition of alkynylboronates and 1,3-dienes provides a convenient and general method for the synthesis of benzene-based aromatic boronic esters. Two complementary aromatization strategies involving in situ elimination and DDQ oxidation were explored, with the latter finding more generality. Finally, the potential of this technique to generate highly functionalized biaryls has been demonstrated via the synthesis of chiral (racemic) DMAP catalysts.

Pinacol Boronates by Direct Arene Borylation with Borenium Cations

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Aryl boronate esters are essential synthetic building blocks, their ubiquity arising from ease of handling combined with high efficacy in CÀX (X = C, N, O) bond-forming reactions. [1][2][3][4][5][6][7] Recent alternative approaches avoiding haloarenes include: [4+2] cycloadditions, [8] aryl-CH deprotonation, [9,10] diazonium ions, [11] and direct arene borylation with sterically hindered iridium catalysts. [1][2][3][4][5][6][7] Recent alternative approaches avoiding haloarenes include: [4+2] cycloadditions, [8] aryl-CH deprotonation, [9,10] diazonium ions, [11] and direct arene borylation with sterically hindered iridium catalysts.

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“…[1] Aryl boronate esters are commonly synthesized from arenes by a multistep process involving haloarene intermediates and either stoichiometric hard organometallic reagents or metalcatalyzed cross-coupling (e.g., Cu, Ni, Co, or Pd). [1][2][3][4][5][6][7] Recent alternative approaches avoiding haloarenes include: [4+2] cycloadditions, [8] aryl-CH deprotonation, [9,10] diazonium ions, [11] and direct arene borylation with sterically hindered iridium catalysts. [12][13][14] The latter represents a considerable advance, being highly generic and proceeding in one step from the parent aryl CH.…”

Synthesis, Characterisation and Reactions of Phosphine-Substituted Alkynylboronates and Alkynyltrifluoroborate Salts

“…Moreover, these reagents are OPEN ACCESS subject to chemistry at the triple bond. For example, hydrogenation [6], metallation [7,8], metathesis [9] and various cycloaddition reactions [10][11][12][13][14][15][16][17][18][19][20] have all been reported. Whilst a range of reactions of alkynylboronates are now documented, there is relatively little in the literature regarding heteroatom-substituted alkynylboronates of the type shown in Figure 1.…”

Regiodiverse Three‐Component Synthesis of Arenes

“…[3] An alternative to the cyclotrimerization of alkynes is the cobalt-catalyzed Diels-Alder reaction of alkynes with 1,3-dienes followed by oxidation of the dihydroaromatic intermediate. [4] Thereby, excellent regiocontrol can be obtained and the dienophile is not limited to acceptor-substituted educts as in thermal Diels-Alder reactions with normal electron demand. In the past we were able to show that the neutral Diels-Alder reaction can be efficiently catalyzed by low valent cobalt complexes under ambient temperature.…”

“…[4,5] Therefore, a bromo-functionalized building block 7e can be generated and converted in a Sonogashira reaction to the alkyne 9 in excellent yield A C H T U N G T R E N N U N G (dppe) (10 mol%), ZnI 2 (20 mol%), Zn powder (20 mol%); third step: 1.2 equiv. DDQ, toluene.…”