Transition-Metal-Free Synthesis of Borylated Thiophenes via Formal Thioboration

Abstract

A simple, regiocontrolled, and transition-metal-free approach to access exclusively 3-borylated thiophene derivatives is reported. The commercially available B-chlorocatecholborane reagent (ClBcat) acts as a carbophilic Lewis acid to activate the alkyne in readily synthesized (Z)-organylthioenyne substrates. This boron-induced activation initiates the formal thioboration and subsequent sulfur dealkylation, leading to the formation of 3-borylated thiophenes in good yields. The resulting borylated thiophenes are isolable as boronic esters (Bpin) and boronamides (Bdan). These borylated products are amenable to diverse downstream functionalization reactions, i.e., C−C bond formation through cross-coupling, azidation, bromination, and C−H activation.

Copper-Catalyzed Aminoboration from Hydrazones To Generate Borylated Pyrazoles

“…On the basis of our previous borylative heterocyclization reactions, ,− we hypothesized that an analogous route to borylated pyrazoles from hydrazones 1 may be amenable to a direct borylation pathway . The aminoboration reaction developed here starts from readily available hydrazones and does not require isolation of synthetic intermediates (Scheme d).…”

Repurposing π Electrophilic Cyclization/Dealkylation for Group Transfer

“…Electrophilic cyclization reactions onto π bonds are a common route for the synthesis of heterocycles [1–11] . These reactions use simple acyclic precursors and employ readily available external electrophiles ranging from dihalides, [1–4] to boron [5–9] and selenium electrophiles, [1, 3, 4] to transition metal catalysts, [10–17] producing a range of corresponding heterocyclic products.…”

“…Electrophilic cyclization reactions onto p bonds are a common route for the synthesis of heterocycles. [1][2][3][4][5][6][7][8][9][10][11] These reactions use simple acyclic precursors and employ readily available external electrophiles ranging from dihalides, [1][2][3][4] to boron [5][6][7][8][9] and selenium electrophiles, [1,3,4] to transition metal catalysts, [10][11][12][13][14][15][16][17] producing a range of corresponding heterocyclic products. Yet for decades since their initial popularization, including substantial work by Larock on sequences of p electrophilic cyclization followed by nucleophilic dealkylation, [3,4,[18][19][20] the focus on forming heterocycles has left the potential of these sequences toward other pathways largely untapped.…”

Six Low-Lying Isomers of C11H8 Are Unidentified in the Laboratory–A Theoretical Study

“…While several derivatives of 7b H -cyclopenta­[ cd ]­indene ( 10 ) have been isolated elsewhere, − the parent molecule remains elusive, which outlines that it is the kinetic control that governs the formation of a particular molecule in the laboratory. Other low-lying neutral molecules of C 11 H 8 that are synthetically characterized are as follows: 1 H -cyclopropa­[ b ]­naphthalene ( 12 ; see Figure ), , 1 H -cyclopropa­[ a ]­naphthalene ( 13 ), penta-1,3-diyn-1-ylbenzene ( 15 ), , 1-(buta-1,3-diyn-1-yl)-4-methylbenzene ( 20 ), , 1-(buta-1,3-diyn-1-yl)-2-methylbenzene ( 22 ), , 1-ethynyl-2-(prop-1-yn-1-yl)­benzene ( 28 ; see Figure ), , 1,4-diethynyl-2-methylbenzene ( 31 ), 1,3-diethynyl-5-methylbenzene ( 33 ), 1,2-diethynyl-4-methylbenzene ( 36 ), and 1,2-diethynyl-3-methylbenzene ( 37 ) …”