Visible‐Light‐Induced Photoreduction of Carborane Phosphonium Salts: Efficient Synthesis of Carborane‐Oxindole‐Pharmaceutical Hybrids

Abstract

Visible‐light‐induced photoreaction of carboranes is an effective approach to prepare carborane‐containing compounds. While several methods involving boron‐centered carboranyl radicals have been established, those for carbon‐centered carboranyl radicals are underdeveloped, except for the UV‐light‐promoted photohomolysis. Herein, we describe a simple but effective approach to access carbon‐centered carboranyl radicals by photoreduction of carborane phosphonium salts under blue light irradiation without using transition metals and photocatalysts. The utility of the method was demonstrated by successfully preparing a range of carborane‐oxindole‐pharmaceutical hybrids by radical cascade reactions. Computational and experimental studies suggest that the carbon‐centered carboranyl radicals are generated by single‐electron transfer of the photoactive charge‐transfer complexes between the salts and the additive potassium acetate.

Charge Transfer Complex-Enabled Synthesis of (Hetero)arylated m-Carboranes from m-Carborane Phosphonium Salts

“…Based on the above results and our previous report, one possible mechanism is shown in Figure . The CTC between m -carborane phosphonium iodide 1 and Na 2 CO 3 or 2 could be excited by blue light to undergo a single electron transfer (SET) process, delivering the m -carboranyl radical species and the iodine radical with PPh 2 Me as the byproduct.…”

“…Recently, our group discovered that phosphonium salts are capable of forming charge transfer complexes (CTCs) with proper electron donors to produce organic radical species under blue-light irradiation without the need of photocatalysts and transition metals (Figure B) . Building upon this finding, here we disclose the use of m -carborane phosphonium salts to form CTCs, resulting in the production of carbon-centered m -carboranyl radicals upon blue-light irradiation (Figure C) . Given the advantages of blue light over UV light, we envisioned that the CTC strategy could enable photoarylation/photoheteroarylation of m -carboranes.…”

“…To understand the possible mechanism of the reaction, a series of mechanistic experiments were carried out (Scheme ). Our previous report showed that m -carborane phosphonium iodide could serve as a photoactive intramolecular CTC (ICTC) for the generation of the carbon-centered m -carboranyl radical . When a solution of m -carborane phosphonium iodide salt 1 was irradiated in the presence of Na 2 CO 3 , an 81% yield of the corresponding m -carborane was observed with a trace of salt 1 recovered (Scheme A).…”

Asymmetric synthesis of metallacarboranes using a traceless chiral auxiliary

“…Carboranes are a class of boron-rich cage-like molecular clusters possessing three-dimensional aromaticity. Due to their unique properties in chemical bonding, electronic regulation, thermal stability, shape, lipophilicity etc., 1 in the past decades, great efforts have been made to functionalize [2][3][4] and employ carboranes as the key structural units in polymers, 5,6 optoelectronic materials, [7][8][9] pharmaceuticals, [10][11][12] coordination chemistry 13 and catalysis. 14 Among the known carboranes, o-carborane (o-C 2 B 10 H 12 ), has been the most easily accessible and most studied precursor.…”

9,9′-Bis-o-carboranes: synthesis and exploration of properties

“…4 Therefore, the selective functionalization of carboranes has received much attention. 5 Generally speaking, the C–H bond is acidic and can be functionalized under strong bases, 6 while B–H bonds are less polar, making the functionalization on the boron vertex more challenging. o -Carborane contains ten B–H bonds in four different electronic environments with the following electron density distributions: B(9,12) > B(8,10) > B(4,5,7,11) > B(3,6).…”