Photooxidizing Chromium Catalysts for Promoting Radical Cation Cycloadditions

Abstract

The photooxidizing capabilities of selected Cr(III) complexes for promoting radical cation cycloadditions are described. These complexes have sufficiently long-lived excited states to oxidize electron-rich alkenes, thereby initiating [4+2] processes. These metal species augment the spectrum of catalysts explored in photoredox systems, as they feature unique properties that can result in differential reactivity from the more commonly employed ruthenium or iridium catalysts.

Excited state decay of cyclometalated polypyridine ruthenium complexes: insight from theory and experiment

“…Additionally, a wide range of complexes of this type have been successfully used as sensitizers in photoredox catalysis. [20][21][22][23][24][25][26][27][28][29][30][31] Moreover, the most fascinating and thoroughly studied property observed for a large number of polypyridine transition metal complexes is their luminescence. 32,33 The first polypyridine complex reported to exhibit luminescence was [Ru(bpy) 3 ] 2+ (bpy = 2,2′-bipyridine).…”

Radical Cation Cycloadditions Using Cleavable Redox Auxiliaries

“…As a first attempt to investigate the generality of this strategy, we studied the use of vinyl sulfides in intermolecular [2+2] cycloaddition reactions. 3d We had previously investigated the factors controlling the crossed selectivity of such reactions involving electron-rich styrenes and were pleased to observe that similar considerations are applicable to [2+2] radical cation cycloadditions of vinyl sulfides (Scheme 2). Thus, when 1 is irradiated in the presence of electron rich monosubstituted alkenes, the corresponding unsymmetrical cyclobutanes are produced in good yield.…”

“…1 In the past several years, there has been a renewed interest in the application of radical ion chemistry to synthesis, due in part to the recognition that photoredox catalysis offers a convenient means to access the distinctive reactivity of these odd-electron intermediates under relatively mild and convenient conditions. 2 Recent reports of synthetic transformations involving photogenerated alkene radical cations have included a variety of cycloaddition reactions 3 and anti-Markovnikov hydrofunctionalization reactions, 4 each of which would be challenging to accomplish using alternate synthetic strategies.…”

“…We wondered if an analogous redox auxiliary strategy might be applied to facilitate photooxidatively initiated organic transformations. We imagined that installation of an electron-rich redox auxiliary onto an otherwise unactivated alkene would facilitate its one-electron oxidation; the resulting radical cations could subsequently be induced to undergo a number of characteristic alkene radical cation reactions, including Diels–Alder 3bfg and [2+2] cycloadditions. 3ad Subsequent cleavage of the redox auxiliary group would afford the products of formal cycloadditions involving unactivated alkene substrates (Figure 1).…”

Al2O3 Surface Complexation for Photocatalytic Organic Transformations

“…[27][28][29][30] As for the photocatalytic redox route, the reaction can proceed only if the redox potential of the photocatalyst is greater than that of the reactants 3 . Conventional strategies to fulfill this condition include using wide band gap semiconductors 31 such as TiO 2 20,24 or Nb 2 O 5 32 , and modifying the cation 3, 33,34 or ligands of transition metal complexes to tune the redox potential.…”