Acyl Radicals from Acylsilanes: Photoredox-Catalyzed Synthesis of

Dec 4, 2017 - (20) Among visible-light photocatalysts, we chose to screen ... As mentioned previously, the crucial point here was the choice of the li...
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Letter Cite This: ACS Catal. 2018, 8, 304−309

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Acyl Radicals from Acylsilanes: Photoredox-Catalyzed Synthesis of Unsymmetrical Ketones Luca Capaldo, Riccardo Riccardi, Davide Ravelli, and Maurizio Fagnoni* PhotoGreen Lab, Department of Chemistry, University of Pavia, Viale Taramelli 12, 27100 Pavia, Italy

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ABSTRACT: Acyl radicals were smoothly generated from acylsilanes under photoredoxcatalyzed conditions. These radicals were formed upon ultraviolet B (UV-B), solar, or visible light irradiation by using decatungstate and acridinium salts as photocatalysts. Acylation of Michael acceptors and a few styrenes resulted in a smooth preparation of unsymmetrical ketones in yields up to 89%.

KEYWORDS: photoredox catalysis, acylsilanes, acyl radicals, decatungstate salts, acridinium salts

A

Scheme 1. Photoredox-Catalyzed Generation of Acyl Radicalsa

cyl radicals are valuable intermediates for the preparation of ubiquitous functional groups in nature, such as unsymmetrical ketones and amides.1 One of the most common ways to generate these intermediates is to start from acyl selenides in the presence of an organotin reagent and a radical initiator.1 A greener approach consists of the use of aldehydes as starting materials and light to promote the C(sp2)−H homolytic cleavage of the formyl hydrogen,2 facilitated by the low bond dissociation energy of this C−H bond (88.7 kcal mol−1 for propanal).3 Our group (and others) demonstrated that tetrabutylammonium decatungstate ((nBu4N)4[W10O32], TBADT)4,5 can be used in the role of photocatalyst for the generation of acyl radicals via a hydrogen atom transfer (HAT) process.6 However, this smooth process may fail if the H atom donor contains other labile C−H bonds, as was observed in the case of piperonal, where the activation of the methylene hydrogens in the benzodioxole ring was exclusive.7 Moreover, the volatility of some aldehydes (e.g., acetaldehyde) may represent a serious drawback of this approach. This calls for alternative strategies for acyl radicals generation and a reasonable choice is photoredox catalysis.8 Unfortunately, this approach appears unsuccessful, because of the electrochemical 9a inertia of aldehydes (Eox 1/2 > +2 V vs SCE, estimated ca. +3.5 V 9b,c red vs SCE; E1/2 ≈ −1.65 V vs SCE for benzaldehyde).9d Nevertheless, the incorporation of a redox active moiety (an electroauxiliary group)10 in the acyl radical precursor may drive the desired oxidation/reduction with positive effects in terms of conditions mildness and selectivity. In fact, some groups recently studied the formation of acyl radicals from RC(O)X derivatives under photoredox-catalyzed conditions (see Scheme 1). α-Keto acids were used as a source of acyl radicals (Scheme 1, case a).11 These compounds are quite easily oxidized when in 11a the anionic form (Eox and Ru(II) 1/2 ≈ +1.0 V vs SCE) 11a 11b 11c−g complexes, Eosin, Ir(III) complexes, and acridinium salts11h promoted the acyl radical formation via oxidation and ensuing elimination of carbon dioxide from the resulting carboxylyl radical. However, most of the reported examples © 2017 American Chemical Society

a

Abbreviations: HLB, household light bulb; DMA, N,N-dimethylacetamide; and DMDC, dimethyl dicarbonate.

have involved aroyl radicals, while only a handful of examples exploiting aliphatic derivatives was reported.11 Acyl radicals were also formed through the photocatalyzed reduction of in situ prepared (mixed) anhydrides (case b12a and case c12b in Scheme 1) or acyl chlorides (Scheme 1, case d12c). However, the existing protocols often made use of expensive transition-metal photocatalysts.11 Accordingly, we started thinking that a less-expensive and more direct method to generate acyl radicals was needed: an interesting and unexplored class of compounds worthy of testing is that of acylsilanes.13 Remarkably, these compounds can be easily Received: October 31, 2017 Revised: December 1, 2017 Published: December 4, 2017 304

DOI: 10.1021/acscatal.7b03719 ACS Catal. 2018, 8, 304−309

Letter

ACS Catalysis Table 1. Optimization of the Reaction Conditionsa

entry

photocatalyst

1a (M)

solvent

time (h)

light source

2a consumption (%)

3 yieldb (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

TBADT (2 mol %) TBADT (2 mol %) TBADT (2 mol %) TBADT (2 mol %) TBADT (2 mol %) TBADT (2 mol %) Acr+-Mes (5 mol %) Acr+-Mes (5 mol %) Acr+-Mes (5 mol %) Acr+-Mes (10 mol %) Acr+-Mes (10 mol %) pyrylium salt (2 mol %) pyrylium salt (2 mol %) Ru(bpz)32+ (5 mol %) DCA (0.75 mol %) DCA (5 mol %)

0.1 0.1 0.12 0.12 0.12 0.12 0.12 0.12 0.15 0.15 0.15 0.12 0.12 0.12 0.12 0.12

MeCN MeCN-H2O 5/1 MeCN-H2O 5/1 MeCOMe-H2O 5/1 MeCN-H2O 5/1 MeCN-H2O 5/1 CHCl3 CH2Cl2-MeOH 1/1 MeOH MeOH MeOH MeCN MeCN-H2O 5/1 MeCN-H2O 5/1 MeCN CHCl3

8 8 8 8 8 8 48 48 48 48 48 24 24 24 24 24

310 nm 310 nm 310 nm 310 nm 366 nm SolarBoxd 410 nme 410 nme 410 nme 410 nme 410 nme,f 410 nme 410 nme 450 nme,g 410 nme 410 nme

30 72 100 100 65 100