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      A dual-catalysis approach to enantioselective [2 + 2] photocycloadditions using visible light.

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          Abstract

          In contrast to the wealth of catalytic systems that are available to control the stereochemistry of thermally promoted cycloadditions, few similarly effective methods exist for the stereocontrol of photochemical cycloadditions. A major unsolved challenge in the design of enantioselective catalytic photocycloaddition reactions has been the difficulty of controlling racemic background reactions that occur by direct photoexcitation of substrates while unbound to catalyst. Here, we describe a strategy for eliminating the racemic background reaction in asymmetric [2 + 2] photocycloadditions of α,β-unsaturated ketones to the corresponding cyclobutanes by using a dual-catalyst system consisting of a visible light-absorbing transition-metal photocatalyst and a stereocontrolling Lewis acid cocatalyst. The independence of these two catalysts enables broader scope, greater stereochemical flexibility, and better efficiency than previously reported methods for enantioselective photochemical cycloadditions.

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          Most cited references23

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          Visible light photoredox catalysis with transition metal complexes: applications in organic synthesis.

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            Efficient visible light photocatalysis of [2+2] enone cycloadditions.

            We report that Ru(bipy)3Cl2 can serve as a visible light photocatalyst for [2+2] enone cycloadditions. A variety of aryl enones participate readily in the reaction, and the diastereoselectivity in the formation of the cyclobutane products is excellent. We propose a mechanism in which a photogenerated Ru(bipy)3+ complex promotes one-electron reduction of the enone substrate, which undergoes subsequent radical anion cycloaddition. The efficiency of this process is extremely high, which allows rapid, high-yielding [2+2] cyclizations to be conducted using incident sunlight as the only source of irradiation.
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              Photophysics, photochemistry and solar energy conversion with tris(bipyridyl)ruthenium(II) and its analogues

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                Author and article information

                Journal
                Science
                Science (New York, N.Y.)
                1095-9203
                0036-8075
                Apr 25 2014
                : 344
                : 6182
                Affiliations
                [1 ] Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706, USA.
                Article
                344/6182/392 NIHMS716467
                10.1126/science.1251511
                24763585
                fcdf8b3e-c971-4d34-9271-7829ac24577f
                History

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