DOI: 10.1021/jo100349h Published on Web 04/02/2010 J. Org. Chem. 2010, 75, 3477–3480 3477 r 2010 American Chemical Society pubs.acs.org/joc Photoarylation of Alkenes and Heteroaromatics by Dibromo-BINOLs in Aqueous Solution Daniela Verga, Filippo Doria, Luca Pretali, and Mauro Freccero* Dipartimento di Chimica Organica, Universit  a di Pavia, V.le Taramelli 10, 27100 Pavia, Italy mauro.freccero@unipv.it Received February 24, 2010 The photochemistry of 6,6 0 -dibromo-BINOL (BINOL = 2,2 0 -dihydroxy-1,1 0 -binaphthyl) under mild basic condi- tions and its methyl and triisopropylsilyl ethers has been investigated in neat and aqueous acetonitrile through product distribution analysis and laser flash photolysis. Arylation and alkylation have been successfully achieved in the presence of allyltrimethylsilane, ethyl vinyl ether, pyrrole, pyridine, thiophene, benzene, and indole. Such a photoreactivity offers a metal and protecting group free synthetic protocol toward mono- and disubstituted 6-aryl/alkyl BINOLs, since the BINOL chirality is pre- served in the photoactivation process. The activation of carbon-halogen bonds in aryl halides, by photoexcitation, has been studied for decades by classic product distribution analysis 1 and fast kinetic techniques, such as laser flash photolysis (LFP), detecting the generated transient species by means of UV/vis. 2 The mechanistic investigations propelled the exploitation of such a reactivity for synthetic applications 3 and photodegradation of organic pollutants in water. 4 Recently, the reactivity has been ex- tended to organic solvents, suggesting new synthetic applica- tions for a metal free arylation/alkylation process. 5 Nevertheless, synthetic protocols based on the mild photo- activation in aqueous solvents are still scarce. 6 The photo- reactivity of the 4-chlorophenol (1) and its methyl ether (2) as prototypes of aryloxy halides has been rationalized as a heterolytic process generating both 4-oxocyclohexa-2,5-die- nylidene carbene (3) and 4-hydroxyphenyl cation (4). The carbene 3 has been proposed to arise from the depro- tonation of the 4 (Scheme 1). The latter has successfully been trapped by several π-nucleophiles. Despite the great effort on mechanistic and synthetic aspects related to the photoreactivity of 1 and 2, only very recently the photoreactivity of both the 6-bromonaphthols 5a-d 7 and the 1,6-dibromonaphthols 6a-d (Scheme 2), 8 has been investigated in the presence of aromatics, heteroaromatic, and alkenes. The reactive triplet carbenes 5C and 6C, photogenerated from the bromonaphthols 5a and 6a (Scheme 2), have been shown to be the key intermediates involved in the reactivity, by LFP and trapping experiments. 7,8 These carbene inter- mediates have been efficiently trapped by O 2 generating the detectable carbonyl oxides 5CO 7 and 6CO, 8 which exhibit a broad absorbance centered at 580 and 680 nm, respectively. From a synthetic point of view, 6,6 0 -dibromo-BINOL (8) is much more interesting than the 6-bromonaphthol, since it is widely used as precursor in the synthesis of both 6,6 0 - disubstituted chiral ligands, 9 and several helicates with dif- ferent cavity sizes and chemical properties. 10 The above aspects, conjugated with the interest of our group in the SCHEME 1. Photogeneration of 4-Oxocyclohexa-2,5-dienyl- idene Carbene (3) and 4-Hydroxyphenyl Cation (4) from Chlorophenol 1 and Chloroanisole 2 (1) (a) Schutt, L.; Bunce, N. J. In CRC Handbook of Organic Photochem- istry and Photobiology, 2nd ed.; Horspool,W. M., Lenci, F., Eds.; CRC Press: Boca Raton, FL, 2004; Chapters 38 and 39 and references cited therein. (b) Grimshaw, J.; De Silva, A. P. Chem. Soc. Rev. 1981, 10, 181–203. (c) Protti, S.; Fagnoni, M.; Mella, M.; Albini, A. J. Org. Chem. 2004, 69, 3465–3473. (2) (a) Bonnichon, F.; Richard, C.; Grabner, G. Chem. Commun. 2001, 73–74. (b) Bonnichon, F.; Grabner, G.; Richard, C.; Lavedrine, B. New J. Chem. 2003, 27, 591–596. (c) Grabner, G.; Richard, C.; Kohler, G. J. Am. Chem. Soc. 1994, 116, 11470–11480. (d) Bonnichon, F.; Grabner, G.; Guyot, G.; Richard, C. J. Chem. Soc., Perkin Trans. 2 1999, 1203–1210. (e) Arnold, B. R.; Scaiano, J. C.; Bucher, G. F.; Sander, W. J. Org. Chem. 1992, 57, 6469– 6474. (3) (a) Lazzaroni, S.; Protti, S.; Fagnoni, M.; Albini, A. Org. Lett. 2009, 11, 349–352. (b) Fagnoni, M.; Albini, A. Acc. Chem. Res. 2005, 38, 713–721. (c) Dichiarante, V.; Fagnoni, M. Synlett 2008, 787–800. (4) (a) Sykora, J.; Pado, M.; Tatarko, M.; Izakovic, M. J. Photochem. Photobiol. A 1997, 110, 167–175. (b) Skurlatov, Y. I.; Ernestova, L. S.; Vichutinskaya, E. V.; Samsonov, D. P.; Semenova, I. V.; Rodko, I. Y.; Shvidky, V. O.; Pervunina, R. I.; Kemp, T. J. J. Photochem. Photobiol. A 1997, 107, 207–213. (5) (a) Protti, S.; Fagnoni, M.; Albini, A. Org. Biomol. Chem. 2005, 3, 2868–2871. (b) Fagnoni, M.; Albini, A. 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