& Photochemistry Competing Pathways in the Photogeneration of Didehydrotoluenes from (Trimethylsilylmethyl)aryl Sulfonates and Phosphates Stefano Crespi, Davide Ravelli, Stefano Protti, Angelo Albini, and Maurizio Fagnoni* [a] Abstract: The scope of the photochemical generation of a,n-didehydrotoluene diradicals from aryl sulfonates and phosphates and their chemistry are explored. The thermally inaccessible a,2- and a,4- intermediates are efficiently ob- tained by irradiation of ortho- and para-(trimethylsilylme- thyl)phenyl triflates through heterolytic splitting of the ester anion from the substrate in the triplet state. Triplet phenyl cations are formed and the loss of trimethylsilyl cation from them affords the desired diradicals ( 3 Me 3 SiCH 2 C 6 H 4 -OZ ! 3 Me 3 SiCH 2 C 6 H 4 + ! · CH 2 C 6 H 4 · ). Triplet sensitization is required, for which acetone is used throughout. Direct irradiation leads, on the contrary, to photo-Fries fragmentation ( 1 Me 3 SiCH 2 C 6 H 4 O-Z !Me 3 SiCH 2 C 6 H 4 O · + Z · ). With mesylates, where ester cleavage is less convenient, a further competi- tion from the triplet is direct desilylation. Didehydrotoluenes are also obtained from the corresponding phosphates, al- though with poor efficiency. Introduction The photochemistry of aromatic compounds seems to provide an inexhaustible source of new reactions, although these often defy simple rationalization. [1] A new development in recent years has been the heterolytic cleavage of aromatic halides. [1e, 2] When irradiated in polar, protic solvents, aryl chlorides (provid- ed that no electron-withdrawing substituent is present) pro- duce phenyl cations in what appears to be a general reaction via the triplet state [Eq. (1)]. [1e, 2] ArX þ hn ! Ar þ þ X  ð1Þ However, the efficiency of the process largely depends on the nature of the leaving anion X  , on the other substituents present on the aromatic ring, and on their relative positions. The chemistry of the triplet cations thus formed has revealed interesting facets. In particular, they behave as selective elec- trophiles, typically adding to alkenes, [3] alkynes, [4] and (hetero)- aromatics, [5] thus leading to convenient metal-free phenylation procedures. [1e, 2] However, when an electrofugal group E is pres- ent in the starting molecule, it may split away directly giving a highly unsaturated neutral species. [6, 7] Thus, a s,p-diradical, a didehydrotoluene (DHT), is formed when E is at the benzylic position (Scheme 1 a, left side). [7] In turn, DHTs undergo a chem- ical reaction that, in accordance with previous investigations, involves a “radical” path, presumably via benzyl radicals, and an “ionic” path. [8, 9] Thus a,n-didehydrotoluenes were formed upon irradiation of E-substituted aryl chlorides (E = Me 3 Si), such as isomeric (n-chlorobenzyl)trimethylsilanes, [7] as an alternative to the Myers–Saito cycloaromatization of enyne–allenes (Scheme 1 a, right side). [8, 10, 11] In the photochemical approach, [7] however, the process starts from a stable aromatic derivative, rather than from a less-easily prepared polyunsaturated molecule. The significance of the photochemical approach obviously also depends on its versatility. However, knowledge is still limited concerning the generation and the chemistry of didehydroto- luenes according to Scheme 1 a (left side) and the factors that affect the points of divergence. This is a key issue when poten- tial applications of such intermediates are considered. As an example, these diradicals appear to be formed as the active moiety in several natural compounds in which their ability to attack DNA endows them with antineoplastic action. [12, 13] Scheme 1. a) Thermal (right) and photochemical (left) generation of a,n- DHTs. b) Photochemistry of aryl sulfonates and phosphates [a] S. Crespi, Dr. D. Ravelli, Dr. S. Protti, Prof. A. Albini, Prof. M. Fagnoni PhotoGreen Lab, Department of Chemistry University of Pavia, Viale Taramelli 12, 27100 Pavia (Italy) E-mail: fagnoni@unipv.it Supporting information for this article is available on the WWW under http ://dx.doi.org/10.1002/chem.201404787. Chem. Eur. J. 2014, 20,1–8  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 && These are not the final page numbers! ÞÞ Full Paper DOI: 10.1002/chem.201404787