Aerobic palladium(II)-catalyzed dehydrogenative heck reaction in the synthesis of pyrenyl fluorophores. a photophysical study of β-pyrenyl acrylates in solution and in the solid state.

An aerobic dehydrogenative Heck reaction of pyrene (1a) and 2,7-di-tert-butylpyrene (1b) with ethyl acrylate is reported. The reaction is catalyzed by a Pd(OAc)2/4,5-diazafluoren-9-one (DAF) system and takes place in acetic or pivalic acid as solvents at 110-130 °C. The reaction of 1a afforded a 6:1 mixture of C-1- and C-4-alkenylated pyrenes (2a and 3a, respectively) in 71% yield. In the case of 1b, only a C-4-substituted product (3b) was formed in 46% yield. Compounds 2a and 3a,b exhibited fluorescence in solution and in the solid state. In chloroform and THF solution the fluorescence maxima were in the range of 440-465 nm, and quantum yields decreased in the order 2a > 3a> 3b. In the solid state, 3a,b showed blue-green fluorescence (ΦF = 0.26 and 0.14, respectively), whereas 2a emitted yellow-green fluorescence) (ΦF = 0.35). Besides blue-emitting monomers, the presence of green-emitting aggregated species (preformed dimers) in the crystals of 3a,b and red-emitting dynamic excimers in the crystals of 2a has been demonstrated. Single-crystal X-ray diffraction analyses of 2a and 3b confirmed π-stacking of pyrenyl moieties in the crystals of the former and the absence of stacking in the crystals of the latter compound.