Over the last two decades the Heck reaction has emerged as one of the most versatile tools for the synthesis of carbonsubstituted alkenes. [1] The mechanism of this reaction is generally accepted to proceed through an oxidative addition/ syn-carbopalladation/syn-b-hydrogen elimination sequence. The Heck reaction is also the origin of a fascinating variety of cascade reactions in which the key common feature is the formation, after the carbopalladation step, of a s-alkylpalladium(ii) intermediate lacking a suitable b-hydrogen syn with respect to the palladium center. [1,2] A particularly interesting case is that in which the s-alkylpalladium(ii) complex undergoes an intramolecular CÀH activation process to give a palladacycle intermediate. [3] In our research into the use of substituted vinyl sulfones in stereoselective synthesis, [4] we envisaged that acyclic b,b'disubstituted-a,b-unsaturated sulfones could be stereoselectively prepared by simple Heck arylation of a,b-unsaturated sulfones through the seemingly evident syn-carbopalladation/ syn-b-hydrogen elimination pathway. [5] However, we describe herein that the palladium-catalyzed reaction of trans-a,bunsaturated phenyl sulfones with excess iodobenzene/Ag 2 CO 3 proceeds by an intermolecular four-component cascade reaction to give 1-phenyl-9-phenylsulfonyl-9,10-dihydrophenanthrenes in which four CÀC bonds are formed in a single step.To test the behavior of vinyl sulfones in intermolecular Heck reactions, [6] we began by treating the model substrate 1 a (see Eq. (1), Table 1, R iPr) [7] with iodobenzene under typical Heck reaction conditions: Pd(OAc) 2 (10 mol %), K 2 CO 3 , N,N-dimethylformamide (DMF), 120 8C. However, only the base-catalyzed isomerization of the vinyl sulfone to form the corresponding allyl sulfone was observed. [8] Therefore, we turned our attention to the use of Ag 2 CO 3 , a milder base that has proved to be very efficient in the Heck reactions of a,b-unsaturated sulfoxides. [9] The reaction of 1 a with iodobenzene (1 equiv) and Ag 2 CO 3 (200 mol %) in DMF at 120 8C for 72 h gave a mixture of four compounds: the starting vinyl sulfone (32 %), the isomerized allyl sulfone (30 %), the expected Heck product 2 a (26 %), and, quite surprisingly, the dihydrophenanthrene 3 a (21 %; Eq. (1)). The yield of 3 a improved progressively in the presence of increasing amounts of iodobenzene since the formation of 3 a requires the participation of three molecules of iodobenzene. The optimal 2 a:3 a ratio was obtained in the presence of ten equivalents of iodobenzene [10] (2 a:3 a 5:95, Table 1, entry 1).In order to determine the versatility of this unprecedented Heck-type cascade process on acyclic alkenes, the arylation of several trans-a,b-unsaturated phenyl sulfones 1 [7] was studied (Table 1, entries 1 ± 5).Remarkably, the cascade product 3 predominated largely over the Heck product 2 (2:3 ratio from 12:88 to 3:97, 82 ± 86 % yields of mixtures of 2 and 3) with both b-alkyl- (Table 1, entries 1 ± 3) and b-aryl-substituted vinyl sulfones (Table 1, ent...
