and dioxygen serve as oxidants in highly stereoselective oxidative Pd‐catalyzed Heck arylations in which aryl boronicacids are used to synthesize triarylalkyl‐substituted olefins. The reactions occur under very mild conditions at room temperature. As an example, the threefold sequential arylation of methyl acrylate is the crucial step in the stereoselective synthesis of Z‐Tamoxifen.