Negishi Coupling for Highly Selective Syntheses of Allenes via Ligand Effect and Mechanistic Study via SAESI‐MS/MS

Abstract

Summary of main observation and conclusionβ‐H elimination is an intrinsic problem in transition metal‐catalyzed reactions. We describe herein an interesting ligand effect for Et2Zn acting as either ethyl provider or H provider, respectively: by applying SPhos or Gorlos‐Phos as the ligand, β‐H elimination has been successfully controlled in the corresponding Negishi coupling reaction affording different poly‐substituted allenes in good yields and excellent selectivities. SAESI‐MS (Solvent Assisted Electrospray Ionization Mass Spectrometry) has been applied to successfully capture the highly reactive organometallic intermediates, which show the different coordination behaviors of Pd with SPhos or Gorlos‐Phos as the ligand in the catalytic cycle. In addition, the different reactivities of Int 1 and Int 2 towards the formation of the final allene products have been demonstrated via SAESI‐MS/MS experiments. These MS studies visualized the whole catalytic cycle for the Negishi coupling reaction while nicely explain the observed reactivity and selectivity.