Synthesis, Structural Elucidation, Cyclic Voltammetry, and Theoretical Modelling of 2‐Ferrocenyl‐4H‐benzo[e][1,3]thiazines and 2‐Aryl‐4H‐ferroceno[e][1,3]thiazines

Abstract

2‐Ferrocenyl‐4H‐benzo[e][1,3]thiazine and its 6,7‐dimethoxy derivative were prepared by a Bischler–Napieralski‐type annulation of the Mannich adducts of ferrocenecarboxamide, formaldehyde, and the corresponding thiophenol. A more efficient synthetic pathway, comprising a directed lithiation/iodination sequence followed by standard functional‐group transformations and a final copper‐catalyzed cyclization, was elaborated to convert [(dimethylamino)methyl]ferrocene into racemic mixtures of the first representatives of planar‐chiral 4H‐ferroceno[e][1,3]thiazines. A similar strategy with 2‐iodobenzyl bromide as the precursor enabled a highly improved synthesis of 2‐ferrocenyl‐4H‐benzo[e][1,3]thiazine. The relative tendency of the new ferrocene‐based thiazines, composed of potential redox sites assembled in different molecular architectures, to behave as donors in single‐electron transfer (SET) reactions was studied by cyclic voltammetry (CV) and DFT calculations. The results disclosed that 2‐ferrocenyl‐4H‐ferroceno[e][1,3]thiazine can undergo two consecutive redox steps and is the most efficient reductant among the prepared models; it has the lowest first half‐cell potential, the highest‐energy highest occupied molecular orbital (HOMO) concentrated on the fused metallocene unit and the lowest first ionization energy.