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Effective stabilization of a planar phosphorus(III) center embedded in a porphyrin-based fused aromatic skeleton

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Abstract

Organophosphorus(III) compounds usually take on stable pyramidal structures with a large inversion barrier of 30–35 kcal mol−1. In contrast, diphenylphosphine-fused Ni(II) porphyrin, where the phosphorus atom is directly attached at the meso-position and embedded in a rigid skeleton, exhibits a considerably planarized phosphorus center. Here we report the synthesis of a mesityl-substituted Ni(II) porphyrin analogue, 6, which allowed an evaluation of the inversion barrier (ΔG203) by variable temperature 1H NMR spectroscopy which showed it to be exceptionally small, at 14.0 kcal mol−1. The observed small inversion barrier has been attributed to conformational constraint imposed by the fused structure. In addition, it was thought that the planar transition state is stabilized by the Ni(II) porphyrin network that allows the contribution of a 22π-aromatic circuit involving phosphorus lone-pair electrons. Along this postulate, we attempted to engineer diarylphosphine-fused porphyrins with smaller inversion barriers by replacing the fused benzene rings with five-membered heterocyclic rings such as thiophene, benzothiophene, benzofuran, indole, benzothiophene 1,1-dioxide, and thiophene 1,1-dioxide. In that order, the aromatic character of the heterocycle decreases, which leads to increasing contribution of the 22π-aromatic circuit. Actually, the inversion barrier of the phosphorus center becomes smaller in this order and reaches 8.7 kcal mol−1 for thiophene 1,1-dioxide-fused Ni(II) porphyrin 15, supporting the postulate.

Graphical abstract: Effective stabilization of a planar phosphorus(iii) center embedded in a porphyrin-based fused aromatic skeleton

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Publication details

The article was received on 05 Sep 2017, accepted on 05 Oct 2017 and first published on 06 Oct 2017


Article type: Edge Article
DOI: 10.1039/C7SC03882H
Citation: Chem. Sci., 2017, Advance Article
  • Open access: Creative Commons BY-NC license
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    Effective stabilization of a planar phosphorus(III) center embedded in a porphyrin-based fused aromatic skeleton

    K. Fujimoto and A. Osuka, Chem. Sci., 2017, Advance Article , DOI: 10.1039/C7SC03882H

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