Synthesis and inversion barriers of undeca- and dodeca-substituted saddle shaped porphyrin complexes q Akito Hoshino, a Yoshiki Ohgo b and Mikio Nakamura a,b, * a Division of Biomolecular Science, Graduate School of Science, Toho University, Funabashi 274-8510, Japan b Department of Chemistry, School of Medicine, Toho University, Tokyo 143-8540, Japan Received 2 May 2005; revised 18 May 2005; accepted 20 May 2005 Available online 9 June 2005 Abstract—Synthesis and barriers to inversion of a series of highly saddle shaped complexes are reported. The DG 5 has decreased by 8 kJ mol 1 at 243 K when the meso phenyl groups are replaced by bulkier 2,6-dichlorophenyl groups, and by 17 kJ mol 1 when one of the peripheral ethyl groups is removed. Ó 2005 Published by Elsevier Ltd. Highly deformed porphyrin complexes have attracted much attention because they serve not only the unique ligand field of the central metal ions but also the unique platform to recognize the ligand molecules. 1 One of the important factors to accomplish the above mentioned properties is the rigidity of porphyrin core. In the course of our research to reveal the effects of porphyrin defor- mation on the electronic structure of metal porphyri- nates, 2 we have found that the rigidity of the saddled porphyrin ring is greatly influenced by the peripheral modification. Here, we report the synthesis of saddle shaped Zn(II)(OETArP)(1a–e) and Zn(II)(HpETPP)(2) together with their activation parameters for ring inver- sion determined by the dynamic 1 H NMR spectroscopy (Scheme 1). 3,4 The free base porphyrins (OETArP)H 2 were prepared from 3,4-diethylpyrrole and the corresponding substi- tuted benzaldehydes in CH 2 Cl 2 solution according to the literature method. 5 Treatment of these porphyrins with zinc acetate in refluxing CH 2 Cl 2 –THF solution in the presence of anhydrous sodium sulfate gave the cor- responding zinc complexes 1a–e. (HpETPP)H 2 was sim- ilarly prepared from benzaldehyde-d 5 and a 3:1 mixture of 3,4-diethylpyrrole and 3-ethylpyrrole in the presence of BF 3 ÆOEt 2 in refluxing CH 2 Cl 2 solution; deuterated benzaldehyde was used to simplify the 1 H NMR spec- trum. The porphyrinogen thus formed was oxidized by DDQ. In addition to (HpETPP)H 2 , several porphyrin compounds represented as (Et x –TPP)H 2 (x = 4–8) were formed. In principle, there are 4, 4, and 5 geometrical 0040-4039/$ - see front matter Ó 2005 Published by Elsevier Ltd. doi:10.1016/j.tetlet.2005.05.100 Keywords: Nonplanar porphyrins; Inversion barriers; Dynamic NMR. q The EyringÕs plots are given as Supplementary data. * Corresponding author. Tel.: +81 3 3762 4151x2551; fax: +81 3 5493 5430; e-mail: mnakamu@med.toho-u.ac.jp N N N N Zn Et Et Et Et Et H Et Et N N N N Zn Et Et Et Et Et Et Et Et Y Y Y Y Y Y Y a X = OCH 3 b X = H c X = CF 3 d X = NO 2 e X = H Y = H Y = H Y = H Y = H Y = Cl Zn(OETArP) (1a-1e) X X X X Y Zn(HpETPP) (2) Scheme 1. Complexes examined in this study. Tetrahedron Letters 46 (2005) 4961–4964 Tetrahedron Letters