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Synthesis and Self‐Assembly of NCN‐Pincer Pd‐Complex‐Bound Norvalines
Author(s) -
Ogata Kazuki,
Sasano Daisuke,
Yokoi Tomoya,
Isozaki Katsuhiro,
Yoshida Ryota,
Takenaka Toshio,
Seike Hirofumi,
Ogawa Tetsuya,
Kurata Hiroki,
Yasuda Nobuhiro,
Takaya Hikaru,
Nakamura Masaharu
Publication year - 2013
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201301513
Subject(s) - pincer movement , chemistry , supramolecular chemistry , dipeptide , crystal structure , crystallography , palladium , stereochemistry , single crystal , peptide , catalysis , organic chemistry , biochemistry
The NCN‐pincer Pd‐complex‐bound norvalines Boc‐ D / L ‐[PdCl(dpb)]Nva‐OMe ( 1 ) were synthesized in multigram quantities. The molecular structure and absolute configuration of 1 were unequivocally determined by single‐crystal X‐ray structure analysis. The robustness of 1 under acidic/basic conditions provides a wide range of N ‐/ C ‐terminus convertibility based on the related synthetic transformations. Installation of a variety of functional groups into the N‐ / C ‐terminus of 1 was readily carried out through N ‐Boc‐ or C ‐methyl ester deprotection and subsequent condensations with carboxylic acids, R 1 COOH, or amines, R 2 NH 2 , to give the corresponding N ‐/ C ‐functionalized norvalines R 1 ‐ D / L ‐[PdCl(dpb)]Nva‐R 2 2 – 9 . The dipeptide bearing two Pd units 10 was successfully synthesized through the condensation of C ‐free 1 with N ‐free 1 . The robustness of these Pd‐bound norvalines was adequately demonstrated by the preservation of the optical purity and Pd unit during the synthetic transformations. The lipophilic Pd‐bound norvalines L ‐ 2 , Boc‐ L ‐[PdCl(dpb)]Nva‐NH‐ n ‐C 11 H 23 , and L ‐ 4 , n ‐C 4 H 9 CO‐ L ‐[PdCl(dpb)]Nva‐NH‐ n ‐C 11 H 23 , self‐assembled in aromatic solvents to afford supramolecular gels. The assembled structures in a thermodynamically stable single crystal of L ‐ 2 and kinetically stable supramolecular aggregates of L ‐ 2 were precisely elucidated by cryo‐TEM, WAX, SAXS, UV/Vis, IR analyses, and single‐crystal X‐ray crystallography. An antiparallel β‐sheet‐type aggregate consisting of an infinite one‐dimensional hydrogen‐bonding network of amide groups and π‐stacking of PdCl(dpb) moieties was observed in the supramolecular gel fiber of L ‐ 2 , even though discrete dimers are assembled through hydrogen bonding in the thermodynamically stable single crystal of L ‐ 2 . The disparate DSC profiles of the single crystal and xerogel of L ‐ 2 indicate different thermodynamics of the molecular assembly process.