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Cage‐Shaped Borate Esters with Tris(2‐oxyphenyl)methane or ‐silane System Frameworks Bearing Multiple Tuning Factors: Geometric and Substituent Effects on Their Lewis Acid Properties
Author(s) -
Yasuda Makoto,
Nakajima Hideto,
Takeda Ryosuke,
Yoshioka Sachiko,
Yamasaki Satoshi,
Chiba Kouji,
Baba Akio
Publication year - 2011
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.201002789
Subject(s) - chemistry , lewis acids and bases , dihedral angle , substituent , pyridine , boron , ligand (biochemistry) , medicinal chemistry , aryl , silane , silanes , bite angle , crystallography , catalysis , stereochemistry , inorganic chemistry , organic chemistry , crystal structure , hydrogen bond , denticity , molecule , alkyl , biochemistry , receptor
Boron complexes that contain new tridentate ligands, tris( o ‐oxyaryl)methanes and ‐silanes, were prepared. These complexes had a cage‐shaped structure around a boron center and showed higher Lewis acidity and catalytic activity than open‐shaped boron compounds. The cage‐shaped ligands determined the properties of the borates by altering the geometry and were consistently bound to the metal center by chelation. The synthesized compounds were L⋅B(OC 6 H 4 ) 3 CH, L⋅B(OC 6 H 4 ) 3 SiMe, and its derivatives (L=THF or pyridine as an external ligand). Theoretical calculations suggested that the cage‐shaped borates had a large dihedral angle (C ipso ‐O‐B‐O) compared with open‐shaped borates. The geometric effect due to the dihedral angle means that compared with open‐shaped, the cage‐shaped borates have a greater Lewis acidity. The introduction of electron‐withdrawing groups on the aryl moieties in the cage‐shaped framework increased the Lewis acidity. Substitution of a bridgehead Si for a bridgehead C decreased the Lewis acidity of the boron complexes because the large silicon atom reduces the dihedral angle of C ipso ‐O‐B‐O. The ligand‐exchange rates of the para ‐fluoro‐substituted compound B(OC 6 H 3 F) 3 CH and the ortho ‐phenyl‐substituted compound B(OC 6 H 3 Ph) 3 CH were less than that of the unsubstituted borate B(OC 6 H 4 ) 3 CH. The ligand‐exchange rate of B(OC 6 H 4 ) 3 SiMe was much faster than that of B(OC 6 H 4 ) 3 CH. A hetero Diels–Alder reaction and Mukaiyama‐type aldol reactions were more effectively catalyzed by cage‐shaped borates than by the open‐shaped borate B(OPh) 3 or by the strong Lewis acid BF 3 ⋅OEt 2 . The cage‐shaped borates with the bulky substituents at the ortho ‐positions selectively catalyzed the reaction with less sterically hindered substrates, while the unsubstituted borate showed no selectivity.