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

Abstract

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.

Synthesis, characterization, and properties of a benzofuran-based cage-shaped borate: photo activation of Lewis acid catalysts

“…The NMR spectroscopy of 4ÁTHF showed typical signals for a cage-shaped borate; the upfield/downfield shift of the d( 1 H)/d( 13 C) of C-H at the tether position was compared with those of the ligand 12: (d( 1 H) 6.69 -6.32 ppm, d( 13 C) 38.9 -43.2 ppm). 3 The coordinating THF showed broad signals at the lower chemical shifts compared with those of free THF. The recrystallization of 4ÁTHF from hexane/CH 2 Cl 2 gave a suitable crystal for X-ray analysis.…”

“…Lewis acidity was also estimated via the NMR chemical shift d( 13 C) and infrared stretching frequency of 2,6-dimethyl-gpyrone 13 ligated to borates 4, as shown in Table 1. 3 The Dd( 13 C) shift of C3 and the stretching frequency of the carbonyl (CQO) groups in 13 clearly show the degree of Lewis acidity. The borate 4 showed a large Dd ( 13 C) and Dn(CQO), which indicated a Lewis acidity that was slightly higher than that of 2.…”

Recognition of Aromatic Compounds by π Pocket within a Cage‐Shaped Borate Catalyst

“…The competitive reaction between butanal ( 1 ) and benzaldehyde ( 2 a ), which have similar steric demands,10 with Danishefsky’s diene 3 11 to produce cycloadducts 4 and 5 a , respectively, was studied (Table 1). 12 The previously reported cage‐shaped borate catalyst 6 B⋅THF (10 mol %), having no π pocket,9a,b,d, 13 gave the products in a 73 % yield with a 5 a / 4 ratio of 0.92:1 when dichloromethane was used as the solvent (entry 1). This result seemed reasonable as the two aldehydes, 1 and 2 a , had similar affinities to the boron center.…”

“…19 Compared with the phenyl groups, the naphthyl groups were expected to interact with the aromatic moiety more effectively because of their large π framework. Crystals of pyridine‐ligated borates 7 B⋅Py,9d 11 B⋅Py,9d and 12 B⋅Py (Py=pyridine), suitable for X‐ray analysis, were grown from a mixture of dichloromethane and n ‐hexane 20. The results of the structural determination are shown in Figure 1.…”

X-ray structure and conformational study of tris(2-(dimethylamino)phenyl)methanol salt derivatives: Roles of anions and hydrogen bond

“…The Si-atrane complexes based on tris(2-methoxyphenyl)methyllithium and tris(2-hydroxyphenyl)methyllithium were proved to be unusually stable to moisture and nucleophilic substitution [16,17]. Likewise, the cage-shaped borate esters based on tris(2-hydroxyphenyl)methane were reported with enhanced lewis acidity and catalytic activity [14,18]. As anion receptors, tris(2-hydroxyphenyl)methane derivatives displayed binding selectivity for chloride (Cl-), in comparison with bromide (Br À ), iodide (I À ) and nitrate ðNO 3 À Þ ions [19,20].…”