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Formation of Hetero-Boroxines: Dynamic Combinatorial Libraries Generated through Trimerization of Pairs of Arylboronic Acids

2007; Elsevier BV; Volume: 74; Issue: 1 Linguagem: Inglês

10.3987/com-07-s(w)61

1881-0942

Yuji Tokunaga, Hiroki Ueno, Youji Shimomura,

Metal-Organic Frameworks: Synthesis and Applications

Condensation of pairs of arylboronic acids provided homo-and hetero-boroxines in solution as evidenced from NMR spectra, and those boroxines were detected in the gas phase by GC-MS spectrometry.Equilibrium constants for the formation of these boroxines in solution were obtained through integration of pertinent signals in the NMR spectra of the mixtures of boronic acids.Boroxines, cyclic anhydrides of boronic acids, are six-membered inorganic heterocycles that have been used as flame retardant materials, 1 dopants that enhance lithium ion transfer in polymer electrolytes, 2 nonlinear optical materials, 3 and agents that expand the functional and structural complexity of polymeric materials. 4Two attractive features of boroxines are their C 3 -symmetric tripodal structures 5 and Lewis acidity, 6 which provides the additional possibility of functionalizing the boroxine core though noncovalent interactions.Recently, we 7 and the Kua group 8 reported the thermodynamics of formation of boroxines from boronic acids.The equilibrium constants between the boronic acids and the boroxines in the presence of water have been measured using NMR spectroscopy, and a computational study has provided details of their thermodynamic and kinetic behavior.Those reports, however, discussed only homo-boroxines assembled from single monomeric boronic acids.In this paper, we describe the formation and stabilities of hetero-arylboroxines (Scheme 1).We employed several symmetrical arylboronic acids (1-5) to determine whether it would be easy to analyze their resulting hetero-and homo-boroxines using 1 H NMR spectroscopy (Figure 1).The rates of formation and hydrolysis of the boroxines were slow on the NMR spectroscopic timescale at room temperature, as reported previously. 7The 1 H NMR spectra (CDCl 3 ) of mixtures of pairs of these boronic This Paper id dedicated to Professor Dr. Ekkehard Winterfeldt on the occasion of his 75 th birthday.acids in the presence of water provided evidence for formation of their corresponding two homo-and two hetero-boroxines, respectively.For example, the 1 H NMR spectrum of a mixture of 1 and 2 (Figure 2) displays an array of well-defined signals.B(OH) 2 B(OH) 2 O B O B O B O B O B