1,2‐Azaborine Cations

Abstract

1,2-Dihydro-1,2-azaborine is a six-membered aromatic heterocycle that is isoelectronic with benzene through the replacement of a C = C unit in benzene with an isoelectronic B À N unit. [1,2] Since the pioneering work by Dewar et al., [3,4] significant advances have been made in the synthesis and reactivity studies of this family of heterocycles. [5][6][7] Our continued exploration of the 1,2-azaborine motif [8][9][10][11][12][13][14][15] has led us to consider the synthesis of cationic 1,2-azaborines, for which no examples have been reported. In particular, we envisioned that substitution of 1,2-azaborine on the boron atom with pyridine derivatives would furnish cationic biaryltype structures [16] having the potential for use in materials applications (Scheme 1). Herein we report the synthesis, structural characterization, and optoelectronic properties of pyridine-substituted 1,2-azaborine cations, including a cationic heterocyclic analogue of para-terphenyl.We have previously established nucleophilic substitution of the B À Cl bond in 1,2-azaborines by anionic nucleophiles (with Cl À serving as the leaving group). [8] Less reactive neutral nucleophiles did not displace the chloride from the boron atom. We hypothesized that a better leaving group on the boron atom (e.g., OTf) could render it susceptible to nucleophilic attack by weaker neutral nucleophiles. In the course of our studies, we discovered that silver reagents facilitate the ligand exchange at the boron position in 1,2azaborines. [13] We were thus pleased to discover that treatment of 1,2-azaborine 1 with AgOTf produced the substituted 1,2-azaborine 2 in 59 % yield as an extremely moisturesensitive liquid (Scheme 2). The 1,2-azaborine 2 was characterized by 1 H, 11 B, and 13 C NMR spectroscopy as well as IR spectroscopy.Heterocycle 2 readily reacts with para-substituted pyridines to form the desired cationic 1,2-azaborines 3. As can be seen from Scheme 3, the substitution reaction is independent of the electronic nature of the nucleophile. Excellent yields have been obtained with both electron-rich and electron-poor pyridines.The 1,2-azaborine cations 3 are highly crystalline solids that fluoresce under UV light. We thus explored the solidstate fluorescence of the pyridine-substituted 1,2-azaborine cations 3. The solid-state fluorescence and quantum yields of aromatic hydrocarbons, including para-terphenyl, have been reported using an integrating sphere. [17] We have recorded the fluorescence spectra of crystalline samples of 3 a-e (Figure 1), all of which were freshly recrystallized prior to making the fluorescence measurements. The solid-state fluorescence of 3 a (R = Me) shows a peak at l em = 436 nm (F PL = 0.03) and is visibly less fluorescent than samples of 3 b and 3 c under a UV lamp (l = 365 nm; see Figure 1, right). The solid-state emission spectrum of 3 b (R = Ph) showed a relatively narrow band at l em = 448 nm (F PL = 0.86). The high quantum yield observed for 3 b is quite similar to the values obtained for Scheme 1. 1,2-Azaborine cations....

Expedient Synthesis of 1,2‐Thiaborines by Means of Sulfur Insertion into Boroles

“…Recently, there have been significant advances in the development of benzene analogues that feature a boron atom with an adjacent lone‐pair bearing atom, effectively acting as a C=C substitute in novel heteroarene systems . In particular, this chemistry has flourished for boron/nitrogen species, namely 1,2‐azaborines ( A , Figure ) . With respect to boron–oxygen analogues (1,2‐oxaborines, B ), numerous fused polycyclic systems are known, as well as five monocyclic species .…”

Jüngste Fortschritte in der Azaborin‐Chemie

“…Liu und Mitarbeiter fanden heraus, dass die Behandlung von 69 mit AgOTf die Verbindung 73 ergibt, welche die wesentlich instabilere B‐OTf‐Bindung enthält. Wie sich zeigte, reagiert 73 sowohl mit elektronenreichen als auch elektronenarmen para ‐substituierten Pyridinen unter Bildung der ersten 1,2‐Azaborin‐Kationen 74 a – e (Schema , oben) 66. Die an 74 a – e im Festkörper gemessene Fluoreszenz wurde mit Ergebnisse für ihre Kohlenstoffanaloga verglichen.…”

Organische Chemie 2010

“…Um neuartige Verbindungen handelt es sich bei den 1,2‐Azaborinkationen (24), die durch ausgeprägte Festkörperfluoreszenz auffallen (Abbildung 12) 31…”