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Incorporation of Boron into p-Conjugated Scaffolds to Produce Electron-Accepting p-Electron Systems

Atsushi Wakamiya

Institute for Chemical Research, Kyoto University, Japan

1.1 Introduction

Boron, a group 13 element, exhibits several characteristic structural and electronic features. One of the most outstanding features, with regard to potential benefits in functional organic materials, is the vacant p-orbital of trivalent boron. By connecting boron with p-conjugated systems, p-p*conjugation can occur effectively, providing unique electronic structures with a high p-electron-accepting ability (Figure 1.1).

Figure 1.1 p-p* Conjugation between trivalent boron and sp2 carbon atoms.

Historically, the development of functional materials using boron as a key element started with the seminal work of Williams and Kaim [1,2]. The vacant p-orbital renders trivalent boron unstable under normal conditions, but once kinetically protected [3] trivalent boron-containing compounds can be applied to functional materials. Williams and co-workers demonstrated that the introduction of two bulky aryl groups, such as mesityl (Mes), on the boron center affords sufficiently stabilized p-electron systems containing trivalent boron. Thus, the Mes2B group can be used as an electron-accepting group. The combination of the Mes2B group with p-electron-donating groups such as NR2 in p-conjugated systems provides intriguing photophysical properties based on intramolecular charge-transfer transitions [1b]. Kaim et al. discovered unique electrochemical properties of p-systems containing trivalent boron based on the fact that trivalent boron is isoelectronic to carbocations [2]. Following their pioneering work, a variety of p-conjugated compounds containing the Mes2B group as an electron-accepting group have been synthesized and applied to various functional materials (Figure 1.2). For example, p-systems containing the Mes2B group together with electron-donating dimethylamine groups show unique photophysical properties, and can be used as non-linear optical [4,5] and two-photon-absorbing materials [6]. The introduction of Mes2B groups at the terminal positions of oligothiophenes, which are generally known as hole-transporting materials, endows the resulting materials with electron-transporting ability on account of the electron-accepting ability of these boryl groups [7]. The introduction of these bulky boryl groups at lateral positions of p-conjugated skeletons [8,9] is an effective way to induce intense fluorescence, even in the solid state [8]. Oligothiophenes bearing Mes2B group at lateral positions are also used as a turn-on type fluorescence sensor for F- or CN- anions [9b].

Figure 1.2 Examples of Mes2B-substituted functional materials.

Some review articles on functional p-conjugated materials using trivalent boryl groups as electron-accepting units, including details and applications, are available [10]. This chapter focuses more specifically on boron-containing p-conjugated systems, where boron is embedded into the p-conjugated scaffold. It aims to combine the underlying chemistry and the fundamental aspects of electronic structure with the recent progress in the development of functional materials using such systems as a key scaffold.

1.2 Boron-Containing Five-Membered Rings: Boroles and Dibenzoboroles

Among the wide variety of boron-containing p-conjugated systems, special attention should be given to borole, which is a five-membered ring system that contains four p-electrons and one boron atom.

Figure 1.3 shows the results of theoretical calculations on polyheteroles that contain various main group elements (E) , reported by Salzner and co-workers [11]. Their results suggest that the electronic structure of polyheteroles strongly depends on the embedded main group element. In particular, polyboroles exhibit very low-lying LUMO levels compared to the other polyheteroles. However, regarding the electronic structure of borole, it is important to consider that borole is isoelectronic with the cyclopentadienyl cation, whose structure has been discussed intensively (Figure 1.4) [12]. Borole is also interesting from a fundamental perspective, as it should allow elucidation of the nature of antiaromatic singlet 4p-electron systems including the vacant p-orbital on the boron atom. The electronic properties of borole should be clearly manifested in its structure (Figure 1.4a) [13]. The singlet state should, in contrast to the triplet state, exhibit significant bond alternation in the borole moiety, and the results of theoretical calculations on thienylborole oligomers predict significant biradical character (Figure 1.4b) [14]. From a fundamental perspective, it should thus be interesting to investigate the magnitude of the contribution of the triplet state to the ground state of boroles.

Figure 1.3 Calculated HOMO (white rectangles) and LUMO (black rectangles) levels for polyheteroles (B3P86-30%/CEP-31G*).

Figure 1.4 (a) Electronic structure of borole and (b) the structure of thienylborole oligomers.

Pentaphenylborole (1) was first synthesized in 1969 by Eisch et al. [15]. Boroles exhibit strong Lewis acidity, and they thus undergo Diels-Alder reactions and other related dimerizations [16]. More importantly, boroles are extremely sensitive to air and moisture, and this high reactivity has most likely slowed down progress of borole chemistry for a long time. In 2008, the X-ray diffraction structures of pentaarylboroles 1 and 2, which were prepared from 1,1,-dimethyl-2,3,4,5-tetraphenylstannole (Scheme 1.1), were independently determined by Braunschweig et al. [17] and Yamaguchi et al. [18].

Scheme 1.1

In these solid-state structures, pentaphenylborole (1) exhibited a significantly lower degree of bond alternation in the butadiene moiety than expected for the singlet state (Figure 1.5), which is probably due to an intermolecular interaction between the boron atom and the substituents of the neighboring borole compounds [17]. In contrast to 1, pentaarylboroles with p-substituted phenyl groups (2a-c) on the boron atom showed a distinct bond alternation (Figure 1.5) [18]. Theoretical calculations suggested energy differences between the singlet and triplet states in 2a-c of +15.9, +15.4, and +15.7?kcal?mol-1, respectively [18]. These results clearly demonstrate that the triplet state does not contribute to the bonding situation in the ground state of these pentaarylboroles. For the singlet-state boroles 2a-c, NICS(0) values of +12.65, +12.78, and +12.94?ppm were calculated, respectively, indicating an antiaromatic character [18].

Figure 1.5 Structural parameters for calculated (B3LYP/6-31G(d)) and X-ray crystallographically determined structures of pentaarylboroles. Numerical values refer to bond lengths (Å).

Following these reports, various other borole derivatives have been synthesized and isolated [19-21]. The transmetalation of stannoles with BX3 or RBX2 (R?=?aryl or heteroaryl) has emerged as the most straightforward route for the construction of borole rings, and transmetalations from other metalloles, such as zirconacycles [16b] and plumbacycles [19], offer alternative synthetic routes to boroles. The synthesis of B-F-containing borole 3 from the reaction between plumbacyclopentadienylidene and BF3?·?OEt2 has been reported by Saito et al. (Scheme 1.2) [19].

Scheme 1.2

Wrackmeyer et al. have reported the construction of borole rings from the 1,1-carboboration of aminobis(alkynyl)boranes with BEt3 [20]. Subsequently, this method was expanded by Erker et al. [21], who demonstrated that the use of the stronger Lewis acid B(C6F5)3 can be applied to substrates containing silyl groups, which renders this reaction a more convenient synthetic route to aryl-substituted boroles (Scheme 1.3).

Scheme 1.3

For the further modification of boroles, Braunschweig et al. have used 1-chloroborole 4 as a key intermediate in the synthesis of new borole derivatives [22-26] such as amino- [22], aryl- [23], or heteroaryl-substituted boroles 5-7 [22], as well as carbene adduct 8 [23] or metal complex 9 [25] (Figure 1.6). Following the development of synthetic routes to boroles, the redox chemistry of boroles to form radical anion and dianion species has been intensively explored, which has been summarized nicely in a recently published book [27].

Figure 1.6 Examples of borole derivatives prepared from 1-chloroborole.

Peripheral substituents should enhance the electron-accepting ability...