Benzene (c-C6H6) is arguably one of the most fundamentally significant small molecules in chemistry. First discovered by Faraday in 1825, the study of benzene introduced the basic concept of aromaticity and delocalization. In addition to its fundamental importance, benzene and its derivatives (arenes) are ubiquitous in chemical research with numerous applications ranging from biomedical research to materials science. The inorganic isoelectronic relative of benzene, borazine (cB3N3H6), [4] has also played a pivotal role in fundamental as well as applied chemistry. The isoelectronic and isostructural relationship between the B N and C=C bond and its consequence on the aromaticity of borazine has been a topic of discussion. From a more applied perspective, borazine serves as a precursor to BN-based ceramic materials. More recently, borazine has been implicated in chemical hydrogen storage applications because it is formed as an intermediate in the hydrogen release from ammonia– borane. Both benzene and borazine have been known for more than 80 years, and consequently, their chemical and physical properties have been thoroughly investigated. The corresponding organic/inorganic (or organometalloidal) hybrid structure containing carbon, boron, and nitrogen, that is, 1,2-dihydro-1,2-azaborine 1, has thus far eluded characterization. The development of boron–nitrogen heterocycles such as 1,2-dihydro-1,2-azaborines (from hereon in, abbreviated as 1,2-azaborines) has been a relatively unexplored area of research. Dewar and White pioneered the chemistry of monocyclic and ring-fused polycyclic 1,2-azaborine derivatives in the 1960s. Recently, contributions by the groups of Ashe, Piers, and Paetzold, as well as our group have further advanced the preparation of novel BN heterocycles and sparked a renewed interest in the chemistry and properties of these compounds. Despite the advances achieved to date, and given the powerful tools made available by modern chemical synthesis, it is surprising that a simple heterocycle such as the parent 1,2-dihydro-1,2-azaborine 1 has remained elusive. Dewar attempted its synthesis and isolation in 1967 but ultimately concluded that it “seems to be a very reactive and chemically unstable system, prone to polymerization and other reactions.” Herein, we describe the first isolation and characterization of 1,2-dihydro-1,2-azaborine. Its successful preparation allows a direct comparison of the physical and spectroscopic properties of the series of an organic, inorganic, and now, an organometalloidal benzene. The present study demonstrates that 1,2-dihydro-1,2-azaborine 1 is not only isolable but it actually exhibits remarkable stability, consistent with substantial aromatic character. Our experimentally determined structural and spectroscopic properties are consistent with values derived from high-level computations. Scheme 1 illustrates our synthetic route to compound 1. Coupling of the in situ-generated allylboron dichloride with tert-butyldimethylsilyl allyl amine (TBS allyl amine) furnished diene 2. Ring-closing metathesis of this intermediate with the first-generation Grubbs catalyst produced an isomeric mixture of 3 and 3’ (60:40 ratio) in 82 % yield. Dehydrogenation of this mixture was carried out in the presence of catalytic amounts of Pd/C to generate 4. Treatment of heterocycle 4 with LiBHEt3 installed the B H functionality to give 5 in quantitative yield. Complexation of 1,2-azaborine 5 to {Cr(CO)3} produced the piano-stool adduct 6. Subsequent removal of the N-protecting group gave 7 in 76% yield. Finally, decomplexation of 1 from {Cr(CO)3} was accomplished using triphenylphosphine. The use of {Cr(CO)3} as a temporary “protecting group” was necessary because efforts toward cleaving the N TBS bond directly from 5 were unsuccessful. Compound 1 proved to be difficult to isolate, owing to its high volatility. However, we ultimately accomplished its isolation (10 % yield) by fractional vacuum transfer in the presence of a low-boiling [*] M. H. Matus, Prof. Dr. D. A. Dixon Department of Chemistry, University of Alabama Tuscaloosa, AL 35487 (USA) E-mail: dadixon@as.ua.edu

A Hybrid Organic/Inorganic Benzene

The first examples of “pre-aromatic” 1,2-dihydro-1,2-azaborine heterocycles have been structurally characterized, enabling the direct comparison of delocalized bonds of 1,2-dihydro-1,2-azaborines to their corresponding formal double and single bonds in nonaromatic systems. The crystallographic data provide an unprecedented look into the structural changes that occur in six-membered BN-heterocycles on their road to aromaticity, and they establish with little ambiguity that 1,2-dihydro-1,2-azaborines possess delocalized structures consistent with aromaticity.

Crystal clear structural evidence for electron delocalization in 1,2-dihydro-1,2-azaborines.

The synthesis of two parental BN anthracenes, 1 and 2, was developed, and their electronic structure and reactivity behavior were characterized in direct comparison with all-carbon anthracene. Gas-phase UV-photoelecton spectroscopy studies revealed the following HOMO energy trend: anthracene, −7.4 eV; BN anthracene 1, −7.7 eV; bis-BN anthracene 2, −8.0 eV. The λmax of the lower energy band in the UV–vis absorption spectrum is as follows: anthracene, 356 nm; BN anthracene 1, 359 nm; bis-BN anthracene 2, 357 nm. Thus, although the HOMO is stabilized with increasing BN incorporation, the HOMO–LUMO band gap remains unchanged across the anthracene series. The emission λmax values for the three investigated anthracene compounds are at 403 nm. The pKa values of the N-H proton for BN anthracene 1 and bis-BN anthracene 2 were determined to be approximately 26. BN anthracenes 1 and 2 do not undergo heat- or light-induced cycloaddition reactions or Friedel–Crafts acylations. Electrophilic bromination of BN anthracen...

Two BN isosteres of anthracene: synthesis and characterization.

with minimal perturbation oftheir structures. 1,2-Dihydro-1,2-azaborines (abbreviated as 1,2-azaborines) serve as aunique structural platform to accomplish this goal because of their isostructural relationshipwith arenes, a ubiquitous motif in living organisms and in pharmaceuticals. Furthermore, thebonding of arenes with cations and other arenes through cation-π

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Boron mimetics: 1,2-dihydro-1,2-azaborines bind inside a nonpolar cavity of T4 lysozyme.

We present a comprehensive electronic structure analysis of structurally simple BN heterocycles using a combined UV-photoelectron spectroscopy (UV-PES)/computational chemistry approach. Gas-phase He I photoelectron spectra of 1,2-dihydro-1,2-azaborine 1, N-Me-1,2-BN-toluene 2, and N-Me-1,3-BN-toluene 3 have been recorded, assessed by density functional theory calculations, and compared with their corresponding carbonaceous analogues benzene and toluene. The first ionization energies of these BN heterocycles are in the order N-Me-1,3-BN-toluene 3 (8.0 eV) 2 (2.209 D) > 1 (2.154 D) > toluene (0.349 D) > benzene (0 D) and are consistent with experimental observations. The λmax in the UV–vis absorption spectra are in the order 3 (297 nm) > 2 (278 nm) > 1 (269 nm) > toluene (262 nm) > benzene (255 nm). We also es...

UV-Photoelectron Spectroscopy of 1,2- and 1,3-Azaborines: A Combined Experimental and Computational Electronic Structure Analysis

Literature data and the investigation results of the present authors on the reduction of heterocyclic aldehydes and ketones using catalytic hydrogen transfer have been analyzed.

M. Shimanska

Papers

Catalytic intermolecular hydrogen transfer in the hydrogenation of heterocyclic aldehydes and ketones. A review