Adam J. V. Marwitz, M. Matus, L. Zakharov
Jan 19, 2009
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Angewandte Chemie
Abstract
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