Heteroaromatic boron compounds

Dewar MJS, Rogers H (1962) Pi-complexes. 2. Charge transfer spectra of pi-complexes formed by tetracyanoethylene with polycyclic aromatic hydrocarbons and with heteroaromatic boron compounds. J Am Chem Soc 84 395... 

Dewar et al have examined the B shifts of several heteroaromatic boron compounds and suggested that the results yielded additional evidence of the aromaticity of the compounds. From the order of the chemical shifts, it was inferred that the 7t-bonding between the boron and the adjacent heteroatom was more important than in the... 

S. Gronowitz and C. Glennow. Heteroaromatic boron compounds. XVII. On the synthesis of bromoborazarothienopyridines and their reactions. Chem. Scripta, 1977, 11, 76. 

Costanzo et al. <2002MI87> published the synthesis of the furyl-substituted pyrazolo[3,2-f][l,2,4]triazine derivative 146 by Suzuki coupling of the iodo compound 145 with 2-furylboronic acid. The yield was found to be moderate (38%). It may be important to mention that these authors also tried to transform 145 to a heteroaromatic boronic acid and to carry out cross-coupling of this compound with 3-bromofuran. Unfortunately, however, this approach failed and only homo-coupling occurred. 

This reaction has the advantage that only one product (11) (head-to-head coupling product) is formed. Additional results are shown in Table 5. Aromatic bromides and iodides easily react with vinylic boron compounds, but organic chlorides do not participated, except the reactive allylic and benzylic derivatives. Heteroaromatic halides can be used as coupling partners. Or// o-substituents on benzene ring do not give difficulty. Thus, the cross-coupling reaction is used for the synthesis of benzo-fused heteroaromatic compounds (eq 5) (6). 

One of the most reliable methods for the construction of an oligoheteroarene structure is the transition-metal-catalyzed cross-coupling reaction [26]. However, a problem to be overcome remains in the cross-coupling. The installation of a metal group into heteroaromatic compounds is often dififlcult because of problems with the stability of the resulting heteroaromatic metal reagent [27]. For example, 2-pyridyl boronic acid and its esters are easily decomposed by proton [28]. In addition to this problem with stability, the transmetallation of an electron-deficient heteroaromatic boron reagent to palladium is relatively slow [29]. 

Borazarophenanthrene was the first representative of a new class of heteroaromatic compounds containing boron atoms in six-membered aromatic rings.4 These compounds are of a different order of stability from previously known types of organoboron compounds, being chemically similar to normal aromatics, and their discovery has opened up a new field of aromatic chemistry. The procedure indicated here has been used to prepare a large number of related aromatic systems. 

Terashima, M., Ishikura, M., Boron-substituted Heteroaromatic Compounds, 46, 143. 

Biological pyrimidines, tautomerism and electronic structure of, 18, 199 Bipyridines, 35, 281 Boron-substituted heteroaromatic compounds, 46, 143 Bridgehead nitrogen saturated bicyclic 6/5 ring-fused systems with, 49, 193... 

Most attention has been devoted to an exploration of the chemistry of the borazaromatics. In these compounds, two neighboring carbon atoms of an aromatic system are replaced by the isoelectronic boron-nitrogen groups as illustrated by the 2,1-borazaronaphthalene, 7 2>3>. Their great chemical stability is a distinct feature of heteroaromatic materials and several reviews of their chemistry have been published 4 6>. 

Both acid and base catalysis have been used extensively to catalyze exchange in aromatic, and to a lesser extent, heterocyclic molecules. In acid exchange, the most widely used catalysts are sulfuric acid,122,129, 131 phosphoric acid,132 trifluoroacetic acid5133 perchloric acid,134 aluminum chloride,135 and the phosphoric acid-boron trifluoride complex.132 These reactions constitute the simplest electrophilic substitution. The mechanism for such substitution in benzenoid compounds is now comparatively well understood 122 however, the problem of heteroaromatic electrophilic substitution is still being clarified and has led to renewed interest in acid-catalyzed exchange in heterocyclic compounds.122... 

Boron-substituted heteroaromatic compounds 89AHC(46)143. Dithiocarboxyl esters 88SR(8)155. 

The Suzuki coupling reaction is a powerful tool for the construction of biaryl compounds and their homologues, which are key structural elements of various natural products, polymers, and compounds of medicinal interest. Aryl boronic acids and their esters are the usual substrates in reactions with aryl or heteroaromatic halides and... 

The chemistry of boron-containing heterocycles is diverse. A variety of heteroaromatic compounds having a boron atom in the ring, borazaro-matics for example, has been investigated, mostly due to their interesting... 

This article, therefore, will deal only with the chemistry of heteroaromatic compounds directly substituted on a ring carbon with a boron atom. The primary chemical literature published between 1965 and the middle of 1987 is surveyed, but patents are not included. Earlier relevant literatures are also referenced. 

The application of general reactions available for the synthesis of aryl-boranes to synthesis of boron-substituted heteroaromatic compounds is somewhat restricted by the presence of a sensitive heteroaromatic ring. The substitution reaction of heteroaryl-metallic compounds (1) with halo-boranes (2) has been used to synthesize five-membered ir-excessive het-eroarylboranes (3) (76CB1075) (Scheme 1). 

The reactions of boron-substituted heteroaromatic compounds may be formally classified into two types (1) reactions relating to the carbon-boron bond, and (2) all other reactions. Very few investigations have been reported concerning the latter. 

Section III,A emphasizes versatile and convenient reactions of boron-substituted heteroaromatic compounds for synthetic heterocyclic chemistry. 

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