Boron-substituted heteroaromatic compounds

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... 

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

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. 

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. 

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. 

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... 

Further interesting examples of six-membered heteroaromatic systems including boron atoms are boranthracene derivatives, the basic structure of which consists of a 9,10-dihetero-substituted anthracene skeleton. The details of the structural analysis of such compounds are reported in a series of papers <1995ZNB1476, 1995SM1109,... 

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