Boron heterocycles, literature

This contribution first surveys some of the attractive properties of boron, briefly describing applications that have been developed mostly with non-aromatic boron-containing compounds. It then examines many of the stable, formally aromatic boron heterocycles that have been reported to dale, covering much of the pertinent literature through the end of 1999. With the sum of these two parts, I hope the reader will gain an appreciation of the untapped potential held by boron heterocycles, especially for constructing new bioactive agents. 

Compounds with Isolated Trigonal Boron Atoms and Covalent Boron-Nitrogen Bonding (Aminoboranes and B-N Heterocycles). Literature coverage from 1950 to the end of 1973. 

There now exists a vast literature on inorganic heterocycles, and much of this has been reviewed in the two recent volumes edited by Haiduc and Sowerby [19], The most extensively studied of the boron-nitrogen heterocycles, and probably the system... 

For the last four decades, there has been an exponential increase in the synthesis and usage of a number of organic and inorganic boron compounds in industry and academia. Due to the explosive growth of heterocyclic boron-containing compounds, this topic is covered as a separate chapter here. Previously, the subject of this chapter was covered in a subsection of Chapter 4.24 in CHEC-II(1996) (Sections 4.24.1.3.6 and 4.24.1.3.7). This chapter covers as much literature as possible, but the space restraints did not permit a comprehensive coverage of all literature. 

Some noteworthy intramolecular nucleophilic ring openings have been reported in the recent literature, which can be used to prepare functionalized heterocycles of synthetic interest. For example, the highly oxygenated epoxide 100 undergoes rearrangement induced by boron trifluoride etherate, whereby anchimeric assistance from the pendant phenylthio substituent leads to an intermediate episulfonium ion 101 which subsequently suffers 5-e o-tet cyclization to form the tetrahydrofiiran derivative 102 <03TL5547>. 

All heterocycles of this class found in the literature have pure inorganic character, and contain phosphorus, boron, silicon, etc., as the fourth atom of the ring [see references included in (83IC2095)]. 

Five-membered heterocycles with two adjacent heteroatoms with at least one boron atom were discussed very sparsely in the first edition of Comprehensive Heterocyclic Chemistry (CHEC-I) <84CHEC-l(l)637>. Only the class of 1,2-azaborolines was described a little more comprehensively. Other types such as 1,2-azaborolidines, the 1,2-oxaborolanes, and the 1,2-oxaboroles were completely neglected. For that reason it seems appropriate to discuss these heterocycles in more detail. This requires partial consideration of literature before 1982. 

There is a lot of information on the mercuration of heterocycles in the old literature, but it is seldom used nowadays due to the major disadvantages of toxicity and associated waste management it can be, however, a very useful reaction. The advantages of mercuration are that it can be carried out in hydroxylic and acidic solvents and in the presence of air, and that mercury in the product is easily replaced by ipso-substitution with other electrophiles, such as halogens, and gives boronic acids by reaction with borane. 

A large number of heterocyclic boronates and, to a lesser degree, stannanes are available conunercially. Included in the suppliers lists are many compounds, the preparations and properties of which do not appear in the literature. They are often noted in papers just as reagents for syntheses of libraries for biological testing and, although this may be a practical approach, it is scientifically unsatisfactory because there is no trail of characterization, particularly as some of these compounds, for example azine a-boronates, are significant from a theoretical viewpoint. 

An extensive review of arylboronic acid chemistry in older literature was written by Lappert [177], while an excellent and comprehensive review of heterocyclic boronic acids chemistry was summarized by Tyrrell and Brookes [178]. Since the arylboronic acids and esters became an important chemical intermediates, a number of these substances can be purchased. However, they are still quite expensive materials. 

Boron Nitride. B-N-C Heterocycles. Polymeric B-N Compounds. Literature coverage from 1950 up to 1972. 

Diverse N-heterocyclic carbenes containing Lewis-acidic boron atoms within the carbene ring have been described in the literature. Carbene 128 derived from a four-membered heterocycle (Figure 1.11) showed several similarities with the previously discussed carbene 77b (see Scheme 1.11). Substitution of the... 

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