Boron-containing Heterocycles

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. 

Interactions between non-halogen-containing IIIB compounds and transition-metal complexes are found in 6.5.3. Most of these IIIB compounds are boron-containing heterocycles. A series of interesting sandwich compounds, including triple- and tetradecked complexes, are synthesized by methods in 6.5.2.1-6.5.3. 

In Chapter 8.34 of CHEC-II(1996) synthesis and properties of the boron-containing heterocycles with ring junction boron atom 1-6 have been considered <1996CHEC-II(8)889>. However, only a few papers have been published during the last decade concerning the systems 1-4 (Figure 1) and so they are discussed in Section 12.13.6. 

Boron, D. J. Atlas, R. M. Johnson, A. R., et ah, Method for removing organic sulfur from heterocyclic sulfur containing organic compounds. Patent No. USH1986H. 

Thus, existing methods give the possibility of obtaining various heterocyclic compounds containing the P—C—O—B fragment with phosphorus and boron atoms being in a different coordination. 

Bis(a-aminomethyl)phosphines having a mobile hydrogen atom could be expected to undergo a borylation reaction with borane, hydrogen being evolved. In fact, bis(Af-phenylaminomethyl)-phenylphosphine (199) and its sulfide (200) appeared to interact with borane under mild conditions, yielding a new type of phosphorus-boron-containing heterocycle—2-... 

Boryl nitronates (75-78), (217, 229-231) which have been poorly studied, are as a rule unstable. It should be noted that dimers (75) are crystalline compounds, which decompose in the solid state within a few days to give boron-containing heterocycles (79) (Scheme 3.74). 

Boron. Neutral boron-containing systems with a //-electron doublet (heterocyclic counterparts of the cyclopropenyl cation55 are called borirenes (64—66, Scheme 32) and azadiboriridines (67, Scheme 33)80 81 Scheme 32. Borirenes... 

FIGURE 27. Solid-state structures of Ethium organics with heterocyclic perimeters containing boron as a group 13 element... 

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. 

---