Borepin ring

The UV spectrum closely resembles that of tropone and exhibits a batho-chromic shift over those of cycloheptatriene (unconjugated). Although the "B signal at 53.6 ppm is not much shifted over that in 1-phenyl-4,5-dihydroborepin (54.6 ppm), the borepin ring protons occur at markedly... 

In a series of papers some thieno fused boron compounds have been subjected to structure determination by X-ray methods. Thus the bis(dithienoborepinyl) ether (26 R = C10H6BS2O) has dimensions close to those of the isoelectronic dithienotropylium ion (74ACS(B)998>. The borepin ring is almost planar, and the B—C bond length was found to be 1.533 A. 

The first synthesis of the borepin ring was performed in 1960 by van Tamelen and coworkers starting from 2,2 -dilithiobibenzyl according to Scheme 39 (60TL(8)14). The ethanolamine derivative (203) was isolated and characterized. Reduction with LAH gave a product, isolated as an unstable pyridine complex. The Lewis acid part of the complex was considered to have structure (204), but the conclusions were tentative, partly due to its re-oxidation to (20S). The corresponding synthetic strategy to non-fused borepins met with difficulties 4,5-dihydroborepin (206) could be prepared, but it was not possible to introduce the third double bond. 

As indicated in Section 14.20.3.4, borepins can react with organometallics to form complexes in which the borepin ring serves as an rf ligand to the metal. Thus, 1-methylborepin 15 reacted with tris(pyridine)molybdenum tricarbonyl to afford the corresponding molybdenum complex 16 as a red, air-sensitive oil, as in Equation (1) <19970M1884>. 

This section briefly covers some recent examples of conversion of a benzostannepin to corresponding benzoborepin systems. The reader is also referred to Section 14.20.6.2 for examples of formation of larger ring borepanes and benzoborepanes via some recently reported ring-expansion reactions. No recent examples of synthesis of simple borepin ring systems have been reported. 

There have been very few studies on the borepin ring system and accordingly discussions of its aromatic character have been sparse. Balaban and Simon169 calculated a K value (aromaticity constant) of +28, which is much lower than for tropylium cation (+100). Further evidence of aromaticity has been based very much on qualitative interpretations of spectral features. 

Until the early 1990s, x-ray analysis of the borepin ring system geometry had been confined to examination of stable metal complexes such as Mo(CO)3 complex (8) and Cr(CO)3 complex (9) <900M2944, 92AG(E)1255>. 

Benzoborepin-Cr(CO)3 complex (9) is readily formed and shows no tendency toward haptotropic migration from the borepin ring to the benzene moiety (Equation (6)) <90OM2944>. 

One of the earliest examples of an approach to the borepin ring system was synthesis of (19) from (30) <60TLI4), which was prepared from o,o -dilithiobibenzyl as shown in Scheme 4 <55JA5176>. 

XXXI) may be brominated with Af-bromosuccinimide (NBS) and treated with NaOCHs in CH3OH to give 5-hydroxydibenzo[b,f]borepin (XXXII), which is of theoretical interest due to the possible aromatic character of the borepin ring (41, 42). 

Among boron-carbon ring systems, the unsaturated seven-membered borepin ring is interesting in that it is isoelectronic with the tropylium cation and has aromatic character. The benzoborepin 3.8 which in its chemical, u.v. and n.m.r. spectroscopic properties shows typical aromatic characteristics, can be prepared via the tin analogue (which is not aromatic) ... 

---