Borepins, aromaticity

This apparently moderate aromatic stabilization for the planar borepin (114) should therefore be only a small barrier for isomerization into the 7-borabicyclo[4.1.0]heptadiene system (115) (Eq. 35). 

The determination of the energy of aromatic stabilization of borepin by calculating the energies of isodesmic reactions (62) and (63) with a correction for the strain leads to the value 12.7 kcal/mol (89MI6). The same energy calculated with the same basis set (6-31G ) is for benzene... 

The MP2/4-31G/STO-3G calculations show (83TL1863) the tropylium cation to be 112.7 kcal/mol more stable than the hypothetical bicyclo [4.1.0]hepta-2,4-dien-7-yl cation. This difference, attributed to the effects of aromatic stabilization, should be compared with the value of AE (172) - (124) = 44.8 kcal/mol (at the same computational level). Therefore, the energy of aromatic stabilization of borepin comes to about 40% of that of the tropylium cation. The 6-31G /6-31G calculated AE (172) -(124) = 37.6 kcal/mol, whereas nonaromatic cycloheptatriene is a mere... 

The seven-membered fully unsaturated boron heterocycle is named l//-borepin or, more frequently, borepin. It has six 7r-electrons and is therefore a potential Hiickel aromatic, isoelectronic with the tropylium ion. This similarity has induced several groups to search for a synthesis. Recent theoretical calculations have predicted the -stabilization to be less than that of the tropylium ion. On the other hand, borepins are organoboranes and therefore expected to be sensitive to oxygen. 

The question of ring planarity has always been closely associated with the assignment of aromatic character to borepins, and a recently obtained microwave spectrum of 1-chloroborepin has allowed the derivation of its ground-state rotational constants. Calculation of the inertia defect (A = —0.19 uA2) from these data led to the conclusion that... 

The aromatic borepins and benzoborepins have been prepared primarily as compounds of theoretical interest and no significant applications have been reported to date. Saturated 1-alkylborepanes, such as 23 and similar large ring boron compounds, are of use primarily as precursors in the synthesis of cyclic ketones by treatment with carbon monoxide followed by alkaline hydrogen peroxide <1982JOC1792>. 

Simple applications of the Hiickel rule predict that borepin should be aromatic, it being isoelectronic with cycloheptatrienyl (tropylium) cation, while the unsaturated seven-membered rings containing an oxygen, nitrogen, or sulfur atom are potentially antiaromatic. Discussion of antiaromaticity is outside the scope of this review but because of the... 

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. 

The tellurepin (29) spontaneously eliminates tellurium over 2-3 days in solution at room temperature to give naphthalene in a manner similar to borepins and benzophosphepines <91CC802>. Higher temperatures (about 200 °C) are required to eliminate selenium from (30) <72JHC1427> and from (19) <84JHC1321> to give the aromatic products (31) and (32), respectively. 

From a theoretical standpoint, borepins (1) are of considerable interest as they are 6jc-electron systems that are isoelectronic with the tropylium cation (4) and therefore should have at least some aromatic character. Initial ab initio calculations using the STO-3G basis set indicated that H-borepin (5), the boron analogue of (7), should be planar, and it was calculated to be about 37 kcal mol (155 kJ mol ) more stable than boranorcaradiene (6) <82JA3785,83TLI863,84JA7696). 

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). 

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