Borepins

UV spectrum, 4, 1031 Borazaroselenolopyridines synthesis, 4, 1036 Borepin, 4,5-dihydro-synthesis, 1, 660 Borepin, 4,5-dihydro-l-phenyl- H NMR, 1, 635 Borepin, heptaphenyl-synthesis, 1, 638, 660 Borepin, 1-methyl-semi-empirical calculations, 1, 633 synthesis, 1, 661 IH-Borepin, 1-methyl- H NMR, 1, 635... 

Borepino[3,2-6 6,7-6 ]dithiophene, 4-methyl- H NMR, 1, 636 Borepinodithiophenes theoretical methods, 1, 633 Borepins... 

Dithienobenzene synthesis, 4, 720 Dithieno[3,2-fc 2,3-/]borepin molecular dimensions, 1, 633 Dithienoborepins stability, 1, 660... 

The series of neutral B-containing carbocycles as complex ligands within the theme of metal-boron donor-acceptor relationships concludes with the 4,5-dihydro-borepines. l-Phenyl-4,5-dihydroborepine is able to substitute pyridine, acetonitrile or CO... 

The first generation of the borepin nucleus without annulated substituents occurred during the heating of colorless heptaphenyl-7-borabi-cyclo[2.2.1]heptadiene (46a) into refluxing toluene.103 104 The appearance... 

Successful syntheses of monoannulated borepins were achieved by Ashe and co-workers by tin-boron exchanges with the tin heterocyclic precursors.105 106 Particularly informative were the properties of 113, which has no benzoannulation (Eq. 34) ... 

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... 

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). 

Under mild conditions 1-phenyl-4,5-dihydro-lH-borepine (55) undergoes complex formation without rearrangement (92,93). However, under forcing conditions, typically in boiling mesitylene, skeletal rearrangements take place and borole as well as borabenzene ligands are formed (52,94). ... 

Borepin is the heterocyclic analogue of the tropyl-ium cation with a jr-electron sextet. Few borepin derivatives have been reported a monocyclic 1-meth-ylborepin 202,245 benzo derivatives 204,246 and a dithienoborepin 205247 (Scheme 77). Heptaphenyl-... 

In order to allow for a closer approach of the boron centers, the introduction of flat cyclic boryl moieties with reduced steric hindrance has also been pursued. Thus, the reaction of 26 with 9-chloro-9-borafluorene and 5-bromo-10,l 1-di-hydrodibenzo[b,f]borepin resulted in the formation of diboranes 31 and 32 which bear two different boryl moieties at the pen-positions of naphthalene (Scheme 13). " These diboranes have been characterized by multinuclear NMR spectroscopy and X-ray single-crystal analysis. In 31, the boron center of the borafluorenyl moiety is 7i-coordinated by the zp o-carbon of a mesityl group with which it forms a contact of 2.730(3) A (Fig. 8). As a result of this interaction, the boron center involved in this contact is slightly pyramidalized (Xangie = 355.7°). In the case of 32 (Fig. 9), the distance between the boron center of the boracylic moiety and the zpio-carbon of... 

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... 

Transition from the tropylium ion to its neutral heteroaromatic counterparts is possible by replacement of a CH+ group by a heteroatom with a vacant p-orbital. The latter effectively accepts tt-electrons, thus providing ring electron delocalization. A typical example is the boron atom in a seven-membered borepine (8) (92AG1267). Correspondingly, this type of heteroatom can be referred to as borepine-like . Other little-known representatives of this family are alumopine (8, Z = A1H) or gallepine (8, Z = GaH). 

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 only NMR spectrum of a non-condensed borepin was recorded on a mixture, probably containing 1 -methyl- 1H-borepin (42) and 7-methyl-7-borabicyclo[4.1.0]hepta-2,4-diene. Signals at S 6.9-8.3 were attributed to the borepin hydrogens (sijom(215)303). However, some fused borepins have been isolated and their spectra recorded. 3-Phenyl-benzo[d]borepin (43) has been compared with 1-phenyl-4,5-dihydroborepin (44). The benzoborepin hydrogens give resonance at S 7.72 (Ha) and 8.22 (Hp) while the alkenic... 

Some data have been recorded for dithieno fused borepins. The resonance occurred at 8 38.9 for the ether (26 R = 0) but the corresponding partially hydrogenated species absorbed at 8 36.8, i.e. no large difference was found (73CS(4)183). 

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 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. 

Monobenzo fused borepins have been prepared by tin-boron exchange reactions (equation 18). Different stabilities to air were noted (208 R = OH) was air-sensitive <7lCC29l), but (208 R = Ph) had an unusual stability (67TL1263). 

The first non-fused borepin to be prepared was heptaphenylborepin (56), starting from pentaphenylborole and diphenylacetylene according to Scheme 3. The intermediate 7-borabicyclo[2.2.1]heptadiene was isolated and heated in toluene for 24 h to give 84% yield... 

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