Borepin derivatives

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-... 

Although ring-annulated and highly substituted borepins have been known for some time, simple 1-substituted borepins have only been thoroughly studied over the past decade. The discovery of a facile synthetic ronte to 1-snbstituted borepins through tin-boron exchange has much opened up the chemistry of these 7-membered boracycles. The chloro-substituted borepin (145) serves as a highly useful precursor to many other borepin derivatives via substitution reactions with nucleophiles (Scheme 20). In addition, several transition metal complexes of 1-borepins have been reported. 

The properties of the borepin derivatives are discussed in Section VI. Hydrolysis of chloride. 

The first borepin derivative to be prepared was the dibenzoderivative 136363 and subsequently Dewar commented364 that it was unlikely to be... 

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. 

The longest wavelength band in the ultraviolet (UV) spectrum of the 1 -phenyl-fV-methyl derivative of 9 was reported to occur at 356 nm in cyclohexane and displayed only slight solvatochromism, while the fluorescence spectrum ( = 0.13, cyclohexane) showed marked solvatochromism, for example, 87 nm with a change from cyclohexane to dimethylforma-mide (DMF) <1995CC1249>. No infrared (IR) data have been reported recently for borepins or their analogues. 

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... 

Some replacement reactions of l-chloroborepin(tricarbonyl)molybdenum 42 are summarized in Scheme 3, illustrating its utility as a precursor to certain derivatives 43-46, which may be unavailable directly from the corresponding borepin. Subsequent treatment of 45 with MgSC>4 in ether gave the corresponding 1,1 -bis[tricarbonyl(borepin)mo-lybdenum] oxide <19970M1884>. 

Bisethynylthiophene has been converted to the tin derivative (397) by treatment with dibutyltin hydride, KOH and 18-crown-6 in benzene (85% yield). Reaction of this with PhBCl2 for 10 min gave 1-phenylthieno [3,4- i]borepine (398) in 40% yield (Scheme 81) <92JA1479>. 

Chloroborepin (17) is readily converted to the corresponding Mo(CO)3 complex (Equation (4)) by reaction with tricarbonyl-tris(pyridine)molybdenum and BF3 Et20 in ether. This Mo(CO)3 derivative is smoothly converted to the corresponding 1 /f-borepin-molybdenum complex (18) by reduction with lithium triethylborohydride (Super-Hydride) in THF (Equation (5)). This is noteworthy as (18) cannot be prepared directly from the highly labile 1//-borepin itself <93AG(E)1065>. 

Although there is httle data comparing the relative reactivity of borepins with corresponding nonaromatic derivatives, the unusual air-stability of benzoborepin (2) compared to that of dihydro-borepin (10) has been attributed to resonance stabilization <67TL1263>. 

New zwitterionic heterocycles have been derived from l-chloro(dimethyl)silyl-1-borylalkenes and 2-lithio-l-methylimidazole or 1-lithio-indazole. The zwitterionic products are obtained amongst other minor products. A DFT theoretical study has been made of borepin and its analogues the aminoborepins, methylated borepins, dibenzoborepins and tribenzoborepin. The interest here is to determine which of these compounds are planar and which are non-planar. Some of these compounds are, of course, known and here measured and B NMR chemical shifts agree well with the calculations. The calculations predict that tribenzoborepin and two of the dibenzoborepins are non-planar. 

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