Borepine Complexes

In recent work, Ashe et al. have prepared a series of molybdenum tricarbonyl complexes of C6H6B-R ligands.142 X-ray diffraction data on [H6C6B-N(CHMe2)2]Mo(CO)3 123 revealed that the metal is coordinated to the six ring carbons but not to boron. 

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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... 

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). ... 

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. 

Reactions of B-Substituted Borepin Molybdenum Tricarbonyl Complexes 1045... 

Aside from the structure obtained for 1-chloroborepin referred to in Section 14.20.3.3, no other X-ray studies of simple borepins or benzoborepins have been reported however, an X-ray structure for molybdenum carbonyl complex 12 has been obtained <19970M1884> and was compared to previously reported structures for 13 and 14 <1992AGE1255> shown in Figure 3. 

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>. 

No new studies of reactivity of substituents on boron have been reported for borepins themselves. A few selected examples of 7 -substituent reactivity of borepin metal carbonyl complexes and of borepanes and borocanes follow. Section 9.37.5.4 in CF1EC-II(1996) provides some additional relevant examples. 

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. 

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>. 

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>. 

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>. 

Fluorescent, greenish yellow heptaphenyl borepin (22) reportedly formed a colorless complex with pyridine that dissociated in warm toluene solution. Mercurideborination of (22) and treatment of the crude organomercury product with -butyllithium, followed by hydrolysis (Equation (9)), afforded hexaphenyldihydrobenzenes (23) and (24), along with some hexaphenylbenzene <75JA4436>. 

Reaction of 1-chloroborepin (17) with methanol or tri-n-butyltin hydride (Scheme 2) afforded the corresponding 1-methoxyborepin (27) or l f-borepin (5) <92AG(E)1255>. The Mo(CO)j complex of (17) undergoes replacement of Cl by hydride using LiEtsBH, as a reducing agent <93AG(E)1065>. 

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