Lithium 1 - boratabenzene

The first approach to lithium boratabenzenes (Scheme 3) was an elegant adaption of a more general synthetic strategy (81). Hydrostanna-tion of 1,4-pentadiyne (82) with Bu2SnH2 gives the stanna-2,5-cyclohexadiene 24 (83,84), which on treatment with PhBBr2 produces... 

Starting from stannacyclohexadienes and BHaL, Ashe and coworkers reported in 1971 a general synthesis of lithium boratabenzenes, which can easily be converted into other metal complexes (75PAC513). The zirconium(IV) complexes with two boratabenzene rings and two halogen... 

Herberich s 1970 report of the synthesis of a cobalt-bound boratabenzene1 and Ashe s 1971 description of the synthesis of lithium 1-phenylboratabenzene2 marked the starting point of the study of borabenzenes, a family of molecules that includes neutral borabenzene-ligand adducts as well as anionic boratabenzenes (Scheme 1). 

The X-ray structure of lithium l-(dimethylamido)boratabenzene, reported in 1993, provided the first crystallographic characterization of a transition metal-free boratabenzene (Scheme 13).18a The observed bond lengths are consistent with a delocalized anion and with significant B—N double-bond character. In a separate study, the B—N rotational barrier of [C5H5B—NMeBnjLi has been determined to be 10.1 kcal/mol, and it has been shown that TT-complexation to a transition metal can increase this barrier (e.g., 17.5 kcal/mol for (C5H5B-N(i-Pr)2)Mn(CO)3).24... 

Lithium 2-diisopropylamino-2-boratanaphthalene (cf. 28) with Me2-NCH2CH2NMe2 (TMEDA) forms a crystalline derivative Li(TMEDA) (C9H7BNPr 2) which has been characterized by X-ray structure determination (102). The lithium cation is situated above the center of the boratabenzene moiety of the anion and, in addition, is chelated by one TMEDA ligand. Thus, the crystal consists of discrete tight ion-pair molecules. 

Metal complexes of pinene-fused boratabenzene ligands, analogous to chiral metallocenes that have found application in catalysis and enantioselective synthesis, have been prepared.122-124 With late transition metals such as Mn and Fe, the complexes are obtained as mixtures of diastereomers (e.g., 97) with the sterically less congested exo form predominating, but the bis(ligand) Zr complex 98 was obtained as the pure exo,exo product.124 A lithium... 

The analogous reactions of PbCl2 with lithium l-(dimethylamino)-3,5-dimethylboratabenzene or l-[bis(trimethyl-silyl)amino]-3,5-dimethylboratabenzene yield the corresponding sandwich compounds 44 and 45. The lead atom of 45 displays a rather asymmetric bonding to the boratabenzene rings as evidenced by the fact that the lead-to-carbon/ boron distances span the large range from 2.519 to 3.033 A.7... 

There is a certain analogy between the aromatic anions of cyclopentadienide (C5H5 ) and boratabenzene (C5H6B ). l-Methylbora-2,5-cyclohexadienehas a more acidic proton connected to the, sp3-hybridized ring carbon atom than cyclopentadi-ene, due to the same tendency of aromatic anion formation [252, 253]. The related 1-phenyl-1,4-dihydroborabenzene affords the lithium salt of 1-phenylborataben-zene on treatment with tert-butyllithium. Like metallic complexes such as ferrocene formed by cyclopentadiene, boratabenzene also forms such sandwich -complexes with iron and cobalt. The iron complex can be acetylated under Friedel-Crafts conditions. 

Herberich etal. have shown that boratabenzenes, for example, lithium salt 68 (Equation 1), behave as nucleophiles in the presence of Me3XCl and undergo addition at C-2 to yield 1-methyl-1,2-dihydroborinines such as 69 <19970M926>. X-Ray diffraction studies on the products have been discussed earlier (see Section 7.14.3). 

New convenient routes to aminoboratabenzenes from 1,3-pentadienes were developed by Herberich. Ring-closure of [bis(dialkylammoboryl)]pentadienes (131) was accomplished with lithium amides, thereby generating the aminoborataben-zene (132) (Scheme 19). Another route from dipotassium salts was also reported. Thus a large number of borabenzene adducts and boratabenzene complexes including annulated boratanaphthalenes and borataanthracenes are readily available now. ... 

Lithium salts of 1-methylboratabenzene, l-dimethylamino-3,5-dimethylborata-benzene and l-bis(trimethylsilyl)amino-3,5-dimethylboratabenzene react with ScQs in toluene at elevated temperature to yield complexes 11, 12 and 13 having different structures and compositions (03OM5496). The structure 11 is characterized by two sandwich units doubly bridged by the chloride moieties. Compound 12 seems similar to 11 but one boratabenzene ligand is coordinated in an unusual (NBC) mode, while the second one is -coordinated. Compound 13 is mononuclear. 

Boratabenzene lithium with [(/ -Cp )ZrCl3] gives the fj -coordinated species 19 and with zirconium chloride - the full sandwich 20 (96JA2291, 970M2492). The latter... 

JOM(642)275). Sodium boratastilbene with [ZrCl4(THF)2] forms sandwich 40. Species 37 and 38 are efficient catalysts for ethylene polymerization. Interaction of the appropriate boratabenzene ligand with lithium di-wo-propylamide and then zirconi-um(IV) chloride gives 41 (030M203). Similarly, 42 can be obtained. 

---