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Borabenzene Derivatives. 23. New Synthetic Entry into Borabenzene Chemistry via Doubly Kaliated Pentadienes: Synthesis of 1-(Dimethylamino)-3-methylene-1,2,3,6-tetrahydroborinines and Lithium 1-(Dimethylamino)boratabenzene Derivatives

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journal contribution
posted on 11.06.1996, 00:00 by Gerhard E. Herberich, Ulli Englert, Martin U. Schmidt, Regina Standt
Dicarbanions obtained by dikaliation from suitable dienes 1 such as (E)-2-methyl-1,3-pentadiene, 2,4-dimethyl-1,3-pentadiene, and 2-tert-butyl-4-methyl-1,3-pentadiene react with BCl2(NMe2) to form 1-(dimethylamino)-3-methylene-1,2,3,6-tetrahydroborinines 3a (5-R = H), 3b (5-R = Me), and 3c (5-R = But) in moderate yields. The compounds 3a−c dimerize reversibly to give crystalline dimers (3a)2, (3b)2, and (3c)2 as C2v and C2h isomers. The structure of C2v-(3a)2 was determined by X-ray diffraction. In solution a monomer/dimer equilibrium is established with K = 49.0 mol L-1 for dissociation at 22.0 °C, ΔH = (70 ± 2) kJ mol-1, and ΔS = (270 ± 8) J K-1 mol-1. The methylene-1,2,3,6-tetrahydroborinines 3 isomerize thermally at 100−120 °C to give dihydroborinines. Thus 3b affords a mixture (3/2) of 3,5-dimethyl-1,2-dihydroborinine 5b and 3,5-dimethyl-1,4-dihydroborinine 6b. The isomerization is catalyzed by acids as HCl in Et2O and then may take place at ambient temperature. Metalation of 3a−c with LDA in THF gives high yields of lithium 1-(dimethylamino)boratabenzene salts which can be isolated as TMEDA solvates 4a−c. The structure of [Li(TMEDA)](3-But-5-MeC5H3BNMe2) (4c) is that of a contact ion pair.