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

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(NMe(2)) to form 1-(dimethylamino)-3-methylene-1,2,3,6-tetrahydroborinines 3a (5-R = H), 3b (5-R = Me), and 3c (5-R = Bu(t)) in moderate yields. The compounds 3a-c dimerize reversibly to give crystalline dimers (3a)(2), (3b)(2), and (3c)(2) as Ct, and Cah isomers. The structure of C-2v-(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 degrees C, Delta H = (70 +/- 2) kJ mol(-1), and Delta S = (270 +/- 8) J K-1 mol(-1). The methylene-1,2,3,6-tetrahydroborinines 3 isomerize thermally at 100-120 degrees 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 Et(2)O 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-Bu(t)-5-MeC(5)H(3)BNMe(2)) (4c) is that of a contact ion pair.