BORABENZENE DERIVATIVES .24. FROM LITHIUM 1-METHYLBORATABENZENE TO 2-MONOSUBSTITUTED AND 2,2-DISUBSTITUTED 1-METHYL-1,2-DIHYDROBORININES WITH ME(3)SI, ME(3)GE, ME(3)SN, AND ME(3)PB SUBSTITUENTS - DEGENERATE SIGMATROPIC REARRANGEMENTS WITH EXCEPTIONALLY LOW BARRIERS AND THE STRUCTURE OF 2-(ME(3)SN)C(5)H(5)BME

The reaction of Li(C(5)H(5)BMe) (1) with electrophiles Me(3)ECl (E = S i, Ge, Sn, Pb) produces 1,2-dihydroborinines 2-(Me(3)E)C(5)H(5)BMe (2- 5). The structure of the Sn compound 4 shows a lengthened Sn-C(ring) b ond [228.7(2) pm]. In solution the compounds 2-5 are fluxional and sho w [1,3] sigmatropic migrations of the Me(3)E groups from C-2 to C-6. B arriers to the degenerate sigmatropic rearrangements for 2 and 3 have been determined from NMR spectroscopy [for 2 Delta G(300)(double dagge r) = 58.9(11) kJ mol(-1), Delta H-double dagger = 43.3(6) kJ mol(-1), and Delta S-double dagger = -52(2) J K-1 mol(-1); for 3 Delta G(300)(d ouble dagger) = 43.4(9) kJ mol(-1), Delta H-double dagger = 27.8(3) kJ mol(-1), and Delta S-double dagger = -52(2) J K-1 mol-l] and are lowe r than those found for previously investigated systems. The Sn and Pb satellites observed for 4 and 5 in THF solutions are quenched by catal ytic amounts of DMSO while chemical shifts are not affected even at hi gher concentrations. The Si compound 2 can be deprotonated to give Li[ 2-(Me(3)Si)C(5)H(4)BMe] (6). Silylation of 6 affords (Me(3)Si)(2)C(5)H (4)BMe (7) as a slowly interconverting equilibrium mixture of 2,2- and 2,6-isomers whereas stannylation gives the fluxional derivative (Me(3 )Si)(Me(3)Sn)C(5)H(4)BMe (8).