Borabenzene Derivatives. 24.1From Lithium 1-Methylboratabenzene to 2-Mono- and 2,2-Disubstituted 1-Methyl-1,2-dihydroborinines with Me3Si, Me3Ge, Me3Sn, and Me3Pb Substituents. Degenerate Sigmatropic Rearrangements with Exceptionally Low Barriers and the Structure of 2-(Me3Sn)C5H5BMe

The reaction of Li(C 5 H 5 BMe) ( 1 ) with electrophiles Me 3 ECl (E = Si, 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) bond [228.7(2) pm]. In solution the compounds 2 − 5 are fluxional and show [1,3] sigmatropic migrations of the Me 3 E groups from C-2 to C-6. Barriers to the degenerate sigmatropic rearrangements for 2 and 3 have been determined from NMR spectroscopy [for 2 Δ G 300 ⧧ = 58.9(11) kJ mol -1, Δ H ⧧ = 43.3(6) kJ mol -1, and Δ S ⧧ = −52(2) J K -1 mol -1; for 3 Δ G 300 ⧧ = 43.4(9) kJ mol -1, Δ H ⧧ = 27.8(3) kJ mol -1, and Δ S ⧧ = −52(2) J K -1 mol -1 ] and are lower than those found for previously investigated systems. The Sn and Pb satellites observed for 4 and 5 in THF solutions are quenched by catalytic amounts of DMSO while chemical shifts are not affected even at higher 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 ).