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

Abstract

The reaction of Li(C5H5BMe) (1) with electrophiles Me3ECl (E = Si, Ge, Sn, Pb) produces 1,2-dihydroborinines 2-(Me3E)C5H5BMe (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 Me3E 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 ΔG300⧧ = 58.9(11) kJ mol-1, ΔH⧧ = 43.3(6) kJ mol-1, and ΔS⧧ = −52(2) J K-1 mol-1; for 3 ΔG300⧧ = 43.4(9) kJ mol-1, ΔH⧧ = 27.8(3) kJ mol-1, and ΔS⧧ = −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-(Me3Si)C5H4BMe] (6). Silylation of 6 affords (Me3Si)2C5H4BMe (7) as a slowly interconverting equilibrium mixture of 2,2- and 2,6-isomers whereas stannylation gives the fluxional derivative (Me3Si)(Me3Sn)C5H4BMe (8).