A zwitterionic silylene as reactive intermediate and its unusual dimerization to an N-heterobicyclic disilane

A way to synthesize the transient zwitterionic silylene L'Si: 8 {L'=CH[(C=CH(2))CMe(N(tBu))(2)]} and achieve its facile dimerization to the remarkable N-heterobicyclic disilane 8(2) is described. At first, employing the beta-diketiminate ligand L [L=CH(CMeN(tBu))(2)], both starting materials LH (2) and its N-lithium salt LLi (3) can react with SiBr(4) to yield the silylene precursor L'SiBr(2) (4) by silicon-induced C-H activation at an exocyclic methyl group on the backbone of the ligand. Compound 4 reacts with SiBr(4) above room temperature to afford the unexpected terminal CH(SiBr(3))-substituted dibromosilane 6 along with the unique tricyclic trisilane 7. Reduction of 4 with KC(8) at 0 degrees C furnishes the novel N-heterobicyclic disilane 8(2), which is a formal dimer of the desired zwitterionic silylene L'Si: (8). It has been reasoned that compound 8(2) may results from [4+1] cycloaddition of two molecules of 8 to give the transient dimer 8(2)', which subsequently undergoes hydrogen transfer from a terminal methyl group on the backbone of the C(3)N(2)Si ligand to the low-coordinate Si atom. The latter dimerization can be rationalized by the intrinsic zwitterionic character of 8 and insufficient steric protection through the tBu groups at the nitrogen atoms. The novel compounds 3, 4, 6, 7, and 8(2) have been characterized by (1)H, (13)C, and (29)Si NMR spectroscopy, mass spectrometry, and elemental analysis. Additionally, the structures of 3, 6, 7, and 8(2) were also established by single-crystal X-ray diffraction analyses.