Synthesis of Heavier Analogues of Alkenes, R₂E=CR’₂ (E= Si, Ge, Sn), via Lithium Silanolate Elimination

We report here the reactions of tri-silyl-substituted lithium anions (R₃Si)₃ELi•nTHF 1, 3, 5, 6 (E = Si, Ge, Sn) with 2-adamantanone, in order to prepare stable heavier analogues of alkenes (R₃Si)₂E=CR’₂ (E= Si, Ge, Sn). We find that larger silyl-substituents on (R₃Si)₃ELi facilitate their oxidation, but at the same time provide stability to the desired doubly-bonded product. In this work we aimed to find the best combination of silyl-substituents, to obtain stable heavier analogues of alkenes.

The reaction of the most bulky silyl-branched (R₃Si)₃ELi 1 with 2-adamantanone leads to the corresponding E-radicals 2 i.e., only electron-transfer occurs. Decrease in the size of the silyl-substituents, e.g., 3a and 5a yields a stable silene 4a. In contrast, the reaction of analogous stannyl lithium 3b leads to corresponding radical. Reaction of silyl-branched stannyl lithium 5b with 2-adamantanone yields the stable stannene 4c. Reaction of smallest silyl-branched (t-BuMe₂Si)₃E-Li 6 with 2-adamantanone leads to the corresponding stable silene 7a and germene 7b, but to a transient stannene 7c, trapped by reaction with p-quinone. The obtained products 2, 4, 7, 8 were characterized by NMR, EPR spectroscopy and some by X-ray crystallography.

Scheme 1. The reactions of (R₃Si)₃ELi•nTHF (E=Si, Ge, Sn) with 2-adamantanone.