Synthesis, Reactivity, and NMR Studies of Dilithiated (E)-1, 4-Bis(trimethylsilyl)but-2-ene and (E)-1,4-Bis(trimethylsilyl)-2,3-dimethylbut-2-ene

(Z)-1, 4-Dilithio-1,4-bis(trimethylsilyl)but-2-ene and 1,4-dilithio-1,4-bis(trimethylsilyl)-2,3-dimethylbut-2-ene adducts of TMEDA (≡N,N,N′,′-tetramethylethylenediamine), [{Li(TMEDA)}₂{(Me₃SiCHCH)₂}] (3) and [{Li(TMEDA)}₂((Me₃SiCHCMe)₂}] (4), respectively, have been prepared by dimetalation of the corresponding (E)-but-2-ene using n-BuLi(TMEDA) in hexane. Assignment of the Z configuration of the dianions with Li atoms bridging the ipso carbons (C_1,4) is based on NMR data: Magnetically equivalent protons attached to the ipso carbons (C_1,4) couple to ⁷Li (²J_Li-H = ca. 1 Hz) in both compounds, lack of coupling of the protons attached to C_2,3 with the lithiums in 3, and NOE enhancements for ¹H⁻⁷Li HOESY experiments. The Z configuration is also consistent with the structures of two isolated silyl derivatives of 3; treatment with ClSiMe₃ gives (Z)-1,1,4, 4-tetrakis(trimethylsilyl)but-2-ene (5), and with Cl₂SiMe₂ it yields the (Z, Z)-1, 6-disilacyclodeca-3, 8-diene [(Me₃SiCHC(H)=C(H)CH(SiMe₃)SiMe₂}₂] (6). Compound 3 is a potent reducing agent, affording (E, E)-1, 4-bis(trimethylsilyl)buta-1, 3-diene (7) and metal when treated with HgCl₂ (yield of 7 71%) or CaCl₂. X-ray diffraction data for 5 revealed a distorted Z double bond, the C-C-C angles at the sp² carbon centers being “opened up” to 130.2° (mean): Crystals are monoclinic, space group P2₁/n, with α = 11.595 (4) Å, b = 12.205 (1) Å, c = 17.569 (6) Å, β = 107.33 (2)°, V = 2373 (1) ų, and Z = 4.