Cycloadditions with a Stable Charge-Separated Cyclobutadiene-Type Amido-Substituted Silicon Ring Compound

Abstract

Reductive debromination of {N(SiMe₃ )₂ }SiBr₃ with Rieke magnesium results in the formation of the five-vertex silicon cluster with one bromine substituent Si₅ {N(SiMe₃ )₂ }₅ Br, 1, and the cyclobutadiene analogue 2 in a 1:1 ratio. The latter features a planar four-membered silicon ring with a charge-separated electronic situation. Two silicon atoms in 2 are trigonal planar and the other two trigonal pyramidal. In cycloadditions with ethylene, diethylacetylene, 1,5-cyclooctadiene, and 2,3-dimethyl-1,3-butadiene cyclic unsaturated ring compounds (3-6) were formed at room temperature in quantitative reactions. Two of the products (3 and 6) show photochemical isomerization with LED light (λ=405 nm) to afford saturated ring compounds 4 e and 6'.

Keywords: charge separation; cycloaddition; photochemical isomerization; silicon amides; tetrasilacyclobutadiene.

Figure 1

Unsaturated silicon ring compounds (Dipp=2,6‐iPr₂C₆H₃).

Figure 2

Top: Synthesis of 1 and 2. Bottom: Molecular structures of 2 (thermal ellipsoids set at 50 % probability level). Selected distances [Å] and angles [°]: Si1–Si2 2.282(7), Si1–Si2′ 2.280(7), Si2⋅⋅⋅Si2′ 2.800(1), Si1⋅⋅⋅Si1′ 3.602(1), Si1–N1 1.7617(17), Si2–N2 1.7333(16); Si2‐Si1‐Si2′ 75.72(3), Si1‐Si2‐Si1′ 104.28(3).

Figure 3

A) Schematic orbital diagram of the π electrons in rectangular and square planar cyclobutadiene species (C₄R₄). B) Kohn–Sham molecular orbitals of 2 (CAM‐B3LYP‐D3(BJ)/def2‐QZVP, isovalue set at ±0.04 a.u.) with schematic representation of the MOs.

Scheme 1

Cycloadditions with 2 at room temperature (R=SiMe₃).

Scheme 2

DFT‐calculated mechanism for the formation of 3.

Scheme 3

DFT‐calculated mechanism for the formation of 4 (R=Et).

Scheme 4

Photochemical isomerization of 3 into 4 e and partial thermal reconversion to 3.