Synthesis and Reactivity of a Neutral Homocyclic Silylene

Abstract

Isolation of the neutral homocyclic silylene 2 is possible via amine ligand abstraction with potassium graphite (KC₈ ) and subsequent reaction with SiMe₃ Cl from a bicyclic silicon(I) amide J. This reaction proceeds via an anionic homoaromatic silicon ring compound 1 as an intermediate. The twofold-coordinated silicon atom in the homocyclic silylene 2 is stabilized by an allyl-type π-electron delocalization. 2 reacts in an oxidative addition with two equivalents of MeOH and in cycloadditions with ethene, phenylacetylene, diphenylacetylene and with 2,3-dimethyl-1,3-butadiene to afford novel functionalized ring compounds.

Keywords: Homoaromaticity; Homocyclic; N ligands; Silylene; Small molecule activation.

Figure 1

Isolable heterocyclic silylenes B–H and acyclic silylenes A and I that are in equilibrium with Si^IV species. Ar=2,6‐iPr₂C₆H₃).

Scheme 1

Synthesis of 1 and 2.

Figure 2

Left: Molecular structure of 1. Thermal ellipsoids are set at 50 % probability level. H atoms are not shown. Selected bond lengths [Å] and angles [°] for 1: Si1–Si2 2.274(8), Si2–Si3 2.411(8), Si3–Si4 2.447(8), Si4–Si1 2.275(8), Si2⋅⋅⋅Si4 2.451(8), Si1⋅⋅⋅Si3 3.951(1), Si2–N1 1.766(2), Si3–N2 1.848(2), Si4–N3 1.774(2); Si2‐Si1‐Si4 65.21(3), Si2‐Si3‐Si4 60.60(2), N1‐Si2‐Si4 149.75(7), N3‐Si4‐Si2, 151.11(7), Si1‐Si2‐Si4‐Si3 159.864(5). Center and Right: Frontier molecular orbitals of 1‐opt with orientation of the molecule identical to that shown on the left (TPSS‐D3/def2‐TZVP, Isovalue set at +/− 0.05 a.u.).

Figure 3

Top: Molecular structure of 2. Thermal ellipsoids are set at a 50 % probability level. H atoms are not shown. Selected bond lengths [Å] and angles [°] for 2: Si1–Si2 2.248(9), Si2–Si3 2.408(1), Si3–Si4 2.400(5), Si2⋅⋅⋅Si2′ 2.682(1), Si1⋅⋅⋅Si3 3.779(1), Si2–N1 1.750(2), Si3–N2 1.759(3), Si2‐Si1‐Si2′ 73.24(4), Si1‐Si2‐Si3 108.44(3), Si2‐Si3‐Si2′ 67.69(4), N1‐Si2‐Si2′ 167.72(6), Si1‐Si2‐Si2′‐Si3 166.633(2). Bottom: Frontier molecular orbitals of 2‐opt with orientation of the molecule identical to that shown on the left (TPSS‐D3/def2‐TZVP, Isovalue set at +/− 0.05 a.u.).

Scheme 2

Reactivity of 2, R=N(SiMe₃)Dipp.

Figure 4

Cutouts of molecular structures 3–5. Thermal ellipsoids are set at a 50 % probability level. SiMe₃ and Dipp groups are not shown. Selected bond lengths [Å] for 3: Si1–O1 1.633(4), Si1–O2 1.643(4), Si1–Si2 2.3709(19), Si2–Si3 2.3831(19), Si3–Si4 2.4266(19), Si4–Si1 2.4026(18), Si4–Si8 2.392(2), Si1⋅⋅⋅Si3 3.114(2), Si2⋅⋅⋅Si4 3.468(2). For 4: Si1–Si2 2.3649(8), Si2–Si3 2.4178(7), Si3–Si4 2.3914(8), Si4–Si1 2.3638(8), Si2–Si4 2.3493(8), Si1⋅⋅⋅Si3 2.9819(2), Si1–C1 1.922(2), Si3–C2 1.896(2), C1–C2 1.504(3). For 5: Si1–Si2 2.2840(11), Si2–Si3 2.2302(11), Si3–Si4 2.3634(11), Si4–Si1 2.3640(11), Si1⋅⋅⋅Si3 3.0701(1), Si2^…Si4 3.328(1), Si1–C1 1.818(3), Si1–C2 1.838(3), C1–C2 1.331(5).

Figure 5

Cutouts of molecular structures 6 and 7. Thermal ellipsoids are set at a 50 % probability level. SiMe₃ and Dipp groups are not shown. Selected bond lengths [Å] For 6: Si1–Si2 2.3025(9), Si2–Si3 2.2600(8), Si3–Si4 2.3705(8), Si4–Si1 2.3729(9), Si1⋅⋅⋅Si3 3.1195(1), Si2⋅⋅⋅Si4 3.370(1), Si1–C1 1.840(2), Si1–C2 1.839(2), C1–C2 1.341(3). For 7: Si1–Si2 2.3021(15), Si2–Si3 2.2193(15), Si3–Si4 2.3642(16), Si4–Si1 2.3815(15), Si1^…Si3 3.1579(2), Si2⋅⋅⋅Si4 3.278(2), Si1–C1 1.893(5), C1–C2 1.523(7), C2–C3 1.322(7), C3–C4 1.506(6), Si1–C4 1.903(5).

Figure 6

Calculated isomerization of 2/2′/2′′ and intermediates and transition structures in the reaction of 2/2′ with ethene. In brackets: calculated free energies relative to isolated reactants (2 and ethene) in kcal mol⁻¹.

Figure 7

Calculated product 5 and hypothetical products 8 and 9 a. In brackets: calculated free energies relative to isolated reactants (2 and phenylacetylene) in kcal mol⁻¹.

Figure 8

Calculated intermediates and transition states during the formation of 3. In brackets: calculated free energies relative to isolated reactants (2 and two MeOH molecules) in kcal mol⁻¹.