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Valence isomerization of 2-phospha-4-silabicyclo[1.1.0]butane: a high-level ab initio study - PubMed

Valence isomerization of 2-phospha-4-silabicyclo[1.1.0]butane: a high-level ab initio study

J Chris Slootweg et al. J Mol Model. 2006 Jul.

Abstract

The rearrangements for 2-phospha-4-silabicyclo[1.1.0]butane, analogous to the valence isomerization of the hydrocarbons bicyclobutane, 1,3-butadiene, and cyclobutene, were studied at the (U)QCISD(T)/6-311+G**//(U)QCISD/6-31G* level of theory. The monocyclic 1,2-dihydro-1,2-phosphasiletes are shown to be the thermodynamically preferred product, in contrast to the isomerization of the hydrocarbons, which favors the 1,3-butadiene structure. Furthermore, an unprecedented direct isomerization pathway to the 1,2-dihydro-1,2-phosphasiletes was identified. This pathway is competitive with the isomerization via the open-chain butadienes and becomes favorable when electron-donating substituents are present on silicon. Figure 2-Phospha-4-silabicyclo[1.1.0]butane can isomerize directly into the more stable P,Si-cyclobutene via an unprecedented [sigma2s+sigma2a] process, which becomes favorable over the isomerization via the P,Si-butadiene when electron-donating substituents are present on silicon.

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Figures

**Figure**
2-Phospha-4-silabicyclo[1.1.0]butane can isomerize directly into the more stable P,Si-cyclobutene via an unprecedented [σ2s+σ2a] process, which becomes favorable over the isomerization via the P,Si-butadiene when electron-donating substituents are present on silicon

**Scheme 1**
Ring opening of cyclobutene 5

**Scheme 2**
Isomerization of bicyclo[1.1.0]butane 9

**Fig. 1**
Relative QCISD(T)/6-311+G**//QCISD/6-31G* (UQCISD(T)/6-311+G**//UQCISD/6-31G* in parenthesis) energies (ZPE corrected, in kcal mol⁻¹) for the rearrangements of 1 and 4 into 2. Selected bond lengths [Å], angles and torsion angles [°] of 1 (C_2v): C1–C2 1.498, C2–C3 1.494, C2–C1–C3 59.8, C1–C2–C3–C4 121.9; 2 (C₂): C1–C2 1.342, C2–C3 1.479, C3–C4 1.342, C1–C2–C3–C4 37.9; 4 (C_2v): C1–C2 1.520, C1–C4 1.570, C2–C3 1.346, C1–C2–C3 94.2; **TS1–2** (closed-shell): C1–C2 1.403, C1–C3 2.344, C2–C3 1.542, C2–C4 1.569, C2–C1–C3 39.4; **TS4–2** (C₂): C1–C2 1.430, C1–C4 2.150, C2–C3 1.379, C1–C2–C3–C4 21.7

**Fig. 2**
Relative QCISD(T)/6-311+G**//QCISD/6-31G* energies (ZPE corrected, in kcal mol⁻¹) for the rearrangements of 2. Selected bond lengths [Å], angles and torsion angles [°] of 3 (C_2 h): C1–C2 1.343, C2–C3 1.467, C1–C2–C3 123.8; **TS2–3** (C₂): C1–C2 1.340, C2–C3 1.490, C1–C2–C3–C4 101.9; **TS2–2**′ (C_2v): C1–C2 1.430, C2–C3 1.379, C1–C2–C3–C4 0.0

**Fig. 3**
Relative QCISD(T)/6-311+G**//QCISD/6-31G* (UQCISD(T)/6-311+G**//UQCISD/6-31G* in parenthesis) energies (ZPE corrected, in kcal mol⁻¹) for the rearrangements of 11 into 14. Selected bond lengths [Å], angles and torsion angles [°] of 11 (C_s): P1–C1 1.852, Si1–C1 1.840, C1–C2 1.548, C1–P1–C2 49.4, C1–Si1–C2 49.7, P1–C1–C2–Si1 119.0; **TS11–12**: P1–C1 1.782, P1–C2 2.664, Si1–C1 1.982, Si1–C2 1.785; 12 (C_s): P1–C1 1.708, Si1–C2 1.741, C1–C2 1.443; **TS12–13**: P1–C1–C2–Si1 103.3; 13: P1–C1–C2–Si1 36.3; **TS13–14**: P1–C1 1.736, P1–Si1 3.001, Si1–C2 1.774, C1–C2 1.414, P1–C1–C2–Si1 34.1; 14: P1–C1 1.869, P1–Si1 2.290, Si1–C2 1.872, C1–C2 1.354

**Fig. 4**
Relative QCISD(T)/6-311+G**//QCISD/6-31G* energies (ZPE corrected, in kcal mol⁻¹) for the direct rearrangement of 11 into 14. Selected bond lengths [Å] and torsion angles [°] of TS11–14: P1–C1 1.834, P1–Si1 2.431, Si1–C2 1.800, C1–C2 1.422, P1–C1–C2–Si1 76.0

**Fig. 5**
Relative QCISD(T)/6-311+G**//QCISD/6-31G* (UQCISD(T)/6-311+G**//UMP2/6-31G* in parenthesis) energies (ZPE corrected, in kcal mol⁻¹) for the rearrangements of 15 into 18. Selected bond lengths [Å], angles and torsion angles [°] of 15 (C_s): P1–C1 1.852, Si1–C1 1.823, Si1–N1 1.730, C1–C2 1.613, C1–P1–C2 51.6, C1–Si1–C2 52.5, P1–C1–C2–Si1 122.1; **TS15–16**: P1–C1 1.768, P1–C2 2.590, Si1–C1 1.977, Si1–C2 1.748, Si1–N1 1.726; 16 (C_s): P1–C1 1.718, Si1–C2 1.724, Si1–N1 1.717, C1–C2 1.434; **TS16–17**: P1–C1–C2–Si1 98.2; 17 (C_s): P1–C1 1.727, Si1–C2 1.732, Si1–N1 1.710, Si1–N2 1.717; **TS17–18**: P1–C1 1.743, P1–Si1 3.103, Si1–C2 1.765, Si1–N1 1.719, C1–C2 1.406, P1–C1–C2–Si1 24.8; 18: P1–C1 1.867, P1–Si1 2.309, Si1–C2 1.867, Si1–N1 1.741, C1–C2 1.356

**Fig. 6**
Relative QCISD(T)/6-311+G**//QCISD/6-31G* energies (ZPE corrected, in kcal mol⁻¹) for the direct rearrangement of 15 into 18. Selected bond lengths [Å] and torsion angles [°] of **TS15–18**: P1–C1 1.841, P1–Si1 2.458, Si1–C2 1.777, Si1–N1 1.742, Si1–N2 1.736, C1–C2 1.444, P1–C1–C2–Si1 78.0

**Fig. 7**
Relative QCISD(T)/6-311+G**//QCISD/6-31G* energies (ZPE corrected, in kcal mol⁻¹) for the direct rearrangement of 19 into 20. Selected bond lengths [Å] and torsion angles [°] of 19 (C_s): P1–C1 1.852, Si1–C1 1.797, Si1–F1 1.600, C1–C2 1.631, C1–P1–C2 52.2, C1–Si1–C2 54.0, P1–C1–C2–Si1 122.0; **TS19–20**: P1–C1 1.834, P1–Si1 2.383, Si1–C2 1.758, Si1–F1 1.613, Si1–F2 1.610, C1–C2 1.457, P1–C1–C2–Si1 77.5; 20: P1–C1 1.879, P1–Si1 2.252, Si1–C2 1.841, Si1–F1 1.607, C1–C2 1.357

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Valence isomerization of 2-phospha-4-silabicyclo[1.1.0]butane: a high-level ab initio study - PubMed