New Insights in Frustrated Lewis Pair Chemistry with Azides

Abstract

The geminal frustrated Lewis pair (FLP) tBu₂ PCH₂ BPh₂ (1) reacts with phenyl-, mesityl-, and tert-butyl azide affording, respectively, six, five, and four-membered rings as isolable products. DFT calculations revealed that the formation of all products proceeds via the six-membered ring structure, which is thermally stable with an N-phenyl group, but rearranges when sterically more encumbered Mes-N₃ and tBu-N₃ are used. The reaction of 1 with Me₃ Si-N₃ is believed to follow the same course, yet subsequent N₂ elimination occurs to afford a four-membered heterocycle (5), which can be considered as a formal FLP-trimethylsilylnitrene adduct. Compound 5 reacts with hydrochloric acid or tetramethylammonium fluoride and showed frustrated Lewis pair reactivity towards phenylisocyanate.

Keywords: azides; density functional calculations; frustrated Lewis pairs; heterocycles; isocyanates.

Figure 1

Examples of NHC adducts with azides (top) and FLP adducts with azides (bottom) (EDG=electron‐donating group, EWG=electron‐withdrawing group).

Scheme 1

Reactivity of FLP 1 with phenyl azide (2_Ph), tert‐butyl azide (3 _tBu), and mesityl azide (3_Mes, 4_Mes) and trimethylsilyl azide (5).

Figure 2

Molecular structures of 2_Ph, 3 _tBu, 3_Mes, and 4_Mes (ellipsoids at 50 % probability, hydrogen atoms and for 2_Ph and 4_Mes second crystallographic independent molecules omitted for clarity). Selected bond lengths [Å] and angles [°] for 2_Ph (structural parameter for the second crystallographic independent molecule in brackets): P1−N1 1.687(2) [1.686(2)], N1−N2 1.290(3) [1.291(3)], N2−N3 1.323(3) [1.329(3)], N3−B1 1.597(3) [1.603(3)]. 3 _tBu: P1−N1 1.6675(12), N1−N2 1.3573(16), N2−N3 1.2502(17), N1−B1 1.6066(19); P1‐N1‐N2‐N3 −1.66(19). 3_Mes: P1−N1 1.6838(19), N1−N2 1.342(3), N2−N3 1.259(3), N1−B1 1.613(3); P1‐N1‐N2‐N3 0.000(1). 4_Mes: P1−N1 1.6592(13) [1.6557(13)], N1−N2 1.3364(17) [1.3397(17)], 1.2733(17) [1.2695)], N2−B1 1.668(2) [1.655(2)]; P1‐N1‐N2‐N3 −176.26(11) [175.08(11)].

Scheme 2

Complete energy profile (ΔE) in kcal mol⁻¹ for the formation of 2_Ph (blue), 2 _tBu (red) and 2_Mes (green) through initial P−N bond formation (interrupted Staudinger reaction, shown in red) versus initial B−N bond formation and rearrangement to heterocycle 3. *TS4_Ph could not be located.

Figure 3

Four different isomers of FLP–azide adducts (R=H, Ph, tBu, Mes, and TMS, see Table 1).

Scheme 3

The reaction of 1 with TMS−N₃ (top) and the molecular structure of 5 (bottom; ellipsoids at 50 % probability, hydrogens are omitted for clarity). Selected bond lengths [Å] and angles [°] for 5: P1−N1 1.6366(11), B1−N1 1.6802(18), N1−Si1 1.7348(12); P1‐N1‐B1 90.96(7), B1‐N1‐Si1 132.33(8), Si1‐N1‐P1 135.66(7).

Scheme 4

The reaction of 5 with HCl and TMAF (top) and the molecular structure of 7 (bottom; ellipsoids at 50 % probability, hydrogens Me₄N cation and a second ion pair of 7 are omitted for clarity). Selected bond lengths [Å] and angles [°] for 7 (structural parameter for the second crystallographic independent molecule in brackets): P1−N1 (1.543(3) [1.545(3)], B1−F1 1.470(4) [1.473(4)], N1−Si1 1.653(3) [1.648(3)]; P1‐C1‐B1 131.9(2) [130.9(2)], P1‐N1‐Si1 171.5(2) [174.6(2)], Σ(C‐B1‐C) 331 [332]).

Scheme 5

The equilibrium between the open (5′) and closed form of 5 and the corresponding energies.

Scheme 6

The reaction of 5 with phenylisocyanate.