A Phosphinine-Derived 1-Phospha-7-Bora-Norbornadiene: Frustrated Lewis Pair Type Activation of Triple Bonds

Abstract

Salt metathesis of 1-methyl-2,4,6-triphenylphosphacyclohexadienyl lithium and chlorobis(pentafluorophenyl)borane affords a 1-phospha-7-bora-norbornadiene derivative 2. The C≡N triple bonds of nitriles insert into the P-B bond of 2 with concomitant C-B bond cleavage, whereas the C≡C bonds of phenylacetylenes react with 2 to form λ⁴ -phosphabarrelenes. Even though 2 must formally be regarded as a classical Lewis adduct, the C≡N and C≡C activation processes observed (and the mild conditions under which they occur) are reminiscent of the reactivity of frustrated Lewis pairs. Indeed, NMR and computational studies give insight into the mechanism of the reactions and reveal the labile nature of the phosphorus-boron bond in 2, which is also suggested by detailed NMR spectroscopic studies on this compound. Nitrile insertion is thus preceded by ring opening of the bicycle of 2 through P-B bond splitting with a low energy barrier. By contrast, the reaction with alkynes involves formation of a reactive zwitterionic methylphosphininium borate intermediate, which readily undergoes alkyne 1,4-addition.

Keywords: alkyne; frustrated Lewis pair; nitrile; norbornadiene; phosphinine.

Figure 1

A generic 1‐R‐phosphacyclohexadienyl anion A and its reactivity toward various electrophiles; the first reported P/B FLP (F); a triphosphabenzene derivative that displays FLP‐like reactivity (G). Reagents: i) CH₃I, ii) R′COCl, iii) H⁺, iv) M¹L_n=M²L_n=Rh(1,5‐cod), cod=cycloocta‐1,5‐diene; R=aryl or alkyl substituents.

Scheme 1

Synthesis of 2 through 1 and Iso‐2 a; reagents and conditions: i) MeLi (Et₂O, −78 °C), ii) (C₆F₅)₂BCl/−LiCl (RT, n‐hexane). Relative wB97X‐D/6–311+G** electronic energies (ΔE in kcal mol⁻¹) and free energies (ΔG in kcal mol⁻¹) for possible isomers of 2 (Iso‐2 a–Iso‐2 d).

Figure 2

Molecular structure of 2 in the single crystal. Displacement ellipsoids are shown at the 40 % probability level; H atoms are omitted for clarity; selected bond lengths [Å] and angles [°]: P1−B1 1.9894(19); P1−C1 1.8025(17); B1−C4 1.721(2); P1−C2 1.8146(17); P1−C6 1.8081(17); C2−C3 1.344(3); C6−C5 1.336(2); C1‐P1‐B1 129.51(8); C2‐P1‐C6 101.44(8); P1‐B1‐C4 84.58(10).

Figure 3

a) ³¹P{¹H} NMR monitoring (starting from 193 K and warming up to 300 K) of the reaction of 1 with (C₆F₅)₂BCl (see Supporting Information for full spectra, Figure S79). The spectral regions of 2 and Iso‐2 a are displayed with the same expansion and intensity scaling. b) Overlaid F1 (indirect dimension) projections of a ¹H–³¹P HSQC with simultaneous ¹¹B and ¹H decoupling in F1 and ³¹P decoupling in F2 (direct dimension) acquired in the region of 2 (better signal/noise compared to 1d ³¹P spectrum, see S81 in Supporting Information for pulse program). ¹¹B decoupling was applied on resonance (red) and off resonance (black). The spectra were acquired on a TBI‐P probe. c) Overlaid 1d ³¹P spectra with simultaneous ¹¹B and ¹H decoupling. ¹¹B decoupling was applied on resonance (red) and off resonance (black). The spectra were adjusted to same height for a better comparison of line widths. The spectra were acquired on a TBI‐P probe. d) Overlaid 1d ³¹P spectra with ¹H decoupling only (black) and simultaneous ¹⁹F and ¹H decoupling (blue). The spectra were adjusted to same height for a better comparison of line widths (see also S80 in Supporting Information). The spectra were acquired on a TBI‐F probe. e) Overlaid F1 projections of a ¹H–³¹P HSQC with simultaneous ¹¹B and ¹H decoupling in F1 and ³¹P decoupling in F2 acquired in the region of Iso‐2 a. The spectra were acquired on a TBI‐P probe.

Scheme 2

Synthesis of nitrile insertion products 3 a–3 e (yields of isolated compounds are given in parentheses in this caption): 3 a: R=Me (33 %), 3 b: R=Ph (64 %), 3 c: R=3,5‐Br₂C₆H₃ (43 %), 3 d: R=CH₂Cl (39 %), 3 e: R=Et (41 %).

Figure 4

Molecular structure of 3 a in the single crystal. Displacement ellipsoids are shown at the 40 % probability level; H atoms are omitted for clarity; phenyl and C₆F₅ groups are shown in wireframe for clarity; selected bond lengths [Å] and angles [°]: P1−C1 1.8036(18); P1−C7 1.8822(17); P1−C2 1.7475(16); P1−C6 1.7431(17); C2−C3 1.397(2); C3−C4 1.399(2); C5−C6 1.378(2); C7−N1 1.251(2); N1−B1 1.367(2); C7−C8 1.492(2); C1‐P1‐C7 102.58(8); P1‐C7‐N1 120.29(13); C7‐N1‐B1 172.06(18); C8‐C7‐N1 124.56(16).

Scheme 3

Reaction of 2 with 2‐(dimethylamino)acetonitrile.

Figure 5

Molecular structure of 4 in the crystal. Displacement ellipsoids are shown at the 40 % probability level; H atoms are omitted for clarity; phenyl and C₆F₅ groups are shown in wireframe for clarity; selected bond lengths [Å] and angles [°]: P1−C7 1.8435(12); P1−C2 1.7631(12); P1−C6 1.7612(12); P1−C1 1.8231(12); C2−C3 1.3916(17); C3−C4 1.3985(18); C4−C5 1.4070(17); C5−C6 1.3822(17); C7−C8 1.5084(16); N2−B1 1.7322(16); B1−N1 1.5237(16); N1−C7 1.2578(16); C7‐N1‐B1 111.27(10); N1‐B1‐N2 100.38(9); P1‐C7‐N1 125.34(9); C8‐N2‐B1 103.

Figure 6

Relative ωB97X‐D/6–311+G** energies (calculated free energies ΔG in kcal mol⁻¹) for the conversion of 2 into 3 a.

Scheme 4

Reaction of 2 with phenylacetylene derivatives, leading to the formation of 5 a–5 c, thermal rearrangement of 2 to 6, and relative ωB97X‐D/6–311+G** energies (calculated free energies ΔG in kcal mol⁻¹); 5 a: R=Ph (61 %); 5 b: R=4‐CF₃‐C₆H₄ (62 %); 5 c: R=4‐Br‐C₆H₄ (72 %).

Figure 7

Molecular structure of 5 b in the crystal. Displacement ellipsoids are drawn at the 40 % probability level; H atoms are omitted for clarity; phenyl; C₆H₄CF₃ and C₆F₅ groups are shown in wireframe for clarity; a disordered n‐hexane molecule is omitted for clarity; selected bond lengths [Å] and angles [°]: P1−C8 1.783(4); C4−C7 1.577(5); P1−C2 1.799(4); P1−C6 1.816(4); P1−C1 1.781(4); C2−C3 1.335(5); C3−C4 1.560(5); C4−C5 1.541(5); C5−C6 1.335(5); C7−C8 1.334(5); C2−B1 1.636(5); P1‐C8‐C7 112.1(3); C8‐C7‐C4 116.4(3); C2‐P1‐C6 103.36(17); C3‐C4‐C5 107.2(3).

Figure 8

Relative ωB97X‐D/6–311+G** energies (calculated free energies ΔG in kcal mol⁻¹) for the conversion of 2 into 5 a.