Synthesis and Ambiphilic Reactivity of Metalated Diorgano‐Phosphonite Boranes

Abstract Unprecedented metalated phosphonite boranes were prepared from PH‐substituted precursors and silyl amides. Although potassium derivatives were thermally stable and could even be isolated and structurally characterised, lithiated analogues proved to be unstable towards self‐condensation under cleavage of LiOR at ambient temperature. Reaction studies revealed that the metalated phosphonite boranes exhibit ambiphilic character. Their synthetic potential as nucleophilic building blocks was demonstrated in the synthesis of the first stannylated phosphonite representing a new structural motif in phosphine chemistry.


Lithium-1,1,3,3-tetraethoxy-triphosphide 1,3-bis-borane (Li[6b])
A solution of LiHMDS (50 mg, 0.30 mmol) in THF (4 mL) was cooled to -78°C. Phosphine borane 2b (40 mg, 0.30 mmol) was added dropwise and the mixture was warmed to room temperature. Volatiles were removed in vacuum. The oily residue was treated with hexane (1mL), and Et 2 O was added until the product had completely dissolved. Storage at -25 °C produced single crystals suitable for an XRD study. For spectroscopic characterisation, the crude oily product obtained in another experiment was dissolved in THF-d 8

Bis(2,6-diisopropylphenoxy)triphenylstannylphosphine borane (9d)
Phosphine borane 2d (250 mg, 0.62 mmol) and KHMDS (125 mg, 0.62 mmol) were dissolved in toluene (12 mL) at -78°C. The mixture was stirred for 1h at -78 °C and then allowed to warmed to room temperature until it became homogeneous. After re-cooling to -78 °C, a solution of Ph 3 SnCl (241 mg, 0.62 mmol) in toluene (2 mL) was slowly added. The mixture was stirred for 1 h at -78°C and then for 1 h at room temperature. Volatiles were removed in vacuum and the residue treated with hexane (5 mL). The resulting suspension was filtered. Evaporation of the filtrate to dryness afforded a colourless solid (285 mg, 380 µmol, 60%). Single crystal suitable for X-Ray crystallography were grown from a saturated hexane solution.

Diisopropoxytriphenylstannylphosphine (10c)
The experiment was carried out as described for 10b using 9c

Bis(2,6-diisopropylphenoxy)triphenylstannylphosphine (10d)
A solution of 9d (50 mg, 67 μmol) in toluene (3 mL) and NEt 3 (1.5 mL) was stirred for 16 h. Volatiles were removed under reduced pressure and the residue treated with pentane (3 mL). Insoluble components were removed by filtration and the volume of the filtrate was reduced to 0.5 mL. The product separated as colourless crystals (40 mg, 54 μmol, 82 %). Crystallographic studies X-ray diffraction data were collected on a Bruker Kappa Apex II Duo diffractometer equipped with an APEX II CCD-detector and a KRYO-FLEX cooling device with Mo-radiation ( = 0.71073 Å) at 130 (2) 10d), respectively. The structures were solved with direct methods (SHELXS-2014 [7] ) and refined with a fullmatrix least squares scheme on F 2 (SHELXL-2014 [7] ). Semi-empirical or numerical absorption corrections (see Table S1) were applied. Non-hydrogen atoms were refined anisotropically and hydrogen atoms except those bound to phosphorus and boron using a riding model. One SiMe 3 moiety and the THF moieties in K[3b]-KNTms 2 , the ethoxy groups in 9b, and two iPr-groups in 9d as well as all iPr-groups in 10d are disordered. Further details on the refinement of the disorder is given in the cif-files and the incorporated resfiles. CCDC-2046650 to CCDC-2046657 contain the crystallographic data for this paper, which can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.              Figure S12: 31 P NMR spectrum of 2d in C 6 D 6 .

Computational Studies
General remarks. DFT calculations were performed with the Gaussian 16 program package [8] using the B3LYP functional, [9] which is an established standard in main group element chemistry, with basis sets from Weigend's and Ahlrichs' def2-family, [10] and application of the D3 version of Grimme's dispersion correction with Becke-Johnson damping [11] and the PCM formalism (keyword scrf, solvent=THF) to model solvation. The molecular structures were established by full energy optimization at the PCM-B3LYP-D3BJ/def2-svp level and identified as local minima on the potential energy hypersurface by subsequent harmonic vibrational frequency calculations. Magnetic shieldings were obtained from single point calculations at the PCM-B3LYP-D3BJ/def2-tzvpp level at the optimised geometries. NBO population analyses of electron densities were carried out using the NBO module implemented in the Gaussian package. The analysis of magnetic shieldings was carried out with NBO6. [12] Chemical shifts were computed as δ s = (σ ref -σ s − 266.1) relative to 85% H 3 PO 4 [13] using the magnetic shielding constants of PH 3 (σ ref = 590.5 ppm) calculated at the same computational level for referencing. ) calculated at the PCM-B3LYP-D3BJ/def2-tzvpp//PCM-B3LYP-D3BJ/def2-svp level of theory; b) σ iso ( 31 P) and δ 31 P for 10b were calculated as -39.3 and 315.9 ppm but the limitations of the available NBO module precluded further analysis. ∆E HOMO,LUMO+6 of 10b (the LUMO+6 is in this case the lowest unoccupied MO with a significant local contribution at phosphorus) was computed as 5.98 eV.