Hydrophosphination of Boron–Boron Multiple Bonds† Cite This: Chem
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Chemical Science EDGE ARTICLE View Article Online View Journal | View Issue Hydrophosphination of boron–boron multiple bonds† Cite this: Chem. Sci., 2020, 11,1335 ab ab ab ab All publication charges for this article Tom E. Stennett, Arumugam Jayaraman, Tobias Bruckner,¨ Lea Schneider have been paid for by the Royal Society and Holger Braunschweig *ab of Chemistry Five compounds containing boron–boron multiple bonds are shown to undergo hydrophosphination reactions with diphenylphosphine in the absence of a catalyst. With diborenes, the products obtained are Received 21st November 2019 highly dependent on the substitution pattern at the boron atoms, with both 1,1- and 1,2- Accepted 13th December 2019 hydrophosphinations observed. With a symmetrical diboryne, 1,2-hydrophosphination yields DOI: 10.1039/c9sc05908c a hydro(phosphino)diborene. The different mechanistic pathways for the hydrophosphination of rsc.li/chemical-science diborenes are rationalised with the aid of density functional theory calculations. Introduction require a catalyst in the form of a base or metal complex, or Creative Commons Attribution-NonCommercial 3.0 Unported Licence. a radical initiator, and the scope is somewhat limited in terms Compounds containing covalent bonds between phosphorus of which secondary phosphines can be used. Whereas P–H 25 26 and boron, while not enjoying the ubiquity of their nitrogen– bond activation at transition and main-group metal centres boron analogues, display a rich chemistry.1–3 Phosphinoboranes is well documented, to the best of our knowledge the sole re- – ] 0 4 that is, compounds of the form R2B PR2 with a planar ported example of hydrophosphination of a homonuclear geometry at phosphorus and partial double bond character5 – double bond comprising elements other than carbon is the 6,7 can undergo addition reactions with such substrates as H2, 2016 report by Wang and Wu of hydrophosphination of the ketones6 and dienes6 and coordinate in an h2 fashion to tran- sition metals.8 Borylphosphines, in which the boron atom is not This article is licensed under a p-acidic (e.g. due to p-donor or carborane substituents) and phosphorus thus retains its lone pair and tetrahedral geometry, are highly electron rich h1 ligands for transition metal 9–11 Open Access Article. Published on 18 December 2019. Downloaded 9/26/2021 12:33:42 AM. complexes. By far the most common route for the construc- tion of P–B bonds is salt elimination12–19 using an alkali metal phosphide, MPR2, and a haloborane (Scheme 1A), although other routes are known, including nucleophilic addition of a boryl anion to a chlorophosphine (B),20 elimination of silyl halide (C),10,11,21 and cross-coupling of a B–I species with secondary phosphines using a palladium catalyst (D).9 Of the different synthetic options for the construction of phosphorus–carbon bonds, hydrophosphination of unsatu- rated carbon substrates has gained popularity in recent years.22–24 Advantages of this method over typical nucleophilic addition reactions include reduced waste (hydrophosphination is a 100% atom-economical process) and increased functional group tolerance due to the avoidance of highly reactive Grignard or organolithium reagents. However, these reactions typically aInstitut fur¨ Anorganische Chemie, Julius-Maximilians-Universitat¨ Wurzburg,¨ Am Hubland, 97074 Wurzburg,¨ Germany. E-mail: [email protected] bInstitute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians- Universitat¨ Wurzburg,¨ Am Hubland, 97074 Wurzburg,¨ Germany † Electronic supplementary information (ESI) available. CCDC 1954176–1954180. For ESI and crystallographic data in CIF or other electronic format see DOI: Scheme 1 Reported methods for the construction of covalent 10.1039/c9sc05908c boron–phosphorus bonds. This journal is © The Royal Society of Chemistry 2020 Chem. Sci.,2020,11, 1335–1341 | 1335 View Article Online Chemical Science Edge Article N]N bond in azobenzene with diphenylphosphine oxide.27 Recent studies in our laboratories have demonstrated the feasibility of adding H–H,28,29 B–H,30–32 B–B,33,34 S–S35,36 and Se– Se35 bonds across boron–boron multiple bonds. In this work, we show that uncatalysed hydrophosphination of both diborenes and diborynes can be used as a mild, selective and catalyst-free route to construct B–P bonds. Results and discussion Diborenes based on a chelating benzylphosphine group37 that holds the aryl substituent in the plane of the boron–boron p- bond have shown particularly high reactivity towards dienes38 and diboranes,33 so we chose these compounds as a starting point. The symmetrical diborene 1 was treated with diphenyl- phosphine at room temperature, leading to slow conversion to a compound with signals in the 11B NMR spectrum at À15.9 and À27.2 ppm, both in the region expected for four-coordinate boron atoms. Heating the mixture to 60 C for 1 h resulted in the decolouration of the solution and full conversion to the product, which was isolated as a colourless solid aer workup in 79% yield. X-ray diffraction on single crystals grown from Creative Commons Attribution-NonCommercial 3.0 Unported Licence. a diethyl ether solution conrmed the structure as that of product 5, the result of a 1,2-hydrophosphination across the B]B bond (Scheme 2 and Fig. 1). The PPh2 and hydrogen substituents display a gauche conformation, while the benzyl- phosphine units have reverted to a vicinal (1,2-) coordination mode; in contrast to other compounds bearing this This article is licensed under a Open Access Article. Published on 18 December 2019. Downloaded 9/26/2021 12:33:42 AM. Fig. 1 Molecular structures of (top to bottom) 5, 6, 7 and 8, with selected atomic displacement ellipsoids at the 50% probability level. Hydrogen atoms except for those bound to boron, a bromide coun- terion (compound 7) and co-crystallised solvent molecules are omitted for clarity. Selected bond lengths (A),˚ angles and torsions (): compound 5:B1–B2 1.780(2), B1–P1 2.051(2), B1–P2 2.008(2), B2–P3 1.928 (2), P1–B1–B2–H1 24.7(9); compound 6:B1–B2 1.777(3), B1–P1 1.974(2), B1–P2 1.932(2), B2–P3 1.953(2); compound 7:B1–B2 1.772(5), B1–P1 1.918(4), B2–P1 1.948(4), B1–P2 1.885(4), B2–C1 1.601(5), B1– P1–B2 54.6(2) P2–B1–B2–C1 141.1(3); compound 8:B1–B2 1.789(4), B2–P1 1.986(4), B1–P2 1.938(3), B1–P3 1.935(4). substituent,38 no conversion to the geminal isomer was observed. At 2.051(2) A,˚ the B1–P1 bond is in the range of published base-stabilised borylphosphines,15 and markedly longer than both dative B–P bonds in the molecule (B1–P2 ¼ ˚ Scheme 2 Hydrophosphination of diborenes and a borylborylene with 2.008(2), B2–P3 ¼ 1.928(2) A). As expected, P1 is highly pyra- i diphenylphosphine. I Pr ¼ 1,3-diisopropylimidazol-2-ylidene. midal, with a sum of angles of 312.4 . In the 1H NMR spectrum, 1336 | Chem. Sci.,2020,11, 1335–1341 This journal is © The Royal Society of Chemistry 2020 View Article Online Edge Article Chemical Science a multiplet centred at 3.08 ppm becomes visible upon 11B with the diboration product of 4, compound 8 can be drawn as decoupling, corresponding to the newly formed B–H group. Two a borylene–borane adduct. However, it is notable that the B–B signals at 10.4 ppm and 6.1 ppm in the 31P NMR spectrum distance (1.789(4) A)˚ and the B–P distances (B2–P1 ¼ 1.986(4), correspond to the now inequivalent benzylphosphine phos- B1–P2 ¼ 1.938(3), B1–P3 ¼ 1.935(4) A)˚ are statistically indis- phorus atoms, whereas the PPh2 group is represented by tinguishable from those in its constitutional isomer 6, indi- a broad signal at À13.3 ppm. cating that the electronic situation at the B2 units may not be as Unsymmetrically-substituted diboron compounds are different as these formalisms suggest. known to display higher reactivity than their symmetrical To gain mechanistic insight into how different hydro- counterparts.39,40 We have previously shown that unsymmetrical phosphination outcomes were obtained using the diboron diborenes 2 and 3 (Scheme 2) react more quickly than 1 in compounds 1–4, DFT computations at the uB97XD/6- 33 2 u diboration reactions with B2cat2. Treatment of compound 31++g(d,p)/SMD// B97XD/6-31g(d,p) level of theory were per- A1– with HPPh2 resulted in complete conversion to a new formed. For computations, truncated models of diborenes, compound within 3 h at room temperature. Signals in the 11B D1, in which the cyclohexyl substituents of the chelating NMR spectrum at À7.7 and À34.2 ppm again indicated the phosphines were replaced with methyl groups, were examined. presence of inequivalent, four-coordinate boron atoms. Three The 1,1- and 1,2-hydrophosphination reactions observed are broad signals in the 31P NMR spectrum at 23.1, 13.8 and signicantly exergonic, with DG values ranging from À23.9 to À À6.9 ppm also suggested three boron-bound phosphorus À38.4 kcal mol 1 (Fig. 2 and 3). For the 1,2-hydrophosphination environments. Measurement of a 1H NMR spectrum with of diborene 1, yielding product 5, the likely mechanism is 11 d ff ¼ selective B decoupling ( 11B o set 34 ppm) again revealed shown in Fig. 2a. Initially, coordination of HPPh2 to one of the a complex multiplet for a B–H moiety at 2.15 ppm. X-ray boron atoms affords adduct A2. This is followed by a 1,3-H shi diffraction conrmed the product as 6, the result of a formal from the coordinated HPPh2 to the distal boron atom, leading 1,1-hydrophosphination rather than the expected 1,2-addition to the formation of a 1,2-hydrophosphinated intermediate A3, – ˚ Creative Commons Attribution-NonCommercial 3.0 Unported Licence.