B-Centered Reactivity of Persistent P-Stabilized Boryl Radicals

Article pubs.acs.org/Organometallics

B‑Centered Reactivity of Persistent P‑Stabilized Boryl Radicals Amos J. Rosenthal,† Sonia Mallet-Ladeira,‡ Ghenwa Bouhadir,*,† Eric-Daiann Sosa-Carrizo,§ Karinne Miqueu,*,§ and Didier Bourissou*,† †

CNRS, Université Paul Sabatier, Laboratoire Hétérochimie Fondamentale et Appliquée (LHFA, UMR 5069), 118 Route de Narbonne, 31062 Toulouse Cedex 09, France ‡ Institut de Chimie de Toulouse (FR 2599), 118 Route de Narbonne, 31062 Toulouse Cedex 09, France § CNRS/UNIV PAU & PAYS ADOUR, Institut des Sciences Analytiques et de Physico-Chimie pour l’Environnement et les Matériaux, Hélioparc, 2 Avenue du Président Angot, 64053 Pau Cedex 09, France S Supporting Information *

ABSTRACT: A new P-stabilized boryl radical [iPr2P(naph)BMes]• 2a was obtained by reduction of the corresponding phosphinobromoborane 1a with Na(Hg). The persistent radical 2a has been characterized by EPR, and its structure has been thoroughly studied by DFT. The corresponding Gomberg-type dimer has been analyzed by NMR and XRD, and the Gibbs free energy associated with the dimerization process has been evaluated by VT EPR. The replacement of the Ph substituents at phosphorus for iPr groups has a slight but noticeable impact: it increases the spin density at boron and favors the radical over its Gomberg-type dimer. An original cross-coupling product between 2a and the trityl radical Ph3C• has also been authenticated crystallographically. The Pstabilized boryl radicals 2a,b are readily trapped by TEMPO to give the corresponding B−O adducts 3a,b (naphthyl-bridged phosphine-boranes without P → B interaction). The reaction of 2a,b with Ph3CCl substantiates their ability to participate in halogen transfer reactions.

■

INTRODUCTION Boron-containing radicals, in particular radical anions BR3•− and neutral Lewis base-stabilized boryl radicals [L → B10 kcal/mol (ΔG).12 Equilibration of 2a and [2a]2 is supported by variable-temperature EPR measurements, and the corresponding dimerization energy ΔG was estimated to −12 kcal mol−1.12 This value is slightly lower than that measured for the dimerization of 2b (−15 kcal mol−1) and actually very close to that of the trityl radical Ph3C• (−11 kcal mol−1).18 Thus, the replacement of the Ph substituent at P for iPr groups has a slight but noticeable impact: it increases the spin density at B and favors the radical over its Gomberg-type dimer. The homocoupling of persistent radicals is generally favored over heterocoupling due to orbital considerations, but the similar dimerization energies of 2b and Ph3C• prompted us to envision their cross-coupling. The two species were mixed in benzene, and the solution was let to equilibrate at room temperature for several hours. A few crystals suitable for X-ray diffraction analysis deposited spontaneously (Scheme 3). They correspond to the heterodimer 2c resulting from the coupling of the Cnapht atom of 2a in the para position to B with the central carbon atom of the trityl radical Ph3C•. The ensuing C− C bond is very long at 1.605(1) Å. It is significantly longer than

phosphines, NHC, pyridine) to the backbone of the borocyclic radicals C deriving from ortho-quinones, leading to zwitterions (C−Nu bond formation).7 Another relevant contribution is that by Tamm et al. on the NHC-stabilized boryl radical D, a rare example of a persistent boron-centered radical (the spin density at boron reaches ∼40%). It was readily trapped by parabenzoquinone, but instead of the double-addition product (B− O bond formation), hydrogen abstraction from a tBu group of the NHC was observed, leading to a bicyclic zwitterion.8 Recently, we have reported a new class of P-stabilized boreniums E9 and boryl radicals F10 (Chart 2). According to Chart 2. Schematic Representation of the Naphthyl-Bridged P-Stabilized Boreniums and Boryl Radicalsa

a

Borenium: E (R = Ph, iPr; R′ = Mes, Cy, NiPr2, NPh2). Boryl radical: F (R = Ph; R′ = Mes).

EPR data and DFT calculations, the spin density at boron is high in radicals of type F (60−70%), and an original Gombergtype dimerization process has been evidenced (i.e., the formation of a quinoid-type structure by coupling between the central carbon atom of one trityl radical with one of the para carbon atoms of another Ph3C• molecule). We are now interested in exploiting the structural modularity of these Pstabilized boryl radicals and have varied the substituent at phosphorus. This work was stimulated by our studies on related boreniums E,9 for which the substituents at phosphorus (Ph/ iPr) were found to significantly influence the boron Lewis acidity and the reactivity toward dihydrogen. Here, the impact of the substitution pattern at P on the electronic structure and Gomberg-type dimerization of radicals F has been evaluated both experimentally and computationally. The persistent character of the P-stabilized boryl radicals has also prompted us to study their reactivity. We report here the results we obtained in radical trapping with TEMPO (which is extensively used to trap carbon-centered radicals but has never been combined with boryl radicals). The ability of radicals F to participate in atom transfer reactions, in particular halogen transfer, is also substantiated.

■

RESULTS AND DISCUSSION The phosphine−bromoborane 1a11 was chemically reduced using 1% Na(Hg) amalgam in THF (Scheme 1). After filtration, evaporation under vacuum, and precipitation with pentane, the resulting product was isolated as a dark-red powder in 80% yield. Characterization by EPR spectroscopy in Scheme 1. Synthesis of the P-Stabilized Boron Radicals 2 by Chemical Reduction of the Phosphino-Bromoboranes 1

B

DOI: 10.1021/acs.organomet.7b00598 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Figure 1. Experimental and simulated EPR spectra of 2a (magnetic field in gauss), including variable-temperature measurements (on top left is shown the variation of the integral of the EPR signal with temperature); SOMO (cutoff: 0.04, spin alpha) computed at the B3PW91-D3(BJ)/631G** level of theory with the relative contributions (in %) of the different atoms.

Scheme 3. Heterocoupling of 2a and Ph3C• and X-ray Structure of 2ca

Scheme 2. Equilibrium between 2a and Its Gomberg-Type Dimera

a

With 31P and 11B NMR data as well as key metrical data.

a

Ellipsoids set at 50% probability; the iPr, Mes, and Ph groups are simplified and H atoms are omitted for clarity.

the C−C bond formed upon Gomberg-type dimerization of Ph3C• (1.584(3) Å)19 but much shorter than the central C−C bonds of the sterically congested hexaphenylethane derivatives (1.67(3) Å).20 The boron center is in a trigonal planar environment and engaged in strong P → B interaction (the P− B distance is short at 1.947(1) Å). The B−Cnapht bond is also short at 1.466(2) Å, in line with double-bond character.21 To the best of our knowledge, 2c is the first heterocoupling product of the trityl radical with a persistent radical of a p block element.

DFT calculations were also carried out on 2c*, and the optimized geometry nicely reproduced that determined crystallographically (in particular the long C−C bond 1.619 Å as well as the short P → B and B = Cnapht bonds 1.938 and 1.465 Å). Another energy minimum was located on the potential-energy surface. It is associated with another heterodimer 2d* resulting from the coupling of the B atom of 2a with the central carbon atom of the trityl radical Ph3C• (Figure 2). Due to steric crowding, it displays a long B−CPh3 C

DOI: 10.1021/acs.organomet.7b00598 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

species, this suggests that 3a,b adopt open structures without intramolecular P → B interaction. This assignment was unambiguously confirmed by X-ray diffraction studies (suitable crystals were grown from DCM/pentane solutions). Compounds 3a,b exhibit very similar geometries, and thus, only that of 3a will be discussed. In agreement with the NMR data in solution, the P and B atoms are too far away (2.901(2) Å) to be engaged in P → B interaction, and the environment around boron is close to trigonal planar (ΣBα = 357.9°). The presence of two sterically demanding substituents (the iPr2P moiety and the BMes(TEMPO) group) on adjacent peri positions of the naphthyl moiety induces important steric crowding, as apparent from the rather large out-of-plane distortion of the linker (as apparent from the PC1C3B torsion angle of 16.63°). The naphthyl spacer has been shown to enforce strong P → B interaction even when the two sites are sterically hindered,23 but it retains some flexibility and accommodates open structures when the Lewis acidity of the boron center is reduced by π-donating substituents such as amino groups11,24 or TEMPO in the case of 3a,b. Interestingly, the trapping of 2a,b with TEMPO is another type of heterocoupling, and in this case, the coupling occurs selectively at the boron center where most of the spin density is concentrated. No traces of coupling products involving the Cnapht atom in para position to B were detected. Note that reactions of radicals A and B with benzoquinone and dibenzoylperoxide also led to B−O bond formation, as mentioned above.5,6 The formation of the TEMPO adducts 3a,b spurred us to assess the ability of 2a,b to participate in another type of radical reactivity, namely, chloride transfer. The boron-centered radicals 2a,b were thus reacted with 1 equivalent of Ph3CCl in THF (Scheme 4). Over 24 h at room temperature, the deepred solutions of 2a,b progressively faded. The phosphine− chloroboranes 4a,b were isolated as colorless crystals (∼80% yield) after workup. The 31P NMR signatures of 4a,b (δ 15.4 and 2.1 ppm, respectively) are very similar to those of the bromoborane precursors 1a,b (δ 14.1 and 1.8 ppm, respectively), and the tetracoordinate environment around boron was clearly apparent by 11B NMR spectroscopy (δ∼ 3 ppm). In addition, 4b was characterized by X-ray diffraction.12 Its structure parallels that of 1b, the most noticeable feature being the short P−B distance (2.063(2) Å) characteristic of strong P → B interaction. The formation of 4a,b represents a rare example of halogen transfer to a boron-centered radical. Chloride abstraction has been occasionally reported for transient in situ-generated L → BH2• radicals (L = R3P, R3N, NHC).2a,25 Another related reaction is the double chloride abstraction observed upon reacting (iPr2PBtBu)2 (a stable fourmembered ring 1,3-diradicaloid)26 with chloroform.27

bond (1.760 Å) and is located 6.2 kcal/mol (ΔG) above 2c* in energy.

Figure 2. DFT-optimized structure of the heterocoupling product 2d* (the iPr groups at P are simplified for clarity).

The characterization of the heterocoupling product 2c prompted us to investigate then the trapping of 2a,b with the stable 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical.22 The dark-red color characteristic of 2a,b disappears within minutes upon addition of TEMPO in THF (Scheme 4). Mass Scheme 4. Boron-Centered Reactivity of 2a,b Radicals with TEMPO (Radical Coupling) and Ph3CCl (Chlorine Abstraction) and X-ray Structure of 3aa

■

CONCLUSION A new persistent P-stabilized boryl radical 2a has been prepared. Its electronic structure has been thoroughly analyzed by EPR spectroscopy and DFT calculations. The corresponding Gomberg-type dimer [2a]2 has been characterized by multinuclear NMR and XRD. In contrast with most persistent boryl radicals, which are stabilized by π delocalization, the spin density is highly concentrated on boron in 2a,b. The replacement of the Ph substituents at phosphorus for iPr groups lowers the dimerization energy, and that evaluated for 2a by VT EPR is actually very close to that of the trityl radical. Besides this Gomberg-type homocoupling, a heterocoupling product between 2a and Ph3C• has been authenticated

a

Ellipsoids set at 50% probability; the iPr, Mes, and tetramethylpiperidino groups are simplified and H atoms are omitted for clarity.

spectrometry analyses (HRMS, electrospray ionization) of the resulting products indicate the formation of 1:1 adducts [R2P(naph)BMes/TEMPO]+. Compounds 3a,b were isolated as white powders in ∼85% yield. The 31P NMR resonance signals (δ − 5.3 and −9.5 ppm, respectively) appear at high field compared to those of the phosphine−bromoboranes 1a,b, which display P → B interactions (Δδ1a−3a= 19.4 and Δδ1b−3b = 11.3 ppm). In combination with the 11B NMR signals at ∼42 ppm, which is in the typical range for tricoordinate Ar2BOR′ D

DOI: 10.1021/acs.organomet.7b00598 Organometallics XXXX, XXX, XXX−XXX

Organometallics crystallographically. Radical coupling also readily occurs with the persistent nitroxyl radical TEMPO, providing the first evidence for such a radical trapping with a boryl radical. In addition, the reaction of 2a,b with Ph3CCl substantiates their ability to participate in halogen transfer reactions. This works demonstrates that the stabilization of boryl radicals is not detrimental to their reactivity. Boron-centered reactivity seems to be favored for 2a,b in line with the high spin density at boron. The study of the reactivity of persistent boron radicals is still in its infancy, and we do believe much remains to be discovered. In our group, efforts will be focused on Pstabilized boryl radicals. Variations of the substituent at boron and of the organic backbone should have a significant impact and are certainly worthwhile to explore.

■

■

ABBREVIATIONS

■

REFERENCES

NHC, N-heterocyclic carbene; NMR, nuclear magnetic resonance; EPR, electron paramagnetic resonance

(1) Power, P. P. Chem. Rev. 2003, 103, 789−810. (2) (a) Baban, J. A.; Roberts, B. P. J. Chem. Soc., Perkin Trans. 2 1984, 1717−1722. (b) Ueng, S. H.; Solovyev, A.; Yuan, X.; Geib, S. J.; Fensterbank, L.; Lacôte, E.; Malacria, M.; Newcomb, M.; Walton, J. C.; Curran, D. P. J. Am. Chem. Soc. 2009, 131, 11256−11262. (c) Matsumoto, T.; Gabbaï, F. P. Organometallics 2009, 28, 4252− 4253. (d) Walton, J. C.; Brahmi, M. M.; Fensterbank, L.; Lacôte, E.; Malacria, M.; Chu, Q.; Ueng, S. H.; Solovyev, A.; Curran, D. P. J. Am. Chem. Soc. 2010, 132, 2350−2358. (e) Dahcheh, F.; Martin, D.; Stephan, D. W.; Bertrand, G. Angew. Chem., Int. Ed. 2014, 53, 13159− 13163. (f) Bissinger, P.; Braunschweig, H.; Damme, A.; Krummenacher, I.; Phukan, A. K.; Radacki, K.; Sugawara, S. Angew. Chem., Int. Ed. 2014, 53, 7360−7363. (g) Bertermann, R.; Braunschweig, H.; Dewhurst, R. D.; Hö rl, C.; Kramer, T.; Krummenacher, I. Angew. Chem., Int. Ed. 2014, 53, 5453−5357. (h) Braunschweig, H.; Krummenacher, I.; Légaré, M. A.; Matler, A.; Radacki, K.; Ye, Q. J. Am. Chem. Soc. 2017, 139, 1802−1805. (i) Wu, C.; Hou, X.; Zheng, Y.; Li, P.; Lu, D. J. Org. Chem. 2017, 82, 2898− 2905. (3) In particular, in situ-generated NHC-stabilized boryl radicals have been used in radical polymerizations and Barton-McCombie reductions, see: (a) Roberts, P. Chem. Soc. Rev. 1999, 28, 25−35. (b) Curran, D. P.; Solovyev, A.; Makhlouf Brahmi, M.; Fensterbank, L.; Malacria, M.; Lacôte, E. Angew. Chem., Int. Ed. 2011, 50, 10294− 10317. (4) Very recent developments of transient pyridine-stabilized boryl radicals include: (a) the reduction of azo compounds: Wang, G.; Zhang, H.; Zhao, J.; Li, W.; Cao, J.; Zhu, C.; Li, S. Angew. Chem., Int. Ed. 2016, 55, 5985−5989. (b) the borylation of aryl halides: Zhang, L.; Jiao, L. J. Am. Chem. Soc. 2017, 139, 607−610. and (c) addition reactions to α,β-unsaturated ketones: Wang, G.; Cao, J.; Gao, L.; Chen, W.; Huang, W.; Cheng, X.; Li, S. J. Am. Chem. Soc. 2017, 139, 3904−3920. (5) Braunschweig, H.; Dyakonov, V.; Jimenez-Halla, J. O.; Kraft, K.; Krummenacher, I.; Radacki, K.; Sperlich, A.; Wahler, J. Angew. Chem., Int. Ed. 2012, 51, 2977−2980. (6) Aramaki, Y.; Omiya, H.; Yamashita, M.; Nakabayashi, K.; Ohkoshi, S. i.; Nozaki, K. J. Am. Chem. Soc. 2012, 134, 19989−19992. (7) Longobardi, L. E.; Zatsepin, P.; Korol, R.; Liu, L.; Grimme, S.; Stephan, D. W. J. Am. Chem. Soc. 2017, 139, 426−435. (8) Silva Valverde, M. F.; Schweyen, P.; Gisinger, D.; Bannenberg, T.; Freytag, M.; Kleeberg, C.; Tamm, M. Angew. Chem., Int. Ed. 2017, 56, 1135−1140. (9) Devillard, M.; Brousses, R.; Miqueu, K.; Bouhadir, G.; Bourissou, D. Angew. Chem., Int. Ed. 2015, 54, 5722−5726. (10) Rosenthal, A. J.; Devillard, M.; Miqueu, K.; Bouhadir, G.; Bourissou, D. Angew. Chem., Int. Ed. 2015, 54, 9198−9202. (11) Devillard, M.; Mallet-Ladeira, S.; Bouhadir, G.; Bourissou, D. Chem. Commun. 2016, 52, 8877−8880. (12) See Supporting Information for details. (13) Most a(11B) values reported for persistent boryl radicals fall in the range of 8.0 ± 0.5 G.2c,d,6,8 (14) Coupling to 31P and two H atoms indicates weak delocalization of the spin density over the phosphorus atom and the naphthyl moiety. (15) The hybrid functional B3PW91 with Grimme’s D3 dispersion corrections in the Becke−Johnson damping variant was used for all calculations associated with the 6-31G(d,p) basis set. The EPR data were computed using the GIAO formalism with the EPR-II basis set for H, C, and B atoms and the IGLO-II basis set for phosphorus. (16) The Lewis acidity of the corresponding P-stabilized boreniums, as evaluated by the Gutmann−Beckett method, was shown to decrease from Ph2P to iPr2P.11

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00598. Synthetic procedures; NMR spectra for compounds [2a]2, 3a,b, and 4a,b; EPR spectrum of 2a; theoretical details, geometrical structures, spin densities, and NBO analyses of 2a,b; relative stabilities of the different isomers of [2a]2; dimerization energy of the Gombergtype dimers [2a,b]2 (PDF) Cartesian coordinates of optimized structures (XYZ) Accession Codes

CCDC 1565498−1565502 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

Article

AUTHOR INFORMATION

Corresponding Authors

*G.B.: Web: http://lhfa.cnrs.fr/index.php/equipes/lbpb *K.M.: E-mail: [email protected]−tlse.fr *D.B.: E-mail:[email protected]−tlse.fr; Web: http://lhfa. cnrs.fr/index.php/equipes/lbpb ORCID

Eric-Daiann Sosa-Carrizo: 0000-0001-5177-8865 Didier Bourissou: 0000-0002-0249-1769 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The Centre National de la Recherche Scientifique (CNRS) and the Université Paul Sabatier (UPS) are acknowledged for financial support of this work. UPPA, MCIA (Mésocentre de Calcul Intensif Aquitain), and CINES under allocation 2017 (A002080045) made by Grand Equipement National de Calcul Intensif (GENCI) are acknowledged for computational facilities. E.-D. S.-C. thanks CDAPP for funding part of his postdoctoral contract. Lionel Rechignat (LCC Toulouse) is acknowledged for his assistance with the EPR measurements.

■

DEDICATION Dedicated to Professor Herbert W. Roesky. E

DOI: 10.1021/acs.organomet.7b00598 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics (17) Bis(benzazaborole) compounds have been recently reported to split upon heating and irradiation (C−C bond cleavage). The resulting transient radicals have been characterized by EPR: Hejda, M.; Lyčka, A.; Mikysek, T.; Jambor, R.; Růzǐ čka, A.; Vinklárek, J.; Wilfer, C.; Hoffmann, A.; Herres-Pawlis, S.; Dostál, L. Chem. - Eur. J. 2016, 22, 15340−15349. (18) Colle, T. H.; Glaspie, P. S.; Lewis, E. S. J. Org. Chem. 1978, 43, 2722−2725. (19) Lichtenberg, C.; Viciu, L.; Vogt, M.; Rodriguez-Lugo, R. E.; Adelhardt, M.; Sutter, J.; Khusniyarov, M. M.; Meyer, K.; de Bruin, B.; Bill, E.; Grützmacher, H. Chem. Commun. 2015, 51, 13890−13893. (20) (a) Stein, M.; Winter, W.; Rieker, A. Angew. Chem., Int. Ed. Engl. 1978, 17, 692−694. (b) Kahr, B.; Van Engen, D.; Mislow, K. J. Am. Chem. Soc. 1986, 108, 8305−8307. (c) Grimme, S.; Schreiner, P. R. Angew. Chem., Int. Ed. 2011, 50, 12639−12642. (21) Similarly short P−B and B−Cnapht bonds were observed in the Gomberg-type dimer of 2b: 1.933(6) and 1.463(7) Å, respectively.10 (22) Literature survey including a Cambridge database search gave no hint for the coupling of a trityl radical with TEMPO. This is probably due to electronic mismatch. (23) (a) Bontemps, S.; Devillard, M.; Mallet-Ladeira, S.; Bouhadir, G.; Miqueu, K.; Bourissou, D. Inorg. Chem. 2013, 52, 4714−4720. (b) Li, Y. F.; Kang, Y.; Ko, S. B.; Rao, Y.; Sauriol, F.; Wang, S. Organometallics 2013, 32, 3063−3068. (c) Beckmann, J.; Hupf, E.; Lork, E.; Mebs, S. Inorg. Chem. 2013, 52, 11881−11888. (24) Devillard, M.; Bouhadir, G.; Mallet-Ladeira, S.; Miqueu, K.; Bourissou, D. Organometallics 2016, 35, 3788−3794. (25) (a) Walton, J. C.; Brahmi, M. M.; Fensterbank, L.; Lacôte, E.; Malacria, M.; Chu, Q.; Ueng, S. H.; Solovyev, A.; Curran, D. P. J. Am. Chem. Soc. 2010, 132, 2350−2358. (b) Walton, J. C.; Brahmi, M. M.; Monot, J.; Fensterbank, L.; Malacria, M.; Curran, D. P.; Lacôte, E. J. Am. Chem. Soc. 2011, 133, 10312−10321. (26) Scheschkewitz, D.; Amii, H.; Gornitzka, H.; Schoeller, W. W.; Bourissou, D.; Bertrand, G. Science 2002, 295, 1880−1881. (27) Amii, H.; Vranicar, L.; Gornitzka, H.; Bourissou, D.; Bertrand, G. J. Am. Chem. Soc. 2004, 126, 1344−1345.

F

DOI: 10.1021/acs.organomet.7b00598 Organometallics XXXX, XXX, XXX−XXX