Transient phosphenium and arsenium ions, generated by fluoride abstraction from bis(m-terphenyl)fluoropnictogens, underwent intramolecular electrophilic attack prior to methyl group migration and gave rise to isolable 9-phospha- and 9-arsena-fluorenium ions

Beckmann, Jens

Duvinage, Daniel

Malaspina, Lorraine A.

Mebs, Stefan

Nass, Yannik

Olaru, Marian

English

Institutional Repository of the Freie Universität Berlin

Pnictogenium ionsIntramolecular Reaction of Transient Phosphenium and Arsenium IonsGiving Rise to Isolable 9-Phospha- and 9-Arsena-Fluorenium IonsMarian Olaru, Daniel Duvinage, Yannik Naß, Lorraine A. Malaspina, Stefan Mebs, andJens Beckmann*In memory of Professor Robert R. HolmesAbstract: Transient phosphenium and arsenium ions, gener-ated by fluoride abstraction from bis(m-terphenyl)fluoropnic-togens, underwent intramolecular electrophilic attack prior tomethyl group migration and gave rise to isolable 9-phospha-and 9-arsena-fluorenium ions.Arenium ions are well-recognized key intermediates ofelectrophilic aromatic substitution reactions.[1] Althoughoften referred to as Wheland intermediates due to his seminaltheoretical publication of 1942,[2] the interest in arenium ionsdates back earlier to pioneering experimental work byPfeiffer and Wizinger, published already in 1928.[3] Over theyears a number of arenium ions have been isolated and fullycharacterized (Scheme 1). For instance, the protonation ofbenzene provided the parent benzenium ion [C6H7]+ (I).[4]Starting from hexamethylbenzene, substituted hexamethyl-benzenium ions [C6Me6R]+ (II, R = Me,[5] Cl,[6] Br[7]) wereobtained. Attempts to afford silyl cations [R3Si]+ have led tothe isolation of silyl-substituted toluenium ions[C6H5MeSiR3]+ (III, R = Me, Et).[8]In their native form, phosphenium ions [R2P]+ andarsenium ions [R2As]+ are highly reactive six-valence-elec-tron species that have never been isolated without the supportof electron-rich ligands.[9] Our efforts to prepare kineticallystabilized pnictogenium ions [(2,6-Mes2C6H3)2E]+[10] by fluo-ride abstraction from bis-m-terphenylpnictogen fluorides(2,6-Mes2C6H3)2EF (E = P, As) yielded 1,2,4-trimethyl-6-mesityl-5-m-terphenylbenzo[b]phospholium and -arsoliumions instead.[11] The formation of these protonated phospholeand arsole structures involved a methyl group migration fromthe ortho-position of a flanking mesityl group to the meta-position prior to a hydrogen transfer from that position tophosphorus and arsenic. For the mechanism of this rearrange-ment we proposed transient short-lived arenium ions. Wefurther concluded that reactive hydrogen atoms need to beavoided in the m-terphenyl substituents in order to isolatekinetically stabilized phosphenium and arsenium ions by thisroute.[11]We have therefore adjusted our strategy and turned ourattention to a related m-terphenyl substituent in which thetwo flanking aryl groups are permethylated. Starting from 2,6-(Me5C6)2C6H3Li[12] and ECl3 (E = P, As), we have nowprepared the bis(m-terphenyl)fluoropnictogens (2,6-{Me5C6}2C6H3)2EF (1a, E = P; 1b, E = As). The synthesis,solution NMR, and solid-state structural features of 1a and1b resemble those of (2,6-Mes2C6H3)2EF (E = P,[13] As[11])reported previously. In solution, 1a and 1b are characterizedby 19F NMR chemical shifts (CD2Cl2) at d =200.3 (E = P)and 216.0 ppm (E = As), while 1a gives a 31P NMR signal atd = 193.8 ppm. Fluoride ion abstraction from 1a and 1b usingEtAlCl2 and AlCl3, respectively, in CH2Cl2 proceeded invirtually quantitative yield to afford two-isomer mixtures ofphospha-fluorenium ions [5a]+ and [7a]+ (ca. 0.65:0.35 molarratio) and arsa-fluorenium ions [5 b]+ and [7b]+ (ca. 0.70:0.30molar ratio), species isolated as dark-red crystalline solids(Figure 1). The 31P NMR spectrum (CD2Cl2) showing twoclose resonances at d =26.9 ppm for [5 a]+ and 22.2 ppmfor [7a]+ confirmed that phosphorus was in a very similarcoordination environment for both isomers. The 1H NMRspectra of these mixtures were complex; as a consequence ofcomplete desymmetrization of the molecules, each speciesdisplayed six signals assigned to the aromatic protons andindividual resonances for all methyl groups in the aliphaticarea which only overlapped by chance. The similar distribu-tion pattern of the resonances and of the coupling constantsindicated that the major and minor species were closelyrelated isomers. In both the phosphorus and arsenic cases, the1H NMR spectra displayed sharp signals with virtually nochange when measured at lower temperatures, signalingScheme 1. Examples of previously known arenium ions.[*] Dr. M. Olaru, D. Duvinage, Y. Naß, Dr. L. A. Malaspina,Prof. Dr. J. BeckmannInstitut fr Anorganische Chemie und KristallographieUniversitt BremenLeobener Straße 7, 28359 Bremen (Germany)E-mail: j.beckmann@uni-bremen.deDr. S. MebsInstitut fr Experimentalphysik, Freie Universitt BerlinArnimallee 14, 14195 Berlin (Germany)Supporting information and the ORCID identification number(s) forthe author(s) of this article can be found under:https://doi.org/10.1002/anie.202006728. 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co.KGaA. This is an open access article under the terms of the CreativeCommons Attribution License, which permits use, distribution andreproduction in any medium, provided the original work is properlycited.AngewandteChemieCommunicationsHow to cite: Angew. Chem. Int. Ed. 2020, 59, 14414–14417International Edition: doi.org/10.1002/anie.202006728German Edition: doi.org/10.1002/ange.20200672814414  2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2020, 59, 14414 –14417configurationally stable structures. NOESY spectra did notreveal any exchange between the major and the minor speciesin either phosphorus or arsenic cases, even at very shortmixing times (see the Supporting Information).[15] Theidentities of the major phosphole [5a]+ and arsole [5b]+components of the mixtures were confirmed by single-crystalX-ray diffraction (Figure 1),[14] and after the full NMRassignment it was clear that the solid-state structures wereretained in solution. The molecular structures of [5a]+ and[5b]+ reveal that after abstraction of the fluoride ion, thecorresponding pnictogenium ions [(2,6-{Me5C6}2C6H3)2E]+([2a]+, E = P; [2b]+, E = As) immediately underwent intra-molecular electrophilic substitution, which caused the dis-placement of one of the original ortho-methyl groups ofa flanking C6Me5 group and formation of arenium ions. This isreminiscent of similarly behaving unstable cations [(2,6-Mes2C6H3)2E]+ (E = P, As), which were previously shown toundergo a 1,2-methyl shift, resulting in formation of proton-ated phospholium and arsolium ions.[11] The molecularstructures of [[5a] and [5b]+ compare well with those of therelated phospholes and arsoles,[11] with very similar bondlengths and angles around the pnictogen atoms. The dearom-atized rings in [5a]+ and [5b]+ contain two single CC bonds(C22,24C23 1.499(3)  for both [5a] and [5b]+) while therest of the CC bond lengths range between 1.359(3)  (C21C22, [5a]+) and 1.436(2)  (C20C25, [5b]+). Despitesignificant efforts we could not obtain suitable single crystalsof [7a]+ and [7b]+ but the solution structures were unambig-uously inferred from NMR spectroscopy and identified aspositional isomers resulting from the migration of thedisplaced methyl group in a position ortho relative to thecentral aromatic ring.In dichloromethane solution the mixture of [5a]+ and[7a]+ underwent a slow reaction in which a methyl groupmigrated to phosphorus, over the course of ca. one month,yielding [8a]+ as the major product.[16] Such a reaction did notFigure 1. Fluoride abstraction from bis(m-terphenyl)element fluorides 1a and 1b and subsequent reactions of arenium ions [5a]+ and [5b]+ aswell as [7a]+ and [7b]+. Molecular structures of [5a]+, [5b]+, [8a]+, 9a, and 9b showing 30% probability ellipsoids and the essential atomnumbering scheme. For clarity, the counter ions of [5a]+ ([0.75·AlCl4 + 0.25·EtAlCl3]), [5b]+ ([AlCl4]), and [8a]+ ([AlCl4]) were omitted from boththe reaction scheme and structure depictions. Selected bond parameters [, 8] of [5a]+: P1-C10 1.831(2), P1-C21 1.836(2), P1-C60 1.861(2), C20-C21 1.432(3), C21-C22 1.359(3), C22-C23 1.500(3), C24-C23 1.499(3), C25-C24 1.370(3), C20-C25 1.429(3), C10-P1-C21 88.40(8), C10-P1-C60104.77(8), C21-P1-C60 107.63(8), C28-C23-C27 108.69(16). Selected bond parameters [, 8] of [5b]+: As1-C10 1.950(1), As1-C21 1.956(1), As1-C601.982(1), C20-C21 1.432(2), C20-C25 1.436(2), C21-C22 1.362(2), C22-C23 1.499(2), C24-C23 1.499(2), C25-C24 1.376(2), C10-As1-C21 84.64(5),C10-As1-C60 103.21(5), C21-As1-C60 104.80(5), C27-C23-C28 109.12(11). Selected bond parameters [, 8] of [8a]+: P1-C1 1.7962(9), P1-C101.7997(8), P1-C21 1.7825(8), P1-C60 1.8269(8), C21-P1-C1 110.38(4), C21-P1-C10 93.86(4), C1-P1-C10 109.84(4), C21-P1-C60 114.40(4), C1-P1-C60114.96(4), C10-P1-C60, 111.45(4). Selected bond parameters [, 8] of 9a : C10-P1 1.836(2), C21-P1 1.824(2), C60-P1 1.865(2), C20-C21 1.352(3),C21-C22 1.459(3), C22-C26 1.340(3), C22-C23 1.469(3), C23-C24 1.348(3), C24-C25 1.518(3), C20-C25 1.520(3) C21-P1-C10 89.78(9), C21-P1-C60105.15(8), C10-P1-C60 111.59(8), C29-C25-C30 109.17(17). Selected bond parameters [, 8] of 9b : As1-C10 1.970(1), As1-C21 1.946(2), As1-C602.004(1), C21-C20 1.356(2), C21-C22 1.465(2), C22-C26 1.360(2), C22-C23 1.460(2), C23-C24 1.352(3), C25-C24 1.518(3), C20-C25 1.522(2), C21-As1-C60 103.53(6), C21-As1-C10 85.58(6), C10-As1-C60 106.33(6), C29-C25-C30 110.6(1).AngewandteChemieCommunications14415Angew. Chem. Int. Ed. 2020, 59, 14414 –14417  2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.orgtake place for [5b]+ and [7b]+ even after six months. Themethyl-phospholium ion [8a]+ gave a 31P NMR resonancesignal at d = 21.7 ppm. Inspection of its molecular structurereveals that upon methylation of phosphorus, the C6Me4 ringrearomatized. The coordination geometry around the phos-phorus atom is disordered tetrahedral with bond lengths andangle values that are remarkably close to those of the simplestcongener 2,2’-biphenylenemethylphenylphosphonium iodidedespite the steric bulk of the substituents in [8a]+.[17] Withtetrahydrofuran, mixtures of [5a]+ and [7a]+, as well as [5b]+and [7b]+, reacted within minutes to afford 9a and 9b,respectively, as the major species, as a result of a methyl groupdeprotonation in trans-position of the dimethyl group.[18] Thistransformation took place in a seemingly complex equilibri-um; the reversal of the reaction by evaporation of THF gave,aside from [5a]+ and [7a]+, as well as [5b]+ and [7b]+,a complex mixture of by-products. In solution, 9a is charac-terized by a 31P NMR chemical shift ([D8]THF) of d =24.2 ppm. The molecular structure of neutral species 9a(and similarly that of 9b) shows that the dearomatized C20–C25 ring contains two parallel double bonds (C23C241.348(3) and C21C20 1.352(3) ) and four single bondswith CC distances ranging between 1.459(3) (C21C22) and1.520(3)  (C20C25). The C22C26 distance (9a : 1.340-(3) ; 9b : C22C26 1.360(2) ) is consistent with a doublebond character.From the very different structures of the targetedpnictogenium ions [2a]+ and [2 b]+ and the actually isolated9-pnictogena-fluorenium ions [5a]+ and [5b]+ it becomesobvious that the mechanism of the rearrangement involvedunobserved cationic intermediates. In an effort to shed somelight on the possible mechanism of the rearrangement, DFTcalculations were carried out at the B3PW91/6-311 + G(d)level of theory, whereby the relative energies of the isolatedarenium ions [5a]+ and [5b]+ were arbitrarily set to 0 kJmol1(Figure 2). The energetically least favorable cation is theinitially formed phosphenium ion [2a]+, which is 90.9 kJmol1higher in energy. Notably, all attempts to geometricallyoptimize the arsenium ion [2b]+ failed and gave the productof the electrophilic attack at the ortho-position, namely, thearenium ion [3b]+, which is 3.7 kJmol1 more stable than theisolated [5b]+. The related arenium ion [3a]+ is 82.4 kJmol1more stable than the phosphenium ion [2a]+, but 8.5 kJmol1less stable than reference [5a]+. From the arenium ions [3a]+and [3b]+, formal 1,2-, 1,3-, 1,4-, and 1,5-methyl shifts give riseto the conceivable arenium ions [4a]+ and [4b]+ (notobserved), [5a]+ and [5b]+ (observed), [6a]+ and [6b]+ (notobserved), as well as [7a]+ and [7 b]+ (observed), whereby theFigure 2. Suggested mechanism of the rearrangement, including relative energies of the pnictogenium ions [2a]+, the 9-pnictogena-fluoreniumions [5a]+ and [5b]+ as well as [7a]+ and [7b]+ and the phospholium ion [8a]+. Attempts to optimize [2b]+ always resulted in the conversion into[3b]+.AngewandteChemieCommunications14416 www.angewandte.org  2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2020, 59, 14414 –14417relative energies correctly reflect the experimental observa-tions. Mechanistically, the circumambulation of the methylgroups might involve intermediate bicycle[3.1.0]hexenyl cat-ions as suggested by Childs and Winstein for the parentheptamethylbenzenium ions [C6Me7]+.[19] This might alsoexplain why the global energetic minimum, the phospholiumion [8a]+, which is 57.1 kJmol1 more stable than thereference [5a]+, is not immediately formed from [3 a]+,despite the fact that the volatile methyl group is already inclose proximity to the phosphorus atom. In the same contextit is surprising that the related arsonium ion [8b]+ is notexperimentally observed, although the energy gain relative toreference [5b]+ would be 106.5 kJ mol1. In light of theseenergy values, the observed slow transfer of the methyl groupto the phosphorus might occur by an intermolecular transfermechanism, perhaps involving traces of aluminum species astransfer reagents. Overall, the rearrangement of the phos-phenium ion [2a]+ to the methyl-phospholium ion [8a]+ isenergetically favored by 248.0 kJmol1. This might beexplained by the fact that the permethylphenyl group iselectron-richer than the mesityl group, which makes it moresusceptible to an electrophilic attack. Therefore, this valueeven exceeds the energy gain (209.6 kJmol1) associated withthe rearrangement of the previously studied [(2,6-Mes2C6H3)2P]+ into the related protonated 9-phospha-fluo-rene.[11]In summary, attempts to prepare the kinetically stabilizedphosphenium and arsenium ions [(2,6-{Me5C6}2C6H3)2E]+ (2a,E = P; 2b, E = As) lead to the isolation of two-isomermixtures of phospha-fluorenium ions [5a]+ and [7a]+ (ca.0.65:0.35 molar ratio) and arsena-fluorenium ions [5b]+ and[7b]+ (ca. 0.70:0.30 molar ratio), respectively. Compared tothe methyl-phospholium ion [8a]+, the arenium ions [5a]+and [7a]+ are high-energy species; their conversion is slowenough (ca. one month) on the laboratory time scale, whichenabled their full characterization. Although thermodynami-cally also favored, the conversion of the arsena-fluoreniumions [5 b]+ and [7 b]+ into the elusive methyl-arsolium ion[8b]+ is not observed.AcknowledgementsThe Deutsche Forschungsgemeinschaft (DFG) is gratefullyacknowledged for financial support. We thank JohannesStelten for his help with the acquisition of some NMR data.Open access funding enabled and organized by ProjektDEAL.Conflict of interestThe authors declare no conflict of interest.Keywords: arenium ions · arsoles · m-terphenyl compounds ·phospholes · pnictogenium ions[1] D. Lenoir, Angew. Chem. Int. Ed. 2003, 42, 854 – 857; Angew.Chem. 2003, 115, 880 – 883.[2] G. W. Wheland, J. Am. Chem. Soc. 1942, 64, 900 – 908.[3] P. Pfeiffer, R. Wizinger, Liebigs Ann. Chem. 1928, 461, 132 – 154.[4] a) C. A. Reed, K.-C. Kim, E. S. Stoyanov, D. Stasko, F. S. Tham,L. J. Mueller, P. D. W. Boyd, J. Am. Chem. Soc. 2003, 125, 1796 –1804; b) F. Scholz, D. Himmel, L. Eisele, W. Unkrig, I. Krossing,Angew. Chem. Int. Ed. 2014, 53, 1689 – 1692; Angew. Chem.2014, 126, 1715 – 1718.[5] N. C. Baenziger, A. D. Nelson, J. Am. Chem. Soc. 1968, 90,6602 – 6607.[6] R. Rathore, J. Hecht, J. K. Kochi, J. Am. Chem. Soc. 1998, 120,13278 – 13279.[7] A. V. Vasilyev, S. V. Lindeman, J. K. Kochi, New J. Chem. 2002,26, 582 – 592.[8] a) J. B. Lambert, S. Zhang, C. Stern, J. Huffman, Science 1993,260, 1917 – 1918; b) M. F. Ibad, P. Langer, A. Schulz, A.Villinger, J. Am. Chem. Soc. 2011, 133, 21016 – 21027.[9] a) A. H. Cowley, R. A. Kemp, Chem. Rev. 1985, 85, 367 – 382;b) D. Gudat, Coord. Chem. Rev. 1997, 163, 71 – 106; c) N.Burford, P. J. Ragogna, Dalton Trans. 2002, 4307 – 4315;d) A. P. M. Robertson, P. A. Gray, N. Burford, Angew. Chem.Int. Ed. 2014, 53, 6050 – 6069; Angew. Chem. 2014, 126, 6162 –6182; e) M. Olaru, A. Mischin, L. A. Malaspina, S. Mebs, J.Beckmann, Angew. Chem. Int. Ed. 2020, 59, 1581 – 1584; Angew.Chem. 2020, 132, 1597 – 1600.[10] M. Olaru, D. Duvinage, S. Mebs, J. Beckmann, Angew. Chem.Int. Ed. 2018, 57, 10080 – 10084; Angew. Chem. 2018, 130, 10237 –10241.[11] M. Olaru, D. Duvinage, E. Lork, S. Mebs, J. Beckmann, Chem.Eur. J. 2019, 25, 14758 – 14761.[12] S. Hino, M. M. Olmstead, J. C. Fettinger, P. P. Power, J. Organo-met. Chem. 2005, 690, 1638 – 1644.[13] M. Olaru, A. Schrçder, L. Albers, D. Duvinage, S. Mebs, J.Beckmann, Chem. Eur. J. 2019, 25, 9861 – 9865.[14] Deposition numbers 1993654 (1a), 1993655 [5a]+, 1993656 [5b]+,1993657 [8a]+, 1993658 (9a), and 1993659 (9b) contain thesupplementary crystallographic data for this paper. These dataare provided free of charge by the joint Cambridge Crystallo-graphic Data Centre and Fachinformationszentrum KarlsruheAccess Structures service[15] This observation is in contrast to the heptamethylbenzenium ion[C6Me7]+, which undergoes a methyl shift making all sevenmethyl groups equivalent at sufficiently high temperatures. B. H.Meier, R. R. Ernst, J. Am. Chem. Soc. 1979, 101, 6441 – 6442.[16] The reaction appeared to be faster and cleaner when the solutionwas exposed to sunlight. A control reaction that was ran inparallel, in complete darkness, was significantly slower (see theSupporting Information).[17] P. Adkine, T. Cantat, E. Deschamps, L. Ricard, N. Mzailles, P.Le Floch, M. Geoffroy, Phys. Chem. Chem. Phys. 2006, 8, 862 –868.[18] The same regioselectivity was observed for the deprotonation ofheptamethylbenzenium ion and the protonation of 4-methylene-1,1,2,3,5,6-hexamethylcyclohexadiene-2,5; a) W. von Doering,M. Saunders, H. G. Boyton, H. W. Earhart, E. F. Wadley, W. R.Edwards, G. Laber, Tetrahedron 1958, 4, 178 – 185; b) H. Hart,P. M. Collins, A. J. Waring, J. Am. Chem. Soc. 1966, 88, 1005 –1013; c) M. Attina, F. Cacece, G. de Petris, S. Fornarini, P.Giacomello, J. Am. Chem. Soc. 1985, 107, 2297 – 2302.[19] R. F. Childs, S. Winstein, J. Am. Chem. Soc. 1974, 96, 6409 – 6417.Manuscript received: May 9, 2020Accepted manuscript online: May 27, 2020Version of record online: July 6, 2020AngewandteChemieCommunications14417Angew. Chem. Int. Ed. 2020, 59, 14414 –14417  2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org

Intramolecular Reaction of Transient Phosphenium and Arsenium Ions Giving Rise to Isolable 9-Phospha- and 9-Arsena-Fluorenium Ions

Transient phosphenium and arsenium ions, gener-ated by fluoride abstraction from bis(m-terphenyl)fluoropnic-togens, underwent intramolecular electrophilic attack prior tomethyl group migration and gave rise to isolable 9-phospha-and 9-arsena-fluorenium ion

Naß, Yannik

Bern Open Repository and Information System (BORIS)

Intramolecular Reaction of Transient Phosphenium and Arsenium Ions Giving Rise to Isolable 9‐Phospha‐ and 9‐Arsena‐Fluorenium Ions

https://boris.unibe.ch/154633/1/Malaspina_17.pdf

Intramolecular Reaction of Transient Phosphenium and Arsenium Ions Giving Rise to Isolable 9-Phospha- and 9-Arsena-Fluorenium Ions

Abstract

Similar works

Full text

Available Versions

Institutional Repository of the Freie Universität Berlin

Bern Open Repository and Information System (BORIS)