Facile Cleavage of the P=P Double Bond in Vinyl

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International Edition: DOI: 10.1002/anie.201812592 German Edition: DOI: 10.1002/ange.201812592

Organophosphorus Chemistry

Facile Cleavage of the P=P Double Bond in Vinyl-Substituted Diphosphenes Liu Leo Liu, Levy L. Cao, Jiliang Zhou, and Douglas W. Stephan* Abstract: The reactions of the cyclic alkyl amino carbene (CAAC) 1 with phosphaalkynes generate the kinetically unstable CAAC-derived phosphirenes 4 and 5, which undergo rearrangement/dimerization reactions to give the vinyl-substituted diphosphenes 2, 3, and 6. The P=P double bond scission of 2 or 3 is unprecedentedly effected by S8, [AuCl(tht)], or MeOTf at room temperature, which affords a dithiophosphorane 7, a phosphepine Au complex 8, or phosphepinium cations 9 and 10, respectively. The cationic species feature little homoaromaticity while representing the first examples of the phosphorus-containing analogue of the tropylium ion.

Unsaturated organic compounds containing C=C double

bonds are pervasive in organic chemistry, whereas the heavier Group 15 element double bonded compounds, namely dipnictenes (R-Pn=Pn-R, Pn = P, As, Sb, Bi), are far less common.[1] Such dipnictenes, exemplified by YoshifujiQs milestone diphosphene Mes*P=PMes* (Mes* = 2,4,6-tBu3C6H2),[2] have been shown to feature a conventional double bond between the pnictogen (Pn) atoms with s- and pcomponents (Figure 1 a), and their reactivity pattern resembles that of olefins.[3] The smaller HOMO–LUMO gap of dipnictenes compared to that of olefins results in unique photophysical properties, leading to various applications ranging from synthetic chemistry to material science.[3a,b, 4] While conjugated olefin systems have been extensively explored for decades,[5] alkenyl- or alkynyl-substituted dipnictenes are hitherto unknown. Synthetic approaches to dipnictenes remain quite limited.[3a,b, 6] In the case of diphosphenes, the most commonly used strategies for the preparation of diphosphenes involve the reduction or photolysis of suitable precursors, including P-substituted dichlorophosphines,[2, 7] phospha-Wittig reagents[8] and (phosphanyl)phosphaketene.[9] To date, there is only one example of a dimetalladiphosphene prepared by a rearrangement reaction of a metallaphosphaketene described by Grgtzmacher, Bertrand et al.[10] These stand in remarkable contrast to numerous synthetic methods involving rearrangement reactions for constructing C=C double bonds.[11] The cleavage and redistribution of a C=C double bond are of paramount significance in organic synthesis.[12] However, [*] Dr. L. L. Liu, L. L. Cao, Dr. J. Zhou, Prof. Dr. D. W. Stephan Department of Chemistry University of Toronto 80 St. George Street, Toronto, Ontario, M5S3H6 (Canada) E-mail: [email protected] Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.1002/anie.201812592. Angew. Chem. Int. Ed. 2019, 58, 273 –277

Figure 1. a) Isolobal relationship between olefins and dipnictenes. b) A transient vinylphosphinidene. c) A stable DAC-derived 2H-phosphirene. d) Present work.

the scission of P=P bonds has been seldom encountered. Although a variety of transition-metal diphosphene complexes featuring h1- or h2-type coordination modes have been explored,[1a,b, 6] there are hitherto only two examples of scission of a P=P double bond in R@P=P@R (R = Ph, Mes* or 2,4,6-CF3-C6H2) by multi-nuclear transition-metal carbonyl complexes at elevated temperature.[13] In 2017, Matsuo, Hatanaka and co-workers described a facile cleavage of a P=P double bond of bulky diphosphenes (Rind)P=P(Rind) (Rind = 1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacen-4-yl) employing N-heterocyclic carbenes (NHCs) via a highly polarized diphosphene intermediate.[14] More recently, Jana, Chandrasekhar, Scheschkewitz, Yildiz et al. reported the reactivity enhancement of (TerMes)P=P(TerMes) (TerMes = 2,6Mes2-C6H3) upon NHC coordination.[15] In 2000, Hahn and co-workers reacted a N,N’-bis(2,2dimethylpropyl)-substituted NHC with tBuC/P or (iPr)2NC/ P to give a 1,2,4-triphosphole or a 1,2,3-triphosphetene derivative, respectively.[16] These reactions proceed via a transient vinylphosphinidene intermediate (Figure 1 b). In a similar fashion, Hahn et al. also described the reaction of N,N’bismethyl-substituted NHC with (iPr)2NC/P.[17] We recently prepared a stable 2H-phosphirene via a [1+ +2] cycloaddition

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Communications reaction of an electrophilic diamidocarbene (DAC, 1,3dimesityl-5,5-dimethyl-4,6-dioxo-3,4,5,6-tetrahydropyrimidin-1-ium-2-ide)[18] with tBuC/P (Figure 1 c).[19] This phosphirene undergoes diverse rearrangements via the phosphirene ring opening.[19, 20] Herein, we report the synthesis, characterization, and reactivity of the first examples of vinyl-substituted diphosphenes 2, 3, and 6 via a rearrangement/dimerization reaction of the short-lived cyclic alkyl amino carbene (CAAC) derived phosphirenes 4 and 5 (Figure 1 d). These dimerization reactions represent the first examples of rearrangement reactions leading to organic diphosphenes. Reactivity studies of 2 or 3 toward S8, [AuCl(tht)] or MeOTf show that the P=P double bond is quite labile, allowing for the synthesis of a series of novel vinylsubstituted phosphorus derivatives. While the phosphirene was isolable using a DAC skeleton, we speculated that a corresponding singlet phosphriene would dimerize if the p-accepting ability of the carbene center and the steric hindrance of the substituents were suitably reduced. To test this, we exploited CAACs discovered by Bertrand et al.[21] Treatment of the EtCAAC 1 with tBuC/P in a molar ratio of 1:1 in pentane at room temperature immediately gave rise to a red solution (Scheme 1 a). After stirring for 3 h, a new species 2 was isolated as a reddish powder in 74 % yield. The 31P NMR spectroscopy of 2 showed a broad singlet resonance at 530.1 ppm in CDCl3, which is significantly downfield shifted compared to that of tBuC/P (@69.2 ppm). This diagnostic resonance is comparable to those observed for carbon-substituted symmetrical transdiphosphenes (477.0–598.6 ppm).[6a] In addition, the 13C NMR spectroscopy of 2 revealed a pseudo-triplet at 124.4 ppm (JC-P = 26 Hz) assigned to P-bound vinyl carbon atoms. These data indicated the formation of a vinyl-substituted diphosphene 2. The analogous reaction of 1 with AdC/P (Ad = adamantyl) gave a diphosphene 3 as a pink powder in 68 % yield (31P NMR: 524.0 ppm).

Scheme 1. a) Synthesis of symmetrical diphosphenes 2 and 3 via the dimerization reaction of 4 and 5, respectively. b) Formation of the unsymmetrical diphosphene 6.

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The structures of 2 and 3 were unambiguously confirmed by single crystal X-ray diffraction (Figure 2). In the case of 2, the P(1)-P(1a) bond length (2.0453(6) c) is in the typical range of diphosphenes.[6a] The bond lengths of C(1)-P(1), C(1)-C(2), and C(2)-N(1) are 1.854(2) c, 1.372(2), and 1.433(2) c, respectively, while the observed C(1)-P(1)-P(1a) angle is 104.45(5)88. In the case of 3, the structural parameters (Figure 2) are comparable to those of 2.

Figure 2. POV-ray depiction of the X-ray structures of 2 (a) and 3 (b). C black, N blue, P orange. Hydrogen atoms are omitted for clarity. Selected distances [b] and angles [88]: 2: P(1)–P(1a) 2.0453(6), C(1)– P(1) 1.854(2), C(1)–C(2) 1.372(2), C(2)–N(1) 1.433(2); C(1)-P(1)-P(1a) 104.45(5), C(2)-C(1)-P(1) 116.7(1). 3: P(1)–P(2) 2.041(1), C(1)–P(1) 1.864(3), C(1)–C(2) 1.372(4); C(1)-P(1)-P(2) 105.9(1), C(2)-C(1)-P(1) 117.8(2).

Monitoring the reaction of 1 with an equal molar portion of tBuC/P or AdC/P in toluene at @20 8C by 31P NMR spectroscopy showed the initial formation of an intermediate 4 (199.8 ppm) or 5 (194.7 ppm) that smoothly convert into 2 or 3, respectively (Scheme 1 a). The composition of 4 and 5 as (EtCAAC)(tBuCP) and (EtCAAC)(AdCP), respectively, were evidenced by high-resolution mass spectrometry studies. Moreover, the observed 31P NMR resonances of 4 and 5 are in excellent agreement with the calculated values (4: 197.6 ppm; 5: 194.1 ppm; See the Supporting Information for details). These data support generation of the kinetically unstable phosphirenes 4 and 5 followed by rapid dimerization reactions to give diphosphenes. Interestingly, upon mixing 1, tBuC/P and AdC/P in a molar ratio of 2:1:1, an unsymmetrical divinyl diphosphene 6 (31P NMR: d = 545.6 and 521.7 ppm, JPP = 630 Hz) was formed in 46 % yield along with lesser quantities of 2 (28 % yield) and 3 (26 % yield) (Scheme 1 b). The combination of 2 and 3 showed no evidence of conversion into 6 over 5 days at room temperature, indicating that the phosphirene dimerization reactions are kinetically irreversible. The bonding scenario in 3 was also investigated by DFT calculations (M06-2X/Def2-SVP). The HOMO@2 (@6.99 eV) primarily involves the p orbitals of the P=P double bond


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(Figure 3 a), while the HOMO (@5.88 eV) corresponds to the non-bonding lone pairs on P and the p orbitals of the vinyl substituents (Figure 3 b). The LUMO (@0.87 eV) predominantly exhibits features of p* orbitals of the P=P double bond (Figure 3 c), while LUMO + 1 (0.48 eV) displays p* orbitals of the vinyl and Dipp (2,6-diisopropylphenyl) substituents (Figure 3 d).

of 2 and 3. The small HOMO–LUMO gaps in 2 and 3 indicate that these species should be highly reactive. Generally, sulfurization reactions of aryl-substituted diphosphenes are base-promoted and give diphosphene monosulfides that transform into thiadiphosphiranes upon photolysis or thermolysis.[23] In the absence of base, no reaction is observed even under severe conditions.[23b] In sharp contrast, compound 3 reacts with S8 at room temperature to afford a dithiophosphorane 7 quantitatively (31P NMR: 283.2 ppm) (Scheme 2). The formulation of 7 was further confirmed crystallographically (Figure 5 a), which represents the first example of a non-aryl-substituted dithiophosphorane.[24] The formation of this vinyl dithiophosphorane is unique as the corresponding reaction of the DAC-derived phosphirene (Figure 1 c) with S8 did not react. The slow addition of 2 equivalents of [AuCl(tht)] to a CH2Cl2 solution of 3 at room temperature rapidly forms a yellow solution from which species 8 was isolated in 81 % yield (Scheme 2). The CDCl3 solution of 8 revealed a 31P NMR spectrum consisting of a doublet of doublets (d = 34.7 ppm, dd, JPH = 34 and 11 Hz), which collapses into a singlet upon proton decoupling. Single-crystal X-ray diffraction of yellow Figure 3. a) HOMO@2 of 3. b) HOMO of 3. c) LUMO of 3. d) LUMO + 1 of 3. crystals of 8 showed the P=P double bond was completely cleaved by the coordination of AuCl and Compounds 2 and 3 exhibit two absorption maxima in the the resulting phosphinidene inserted into an aromatic C@C UV/Vis spectra in THF at 443 and 564 nm or 430 and 560 nm, bond of the Dipp group (Figure 5 b). Such P=P bond cleavage respectively. These absorptions were attributed to the s-p* is reminiscent of the splitting of a C=C bond in an enetetrand p–p* transitions according to TD-DFT calculations amine by [Mo(nor)(CO)4] (nor = norbornadiene) reported by (Figures S36 and S37). Of the known carbon-substituted Hahn et al.[25] It is noteworthy that this reaction is the first symmetrical diphosphenes (Table S3), compound 2 features example of a P=P double bond cleavage by a mono-nuclear the narrowest known HOMO–LUMO gap (4.93 eV), while transition-metal complex. This reactivity stands in contrast to the HOMO energy (@5.88 eV) of 3 is the highest (Figure 4). the 2009 report from Protasiewicz et al. who described monoInterestingly, the corresponding HOMO–LUMO gaps of two or di-auration of Mes*P=PMes*.[26] additional model species, one in which the olefinic fragment is The reaction of 2 or 3 with excess MeOTf (10 equivalents) reduced, 3 a (5.72 eV), and one in which the nitrogen atom in was examined at room temperature and provided species 9 or the CAAC is replaced by a quaternary carbon, 3 b (5.48 eV), 10, respectively (Scheme 2). This was evidenced by a doublet were computed to be much larger than those of 2 and 3. These of pseudo-quintets at 38.6 (9: JPH = 41 and 13 Hz) or 38.7 (10: data suggest that the strong p-donating N-heterocyclic olefin JPH = 42 and 13 Hz) ppm in the 31P NMR spectrum, respec[22] (NHO) substituents efficiently boost the HOMO energies tively, which collapses into a singlet upon proton decoupling. Recrystallization of 10 from chlorobenzene/pentane mixture

Figure 4. Energy [eV] of the HOMO and LUMO of 2, 3, 3 a, and 3 b. Angew. Chem. Int. Ed. 2019, 58, 273 –277

Scheme 2. Reactivity of 2 or 3 toward S8, [AuCl(tht)], and MeOTf. DCM = CH2Cl2.


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C6P ring of 10 features little six-electron homoaromaticity,[30] as suggested by the computed nucleus independent chemical shift (NICS) values (NICS(0): @4.5 ppm; NICS(1): @6.4 ppm) (M06-2X/Def2-TZVP).[31] For comparison purpose, the calculated NICS(0) and NICS(1) for the planar Hgckel aromatic parent tropylium ion are @5.0 and @9.1 ppm, respectively. These computations are consistent with the observed downfield shifts of the three protons on the C6P rings (above 7 ppm) in both 9 and 10. In summary, we have reported the first examples of vinylsubstituted symmetrical diphosphenes 2 and 3, as well as an unsymmetrical diphosphene 6. These diphosphenes feature small HOMO–LUMO gaps, showcasing the impact of the NHO substituents. This impact is also reflected in the reactivity of 2 or 3 toward S8, [AuCl(tht)] and MeOTf which proceeds with the facile P=P bond cleavage at room temperature. Notably, compounds 9 and 10, the first phosphoruscontaining analogues of tropylium ions, feature little homoaromaticity. Studies on the reactivity of 2 and 3 targeting the further elaboration of these vinyl-substituted phosphorus derivatives are the subject of ongoing research.

Acknowledgements D.W.S. gratefully acknowledges the financial support from NSERC Canada and the award of Canada Research Chair. D.W.S. is also grateful for the award of an Einstein Fellowship at TU Berlin. Prof. C. A. Russell at University of Bristol is thanked for providing tBuC/P. Figure 5. POV-ray depiction of the X-ray structures of 7 (a), 8 (b), and the cation of 10 (c). C black, N blue, P orange, S yellow, Cl aquamarine, Au gold. Hydrogen atoms and triflate anion are omitted for clarity.

gave yellow single crystals suitable for X-ray diffraction, revealing a system containing a cationic C6P seven-membered ring (Figure 5 c). The C6P seven-membered ring appears to be a boat conformer, while the phosphonium cation adopts a pyramidalized phosphorus center. The dihedral angles of C(12)-C(13)-C(14)-C(15) and C(14)-C(15)-C(16)-C(17) are @21.4(5) and 32.0(5)88, respectively. It is noteworthy that the DAC-derived phosphirene (Figure 1 c) does not react with MeOTf. As a point of comparison, we note that, in 1999, Grgtzmacher et al. described the methylation of Mes*P= PMes* to give a stable phosphanyl phosphenium salt [Mes*P=P(Mes*)-Me][OTf].[27] Calculations showed that double protonation of HP=PH to generate the dication [H2P=PH2]2+ is thermodynamically predisposed towards dissociation to H2P+ as a result of Coulombic explosion.[27, 28] Presumably, the higher HOMO energies of 2 (@5.93 eV) and 3 (@5.88 eV) compared to that of Mes*P=PMes* (@6.85 eV) prompts a similar dissociation to transient phosphenium cations which effect the observed ring expansion. This mechanism is supported by DFT calculations (Figure S38). The cationic C6P seven-membered rings of 9 and 10 can be considered as the phosphorus-containing analogues of the tropylium ion.[29] While the NBO analysis shows the P center of 10 carries a positive charges of 1.59 a.u., the non-planar

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Conflict of interest The authors declare no conflict of interest. Keywords: diphosphenes · N-heterocyclic olefins · P=P bonds · phosphinium · phosphirene How to cite: Angew. Chem. Int. Ed. 2019, 58, 273 – 277 Angew. Chem. 2019, 131, 279 – 283

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Manuscript received: November 2, 2018 Accepted manuscript online: November 16, 2018 Version of record online: December 6, 2018


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