A Room-Temperature-Stable Phosphanorcaradiene - Journal of the

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A Room-Temperature-Stable Phosphanorcaradiene Liu Leo Liu, Jiliang Zhou, Ryan Andrews, and Douglas W. Stephan J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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Journal of the American Chemical Society

A Room-Temperature-Stable Phosphanorcaradiene Liu Leo Liu, Jiliang Zhou, Ryan Andrews and Douglas W. Stephan* Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ontario, Canada M5S3H6 Supporting Information Placeholder ABSTRACT: A room-temperature-stable crystalline phosphanorcaradiene 2 has been synthesized via a C-C bond forming strategy induced by the demetalation of a phosphepine-Au complex 1 using a Lewis base (Ph3P, IDipp or CyNC). Compound 2 undergoes photolysis to afford a 2H-phosphirene 4. Ru(PPh3)3Cl2 is shown to catalyze the equilibration of 2 and 4 in solution. In addition, the transformation of 2 to 4 can be achieved by a twostep strategy, namely cyclopropenylidene-induced ring opening to give cyclopropenylidene-stabilized vinylphosphinidene 7 followed by elimination of cyclopropenylidene by an electrophile including TMSOTf and Me2SBH3.

The concept of valence tautomerism, pioneered by Ingold in 1922,1 has continued to inspire, evolve and impact modern organic and organometallic chemistry for almost a century.2 Valence tautomerization involves the reorganization of bonding electrons via the rupture of single or double bonds without an atom or group migration. This phenomenon is pervasive in organic chemistry.2-3 One such example is the valence isomers of cycloheptatriene (CHT) (Figure 1a, 1-CR2),4 including norcaradienes (NCDs, 2-CR2) and norbornadienes (NBDs, 3-CR2). Such structures have been known since the 1880s,5 and the equilibria governing the valence tautomerism of CHTs and NCDs have been unambiguously confirmed and extensively explored.6 Nonetheless, much less is known about their heterocyclic derivatives. In case of analogs incorporating NR7 or S8, the monocyclic forms 1E are stabilized by bulky substituents or extended conjugation. One highly rigid derivative of 2-NR has been described.9 In the case of benzene oxide, 2-O is the thermodynamically stable tautomer, while entropy facilitates a shift towards oxepin 1-O at ambient temperatures.10 For PR derivatives, Märkl et al. reported the 2,7-dialkyl substituted11 and annelated12 phosphepines (Figure 1b, 1-PPh and 1-PPh’). Benzannulated phosphepines were investigated by the groups of Tsuchiya13 and Lammertsma.14 Bickelhaupt, Lammertsma et al. isolated the first 7phosphanorbornadiene 3-PPh with a highly strained skeleton,15 while Cummins et al. described related dibenzo-7phosphanorbornadiene compounds 3-PR (Figure 1b).16 In addition, Lammertsma et al. showed that 3H-benzo-phosphepine transition-metal complexes act as versatile phosphinidene precursors.17 However, there are no experimental reports describing phosphanorcaradiene 2-PR (Figure 1a).18 Indeed, such phosphanorbornadienes are thought to be highly unstable transient intermediates that rapidly dissociate into aromatic rings and phosphinidenes.12-13,17-19 Very recently, we described the first phosphorus analog of the Büchner ring expansion reaction, in which a transient phosphinidene-Au complex undergoes facile intramolecular phosphinidene-insertion into an aromatic C−C bond of a mesityl

group.20 In the present work, we report the synthesis and characterization of the first isolable phosphanorcaradiene 2 (Figure 1c) via an intramolecular C-C bond formation reaction. This species is shown to undergo rearrangements leading to the corresponding phosphepine, phosphinidene and phosphirene.

Figure 1. (a) Valence isomers of cycloheptatriene and the corresponding heterocyclic analogs. (b) Märkl’s phosphepines (1-PPh and 1-PPh’), Lammertsma’s 7-phosphanorbornadiene (3-PPh) and Cummins’ 7-phosphanorbornadienes (3-PR). (c) Present work. The interaction of Walsh molecular orbitals of the norcaradienecyclopropane ring with the substituents at C7 plays a crucial role in stabilizing the norcaradiene.21 In a related sense, Lammertsma and co-workers showed interaction of phosphanorcaradiene with W(CO)5 results in σ,π-interactions that weaken the distal C-C bond thus favoring the formation of phosphepine-W(CO)5.17b,18a This prompted us to question whether demetalation of phosphepine-AuCl complex 1 (Scheme 1)20 would lead to the tautomerism of the phosphepine and offer access to a free phosphanorcaradiene. To support this hypothesis, we initially carried out density functional theory (DFT) calculations at SMD-M06-2X/Def2TZVP//M06-2X/Def2-SVP level of theory (Figure 2). Removal of the metal center with a strong base would give rise to the intermediate IN1. Gratifyingly, IN1 is found to be kinetically and thermodynamically unstable in the absence of the coordination of Au at phosphorus and consequently will undergo facile tautomerism (TS1) with an activation barrier of 9.0 kcal/mol to produce the thermodynamically more stable phosphanorcaradiene 2 (-7.5 kcal/mol). Further rearrangement (TS2) to the phosphirene 4 is kinetically unfavorable with an activation barrier of 46.4 kcal/mol. Encouraged by these computational results, the synthesis of 2 was targeted (Scheme 1). Indeed, treatment of 1 with an equimolar portion of a Lewis base including Ph3P, IDipp (1,3-bis(2,6-

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diisopropylphenyl)imidazol-2-ylidene) or CyNC (Cy = cyclohexyl) gave rise to the generation of the same new species 2 within seconds. This was evidenced by a sharp singlet appearing in the 31 P NMR spectra (-177.4 ppm in C7D8; -176.4 ppm in CD2Cl2). This is similar to chemical shifts previously observed for trivalent phosphiranes22 but not for phosphepines11-14,23. Concurrently, the known species Ph3PAuCl, IDippAuCl and CyNCAuCl were isolated from these reactions by recrystallization. In all cases, 2 was isolated as a yellowish powder in good yields (>70%). The 1H NMR spectrum of 2 displays two diagnostic singlets at 5.82 and 5.39 ppm, integrating for one proton each in the alkene region.

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ly, while the angle of C(1)-P(1)-C(2) is 47.5(1)o. These data are similar to those previously observed for phosphiranes.22a Interestingly, addition of (tht)AuCl to a toluene solution of 2 led to the regeneration of 1. Collectively, the results demonstrate the first reversible intramolecular C-C bond activation via a metalation/demetalation method on P. The reversibility further demonstrates that coordination of phosphorus to AuCl destabilizes the distal C-C bond thus favoring the phosphepine tautomer. DFT calculations reveal that the LUMO (-0.26 eV) of 2 primarily involves the P(1)-C(2) σ*-antibonding and C(9)-C(10) π*antibonding orbitals (Figure 4a). The HOMO (-6.35 eV) of 2 involves the degenerate bonding orbitals of the C2P ring, as well as the C-C π-orbitals over the C6 and azaphosphole rings (Figure 4b), whereas the lone pair of the P atom is in the HOMO-1 (-7.36 eV) (Figure 4c). Moreover, natural bond orbital (NBO) analysis (M062X/cc-pVTZ//M06-2X/Def2-SVP) shows a positively charged P center (0.72 a.u.), which is also in agreement with the observations from the electrostatic potential analysis (Figure 4d). The P-C bonds of the C2P ring (P(1)-C(1): 0.80; P(1)-C(2): 0.78) are relatively weaker compared to that (0.93) of the P(1)-C(9) bond, supported by the smaller Wiberg bond indices (WBIs).

Figure 2. DFT prediction of the kinetic and thermodynamic stability of 2. Energies are given in kcal/mol.

Figure 4. (a) LUMO of 2. (b) HOMO of 2. (c) HOMO-1 of 2. (d) Molecular electrostatic potential (MEP) map for 2. The yellow and red areas indicate repulsive and attractive MEPs towards a positive point charge, respectively. Scheme 1. Synthesis of 2 and conversion to 4.

Figure 3. POV-ray depiction of the molecular structure of 2 with H atoms omitted for clarity. C, black; N, blue; O, red; P, orange. Single crystals suitable for X-ray diffraction studies were obtained by slow evaporation of a concentrated pentane solution of 2 in the dark at -15 oC, unambiguously confirming the phosphanorcaradiene structure of 2 (Figure 3). Compound 2 features a tetracyclic system with a 2,3-dihydro-1H-1,3-azaphosphole ring fused to the bicycle phosphanorcaradiene ring as well as to the N,N’diamidocarbene (DAC) ring. The C6 ring capped by the PR fragment is close to planar. The bond lengths of P(1)-C(1), P(1)-C(2) and C(1)-C(2) are 1.874(2), 1.916(3) and 1.527(4) Å, respective-

We have previously shown that the formation of 1 proceeds via the addition of an extremely reactive phosphinidene-Au complex derived from 4 to an aromatic C-C bond.20 Thus we speculated that a reversible binding of 2 to a sterically bulky transition-metal complex could interconvert 2 and 4. Ru(PPh3)3Cl2 is known to readily equilibrate to provide Ru(PPh3)2Cl2 and free PPh3 in solution.24 Treatment of 2 with 10 mol% Ru(PPh3)3Cl2 in DCM prompts the establishment of an equilibrium mixture of 2 and 4 (Scheme 1). The reaction reaches equilibrium in 5 h at 20 oC with the molar ratio of 56:44 (2:4) based on 31P NMR data. Increasing the catalyst loading had no effect, but lowering the temperature of the solution to -40oC increased the 2:4 ratio to 61:39. Indeed, DFT calculations using Ru(PMe3)2Cl2 as a simplified model catalyst show that both 2 and 4 weakly coordinate to the Ru center forming IN2 (-6.6 kcal/mol) and IN4 (-5.7 kcal/mol) with the Ru-P(1) bond lengths of 2.637 and 2.538 Å, respectively (Figure 5). The approach of the Ru center toward P(1) (Ru-P(1): 2.193 Å) significantly stabilizes the TS3 of phosphanorcaradiene-phosphirene rearrangement as indicated by an activation barrier (27.0 kcal/mol, 2→TS3) that is much lower than the catalyst-free TS2 (46.4 kcal/mol, Figure 2). This stabilization is attributed to the metalphosphinidene interaction.19c,25 Indeed, the HOMO and HOMO-1

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Journal of the American Chemical Society of TS3 reveal orbital interactions of Ru and P centers (Figure S29 in the SI). In addition, since the phosphirane circumambulation for 9-phenyl-9-phosphabicyclo[6.1.0]nona-2,4,6-triene was described by Lammertsma et al.,26 we investigated pathways leading to phosphanorbornadienes IN3 and 3 via a [1,3]-sigmatropic shift from 2 (Figure S30 in the SI). These processes are computed to be unfavorable as the activation barrier is 44.0 (2→TS4) or 41.7 kcal/mol (2→TS4’) with or without the presence of Ru(PMe3)2Cl2, respectively.

PMe3 Me3P RuCl2

2.193

tBu P N

N

O

2.633 2.736

O

PMe3 Ru(PMe3)2Cl2 2 Me3P RuCl2 (-7.5) 2.637

P N O

TS3 Mes (19.5)

IN2 (-6.6) tBu N

Scheme 2. Reactions of 2.

PMe3 IN4 Me P RuCl 3 2 (-5.7) 2.538

tBu P

Mes

N

N

Mes

Mes O

O

O

Figure 5. DFT mechanistic study of the Ru-catalyzed reversible rearrangement between IN2 and IN4. Optimized structure of TS3. Energies are given in kcal/mol. Selected bond lengths are given in angstroms. The species 2 can be stored in the solid state under dry air in the dark over a week without noticeable decomposition. DFT calculations suggest that the additional linkage (i.e., C(1)-N(1) bonding) of 2 plays a pivotal role in the remarkable stability of 2 toward the rearrangement (Figure 2).15 Without the C(1)-N(1) linkage, the departure of the transient phosphinidene leading to a phosphirene 4’’ is favorable via a [2+1] cyclo-reversion with an activation barrier of 7.1 kcal/mol (2’→TS2’, Figure S31 in the SI).18a The UV-visible spectrum of 2 in DCM shows two broad absorption bands (maxima at 283 and 340 nm, Figure S4 in the SI). TDDFT calculations at the TD-M06-2X/cc-pVTZ//M06-2X/Def2SVP level of theory suggest that the band observed at 340 nm primarily corresponds to the transition with configuration of HOMO to LUMO, while the transition at 283 nm is primarily assigned to the excitation of electrons from HOMO-1 to LUMO (Figure S5 in the SI). Photolysis of 2 in CDCl3 led to the complete conversion to the phosphirene 4 at room temperature within 1 h as evidenced by NMR spectroscopy. This rearrangement is also achieved using sunlight but requires longer time (5 h). In addition, 4 was shown to be inert upon exposing to irradiation for 3 h. The conversion of 2 to 4 supports the notion of a transient phosphinidene intermediate, leading to a photolytically stable 4. This view is consistent with the known ability of phosphiranes to participate in thermally or photochemically induced [2+1] cycloreversions.27 The access to phosphinidene intermediates from compound 2 prompted an exploration of reactivity. Species 2 was shown to react with 1,10-phenanthroline-5,6-dione or phenanthrene-9,10dione smoothly to afford the corresponding dioxaphosphole 5 or 6 in 76 and 72 % yields, respectively (Scheme 2). The formulation of 5 and 6 was confirmed spectroscopically and crystallographically for 6 (Figure S28 in the SI). It is noteworthy that similar reactivity was exhibited by 4,20 inferring that both 2 and 4 can access the intermediate phosphinidene (Figure S32 in the SI).

Efforts to trap the phosphinidene intermediate from 2 were also undertaken. Compound 2 was inert in the presence of IDipp, however it reacted with a diisopropyl cyclopropenylidene described by Bertrand et al.28 Addition of this carbene to 2 in toluene gave rise to a deep red solution immediately, from which 7 was isolated in 80% yield as a red solid (Scheme 2). DFT calculations showed that the transformation of 2 to 7 involves an incipient phosphinidene with the activation barrier of 19.2 kcal/mol (Figure S33 in the SI). While efforts to obtain crystals of 7 were unsuccessful, the 31P NMR spectrum of 7 showed a sharp singlet at -17.7 ppm, which is downfield shifted relative to the values observed for diisopropyl cyclopropenylidene-stabilized phenyl phosphinidene (-34.9 ppm) and IDipp-stabilized phenyl phosphinidene (-18.9 ppm).29 This value is in good agreement with that calculated for 7 (-19.5 ppm). Treatment of 7 with MeSiOTf or Me2SBH3 in toluene in the dark immediately prompted the color to fade to a slightly yellow. The 31P NMR spectra in both reactions revealed the generation of 4 quantitatively. Concurrent with this were the formations of the silylated-cyclopropenyl salt 8 and a cyclopropenylidene-BH3 adduct 9, respectively. The structures of these latter species 8 and 9 were unambiguously confirmed by single crystal X-ray diffraction studies. These reactions of 7 represent rare examples of ligand exchange reactions at a main group element center.30 Moreover, these stand in contrast to corresponding reactions of NHC-stabilized phosphinidenes which afforded alkylation or borylation at P affording the corresponding Pfunctionalized adducts.31 In summary, we have described the room-temperature-stable phosphanorcaradiene 2, the first example of a phosphorus analog of norcaradienes. Synthesized by a demetalation strategy, the spontaneous C-C bond formation proceeds via a phosphepine to phosphanorcaradiene rearrangement. Compound 2 reacts with diones to give dioxaphospholes 5 and 6 via a phosphanorcaradiene-phosphinidene rearrangement. A phosphanorcaradienephosphirene rearrangement affording 4 is induced by UV irradiation of 2. Alternatively, this can be catalyzed by Ru(PPh3)3Cl2. Such phosphanorcaradiene-phosphirene transformation is also achieved via a cyclopropenylidene-induced rearrangement followed by treatment with Me3SiOTf or Me2SBH3. The reactivity of 2 toward other substrates and more detailed mechanistic studies are the subjects of ongoing research.

ASSOCIATED CONTENT Supporting Information

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Synthetic, spectroscopic, crystallographic and computational data (PDF); Structure data in cif format (CIF). The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author

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D. Stephan, [email protected]

Notes The authors declare no competing financial interests.

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ACKNOWLEDGMENT 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. E. Mosaferi is thanked for the preparation of Ru(PPh3)3Cl2. Prof. C. A. Russell at University of Bristol is thanked for providing tBuC≡P. This work is dedicated to Professor Guy Bertrand on the occasion of his 66th birthday.

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