Î»5-Phosphane Pair and Its Ambiphilic Reactivity

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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A Tautomeric λ3/λ5‑Phosphane Pair and Its Ambiphilic Reactivity Jean-Marc Mörsdorf, Hubert Wadepohl, and Joachim Ballmann* Anorganisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 276, 69120 Heidelberg, Germany

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF GLASGOW on 02/19/19. For personal use only.

S Supporting Information *

ABSTRACT: The central phosphorus atom of a novel hydroxyl-functionalized triarylphosphane was shown to reversibly insert into one of the molecule’s O−H bonds, which forms the basis for a tautomeric λ3/λ5-phosphane equilibrium. For the first time, this equilibrium was detected for a λ3triarylphosphane and the underlying dynamic process was elucidated by NMR spectroscopy. On the basis of reactivity studies, a nucleophilic character was assigned to the minor species present in solution, the λ3-phosphane. Upon methylation, for example, the λ3-form was selectively removed from the equilibrium and converted to the corresponding phosphonium salt. However, upon generation of an alkoxide via proton abstraction, the electrophilic character of the λ5-phosphane in the equilibrium became evident since the alkoxide was found to attack the molecule’s phosphorus atom. This intramolecular reaction led to the selective formation of a new anionic λ6hydridospirophosphane.

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INTRODUCTION Ambiphilic molecules, i.e., compounds that combine electrophilic and nucleophile moieties within the same framework, are known to exhibit unusual reactivities, which may be dominated either by their electrophilic or by their nucleophilic subunits or by both of them.1 Over the past decades, frustrated Lewis acid/base pairs containing a rigid backbone to separate the nucleophilic donor and electrophilic acceptor units (commonly phosphines and boranes) have been explored in detail.2 More recently, ambiphilic reactivity patterns have been discovered for a number of organophosphorus compounds incorporating donor and acceptor units that are directly bound to each other.3 In the case of the diazadiphosphapentale A (see Chart 1), the amido-substituted phosphorus atom was found to react as an electrophile, for example, upon σ-bond metathesis with NH3,3a while the carbon-substituted phosphorus atom was found to exhibit nucleophilic character, for example, upon alkylation with MeI.3d,4 A similar ambiphilicity was also reported for the structurally related phosphoramidite B (see Chart 1).3e Recently, the groups of Kano and Morokuma reported on the [OPO]-coordinated boronate C (see Chart 1)5 and its preparation starting from BH3, [C7H7]BF4 (i.e., a hydride abstractor) and Martin’s well-known ring-closed λ4phosphoranide anion.6 In the synthesis of C, a unique rearrangement via a two-fold ring expansion has been exploited,5 which demonstrates that coordinatively stabilized λ3-phosphanes are actually readily accessible starting from the corresponding λ5-phosphanes. Considering that numerous λ5dioxyspirophosphanes with fascinating stereochemical properties are known,7it is not surprising that this research field is rapidly emerging at presentin particular with respect to the development of novel organic transformations.8 In view of the unusual structure of C5 and its novelty in main group © XXXX American Chemical Society

Chart 1. Ambiphilic Phosphorus Containing Compounds A and B (D = Donor, A = Acceptor) together with the System Reported Herein (D)a

a

In the inset, the [OPO]-coordinated boronate C is shown with its P−B bond represented as dative D → A interaction.

chemistry, it needs to be noted that formally related [ONO]coordinated metal complexes have been explored in detail by Veige and co-workers.9 However, there is a fundamental difference between these [OPO]- and [ONO]-systems, which becomes obvious upon inspection of the respective protonated Received: January 9, 2019

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DOI: 10.1021/acs.inorgchem.9b00076 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry forms. In H3[ONO], hypervalency does not play a role in general,10 which is also true for the deprotonated forms (e.g., for [ONO]3−) and for the related [NNN]-systems.11 A similar situation is found for protonated [NPN]-derivatives, while H2[OPO]-frameworks are known to form ring-closed λ5phosphanes, as the central phosphorus atom tends to insert into one of the O−H bonds.12 Most often, the ring-closed λ5[OPO]-phosphanes are thermodynamically favored over the corresponding open-chain λ3-[OPO]-phosphanes, although a few exceptions have been reported.13 Interestingly, the reverse reaction, i.e., the ring-opening of λ5-[OPO]-phosphanes, is not easily achieved and the yet known λ5-hydrido-triarylphosphanes are not even converted to the open-chain λ3-form upon deprotonation, as λ4-phosphanide anions are generated preferentially.6 Nearly half a century ago, Burgada and coworkers have shown that cyclic λ5-tetraoxophosphanes may coexist in an equilibrium with the corresponding open-chain λ3-phosphites.14 However, equilibria of this type have not been established for hydroxyl-functionalized λ3-triarylphosphanes. Herein, a de novo synthesis of the λ3-[OPO]-system depicted in Figure 1 was developed, which was found to coexist in a

Scheme 1. Synthesis of (S)-5/(R)-5 and Equilibrium between (S)-5/(R)-5 and 5i in Solution

PhPCl2, the acetal impeded a premature formation of an airsensitive cyclic λ5-phosphane, which significantly simplified the workup procedure. After purification, the λ3-phosphane 4 was easily converted to the cyclic λ5-phosphane 5 via deprotection with methanolic HCl. The desired product was isolated as a racemic mixture of (S)-5 and (R)-5. Upon crystallization from MeOH, single crystals of 5 were obtained and subjected to XRD (see Figure 1). As expected, 5 was found to crystallize as a racemic pair in a centrosymmetric space group (P21/n). Routine NMR spectroscopic measurements suggested that an equilibrium between the λ5-phosphane 5 and the λ3triarylphosphane 5i was present in THF-d8 (see inset in Scheme 1). For the λ5-phosphanes (S)-5 and (R)-5, a singlet at −52.6 ppm was observed in the 31P{1H} NMR spectrum. In the corresponding 19F{1H} NMR spectrum, the four chemically inequivalent CF3 groups in 5 gave rise to four multiplet signals in a 1:1:1:1 ratio. For 5i, a singlet was found in the 31 1 P{ H} NMR spectrum at −7.9 ppm, while two broad equally intense signals were detected in the 19F{1H} NMR spectrum, due to the diastereotopic, but otherwise equivalent, CF3 groups. To substantiate that both species are indeed in equilibrium with each other, variable-temperature 31P{1H} and 19 1 F{ H} NMR spectra were recorded over a temperature range from −60 to +20 °C.16 The thermodynamic state variables ΔRH, ΔRS, and ΔRG (293 K) were determined by variabletemperature 31P{1H} NMR spectroscopy using Ph3PO as internal standard. Fitting the van’t Hoff plot for 5 → 5i revealed that ΔRH = 7.68 ± 0.20 kJ/mol, ΔRS = 10.72 ± 0.77 J/(K·mol), and ΔRG (293 K) = 10.82 ± 0.30 kJ/mol, which is in line with an endergonic ring-opening of 5. A dynamic P−H/ O−H exchange was observed in the 1H−1H EXSY spectrum, which agrees well with a fully reversible ring-opening/ringclosure process, i.e., with the equilibrium between 5 and 5i. Upon dissolution of 5 in MeOH-d4, a rapid and quantitative (>98%) deuterium exchange reaction was observed. Within 1

Figure 1. ORTEP plot of the molecular structure of (S)-5 (displacement ellipsoids set at 50% probability level, all carbonbound hydrogen atoms omitted). Selected bond lengths (Å) and angles (deg): P−H 1.335(19), P−O1 1.8463(10), O2−H2 0.87(3), O1−P−H 174.2(8).

tautomeric equilibrium with its λ5-alkoxy-hydrido-triarylphosphane (cf. equilibrium D in Chart 1). Given that equilibria of this type were unprecedented for triarylphosphanes, the novel λ3/λ5-phosphane pair was examined in detail, in particular with respect to its dynamics in solution and with respect to its (ambiphilic) reactivity.

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RESULTS AND DISCUSSION The aforementioned tautomeric λ3-triarylphosphane/λ5-alkoxy-hydrido-triarylphosphane couple 5/5i was prepared in five steps starting from the commercially available alcohol 1 (see Scheme 1). Employing a well-documented ortho-lithiation procedure,15 1 was converted to the corresponding dilithium derivative and quenched with an excess of I2 to afford 2. The hydroxyl group in 2 was then protected with ClCH2OEt/DIPEA and the acetal 3 isolated in 82% yield. The use of this particular protective group allowed for the desired lithium-halogen exchange via reaction with PhLi at 0 °C, which was completed within a few minutes due to the ortho-directing influence of the acetal functionality. After reaction of the lithiated intermediate with B


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Inorganic Chemistry min, this H/D exchange led to the formation of 5-d2, which was shown unambiguously by 31P NMR spectroscopy (see toprow spectra in Figure 2). As expected, the deuterated species

Scheme 2. Synthesis of 6, 7, and 8 Exploiting the Nucleophilic Reactivity of 5i

Figure 2. Top: 31P NMR spectrum of 5/5i in THF-d8 at 25 °C (spectrum 1) and 31P{1H} NMR spectrum of 5-d2/5i-d2 in THF-d8 at 25 °C (spectrum 2). The doublet and triplet patterns at −52.6 ppm and −53.5 ppm, respectively, are due to spin−spin coupling (1JH,P = 308.6 Hz and 1JD,P = 47.5 Hz). Bottom: 19F−19F EXSY/NOESY spectrum of 5-d2/5i-d2 in THF-d8 at 25 °C.

Figure 3. ORTEP plot of the molecular structure of 6 as its Et2O adduct (displacement ellipsoids set at 50% probability level, all carbon-bound hydrogen atoms omitted). Selected bond lengths (Å) and angles (deg): P−Li2 2.504(4), Li2−O1 1.865(4), Li2−O2 1.914(4), Li1−O2 1.822(4), Li1−O1 1.889(4), Li1−O2−Li2 88.12(16), Li2−O1−L1 87.61(16).

5i-d2 and 5-d2 were in equilibrium as well, which was shown by 19 F−19F EXSY spectroscopy. In the correlation spectrum (see Figure 2, bottom), exchange cross-peaks were detected for the interchanging and therefore mutually related CF3 groups of 5id2 and 5-d2. Thus, an equilibrium between 5i-d2 and 5-d2 needs to be present in solution. To further probe the equilibrium between the λ3/λ5phosphane couple 5/5i, reactivity studies were conducted. Upon reaction with LiHMDS, the equilibrium, which lies on the side of the cyclic λ5-phosphane, was shifted selectively to the open-chain λ3-phosphane by selectively removing the λ3phosphane as its doubly deprotonated derivative 6 (see Scheme 2). Note that 6 was isolated in a solvent-free form, but crystallized as its etherate (see Figure 3). A λ4-phosphanide anion, similar to the one reported by Martin and coworkers,6c,d i.e., deprotonation of the P−H group in 5, was not observed in our case. Akin to metalation, protonation was found to expose the nucleophilic reactivity of the ambiphilic couple as well. Upon reaction with HNTf2 in CH2Cl2, 5i was removed from the tautomeric equilibrium and isolated in form of its phosphonium salt 7 (see SI for an ORTEP plot of the molecular

structure of 7). A similar reactivity was found upon alkylation of 5i with [Me3O]BF4], which led to the isolation of the corresponding phosphonium salt 8 (see Scheme 2). In the 1H NMR spectrum of 8, the resonance of the P-bound methyl group was unambiguously identified on the basis of its characteristic 2JH,P coupling constant (2JH,P = 14.0 Hz). With the nucleophilicity of the phosphorus atom in the λ3triarylphosphane 5i exposed, we posed the question whether an electrophilic character was found for the corresponding λ5phosphane 5. This type of reactivity was indeed observed upon treatment of the λ3/λ5-phosphane couple with 1,8-bis(dimethylamino)naphthalene (BDMAN, “proton sponge”). In Et2O, a proton was abstracted from the hydroxyl group in 5, generating an alkoxide, which was found to attack the electrophilic phosphorus atom of the λ5-phosphane. This intramolecular reaction proceeded selectively and led to the bicyclic λ6-hydridospirophosphane (R)-9/(S)-9 (see Scheme 3). In the 31P NMR spectrum of 9, a characteristically8b,17 upfield shifted doublet with a large 1JH,P coupling constant of 705.7 Hz was detected at −139.2 ppm. In the crystalline state, the λ6-spirophosphane anion in 9 was found to adopt a C


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Inorganic Chemistry Scheme 3. Synthesis of (R)-9/(S)-9 Exploiting the Electrophilic Reactivity of (R)-5/(S)-5

distorted octahedral structure with an H1−P−C1 angle of 168.0° between the axially oriented atoms (see Figure 4).

Figure 5. Optimized structures and Gibbs free energy profile for the ring-opening reaction 5 → 5i taking one molecule of THF into account (B3LYP/6-311+G(d,p), DFT-D3 dispersion, and PCM solvation correction for THF, 5trans O‑P‑H set to 0.0 kcal/mol). The barrier for the Berry-pseudorotation was not determined as indicated by the dashed line interconnecting the energy levels between 5trans O‑P‑H and 5cis O‑P‑H.

Figure 4. ORTEP plot of the anion in 9 (displacement ellipsoids set at 50% probability level, all carbon-bound hydrogen atoms omitted). Selected bond lengths (Å) and angles (deg): P−H1 1.325(19), P−O1 1.8386(14), P−O2 1.8415(14), P−C10 1.8760(19), P−C19 1.895(2), P−C1 1.8780(19), O1−P−O2 84.59(6), C10−P−O2 85.71(7), C19−P−O1 92.00(7), C19−P−C10 97.64(9), O1−P− C1 85.32(7), O2−P−C1 90.62, C10−P−C1 94.74(8), C19−P−C1 95.43(8), H1−P−C1 168.0(8).

corresponding cis-configured species, presumably via Berrypseudorotationan isomerization pathway, which is welldocumented for λ5-phosphanes.7a,21 Next, the transition state containing a six-membered ring with an O···H···P motif is reached, before the O−H bond in 5i is formed. In the optimized structure of 5i, hydrogen bonds between the two hydroxyl groups were detected in silico and the THF molecule was found to serve as a hydrogen bonding acceptor in all the optimized structures. This is in line with the experimental finding that 5 was isolated as its THF solvate (and crystallized as MeOH solvate). The aforementioned significantly more positive ΔRG values, which were calculated without an explicitly included THF molecule, are considered meaningful as well: Upon dissolution of rigorously dried samples of 5 in CD2Cl2, no signals corresponding to 5i were detected by NMR spectroscopy, indicating that intermolecular hydrogen bonding interactions to the surrounding solvent are of crucial importance for the detection of the λ3/λ5-phosphane equilibrium described in this Article.

Interestingly, an aryl-substituent is trans to H1 in 9, while an alkoxide was found in trans-position to the corresponding hydrogen atom in 5. As expected, 9 crystallized as a racemic mixture in a centrosymmetric space group (P1̅), which is also the case for 5 (P21/n) (vide supra). With the reactivity of the tautomeric λ3-triarylphosphane/λ5alkoxy-hydrido-triarylphosphane pair explored in detail, light was shed on the underlying ring-opening reaction 5 → 5i by modeling the reaction pathway computationally.18 At the B3LYP/6-311+G or B3LYP/6-311++G-(2df, 2p) level of theory,18a,b endergonic reaction profiles were found, but exceedingly large ΔRG values were obtained, which varied between 6.9 and 10.6 kcal/mol (exp. ΔRG ≈ 2.6 kcal/mol) depending on the basis set and the employed solvation model (no solvation, polarizable continuum model, or SMD model).18c,d This situation improved significantly when dispersion corrections were taken into account,18f although overcompressed structures were obtained,19 which was corrected by including one THF molecule into the model. This approach led to reasonable ΔRG values of 4.0 kcal/mol and a barrier of approximately 14.6 kcal/mol at the B3LYP/6311+G(d,p) level of theory (see Figure 5).20 Starting from the thermodynamic minimum, i.e., from 5 with the P-bound oxygen and hydrogen atoms mutually transpositioned, the system was found to isomerize to the

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CONCLUSIONS In summary, two molecules with opposing reactivities were found to coexist in an equilibrium, namely, the hydroxylfunctionalized λ3-phosphane 5i and the λ5-alkoxy-hydridotriarylphosphane 5. While intra- and intermolecular equilibria between tri- and penta-coordinated phosphorus species are well-known for phosphites and phosphoramidites (e.g., compound B, cf. Chart 1), equilibria of this type were hitherto undocumented for hydroxyl-functionalized triarylphosphanes. D


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Inorganic Chemistry

support, fruitful discussions, and continued interest in our work.

The experimental results and the insights gained from DFT calculations both suggest that the choice of the solvent plays an important role for the equilibrium between 5 and 5i, as hydrogen bonding interactions significantly alter the thermodynamics of the ring-opening reaction 5 → 5i, which is reflected by the ratio between both species in solution (CD2Cl2 vs THF-d8). Reactivity studies revealed that the minor species in equilibrium (5i) may be either deprotonated or selectively protonated (or methylated) to afford either the dilithium salt 6 or the phosphonium salt 7 (or 8). To supplement the latter P-nucleophilic reactivities of the tautomeric 5/5i pair, a proton was selectively abstracted from the major species in equilibrium (5), which exposed the electrophilicity of 5 and led to the preparation of the new anionic λ6-hydridospirophosphane 9.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00076. Experimental procedures for the preparation of 2−9, selected NMR and IR spectra, detail on DFT methods and optimized coordinates for selected structures, crystallographic data and ORTEP plot of the molecular structures of 7 (PDF) Accession Codes

CCDC 1863225, 1863226, 1863230, and 1863231 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]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (+49) 6221-54-8596. ORCID

Joachim Ballmann: 0000-0001-6431-4197 Author Contributions

The experiments were designed and carried out by J.-M.M.; DFT calculations were carried out and analyzed by J.B.; H.W. conducted all the crystallographic studies. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

We thank the Bernthsen Foundation (University of Heidelberg) for funding of this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Dr. Markus Enders and Dr. Jürgen Graf for their support with NMR measurements. Patrick W. Antoni is gratefully acknowledged for his help with the DFT calculations. The authors acknowledge support by the state of BadenWürttemberg through bwHPC and the German Research Foundation (DFG) through grant no. INST 40/467-1 FUGG (JUSTUS cluster). We thank Prof. Dr. L. H. Gade for generous E


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Inorganic Chemistry

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Î»5-Phosphane Pair and Its Ambiphilic Reactivity

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