Facile Phosphorus–Carbon Bond Formation using a Tungsten

Communication pubs.acs.org/Organometallics

Facile Phosphorus−Carbon Bond Formation using a TungstenCoordinated Phosphirenyl Cation Arumugam Jayaraman and Brian T. Sterenberg* Department of Chemistry and Biochemistry, University of Regina, 3737 Wascana Parkway, Regina, Saskatchewan, Canada S4S 0A2 S Supporting Information *

ABSTRACT: Reaction of K2[W(CO)5] with Cl2PN-i-Pr2 in the presence of diphenylacetylene leads to the tungsten aminophosphirene complex [W(CO)5{P(N-i-Pr2)C(Ph)C(Ph)}] (1), which can be converted to the chlorophosphirene complex [W(CO)5{P(Cl)C(Ph)C(Ph)}] (2) by reaction with HCl. Chloride abstraction from 2 with excess AlCl3 leads to the tungsten-complexed phosphirenyl cation [W(CO)5{PC(Ph)C(Ph)}][AlCl4] (3). Compound 3 reacts with PPh3 to form a phosphoniophosphirene complex and undergoes electrophilic aromatic substitution with ferrocene to form a ferrocenylphosphirene complex. Chloride abstraction from 2 with silver triflate leads to a phosphirenyl triflate complex, which reacts with PPh3 and ferrocene in the same fashion as 3 but also reacts cleanly with a wider range of substrates, including phenylacetylene and allyltrimethylsilane, to form respectively a phenylalkynylphosphirene complex and an allylphosphirene complex.

P

hosphorus−carbon bond formation is a fundamental step in the synthesis of organophosphorus compounds. Nucleophilic attacks on phosphorus electrophiles constitute a significant proportion of P−C bond forming reactions.1 Phosphorus electrophilicity can be enhanced via anion abstraction, broadening the range of potential nucleophiles. For example, chloride abstraction from P−Cl bonds has been used extensively to form cationic, electron-deficient phosphorus species including phosphenium ions2 and transition-metal phosphinidene complexes,3 both of which can be used in P− C bond forming reactions. We were interested in extending this methodology to chloride abstraction from metal-coordinated chlorophosphines, to form metal-coordinated phosphenium ions. Although halide abstraction has been applied to the generation of metal-coordinated phosphenium ions, examples are limited to heteroatom-stabilized species, and very little is known about their reactivity.4 Metal coordination has the potential to stabilize otherwise unstable phosphenium ions, and by generating them in the coordination sphere of a metal complex we may be able to observe otherwise unstable species. Metal coordination is also expected to alter the reactivity by protecting the phosphorus lone pair and by altering electrophilicity through P to M donation or M to P back-donation, depending on the nature of the metal fragment. Our interest was drawn to the phosphirenyl cation (Figure 1a), because the ring unsaturation provides potential aromatic stabilization,5 which may allow us to form stable phosphenium complexes without the need for heteroatom stabilization. Transitionmetal-coordinated phosphirenyl cations were first proposed as intermediates in anion exchange reactions of phosphirene complexes.6,7 The first stable transition-metal-coordinated phosphirenyl cation was an η3 nickel complex obtained by condensation of vaporized atomic nickel with tert-butylphosphaalkyne (Figure 1b).8 A pentacarbonyltungsten-complexed © 2013 American Chemical Society

Figure 1. Phosphirenyl cation and phosphirenyl cation metal complexes.

phosphirenyl cation was prepared by abstraction of triflate from a phosphirenyl triflate complex using B(OTf)3 in liquid SO2 at −78 °C (Figure 1c).9 However, the reactivity of these metalcoordinated phosphirenyl cations remains completely unexplored, likely in part due to the difficult synthetic routes used to form them. We report here the facile generation of a tungstencoordinated phosphirenyl cation and its versatile reactivity toward P−C bond formation. To generate a coordinated phosphirenyl cation, we chose the known chlorophosphirene complex [W(CO)5{P(Cl)C(Ph)C(Ph)}] (2)7,10 as an ideal precursor. Synthesis of 2 was achieved in two steps. Reaction of K2[W(CO)5] with Cl2PN-iPr2 and diphenylacetylene leads to the aminophosphirene complex 1 (Scheme 1). This reaction involves a (1 + 2)cycloaddition between diphenylacetylene and an electrophilic aminophosphinidene complex generated in situ from the tungsten dianion and Cl2PN-i-Pr2. This route to 1 is simpler and more efficient than other reported methods7,10 and is similar to the method used for the generation of a transient iron aminophosphinidene complex from Collman’s reagent and Received: December 31, 2012 Published: January 23, 2013 745

dx.doi.org/10.1021/om301266x | Organometallics 2013, 32, 745−747

Organometallics

Communication

Scheme 1a

the optimized geometry is shown in Figure 3.13 Shortened phosphirenyl ring P−C distances (1.738 vs 1.784 Å in

a Reagents and conditions: (i) Cl2PN-i-Pr2, PhCCPh, THF, room temperature; (ii) HCl, Et2O, room temperature.

Cl2PN-i-Pr2.11 Reaction of 1 with hydrogen chloride gave the desired chlorophosphirene complex 2. Treatment of 2 with 4 equiv of AlCl3 resulted in an immediate color change to deep red. Note that reaction of 2 with 1 equiv of AlCl3 does not generate a detectable reaction.6 The 31P NMR spectrum of the resulting solution showed a single phosphorus resonance at δ 171.7, deshielded by 280.7 ppm from the precursor, consistent with previously observed phosphirenyl chemical shifts.9 The 13C NMR spectrum showed a peak for the phosphirene ring carbons at δ 180.5, similarly deshielded by 37.3 ppm from 2 (143.2 ppm). The electrospray mass spectrum showed an ion cluster centered at m/z 533, which matches the pattern predicted for a cation with the formula C19H10O5PW. These data are consistent with formulation of the new compound as [W(CO)5{PC(Ph)C(Ph)}][AlCl4] (3), a tungsten complex of a phosphirenyl cation, formed by chloride abstraction from 2 (Scheme 2).

Figure 3. Optimized structures of 2 and 3. Selected distances (Å): 2, W−P = 2.541, P−C = 1.784, CC = 1.343, C−Ph = 1.449 Å; 3, W−P = 2.378 Å, P−C = 1.738 Å, CC = 1.393 Å, C−Ph = 1.434 Å.

precursor 2), along with a longer CC distance (1.393 vs 1.343 Å), show that the empty phosphorus p orbital is stabilized by resonance delocalization. A shortened P−W bond (2.378 vs 2.541 Å in 2) also indicates that W-to-P π backdonation is significant. However, this distance is much longer than W−P double bonds14 and cannot be considered a true double bond with full phosphido character. Shortened C−Ph bonds suggest that charge is also being delocalized onto the phenyl rings. In the optimized structure, the phenyl rings are nearly coplanar with the PC2 ring, providing further evidence for this delocalization. Optimization of both η1- and η3phosphirenyl coordination shows that η1 coordination is more stable by 11.0 kcal/mol than η3 coordination, which again demonstrates that the phosphirenyl cation must be η 1 coordinated. The calculated parameters suggest that all three of the forms shown in Figure 2 make a significant contribution to 3. Attempts to isolate and crystallize 3 resulted in reversion to 2 as excess AlCl3 crystallizes from solution. As a result, it was trapped with PPh3, resulting in phosphine coordination to the phosphirenyl phosphorus to form the P−P-bonded adduct 4 (Scheme 2), which was characterized by X-ray crystallography (Figure 4). This reactivity demonstrates that 3 is electrophilic at phosphorus. Compound 3 also undergoes rapid electrophilic aromatic substitution with ferrocene to form the ferrocenylphosphirene complex 5 in 51% isolated yield (Scheme 2), again illustrating the strongly electrophilic phosphorus center. Clearly, 3 reacts as a phosphenium ion. Reaction of 3 with other substrates, including arenes, alkynes, and alkenes, led to decomposition and no observable phosphorus-containing products. One possible reason for the decomposition is the excess AlCl3 needed to form 3, which may react with both the substrate and the product. Similarly, the HCl generated as the byproduct of an electrophilic substitution reaction may be contributing to product decomposition. As a result, we looked for alternative chloride abstractors. Reaction of 2 with silver tetrafluoroborate resulted in chloride abstraction; however, the observed product was the unreactive fluorophosphirene complex [W(CO)5{P(F)C(Ph)C(Ph)}]. Silver triflate led to the phosphirenyl triflate complex 6 (Scheme 3). Compound 6 reacts rapidly with ferrocene to give 5, the same product formed in the reaction of 3 with ferrocene, in 86% yield (Scheme 3). On the basis of this reactivity, we suggest that 6 is in equilibrium with the

Scheme 2a

a

Reagents and conditions: CH2Cl2, room temperature; (i) AlCl3, 4 equiv; (ii) PPh3; (iii) Cp2Fe. [W] = W(CO)5.

Furthermore, the large 392 Hz W−P coupling and the lack of W coupling in the ring carbon resonances indicate that the phosphirenyl cation is coordinated to W in an η1 fashion through P. The bonding between the transition metal and phosphorus in planar M−PR2 complexes can range between two extremes, phosphenium and four-electron phosphido (Figure 2), depending on the extent of metal-to-P π back-donation.12 For the phosphirenyl cation specifically, an additional contributing form is delocalized (aromatic) overlap of the empty phosphorus p orbital with the alkenyl π bond. A computational study has been carried out to address the nature of the bonding in 3, and

Figure 2. Bonding between transition metal and phosphorus: (a) fourelectron phosphido; (b) phosphenium. (c) aromatic phosphirenyl cation. 746

dx.doi.org/10.1021/om301266x | Organometallics 2013, 32, 745−747

Organometallics

Communication

are applicable to any W(CO)5(PClR2) complex and will constitute a powerful new method for P−C bond formation.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

Text, figures, tables, and a CIF file giving experimental details and compound characterization data, computational details, energies, and Cartesian coordinates of optimized geometries, and crystallographic data for 4. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Notes

Figure 4. ORTEP diagram of the cation of compound 4. The counterion and hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): W1−P1 = 2.4612(4), P1−P2 = 2.2526(6), P1−C6 = 1.796(2), P1−C7 = 1.792(2), C6−C7 = 1.319(2); W1−P1−P2 = 124.90(2), W−P1−C6 = 129.22(6), W1− P1−C7 = 124.86(6), C6−P1−C7 = 43.14(8), P1−C7−C6 = 68.6(1), P1−C6−C7 = 68.3(1).

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Bob McDonald and Mike Ferguson (University of Alberta) for X-ray data collection, Compute Canada (Westgrid) for computing resources, and the University of Regina for funding.

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Scheme 3a

REFERENCES

(1) Engel, R.; Cohen, J. I. Synthesis of Carbon-Phosphorus Bonds; CRC Press: Boca Raton, FL, 2004. (2) Abrams, M. B.; Scott, B. L.; Baker, R. T. Organometallics 2000, 19, 4944. Burford, N.; Herbert, D. E.; Ragogna, P. J.; McDonald, R.; Ferguson, M. J. J. Am. Chem. Soc. 2004, 126, 17067. Spinney, H. A.; Yap, G. P. A.; Korobkov, I.; DiLabio, G.; Richeson, D. S. Organometallics 2006, 25, 3541. Reed, R. W.; Xie, Z.; Reed, C. A. Organometallics 1995, 14, 5002. (3) Rajagopalan, R. A.; Sterenberg, B. T. Organometallics 2011, 30, 2933. Vaheesar, K.; Bolton, T. M.; East, A. L. L.; Sterenberg, B. T. Organometallics 2010, 29, 484. Graham, T. W.; Cariou, R. P. Y.; Sanchez-Nieves, J.; Allen, A. E.; Udachin, K. A.; Regragui, R.; Carty, A. J. Organometallics 2005, 24, 2023. Graham, T. W.; Udachin, K. A.; Carty, A. J. Chem. Commun. 2005, 5890. Sterenberg, B. T.; Udachin, K. A.; Carty, A. J. Organometallics 2001, 20, 2657. (4) Cowley, A. H.; Kemp, R. A.; Wilburn, J. C. Inorg. Chem. 1981, 20, 4289. Montemayor, R. G.; Sauer, D. T.; Fleming, S.; Bennett, D. W.; Thomas, M. G.; Parry, R. W. J. Am. Chem. Soc. 1978, 100, 2231. (5) Eisfeld, W.; Regitz, M. J. Org. Chem. 1998, 63, 2814. (6) Deschamps, B.; Mathey, F. New J. Chem. 1988, 12, 755. (7) Deschamps, B.; Mathey, F. Tetrahedron Lett. 1985, 26, 4595. (8) Avent, A. G.; Cloke, F. G. N.; Flower, K. R.; Hitchcock, P. B.; Nixon, J. F.; Vickers, D. M. Angew. Chem., Int. Ed. 1994, 33, 2330. (9) Simon, J.; Bergsträsser, U.; Regitz, M.; Laali, K. K. Organometallics 1999, 18, 817. (10) Mercier, F.; Deschamps, B.; Mathey, F. J. Am. Chem. Soc. 1989, 111, 9098. (11) Wit, J. B. M.; van Eijkel, G. T.; Schakel, M.; Lammertsma, K. Tetrahedron 2000, 56, 137. Wit, J. B. M.; van Eijkel, G. T.; de Kanter, F. J. J.; Schakel, M.; Ehlers, A. W.; Lutz, M.; Spek, A. L.; Lammertsma, K. Angew. Chem., Int. Ed. 1999, 38, 2596. (12) Rosenberg, L. Coord. Chem. Rev. 2012, 256, 606. (13) DFT, B3LYP, mixed basis sets, LANL2DZ for W, 6-31G(d,p) for H, and 6-31G(d) for other atoms. (14) Gross, E.; Jörg, K.; Fiederling, K.; Göttlein, A.; Malisch, W.; Boese, R. Angew. Chem., Int. Ed. 1984, 23, 738. Jörg, K.; Malisch, W.; Reich, W.; Meyer, A.; Schubert, U. Angew. Chem., Int. Ed. 1986, 25, 92. Burck, S.; Gudat, D.; Nieger, M. Angew. Chem., Int. Ed. 2007, 46, 2919. Balázs, G.; Green, J. C.; Scheer, M. Chem. Eur. J. 2006, 12, 8603. (15) Tran Huy, N. H.; Ricard, L.; Mathey, F. Angew. Chem., Int. Ed. 2001, 40, 1253.

a

Reagents and conditions: CH2Cl2, room temperature; (i) AgOSO2CF3; (ii) Cp2Fe; (iii) HCCPh; (iv) CH2CHCH2Si(CH3)3. [W] = W(CO)5.

phosphirenyl cation complex, as has previously been proposed for a related phosphirenyl triflate.9 In contrast to 3, 6 also reacts rapidly and cleanly with a wide range of organic substrates. Two examples are given here. Reaction with phenylacetylene leads to the alkynylphosphirene complex 7 in 78% yield (Scheme 3). Compound 7 is known, having been previously formed via trapping of a transient alkynylphosphinidene with diphenylacetylene.15 Reaction with allyltrimethylsilane gave the allylphosphirene complex 8 in 89% yield. These reactions reveal not only the high electrophilicity of the phosphirenyl cation but also its versatility in forming P− C bonds. In summary, the straightforward formation of the η1phosphirenyl cation complex 3 was achieved by simple chloride abstraction using AlCl3 in CH2Cl2 at room temperature from the precursor chlorophosphirene complex 2. We have also demonstrated that the phosphirenyl triflate complex 6 is an appropriate surrogate for 3 and reacts cleanly with a wider range of substrates. The rapid and high-yielding reactions of 3 and 6 with organic substrates will open up new prospects in synthesizing various phosphirene-containing organophosphorus compounds. Furthermore, the methodologies described here 747

dx.doi.org/10.1021/om301266x | Organometallics 2013, 32, 745−747