A facile route to cis-olefin-linked phosphino-phosphonium salts of the form: [Ph₂PC(R)C(H)P(R′)₂H][AlCl₄]

Abstract

The diphenylphosphirenium salts of the form [Ph₂PC(R)C(H)][AlCl₄] are readily generated and select examples react with secondary phosphines, R₂PH, to give dissymmetric bidentate phosphonium salts, [Ph₂PC(R)C(H)P(R′)₂H][AlCl₄]. While these reactions work well for sterically encumbered combinations of the phosphirenium cations and secondary phosphines (R = tBu, Cy, Mes), less encumbered combinations provide a mixture of products arising from alkyne displacement. As expected the protonated bis-phosphine salts are easily deprotonated, demonstrating easy access to a rare class of dissymmetric bidentate phosphine ligands.

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From the advent of organometallic chemistry and the development of homogeneous catalysts in 1960s, phosphines have been a dominant class of ligands that have been widely employed. While simple monodentate phosphines were used initially, chelating bidentate phosphines emerged to provide additional complex stability by the chelate effect.¹ Subsequent modifications to introduce optically pure chiral centers were essential to the successful implementation of catalytic asymmetric reductions.^2–6 While modifications of the substituents on monodentate phosphines are achieved by treatment of phosphorus halides with selected organic nucleophiles, similar control of the substituents on bidentate ligands is more challenging. Thus, the vast majority of commercially available bidentate ligands incorporate two PPh₂ groups, while a few bidentate phosphines include two P-centers with substituents other than Ph groups.¹ Rarer still, are bidentate phosphines that incorporate differing phosphine fragments. Such ligands are very challenging to prepare by known methods as such dissymmetric bidentate phosphines typically require consecutive additions of differing phosphide nucleophiles,^7–10 or require metal catalyzed, photochemical or radical based reaction methods.^11–20

In a recent effort targeting new routes to dissymmetric bidentate phosphines, we^21,22 recognized that the phosphino-phosphenium cation (PPC) originally described by Burford and coworkers^23–27 and others^28,29 can act as frustrated Lewis pairs (FLPs) and provide access to both phosphorus Lewis acidic and basic sites. As such, they undergo addition to alkynes to afford cis-substituted olefins to give cations of the form [cis-R₃PCHC(R′′)PR′₂]⁺.²¹ Suitable modification of the precursors afforded [cis-R₂ClPCHC(R′′)PR′₂]⁺ which were readily reduced by nBu₃P to give the dissymmetric bidentate phosphine in good yields (Scheme 1).²¹

Scheme 1 Previously reported FLP synthesis of dissymmetric bidentate phosphines. ODFB = o-F₂C₆H₄.

While this new protocol provides access to a rare set of bidentate phosphines, we sought to further improve the generality and facility of the synthetic protocol. We recognized that one of the limitations of the above method is the need for reduction of the intermediate phosphorus(V) halide salt (Scheme 1). We envisioned deprotonation as a simpler, cheaper process. Moreover, this avenue offers increased variability as a broad selection of secondary phosphines are commercially available. However, we also recognized that the reactivity of PPCs prepared using a secondary phosphine as the donor are readily deprotonated affording the neutral diphosphine R₂PPR′₂. Our previous mechanistic study revealed that phosphino-phosphination was initiated by alkyne displacement of the phosphine donor in the PPC generating a three membered phosphirenium cationic ring,²¹ which undergoes subsequent ring opening by the chlorophosphines. Herein, we explore the reactivity of phosphirenium cations targeting ring-opening by secondary phosphines as a route to protonated dissymmetric bidentate phosphine cations, species which are readily deprotonated to give olefin-linked dissymmetric bidentate phosphines.

To initiate this study, the phosphirenium salt of the form [R₂P(R′CCH)][AlCl₄] were prepared according to literature procedures^30–34via a 1 : 1 : 1 reaction of the chlorodiphenylphosphine, terminal alkyne and AlCl₃ (Table 1). Initially using tBuCCH, the species [Ph₂PC(tBu)C(H)][AlCl₄] 1 was generated as evidenced by the ³¹P NMR signal at 104.6 ppm, characteristic of the phosphirenium cation.

Table 1 Generation of phosphirenium cation salts

Cmpd	R	T (h)	Solvent	^31P shift
1	tBu	0.3	CH2Cl2	−104.6
2	Me3Si	1	CH2Cl2	−122.0
3	Ph	94	o-DFB	−105.5
4	p-tol	0.3	CH2Cl2	−107.3
5	p-(HCC)C6H4	24	o-DFB	−105.9
6	p-IC6H4	48	CH2Cl2	−106.2
7	p-BrC6H4	0.3	CH2Cl2	−105.8
8	C4H9	95	o-DFB	−106.9
9	c-C3H5	0.5	CH2Cl2	−105.8
10	c-C5H9	2	C6H5Me	−107.2
11	Cy	1	CH2Cl2	−107.2

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In a similar fashion, this protocol was used to generate a series of related phosphirenium cations [Ph₂PC(R)C(H)][AlCl₄] (R = SiMe₃2, Ph 3, p-tol 4, p-(HCC)C₆H₄5, p-IC₆H₄6, p-BrC₆H₄7, C₄H₉8, C₃H₅9, C₅H₉10, C₆H₁₁11). While these species were not isolated all reactions showed a clean singlet in the ³¹P NMR spectrum in the range of 104–122 ppm, typical of phosphirenium cations.

As these phosphirenium cations proved to be readily generated, we next assessed the ability of a secondary phosphine to effect ring opening of these phosphacyclic cations. To this end, we first generated a CH₂Cl₂ solution of 1 and added an equivalent of tBu₂PH. Stirring the mixture 20 minutes followed by subsequent workup afforded a reddish-brown sticky solid 12 in 85% yield. This species shows a ³¹P{¹H} NMR spectrum consisting of two doublets at 28.6 and −1.1 ppm with a coupling constant of 86 Hz. The corresponding ³¹P NMR spectrum revealed that the downfield signal also showed P–H coupling of 474 Hz consistent with a one-bond P–H coupling constant. Thus, the upfield resonance was attributed to a PPh₂-fragment, while the downfield signal was assigned to the protonated tBu₂PH-fragment. These data together with ¹H NMR and MS data supported the formulation of 12 as [Ph₂PC(tBu)C(H)(HPtBu₂)][AlCl₄] (Scheme 2). On standing slow vaporization of CH₂Cl₂ from the reaction mixture provided crystals of 12 suitable for an XRD study. The structural data confirmed the formulation of the salt 12 (Fig. 1) in which cation and anion are well separated. In the cation, the phosphorus centers adopt a cis-disposition with P–C bond distances to the linking olefinic fragment found to be 1.847(6) Å and 1.793(6) Å. The shorter of the two is associated with the cationic tBu₂PH fragment. Despite the phosphonium center, the tBu₂PH fragment affords the shorter P–C bond, consistent with the presence of the electron donating alkyl substituents and the less hindered carbon center. The olefin linker showed the typical C–C distance of 1.336(9) Å.

Scheme 2 Synthesis of 12–20.

Fig. 1 POV-ray depiction of the crystallographic structure of the cation of 12. Hydrogen atoms except for the PH unit are omitted for clarity. C: black, P: orange, H: white.

In a similar fashion following this protocol, reaction of 1 with Cy₂PH, afforded a sticky solid in 73% yield. This product 13 showed ³¹P{¹H} NMR resonances at 9.5 and −2.8 ppm with a P–P coupling constant of 57 Hz. The corresponding ¹H coupled ³¹P NMR data revealed the downfield signal was also coupled to proton with a coupling constant of 479 Hz. These data as well as mass spectral data affirmed the formulation of 13 as [Ph₂PC(tBu)C(H)(HPCy₂)][AlCl₄] (Scheme 2).

In the case of the analog using the phosphirenium salt 2, reaction with tBu₂PH was performed on a 2 g scale following a similar procedure. The ultimate yield of the [Ph₂PC(Me₃Si)C(H)(HPtBu₂)][AlCl₄] 14 was 91% yield (Scheme 2). This species showed the two expected resonances in the ³¹P{¹H} NMR spectrum at 20.9 and −3.39 ppm with a ³J_PP of 81 Hz, while the ³¹P NMR spectrum revealed a ¹J_PH of 466 Hz for the upfield resonance, again consistent with the formation of 14.

We next examined the reactions of related aryl-substituted phosphirenium cations. Thus, 4 was independently reacted with tBu₂PH or Cy₂PH. These reactions proceeded similar to those above affording reddish-brown and yellowish-orange products 15 and 16. These products were further purified by reprecipitation from C₆H₅Cl and isolated in 89% and 62% yields respectively. Compound 15 exhibited ³¹P resonances at 26.6 and −4.5 ppm with ³J_PP of 74 Hz and the downfield signal exhibiting a ¹J_PH of 461 Hz. The corresponding data for 16 showed ³¹P signals at 9.6 and −2.6 ppm with ³J_PP of 70 Hz with the downfield signals also exhibiting 475 Hz, respectively. These data again affirmed the formulation of these products as [Ph₂PC(p-tol)C(H)(HPR₂)][AlCl₄] (R = tBu 15, Cy 16) (Scheme 2).

Switching the phosphirenium cation to 7, the corresponding derivative [Ph₂PC(p-BrC₆H₄)C(H)(HPtBu₂)][AlCl₄] 17 was formed in 76% yield (Scheme 2) as evidenced by the ³¹P resonances at 27.0 and −4.0 ppm with ³J_PP and ¹J_PH of 77 and 450 Hz, respectively. While this formulation was also supported by ¹H and MS data (see SI) it was also supported by a crystallographic study. This unambiguously confirmed the formation of 17 where two phosphorus atoms on an olefinic linker are cis-disposed (Fig. 2). Further the PPh₂ fragment resides on the substituted carbon of the olefinic link, and the PtBu₂ fragment is protonated. The resulting P–C bond distances to the linking olefin were found to be 1.862(2) Å and 1.780(2) Å, with the shorter P–C bond arising from tBu₂P-fragment. The remainder of the metric parameters are unexceptional.

Fig. 2 POV-ray depiction of the crystallographic structure of 17. Hydrogen atoms except for the PH unit are omitted for clarity. C: black, P: orange, Cl: green, Al: white, Br: red.

In a similar fashion reaction of 7 with Mes₂PH gave an orange-yellow solid in 91% yield. Two ³¹P resonances at −2.3 and −31.6 ppm showed a ³J_PP of 100 Hz and the upfield signals showed a ¹J_PH of 497 Hz, consistent with the formulation of 18 as [Ph₂PC(p-BrC₆H₄)C(H)(HPMes₂)][AlCl₄] (Scheme 2) which was also supported by ¹H and MS data (see SI).

Probing the generality of this reactivity further, the corresponding reaction of 7 with Ph(tBu)PH was found to give a complex mixture of products including unreacted phosphirenium cation, the expected product [Ph₂PC(p-BrC₆H₄)C(H)(HP(tBu)(Ph)][AlCl₄], [Ph₂P(PHPh(tBu))]⁺, [Ph(tBu)P(PHPh₂)]⁺ and [H₂P(tBu)(Ph)]⁺ (Scheme 3) as evidenced by the ³¹P NMR signals. The complexity of this mixture precluded the isolation of individual products, but did provide some mechanistic insights.

Scheme 3 Divergent reaction pathways for less hindered combinations of phosphirenium cations and secondary phosphines.

The product mixture is consistent with two reaction pathways. Reaction of the phosphirenium cation with Ph(tBu)PH can proceed via ring-opening to protonated bisphosphine salt, alternatively alkyne displacement affords phosphino-phosphenium cations which in the presence of secondary phosphine also generates [H₂P(tBu)(Ph)]⁺. We note that no evidence of this latter pathway was seen in the previous examples where either bulky substituents were incorporated into the secondary phosphine or the alkyne. These observations can be rationalized on the basis of steric demands associated with the nucleophilic attack of the phosphirenium cation as well as the basicity of the alkyne used to generate the phosphirenium cation. While sterically demanding secondary phosphines lead to attack at the unsubstituted carbon of the phosphirenium cation, sterically less encumbered phosphines can access the σ*-orbital on phosphorus, prompting alkyne displacement and the formation of phosphino-phosphenium cations.

To further probe this notion, the phosphirenium salt 11 was reacted with tBu₂PH affording the dissymmetrically substituted [Ph₂PC(Cy)C(H)(HPtBu₂)][AlCl₄] 19 (Scheme 2) as evidenced by the ³¹P NMR signals at 28.5 and −6.5 ppm with a P–P coupling constant of 93 Hz and the P–H coupling of the downfield signal of 474 Hz. In contrast, reaction of 11 with the less sterically encumbered Cy₂PH generated a mixture of products including [Ph₂PPCy₂H]⁺, [Ph₂PHPCy₂]⁺ and [Cy₂PH₂]⁺ (see SI) as evidenced by ³¹P NMR spectroscopy (Scheme 3).

Collectively, these data affirm that less hindered combinations of phosphirenium cations and secondary phosphines react via divergent pathways to afford a mixture of products. On the other hand, more sterically demanding systems are shown to readily afford protonated, dissymmetrically substituted, olefin-linked phosphino-phosphonium cations 12–19. To confirm that such cations can be deprotonated in a trivial fashion, compound 18 was treated with an equivalent of 1,8-bis(dimethylamino)-naphthalene (proton sponge) for 30 minutes. Following solvent removal under vacuum, the product 20 (Scheme 2) was obtained in a 90% yield. The ³¹P NMR spectrum of 20 shows resonances at −7.6 and −35.5 ppm with a coupling constant of 148 Hz consistent with the formulation of 20 as the bidentate phosphine, Ph₂PC(p-BrC₆H₄)C(H)(PMes₂). This formulation was also confirmed via a crystallographic study (Fig. 3). The cis-bisphosphine and dissymmetric nature of 20 revealed P–C distances to the linking olefinic fragment of 1.841(4) Å and 1.824(4) Å for the PMes₂ and PPh₂ fragments respectively, with a C=C bond length of 1.349(6) Å.

Fig. 3 POV-ray depiction of the crystallographic structure of 20. Hydrogen atoms are omitted for clarity. C: black, P: orange, Br: red.

Herein, we demonstrated that the salts [Ph₂PC(R)CH][AlCl₄] are readily generated using a variety of terminal alkynes. The reactions of such species with secondary phosphines showed that less encumbered combinations give rise to mixtures of ring-opened and alkyne displacement products, while bulkier combinations afford exclusively the ring-opening products, [Ph₂PC(R)C(H)PR′₂H][AlCl₄]. Facile deprotonation affording a dissymmetric bidentate phosphine ligand was demonstrated confirming that this new synthesis route provides ready access to a class of compounds that are generally rare and otherwise challenging to prepare. We are continuing to examine the reactions of phosphirenium cations as well as new strategies to desirable phosphorus-based compounds.

NSERC of Canada and Syensqo are thanked for financial support. Drs E. Conrad, J. Zheng, and Z. Yan of Syensqo are thanked for helpful discussions.

NA, SS and MDS performed the experimental work, AL performed one of the X-ray structural studies while DWS wrote the manuscript. All authors edited the draft.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included in the SI. Supplementary information: Synthetic details and spectral data. See DOI: https://doi.org/10.1039/d5cc03456f.

CCDC 2444558 (17), 2444559 (12) and 2444560 (20) contain the supplementary crystallographic data for this paper.^35–37

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