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. 2020 Jun 23;5(26):16010–16020. doi: 10.1021/acsomega.0c01413

Synthesis of Aryldiphenylphosphine Oxides by Quaternization of Tertiary Diphenylphosphines with Aryl Bromides Followed by the Wittig Reaction

Chun-Hong Zhong 1, Wenhua Huang 1,*
PMCID: PMC7346246  PMID: 32656422

Abstract

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A two-step method for the synthesis of aryldiphenylphosphine oxides from tertiary diphenylphosphines and aryl bromides is developed. The first step is the quaternization of methyldiphenylphosphine or benzyldiphenylphosphine with aryl bromides. This quaternization can be nickel-catalyzed (metal-free in some cases), and tolerate of a variety of functional groups, furnishing quaternary phosphonium salts in 48–90% yields. The second step is Wittig reactions of these quaternary phosphonium salts with furan-2-carbaldehyde or p-chlorobenzaldehyde to provide aryldiphenylphosphine oxides in 27–90% yields. The use of the Wittig reaction for the synthesis of tertiary phosphine oxides is in contrast to its traditional use for the synthesis of olefins, leaving tertiary phosphine oxide as a byproduct. This quaternization–Wittig method can be applied to synthesize aryldiphenylphosphine oxides that are difficult to access by the alkaline hydrolysis of aryltriphenylphosphonium salts, especially those bearing an electron-deficient aryl group. The ligand-coupling mechanism for the alkaline hydrolysis of (p-acylphenyl)triphenylphosphonim salts is also discussed.

Introduction

Aryldiphenylphosphine oxides have been widely used in optoelectronic materials,1 and as chiral auxiliary2 and a catalyst3 in organic synthesis. They are also precursors for the synthesis of aryldiphenylphosphine by reduction.4−6 For the synthesis of aryldiphenylphosphine oxides, there are mainly five methods: (1) the transition metal-catalyzed coupling of diphenylphosphine oxide with aromatic derivatives such as aryl halides or pseudohalides, that is, the Hirao reaction,7,8 aroylhydrazides,9 and arylboronic acids,10 (2) the oxidation of aryldiphenylphosphine,11,12 (3) carbophosphinylation of arynes,13,14 (4) the reaction of Ph2P(O)Cl with Grignard reagents,15,16 and (5) the alkaline hydrolysis of aryltriphenylphosphonium salts.17,18 Although these methods are available, none solves all challenges of the synthesis of aryldipheylphosphine oxides. The last method is one of applications17−21 based on a three-step mechanism22,23 for the alkaline hydrolysis of quaternary phosphonium salts: (1) nucleophilic attack by OH− to form a hydroxyphosphorane, (2) deprotonation by OH− to form an oxyanionic phosphorane, and (3) the fragmentation of the oxyanionic phosphorane to produce a phosphine oxide and concurrently expel a carbanion, which is called “apical entry and apical departure” on pentacoordinate phosphorus compound, giving an alkane or arene after protonation of the corresponding leaving group (Scheme 1A).24 According to this mechanism, the alkaline hydrolysis of aryltriphenylphosphonium bromides 1 could produce tertiary phosphine oxides through two pathways: the loss of the aryl group to form Ph3PO (path a in Scheme 1B), and the loss of a phenyl group to form aryldiphenylphosphine oxides 2 (path b). Therefore, path b would provide a facile access to aryldiphenylphosphine oxides if path a could be inhibited.

Scheme 1. (A) Hydrolysis Mechanism of Quaternary Phosphonium Salts; (B) Alkaline Hydrolysis of ArP+Ph3Br–

Scheme 1

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Results and Discussion

For alkaline hydrolysis of a variety of aryltriphenylphosphonium bromides, Table 1 shows the results reported (1) by Horner and co-workers,18 (2) by Allen and Benke,25 and (3) herein by us. These phosphonium salts can be readily prepared by our recently developed metal-free method,26 so we anticipated their alkaline hydrolysis provides a two-step, transition metal-free route to the synthesis of aryldiphenylphosphine oxides. From the data in Table 1, four generalizations can be drawn. (1) The yields of phosphine oxides 2 strongly depends on the position, electronic effect of substituents of the arene ring. (2) For para-substituents, electron-donating groups would be expected to facilitate and electron-withdrawing groups to retard the leave of a phenyl group (2a–e, 2j, and 2k–o). It seems that electron-donating meta-substituents are also favorable for the formation of phosphine oxide 2 (2f–h). Phosphine oxide 2t bearing a heteroaromatic group was isolated in a high yield probably because of the presence of electron-donating NH at the para-position of the benzene ring. This means that aryldiphenylphosphine oxides bearing an electron-donating group at para- or meta-position of the aryl ring could be efficiently synthesized by the alkaline hydrolysis of aryltriphenylphosphonium salts. (3) ortho-Substituents, whether electron-donating or electron-withdrawing, would facilitate the leave of the aryl group and the formation of Ph3PO (2p–s), partially because of neighboring-effects.25 (4) If the aryl group is polyaromatic without substituents and heteroatoms, the alkaline hydrolysis of the corresponding aryltriphenylphosphonium salts favors the departure of this aryl group, leading to the formation of Ph3PO and arenes (2i and 2w); if the polyaromatic aryl bears an electron-donating group, aryldiphenylphosphine oxides 2 could also be obtained in high yields (2u and 2v). These observations are generally in agreement with the apicofilicity rule27,28 that electronegative and small substituents prefer apical positions in the trigonal bipyramidal structure of pentacoordinate phosphorus compounds.

Table 1. Alkaline Hydrolysis of Aryltriphenylphosphonium Bromides.

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a

Reaction conditions:18 reflux, 20% NaOH. The yields of Ph3PO are in the parentheses.

b

Reaction conditions:25 reflux, 1 M NaOH. nd = not detected.

c

Reaction conditions: phosphonium bromide 1 (0.5 mmol), 3 M NaOH (2 mL), rt, overnight or refluxed for 3 h.

d

PhCOOH was also isolated in 97% yield.

e

Phenanthracene was also isolated in 93%.

One additional point is worthy of comment on regarding the alkaline hydrolysis of phosphonium salts 1j and 1o. For 1o, besides Ph3PO, we also isolated three products: p-PhC6H4Ac (47%), PhAc (20%), and Ph2HPO (21%); for 1j, Allen and Benke have reported25 that Ph3PO, PhCOPh, and 4-phenylbenzophenone were isolated in 26%, 24%, and 28% yields, respectively (Scheme 2A). They proposed a ligand-coupling mechanism via penta-valent phosphorus for the formation of 4-phenylbenzophenone (Scheme 2B). The formation of PhAc, PhCOPh, and Ph3PO is expectedly from path a shown in Scheme 2C, according to the general mechanism shown in Scheme 1A. In the mechanism shown in Scheme 2B, Ph2PO–, which subsequently forms Ph2HPO after protonation, should be concurrently formed with the ligand-coupling product. However, Ph2HPO was not isolated and observed only in the crude 31P NMR spectrum by Allen and Benke.25 The isolation of Ph2HPO by us provides further evidence for this ligand-coupling mechanism. Because only Ac and COPh substituents led to the coupling products, it seems that a ketone carbonyl, or its likeness, is crucial for the formation of the coupling products. Therefore, we tentatively suggest a more-detailed mechanism (path b, Scheme 2C): one phenyl group of anionic oxyphosphorane II, which is the same intermediate for the path a, undergoes a migration or addition as a phenyl anion to the benzene ring bearing the acyl group to form enolate III, which loses Ph2PO– to produce Ph2HPO after abstracting a proton from water, and p-PhC6H4Ac or 4-phenylbenzophenone. The recovery of aromaticity of the arene ring from enolate III might be one of the driving forces to expel Ph2PO–.

Scheme 2. (A) Alkaline Hydrolysis of 1o and 1j; (B) Ligand Coupling Proposed by Allen and Benke; (C) Hydrolysis Mechanism of 1o or 1j Proposed by Us.

Scheme 2

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Almost half of aryltriphenylphosphonium bromides in Table 1, especially those bearing an electron-drawing para-substituent and those bearing an ortho-substituent, failed to efficiently produce aryldiphenylphosphine oxides by alkaline hydrolysis. Therefore, we sought to an alternative strategy, that is, a quaternization–Wittig sequence, for the synthesis of aryldiphenylphosphine oxides. In this strategy, the quaternization of MePPh2 or BnPPh2 with aryl bromides produces phosphonium salts 3 or 4, which subsequently undergo the Wittig reaction with an aldehyde to furnish phosphine oxides 2 with the concomitant formation of olefins 5 (Scheme 3). The overall result is that phosphonium salts 3 and 4 lose a methyl and benzyl group, respectively, and gain an oxygen from the aldehyde. It is noteworthy that phosphine oxides 2 are desired products and the olefins 5 are undesired products, in contrast to the traditional utilization of the Wittig reaction for the synthesis of olefins.29 The Wittig reaction has been used in the oxidative resolution of P-stereogenic tertiary phosphines.30−32 The synthesis of phosphine oxide-based surfactants via the ring-opening Wittig olefination of a macrocyclic phosphoranylidene has also been reported.33

Scheme 3. Quaternization–Wittig Strategy.

Scheme 3

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To realize this strategy, we at first explored the synthesis of phosphonium salts 3 and 4 by reactions of aryl bromides with commercially available MePPh2 and BnPPh2, respectively. We found that these phosphonium salts could be readily prepared by adapting the Ni-catalyzed method reported34 by Marcoux and Charette for the synthesis of aryltriphenylphosphonium salts and our metal-free method.26 When a mixture of MePPh2 and p-bromotoluene was refluxed in phenol for 5 h in the presence of 6 mol % NiBr2, phosphonium salt 3a was isolated in 75% yield (Table 2). A variety of functional groups can be tolerated, including electro-withdrawing groups (p-Ac, p-Br, o-F, and o-COOH), electron-donating groups (p-OMe, p-OPh, o-OH, o-NH2, m-Me, and m-OMe). Polyaromatic aryl bromides also underwent quaternization of MePPh2 smoothly to afford phosphonium salts 3d, 3m, and 3n in 73–90% yields. It is particularly noteworthy that 3l and 3n were obtained in the absence of NiBr2, although longer reaction times were required (16 and 8 h, respectively). Phosphonium salt 3d could also be obtained in a comparable yield (86% vs 90%) in the absence of NiBr2, but also required a longer reaction time (20 h vs 5 h). In addition, phosphonium salts 3b and 3h were obtained in high yields in 15 min, and the demethylation of their methoxy groups was observed by thin layer chromatography (TLC) for longer reaction time, resulting in lower yields. Under the same conditions as MePPh2, the quaternization of BnPPh2 provided phosphonium salts 4a–e in 48–83% yields. Several isolated examples have been reported for the quaternization of BnPPh2 with aryl halides by using the early Ni-catalyzed methods.35−38

Table 2. Quaternization of MePPh2 and BnPPh2 with Aryl Bromidesa.

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a

Quaternization conditions: Ph2PCH2R, ArBr (1 equiv), NiBr2 (6 mol %), phenol, reflux, 5 h.

b

Refluxed for 15 min.

c

In the absence of NiBr2, 3d was isolated in 86% yield after 20 h.

d

Without NiBr2, 16 h.

e

Without NiBr2, 8 h.

After completion of the synthesis of phosphonium salts 3 and 4, we next examined their Wittig reactions in refluxing acetonitrile using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a base. Furan-2-carbaldehyde 6a was selected as the model aldehyde given its cheapness, aromaticity, absence of α-proton, and low molecular weight; more importantly, the resulting byproduct, 2-vinylfuran, has not only a low molecular weight close to that of toluene (94 vs 92), providing a better atom economy, but also a low bp (ca. 100 °C),39 making separation easy. The results for Wittig reactions of phosphonium salts 3 and 4 are summarized in Table 3. Four diphenylphosphine oxides (2a, 2c, 2f, and 2g) could be obtained albeit in lower yields than those obtained by alkaline hydrolysis (entries 1–2 and 8–9). (4-Phenoxyphenyl)diphenylphosphine oxide 2x was obtained in a good yield (entry 3). Methylphosphonium salt 3d gave phosphine oxide 2d in an excellent yield, in contrast to the yield of less than 16% by alkaline hydrolysis (entry 4). The Wittig reaction of phosphonium salt 3e with aldehyde 6a did not produce phosphine oxide 2e, but replacing 3e with 4a provides 2e in an excellent yield (entries 5 and 6). For phosphonium salts 3k and 3m, phosphine oxides 2r and 2i were obtained in moderate yields but still much higher than those by alkaline hydrolysis (entries 13 and 16). Phosphine oxide 2o was obtained in a low yield, possibly because the methyl ketone moiety of 3f might undergo aldol reaction with aldehyde 6a (entry 7). Phosphine oxide 2q was not detected under the standard conditions, but could be obtained by using t-BuOK as a base and tetrahydrofuran (THF) as a solvent at rt, albeit in a low yield (entry 12), possibly because of the side reaction of the amino group of 3j with aldehyde 6a to form imine. A higher yield (60%) of 2q could be obtained by the alkaline hydrolysis of 4c. For both 3i and 3l, their Wittig reactions with aldehyde 6a did not produce the corresponding phosphine oxides 2; however, 4b and 4d underwent the Wittig reaction with aldehyde 6b in refluxing xylene smoothly to give 2p and 2s in 74 and 66% yields, respectively (entries 10–11 and 14–15). Under basic conditions, the deprotonation of ArOH of 3i to form ArO– would increase the electronic density of the arene ring and phosphorus atom, and thus lower the acidity of α-proton of the methyl group attached to phosphorus, making it difficult to form the corresponding nonstabilized ylide.29 The o-COO– group of 3l is likely to have the same effect. Replacing the methyl group with the benzyl group would be expected to make it more readily to form the corresponding semistabilized ylide.29 In addition, using more reactive aldehyde 6b because of electron-withdrawing p-Cl substitution instead of 6a would also facilitate the Wittig reaction. These results indicate that introducing a substituent on the benzene ring of Ph3P would strongly affect the reactivity of the corresponding quaternary phosphonium salts for the Wittig reaction. On the other hand, the robust Wittig reaction allows us to have several options such as aldehyde, alkyl group to form olefin, and temperature to achieve a satisfactory yield. Finally, phosphonium salt 3n gave 2w in 44% yield, and replacing 3n with 4e increased the yield of 2w to 60% (entries 17 and 18).

Table 3. Synthesis of Aryldiphenylphosphine Oxides by the Wittig Reactiona.

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a

Reaction conditions: phosphonium salt 3 or 4, aldehyde 6 (1 equiv), DBU (1.3 equiv), MeCN, reflux, 9 h.

b

Hydrolysis yields from Table 1 are listed in parentheses for comparison (some were estimated by the deduction of the yield of Ph3PO).

c

Xylene as a solvent.

d

Using t-BuOK as base and THF as solvent, rt, 3 h. Under the standard conditions, 2q was not detected. 2q was obtained in 60% yield by the hydrolysis of 4c at reflux for 3 h.

e

NMR yield.

It is noteworthy that phosphine oxides 2e, 2p, and 2s have been used as a ligand in the synthesis of catalysts40−42 or as starting materials in the synthesis of more functionalized phosphine oxides.43−45 It has been reported15 that phosphine oxide 2p could be synthesized starting from 1-bromo-2-methoxybenzene via a three-step route (Grignard-diphenylphosphorylation-deprotection), in which Grignard reagent, Ph2P(O)Cl, and BBr3 all are very sensitive to air and moisture, leading to a total yield of 55%. Our method can directly start from 2-bromophenol via a two-step route, in which only BnPPh2 is slightly sensitive to air and moisture, and thus provides a facile, protection-free access to 2p in a total yield of 61% (quaternization, 83%, 4b in Table 2; Wittig, 74%, entry 11 in Table 3).

In summary, we have demonstrated that aryldiphenylphosphine oxides can be synthesized via a quaternization–Wittig sequence: the nickel-catalyzed (or metal-free) quaternization of MePPh2 or BnPPh2 with aryl bromides provides the corresponding quaternary phosphonium salts followed by their Wittig reactions with furan-2-carbaldehyde or p-chlorobenzaldehyde to afford aryldiphenylphosphine oxides in 27–90% yields. In addition, our studies on the alkaline hydrolysis of aryltriphenylphosphonium bromides strengthen the notion that it is strongly dependent on the position, electronic effect of substituents of the arene ring, and provide further evidence for the ligand coupling mechanism in case of (p-acylphenyl)triphenylphosphonium salts.

Experimental Section

General

All of the reactions were performed under nitrogen by connecting the flask to a nitrogen balloon. Aryltriphenylphosphonium bromides 1 were prepared according to our previously reported method.26 The yield and NMR data of 1o are listed here because it was not reported in our previous paper.26 Petroleum ether (PE, bp 60–90 °C) was used.

(4-Acetylphenyl)triphenylphosphonium Bromide (1o)34

Prepared from p-BrC6H4Ac and Ph3P on a 1 mmol scale according to our previous paper.26 A white solid, yield: 55% (255 mg). 1H NMR (400 MHz, DMSO-d6): δ 2.69 (s, 3 H), 7.73–7.87 (m, 12 H), 7.92 (dd, J1 = 8.1 Hz, J2H–P = 12.1 Hz, 2 H), 7.96–8.02 (m, 3 H), 8.29 (d, J = 8.1 Hz, 2 H); 13C NMR (100 MHz, DMSO-d6): δ 27.7, 117.7 (d, JC–P = 89.3 Hz), 123.0 (d, JC–P = 87.5 Hz), 129.9 (d, JC–P = 13.1 Hz), 131.0 (d, JC–P = 12.9 Hz), 135.1 (d, JC–P = 10.6 Hz), 135.7 (d, JC–P = 10.8 Hz), 136.0 (d, JC–P = 2.8 Hz), 141.9 (d, JC–P = 2.8 Hz), 198.1; 31P NMR (162 MHz, DMSO-d6): δ 22.29.

General Procedure for the Preparation of Phosphonium Salts 3 and 4

A 0.67 M solution of methyldiphenylphosphine or benzyldiphenylphosphine, arylbromide (1 equiv), and NiBr2 (6 mol %) in phenol was refluxed for 15 min to 20 h. After the reaction, water (10 mL × 3) was added to azetropically remove phenol by evaporation under reduced pressure. Phosphonium salt 3 or 4 was isolated from the residue by column chromatography [PE/dichloromethane (DCM) 2:1 then DCM/MeOH 30:1, v/v] and dried at 110 °C till no solvent residue was detected by 1H NMR.

Methyldiphenyl(p-tolyl)phosphonium Bromide (3a)46

The synthesis was run on a 5 mmol scale for 5 h to give 3a as a white solid in 75% (1.400 g) yield. 1H NMR (400 MHz, DMSO-d6): δ 2.46 (s, 3 H), 3.16 (d, JH–P = 14.6 Hz, 3 H), 7.59 (dd, J1H–P = 3.1 Hz, J2 = 8.2 Hz, 2 H), 7.67 (dd, J1 = 8.2 Hz, J2H–P = 12.9 Hz, 2 H), 7.73–7.81 (m, 8 H), 7.85–7.92 (m, 2 H); 13C NMR (100 MHz, DMSO-d6): δ 7.9 (d, JC–P = 55.7 Hz), 21.7, 116.8 (d, JC–P = 90.7 Hz), 120.1 (d, JC–P = 88.3 Hz), 130.5 (d, JC–P = 12.6 Hz), 131.2 (d, JC–P = 13.2 Hz), 133.66 (d, JC–P = 10.8 Hz), 133.70 (d, JC–P = 11.1 Hz), 135.2 (d, JC–P = 2.8 Hz), 146.1 (d, JC–P = 3.0 Hz); 31P NMR (162 MHz, DMSO-d6): δ 23.31.

(4-Methoxyphenyl)(methyl)diphenylphosphonium Bromide (3b)47

The synthesis was run on a 2.5 mmol scale for 15 min to give 3b as a white solid in 81% (0.781 g) yield. 1H NMR (400 MHz, DMSO-d6): δ 3.14 (d, JH–P = 14.6 Hz, 3 H), 3.90 (s, 3 H), 7.32 (dd, J1H–P = 2.5 Hz, J2 = 8.9 Hz, 2 H), 7.70 (dd, J1 = 8.9 Hz, J2H–P = 12.5 Hz, 2 H), 7.73–7.81 (m, 8 H), 7.84–7.91 (m, 2 H); 13C NMR (100 MHz, DMSO-d6): δ 8.21 (d, JC–P = 56.2 Hz), 56.5, 110.1 (d, JC–P = 95.2 Hz), 116.4 (d, JC–P = 13.9 Hz), 121.1 (d, JC–P = 88.7 Hz), 130.5 (d, JC–P = 12.7 Hz), 133.6 (d, JC–P = 10.7 Hz), 135.1 (d, JC–P = 2.9 Hz), 135.9 (d, JC–P = 12.4 Hz), 164.6 (d, JC–P = 3.0 Hz); 31P NMR (162 MHz, DMSO-d6): δ 21.82.

Methyl(4-phenoxyphenyl)diphenylphosphonium Bromide (3c)

The synthesis was run on a 5 mmol scale for 5 h to give 3c as a white solid in 64% (1.440 g) yield. 1H NMR (400 MHz, DMSO-d6): δ 3.13 (d, JH–P = 14.6 Hz, 3 H), 7.20 (d, J = 7.7 Hz, 2 H), 7.25–7.34 (m, 3 H), 7.51 (t, J = 7.9 Hz, 2 H), 7.72–7.82 (m, 10 H), 7.88–7.92 (m, 2 H); 13C NMR (100 MHz, DMSO-d6): δ 8.1 (d, JC–P = 55.8 Hz), 112.9 (d, JC–P = 93.7 Hz), 118.9 (d, JC–P = 14.0 Hz), 120.8, 120.7 (d, JC–P = 88.6 Hz), 125.9, 130.6 (d, JC–P = 12.9 Hz), 131.0, 133.7 (d, JC–P = 10.9 Hz), 135.2 (d, JC–P = 2.9 Hz), 136.4 (d, JC–P = 12.4 Hz), 154.6, 163.1 (d, JC–P = 3.3 Hz); 31P NMR (162 MHz, DMSO-d6): δ 22.06. HRMS-ESI (positive) m/z: calcd for C25H22OP+ [M – Br–], 369.1408; found, 369.1415.

[1,1′-Biphenyl]-4-yl(methyl)diphenylphosphonium Bromide (3d)

The synthesis was run on a 10 mmol scale for 5 h to give 3d as a white solid in 90% (3.893 g) yield. In the absence of NiBr2, 3d was obtained in 86% (0.373 g) yield after a reaction of 1 mmol scale for 20 h. 1H NMR (400 MHz, DMSO-d6): δ 3.23 (d, JH–P = 14.6 Hz, 3 H), 7.46–7.51 (m, 1 H), 7.52–7.58 (m, 2 H), 7.75–7.87 (m, 12 H), 7.88–7.95 (m, 2 H), 8.05–8.11 (m, 2 H); 13C NMR (100 MHz, DMSO-d6): δ 8.0 (d, JC–P = 55.1 Hz), 118.9 (d, JC–P = 90.0 Hz), 120.5 (d, JC–P = 88.4 Hz), 127.7, 128.6 (d, JC–P = 13.1 Hz), 129.6, 129.7, 130.6 (d, JC–P = 12.7 Hz), 133.8 (d, JC–P = 10.9 Hz), 134.4 (d, JC–P = 11.3 Hz), 135.3 (d, JC–P = 2.7 Hz), 138.5 (d, JC–P = 0.9 Hz), 146.5 (d, JC–P = 3.1 Hz); 31P NMR (162 MHz, DMSO-d6): δ 22.54. HRMS-ESI (positive) m/z: calcd for C25H22P+ [M – Br–], 353.1459; found, 353.1454.

(4-Bromophenyl)(methyl)diphenylphosphonium Bromide (3e)46

The synthesis was run on a 1 mmol scale (1,4-dibromobenzene, 2 mmol) for 5 h to give 3e as a white solid in 56% (0.245 g) yield. 1H NMR (400 MHz, DMSO-d6): δ 3.18 (d, JH–P = 14.7 Hz, 3 H), 7.69 (d, J1 = 8.4 Hz, J2H–P = 12.4 Hz, 2 H), 7.73–7.83 (m, 8 H), 7.85–7.93 (m, 2 H), 7.98–8.03 (m, 2 H); 13C NMR (100 MHz, DMSO-d6): δ 7.8 (d, JC–P = 55.2 Hz), 119.9 (d, JC–P = 89.8 Hz), 120.0 (d, JC–P = 88.3 Hz), 130.0 (d, JC–P = 3.5 Hz), 130.6 (d, JC–P = 12.7 Hz), 133.6 (d, JC–P = 13.3 Hz), 133.8 (d, JC–P = 10.9 Hz), 135.4 (d, JC–P = 2.8 Hz), 135.6 (d, JC–P = 11.8 Hz); 31P NMR (162 MHz, DMSO-d6): δ 23.22.

(4-Acetylphenyl)(methyl)diphenylphosphonium Bromide (3f)46

The synthesis was run on a 1 mmol scale for 5 h to give 3f as a white solid in 73% (0.292 g) yield. 1H NMR (400 MHz, DMSO-d6): δ 2.68 (s, 3 H), 3.25 (d, JH–P = 14.7 Hz, 3 H), 7.75–7.85 (m, 8 H), 7.89–7.98 (m, 4 H), 8.25 (dd, J1H–P = 2.8 Hz, J2 = 8.4 Hz, 2 H); 13C NMR (100 MHz, DMSO-d6): δ 7.74 (d, JC–P = 55.1 Hz), 27.7, 119.9 (d, JC–P = 88.5 Hz), 125.3 (d, JC–P = 86.6 Hz), 129.6 (d, JC–P = 13.0 Hz), 130.7 (d, JC–P = 12.9 Hz), 133.8 (d, JC–P = 10.9 Hz), 134.4 (d, JC–P = 11.1 Hz), 135.5 (d, JC–P = 2.9 Hz), 141.6 (d, JC–P = 2.8 Hz), 198.2; 31P NMR (162 MHz, DMSO-d6): δ 23.07.

Methyldiphenyl(m-tolyl)phosphonium Bromide (3g)46

The synthesis was run on a 5 mmol scale for 5 h to give 3g as a white solid in 73% (1.357 g) yield. 1H NMR (400 MHz, DMSO-d6): δ 2.40 (s, 3 H), 3.19 (d, JH–P = 14.6 Hz, 3 H), 7.51 (dd, J1 = 7.7 Hz, J2H–P = 13.2 Hz, 1 H), 7.62–7.73 (m, 3 H), 7.74–7.82 (m, 8 H), 7.86–7.93 (m, 2 H); 13C NMR (100 MHz, DMSO-d6): δ 7.9 (d, JC–P = 55.5 Hz), 21.4, 120.2 (d, JC–P = 87.6 Hz), 120.5 (d, JC–P = 88.1 Hz), 130.4 (d, JC–P = 12.6 Hz), 130.6 (d, JC–P = 12.6 Hz), 131.0 (d, JC–P = 10.8 Hz), 133.6 (d, JC–P = 10.4 Hz), 133.7 (d, JC–P = 10.7 Hz), 135.2 (d, JC–P = 2.9 Hz), 136.0 (d, JC–P = 2.7 Hz), 140.4 (d, JC–P = 12.9 Hz); 31P NMR (162 MHz, DMSO-d6): δ 22.48.

(3-Methoxyphenyl)(methyl)diphenylphosphonium Bromide (3h)

The synthesis was run on a 2.5 mmol scale for 15 min to give 3h as a white solid in 70% (0.681 g) yield. 1H NMR (400 MHz, DMSO-d6): δ 3.21 (d, JH–P = 14.7 Hz, 3 H), 3.83 (s, 3 H), 7.20 (dd, J1 = 7.7 Hz, J2H–P = 13.0 Hz, 1 H), 7.36 (d, JH–P = 14.7 Hz, 1 H), 7.47 (d, J = 8.4 Hz, 1 H), 7.65–7.72 (m, 1 H), 7.73–7.82 (m, 8 H), 7.87–7.93 (m, 2 H); 13C NMR (100 MHz, DMSO-d6): δ 7.8 (d, JC–P = 55.2 Hz), 56.4, 119.0 (d, JC–P = 12.4 Hz), 120.4 (d, JC–P = 88.1 Hz), 120.8 (d, JC–P = 2.8 Hz), 121.6 (d, JC–P = 87.9 Hz), 125.8 (d, JC–P = 10.3 Hz), 130.6 (d, JC–P = 12.8 Hz), 132.1 (d, JC–P = 14.9 Hz), 133.7 (d, JC–P = 10.9 Hz), 135.3 (d, JC–P = 2.8 Hz), 160.4 (d, JC–P = 16.2 Hz); 31P NMR (162 MHz, DMSO-d6): δ 23.04. HRMS-ESI (positive) m/z: calcd for C20H20OP+ [M – Br–], 307.1252; found, 307.1255.

(2-Hydroxyphenyl)(methyl)diphenylphosphonium Bromide (3i)

The synthesis was run on a 5 mmol scale for 5 h to give 3i as a white solid in 69% (1.281 g) yield. 1H NMR (400 MHz, DMSO-d6): δ 3.02 (d, J1H–P = 14.6 Hz, 3 H), 6.98–7.06 (m, 1 H), 7.07–7.13 (m, 1 H), 7.22 (dd, J1H–P = 5.9 Hz, J2 = 8.1 Hz, 1 H), 7.69–7.81 (m, 9 H), 7.83–7.89 (m, 2 H), 11.59 (s, 1 H); 13C NMR (100 MHz, DMSO-d6): δ 8.2 (d, JC–P = 57.3 Hz), 104.7 (d, JC–P = 91.3 Hz), 117.3 (d, JC–P = 6.8 Hz), 120.7 (d, JC–P = 89.8 Hz), 120.8 (d, JC–P = 12.8 Hz), 130.4 (d, JC–P = 12.9 Hz), 133.3 (d, JC–P = 10.8 Hz), 134.8 (d, JC–P = 2.8 Hz), 135.0 (d, JC–P = 9.1 Hz), 137.8 (d, JC–P = 1.4 Hz), 161.6 (d, JC–P = 2.6 Hz); 31P NMR (162 MHz, DMSO-d6): δ 20.80. HRMS-ESI (positive) m/z: calcd for C19H18OP+ [M – Br–], 293.1095; found, 293.1098.

(2-Aminophenyl)(methyl)diphenylphosphonium Bromide (3j)48

The synthesis was run on a 5 mmol scale for 5 h to give 3j as a black solid in 66% (1.223 g) yield. 1H NMR (400 MHz, DMSO-d6): δ 3.10 (d, JH–P = 14.2 Hz, 3 H), 5.84 (br s, 2 H), 6.65–6.75 (m, 2 H), 7.03–7.08 (m, 1 H), 7.45–7.52 (m, 1 H), 7.70–7.82 (m, 8 H), 7.85–7.91 (m, 2 H); 13C NMR (100 MHz, DMSO-d6): δ 7.2 (d, JC–P = 55.5 Hz), 97.1 (d, JC–P = 89.5 Hz), 117.3 (d, JC–P = 12.9 Hz), 117.8 (d, JC–P = 8.1 Hz), 120.0 (d, JC–P = 88.2 Hz), 130.6 (d, JC–P = 12.7 Hz), 133.6 (d, JC–P = 10.8 Hz), 134.8 (d, JC–P = 10.7 Hz), 135.1 (d, JC–P = 2.6 Hz), 136.4 (d, JC–P = 1.8 Hz), 152.6 (d, JC–P = 6.0 Hz); 31P NMR (162 MHz, DMSO-d6): δ 20.38.

(2-Fluorophenyl)(methyl)diphenylphosphonium Bromide (3k)

The synthesis was run on a 1 mmol scale for 5 h to give 3k as a white solid in 90% (0.338 g) yield. 1H NMR(400 MHz, DMSO-d6): δ 3.16 (d, JH–P = 14.7 Hz, 3 H), 7.39–7.50 (m, 1 H), 7.58–7.64 (m, 1 H), 7.64–7.71 (m, 1 H), 7.75–7.87 (m, 8 H), 7.92 (t, J = 6.9 Hz, 2 H), 7.98–8.07 (m, 1 H); 13C NMR (100 MHz, DMSO-d6): δ 8.4 (d, JC–P = 54.1 Hz), 108.8 (d, JC–P = 16.1 Hz), 118.0 (dd, J1C–F = 5.5 Hz, J2C–P = 21.1 Hz), 119.3 (d, JC–P = 90.0 Hz), 126.9 (dd, J1C–F = 3.0 Hz, J2C–P = 11.6 Hz), 130.7 (d, JC–P = 13.1 Hz), 133.5 (d, JC–P = 11.3 Hz), 135.6 (d, JC–P = 2.9 Hz), 135.8 (d, JC–P = 8.6 Hz), 139.3 (d, JC–P = 8.8 Hz), 163.9 (d, JC–F = 249.8 Hz); 31P NMR (162 MHz, DMSO-d6): δ 20.26; 19F NMR (376 MHz, DMSO-d6): δ −98.60. HRMS-ESI (positive) m/z: calcd for C19H17FP+ [M – Br–], 295.1052; found, 295.1050.

2-(Methyldiphenylphosphonio)benzoate (3l)

In the absence of NiBr2, the synthesis was run on a 1 mmol scale for 8 h to give 3l as a white solid and zwitterion in 74% (0.236 g) yield. 1H NMR (400 MHz, DMSO-d6): δ 2.88 (d, JH–P = 13.7 Hz, 3 H), 6.89–7.00 (m, 1 H), 7.4–7.50 (m, 4 H), 7.53–7.70 (m, 7 H), 7.83–7.90 (m, 1 H), 8.16–8.21 (m, 1 H); 13C NMR (100 MHz, DMSO-d6): δ 15.4 (d, JC–P = 73.8 Hz), 122.9 (d, JC–P = 109.3 Hz), 129.5 (d, JC–P = 12.3 Hz), 129.9 (d, JC–P = 6.8 Hz), 130.3 (d, JC–P = 121.9 Hz), 130.8 (d, JC–P = 4.6 Hz), 131.4 (d, JC–P = 9.4 Hz), 132.0 (d, JC–P = 2.4 Hz), 135.1 (d, JC–P = 3.0 Hz), 136.1 (d, JC–P = 15.1 Hz), 144.2 (d, JC–P = 9.1 Hz), 165.3; 31P NMR (162 MHz, DMSO-d6): δ 8.19. HRMS-ESI (positive) m/z: calcd for C20H18O2P+ [M + H+], 321.1044; found, 321.1038.

Methyl(naphthalen-2-yl)diphenylphosphonium Bromide (3m)

The synthesis was run on a 2.5 mmol scale for 5 h to give 3m as a white solid in 88% (0.897 g) yield. 1H NMR (400 MHz, DMSO-d6): δ 3.27 (d, JH–P = 14.6 Hz, 3 H), 7.72–7.88 (m, 11 H), 7.89–7.95 (m, 2 H), 8.10–8.18 (m, 2 H), 8.31 (d, J1H–P = 3.2 Hz, J2 = 8.6 Hz, 1 H), 8.49 (d, JH–P = 15.4 Hz, 1 H); 13C NMR (100 MHz, DMSO-d6): δ 8.1 (d, JC–P = 55.7 Hz), 117.4 (d, JC–P = 89.2 Hz), 120.5 (d, JC–P = 88.3 Hz), 126.9 (d, JC–P = 11.4 Hz), 128.6, 128.7, 129.6, 130.4 (d, JC–P = 12.6 Hz), 130.61 (d, JC–P = 12.8 Hz), 130.62, 132.5 (d, JC–P = 14.6 Hz), 133.8 (d, JC–P = 10.8 Hz), 135.3 (d, JC–P = 2.9 Hz), 135.5 (d, JC–P = 2.6 Hz), 136.7 (d, JC–P = 10.6 Hz); 31P NMR (162 MHz, DMSO-d6): δ 22.94. HRMS-ESI (positive) m/z: calcd for C23H20P+ [M – Br–], 327.1303; found, 327.1298.

Methyl(phenanthren-9-yl)diphenylphosphonium Bromide (3n)

In the absence of NiBr2, the synthesis was run on a 5 mmol scale for 16 h to give 3n as a white solid in 73% (1.680 g) yield. 1H NMR (400 MHz, DMSO-d6): δ 3.34 (d, JH–P = 14.2 Hz, 3 H), 7.59 (t, J = 7.6 Hz, 1 H), 7.70–7.86 (m, 7 H), 7.89–7.97 (m, 6 H), 8.00 (t, J = 7.6 Hz, 1 H), 8.12 (d, J = 7.8 Hz, 1 H), 8.19 (d, JH–P = 18.7 Hz, 1 H), 9.03 (d, J = 8.4 Hz, 1 H), 9.10 (d, J = 8.4 Hz, 1 H); 13C NMR (100 MHz, DMSO-d6): δ 9.8 (d, JC–P = 56.4 Hz), 114.8 (d, JC–P = 86.8 Hz), 120.3 (d, JC–P = 87.9 Hz), 123.8, 125.2, 126.9 (d, JC–P = 6.5 Hz), 128.6, 128.70 (d, JC–P = 9.1 Hz), 128.74, 128.8, 129.6 (d, JC–P = 15.4 Hz), 130.8 (d, JC–P = 12.9 Hz), 131.1 (d, JC–P = 8.7 Hz), 131.2, 131.9, 132.9 (d, JC–P = 2.1 Hz), 133.8 (d, JC–P = 10.8 Hz), 135.4 (d, JC–P = 2.9 Hz), 141.9 (d, JC–P = 10.6 Hz); 31P NMR (162 MHz, DMSO-d6): δ 22.83. HRMS-ESI (positive) m/z: calcd for C27H22P+ [M – Br–], 377.1459; found, 377.1458.

Benzyl(4-bromophenyl)diphenylphosphonium Bromide (4a)

The synthesis was run on a 2.68 mmol scale (1,4-dibromobenzene, 5.36 mmol) for 5 h to give 4a as a white solid in 48% (0.661 g) yield. 1H NMR (400 MHz, DMSO-d6): δ 5.19 (d, JH–P = 15.6 Hz, 2 H), 6.95–7.03 (m, 2 H), 7.21–7.35 (m, 3 H), 7.53–7.62 (m, 2 H), 7.65–7.80 (m, 8 H), 7.88–7.95 (m, 2 H), 7.96–8.02 (m, 2 H); 13C NMR (100 MHz, DMSO-d6): δ 28.5 (d, JC–P = 46.2 Hz), 117.7 (d, JC–P = 87.3 Hz), 117.9 (d, JC–P = 85.8 Hz), 128.2 (d, JC–P = 8.7 Hz), 128.9 (d, JC–P = 3.9 Hz), 129.3 (d, JC–P = 3.1 Hz), 130.4 (d, JC–P = 3.5 Hz), 130.6 (d, JC–P = 12.5 Hz), 131.3 (d, JC–P = 5.7 Hz), 133.7 (d, JC–P = 13.0 Hz), 134.6 (d, JC–P = 10.0 Hz), 135.7 (d, JC–P = 2.6 Hz), 136.3 (d, JC–P = 10.7 Hz); 31P NMR (162 MHz, DMSO-d6): δ 23.46. HRMS-ESI (positive) m/z: calcd for C25H21BrP+ [M – Br–], 431.0564; found, 431.0568.

Benzyl(2-hydroxyphenyl)diphenylphosphonium Bromide (4b)49

The synthesis was run on a 4 mmol scale for 5 h to give 4b as a white solid in 83% (1.489 g) yield. 1H NMR (400 MHz, DMSO-d6): δ 4.98–5.08 (m, 2 H), 6.99 (d, J = 6.9 Hz, 2 H), 7.07–7.38 (m, 6 H), 7.48–7.60 (m, 4 H), 7.64–7.78 (m, 5 H), 7.80–7.88 (m, 2 H), 11.83 (br s, 1 H); 13C NMR (100 MHz, DMSO-d6): δ 29.0 (d, JC–P = 48.6 Hz), 103.1 (d, JC–P = 88.7 Hz), 117.7 (d, JC–P = 7.2 Hz), 118.5 (d, JC–P = 87.4 Hz), 120.9 (d, JC–P = 12.8 Hz), 128.7 (d, JC–P = 3.4 Hz), 129.0 (d, JC–P = 8.5 Hz), 129.2 (d, JC–P = 2.8 Hz), 130.2 (d, JC–P = 12.5 Hz), 131.1 (d, JC–P = 5.7 Hz), 134.0 (d, JC–P = 10.0 Hz), 135.0 (d, JC–P = 2.4 Hz), 135.4 (d, JC–P = 8.4 Hz), 138.1 (d, JC–P = 1.5 Hz), 162.2; 31P NMR (162 MHz, DMSO-d6): δ 20.81.

(2-Aminophenyl)(benzyl)diphenylphosphonium Bromide (4c)48

The synthesis was run on a 2 mmol scale for 5 h to give 4c as a black solid in 70% (0.629 g) yield. 1H NMR (400 MHz, DMSO-d6): δ 5.12 (d, JH–P = 15.4 Hz, 2 H), 5.83 (s, 2 H), 6.71–6.79 (m, 1 H), 6.98–7.11 (m, 4 H), 7.17–7.32 (m, 3 H), 7.49–7.62 (m, 5 H), 7.65–7.75 (m, 4 H), 7.83–7.90 (m, 2 H); 13C NMR (100 MHz, DMSO-d6): δ 28.0 (d, JC–P = 48.0 Hz), 95.0 (d, JC–P = 87.3 Hz), 117.6 (d, JC–P = 12.2 Hz), 118.1 (d, JC–P = 85.2 Hz), 118.2 (d, JC–P = 8.4 Hz), 128.7 (d, JC–P = 3.4 Hz), 129.0 (d, JC–P = 8.2 Hz), 129.3 (d, JC–P = 2.7 Hz), 130.4 (d, JC–P = 12.3 Hz), 131.2 (d, JC–P = 5.8 Hz), 134.2 (d, JC–P = 9.7 Hz), 135.2 (d, JC–P = 9.5 Hz), 135.3 (d, JC–P = 2.6 Hz), 136.7, 153.0 (d, JC–P = 5.7 Hz); 31P NMR (162 MHz, DMSO-d6): δ 19.96.

2-(Benzyldiphenylphosphonio)benzoate (4d)50

The synthesis was run on a 2.5 mmol scale for 5 h to give 4d as a white solid and zwitterion in 75% (0.748 g) yield. 1H NMR (400 MHz, DMSO-d6): δ 4.80 (d, JH–P = 14.1 Hz, 2 H), 6.93–7.03 (m, 2 H), 7.10–7.22 (m, 3 H), 7.23–7.40 (m, 5 H), 7.45–7.55 (m, 4 H), 7.58–7.68 (m, 3 H), 7.84–7.91 (m, 1 H), 8.19–8.24 (m, 1 H); 13C NMR (100 MHz, DMSO-d6): δ 35.7 (d, JC–P = 61.1 Hz), 121.6 (d, JC–P = 103.6 Hz), 127.9 (d, JC–P = 3.3 Hz), 128.5 (d, JC–P = 94.6 Hz), 128.8 (d, JC–P = 2.5 Hz), 129.3 (d, JC–P = 12.3 Hz), 130.2 (d, JC–P = 10.4 Hz), 131.1 (d, JC–P = 6.7 Hz), 131.3, 131.5 (d, JC–P = 8.5 Hz), 132.1 (d, JC–P = 8.7 Hz), 132.2 (d, JC–P = 2.4 Hz), 135.2 (d, JC–P = 2.8 Hz), 136.6 (d, JC–P = 13.3 Hz), 144.1 (d, JC–P = 8.9 Hz), 165.7; 31P NMR (162 MHz, DMSO-d6): δ 10.73.

Benzyl(phenanthren-9-yl)diphenylphosphonium Bromide (4e)

The synthesis was run on a 2.5 mmol scale for 5 h to give 4e as a white solid in 62% (0.826 g) yield. 1H NMR (400 MHz, DMSO-d6): δ 5.46 (d, JH–P = 15.5 Hz, 2 H), 6.90 (d, J = 7.3 Hz, 2 H), 7.16 (dd, J1 = 7.1 Hz, J2 = 7.3 Hz, 2 H), 7.26 (t, J = 7.1 Hz, 1 H), 7.40 (d, J = 7.6 Hz, 1 H), 7.51 (d, J = 8.3 Hz, 1 H), 7.65–7.95 (m, 12 H), 8.02 (t, J = 7.6 Hz, 1 H), 8.31 (d, J = 7.9 Hz, 1 H), 8.93 (d, JH–P = 17.7 Hz, 1 H), 9.03 (d, J = 9.0 Hz, 1 H), 9.05 (d, J = 9.8 Hz, 1 H); 13C NMR (100 MHz, DMSO-d6): δ 30.8 (d, JC–P = 48.1 Hz), 113.2 (d, JC–P = 83.4 Hz), 118.2 (d, JC–P = 84.5 Hz), 123.8, 125.2, 127.0 (d, JC–P = 5.7 Hz), 128.1, 128.4 (d, JC–P = 7.9 Hz), 128.5 (d, JC–P = 6.1 Hz), 128.6, 128.79, 128.83 (d, JC–P = 4.2 Hz), 129.2 (d, JC–P = 2.8 Hz), 129.7 (d, JC–P = 14.9 Hz), 130.6 (d, JC–P = 12.5 Hz), 131.2 (d, JC–P = 8.8 Hz), 131.38 (d, JC–P = 5.3 Hz), 131.41, 132.0, 133.0 (d, JC–P = 2.3 Hz), 134.4 (d, JC–P = 9.9 Hz), 135.5 (d, JC–P = 2.4 Hz), 142.3 (d, JC–P = 7.9 Hz); 31P NMR (162 MHz, DMSO-d6): δ 21.06. HRMS-ESI (positive) m/z: calcd for C33H26P+ [M – Br–], 453.1772; found, 453.1766.

General Procedure for the Alkaline Hydrolysis of Aryltriphenylphosphonium Salts

To a round-bottom flask (5 mL) containing phosphonium salt 1 (0.5 mmol), 3 M NaOH (2 mL) was added. The resulting mixture was stirred at room temperature overnight or refluxed for 3–5 h. Then, the mixture was extracted with EtOAc (10 mL × 3), and the combined extract was evaporated to remove the solvent. The residue was dried in vacuo or further isolated by column chromatography or preparative TLC to give phosphine oxides 2.

General Procedure for the Preparation of Aryldiphenylphosphine Oxides 2 via the Wittig Reaction

Phosphonium salt 3 or 4 (0.5 mmol), aldehyde (0.5 mmol), DBU (0.65 mmol), and acetonitrile or xylene (3 mL) were added to a round-bottom flask (25 mL). After refluxed for 9 h, the mixture was evaporated to remove the solvent and isolated by preparative TLC (DCM/EtOAc, 2/1, v/v) to give phosphine oxides 2.

(4-(Hydroxymethyl)phenyl)diphenylphosphine Oxide (2k)10

The hydrolysis of 1k at reflux for 3 h gave 2k as a white solid in 71% (110 mg) yield. 1H NMR (400 MHz, DMSO-d6): δ 4.57 (d, J = 5.3 Hz, 2 H), 5.39 (d, J = 5.3 Hz, 1 H), 7.49 (d, J = 6.7 Hz, 2 H), 7.47–7.65 (m, 12 H); 13C NMR (100 MHz, DMSO-d6): δ 62.9, 126.9 (d, JC–P = 12.2 Hz), 129.2 (d, JC–P = 11.8 Hz), 131.2 (d, JC–P = 104.3 Hz), 131.89 (d, JC–P = 10.0 Hz), 131.93 (d, JC–P = 9.7 Hz), 132.4 (d, JC–P = 2.3 Hz), 133.4 (d, JC–P = 102.5 Hz), 147.4 (d, JC–P = 2.7 Hz); 31P NMR (400 MHz, DMSO-d6): δ 25.54.

(4-(Dimethylamino)phenyl)diphenylphosphine Oxide (2l)51

The hydrolysis of 1l at reflux for 3 h gave 2l as a white solid in 93% (150 mg) yield. 1H NMR (400 MHz, DMSO-d6): δ 3.00 (s, 6 H), 6.83 (d, J = 8.3 Hz, 2 H), 7.34–7.42 (m, 2 H), 7.52–7.67 (m, 10 H); 13C NMR (100 MHz, DMSO-d6): δ 44.7, 116.6 (d, JC–P = 12.6 Hz), 121.7 (d, JC–P = 114.2 Hz), 133.7 (d, JC–P = 11.5 Hz), 136.6 (d, JC–P = 9.6 Hz), 136.8 (d, JC–P = 2.3 Hz), 138.0 (d, JC–P = 11.0 Hz), 139.3 (d, JC–P = 102.1 Hz), 157.5 (d, JC–P = 2.3 Hz); 31P NMR (162 MHz, DMSO-d6): δ 30.54.

(4-Hydroxyphenyl)diphenylphosphine Oxide (2m)52

The hydrolysis of 1m (2.3 mmol) at reflux for 5 h gave 2m as a white solid in 75% (507 mg) yield. 1H NMR (400 MHz, DMSO-d6): δ 6.92 (d, J = 8.3 Hz, 2 H), 7.42 (d, J1 = 8.7 Hz, J2H–P = 10.8 Hz, 2 H), 7.50–7.65 (m, 10 H), 10.31 (s, 1 H); 13C NMR (100 MHz, DMSO-d6): δ 116.2 (d, JC–P = 13.1 Hz), 122.1 (d, JC–P = 110.6 Hz), 129.1 (d, JC–P = 11.7 Hz), 131.9 (d, JC–P = 9.6 Hz), 132.2 (d, JC–P = 2.0 Hz), 134.00 (d, JC–P = 11.2 Hz), 134.03 (d, JC–P = 102.5 Hz), 161.2 (d, JC–P = 2.8 Hz); 31P NMR (162 MHz, DMSO-d6): δ 25.64.

1-(4-(Diphenylphosphoryl)phenyl)ethanone (2o)53

The hydrolysis of 1o at rt did not produce observable 2o. After the reaction, the mixture was adjusted to pH 6–7 by adding NaHSO4 and then extracted with DCM (10 mL × 3). The combined extract was evaporated to remove DCM at about 5 °C under reduced pressure. The residue was isolated by prep TLC (PE/DCM 2:1 then PE/EtOAc 1:1, v/v) to give PhAc as a colorless oil in 20% (12 mg) yield, p-PhC6H4Ac as a white solid in 47% (46 mg) yield, Ph2HPO as a colorless oil in 21% (21 mg) yield, and Ph3PO as a white solid in 22% (30 mg) yield. These products were identified by comparing with authentic substances. The Wittig reaction of 3f with aldehyde 6a gave 2o as a yellow solid in 28% (44 mg) yield. 1H NMR (400 MHz, DMSO-d6): δ 2.62 (s, 3 H), 7.54–7.70 (m, 10 H), 7.75–7.82 (m, 2 H), 8.09 (d, J = 7.4 Hz, 2 H); 13C NMR (100 MHz, DMSO-d6): δ 27.4, 128.7 (d, JC–P = 11.8 Hz), 129.4 (d, JC–P = 11.9 Hz), 132.0 (d, JC–P = 10.0 Hz), 132.4 (d, JC–P = 10.0 Hz), 132.6 (d, JC–P = 103.3 Hz), 132.8 (d, JC–P = 2.7 Hz), 138.1 (d, JC–P = 99.5 Hz), 139.7 (d, JC–P = 2.8 Hz), 198.2; 31P NMR (162 MHz, DMSO-d6): δ 25.09.

(2-Hydroxyphenyl)diphenylphosphine Oxide (2p)14

The hydrolysis of 1p at reflux for 3 h gave 2p as a white solid in 6% (9 mg) yield and Ph3PO in 52% (72 mg) yield. For the Wittig reaction of 3i with aldehyde 6a using DBU (2.1 equiv), 2p was not detected. However, the Wittig reaction of 4b (0.22 mmol) with aldehyde 6b using DBU (2.2 equiv) in refluxing xylene gave 2p in 74% (48 mg) yield, along with the olefin byproduct (1-chloro-4-styrylbenzene) in 52% (25 mg) yield as a yellow solid of (Z)/(E) mixture (ca. 1:1 ratio estimated roughly by TLC). 1H NMR (400 MHz, DMSO-d6): δ 6.85–6.91 (m, 1 H), 6.94–7.00 (m, 1 H), 7.43–7.49 (m, 1 H), 7.50–7.72 (m, 11 H), 10.50 (br s, 1 H); 13C NMR (100 MHz, DMSO-d6): δ 116.7 (d, JC–P = 102.9 Hz), 117.0 (d, JC–P = 7.1 Hz), 119.7 (d, JC–P = 11.2 Hz), 128.9 (d, JC–P = 12.2 Hz), 131.8 (d, JC–P = 10.2 Hz), 132.2 (d, JC–P = 2.4 Hz), 133.9 (d, JC–P = 7.7 Hz), 133.6 (d, JC–P = 105.0 Hz), 134.7, 160.3 (d, JC–P = 2.5 Hz); 31P NMR (162 MHz, DMSO-d6): δ 27.38.

(2-Aminophenyl)diphenylphosphine Oxide (2q)6

The hydrolysis of 1q at reflux for 3 h gave 2q in 1% (2 mg) yield and Ph3PO in 83% (116 mg) yield. The Wittig reaction of 3j with aldehyde 6a according to the general procedure did not produce 2q, but using t-BuOK as a base and THF as a solvent at rt for 3 h gave 2q as a white solid in 27% (40 mg) yield. 2q could also be obtained in 60% yield by the hydrolysis of 4c at reflux for 3 h. 1H NMR (400 MHz, DMSO-d6): δ 6.15 (br s, 2 H), 6.49–6.55 (m, 1 H), 6.65–6.80 (m, 1 H), 7.19–7.27 (m, 1 H), 7.50–7.66 (m, 11 H); 13C NMR (100 MHz, DMSO-d6): δ 110.5 (d, JC–P = 105.0 Hz), 115.6 (d, JC–P = 12.7 Hz), 116.7 (d, JC–P = 8.3 Hz), 129.2 (d, JC–P = 11.8 Hz), 131.9 (d, JC–P = 9.7 Hz), 132.4 (d, JC–P = 2.6 Hz), 133.0 (d, JC–P = 11.2 Hz), 133.2 (d, JC–P = 102.6 Hz), 133.6 (d, JC–P = 1.8 Hz), 153.8 (d, JC–P = 4.5 Hz); 31P NMR (162 MHz, DMSO-d6): δ 32.83.

(2-Fluorophenyl)diphenylphosphine Oxide (2r)54

The hydrolysis of 1r at rt gave only Ph3PO in 100% (139 mg) yield. The Wittig reaction of 3k with aldehyde 6a gave 2r as a white solid in 48% (71 mg) yield. 1H NMR (400 MHz, DMSO-d6): δ 7.30–7.38 (m, 1 H), 7.39–7.45 (m, 1 H), 7.53–7.69 (m, 11 H), 7.69–7.77 (m, 1 H); 13C NMR (100 MHz, DMSO-d6): δ 116.9 (dd, J1C–P = 5.4 Hz, J2C–F = 22.4 Hz), 120.7 (dd, J1C–F = 18.4 Hz, J2C–P = 99.2 Hz), 125.6 (dd, J1C–P = 3.0 Hz, J2C–F = 10.5 Hz), 129.3 (d, JC–P = 12.3 Hz), 131.6 (d, JC–P = 10.3 Hz), 132.68 (d, JC–P = 106.5 Hz), 132.74 (d, JC–P = 2.6 Hz), 134.4 (dd, J1C–P = 3.9 Hz, J2C–F = 5.9 Hz), 136.0 (d, JC–F = 7.5 Hz), 162.9 (d, JC–F = 249.5 Hz); 31P NMR (162 MHz, DMSO-d6): δ 22.03; 19F NMR (376 MHz, DMSO-d6): δ −100.63.

2-(Diphenylphosphoryl)benzoic Acid (2s)55

The hydrolysis of 1s at reflux for 3 h gave Ph3PO in 100% (139 mg) yield, and 2s was not detected. The further acidification of the aqueous phase with 1 M HCl to pH 2–3 followed by extraction with DCM (10 mL × 3) and evaporation under reduced pressure gave PhCOOH as a white solid in 97% (59 mg) yield. The Wittig reaction of 3l with aldehyde 6a did not produce 2s. The Wittig reaction of 4d with aldehyde 6b using DBU (2.3 equiv) was carried out in refluxing xylene. After reaction, 1 M NaOH (10 mL) was added to the reaction mixture, which was then extracted with DCM (10 mL × 2). The organic phase after evaporation was isolated by preparative TLC (eluted with PE) to afford the olefin byproduct (1-chloro-4-styrylbenzene): (Z)-isomer, a white solid, yield: 26% (27 mg); (E)-isomer, a white solid, yield: 39% (42 mg). These two isomers were identified by TLC with the authentic samples prepared by our previously reported procedure.56 The aqueous phase was adjusted to pH 2–3 with 1 M HCl (15 mL) and then extracted with DCM (10 mL × 3). The combined extract after evaporation was isolated by column chromatography (PE/EtOAc 1:1 then DCM/MeOH 10:1, v/v) to give 2s as a yellow solid in 66% (106 mg) yield. 1H NMR (400 MHz, DMSO-d6): δ 7.47–7.61 (m, 11 H), 7.61–7.67 (m, 1 H), 7.68–7.75 (m, 1 H), 7.85–7.91 (m, 1 H), 13.05 (br s, 1 H); 13C NMR (100 MHz, DMSO-d6): δ 128.8 (d, JC–P = 12.1 Hz), 130.5 (d, JC–P = 8.5 Hz), 131.3 (d, JC–P = 11.6 Hz), 131.7 (d, JC–P = 9.8 Hz), 131.9 (d, JC–P = 2.5 Hz), 132.6 (d, JC–P = 2.2 Hz), 133.2 (d, JC–P = 106.7 Hz), 134.6 (d, JC–P = 10.4 Hz), 134.2 (d, JC–P = 106.9 Hz), 137.3 (d, JC–P = 6.3 Hz), 168.4 (d, JC–P = 2.8 Hz); 31P NMR (162 MHz, DMSO-d6): δ 28.88.

(1H-Indol-5-yl)diphenylphosphine Oxide (2t)57

The hydrolysis of 1t at reflux for 3 h gave 2t as a yellow solid in 81% (128 mg) yield. 1H NMR (400 MHz, DMSO-d6): δ 6.54 (d, J = 2.9 Hz, 1 H), 7.27–7.34 (m, 1 H), 7.45–7.68 (m, 12 H), 7.82 (d, JC–P = 13.1 Hz, 1 H), 11.71 (s, 1 H); 13C NMR (100 MHz, DMSO-d6): δ 102.5, 112.4 (d, JC–P = 13.7 Hz), 122.0 (d, JC–P = 108.6 Hz), 124.0 (d, JC–P = 11.9 Hz), 125.4 (d, JC–P = 11.2 Hz), 127.6, 127.8 (d, JC–P = 14.6 Hz), 129.0 (d, JC–P = 11.6 Hz), 132.0 (d, JC–P = 9.6 Hz), 132.1 (d, JC–P = 2.6 Hz), 134.5 (d, JC–P = 101.7 Hz), 138.0 (d, JC–P = 2.3 Hz); 31P NMR (162 MHz, DMSO-d6): δ 27.55.

(6-Methoxynaphthalen-2-yl)diphenylphosphine Oxide (2u)58

The hydrolysis of 1u at rt gave 2u as a yellow solid 94% (169 mg) yield. 1H (400 MHz, DMSO-d6): δ 3.90 (s, 3 H), 7.25 (d, J = 8.9 Hz, 1 H), 7.41 (s, 1 H), 7.50–7.72 (m, 11 H), 7.96 (t, J = 8.7 Hz, 2 H), 8.22 (d, JH–P = 13.4 Hz, 1 H); 13C NMR (100 MHz, DMSO-d6): δ 55.9, 106.5, 120.2, 127.6 (d, JC–P = 7.8 Hz), 127.7 (d, JC–P = 8.7 Hz), 127.5 (d, JC–P = 82.7 Hz), 128.0 (d, JC–P = 8.8 Hz), 129.2 (d, JC–P = 11.8 Hz), 131.0, 132.0 (d, JC–P = 9.8 Hz), 132.4 (d, JC–P = 2.5 Hz), 133.3 (d, JC–P = 9.5 Hz), 133.6 (d, JC–P = 102.5 Hz), 136.4 (d, JC–P = 2.2 Hz), 159.6; 31P NMR (162 MHz, DMSO-d6): δ 25.67.

(6-Hydroxynaphthalen-2-yl)diphenylphosphine Oxide (2v)59

The hydrolysis of 1v at reflux for 3 h gave 2v as a white solid in 90% (155 mg) yield. 1H NMR (400 MHz, DMSO-d6): δ 7.16–7.21 (m, 2 H), 7.45–7.51 (m, 1 H), 7.52–7.59 (m, 4 H), 7.60–7.70 (m, 6 H), 7.81 (dd, J1C–P = 2.3 Hz, J2 = 8.4 Hz, 1 H), 7.89 (d, J = 8.7 Hz, 1 H), 8.14 (d, JC–P = 13.5 Hz, 1 H), 10.3 (br s, 1 H); 13C NMR (100 MHz, DMSO-d6): δ 109.2, 120.3, 126.4 (d, JC–P = 105.8 Hz), 127.0 (d, JC–P = 11.5 Hz), 127.1 (d, JC–P = 13.2 Hz), 127.2 (d, JC–P = 10.3 Hz), 129.2 (d, JC–P = 11.7 Hz), 131.2, 132.0 (d, JC–P = 9.7 Hz), 132.4 (d, JC–P = 2.3 Hz), 133.5 (d, JC–P = 9.4 Hz), 133.6 (d, JC–P = 102.2 Hz), 136.7 (d, JC–P = 2.1 Hz), 158.1; 31P NMR (162 MHz, DMSO-d6): δ 25.86.

Phenanthren-9-yldiphenylphosphine Oxide (2w)6

The hydrolysis of 1w (0.25 mmol) at rt and isolation by preparative TLC (DCM/MeOH 2:1, v//v) gave 2w as a yellow solid in 2% (2 mg) yield, Ph3PO in 80% (56 mg) yield, and phenanthracene as a white solid in 93% (42 mg) yield. The Wittig reaction of 3n with aldehyde 6a and isolation by preparative TLC (DCM/EtOAc 2:1, v/v) gave 2w as a yellow solid in 44% (84 mg) yield. The Wittig reaction of 4e with aldehyde 6a and isolation by preparative TLC (DCM/EtOAc 15:1, v/v, then PE only) gave an inseparable mixture of 2w and BnP(O)Ph2, whose yields were determined by 1H NMR to be 60% (113 mg) and 14% (20 mg), respectively. 1H NMR (400 MHz, DMSO-d6): δ 7.51–7.61 (m, 5 H), 7.62–7.74 (m, 9 H), 7.80–7.86 (m, 2 H), 8.55 (d, JC–P = 83. Hz, 1 H), 8.88–8.96 (m, 2 H); 13C NMR (100 MHz, DMSO-d6): δ 123.5, 124.2, 127.5, 127.9, 128.1, 128.3 (d, JC–P = 100.3 Hz), 128.4 (d, JC–P = 5.3 Hz), 129.4 (d, JC–P = 12.0 Hz), 129.6 (d, JC–P = 14.5 Hz), 130.1, 130.5, 130.7 (d, JC–P = 8.6 Hz), 130.8 (d, JC–P = 8.2 Hz), 131.9 (d, JC–P = 2.1 Hz), 132.0 (d, JC–P = 9.7 Hz), 132.6 (d, JC–P = 2.5 Hz), 133.1 (d, JC–P = 103.4 Hz), 136.4 (d, JC–P = 11.1 Hz); 31P NMR (162 MHz, DMSO-d6): δ 29.63.

(4-Phenoxyphenyl)diphenylphosphine Oxide (2x)

The Wittig reaction of 3c with aldehyde 6a gave 2x as a yellow solid in 80% (148 mg) yield. 1H NMR (400 MHz, DMSO-d6): δ 7.08–7.16 (m, 4 H), 7.23 (t, J = 7.4 Hz, 1 H), 7.41–7.48 (m, 2 H), 7.51–7.58 (m, 4 H), 7.59–7.67 (m, 8 H); 13C NMR (100 MHz, DMSO-d6): δ 118.1 (d, JC–P = 12.7 Hz), 120.4, 125.2, 126.9 (d, JC–P = 106.4 Hz), 129.2 (d, JC–P = 11.9 Hz), 130.8, 131.9 (d, JC–P = 9.8 Hz), 132.4 (d, JC–P = 2.4 Hz), 133.5 (d, JC–P = 102.8 Hz), 134.3 (d, JC–P = 11.0 Hz), 155.5, 160.8 (d, JC–P = 2.9 Hz); 31P NMR (162 MHz, DMSO-d6): δ 25.08. HRMS-ESI (positive) m/z: calcd for C24H19NaO2P+ [M + Na+], 393.1020; found, 393.1023.

Acknowledgments

We thank the National Natural Science Foundation of China for their financial support of our program (grant no. 21272170).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c01413.

  • 1H and 13C NMR spectra of quaternary phosphonium salts and aryldiphenylphosphine oxides (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c01413_si_001.pdf (15.1MB, pdf)

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