Regioselective and stereoselective cleavages of P–S/C–S bonds by lithium and the formation of P-stereogenic functional phosphine derivatives

Abstract

Various menthyl-containing phosphinothioates were prepared. Using these compounds, an unusual phosphorus-promoted cleavage of the C–S bond with lithium-naphthalene was examined. The C–S/P–S cleavages could be controlled by the amount of naphthalene and temperature applied. Also, the P–S cleavage was confirmed to retain the configuration about the P and this feature was used for achieving a stereospecific formation of the C–P bond.

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Introduction

Because of their important biological activity, namely inhibition of acetylcholine esterase, phosphonothioates and related compounds are widely used as agricultural chemicals such as pesticides, herbicides and insecticides.¹ These compounds are also well known as neurotoxins.² Species containing P–S bonds, especially P-stereogenic species, are usually used as substrates to explore the metabolism and degradation of these substances in organisms or other natural settings.

Besides their applications in physiology and biochemistry, P-stereogenic P–S species also have wide applications in chemistry. For example, they can be used as chemical shift solvating reagents in the analysis of chiral substances.³ Compounds containing P–S bonds (denoted here as simply “P–S species”) can be used as precursors for preparing P-stereogenic compounds via nucleophilic substitutions with alkoxide or alkyl anions as attacking reagents that, respectively, invert or retain the configuration on the phosphorus.⁴ The stereochemical integrity on phosphorus during the reactions of the P–S species enable the use of sulfur to protect the phosphine via formation of a P=S or P–S bond.⁵ Because of the facile formation and cleavage of P–S bonds, the P-stereogenic P–S species have advantages for easy and versatile applications regarding their conversions to various chiral substances.^6,7

Traditionally, P-stereogenic P–S species are prepared via kinetic resolution.^2a H–P(O) compounds such as H-phosphinates and secondary phosphine oxides could be converted to P–S compounds via sulfurization. For example, Mislow and co-workers obtained diastereomerically enriched phosphonothioates from a mixture of two stereoisomers of chiral H–P(O) species.⁸ The very first preparations of (R_P)-t-BuPhP(S)OH involved the sulfurization of racemic H–P species and kinetic resolution of the products.³ Several years ago, we reported the preparations of phosphonothioates via the sulfurization of menthyl H-phenylphosphinate.⁹ The stereoselective conversions of P–H bonds to P–Cl bonds, followed by their reactions with thiols, also afforded optically pure phosphonothioates, as reported by Han.¹⁰

Most of the above preparations relied on the use of optically pure H–P(O) compounds, but the poor availability of these compounds restricted the feasibility to acquire P-stereogenic P–S species.¹¹ Additionally, the application of P–S species usually involved their conversions to P–C bonds via nucleophilic reagents.¹² The reverse conversion of the P–S bond to the P–H bond had rarely been studied.^5a When the optically pure P–S species were obtained conveniently, the conversion of the P–S bond to the P–H bond was hoped to provide an effective route to the preparation of more versatile H–P species.

In the current work, we developed a facile preparation of P-stereogenic P–S species and achieved conversions of these species to functional secondary phosphine oxides. Menthyl(2-hydroxy biphenyl)phosphinothioic acid was obtained as a single stereoisomer, which was alkylated to form the corresponding ester. In contrast to the stability of the C–S bond when in the presence of a metallic reagent, a phosphorus-promoted cleavage of the C–S bond of phosphinothioate, occurring simultaneously with the cleavage of the C–P bond, was observed when the phosphinothioate was treated with lithium (Chart 1). The regioselective cleavage of the P–S bond afforded a secondary phosphine oxide, which was reacted with an electrophilic reagent to form various P-stereogenic functional phosphine derivatives.

Chart 1 Regioselective cleavages of P–S/C–S bonds of phosphonothioate.

Results and discussion

The reaction of 6-chloro-6H-dibenzo[c,e][1,2]oxaphosphinine 1 (CDOP) with (L)-menthyl magnesium chloride,¹³ followed by treatment of the product of this reaction with sulfur, afforded 6-menthyl-6H-dibenzo[c,e][1,2] oxaphosphinine 6-sulfide 2/2′, as a mixture of two diastereomers. After subjecting this mixture to alkali hydrolysis, a mixture of R_P-3′ and S_P-3 in a 35 : 65 ratio was obtained, according to the two peaks observed at 76.3 and 82.7 ppm in the ³¹P-NMR spectrum of the product of the hydrolysis.¹⁴S_P-3, which produced a signal at 82.7 ppm, was obtained from the recrystallization of the mixture (Scheme 1) and its structure was confirmed using X-ray diffraction (Fig. 1).

Scheme 1 Preparation of optically pure S_P-3.

Fig. 1 X-ray diffraction structures of (A) S_P-3, (B) R_P-3′, (C) R_P- and (D) S_P-4e.

When S_P-3 was stirred with ethyl bromide in acetonitrile in the presence of potassium hydroxide, S_P-4b was afforded; here, the alkylation occurred on both sulfur and 2′-OH. In the ³¹P-NMR spectrum of this product, two single peaks were observed, at 65.84 and 60.42 ppm and were assigned to the two stereoisomers resulting from the axial chirality.^13,14 The alkylation did not involve phosphorus.

An intramolecularly cyclized R_P-5 was detected when the alkylation was carried out in the presence of potassium carbonate. In the presence of methyl iodide, R_P-5 formed in near 100% yield, either with potassium hydroxide or potassium carbonate as the base (Scheme 2). It was proposed that sulfur was alkylated before 2′-hydroxyl was alkylated and the resulting alkylthio group was substituted with an oxygen anion to afford R_P-5. In fact, heating the solution of S_P-3 also afforded R_P-5, in which the sulfur atom was similarly substituted as a leaving group. The structure of R_P-5 was confirmed using X-ray diffraction (Fig. 1), which indicated that the substitution of the alkylthio or sulfur group occurred by way of a P-retained mechanism according to Berry pseudo-rotation (BPR) theory.¹⁵

Scheme 2 Cyclization of S_P-3 to form R_P-5.

After the reaction of CDOP with menthyl magnesium chloride, the product was oxidized by exposing it to the air to afford 5/5′ as a mixture of two stereoisomers. Heating the mother liquid shown in Scheme 1 in refluxing ethanol also formed the mixture. After subjecting this mixture to recrystallization, S_P-5′ was obtained and was then converted to R_P-3′ by treating it with Lawson's reagent and subsequent hydrolysis (Scheme 3). The stereochemistry on phosphorus during the conversions was confirmed using X-ray diffraction (Fig. 1).

Scheme 3 Preparation of optically pure R_P-3′.

In addition to methyl iodide, aliphatic alkyl bromides were used for the O,S-alkylation of S_P-3, affording S_P-4 as the major product (entries 2–7, Table 1). During the process, another by-product, namely S_P-6, was detected; here, the oxygen of phosphinothioic acid was alkylated. When benzyl bromide was made to react with S_P-3, S_P-6 was formed as the major product (entries 8 and 9).¹⁶ The benzyl-containing S_P-4 was obtained from the reaction with benzyl chloride (entries 10–12, Table 1).

Table 1 Alkylations of S_P-3 with alkyl halides

Entry	RX	S P-4, Yield % (drA)^a	Other products, yield^a (%)
a Typical procedure: Bromoethane (61.3 μL, 0.822 mmol) was added to a solution of SP-3 (80.0 mg, 0.205 mmol) and potassium hydroxide (14.9 mg, 0.225 mmol) in acetonitrile (1 mL) and the resulting mixture was stirred at 50 °C for 10 hours. The yields and drA values were estimated from the corresponding ^31P{^1H} NMR spectra of 4, which gave two ^31P NMR signals that were assigned as two axial stereoisomers. The axial absolute configuration was not confirmed. b The reaction was carried out at rt in the presence of potassium carbonate. c The O-alkylated product was formed.
1	MeI		R P-5, 99^b
2	EtBr	4b, 98 (80 : 20)
3	nBuBr	4c, 68 (80 : 20)	R P-5, 2; SP-6c, 15
4	iBuBr	4d, 72 (80 : 20)	R P-5, 2; SP-6d, 4
5	iPrBr	4e, 95 (81 : 19)
6	AllylBr	4f, 77 (82 : 18)	S P-6f, 10
7	PhCH2CH2Br	4g, 67 (80 : 20)	R P-5, 22
8	PhCH2Br		S P-6h, 84^c
9	oMeC6H4CH2Br		S P-6i, 29^c; RP-5, 40
10	pMeC6H4CH2Cl	4j, 74 (81 : 19)	S P-6j, 26
11	mMeOC6H4CH2Cl	4k, 43 (83 : 17)	S P-6k, 45
12	pClC6H4CH2Cl	4l, 58 (76 : 24)	S P-6l, 40

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Cleavages of the P–S bond of S_P-4b were attempted with Grignard reagents. When S_P-4b was stirred with n-butyl magnesium bromide in THF at 50 °C, the reaction did not proceed. At 90 °C in toluene, the substitution of the ethylthio group with an n-butyl group afforded R_P-7c in 34% yield. When methyl magnesium iodide was used, R_P-7a was similarly obtained in a low yield of 35%. The retention of the configuration on phosphorus was confirmed by comparing this product to R_P-7b obtained vide infra, which was consistent with the reported substitution of the alkylthio group on the phosphorus (Scheme 4).^12,15

Scheme 4 Cleavages of the P–S bond of S_P-4b with Grignard reagents.

Another strategy explored to break the P–S or C–S bond of S_P-4 involved using lithium or lithium-naphthalene 8. When S_P-4b was treated with lithium, cleavage of the Et–S bond occurred and afforded 9, which after quenching yielded a peak at 97.97 ppm in its ³¹P NMR spectrum. When 9 was subjected to S-alkylation with ethyl bromide, it converted back to S_P-4b, according to the observation of the peaks at 65.84/60.42 ppm. Meanwhile, the cleavage of the P–S bond formed 10, which was confirmed by the detection of secondary phosphine oxide R_P-11 after quenching. R_P-11 showed peaks at 36.63 and 33.87 ppm in its ³¹P NMR spectrum, a result similar to those for the reported compounds.^12,13 Thus the P-stereochemistry-retained cleavage of the P–S bond was confirmed. In the absence of naphthalene, the reaction was sluggish and showed only a 57% conversion of S_P-4b and the formations of 9 and 10 were detected in a ratio of 46 : 54 (entry 1 in Table 2).

Table 2 Cleavages of P–S/C–S bonds of S_P-4 with naphthalene-Li 8

Entry	R of SP-4	Naph. (equiv.)	Temp.	Conversion of SP-4% (SP-9/RP-10)^a
a Typical procedure: a solution of lithium-naphthalene 8 in THF (lithium : naphthalene = 1 : 1.5, 0.23 ml, 1 M, 0.23 mmol) was added to a solution of SP-4b (50 mg, 0.112 mmol) in THF (1 ml) at the indicated temperature. The yield and SP-9/RP-10 ratio were estimated from the peaks in the corresponding ^31P{^1H} NMR spectrum. b Most of the SP-4m was consumed and two unidentified peaks at 52.17 and 49.34 ppm were observed.
1	4b, Et	0	0 °C-rt	57 (46 : 54)
2	4b, Et	0.9	Rt	99 (58 : 42)
3	4b, Et	0.9	50 °C	89 (56 : 44)
4	4b, Et	0.9	−80 °C	69 (99 : 1)
5	4b, Et	1.5	Rt	99 (6 : 94)
6	4b, Et	1.5	50 °C	99 (1 : 99)
7	4b, Et	1.5	−80 °C	84 (99 : 1)
8	4e, iPr	0.9	−80 °C	60 (99 : 1)
9	4j, pMeC6H4CH2	0.9	−80 °C	99 (99 : 1)
10	4g, PhCH2CH2	0.9	−80 °C	99 (99 : 1)
11	4m, Ph	0.9	Rt	98 (1 : 99)
12	4m, Ph	1.5	Rt	93 (1 : 99)
13	4m, Ph	0.9	−80 °C	96^b

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In the presence of 0.9 equivalent naphthalene, naphthalene-Li 8 reacted with S_P-4b at room temperature, giving an excellent conversion, with the detection of 9/10 in a ratio of 50 : 50 (entry 2, Table 2). Carrying out the reaction at an elevated temperature did not change the ratio (entry 3, Table 2). At −80 °C, it was the C–S bond that was predominantly cleaved, as indicated by the observed 99 : 1 ratio of 9/10 and 69% conversion of S_P-4b (entry 4, Table 2).

In the presence of excess naphthalene, the cleavage of the P–S bond predominantly occurred. When 1.5 equivalents of naphthalene-Li 8 reacted with S_P-4b at room temperature, a 6 : 94 ratio of 9/10 resulted. The ratio was improved to 1 : 99 at 50 °C (entries 5 and 6, Table 2). However, when the reaction was carried out at −80 °C, 9 was still formed as the major product (entry 7, Table 2).

Similar to the observation for entry 7 in Table 2, the use of S-isopropyl-substituted S_P-4e predominantly generated 9 at −80 °C (entry 8, Table 2). The cleavage of the C–S bond was also observed for S-phenylethyl- or S-benzyl-substituted S_P-4g or S_P-4j (entries 9 and 10, Table 2). The results indicated that the cleavage of the C–S bond predominantly occurred in the presence of insufficient naphthalene or at low temperature.

The facile cleavage of the C–S bond by lithium was unusual. As a comparison, 1,2-diethyldisulfide was treated with 8 under the same conditions as used in entry 2 of Table 2. The generated lithium ethylthiolate was captured with benzyl bromide to afford ethyl(phenyl)sulfide 12 (Scheme 5). The cleavage of the Et–S bond was not observed during the process.

Scheme 5 Cleavage of the S–S bond of 1,2-diethyldisulfane.

S-Aromatic-substituted S_P-4m, which was prepared from the reaction of 10 with diphenyldisulfide, exhibited a behavior different than the behaviors of S_P-4b. Only the P–S bond of S_P-4m was cut at room temperature, whether or not naphthalene was supplied in excess. When the reaction was carried out at −80 °C, neither the formation of 9 nor of 10 was obvious (entries 11–13, Table 2).¹⁷

The C–S cleavage was further confirmed via the reaction of various phenylphosphinothioates with 8. Similar cleavages of C–S/P–S bonds were observed. At −80 °C, 4n (S-methyl) and 4o (S-Et) predominantly afforded the C–S-cleaved product 9′. For S-phenyl-substituted 4p, 9′ was similarly not detected (Table 3, runs 1–3). When the reaction was carried out at 50 °C, the P–S cleavage took place to form 10′. For each of 4n and 4o, the cleavage of the C–S bond occurred simultaneously with the P–S cleavage, as indicated by the 9′/10′ ratio being about 50 : 50, which was slightly different than the results in Table 2. The reaction using 4p predominantly afforded 10′, although 9′ was also detected (runs 4–6).

Table 3 The cleavages of P–S/C–S bonds of phenyl phosphinothioates

Entry	R of Sp-4	Naph. (equiv.)	Temp.	Conversion of Sp-4% (Sp-9′/Rp-10′)
a Major byproduct was observed at 49 ppm on ^31P NMR spectrum whose structure was not confirmed. b 4o was used in 67 : 33 dr, and 9′ was detected as two stereoisomers in the corresponding ratio.
1	4n, Me	0.9	−80 °C	94 (99 : 1)^a
2	4o, Et	0.9	−80 °C	94 (99 : 1)^a^,^b
3	4p, Ph	0.9	−80 °C	86^a
4	4n, Me	1.5	50 °C	99 (42 : 58)
5	4o, Et	1.5	50 °C	99 (47 : 53)
6	4p, Ph	1.5	50 °C	98 (9 : 91)^a

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The α-carbon of the S-isopropyl group in S_P-4e was thought to be subjected to a level of spatial hindrance greater than that of S-ethyl in S_P-4b. The attack of the α-carbon, or the sulfur, of 4e was supposed to be more hindered than that of 4b. However, S_P-4b and S_P-4e gave similar results when treated with 8 (entries 7 and 8), which indicated that the cleavage of the C–S bond did not occur via a direct nucleophilic attack on the α-carbon or sulfur.

Instead, the adjacent phosphorus was then proposed to be involved in the cleavage of C–S bond. The mechanism for this proposal is shown in Scheme 6. According to this proposal, in the presence of 8, the phosphorus of S_P-4b was attacked by lithium, probably on its vacant d-orbital, forming intermediate 14. After the negative charge on the O substituent of the phosphorus was transferred to the S-alkyl substituent, via a possible cyclic transition state, the alkyl moiety dissociated from the sulfur to afford 15, which was then converted to 9. For 4j, a relatively stable benzyl anion was removed, also affording 9 (entry 9 of Table 2). A similar electron-transfer to the phenylthio group cannot occur, so that S_P-4m did not afford 9 at either −80 °C or room temperature.

Scheme 6 Proposed mechanism for the cleavages of P–S and C–S bonds of S_P-4b.

In the case of P–S cleavage, according to the proposed mechanism, 16 was converted to 16′via a Berry pseudo-rotation (BPR). Dissociation of the alkylthio substituent from the axial position of the bipyramidal structure of 16′ afforded phosphorus anion 10 in a P-stereochemistry-retained manner. As previously reported, the BPR could be stopped at a low temperature, which was consistent with the P–S cleavage having only occurred at a relatively high temperature.¹⁵ The configuration of the phosphorus anion was stabilized by the presence of the menthyl group, so that 10 did not epimerize and 11 formed with excellent stereoselectivity.¹⁸

Metallic lithium has been thought to be more active than naphthalene-lithium 8. However, the poor dispersion of metallic lithium led to a heterogeneous reaction and low conversion of S_P-4b. When naphthalene was used in excess, the reduced activity of lithium also resulted in a more regioselective attack of the P–S/C–S bond.

Subsequent alkylations of S_P-10 with various alkyl halides were performed and the results are summarized in Table 4. Aliphatic primary alkyl halides reacted smoothly with S_P-10 to afford R_P-7a to R_P-7d. As seen in ESI†R_P-7a and R_P-7c obtained herein yielded the same spectral data as that obtained in Scheme 4. Secondary alkyl halides such as cyclohexyl bromide did not react with S_P-10, but allyl chloride could be used for the alkylation and formed R_P-7e. The reaction of S_P-10 with variously substituted benzyl halides afforded 7f to 7k in good to excellent yields.

Table 4 Alkylations of S_P-10 with alkyl halides

R	7 yield %, (drA)	R	7 yield %, (drA)
Me	7a, 82, (69 : 31)	oMeC6H4CH2	7g, 56%, (72 : 28)
Et	7b, 85, (53 : 47)	pMeC6H4CH2	7h, 96, (64 : 36)
nBu	7c, 94, (51 : 49)	p-tBuC6H4CH2	7i, 88, (70 : 30)
iBu	7d, 95, (43 : 57)	mMeOC6H4CH2	7j, 86, (63 : 37)
Allyl	7e, 50, (84 : 16)	oCIC6H4CH2	7k, 72, (74 : 26)
PhCH2	7f, 85, (65 : 35)	pCIC6H4CH2	7l, 90, (64 : 36)

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After cleavage of the P–S bond of S_P-4b, the resulting S_P-10 was reacted with formaldehyde to afford α-hydroxy phosphine oxide R_P-17. The configuration about the phosphorus was thought to have been retained, based on the reported addition of secondary phosphine oxide to aldehyde (Scheme 7).¹⁹

Scheme 7 Reaction of 10 with formaldehyde.

The stereochemistry during the above conversions was confirmed using X-ray diffraction. As seen in Fig. 1, the structures of both S_P-3 and R_P-3′ were confirmed and S_P-3 and R_P-3′ were converted, respectively, to R_P-5 and S_P-5′ upon being heated; and the structure of R_P-5 was also confirmed. During the cyclization, the configuration about the phosphorus was retained. The S-alkylation of S_P-3 did not involve the phosphorus and thus 4e displayed the same S_P-structure as did S_P-3. Upon comparing the X-ray structures of S_P-4e and R_P-7b,¹³ the replacement of the alkylthio group with an alkyl group, via either a Grignard reagent or lithium, was confirmed to have occurred in a P-stereochemistry-retained manner.

Conclusions

Menthyl (2-hydroxy biphenyl)phosphinothioic S-acid 3 was formed from the treatment of a P(III) specie using sulfur and alkali hydrolysis. After subjecting the product of the hydrolysis to recrystallization, the S_P-stereoisomer was successfully obtained and could be converted to various optically pure O,S-alkylated S_P-4 molecules. The regioselectivities of the cleavages of the P–S/C–S bonds of S_P-4 were studied using lithium or naphthalene lithium and were found to depend on the relative amounts of naphthalene/lithium used and on the temperature applied. At low temperature or in the absence of naphthalene, the cleavage of the C–S bond occurred. In the presence of excess naphthalene or at a relatively high temperature, the P–S bond was cleaved. The unusual cleavage of C–S bond was thought to be promoted by the adjacent phosphorus. The cleavage of the P–S bond could be used for the formation of a C–P bond in a P-stereochemistry-retained manner.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the financial support of the Natural Science Foundation of China (grant no. 21802062) and the Natural Science Foundation of Shandong Province (grant no. ZR2016BM18 and ZR2018PB008).

Notes and references

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11. (a) Q. Xu, C.-Q. Zhao and L.-B. Han, Stereospecific Nucleophilic Substitution of Optically Pure H-Phosphinates: A General Way for the Preparation of Chiral P-Stereogenic Phosphine Oxides, J. Am. Chem. Soc., 2008, 130, 12648–12655; (b) W.-M. Wang, L.-J. Liu, C.- Q. Zhao and L.-B. Han, Diastereoselective Hydrolysis of Asymmetric P–Cl Species and Synthesis of Optically Pure (R_P)–(-)–Menthyl H–Phenylphosphinate, Eur. J. Org. Chem., 2015, 2342–2345; (c) J.-J. Ye, B.-X. Yan, J.-P. Wang, J.-H. Wen, Y. Zhang, M.-R. Qiu, Q. Li and C.-Q. Zhao, The construction of three C–P bonds of P-stereogenic tertiary phosphines containing (L)-menthyl, Org. Chem. Front., 2020, 7, 2063–2068.
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14. The two chiral centers on phosphorus and axis derived four stereoisomers. We believed the axial chirality was controlled by P-chirality via the H-bonding between P=O and hydroxyl. Therefore only two signals of 3 were observed on ³¹P NMR spectrum. The similar case could be found in the ref. 13.
15. (a) R. S. Berry, Correlation of Rates of Intramolecular Tunneling Processes, with Application to Some Group V Compounds, J. Chem. Phys., 1960, 32, 933–938; (b) L. Y. Kuo and S. K. Glazier, Inorg. Chem., 2012, 51, 328–335; (c) K. E. Debruin, C. l. W. Tang, D. M. Johnson and R. L. Wilde, Kinetic facial selectivity in nucleophilic displacements at tetracoordinate phosphorus: kinetics and stereochemistry in the reaction of sodium ethoxide with O,S-dimethyl phenylphosphonothioate, J. Am. Chem. Soc., 1989, 111, 5871–5879; (d) J. Seckute, J. L. Menke, R. J. Emnett, E. V. Patterson and C. J. Cramer, Ab Initio Molecular Orbital and Density Functional Studies on the Solvolysis of Sarin and O,S-Dimethyl Methylphosphonothiolate, a VX-like Compound, J. Org. Chem., 2005, 70, 8649–8660; (e) J. L. Menke and E. V. Patterson, Quantum mechanical calculations on the reation of ethoxide anion with O,S-dimethyl methylphosphonothiolate, J. Mol. Struct.: THEOCHEM, 2007, 811, 281–291.
16. The configuration on P of 6 was inferred based on S_P-3 that was converted to 6 without variation of P-configuration. The alkylation product of P–OH probably was P(O)OR or P(S)OR, which was not confirmed.
17. New signals at 52.17 and 49.34 ppm on ³¹P NMR spectrum were observed, which were not ascribed to 9 or 10.
18. S.-Z. Nie, Z.-Y. Zhou, J.-P. Wang, H. Yan, J.-H. Wen, J.-J. Ye, Y.-Y. Cui and C.-Q. Zhao, Nonepimerizing Alkylation of H−P Species to Stereospecifically Generate P-Stereogenic Phosphine Oxides: A Shortcut to Bidentate Tertiary Phosphine Ligands, J. Org. Chem., 2017, 82, 9425–9434.
19. R _P-15 was obtained as a mixture of axial stereoisomers, in a ratio of 53 : 47, as seen its ¹H NMR spectrum. Similar reaction could be found in the reference. H. Zhang, Y.-M. Sun, L. Yao, S.-Y. Ji, C.-Q. Zhao and L.-B. Han, Stereogenic Phosphorus-Induced Diastereoselective Formation of Chiral Carbon during Nucleophilic Addition of Chiral H-P Species to Aldehydes or Ketones, Chem. – Asian J., 2014, 9, 1329–1333.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 1975582, 1975583, 1975584 and 2010827. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0qo00891e

‡ These two authors contributed equally to this work

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