Catalytic Asymmetric Hydrophosphination of ortho-Quinone Methides

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Letter Cite This: Org. Lett. 2018, 20, 7229−7233

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Catalytic Asymmetric Hydrophosphination of ortho-Quinone Methides Xiu Gu,†,§ Hao Yuan,†,§ Jun Jiang,*,† Yi Wu,† and Wen-Ju Bai*,‡ †

School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, P. R. China Department of Chemistry, Stanford University, Stanford, California 94305-5580, United States

‡

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S Supporting Information *

ABSTRACT: An efficient catalytic asymmetric hydrophosphination of ortho-quinone methides with H-phosphine oxides is established. A chiral bifunctional squaramide is superior to catalyze this enantioselective carbon−phosphorus bond formation, delivering optically active α-arylmethyl phosphine oxides in high yields with high enantioselectivities (up to 94% yield, 99:1 er). Additionally, employing in situ-generated o-QMs for this hydrophosphination step economically provides the corresponding phosphine oxides with comparable yield and enantioselectivity.

O

Scheme 1. Asymmetric Conjugate Additions of o-QMs for Construction of Carbon−Carbon and Carbon−Heteroatom Bonds

rganophosphrous compounds, such as phosphine oxides, have wide applications in catalysis,1 medicinal chem2 istry, materials science,3 and organic synthesis. Consequently, efficient construction of C−P bonds in an enantioselective fashion has attracted considerable attention, among which asymmetric hydrophosphination4 of ketones, imines, or α,βunsaturated compounds represents a straightforward and atom-economical5 approach. ortho-Quinone methides (o-QMs), a type of highly reactive and versatile synthetic intermediate, can participate in a variety of catalytic asymmetric conjugate additions and [4 + 2] cycloadditions.6,7 In light of this, we envisioned that employing o-QMs in an asymmetric hydrophosphination reaction would provide a convenient and enantioselective way to build C−P bonds, providing chiral α-arylmethyl phosphine oxides that might serve as bioactive pharmaceutical intermediates.8 Interestingly, although catalytic asymmetric conjugate additions of o-QMs and aza-o-QMs using carbon,9 oxygen,10 sulfur,10a,11 and nitrogen nucleophiles12 have been extensively explored, the corresponding reaction with a phosphorus nucleophile is still missing (Scheme 1). So far, the hydrophosphination of o-QMs has been limited to a diastereoselective manner.13 Herein, we report the first catalytic asymmetric hydrophosphination of o-QMs using a chiral squaramide catalyst to synthesize optically active α-arylmethyl phosphine oxides in good yields with exceptional enantioselectivities. We began to study the catalytic asymmetric hydrophosphination of o-QM 1a using diphenylphosphine oxide 2a as the nucleophile and chiral quinidine-derived squaramide catalyst 4a as the organocatalyst in DCM at 0 °C (Table 1). The reaction gave a satisfying conversion but meager enantioselecivity (entry 1). Screening the organocatalysts revealed that the 3,5-bis(trifluoromethyl)phenyl derived © 2018 American Chemical Society

squaramide catalyst 4d boosted the enantioselectivity to 96:4 er together with improved yield (entries 2−4). Solvent examination suggested that DCM was actually the best choice for this transformation (entries 4−8). Increasing the reaction temperature to ambient temperature hampered both yield and enantioselectivity, whereas dropping temperature to −10 °C led to a lower conversion, albeit with comparable enantioselectivity (entries 9−10). Accordingly, the optimized conditions were established as using 10 mol % of chiral squaramide catalyst 4d in DCM at 0 °C (entry 4). The absolute configuration of the obtained product 3a was established by X-ray single crystal analysis (Figure 1). By Received: October 3, 2018 Published: November 6, 2018 7229

DOI: 10.1021/acs.orglett.8b03158 Org. Lett. 2018, 20, 7229−7233

Letter

Organic Letters Table 1. Optimization of the Reaction Conditionsa

Scheme 2. Scope of H-Phosphine Oxidesa,b,c,d

entry

catalyst

solvent

temp (°C)

yield (%)b

erc

1 2 3 4 5 6 7 8 9 10

4a 4b 4c 4d 4d 4d 4d 4d 4d 4d

DCM DCM DCM DCM CHCl3 DCE THF toluene DCM DCM

0 0 0 0 0 0 0 0 25 −10

77 79 77 91 60 89 66 53 62 71

56:44 86:14 88:12 96:4 95:5 92:8 89.5:10.5 87:13 90:10 95.5:4.5

a Reaction conduction: 1a (0.2 mmol), 2a (0.2 mmol), catalyst (10 mol %), solvent (0.8 mL). bIsolated yield. cDetermined by HPLC on a chiral stationary phase. PMP = p-methoxyphenyl. DCM = dichloromethane. DCE = 1, 2-dichloroethane. THF = tetrahydrofuran.

analogy to (R)-3a, the stereochemistry of products 3 was assigned, respectively.

a

Reaction conduction: 1a (0.2 mmol), 2 (0.2 mmol), catalyst (10 mol %), solvent (0.8 mL). bIsolated yield. cDetermined by HPLC on a chiral tationary phase. dCatalyst (20 mol %), solvent (4.0 mL) at −20 °C.

oxide (DOPO) 2k, product 3k was obtained with poor diastereoselectivity but excellent enantioselectivity for both diastereomers. We next investigated the scope of o-QMs (Scheme 3). Delightfully, o-QMs that bear aryl-, styrenyl-, and vinylsubstituents all tolerated this transformation, producing the desired chiral α-arylmethyl phosphine oxides. o-QMs bearing p-ethoxyphenyl substituent provided 3l in 86% yield with 93:7 er, while a dimethoxylated o-QMs delivered the corresponding product 3m with slightly inferior enantioselectivity (3l−m). A series of styrenyl-substituted o-QMs smoothly generated the corresponding products 3n−3r in 66−92% yields with 94:6 to 97:3 er. Furthermore, a vinylthienyl-substituted o-QM underwent the hydrophosphination to yield product 3s with excellent enantioselectivity. In addition, the o-QM bearing a prenyl group was also able to participate in this transformation to afford product 3t with 98:2 er. Reduction of the obtained prenyl motif would provide an alkyl group at the benzylic position. This is notable considering that it is usually difficult to tame an unstable alkyl-substituted o-QM in a catalytic asymmetric reaction. To further validate the synthetic utility of our newly developed method, the hydrophosphination of 1a and 2i was carried out in 1 mmol scale under the standard reaction conditions (Scheme 4). The anticipated product 3i was obtained in an analogous 98:2 er and moderate yield.

Figure 1. X-ray structure of (R)-3a.

With the optimized conditions in hand, we started to explore the scope of H-phosphine oxides (Scheme 2). Pleasingly, Hphosphine oxides bearing both electron-donating and electronwithdrawing groups provided attractive results. H-Phosphine oxides with an electron-rich motif such as Me, tBu, or fused phenyl on its phenyl core gave slightly diminished enantioselectivity, but >90:10 er was realized in all cases (3b−d). In contrast, an electro-deficient substituent such as F or Cl at the para or meta positions of the phenyl ring afforded excellent yield and outstanding enantioselectivity, ranging from 95.5:4.5 to 97:3 er (3e−g). H-Phosphine oxides that bear trifluoromethyl groups performed remarkably well in this transformation, and the enantioselectivity was enhanced to as high as 99:1 er (3h−j). It was worth noting that by using heterocyclic 10-dihydro-9-oxa-10-phosphaphenanthrene-107230

DOI: 10.1021/acs.orglett.8b03158 Org. Lett. 2018, 20, 7229−7233

Letter

Organic Letters Scheme 3. Scope of o-QMsa,b,c,d

Scheme 5. Asymmetric Hydrophosphination of in SituGenerated o-QMs

Scheme 6. Proposed Mechanism

a

Reaction conduction: 1 (0.2 mmol), 2a (0.2 mmol), catalyst (10 mol %), solvent (0.8 mL). bIsolated yield. cDetermined by HPLC on a chiral stationary phase. dAt −10 °C.

Scheme 4. Scaled-up Reaction

Encouraged by the success of our asymmetric hydrophosphination of o-QMs, we were eager to study this transformation employing in situ-generated o-QMs for synthetic convenience and step-economy.14 Gratifyingly, the o-QMs, formed in situ under oxidative15 and thermal conditions16 both could undergo the corresponding hydrophosphination to give product 3i in moderate yield with excellent enantioselectivity (Scheme 5a,b). Additionally, 2hydroxyalkylnaphthol 7 could also been used as the o-QMs precursor for direct access to the desired product 8 in 87% yield, albeit with 76:24 er (Scheme 5c).17 The proposed catalytic cycle is depicted in Scheme 6.11a,b,18 The P(V) form 2 is known to be in equilibrium with its P(III) tautomer (phosphinous acid) 2′. The subsequent enantioselective addition of nucleophile phosphinous acid 2′ to o-QM 1 is controlled by the bifunctional chiral squaramide 4d, which activates the o-QM 1 by hydrogen-bonding interaction and also deprotonates the nucleophile 2′ with its appended base. The conjugate addition to the o-QM 1 occurs with the restoration of aromaticity as the driving force to provide the

desired chiral α-arylmethyl phosphine oxide 3 and release the catalyst 4d. In conclusion, we have developed the first catalytic asymmetric hydrophosphination of o-QMs using H-phosphine oxides as nucleophiles in the presence of a chiral bifunctional squaramide organocatalyst. This reaction provides an efficient approach to synthesize versatile chiral α-arylmethyl phosphine oxides in good yields with outstanding enantioseletivities. This newly developed method not only expands the repertoire of asymmetric nucleophilic addition of o-QMs but also provides a straightforward way for enantionselective C−P bond formation. 7231

DOI: 10.1021/acs.orglett.8b03158 Org. Lett. 2018, 20, 7229−7233

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Organic Letters

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03158. Experimental procedures, characterization data and NMR spectra of all new compounds; X-ray crystallography of 3a (PDF) Accession Codes

CCDC 1871274 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected] ORCID

Jun Jiang: 0000-0002-5542-7592 Wen-Ju Bai: 0000-0003-1412-8673 Author Contributions §

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the National Natural Science Foundation of China (21762005, 21402032), the Natural Science Foundation of Guangxi Province (2015GXNSFDA139008, 2014GXNSFBA118031), and the Scientific Research Foundation of Guangxi University (XQZ130869).

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