Simple Open-Chain Phosphite-Olefin as Ligand for ... - ACS Publications

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Simple Open-Chain Phosphite-Olefin as Ligand for Rh-Catalyzed Asymmetric Arylation of Cyclic Ketimines: Enantioselective Access to gem-Diaryl α‑Amino Acid Derivatives Yi Li,‡ Yue-Na Yu,‡ and Ming-Hua Xu* State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, 555 Zuchongzhi Road, Shanghai 201203, China S Supporting Information *

ABSTRACT: A new class of open-chain chiral phosphitebased hybrid olefin ligands has been developed and utilized in Rh-catalyzed asymmetric arylation of 1,2,5-thiadiazolidine 1,1dioxide type cyclic ketimines. The reactions generate quaternary carbon-containing, gem-diaryl-substituted sulfahydantoins and 4-ethoxy-2,3-dihydro-1,2,5-thiadiazole 1,1-dioxides with excellent enantioselectivities under exceptionally mild and user-friendly conditions using a H8-binol-derived phosphite-olefin ligand. The method is useful in offering efficient and convenient synthesis of enantioriched α,α-diaryl-α-amino acid amides and nitrogen-containing heterocycles. Furthermore, by taking advantage of the protocol, the first enantioselective synthesis of BACE1 inhibitor (R)-iminohydantoin has been accomplished. KEYWORDS: asymmetric catalysis, rhodium, arylation, cyclic ketimine, P-olefin ligand, α-amino acid membered cyclic N-sulfonyl α-iminoesters, allowing for the construction of gem-diaryl substituted cyclic α-amino acid framework in a highly enantioselective manner. However, the ring cleavage to give the corresponding linear α-amino esters is somewhat problematic because of the loss of enantiopurity encountered under harsh conditions (Scheme 1a). In another recent work with chiral tetrasubstituted 1,2,5-thiadiazoline 1,1dioxide products,8a we found that the removal of sulfonyl group from the five-membered heterocycle can be readily proceeded. Motivated by this success and in conjunction with our interest in nonproteinogenic amino acids,10 we envisioned that a similar strategy might be utilized to enable access to optically active

ptically active gem-diaryl substituted α-amino acids containing a stereogenic quaternary carbon center at the α-position are an intriguing class of non-natural amino acids that are found in a number of naturally occurring products and biologically active compounds, and they are frequently utilized as versatile building blocks in modern organic synthesis and new drug discovery.1,2 Despite the great importance, however, there are few direct methods for their efficient asymmetric synthesis known in the literature.3,4 One major challenge is that the stereofacial differentiation of the two aryl groups particularly with minor electronic or steric differences is of considerable difficulty. Consequently, they were often synthesized in a roundabout way, mainly based on the asymmetric Strecker reaction of diaryl ketimines; however, the process typically requires the presence of an ortho-substituted aryl to promote the enantiocontrol, and the following nitrile hydrolysis usually requires prolonged heating in harsh acidic conditions.4 In recent years, transition-metal-catalyzed asymmetric addition of organoboron reagents to ketimines has emerged and been certified as a powerful and straightforward strategy to obtain chiral α-tertiary amines.5−8 Although this strategy provides a great opportunity, reports on direct asymmetric addition of commercially available arylboronic acids to α-aryl imino esters to access enantioenriched α,α-diaryl-α-amino acid derivatives are rare. Only very recently have a few examples (including ours9a) involving Rh-catalyzed enantioselective arylation of special cyclic N-sulfonyl ketimines been successfully realized, giving α-ester-substituted benzosultams and benzosulfamidates.9 In early 2013, our group9a documented the first Rh-catalyzed asymmetric addition of arylboronic acids to five-

O

© XXXX American Chemical Society

Scheme 1. New Strategy to gem-Diaryl α-Amino Acids

Received: October 26, 2015 Revised: December 7, 2015

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DOI: 10.1021/acscatal.5b02403 ACS Catal. 2016, 6, 661−665

Letter

ACS Catalysis gem-diaryl-substituted acyclic α-amino acids through the ring opening of the corresponding diaryl sulfahydantoins (3-oxo1,2,5-thiadiazolidine 1,1-dioxides), beginning with asymmetric arylation of 4-aryl 3-oxo-1,2,5-thiadiazol 1,1-dioxides. Notably, sulfahydantoins are also important structural units in many compounds that display diverse biological properties, such as inhibition of serine proteases.11 To the best of our knowledge, a sole example of addition of p-tolylboroxine to 4-phenyl 3-oxo1,2,5-thiadiazol 1,1-dioxide using a rhodium/diene catalyst has been shown by Nishimura and Hayashi, but only moderate enantioselectivity (79% ee) was obtained.12 Herein, we describe our discovery of a new and simple chiral phosphite-based olefin ligand and its successful use in Rh-catalyzed asymmetric arylation of 4-aryl 3-oxo-1,2,5-thiadiazol 1,1-dioxides, providing a facile and enantioselective access to gem-diaryl substituted sulfahydantoins, which could be converted to α,α-diaryl-αamino acid derivatives with ease (Scheme 1b). Our initial survey focused on evaluating the possibility of using simple sulfur-olefin ligands L1−413 for the stereoselective addition of p-tolylboronic acid (2a) to 4-phenyl 3-oxo-1,2,5thiadiazol 1,1-dioxide (1a) in the presence of [Rh(COE)2Cl]2 (1.5 mol %). As revealed in Table 1, the reaction proceeded

smoothly in aqueous KF (1.5 M)/toluene at room temperature to afford the corresponding product 3a in 99% yield, unfortunately, only low to moderate enantioselectivities were attained (entries 1−4). In contrast, the use of the equally simple open-chain chiral phosphite-based olefin L5, recently developed by our group,14,15 proved to be more effective in this transformation, affording excellent yield and higher enantioselectivity (84% ee, entry 5). Following up on this encouraging result, a series of (R)-binol-derived phosphite-olefin ligands L6−9 that contained different aromatic moieties attached to the double bond were evaluated (entries 6−9). It was found that the introduction of a bulkier tert-butyl substituent onto the para-position of phenyl ring would be helpful for enhancing the enantioselectivity (L8, entry 8). On the other hand, with regard to the axially chiral backbone, (R)-H8-binol-derived L10 was synthesized. Gratifyingly, L10 exhibited superior enantioselectivity (93% ee) as compared with those derived from (R)-binol, thus indicating that a larger dihedral angle of the two aromatic planes was more beneficial in the chiral induction step of the catalytic process (entry 10). This led us to further design and synthesize a new ligand L11 by combining (R)-H8-binol unit and 4-tert-butyl-substituted styrene element together. As expected, L11 showed great catalytic performance, giving the best enantioselectivity (95% ee) and yield (99%) (entry 11). Subsequently, we attempted to extend the reaction employing less-reactive p-chlorophenylboronic acid 2b. However, a clearly diminished yield (85%) was observed due to incomplete conversion of the imine substrate 1a, albeit with same excellent enantiomeric excess (95% ee, entry 12). To attain higher reactivity, effects related to the additive and solvent were carefully examined by using 2b as the boron component (entries 13−18). To our delight, when the reaction was performed with 1.0 equiv of K3PO4 (2.5 M) in toluene, product 3b can be obtained in a great yield (95%) with slightly improved enantioselectivity (96% ee, entry 15). With the optimal reaction conditions in hand, the scope and generality of the reaction were investigated (Scheme 2). A wide variety of boronic acids with varying electronic and steric demands were successfully reacted with a series of cyclic ketimines 1, providing the corresponding gem-diaryl-substituted chiral sulfahydantoin products (3) mostly in both high yields and enantioselectivities (up to 98% ee). It is important to note that sterically hindered arylboronic acids with ortho substituents on the phenyl rings as well as more challenging heteroarylboronic acids can be well tolerated (3j−m). Interestingly, an (E)-styryl group could also be introduced to produce a unique β,γ-unsaturated α-amino acid derivative 3n, albeit with only moderate enantioselectivity (66% ee). In addition to 1a, substrates with electronically different substituents on the phenyl ring were all found to be suitable, as they also exhibited high reactivity (94−98% yield), leading to the formation of highly enantiomerically enriched products (94−97% ee) (3o−q, 3a′, 3b′, 3d′, 3o′, 3p′). We also found that, under the same catalytic conditions, both (R) and (S)enantiomers of the products can be obtained with the same high level of enantioselectivity by simply switching the corresponding aryl acceptor and donor (3a and 3a′, 3b and 3b′, 3d and 3d′, 3o and 3o′, 3p and 3p′), suggesting great compatibility of the catalytic system. The absolute configuration at the newly created chiral center was determined to be R by X-ray crystallographic analysis of 3e. Assuming an analogous reaction mechanism, the same stereochemistry of the obtained sulfahydantoin products was assigned.

Table 1. Optimization of Conditions for Arylation of 1aa

entry

2

L

additive

solvent

yield (%)b

ee (%)c

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15d 16d 17d 18d

2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2b 2b 2b 2b 2b 2b 2b

L1 L2 L3 L4 L5 L6 L7 L8 L9 L10 L11 L11 L11 L11 L11 L11 L11 L11

KF KF KF KF KF KF KF KF KF KF KF KF KOH K3PO4 K3PO4 K3PO4 K3PO4 K3PO4

toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene dioxane CH2Cl2 THF

99 99 99 90 99 99 95 99 99 99 99 85 82 91 95 77 81 87

47 54 66 20 84 82 62 87 86 93 95 95 96 96 96 96 95 96

a

The reaction was performed with 1a (0.25 mmol), 2 (0.5 mmol), [Rh(COE)2Cl]2 (1.5 mol %), L (3.0 mol %), and 1.0 equiv of additive (1.5 M) in 1 mL of solvent at room temperature. bIsolated yields. c Determined by chiral HPLC analysis. d1.0 equiv of K3PO4 (2.5 M) was used. 662

DOI: 10.1021/acscatal.5b02403 ACS Catal. 2016, 6, 661−665

Letter

ACS Catalysis Scheme 2. Rh/L11-Catalyzed Asymmetric Arylation of 1a,b

Scheme 3. Rh/L11-Catalyzed Asymmetric Arylation of 4a,b

a

Reaction was performed with 4 (0.25 mmol), 2 (0.5 mmol), [Rh(COE)2Cl]2 (1.5 mol %), L11 (3.0 mol %), and 1.0 equiv of KF (1.5 M) in 1 mL of toluene at room temperature. bReaction time: 3 h for 5a, 5c, 5h, 5i, 5k; 6 h for 5d, 5f, 5g; 12 h for 5b, 5e, 5j, 5l.

On the basis of the observed stereochemical outcome, an empirical transition state model is proposed. As shown in Figure 1, the H8-binaphthyl framework provides an excellent

a

Reaction was performed with 1 (0.25 mmol), 2 (0.5 mmol), [Rh(COE)2Cl]2 (1.5 mol %), L11 (3.0 mol %), and 1.0 equiv of K3PO4 (2.5 M) in 1 mL of toluene at room temperature. bReaction time: 3 h for 3a, 3c, 3o′, 3p′; 6 h for 3h, 3i, 3l−n, 3a′, 3b′, 3d′; 12 h for 3b, 3d, 3e−g, 3j, 3k, 3o−q.

The success of enantioselective arylation of 4-aryl 3-oxo1,2,5-thiadiazol 1,1-dioxides led us to extend the method to 3ethoxy-4-aryl-1,2,5-thiadiazole 1,1-dioxides 4 bearing a cyclic Nsulfonyl diketimine framework. To our delight, these ketimines were also viable substrate under similar catalytic system developed. By simply changing the additive to KF, the corresponding addition products (5) could be obtained with ease in high yields and excellent enantioselectivities (94−99% ee) in all cases (Scheme 3). To gain some insight into the coordination mode of the open-chain chiral phosphite-olefin ligand, careful NMR spectroscopy studies were carried out.16 Upon treatment of L11 with [Rh(C2H4)2Cl]2 (0.55 equiv) in CDCl3 at room temperature for 30 min, obvious upfield shifts of the olefinic protons to 4.90 and 4.77 ppm and the olefinic carbons to 61.1 and 60.0 ppm were observed, which clearly suggests that coordination of rhodium to the olefin double bond had taken place. In the meantime, the characteristic free phosphorus resonance (31P NMR: δ 138.0 ppm) disappeared with the concomitant formation of two new peaks (doublet, δ 145.1 ppm, JRh−P = 315 Hz), indicating the involvement of phosphite in metal ligation. Thus, these results disclose that the transition metal Rh is bound to both phosphorus atom and olefinic double bond, forming an exceptional cyclic chelate upon coordination. Unfortunately, attempts to obtain the X-ray crystal structure of the Rh/L11 complex have been unsuccessful so far.

Figure 1. Proposed reaction transition-state model.

stereoenvironment, and the arylrhodium species formed via transmetalation adopt a favorable conformation with the Ar2 positioned trans to the olefin ligand.17 To avoid steric repulsion with the bulky aryl substituent attached to the double bond, rhodium coordination to the imine substrate is preferred as shown in TS-2. Therefore, carborhodation from the re face of the CN bond takes place to give the corresponding (R)products. Following up on our initial plan, the transformation of gemdiaryl-substituted sulfahydantoins into valuable α,α-diaryl-αamino acid derivatives was evaluated. Under a simple protocol of LiAlH4 in refluxing THF, ring cleavage by removal of the sulfonyl group as exemplified by 3a proceeded smoothly and gave the expected α,α-diaryl-α-amino acid amide 6 in 88% yield with untarnished ee (Scheme 4). The usefulness of the current methodology can also be featured by the intriguing synthetic transformation of the addition product 3k. NBS/AIBN-mediated brominationcyclization in CCl4 gave the corresponding tricyclic sulfahydantoin intermediate 7 bearing a fully substituted carbon stereogenic center on the ring in 82% yield without losing 663

DOI: 10.1021/acscatal.5b02403 ACS Catal. 2016, 6, 661−665

Letter

ACS Catalysis Scheme 4. Convenient Access to α,α-Diaryl α-Amino Acid Amide

Scheme 6. Application to Enantioselective Synthesis of (R)Iminohydantoin

optical purity. After recrystallization in the mixture of petroleum ether and ethyl acetate, the optical purity was improved to 99% ee. With this key compound in hand, chiral quaternary carbon-containing isoindoline 8 was afforded in 89% yield by the cleavage of sulfamide ring in 7 under the conditions of sodium naphthalenide. Further treatment of 8 with triphosgene/Et3N in THF or ZnBr2/TBHP in pyridine furnished the corresponding isoindoline-fused hydantoin product 9 and isoindolin-1-one 10 in moderate yields while maintaining the same level of enantioselectivity (Scheme 5). It

a PhB(OH)2 (2 equiv), [Rh(COE)2Cl]2 (1.5 mol %), (S)-L11 (3.0 equiv), K3PO4 (1.5 M, 1 equiv), toluene, rt. bLiAlH4, THF, reflux. c Triphosgene, Et3N, THF, 0 °C to rt. dLawesson’s reagent, toluene, reflux.

Scheme 5. Transformations of Product 3k

provide potential opportunities for future design of more potent and selective BACE1 inhibitors. In summary, we have developed a new series of open-chain chiral phosphite-based hybrid olefin ligands and demonstrated that Rh(I)/phosphite-olefin catalytic system is capable of promoting the highly enantioselective arylation of 4-aryl 3oxo-1,2,5-thiadiazol 1,1-dioxides. Using a simple and readily available H8-binol-derived phosphite-olefin L11 as ligand, a range of quaternary carbon-containing, gem-diaryl-substituted sulfahydantoins as well as 4-ethoxy-2,3-dihydro-1,2,5-thiadiazole 1,1-dioxides were synthesized with excellent enantioselectivities under exceptionally mild and user-friendly conditions. More importantly, the sulfahydantoin products can be easily transformed into enantioriched α,α-diaryl-α-amino acid amides, which have great potential for use in synthetic chemistry and drug discovery. Furthermore, this new method has been successfully applied to the first enantioselective synthesis of BACE1 inhibitor (R)-iminohydantoin. Studies toward applying the novel P-olefin ligand class in other asymmetric catalysis and broadening this methodology in organic synthesis are in progress.

a

NBS (1 equiv), AIBN (10 mol %), CCl4, reflux. bNa naphthalene (5 equiv), DME, −78 °C. cTriphosgene, Et3N, THF, 0 °C to rt. dZnBr2, TBHP, pyridine, 100 °C.

is noteworthy that catalytic asymmetric approaches to access such tetrasubstituted chiral stereogenic center-containing isoindoline units remain considerably scarce,6f,18 and particularly, they are difficult to prepare in enantiomerically pure form using other methods. To further demonstrate the synthetic utility, we carried out a concise and efficient formal asymmetric synthesis of (R)iminohydantoin developed by Merck as a promising BACE1 inhibitor for treating Alzheimer’s disease.2,19 As illustrated in Scheme 6, the key Rh-catalyzed enantioselective addition of phenylboronic acid to cyclic ketimines 1e provided the product 3r in 98% yield with 93% ee. It should be mentioned that (S)H8-binol-derived P-olefin ligand (S)-L11 was used in order to achieve the correct (R)-stereoconfiguration. Subsequently, removal of the sulfonyl followed by triphosgene-mediated cyclization gave the pharmaceutically interesting, anticonvulsant/antiarrhythmic drug phenytoin20 analogue 12. It is worthwhile to mention that ready access to such chiral diarylhydantoin derivatives would provide a useful platform for new biological evaluation.21 Finally, under typical Lawesson reaction conditions, the optically active compound 13 was obtained in excellent yield without erosion of the ee. The intermediate 13 can then be readily transformed into (R)iminohydantoin according to the known procedures.19 To our knowledge, the catalytic enantioselective route to (R)iminohydantoin is unprecedented, and thus, the current strategy represents its first asymmetric synthesis and should

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02403. Experimental procedures, characterization data, and copies of NMR and HPLC spectra (PDF) X-ray data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ‡

These authors contributed equally (Y.L. and Y.-N.Y.).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation of China (21325209, 21472205) and the Shanghai Municipal Committee 664

DOI: 10.1021/acscatal.5b02403 ACS Catal. 2016, 6, 661−665

Letter

ACS Catalysis

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of Science and Technology (Program of Shanghai Academic Research Leader, 14XD1404400) for financial support.

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DOI: 10.1021/acscatal.5b02403 ACS Catal. 2016, 6, 661−665