Ligand-Enabled γ-C(sp3)–H Cross-Coupling of Nosyl-Protected

Research Article Cite This: ACS Catal. 2017, 7, 7777-7782

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Ligand-Enabled γ‑C(sp3)−H Cross-Coupling of Nosyl-Protected Amines with Aryl- and Alkylboron Reagents Qian Shao, Jian He, Qing-Feng Wu, and Jin-Quan Yu* Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, United States S Supporting Information *

ABSTRACT: Pd(II)-catalyzed γ-C(sp3)−H cross-coupling of 4-nitrobenzenesulfonyl (Ns)-protected amines is realized using both arylboron and alkylboron coupling partners. An acetyl-protected aminomethyl oxazoline (APAO) ligand is found to enable the C(sp3)−H arylation reaction, whereas mono-N-protected amino acid (MPAA) ligands promote the C(sp3)−H cross-coupling with various alkylboron reagents. Notably, the APAO-promoted C−H arylation reactions afford high diastereoselectivity (>20:1), providing a useful method for modifying chiral amines. The use of a common nosyl protecting group to direct C(sp3)−H activation significantly improves the practicality of this transformation, as demonstrated by the gram-scale stereoselective synthesis of γ-aryl- and γ-alkyl-α-amino acids. KEYWORDS: amino acids, C−H activation, cross-coupling, palladium, synthetic methods Scheme 1. Pd(II)-Catalyzed γ-C(sp3)−H Cross-Coupling of Alkyl Amines

1. INTRODUCTION Over the past decade, transition-metal-catalyzed C(sp3)−H activation has emerged as a powerful tool for constructing carbon−carbon and carbon−heteroatom bonds in a wide range of carboxylic acid,1,2 amine,3 and oxime substrates.4 While Pd(II)/Pd(IV) catalytic systems have been extensively exploited using organohalides as the coupling partners in carbon−carbon bond-forming reactions,5 the cross-coupling of C(sp3)−H bonds with organometallic reagents via Pd(II)/Pd(0) catalysis remains underexplored due to a lack of suitable ligands.6 To date, three examples of C(sp3)−H cross-coupling reactions of carboxylic acid derivatives have been achieved through the combination of monodentate auxiliaries with MPAA or quinoline ligands.7 Two examples of amine-directed C(sp3)−H cross-coupling are limited to triflamide and 2,2,2,2-tetrasubstituted piperidine substrates, which are not practical or general.8,9 For example, the first example of Pd(II)-catalyzed crosscoupling of γ-C(sp3)−H bonds of triflyl (Tf)-protected amines with arylboron reagents suffers from the difficulty of removing the triflyl directing group with chiral amino acid substrates (Scheme 1, eq 1).8 The moderate diastereomeric ratio (dr) of 4.7:1 is also a drawback for modifying chiral amines. These aforementioned limitations point to the lack of a practical directing group and, most importantly, a new ligand that can accelerate these reactions. We herein report the identification of two classes of ligands (APAO and MPAA) that enable Pd(II)catalyzed C(sp3)−H cross-coupling of nosyl-protected alkyl amines with arylboron and alkylboron reagents, respectively (Scheme 1, eq 2). These ligand-enabled C(sp3)−H activation reactions displayed excellent diastereoselectivity. Notably, although both C(sp3)−H arylation and alkylation of aliphatic amines with aryl and alkyl halides were extensively investigated following Daugulis’ seminal work on picolinamide-directed γ-C(sp3)−H arylation,1a C(sp3)−H alkylation of amines with © XXXX American Chemical Society

alkylborons via Pd(II)/Pd(0) redox catalysis has not been realized to date. The use of a common protecting group as the directing group for C(sp3)−H activation is a significant advantage. These cross-coupling reactions have also been successfully applied to the gram-scale synthesis of chiral γ-aryland γ-alkyl-α-amino acids, which are highly valuable chiral synthons in ligand synthesis and drug discovery.

2. RESULTS AND DISCUSSION Earlier efforts in exploiting the classic cyclopalladation processes have led to the development of numerous Pd-catalyzed C(sp3)−H activation reactions.5 However, these developments were driven primarily by strong coordination of mono- or bidentate substrates, which constitute background reactions and Received: August 11, 2017 Revised: September 18, 2017 Published: September 20, 2017 7777

DOI: 10.1021/acscatal.7b02721 ACS Catal. 2017, 7, 7777−7782

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ACS Catalysis

practicality. While no C(sp3)−H activation occurred in the absence of ligand, we were delighted to find that the acetyl-protected aminoethyl quinoline (APAQ) ligand (L1)10 and Ac-L-Val-OH (L2) provided the desired product in 21% and 50% yields, respectively. Considering that APAO ligands facilitate C(sp3)− H activation/C−C bond-forming reactions via Pd(II)/Pd(IV) catalysis11 as well as enantioselective C(sp3)−H borylation via Pd(II)/Pd(0) catalysis,12 we envisioned that these ligands could be effective in the Ns-protected amine-directed C(sp3)−H cross-coupling, which also proceeds through a Pd(II)/Pd(0) catalytic cycle. Encouragingly, the use of (S,S)-L3 furnished the arylated products in 64% yield. We then systematically surveyed APAO ligands bearing different substituents to further optimize this transformation (L4−L14). Replacing the tert-butyl group on the ligand side chain with an isopropyl group (L4) increased the reaction yield to 67%. Fine tuning of other substituents on the side chain did not provide better results (L5−L8). Substituents on the oxazoline moiety were also important for the ligand reactivity, since the absence of a benzyl group (L9) lowered the yield to 42%. More sterically hindered substituents on the oxazoline moiety proved to be detrimental to the reaction, presumably due to ineffective APAO ligand binding with Pd(II) species (L11−L14). The use of another diastereomer of the optimal ligand (S,R)-L4 drastically inhibited the reaction, reducing the yield to 8%. Another enantiomer or diastereomer, (R,R)-L4 and (R,S)-L4, did not match the substrate either, affording the arylated products in 41% and 43% yields, respectively. These results demonstrated significant chiral recognition between ligands and substrates. Finally, decreasing the loading of (S,S)-L4 from 20 to 15 mol % slightly increased the yield to 68%. Importantly, higher than 99% ee of the monoarylated product was determined by chiral highperformance liquid chromatography, which clearly indicated no racemization at the α-chiral center of α-amino acid substrates under the cross-coupling reaction conditions. With the optimized reaction conditions in hand, we explored this γ-C(sp3)−H cross-coupling reaction of α-amino acid derivative 1b with a wide range of arylboron reagents in the presence of (S,S)-L4 (Table 2). A simple phenylboronate ester provided 4a in a good yield of 77% with greater than 20:1 dr. Replacement of the APAO ligand with either enantiomer of acetylprotected valine resulted in much lower diastereoselectivity (7.7:1 and 3.2:1 dr, respectively), indicating the importance of APAO ligands in this arylation. Gratifyingly, both electrondonating and electron-withdrawing substituents at the para and meta positions of the arylboron reagents are well tolerated, affording the desired products in good yields with high diastereoselectivity. Coupling partners bearing electron-donating methyl, methoxyl, and methylthio groups gave moderate to good yields (55−83%) with excellent diastereoselectivity (4b−d). Electronwithdrawing substituents such as ester, acetyl, trifluoromethyl, fluoro, and nitro groups at the para position also performed well (4e−i). Furthermore, 2-naphthylboronic acid pinacol ester, meta-substituted, and disubstituted arylboron reagents all furnished the desired products in moderate to good yields (4j−n). We were particularly delighted to find that the heteroaryl boronate ester was compatible with this reaction as well (4o). We then explored the amine substrate scope of this ligandenabled cross-coupling protocol (Table 3). The L-tert-leucine derivative was successfully functionalized to give mono- and diarylated products in 63% yield (5a). While a moderate yield of 5c was obtained from the L-isoleucine derivative, together with an 8% yield of δ-C−H arylated product, the L-allo-

prevent the development of ligands that can accelerate the reaction and afford enantioselectivity. We have therefore engineered several classes of weakly coordinating monodentate substrates to provide a platform for achieving ligand-accelerated C−H activations.5g This approach has led to the development of a wide range of ligand-accelerated enantioselective C(sp3)−H activation reactions of aliphatic acid derivatives. In contrast, analogous C−H activation reactions of alkyl amines are highly limited. Influenced by our previously developed triflyl-protected γ-C(sp3)−H arylation of amines,8 we initiated our investigation to the γ-C(sp3)−H cross-coupling of nosyl-protected amine 1a using various types of bidentate ligands (Table 1). Due to the Table 1. Ligand Optimization for C(sp3)−H Cross-Coupling with Arylboron Reagentsa,b

a

Reaction conditions: substrate 1a (0.1 mmol), 2a (3.0 equiv), Pd(OAc)2 (10 mol %), ligand (20 mol %), Ag2CO3 (2.0 equiv), NaHCO3 (4.0 equiv), 1,4-benzoquinone (BQ) (1.0 equiv), H2O (5.5 equiv), t-AmylOH (1.0 mL), 80 °C, 12 h, air. bThe yield was determined by 1 H NMR analysis of the crude product using CH2Br2 as the internal standard.

ease of protecting group removal, the use of a nosyl protecting group constitutes a significant improvement in terms of 7778

DOI: 10.1021/acscatal.7b02721 ACS Catal. 2017, 7, 7777−7782

Research Article

ACS Catalysis Table 2. Scope of Arylboron Reagentsa,b

a

Reaction conditions: substrate 1b (0.1 mmol), 2 (3.0 equiv), Pd(OAc)2 (10 mol %), (S,S)-L4 (15 mol %), Ag2CO3 (2.0 equiv), NaHCO3 (4.0 equiv), 1,4-benzoquinone (BQ) (1.0 equiv), H2O (5.5 equiv), t-AmylOH (1.0 mL), 80 °C, 12 h, air. bIsolated yields. cUsing Ac-L-Val-OH as ligand. dUsing Ac-D-Val-OH as ligand.

Table 3. Substrate Scope of C(sp3)−H Cross-Coupling with Arylboron Reagenta,b

Table 3. continued a

Reaction conditions: substrate 1 (0.1 mmol), 2b (3.0 equiv), Pd(OAc)2 (10 mol %), (S,S)-L4 (15 mol %), Ag2CO3 (2.0 equiv), NaHCO3 (4.0 equiv), 1,4-benzoquinone (BQ) (1.0 equiv), H2O (5.5 equiv), t-AmylOH (1.0 mL), 80 °C, 12 h, air. bIsolated yields. c Including 8% yield of the δ-C−H arylated product. dKH2PO4 (2.0 equiv) was used instead of NaHCO3.

isoleucine derivative gave 5b in 67% yield. This difference is presumably attributed to the more favorable transition state conformation of the cyclopalladation, in which the 2,3-substitutents adopt an anti configuration. Cross-coupling of 1d afforded the corresponding product 5d in 30% yield, while the rest of the starting material was recovered. The lack of a Thorpe−Ingold effect may be attributed to the relatively low reactivity in the activation of 1d. Changing the base to KH2PO4 allowed γ-C−H activation of a cyclopropylmethyl amine derivative in moderate yield (5e). The L-valine benzyl ester derivative was also effective (5f), while other more sterically hindered esters gave lower conversion due to their stronger repulsion with APAO ligands (see Table S3 in the Supporting Information). The parent carboxylic acid derivative is not reactive under these basic conditions (see Table S3), since it can serve as an effective bidentate ligand to outcompete APAO ligands for the coordination sites of Pd(II). The Ns-protected amine derived from a β-amino acid provided the desired products (5g) in 60% yield under the standard conditions. For amino alcohol derivative 1h containing a free hydroxyl group, we were pleased to obtain the corresponding product (5h) in 54% yield with greater than 20:1 dr. Surprisingly, oxidation of the relatively unstable primary 7779

DOI: 10.1021/acscatal.7b02721 ACS Catal. 2017, 7, 7777−7782

Research Article

ACS Catalysis Table 5. Scope of Alkylboron Reagentsa,b

Table 4. Ligand Optimization for C(sp3)−H Cross-Coupling with Alkylboron Reagentsa,b

a

Reaction conditions: substrate 1a (0.1 mmol), 6 (4.0 equiv), Pd(OAc)2 (10 mol %), Ac-D-Val-OH (20 mol %), Ag2CO3 (2.0 equiv), Li2CO3 (3.0 equiv), 1,4-benzoquinone (BQ) (1.0 equiv), DMF (0.1 mL), 1,4-dioxane (1.0 mL), 70 °C, 12 h, N2. bIsolated yields. a Reaction conditions: substrate 1a (0.1 mmol), 6a (4.0 equiv), Pd(OAc)2 (10 mol %), ligand (20 mol %), Ag2CO3 (2.0 equiv), Li2CO3 (3.0 equiv), 1,4-benzoquinone (BQ) (1.0 equiv), DMF (0.1 mL), 1,4-dioxane (1.0 mL), 70 °C, 12 h, N2. bThe yield was determined by 1H NMR analysis of the crude product using CH2Br2 as the internal standard.

trace amounts of 7a, N-acetyl-L-alanine (L17) afforded a 31% yield of the desired products. After systematic modification of the substituents on the ligand side chain, we found that some bulky substituents were beneficial for the cross-coupling reaction, improving the yield from 31% to 63% (L18 and L19). The yield slightly decreased when the isopropyl group was replaced with more sterically hindered groups (L20−L23). Further screening of MPAA ligands revealed an improved efficiency when the D-enantiomer of L19 was employed ((R)-L19). The MPAA ligands derived from quaternary α-amino acids as well as β-amino acids did not offer better results (L25 and L26). After examination of various protecting groups on the amino acid ligands (L27−L30), N-acetyl-D-valine ((R)-L19) still afforded the highest yield of 69%. Notably, this newly developed cross-coupling protocol did not lead to any racemization at the α-chiral center of the alkylated products. While various amino acid substrates from Table 3 are reactive with this alkylation protocol, the MPAA ligand displayed significantly lower diastereoselectivity in comparison to the APAO ligand (S,S)-L4 (see the Supporting Information). We therefore focused on the tert-leucine substrate and performed the γ-C(sp3)−H cross-coupling reaction of α-amino acid derivative 1a with a variety of alkylboron reagents (Table 5). Simple linear coupling partners reacted efficiently with 1a, affording the corresponding alkylated products in good to excellent yields (7a−c). The coupling of 1a with cyclobutylmethyl boron reagent gave a 68% yield (7d). More importantly, alkylboron coupling partners bearing a wide range of functional groups, including ester (7e), phenyl (7f), trifluoromethyl (7g), chloro (7h), and acetoxy (7i), were all well tolerated. Other amino ester and amino

alcohol was not observed. Ac-, Bn-, and TBS-protected amino alcohol derivatives provided the desired products in consistent moderate to good yields and high diastereoselectivity. In contrast to the excellent yield obtained with an L-valine derivative (4b, Table 2), the lack of reactivity of a D-valine derivative (1l) with (S,S)-L4 further illustrates the mismatch between the α-chiral carbon center of the amine substrates and the chiral ligand. While the substrate scope of the majority of the C−H activation reactions is still limited, diverse transformations provide an important dimension to diversify the structures of the C−H activation products. With this thinking in mind, we next investigated the coupling of Ns-protected amines with various alkylboron reagents, which has not been possible to date (Table 4). While the use of boronic acid pinacol esters (Ar-BPin) and NaHCO3 were crucial for arylation, potassium trifluoroborate salts (R-BF3K) and Li2CO3 were optimal reaction components for alkylation. As expected, the reaction did not proceed in the absence of ligand. Initial ligand screening revealed that quinoline-based ligands are ineffective in this transformation, though they are known to promote C(sp3)−H cleavage.10,13 Although APAO ligands were effective for the C(sp3)−C(sp3) bond formation, we could not improve the yield beyond 45% after extensive screening. We then turned our attention to MPAA ligands. While a simple glycine derivative (L16) provided 7780

DOI: 10.1021/acscatal.7b02721 ACS Catal. 2017, 7, 7777−7782

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acids. Without purification, the free amines were subsequently converted to Fmoc-protected amino acids (10a−c).

Scheme 2. Sequential C−H Activation for the Synthesis of γ-Aryl-γ-Alkyl-α-Amino Acids

3. CONCLUSION In summary, we have developed an efficient ligand-enabled Pd(II)-catalyzed C(sp3)−H cross-coupling of alkyl amines directed by a readily removable nosyl protecting group. Key to the success of C(sp3)−H cross-coupling with arylboron reagents is the use of bidentate APAO ligands. The observed excellent diastereoselectivity is also a significant advantage over the previous protocols. This reaction is also compatible with heteroaryl coupling partners. Furthermore, the γ-C(sp3)−H cross-coupling of α-amino acid derivatives with various alkylboron reagents has been realized for the first time using MPAA ligands. These newly developed C(sp3)−H activation protocols have been applied to the gram-scale synthesis of chiral γ-substituted α-amino acids which are not readily accessible by other means.

alcohol derivatives bearing β-tertiary carbon centers were compatible with this reaction as well (7j,k). These functional groups serve as synthetic handles for further functionalizations. With the newly identified bidentate ligands (S,S)-L4 and (R)-L19 (N-acetyl-D-valine) for the cross-coupling with arylboron reagents and alkylboron reagents, respectively, we envisioned the feasibility of sequential incorporation of aryl and alkyl groups onto the γ-methyl C(sp3)−H bonds of amino acid derivative 1a (Scheme 2). By simply switching the order of these two C(sp3)−H functionalization processes, both configurations at the β-stereogenic center of chiral unnatural α-amino acid derivatives can be obtained (8a,b). The use of the Ns-protected amine auxiliary presents significant advantages for the gram-scale preparation of unnatural amino acids, since its installation and removal are simple and highly reliable. In response to current needs for novel bioactive peptide synthesis, amino acid substrate 1a was coupled with alkylboron or arylboron reagents (5 mmol scale) to give the desired products in good yields (Scheme 3). Deprotection of the Ns group could be readily accomplished by using PhSH and K2CO3 in DMSO,14 which afforded the corresponding amines in excellent yields. The resulting crude products were hydrolyzed in the presence of sodium hydroxide to give the desired amino

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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/acscatal.7b02721.

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Experimental details on experimental procedures for the catalytic reactions and spectroscopic data for the products (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail for J.-Q.Y.: [email protected]. ORCID

Jian He: 0000-0002-3388-3239 Jin-Quan Yu: 0000-0003-3560-5774 Notes

The authors declare no competing financial interest.

Scheme 3. Gram-Scale Synthesis and Synthetic Applicationsa

a

Reaction conditions: (a) substrate 1a (5.0 mmol), n-BuBF3K (4.0 equiv), Pd(OAc)2 (10 mol %), Ac-D-Val-OH (20 mol %), Ag2CO3 (2.0 equiv), Li2CO3 (3.0 equiv), 1,4-benzoquinone (BQ) (1.0 equiv), DMF (5.0 mL), 1,4-dioxane (50.0 mL), 70 °C, 12 h, N2; (b) substrate 1a (5.0 mmol), ArBpin (3.0 equiv), Pd(OAc)2 (10 mol %), (S,S)-L4 (15 mol %), Ag2CO3 (2.0 equiv), NaHCO3 (4.0 equiv), 1,4-benzoquinone (BQ) (1.0 equiv), H2O (5.5 equiv), t-AmylOH (50.0 mL), 80 °C, 12 h, air; (c) PhSH (2.0 equiv), K2CO3 (2.0 equiv), DMSO (0.4 equiv), MeCN, 40 °C, 2 h; (d) NaOH (2.0 equiv), THF/H2O/MeOH (3/3/1), 40 °C, overnight; (e) FmocCl (1.0 equiv), 10% aqueous NaHCO3, 1,4-dioxane, room temperature, overnight. 7781

DOI: 10.1021/acscatal.7b02721 ACS Catal. 2017, 7, 7777−7782

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ACKNOWLEDGMENTS We gratefully acknowledge The Scripps Research Institute, the NIH (NIGMS, 2R01GM084019), and Bristol-Myers Squibb for financial support.

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REFERENCES

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DOI: 10.1021/acscatal.7b02721 ACS Catal. 2017, 7, 7777−7782