Letter pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rh(II)/Brønsted Acid Catalyzed General and Highly Diastereo- and Enantioselective Propargylation of in Situ Generated Oxonium Ylides and C‑Alkynyl N‑Boc N,O-Acetals: Synthesis of Polyfunctional Propargylamines Xueling Meng,†,§ Binmiao Yang,†,§ Linxing Zhang,‡,§ Guangyao Pan,† Xinhao Zhang,*,‡ and Zhihui Shao*,†
Org. Lett. Downloaded from pubs.acs.org by MIDWESTERN UNIV on 02/13/19. For personal use only.
†
Key Laboratory of Medicinal Chemistry for Natural Resource, Ministry of Education, School of Chemical Science and Technology, Yunnan University, Kunming 650091, China ‡ Lab of Computational Chemistry and Drug Design, State Key Laboratory of Chemical Oncogenomics, Peking University, Shenzhen Graduate School, Shenzhen 518055, China S Supporting Information *
ABSTRACT: The first metal/organo cooperatively catalyzed asymmetric reaction of C-alkynyl N-Boc-protected N,O-acetals with in situ generated oxonium ylides has been developed. This new type of propargylation allows for the efficient synthesis of structurally diverse unreported chiral propargylamines bearing oxa-quaternary stereocenters. The reaction features unprecedented substrate scope and high diastereo- and enantioselectivity. Theoretical studies suggest a novel cooperative catalysis model and the unique transfer of R2OH.
P
structurally diverse polyfunctional chiral propargylamines bearing oxa-quaternary stereocenters, which cannot be prepared by existing catalytic methodologies. The ylides generated in situ by the transition-metal-catalyzed decomposition of diazo compounds in the presence of a heteroatom nucleophile are known to undergo a variety of transformations.8,9 Recently, the groups of Hu and Gong creatively developed cooperative Rh(II)/chiral phosphoric acid catalyzed multicomponent reactions of diazo compounds and alcohols to afford oxonium ylides, as reactive nucleophilic species, which were trapped with imines to efficiently afford highly functionalized products.10 Unfortunately, applicable imines are limited and were generally limited to aryl imines bearing an N-aryl protecting group. Moreover, 9-anthryl alcohol, a bulky benzyl alcohol was typically required to obtain good stereoselectivity.10 These issues severely limit the diversity of the products as well as their synthetic applications. We envisaged that the strategic and innovative use of C-alkynyl N-Bocprotected N,O-acetals, recently introduced by us6k,m as tailormade electrophiles for ylide trapping processes, may provide a previously unrecognized opportunity for the realization of all the challenges mentioned above and would also enable the first asymmetric propargylation reaction11 of oxonium ylides, a new reaction type in the chemistry of oxonium ylides, as well as provide a novel and efficient approach to polyfunctionalized chiral propargylamines.
ropargylamines are versatile intermediates for the preparation of polyfunctional amine derivatives as well as natural products and bioactive compounds1 because of the rich chemistry associated with the alkynyl group.2 Propargylamines are also important structural motifs, which are present in many natural products and pharmaceuticals.3 The design and development of catalytic asymmetric methods for the preparation of chiral propargylamines is thus a critical objective in chemical synthesis.4 The traditional method of preparing chiral propargylamines involves the asymmetric nucleophilic alkynylation of imines (Scheme 1, path a).5 Recently, a method Scheme 1. Catalytic Asymmetric Approaches to Propargylamines through the C−C Bond Formation
utilizing electrophilic C-alkynyl imines that react with carbon nucleophiles has emerged as an alternative method for the synthesis of chiral propargylamines (Scheme 1, path b).6 Despite potential of this approach, applicable nucleophiles are still rather limited. Moreover, these reported reactions are based on either chiral transition-metal catalysts or organocatalysts alone. Herein, we report for the first time metal/organo cooperatively catalyzed asymmetric reaction of C-alkynyl imines with oxonium ylides as novel nucleophiles.7 This protocol provides © XXXX American Chemical Society
Received: December 19, 2018
A
DOI: 10.1021/acs.orglett.8b04051 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters We initiated our study on the reaction of a C-alkynyl N-Boc N,O-acetal 1a with α-diazoketone 2a, considering that α-aryl-αdiazo ketones have not been employed for the ylide trapping process. When the reaction was performed in the presence of Rh2(OAc)4 (4 mol %), chiral phosphoric acid A112 (6 mol %), and 4 Å molecular sieve (MS), in CH2Cl2 at rt, the desired propargylamine 3a was obtained in 21% yield with 90:10 dr and 95:5 er (Scheme 2). Notably, no 1,4-addition product was
Scheme 3. Propargylation Reaction of C-Alkynyl N-Boc N,OAcetal 1 and α-Diazoketones 2a
Scheme 2. Catalysts and Model Reaction Employed for the Reaction Condition Optimization
observed. After screening a series of reaction conditions (for details, see the Supporting Information), we ultimately determined that the designed reaction could afford 3a in 82% yield with 94:6 dr and 96:4 er in cyclohexane with 8 mol % of A1. With the optimized reaction conditions established, we investigated the substrate scope. First, we studied various Calkynyl N-Boc N,O-acetals (Scheme 3a). Generally, the reaction proceeded smoothly, and the desired products 3a−p were obtained in good yields with high diastereo- and enantioselectivities (up to >95:5 dr and 97:3 er). Notably, this protocol showed the unique scope of alcohols, and a wide range of alcohols, including simple unactivated alcohols such as MeOH and EtOH, can be incorporated into the products with high levels of diastereo- and enantioselectivities (products 3a, 3i−p). Next, we examined the scope of α-aryl-α-diazo ketones 2 and found that various α-aryl-α-diazo ketones reacted with 1a smoothly to afford the corresponding chiral propargylamines 4a−k in good yields with good stereoselectivities (Scheme 3b). This reaction also represents the first general, highly diastereoand enantioselective ylide trapping reaction using N,O-acetals as reaction partners.13 The absolute and relative configuration of 4a was determined by X-ray crystal analysis. The alkynyl group on the obtained chiral propargylamine products can be readily converted into the corresponding alkenyl or alkyl group by controlled hydrogenation, thus providing hitherto unattainable allylic amine 5 and primary alkyl-substituted amine 6 (Scheme 4). The Boc protecting group of 3a can be readily removed to liberate the free amine 7. This new methodology was also applicable to the reaction between a C-aryl N-Boc N,O-acetal, 8, and 2a (eq 1), providing the chiral benzyl amine 9 in good yield with high diastereo- and enantioselectivity.
a
See the Supporting Information for detailed reaction conditions. Diastereomeric ratios were determined by 1H NMR analysis of the crude reaction mixture. Enantiomeric excesses were determined by chiral HPLC analysis.
Scheme 4. Product Transformations
Next, we explored the mechanism of this chemical transformation. Treating the O−H insertion product 10 with 1a under the standard conditions gave no desired product 3a (Scheme 5a), thus ruling out the pathway via an O−H insertion product. In the absence of Rh2(OAc)4 or chiral CPA, neither the desired product 3a nor the corresponding O−H insertion product 10 was observed (Scheme 5b). When the reaction of 1i and 2a with EtOH in the absence of CPA was conducted, the O−H insertion product 10 was obtained in only 37% yield (Scheme 5c). These experiments demonstrate the decisive role of CPA in the cleavage of the C−O bond and the significant effect of CPA on the O−H insertion step. Finally, we performed control reactions of 1i and 2a with CD3OH (from 1.0 equiv to 4.0 equiv) under the standard conditions. Interestingly, the amount of the product 3i was more than the product 3i′ (Scheme 5d). We then performed density functional theory calculations in an effort to understand the mechanism and the origin of the selectivity. Three possible mechanisms are considered (see Figure S1 in Supporting Information for details), and the most B
DOI: 10.1021/acs.orglett.8b04051 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
shuttle in forming the imine and the MeOH in 1-INT2. The reaction between MeOH moiety in 1-INT2 with Rh carbine is barrierless (Figure S3 in Supporting Information), yielding the oxonium ylide 1-INT3.8a Following the release of the [Rh(OAc)2]2 catalyst,14 1-INT4 is ready for the coupling of the imine and the enol via asymmetric Mannich-type reaction.15 Finally, the formation of C−C bond through nucleophilic addition determines the stereoselectivity. To further investigate the origins of the stereoselectivity, a systematic conformational search of the transition structure of 1TS2 was conducted. The bulky chiral phosphoric acid connects the iminium ion and the enol via NH···O and OH···O hydrogen bonding, providing an asymmetric environment for stereoselectivity. In a quadrant projection,16,17 the chiral phosphoric acid occupies two of four quadrants, viewed along the C2-axis (Figure 1b). For the substrates, the iminium ion and the enol both have the Si and Re face for reaction, and the iminium ion can be in either Z or E configuration, leading to eight possible conformations of 1-TS2. The most stable transition states obtained from conformational search (Figure S4 in Supporting Information) is the 1TS2(3R,4R-Z), which leads to the observed 3R,4R-product. Two most stable enantiomers, 1-TS2(3S,4S-Z) and 1-TS2(3R,4R-Z), are depicted in Figure 1c. The iminium ion in the two transition states are both Z conformations. In 1-TS2(3R,4R-Z), the -Boc group of the imine occupies the empty quadrant, avoiding the steric repulsion from the phosphoric acid. In 1-TS2(3S,4S-Z), the bulky -Boc group (orange sphere) is located in a quadrant, which is occupied by the chiral phosphoric acid (black sphere), resulting in the steric hindrance between substrates and catalyst. The free energy difference of these transition states is calculated to be 2.2 kcal/mol corresponding to a theoretical er of 97.5:2.5, in good agreement with experimental observation. Our new methodology was also applicable to cyclic diazo compounds. To date, there is no general imine partner allowing for a highly stereoselective catalytic asymmetric trapping process involving both acyclic and cyclic diazo compounds. We established that a variety of diazooxindoles 11 can react with 1 smoothly to provide structurally diverse chiral propargylamines bearing oxindole scaffolds18 in good yields with high diastereoselectivity (up to >95:5 dr) and enantioselectivity (up to 98:2 er) (Scheme 6). The absolute and relative configuration of 12b was determined by X-ray crystal analysis. In summary, we have developed a different strategy for the reaction of C-alkynyl imines, which, in contrast to the previous approaches utilizing transition-metal catalysis or organocatalysis alone, is based on organo/metal dual catalysis. We have accomplished a novel reaction of C-alkynyl N-Boc N,O-acetals with in situ generated oxonium ylides as nucleophiles. This reaction provides a new and efficient method for the synthesis of polyfunctional chiral propargylamines and also establishes the first asymmetric propargylic alkylation reaction of oxonium ylides, a new reaction type in the chemistry of oxonium ylides. Additionally, this method overcomes many of the hurdles associated with the use of alcoholic oxonium ylides as nucleophiles in Mannich-type trapping processes. A wide range of alcohols, including simple unactivated alcohols, such as MeOH and EtOH, can be successfully incorporated for the first time with both high diastereo- and enantioselectivity. The unprecedented use of easily cleavable Boc as a nitrogen protecting group makes this method particularly attractive. Besides, this protocol can react well with both acyclic and cyclic
Scheme 5. Control Reactions
favorable one is shown in Figure 1a. Complexation of alkynyl N,O-acetal 1i and chiral phosphoric acid A1 leads to the intermediate 1-INT1. The active intermediate 1-INT1 undergoes C−O bond cleavage. The phosphoric acid acts as a proton
Figure 1. (a) Reaction mechanism of propargylation. (b) Model of conformational searching for 1-TS2 and quadrant projection of the chiral phosphoric acid. (c) Structures of 1-TS2(3R,4R-Z) and 1TS2(3S,4S-Z). Free energies and enthalpies (in parentheses) are given in kcal/mol. C
DOI: 10.1021/acs.orglett.8b04051 Org. Lett. XXXX, XXX, XXX−XXX
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ACKNOWLEDGMENTS This work was supported by NSFC (21672184, 21861042), the Program for Changjiang Scholars and Innovative Research Team in University (IRT13095), Shenzhen STIC (JCYJ20170412150343516), the Program for Innovative Research Team (in Science and Technology) in University of Yunnan Province, and Yunling Scholar of Yunnan Province.
Scheme 6. Propargylation Reaction of C-Alkynyl N-Boc N,OAcetals 1 with Diazooxindoles 11a
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See the Supporting Information for detailed reaction conditions. Diastereomeric ratios were determined by 1H NMR analysis of the crude reaction mixture. Enantiomeric excesses were determined by chiral HPLC analysis.
diazo compounds. Theoretical studies reveal a novel cooperative catalysis model involving a ten-membered ring stereochemical transition state and the unique transfer of alcohols. To our knowledge, a theoretical study on such a process has not been reported systematically.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b04051. Experimental details, characterization data for all of the new compounds, DFT calculations, and copies of NMR and HPLC spectra (PDF) Accession Codes
CCDC 1867283 and 1867286 contain 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] (Z.S.). *E-mail:
[email protected] (X.Z.). ORCID
Xinhao Zhang: 0000-0002-8210-2531 Zhihui Shao: 0000-0003-4531-3417 Author Contributions §
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest. D
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