Palladium(0)-Catalyzed Intermolecular Cascade Dearomatization

Letter Cite This: Org. Lett. 2018, 20, 6206−6210

pubs.acs.org/OrgLett

Palladium(0)-Catalyzed Intermolecular Cascade Dearomatization Reaction of β‑Naphthol Derivatives with Propargyl Carbonates Lu Ding‡ and Shu-Li You*,†,‡ †

State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, China ‡ School of Physical Science and Technology, ShanghaiTech University, 100 Haike Road, Shanghai 201210, China

Org. Lett. 2018.20:6206-6210. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/05/18. For personal use only.

S Supporting Information *

ABSTRACT: A Pd(0)-catalyzed intermolecular cascade dearomatization reaction of β-naphthol derivatives with propargyl carbonates was described. In the presence of a Pd complex derived from Pd2dba3 and rac-BINAP, various spironaphthalenones were obtained in excellent yields (up to 95%) and with high chemoselectivity (C/O > 20:1) in most cases.

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chemoselectivity between the C-nucleophile and the Onucleophile is still a great challenge in transition-metalcatalyzed allylic substitution reactions of phenol derivatives. In many cases, the phenolic OH group acts as a nucleophile.7 Herein, we report a Pd-catalyzed intermolecular cascade dearomatization reaction of β-naphthol derivatives with propargyl carbonates. The reaction proceeded with excellent chemoselectivity, favoring C-nucleophilic attack (Scheme 1b). The initial attempt was performed with β-naphthol 1a and methyl prop-2-yn-1-yl carbonate 2a in CH2Cl2 at 50 °C in the presence of 2.5 mol % Pd2dba3 and 5.5 mol % Xantphos. The desired spironaphthalenone product 3aa was obtained in 9% NMR yield together with O-alkylated products 4aa and 5aa. When THF was employed, product 3aa was obtained in 14% NMR yield [see Supporting Information (SI), Scheme S1]. Encouraged by these results, several commercially available phosphine ligands were tested in the presence of 2.5 mol % Pd2dba3 in THF at 50 °C. The results are summarized in Table 1. First, several monodentate phosphine ligands were tested. The desired dearomatized product 3aa was obtained in 20:1). Notably, the 6-Br naphthol 1i was a suitable substrate (83% yield, C/O = 15:1). When the 4pyridyl group and 2-furyl group were installed at the C6 position, 3ma and 3na were obtained in 60% and 83% yield, respectively. The coordination of the pyridine nitrogen with palladium might cause the decreased reactivity, but the C/O ratio was not affected (3ma, C/O = 13.4:1). To the best of our knowledge, the substituted propargyl carbonates usually have a low reactivity.5,6 Methyl carbonate, hydroxy methylene, and phenyl-substituted propargyl carbonates could be successfully converted under the optimized conditions and gave the target products in reasonable yields and C/O ratios (3ab−ad, 52− 94% yields, C/O = 1.5:1−>20:1). Due to the steric hindrance of the phenyl group on propargyl carbonate, the target product was obtained in moderate yield (3ad, 52%). The cis- and transgeometries of olefins were determined by NOE analysis (for details, see SI). 6207

DOI: 10.1021/acs.orglett.8b02681 Org. Lett. 2018, 20, 6206−6210

Letter

Organic Letters

derivative attacks intermediate I with nucleophilic nitrogen (or oxygen) atom to give II (or V). Then upon deprotonation with methoxide, π-allylpalladium complex III (or VI) is formed. Complex III undergoes allylic substitution to deliver product 3 or 5 depending on either carbon or oxygen nucleophilic attack. Product 4 is obtained from complex VI. Finally, the palladium(0) catalyst is liberated for completing the catalytic cycle. In summary, we have realized palladium(0)-catalyzed intermolecular cascade dearomatization reaction of β-naphthol derivatives with propargyl carbonates. A series of βnaphthalenones bearing an all-carbon quaternary stereogenic center at the α-position were obtained with moderate to excellent yields and chemoselectivity under mild conditions. The optimized conditions are compatible with a wide range of substrates and gram-scale reaction. Further studies on the reaction mechanism and development of asymmetric variants are currently under investigation.8

To demonstrate the utility of this method further, a gramscale reaction and several transformations of the spironaphthalenone products have been carried out. As depicted in Scheme 3, the intermolecular dearomatization reaction of 1a Scheme 3. Gram-Scale Reaction and Product Transformations

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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.8b02681. Experimental procedures and compound characterization data (PDF) Accession Codes

CCDC 1861930 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.

with 2a on a 3.0 mmol scale gave the desired product 3aa in 93% yield and with C/O > 20:1. The product 3aa could be transformed to 6 in 97% yield under acidic conditions, and the structure of product 6 was confirmed by an X-ray crystallographic analysis (for details, see SI). In addition, the α,βunsaturated double bond of 6 could undergo 1,4-addition reaction with phenylboronic acid to afford 7 in 91% yield and 5.8/1 dr (improved to 14.4/1 after recrystallization in CH2Cl2). The relative configuration was determined by NOE analysis (for details, see SI). A catalytic cycle was proposed as shown in Scheme 4. The palladium(0) catalyst activates the propargyl carbonate to generate the allenylpalladium intermediate I. The β-naphthol

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shu-Li You: 0000-0003-4586-8359 Notes

The authors declare no competing financial interest.

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Scheme 4. Plausible Catalytic Cycle

ACKNOWLEDGMENTS We thank the National Key R&D Program of China (2016YFA0202900), the National Basic Research Program of China (2015CB856600), NSFC (21332009, 21572252), Program of Shanghai Subject Chief Scientist (16XD1404300), and the CAS (XDB20000000, QYZDYSSW-SLH012) for generous financial support.

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REFERENCES

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(8) The use of (R)-BINAP as the ligand provided the desired product 3aa in 94% NMR yield with 2% ee. The further screening of other chiral phosphine ligands was also attempted, and the optimal result was afforded in 83% NMR yield of 3aa with 38% ee by using (S)-DTBM-SEGphos with t‑BuOK as the base.

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DOI: 10.1021/acs.orglett.8b02681 Org. Lett. 2018, 20, 6206−6210