Synlett 2009(11): 1785-1790  
DOI: 10.1055/s-0029-1217368
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Ni-Catalyzed Carbocyclization of 1,6-Enynes Mediated by Dialkylzinc Reagents: Me2Zn or Et2Zn Makes a Difference

Zhuo Chai, Hai-Feng Wang, Gang Zhao*
Key Laboratory of Synthetic Chemistry of Natural Substances, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, P. R. of China
Fax: +86(21)64166128; e-Mail: zhaog@mail.sioc.ac.cn;
Further Information

Publication History

Received 4 March 2009
Publication Date:
12 June 2009 (online)

Abstract

Unactivated 1,6-enynes were firstly found to undergo different cyclization process under the catalysis of Ni0 species in situ generated from NiII complexes and dialkylzinc reagents. When stoichiometric Me2Zn was used as the reducing agent, only the dimerization products {[2+2+2+2] or [2+2+2]} were obtained. On the other hand, reductive cyclization products were obtained solely when 0.6 equivalents of Et2Zn were employed as the reductant under otherwise the same reaction conditions. In the former case, up to 1:7 selectivity in favor of the [2+2+2] products was also achieved with NiCl2(PPh3)2/Me2Zn.

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4

Dialkylzinc reagents, zinc powder, triethylsilane, triethylboranes, and DIBAL-H have been used as the reducing agent in this type of transformation.

9

For examples of Ni-catalyzed reductive cyclization pro-cesses with the methyl shift involving Me2Zn, see ref. 5a and references cited therein.

13

CCDC 690610(3a) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data-request/cif. See the Supporting Information for a figure of its X-ray structure.

14

The [2+2+2+2] process has been found to be favored at a large loading amount of Ni catalyst for the dimerization of terminal 1,6-diynes, see ref. 10.

15

When 50 mol% or 20 mol% of Me2Zn were used, the reaction failed to proceed.

17

CCDC 690609(2h) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data-request/cif. See the Supporting Information for a figure of its X-ray structure.

19

In ref. 7, the authors also proposed that oligomerization may be the possible reason for the decreased yield with 1j.

21

The addition of 5 mol% of Ph3P, PCy3 or (R)-BINAP did not improve the product selectivity significantly.

22

Typical Procedure for the Dimerization of 1 with Ni II /Me 2 Zn Combination Under an atmosphere of argon, 3.2 mg (0.012 mmol) of Ni(acac)2 were added to a Schlenk tube, and the system was purged with argon three times. Then enyne 1a (86 mg, 0.3 mmol) in 3.0 mL of freshly distilled THF was added via a syringe followed by the addition of Me2Zn 0.3 mmol (1.2 M in toluene) in one portion [in the case of NiCl2(PCy3), Me2Zn was added at reflux]. The reaction mixture was stirred for 1 h at r.t. before being quenched with sat. aq NH4Cl soln. Then, the mixture was extracted with CH2Cl2 (3 × 3 mL), dried with anhyd Na2SO4. After removal of the solvent in vacuum, the residue was purified by column chromatography (silica gel, PE-Et2O = 4:1) to provide the desired products 2a and 3a. (5 E ,10 E )-Tetramethyl-5,10-diphenyl-3a,4,8a,9-tetrahydropyrene-2,2,7,7(1 H ,3 H ,6 H ,8 H )-tetracarboxy-late (2a)
Colorless crystal; mp 193-194 ˚C (hexane-Et2O). ¹H NMR (300 MHz, CDCl3): δ = 7.36-7.17 (m, 10 H), 3.67 (s, 6 H), 3.63 (s, 6 H), 3.05-2.70 (m, 10 H), 2.24 (d, J = 10.6 Hz, 2 H), 1.95-1.89 (dd, J = 13.1, 6.8 Hz, 2 H) ppm. ¹³C NMR (75 MHz, CDCl3): δ = 172.0, 171.7, 145.2, 144.2, 136.1, 128.3, 127.7, 126.3, 59.4, 52.60, 52.59, 46.2, 43.9, 43.0, 40.4 ppm. IR (KBr): 2952, 1734, 1434, 1250, 1205, 1070, 703 cm. MS (EI): m/z = 572 [M+], 167(base). HRMS (EI): m/z calcd for C34H36O8: 572.2410; found: 572.2413. Dimethyl 6-[2,2-Bis(methoxycarbonyl)pent-4-enyl]-5,7-diphenyl-3a,4-dihydro-1 H -indene-2,2(3 H )-dicarboxy-late (3a)
Colorless crystal;¹³ mp 87-89 ˚C (hexane-Et2O). ¹H NMR (300 MHz, CDCl3): δ = 7.39-7.18 (m, 10 H), 4.86-4.74 (m, 1 H), 4.58-4.68 (m, 2 H), 3.77 (s, 3 H), 3.64 (s, 3 H), 3.46 (s, 3 H), 3.36 (s, 3 H), 3.32-3.22 (m, 1 H), 2.97-2.57 (m, 5 H), 2.28 (d, J = 6.9 Hz, 2 H), 2.14 (t, J = 17.4 Hz, 1 H), 2.02-1.95 (dd, J = 13.2, 8.8 Hz, 2 H) ppm. ¹³C NMR (75 MHz, CDCl3): δ = 172.2, 171.9, 171.1, 142.1, 141.4, 139.2, 133.7, 133.1, 130.1, 129.8, 129.6, 128.4, 128.0, 127.8, 127.0, 126.6, 117.7, 60.2, 59.0, 52.9, 52.7, 52.0, 39.6, 39.1, 38.5, 38.4, 37.8, 30.5 ppm. IR (KBr): 2953, 1732, 1491, 1435, 1380, 763, 735, 703 cm. MS (EI): m/z = 572 [M+], 279(base). HRMS (EI): m/z calcd for C34H36O8Na+: 595.2316 ± 0.002; found: 595.2302.