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

Abstract

Unactivated 1,6-enynes were firstly found to undergo different cyclization process under the catalysis of Ni⁰ species in situ generated from Ni^II 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.

organozinc reagents - enynes - cyclizations - nickel

References and Notes

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The combined use of 6 mol% of Cy₃P and 5 mol% of Ni(acac)₂ only led to a ratio of 1.5:1 favoring the [2+2+2] product 3a. Attempts to interject the reaction intermediate with a third alkyne (3 equiv) resulted in a complex mixture (when ethyl 3-phenylpropiolate was used) or the recovery of most of the starting 1,6-enyne 1a (when phenylacetylene or 1-hexyne were used), however, similar strategy was successful with Rh-catalyzed reactions of unactivated 1,6-enynes:

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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.

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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.

<A NAME="RW02909ST-15">15</A>

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

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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.

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

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

<A NAME="RW02909ST-22">22</A>

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^-¹. 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^-¹. MS (EI): m/z = 572 [M⁺], 279(base). HRMS (EI): m/z calcd for C₃₄H₃₆O₈Na⁺: 595.2316 ± 0.002; found: 595.2302.