Synlett 2017; 28(20): 2923-2927
DOI: 10.1055/s-0036-1588528
letter
© Georg Thieme Verlag Stuttgart · New York

Nicholas Reactions of Alkynyl- and Alkenyltrifluoroborates

Brent St. Onge, James R. Green*
  • Department of Chemistry and Biochemistry, University of Windsor, 401 Sunset Ave., Windsor, ON, N9B 3P4, Canada   Email: jgreen@uwindsor.ca
We are grateful to NSERC (Canada) Discovery Grants programme (RGPIN-2016-04946) for support of this research.
Further Information

Publication History

Received: 13 June 2017

Accepted after revision: 04 July 2017

Publication Date:
17 August 2017 (eFirst)

This paper is dedicated to Prof. Victor Snieckus, in consideration of his many years of mentorship and friendship, and for his tireless and continuing dedication to progress in synthetic aromatic chemistry

Abstract

The Lewis acid mediated Nicholas reaction of potassium alkynyltrifluoroborates and propargyl acetate-hexacarbonyldicobalt complexes affords 1,4-diyne dicobalt hexacarbonyl complexes in good yields. The analogous Nicholas reactions of potassium alkenyltrifluoro­borates give 1,3-enyne dicobalt hexacarbonyl complexes in most cases, although the initial site of reaction can vary. Potassium vinyltrifluoroborate itself affords alkynylcyclopropane complexes.

Supporting Information

 
  • References and Notes

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  • 9 Experimental ProcedureTo a solution of 3a (89.7 mg, 0.218 mmol) and 2a (0.113 g, 0.544 mmol) in CH2Cl2 (8 mL) at 0 °C was added BF3·OEt2 (67 μL, 0.54 mmol). After 1.5 h at 0 °C, sat. NH4Cl (aq) was added and the mixture subjected to conventional extractive workup (CH2Cl2). Flash chromatography (PE–Et2O, 50:1) afforded 4aa (90.3 mg, 91%) as a viscous red oil.
  • 10 Representative Characterization DataCompound 4aa: 1H NMR (CDCl3, 300 MHz): δ = 7.30–7.44 (m, 5 H), 4.00 (s, 2 H), 2.92 (q, J = 7.3 Hz, 2 H), 1.35 (t, J = 7.3 Hz, 3 H). 13C NMR (CDCl3, 75 MHz): δ = 199.7, 131.5, 128.3, 128.0, 123.3, 101.6, 93.7, 86.5, 82.3, 26.9, 25.0, 15.6. IR: νmax = 2971, 2088, 2043, 1989, 1600, 1490 cm–1. HRMS: m/e calcd for C19H12Co2O6 [M+ – CO + H]: 454.9376; found: 454.9367.Compound 4ba: 1H NMR (CDCl3, 300 MHz): δ = 3.72 (t, J = 2.3 Hz, 2 H), 2.87 (q, J = 7.4 Hz, 2 H), 2.17 (m, 2 H), 1.31 (t, J = 7.4 Hz, 3 H), 0.90 (t, J = 7.1 Hz, 3 H). 13C NMR (CDCl3, 75 MHz): δ = 200.0, 101.5, 95.3, 82.6, 76.7, 30.6, 26.8, 24.3, 21.9, 18.3, 15.5, 13.5. IR: νmax = 2964, 2874, 2087, 2043, 1990, 1457. HRMS: m/e calcd for C17H16Co2O6 [M+ – CO + H]: 406.9740; found: 406.9733.Compound 4ca: 1H NMR (CDCl3, 300 MHz): δ = 3.76 (d, J = 2.6 Hz, 2 H), 2.88 (q, J = 7.4 Hz, 2 H), 2.19 (t, J = 2.6 Hz, 1 H), 1.32 (t, J = 7.4 Hz, 3 H). 13C NMR (CDCl3, 75 MHz): δ = 200.2, 128.7, 128.0, 101.3, 98.9, 37.0, 27.1, 17.5, 15.6. IR: νmax = 3314, 2972, 2877, 2089, 2044, 1994 cm–1. HRMS: m/e calcd for C13H8Co2O6 [M+ + H]: 378.9063; found: 378.9063.
  • 11 Nicholas reactions for γ-carbonyl cation equivalents routinely give more rapid reaction with Bu2BOTf as Lewis acid relative to BF3·OEt2. See: Taj R. Green JR. J. Org. Chem. 2010; 75: 8258
  • 12 Representative Characterization Data for Compound 7ab 1H NMR (CDCl3, 300 MHz): δ = 7.29–7.44 (m, 5 H), 6.61 (d, J = 15.7 Hz, 1 H), 6.31–6.41 (dt, J = 15.7, 7.2 Hz, 1 H), 3.77–3.79 (d, J = 7.2 Hz, 2 H), 2.92 (t, J = 7.6 Hz, 2 H), 1.48–1.77 (m, 4 H), 1.00 (t, J = 7.2 Hz, 3 H). 13C NMR (CDCl3, 75 MHz): δ = 200.3, 137.0, 132.3, 128.6, 127.5, 127.4, 126.2, 99.4, 97.7, 37.4, 33.8, 22.8, 13.9. IR: νmax = 3084, 3062, 3028, 2960, 2874, 2085, 2040, 1994, 1495 cm–1. HRMS: m/e calcd for C21H18Co2O6 [M+ – CO + H]: 456.9896; found: 456.9907.
  • 13 Representative Characterization DataCompound ( Z )-7bc: 1H NMR (CDCl3, 300 MHz): δ = 5.50–5.67 (m, 2 H), 3.68 (d, J = 6.7 Hz, 2 H), 1.70 (d, J = 5.9 Hz, 3 H), 0.31 (s, 9 H); resonances from the minor isomer were observed at δ = 3.56 (d, J = 5.9 Hz, 2 H), 0.30 (s, 9 H).13C NMR (CDCl3, 75 MHz): δ (major isomer only) = 200.4, 128.0, 125.9, 111.2, 78.7, 32.3, 12.9, 0.6. IR: νmax = 3025, 2959, 2984, 2085, 2041, 2000, 1581. HRMS: m/e calcd for C15H16Co2O6Si [M+ – CO + H]: 410.9509; found: 410.9501.Compound 8db: 1H NMR (CDCl3, 300 MHz): δ = 2.78 (t, J = 8.0 Hz, 2 H), 2.15 (tt, J = 7.4, 4.2 Hz, 1 H), 1.43–1.67 (m , 4 H), 1.11 (ddd, J = 7.4, 6.6, 4.3 Hz, 2 H), 0.97 (t, J = 7.2 Hz, 3 H), 0.72–0.77 (ddd, J = 6.6, 4.3, 4.2 Hz, 2 H). 13C NMR (CDCl3, 75 MHz): δ = 200.1, 103.8, 98.4, 33.9, 33.6, 22.7, 15.2, 13.8, 12.5. IR: νmax = 2962, 2876, 2086, 2040, 2005, 1450 cm–1. HRMS: m/e calcd for C16H14Co2O6 [M+ – CO + H]: 380.9583; found: 380.9589.
  • 14 Experimental ProcedureComplex 4bd (47.6 mg, 0.110 mmol) was dissolved in acetone (8 mL) and the solution cooled to –78 °C. CAN (0.302 g, 0.551 mmol, 5 equiv) was added and the solution allowed to warm to –30 °C (2 h), with monitoring by TLC. A sat. NaCl solution was added and the mixture subjected to a conventional extractive workup (Et2O). The product was filtered through a silica plug using Et2O and concentrated under reduced pressure to give 9bd (13.9 mg, 86%).
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  • 16 Our operating hypothesis for the predominant Z-isomer formation comes from the proposed larger size of the BF3 unit relative to a methyl group. Initial reaction consequently gives a cation 11 with the methyl nearly eclipsed to the homopropargyl-Co2(CO)6 unit (Scheme 4); subsequent hydride migration then gives a cation 12 where rotation of the C–B bond to a colinear orientation relative to the empty p-orbital of the cation is of lower energy than a rotation of the methyl group anti to the homopropargyl-Co2(CO)6 group.
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  • 18 For analogous reactivity of vinylsilanes with more electrophilic carbocations, see: Laub HA. Mayr H. Chem. Eur. J. 2014; 20: 1103
  • 19 El-Amouri H. Gruselle M. Jaouen G. Daran JC. Vaissermann J. Inorg. Chem. 1990; 29: 3238