Preparation of 2-(Tetrahydrofuran-2′-YL)-l,4-Pentadienes From γ-Allenyl Alcohols via Oxymercuration Followed by Palladium(II)-Medlated all Ylation

References

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• 4,5-Hexadien-l-ol was prepared by the method of Gore (Arsineyadis, S., Gore, J. and Roumestant, M.L.m Tetrahedron 1979, 35, 353) or by applying Crabbe's “homologation” procedure to 4-pentyn-1-ol (Crabbe, P., Nassim, B. and Robert-Lopes, M.-T.m Org, Syn. 1984, 63, 203)
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• Preparation of 4 (X=Cl): 4,5-Hexadien-1-ol (0.207 g., 2.107 mmol) was stirred in 4 mL of dichloromethane with 0.67 g. (2.06 mmol) mercury(II) acetate, under a CaCl2 drying tube at 25° C, for 6 hr. The solvent was removed by rotary evaporation [to yield the vinylmercuric acetate which could be used without further ado for coupling reactions], then 10 mL of 2 M sodium chloride was added and the mixture was stirred at room temperature for 1 hr. Extraction with ether, then drying of the extracts (brine wash followed by MgSO4 treatment) yielded the crude vinylmercuric chloride, which could be used without further purification. Chromatography (20 g. 230–400 mesh silica gel, 80:20 hexanes:ethyl acetate eluent) produced 0.346 g. (51%) of high purity product. 1H-NMR (CDCl3, TMS standard): δ 5.75 (1H, appearance of singlet, presence of peaks due to 199Hg-1H splitting (J=594 Hz)), 5.15 (IH, appearance of singlet, presence of peaks due to 199Hg-1H splitting (J=291 Hz)), 4.42 (1H, t, J=7 Hz), 3.9 (2H, m), 1.7–2.3 (4H, m). 13C-NMR (CDC13): δ 166.67, 123.68, 83.97, 33.65, 25.90, 68.06
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• Typical procedure: The vinylmercuric chloride 4 (X=C1) (0.233 g., 0.70 mmol)was dissolved in 5 mL of dry THF and added to a cooled (-78° C.) solution of 0.9 g. (7.0 mmol) of allyl bromide, 0.124 g. (0.7 mmol) of palladium(II) chloride, and 0.06 g. (1.4 mmole) of lithium chloride in 5 mL THF, stirring under a nitrogen atmosphere. The reaction mixture was allowed to warm to room temperature overnight, then 20 mL of saturated ammonium chloride solution was added, and the product was extracted into hexane. Drying (brine wash, MgSU4 treatment), concentration, and chromatography (20 g. 230–400 mesh silica gel, 90:10 hexanes:ethyl acetate eluent) yielded 0.10 g. (∼100%) of the pure diene 5
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• NMR data for pentadienes 5, 6, 7, 10, 11, and 12 (measured in CDCl3 using TMS as the standard) corresponded with the assigned structures. Diagnostic 1H-NMR signals included doublets (J∼1 - 1.5 Hz) at ∼5.0 and 4.8 ppm for the “vinylidene” (=CH2) group, and appropriate signals for the hydrogens on the ring carbons adjacent to the oxygen (∼4.3 - 4.5 ppm for the allylic CH next to the ring O, ∼3.9 - 4.1 for the nonallylic CH or CH2 next to the ring O). Assignments of cis and trans geometries were made on the basis of the relative chemical shifts of the signals for the methyl group and the allylic ring hydrogen which appeared in the mixture of diastereomers: downfield ⇒ cis, upfield ⇒ trans
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• 4,5-Hexadienal, a known compound (Coates, R.M., Senter, P.D. and Baker, W.R., J. Org. Chem., 1982, 47, 3597) was prepared from 4,5-hexadien-1-ol by oxidation using PCC. In a typical preparation, approximately 5 mmol of the crude volatile aldehyde, in 10 mL of dry THF at 0° C. under nitrogen, were treated with 1 molar equivalent of methylmagnesium chloride (1.7 mL of a 3 M solution in THF) at 0° C. for 1 hr, then worked up in the usual manner to yield 0.42 g. (75%) of crude 5,6-heptadien-2-ol (1H-NMR (CDC13/TMS): δ 5.05 (1H, m), 4.68 (2H, m), 3.65 (1H, m), 2.63 (variable, OH), 1.95 (2H, m), 1.52 (2H, m), 1.14 (3H, d, J=6.6 Hz). 13C-NMR: δ 208.27, 89.48, 74.81, 67.03, 38.06, 24.25, 23.15). This alcohol (0.119 g., 1.06 mmol) was stirred under a CaCl2 drying tube in 4 mL dry dichloromethane, at 0° C. with 0.13 mL (approximately 2.1 mmol) 2,6-lutidine, then 0.37 mL (approximately 1.6 mmol) of tert-butyldimethylsilyl trifluoromethanesulfonate was added. Workup in the usual manner yielded the γ-silyloxyallene 9 (0.24 g., ∼100%) in high purity (1H-NMR (CDCl3/TMS): δ 5.06 (1H, pent, J=6.6 Hz), 4.63 (2H, m), 3.72 (1H, m), 2.5 (2H, m), 1.50 (2H, m), 1.12 (3H, d, J=8.0 Hz), 0.86 (9H, s), 0.04 (6H,s). 13C-NMR: δ 208.54, 89.94, 76.99, 67.85, 38.88, 25.89, 24.39, 18.09, -2.95, -4.75). The silyl ether 9 was then stirred in 10 mL dry dichloromethane with 0.68 g. (1.6 mmol) mercuric trifluoroacetate, at room temperature under N2, for 6 hours. Concentration in yacuo gave the vinylmercuric trifluoroacetate 8 (0.34 g., 80%). 1H-NMR of the major (cis) isomer (CDC13/TMS): δ 5.64 (1H, m), 5.13 (1H, m), 4.30 (1H, t, J=6.4 Hz), 3.82 (1H, m), 1.95 (2H, m), 1.57 (2H, m), 1.22 (3H, d, J=6.2 Hz). 13C-NMR of the major (cis) isomer: δ 160.80 (q, Jc-CF=39.3 Hz), 157.32, 118.00(q,Jc-F=286.34Hz), 125.35, 83.68, 68.06, 33.26, 25.81, 21.24
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• Larock observed the formation of 1,4-diene byproducts formed by a dimeriza-tion of the vinylmercuric halides in some of his allylation reactions (ref. 4). We have not observed such dienes, although we have found that treatment of 4 with palladium(II) chloride/lithium chloride (1 equivalent), in the absence of an allyl-ic halide (HMPA solvent, 25° C, overnight) produced the dimer in 73% yield. Details of this precedented dimerization (similar to those reported separately by Larock, R.C. and Riefling, B., J. Org. Chem., 1978, 43, 1468) to bis(tetra-hydrofuranyl)-l,3-butadienes, will be reported in a separate communication
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• This research was made possible by funding from the Robert A. Welch Foundation. NMR spectra were measured using instrumentation purchased using funds provided by the NSF (#CHE-851404)
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