Reduction of alkynes with Red-Al; Alkenes

Ana de Andres Gomez (Ana.de-Andres-Gomez@bris.ac.uk),
A contribution from the Cox Group, Bristol

Chemicals Used
2-Pentyne-1,4-diol (made in house) Red-Al ([(CH₃OCH₂OCH₂)₂AlH₂]Na, Lancaster, 70% w/w soln. in toluene) THF (dry)

Procedure
Red-Al® (2 mL of a 70% w/w solution in toluene, 3.6 mmol, 1.8 eq.) was added dropwise to a stirred solution of pent-2-yne-1,4-diol (200.0 mg, 2.0 mmol, 1.0 eq.) in dry THF (13.3 ml) at RT under nitrogen. The reaction mixture was stirred at RT for 1h, then quenched by dropwise addition of water, resulting in a small amount of gum formation. NaHCO₃ (solid) was added, the mixture was vigorously stirred for 30min and then filtered under vacuum. The filter residues were exhaustively washed with ethyl acetate, the layers separated and the organic phase dried (MgSO₄) and concentrated. The product was dried under vacuum overnight, in order to remove the methoxyethanol. The viscous oil obtained was purified by Kügelrohr distillation to give the desired diol (0.2 g, 85%).

Author's Comments
Red-Al is quite a viscous material. The by-product (methoxyethanol) was difficult to remove, but it did not affect the outcome of the reaction. Reactions performed at small (200mg) and large scale (8g) gave high yields (70-85%). In our hands the reaction has been highly reproducible.

Data
¹H NMR (270 MHz; CDCl₃): 5.90-5.81 (1 H, dt, ³J_HH (d trans) 15.2 and ³J_HH (t) 4.3, C=CH), 5.82-5.74 (1 H, dd, ³J_HH (d trans) 15.5 and ³J_HH (d) 4.6, (-HC=C), 4.39-4.31 (1 H, qd, ³J_HH (q) 6.3 and ³J_HH (d) 4.6, CH₃-CH(OH)C=), 4.17-4.15 (2 H, d, ³J_HH 4.3, =C-CH₂-OH), 1.77 (2 H, br, s, 2 X OH) and 1.29 (3 H, d, ³J_HH 6.6, CH₃).

Lead Reference
Bates R.W., Diez-Martin D., Kerr W.J, Knight J.G., Ley S.V. and Sakellaridis A., Tetrahedron, 1990, 46, 4063.

Other References
Coll. Czech. Chem. Commun., 1969, 34, 118. Org. Synth. Coll., 1990, 7, 524. Synthesis, 1976, 526.

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