Highly Regio- and Stereoselective Hydrostannylation of Propargyl Alcohols and Ethers Using Dibutylchlorostannane and Lithium Chloride

Abstract

In the presence of LiCl, dibutylchlorostannane (Bu₂SnClH) reacted with γ-unsubstituted propargyl alcohols and ethers to give γ-stannylated allyl alcohols and ethers, respectively, with high regio- and stereoselectivity. These vinylstannane products could be successfully utilized for the Pd-catalyzed cross-coupling reaction.

Key words

alcohols - alkynes - cross-coupling - hydrostannations - radical reactions

References

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10

For the Et₃B-initiated reaction, see ref. [3b]

13

General Procedure for the Hydrostannylation of 1 with Bu ₂ SnClH-LiCl Followed by Butylation:
Under N₂, THF (1 mL) was added to a mixture of Bu₂SnCl₂ (83.5 mg, 0.275 mmol) and LiCl (21.2 mg, 0.500 mmol) at 0 °C. Bu₂SnH₂ (64.6 mg, 0.275 mmol) was added to the THF solution, and the mixture was stirred for 10 min at 0 °C. A propargyl alcohol 1 (0.500 mmol) was added to the resultant Bu₂SnClH-LiCl solution. The mixture was warmed to r.t. and stirred for 3 h. The resultant mixture was cooled to 0 °C and treated with BuMgBr (1.00 M in Et₂O, 1.30 mmol). After 20 min, the resultant mixture was poured into a 1:1 mixture of sat. aq NH₄Cl and H₂O. The extract with
t-BuOMe was dried over Na₂SO₄ and evaporated. Purification of the residual oil was performed by silica gel column chromatography (hexane-t-BuOMe 20:1).

18

The formation of Bu₂SnClH from Bu₂SnCl₂ and Bu₂SnH₂ is known to be reversible. The initiation of the present homolytic hydrostannylation may be caused by electron transfer from Bu₂SnH₂ to Bu₂SnCl₂, which forms the anion radical of Bu₂SnCl₂. See ref. [12b]

21

General Procedure for the Subsequent Pd-Catalyzed Cross-Coupling:
PPh₃, Pd₂(dba)₃·CHCl₃, TBAF (1.0 M in THF, 1.5 mmol), and a haloarene (0.550 mmol) were added successively to the reaction mixture obtained by the hydrostannylation of 1a or 8c (0.500 mmol). After being stirred at 60 °C for a given time, the resultant mixture was cooled to r.t. and diluted with t-BuOMe (10 mL). After addition of DBU (0.5 mL), the mixture was stirred for 5-10 min, passed through a short silica gel column, and evaporated. Purification of the residual oil was performed by silica gel column chromatography.