Indium(III) Salt Promoted Intramolecular Addition of Allylsilanes to Unactivated Alkynes

Abstract

In the presence of a stoichiometric or catalytic amount of a Lewis acidic indium(III) salt, allylsilanes reacted intramolecularly with unactivated terminal alkynes to give cyclized products in good to high yields. The fact that the reaction proceeded in a trans-addition mode suggests a reaction mechanism via electrophilic activation of the triple bond by the indium salt.

Key words

alkynes - catalysis - cyclizations - indium - silicon

References and Notes

• 1a Miura K. Hosomi A. In Main Group Metals in Organic Synthesis   Vol. 2:  Yamamoto H. Oshima K. Wiley-VCH; Weinheim Germany: 2004.  Chap. 10. p.409 
• 1b Hosomi A. Acc. Chem. Res.  1988,  21:  200 
• For Lewis acid promoted allylsilylations of alkynes and alkenes, see:
• 2a Yamamoto Y. Asao N. Bull. Chem. Soc. Jpn.  2000,  73:  1071 
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• 6 Miura K. Fujisawa N. Hosomi A. J. Org. Chem.  2004,  69:  2427 
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• 7b

  The original method, in which CuCN was not used, resulted in low reproducibility. Use of the copper catalyst achieved high yield of the desired adduct with good reproducibility.

• 8 The competitive desilylation is attributable to higher nucleophilicity of the methallylsilane moiety. See: Hagen G. Mayr H. J. Am. Chem. Soc.  1991,  113:  4954 
• 9a

  The formation of Me₃SiOTf is expected in the In(OTf)₃-promoted reaction (Scheme [3] ). A strong Brønsted acid generated by reaction of In(OTf)₃ or Me₃SiOTf with adventitious water [e.g., TfOH, H₂O·In(OTf)₃] would cause the competitive desilylation. In the case of InCl₃, its moderate Lewis acidity and water-tolerance may suppress the generation of a strong Brønsted acid causing the desilylation, see ref. 9b. Me₃SiCl, a by-product, can be a source of HCl, which has enough ability to desilylate allylsilanes. However, hydrolysis of Me₃SiCl is expected to be slower than that of Me₃SiOTf, a strong Lewis acid. The slow generation of HCl from Me₃SiCl may allow the efficient cyclization of 1e with InCl₃.

• 9b Babu SA. Synlett  2002,  531 
• For synthetic reactions involving electrophilic activation of alkynes by an indium salt, see:
• 10a Tsuchimoto T. Maeda T. Shirakawa E. Kawakami Y. Chem. Commun.  2000,  1573 
• 10b Tsuchimoto T. Hatanaka K. Shirakawa E. Kawakami Y. Chem. Commun.  2003,  2454 
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• 10e Takita R. Fukuta Y. Tsuji R. Ohshima T. Shibasaki M. Org. Lett.  2005,  7:  1363 

11

In the InCl₃-promoted reaction of allylstannanes (Scheme [1] and ref. 6), a similar vinylindium species survived under almost the same conditions, and could be deuterated with good D-content. The present InCl₃-promoted reaction should form Me₃SiCl as well as the cyclized products. Me₃SiCl is more sensitive toward hydrolysis than Bu₃SnCl, a by-product of the reaction of allylstannanes. HCl generated from Me₃SiCl and adventitious water may cause the in situ protonation of vinylindium 4.

12

We tried the In(OTf)₃-promoted reaction of 1a using CaH₂ (1 equiv) as a dehydration agent; however, treatment of the reaction mixture with DCl-D₂O resulted in low D-content (<5%) of 2a.