Synlett 2014; 25(19): 2738-2742
DOI: 10.1055/s-0034-1379219
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© Georg Thieme Verlag Stuttgart · New York

Nitrate-Enhanced NHC-NiH Catalyzed Tail-to-Tail Vinyl Ether Dimerization

Lisi He
a  Department of Chemistry, Shenzhen Research Institute, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong   eMail: jasonhcy@cuhk.edu.hk
,
Chun-Yu Ho*
a  Department of Chemistry, Shenzhen Research Institute, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong   eMail: jasonhcy@cuhk.edu.hk
b  Department of Chemistry, South University of Science and Technology of China (SUSTC), Shenzhen 518055, P. R. of China   eMail: jasonhcy@sustc.edu.cn
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Publikationsverlauf

Received: 23. Juli 2014

Accepted after revision: 07. September 2014

Publikationsdatum:
07. Oktober 2014 (online)


Abstract

The first tail-to-tail vinyl ether dimerization has been developed by using a newly identified synergistic effect between the NHC-NiH catalyst and organonitrate additive (PhNO2 or t-BuNO2). The additive enhanced the desired C–C bond-forming reactivity by a proposed competitive coordination with the alkoxyl group of the hydrometallated vinyl ether species to the Ni center. This combination addressed complications related to β-alkoxy (β-OR) elimination and polymerization, and gave unexpected dimerization regioselectivity.

Supporting Information

 
  • References and Notes


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  • 17 [NHC–NiH(OTf)] Catalyst Generation: In a glove box, Ni(cod)2 (0.05 mmol) and IPr, SIPr, or IMes (0.05 mmol) were added to an oven-dried test tube equipped with a stir bar. After sealing with a septum, the tube was brought out of the glove box and connected to an N2 line. The mixture was dissolved in dried degassed toluene (2 mL, or indicated amount) and stirred at r.t. for 1 h. 1-Octene (0.2 mmol), Et3N (0.3 mmol), p-anisaldehyde (0.05 mmol), and TESOTf (0.1 mmol) were then added sequentially and the mixture was stirred at r.t. for 45 min. Catalytic Tail-to-Tail Vinyl Ether Dimerization by [IPr-NiH(OTf)] and PhNO2; General Procedure: Catalyst (0.05 mmol) was generated according to the above general procedure except that only 0.5 mL toluene was used. PhNO2 (1.5 mL) was then added to the above directly before adding vinyl ether (1.0 mmol for 5 mol% reactions or 2.0 mmol for 2.5 mol% reactions). After stirring at r.t. for 24 h, a spatula of Na2CO3 (s) was added and the mixture was diluted with hexane (4 mL), and stirred in open air for 30 min. The mixture was then filtered through a short plug of silica gel and rinsed with diethyl ether (75 mL). The solvent was removed under vacuum and the reside was purified by silica gel column chromatography (hexane–CH2Cl2 5:1, 4:1, 3:1, 2:1, then 1:1, all with 1% Et3N buffer) to give the dimers as colorless oils. When the vinyl ether dimers had boiling points similar to that of PhNO2, the pure vinyl ether dimers for characterization were obtained by using t-BuNO2 as additive.
  • 18 The polymerization was also detected by 1H NMR spectroscopic analysis and found to be comparable1–4 with those previously reported.

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  • 21 This might be due to catalyst deactivation or decomposition in the presence of those nitroalkanes. It was suggested by a control experiment: After adding 0.5 mL MeNO2 to the cross-hydroalkenylation system reported in ref. 12a, no substrate conversion was observed.