Behaviour of 1-butoxy-9-tetradecyne in superbasic MEDA/EDA media/1-butoksu-9-tetradetsuuni kaitumine MEDA/EDA superaluselises keskkonnas.
The aim of our present work was to study the isomerization of nonpropargylic alkoxyalkyne, to determine the extent of possible elimination, and to demonstrate the influence of the type of metal in MEDA/EDA superbase.
The model compound, 1-butoxy-9-tetradecyne (1), was effectively (reflux 8 h, 98% yield) synthesized from 9-tetradecyn-1-ol and 1-bromobutane using the solid-liquid phase transfer catalysis technique (NaOH/[K.sub.2]C[O.sub.3]/[Bu.sub.4]NHS[O.sub.4]/ toluene) .
For the isomerization of (1) 7 equivalents of LiEDA or NaEDA in EDA as 1.4 M suspension and solution, respectively, were used. The reaction was relatively fast and the starting compound disappeared after 15 min reaction with NaEDA/EDA at room temperature and LiEDA/EDA at 50 [degrees]C.
The proposed reaction scheme is as follows:
It was determined by GLC (fused silica capillary column with OV-101 liquid phase) that after the consumption of the starting material two groups of compounds appeared. One group corresponds to the isomeric alkoxyalkynes and the other to isomeric enynes. When NaEDA/EDA was used, after 15 min of isomerization several isomeric enynes and only 1-butoxy-13-tetradecyne (2) with 39% yield were detected. After stirring the reaction mixture for 1 h in the region of enynes only two peaks remained (1 : 1), which were separated by column chromatography on silica and identified by NMR and IR spectroscopy as Z and E isomers of 11-tetradecen-13-yne (3). LiEDA/EDA is a weaker base than NaEDA/EDA . As a consequence, the isomerization of compound (1) according to the above scheme with LiEDA/EDA does not afford the pure final products (2) and (3). So, in the region of alkoxyalkynes in GLC, beside 1-butoxy-13-tetradecyne (42% yield) some small peaks (probably other positional isomers) were detected even after several hours. After 15 min of reaction ten peaks in the region of enynes were detected. Within 1 h the relative intensity of the peaks of Z and E isomers of 3-tetradecen-1-yne had substantially increased but the product was still contaminated with other compounds.
Isomerization of the triple bond consists of many equilibrium steps. One of them affords the corresponding propargylic alkoxide, which gives after elimination the corresponding enyne. The formed enyne terminates the isomerization and is not in equilibrium with alkoxyalkynes. Therefore, all isomeric alkoxyalkynes will gradually change to enynes and longer reaction times have to be avoided to give the maximal yield of isomerization product with a terminal triple bond. From preparative point of view NaEDA/EDA should be preferred for the isomerization of alkoxyalkynes.
We acknowledge the Estonian Science Foundation (grant No. 3368) for the financial support.
Received 9 May 2001, in revised form 23 May 2001
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Anton KOTLJAROV, Artur JOGI, Sirje MAEORG, and Uno MAEORG *
Institute of Organic and Bioorganic Chemistry, University of Tartu, Jakobi 2, 51014 Tartu, Estonia
* Corresponding author, email@example.com