Metallation of 1-Alkenyl Ethers, -Thioethers and Related Systems

a-Metallation of 1-Alkenyl Ethers, -Thioethers and Related Systems 

Metallation of the most simple vinyl ether H2C=CHOCH3 in the a-position has been carried out with Fbutyllithium in THF at low temperatures [81]. Ethene is deprotonated not at all by FBuLi or other strongly basic systems under the same conditions. Sec-butyllithium has been used to metallate alkyl vinyl sulfides H2C=CH—SR [82]. We found that both vinyl ethers and vinyl thioethers can be metallated with excellent results using the cheaper combination of rc-BuLi and fBuOK [9]. The cyclic vinyl ethers 2,3-dihydrofuran and 2,3-dihydropyran can be successfully metallated under the same conditions  

The potassio intermediates give excellent results in a number of derivatizations. For other functionalizations, particularly those with carbonyl compounds, potassium has to be replaced by lithium, this can be done most simply by addition of an equivalent amount of anhydrous lithium bromide, dissolved in THF. Although both vinyl ethers and the corresponding sulfides can be metallated in a short time, it is quite clear from the preparative experiments that the sulfides react faster [9]. A similar difference between oxygen and sulfur compounds has been observed in retaliations of other types of substrates. Some authors have invoked d-orbital effects to explain the easier retaliations of sulfur compounds [83], but an explanation on the basis of polarizability seems more satisfactory [84]. Using the kinetically very active bases mentioned, clean a-metallations of simple vinylic ethers and -thioethers can be realized at low temperatures. Under modified conditions two other processes have been observed [9,85]  

At room temperature butyllithium adds across the double bond of vinyl alkyl sulfides  

2-Elimination of thiol occurred if ethyl vinyl sulfide was allowed to interact with potassium amide in liquid ammonia [9]. The same reaction may be expected with 

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