Reaction of Lithiated Bis methylthio methane with Alkyl Halides

3 Reaction of Lithiated Bis(methylthio)methane with Alkyl Halides 

In Seebach s reviews [1,2] several examples of successful alkylation reactions with lithiated S,S-acetals (mainly 1,3-dithianes) are mentioned. These data and our experimental results gave rise to the following summary of the reactivities of lithiated S,S-acetals in alkylations and the result  

Lithiated S,S-acetals in THF-hexane mixtures discriminate excellently between chlorides and iodides (and probably also bromides), allowing very selective couplings with Cl(CH2)nI or Cl(CH2)nBr (n 3). 

Allyl bromide and other compounds with the system C=C—C—Br react very smoothly and cleanly at temperatures in the region of — 80 °C. Benzyl bromide reacts easily at about — 80 °C to give the expected product in an excellent yield. The chloride reacts smoothly between — 60 and — 80 °C, but the product ( 50% yield) is very impure. 

Secondary alkyl bromides (e.g., isopropyl bromide) react less readily and higher temperatures are required. To avoid any decomposition of the lithiated thioacetal or its reaction with THF, the alkylations are usually carried out with the iodides [1]. Cyclopentyl bromide (reaction carried out at + 10°C) and LiCH(SCH3)2 gave the expected product in a moderate yield [3]. With cyclohexyl bromide the predominant reaction is dehydrobromination [1]. 

---