Synthesis and Properties of Secondary Thiocarbamoylsilanes

Secondary thioformamides were reacted with excess LDA and Me3SiCl at -78 degrees C. The silyl group was selectively introduced to the thiocarbonyl carbon atom to give secondary thiocarbamoylsilanes. The use of Me2PhSiCl and t-BuMe2SiCl gave similar products in reduced yields. While a variety of N-arylmethylthioformamides participated in the silylation reaction, the yields of the products were influenced by the substituents on the aromatic ring: electron-withdrawing groups decreased the yield. No reaction occurred with thioformamides having secondary alkyl groups on the nitrogen atom, whereas the reaction of those with primary alkyl groups proceeded smoothly to form the corresponding thiocarbamoylsilanes. Initially, deprotonation takes place at the nitrogen atom to generate lithium thioimidates, which are then silylated with chlorosilanes to form silyl thioimidates. Excess LDA deprotonates silyl thioimidates at the imidate carbon atom to generate lithium silyl thioimidates, which undergo reverse Brook rearrangement to form lithium thioimidates bearing a silyl group at the imide carbon atom. Hydrolysis of the intermediates may lead to the formation of secondary thiocarbamoylsilanes. In UV visible spectra of thiocarbamoylsilanes, the absorptions ascribed to n-pi*transitions were shifted to longer wavelengths by ca. 40-50 nm compared to those of thioformates. Finally, the structures of thioformamide and silylated product and their oxygen counterparts were elucidated by DFT calculations. Elongation of the C=0 and C=S bonds in carbamoyl- and thiocarbamoylsilanes is discussed in relation to secondary orbital interactions.