Bis(dimesitylmethylene)-1,3-dithietane, -1,2,4-trithiane, and -1,2,4,5-tetrathiane. Conformation and DNMR Study. Observable Dimesityl Thioketene

The reaction of dimesityl ketene with P₂S₅ and pyridine gives 2,4-bis(dimesitylmethylene)-1,3-dithietane (<b>3</b>), 3,6-bis(dimesitylmethylene)-1,2,4,5-tetrathiane (<b>4</b>), 3,5-bis(dimesitylmethylene)-1,2,4-trithiane (<b>5</b>), and dimesityl thioketene <b>2</b> as a transient. The structures of <b>3</b> and <b>4</b> were determined by X-ray crystallography. The dimesitylmethylene moieties in <b>3</b> and <b>4</b> have a propeller conformation, and the tetrathiane ring in <b>4</b> has a twist-boat conformation. Static NMR data are consistent with the presence of two enantiomers and one meso form for <b>3</b> and of three pairs of enantiomers for <b>4</b>. The several aromatic signals observed for <b>3</b> and <b>4</b> at slow exchange at 160 K coalesce to a single signal at higher temperatures. The threshold barriers for these dynamic processes are 12.7 (<b>3</b>) and 13.3 (<b>4</b>) kcal mol^-1, and the dynamic behavior was analyzed in terms of flip processes. On standing, a solution of <b>3</b> develops a blue color which is attributed to formation of <b>2</b>, by retro-dimerization of <b>3</b>. Diphenylacetyl chloride gives with P₂S₅ the analog of <b>5</b> and its one-double bond reduction product. Ditipylketene forms a product identified tentatively as the analog of <b>3</b>.