A STABLE THIAZOL-2-YLIDENE AND ITS DIMER

The synthesis, characterization and X-ray crystallographic structure d etermination are described for a stable thiazol-2-ylidene 2. This thia zol-2-ylidene is the first example of a stable, crystalline carbene in which the singlet carbene center bears a sulfur substituent. The carb ene 2 is the closest stable analog of the important thiamin (vitamin B -1) carbene. Although the thiazol-2-ylidene 2 is sufficiently stable t o isolate al room temperature in the absence of moisture and oxygen, i t will dimerize to form a normal carbene-carbene dimer (2)(2). With th e isolation and structure determinations of both 2 and (2(2)), this sy stem is the first in which a stable carbene and its corresponding dime r have been isolated and characterized. Additionally reported is the ' 'reduced'' 2H-thiazoline 2 HZ in which the former carbene center in 2 has been reduced to a methylene (CH2) group. Thiazol-2-ylidenes with s maller groups on nitrogen (e.g. mesityl or methyl) are too unstable to allow easy isolation. The dimer from 3,4,5-trimethylthiazol-2-ylidene possesses a very unusual geometry in which one of the former carbene centers is planar while the second is pyramidal.