AbstractThe synthesis, characterization and X‐ray crystallographic structure determination are described for a stable thiazol‐2‐ylidene 2. This thiazol‐2‐ylidene is the first example of a stable, crystalline carbene in which the singlet carbene center bears a sulfur substituent. The carbene 2 is the closest stable analog of the important thiamin (vitamin B1) carbene. Although the thiazol‐2‐ylidene 2 is sufficiently stable to isolate at room temperature in the absence of moisture and oxygen, it will dimerize to form a normal carbene–carbene dimer {2}2. With the isolation and structure determination of both 2 and {22}, this system is the first in which a stable carbene and its corresponding dimer have been isolated and characterized. Additionally reported is the “reduced” 2H‐thiazoline 2 · H2 in which the former carbene center in 2 has been reduced to a methylene (CH2) group. Thiazol‐2‐ylidenes with smaller 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.

Addition of a sterically demanding cyclic (alkyl)(amino)carbene (CAAC) to AuCl(SMe 2 ) followed by treatment with [Et 3 Si(Tol)] + [B(C 6 F 5 ) 4 ] − in toluene affords the isolable [(CAAC)Au(η 2 -toluene)] + [B(C 6 F 5 ) 4 ] − complex. This cationic Au(I) complex efficiently mediates the catalytic coupling of enamines and terminal alkynes to yield allenes and not propargyl amines as observed with other catalysts. Mono-, di-, and tri-substituted enamines can be used, as well as aryl-, alkyl-, and trimethylsilyl-substituted terminal alkynes. The reaction tolerates sterically hindered substrates and is diastereoselective. This general catalytic protocol directly couples two unsaturated carbon centers to form the three-carbon allenic core. The reaction most probably proceeds through an unprecedented “carbene/vinylidene cross-coupling.”