THE THIAZOLE YLIDE - A FREQUENTLY INVOKED INTERMEDIATE IS A STABLE SPECIES IN THE GAS-PHASE

The 1,2-hydrogen shift isomers of neutral (singlet and triplet) thiazo le (1) and its radical cation have been investigated by a combination of mass spectrometric experiments and hybrid density functional theory calculations. The latter were used to probe the structures and stabil ities of selected C3H3NS and C3H3NS.+ isomers and transition state str uctures. Although 3H-thiazole-2-ylidene (2) is less stable than 1, by 31.5 kcal mol(-1), it is expected to be capable of independent existen ce, since the 1,2-hydrogen shift from carbon to nitrogen involves a ve ry large energy barrier of 72.4 kcal mol(-1). The other 1,2-hydrogen s hift reaction from C(2) leads not to the expected cyclic 1H-thiazole-2 -ylidene structure (3), which is apparently unstable, but rather to th e ring-opened species HSCH=CHNC (4), which is 34.5 kcal mol(-1) higher in energy than 1. The barrier in this case is lower but still large ( 54.9 kcal mol(-1)). The tripler ground states of 1, 2 and 4 are consid erably destabilised (69.5, 63.2 and 58.7 kcal mol(-1)) relative to the ir singlet states. Interestingly, in addition to 2(.+) and 4(.+), the cyclic radical cation 3(.+) is predicted to be stable although it is s ubstantially higher in energy than ionised thiazole 1(.+) (by 53.9 kca l mol(-1)), whereas 2(.+) and 4(.+) are much closer in energy (only 10 .2 and 27.0 kcal mol(-1) higher, respectively). Dissuading 2(.+) and 3 (.+) from isomerising to 1(.+) are energy barriers of 52.6 and 15.3 kc al mol(-1), respectively. Experimentally, dissociative ionisation of 2 -acetylthiazole enabled the generation of 2(.+), which could be differ entiated from 1(.+) by collisional activation mass spectrometry. Reduc tion of the ylide ion 2(.+) in neutralisation-reionisation mass spectr ometry experiments yielded the corresponding neutral molecule 2. This direct observation of a thiazolium ylide provides support for postulat es of such species as discrete intermediates in a variety of biochemic al transformations.