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) thiazole (1) and its radical cation have been investigated by a combination of mass spectro‐metric experiments and hybrid density functional theory calculations. The latter were used to probe the structures and stabilities of selected C 3 H 3 NS and C 3 H 3 NS .+ isomers and transition state structures. Although 3 H ‐thiazole‐2‐ylidene (2) is less stable than 1 , by 31.5 kcalmol −1 , it is expected to be capable of independent existence, since the 1, 2‐hydrogen shift from carbon to nitrogen involves a very large energy barrier of 72.4 kcalmol −1 . The other 1, 2‐hydrogen shift reaction from C(2) leads not to the expected cyclic 1 H ‐thiazole‐2‐ylidene structure (3) , which is apparently unstable, but rather to the ring‐opened species HSCHCHNC (4) , which is 34.5 kcalmol −1 higher in energy than 1 . The barrier in this case is lower but still large (54.9 kcalmol −1 ). The triplet ground states of 1 , 2 and 4 are considerably destabilised (69.5, 63.2 and 58.7 kcalmol −1 ) relative to their singlet states. Interestingly, in addition to 2 .+ and 4 .+ , the cyclic radical cation 3 .+ is predicted to be stable although it is substantially higher in energy than ionised thiazole 1 .+ (by 53.9 kcalmol −1 ), whereas 2 .+ and 4 .+ are much closer in energy (only 10.2 and 27.0 kcalmol −1 higher, respectively). Dissuading 2 .+ and 3 .+ from isomerising to 1 .+ are energy barriers of 52.6 and 15.3 kcalmol −1 , respectively. Experimentally, dissociative ionisation of 2‐acetylthiazole enabled the generation of 2 .+ , which could be differentiated from 1 .+ by collisional activation mass spectrometry. Reduction of the ylide ion 2 .+ in neutralisation‐reionisation mass spectrometry experiments yielded the corresponding neutral molecule 2 . This direct observation of a thiazolium ylide provides support for postulates of such species as discrete intermediates in a variety of biochemical transformations.