The Thiazole Ylide: A Frequently Invoked Intermediate Is a Stable Species in the Gas Phase
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AbstractThe 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 C3H3NS and C3H3NS.+ isomers and transition state structures. Although 3H‐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 1H‐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.
