The thiazole ylide: a frequently invoked intermediate is a stable species in the gas phase.

Autor: McGibbon GA; Department of Chemistry, University of Waterloo (Canada)., Hrušák J, Lavorato DJ, Schwarz H, Terlouw JK
Jazyk: angličtina
Zdroj: Chemistry (Weinheim an der Bergstrasse, Germany) [Chemistry] 1997 Feb; Vol. 3 (2), pp. 232-6.
DOI: 10.1002/chem.19970030211
Abstrakt: 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 C3 H3 NS and C3 H3 NS(.+) 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 HSCHCHNC (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.
(Copyright © 1997 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)
Databáze: MEDLINE
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