Electronic structure thiazole

The first empirical and qualitative approach to the electronic structure of thiazole appeared in 1931 in a paper entitled Aspects of the chemistry of the thiazole group (115). In this historical review. Hunter showed the technical importance of the group, especially of the benzothiazole derivatives, and correlated the observed reactivity with the mobility of the electronic system. In 1943, Jensen et al. (116) explained the low value observed for the dipole moment of thiazole (1.64D in benzene) by the small contribution of the polar-limiting structures and thus by an essentially dienic character of the v system of thiazole. The first theoretical calculation of the electronic structure of thiazole. benzothiazole, and their methyl derivatives was performed by Pullman and Metzger using the Huckel method (5, 6, 8). 

Electronic structure, 26 aminothiazoles, 44 halogenothiazoles, 44 methylthiazoles, 40 thiazole, 27... 

Pka, of alkylthiazoles, 355 conelation with electronic structure, 93 of thiazoles, 91... 

From its structural and spectroscopic data, thiazole fulfills the criteria for aromaticity. In its electronic structure, four 2pz-orbitals and one 3pz-orbital form delocalized it-MOs to which the three C-atoms and the N-atom each contribute one electron and the S-atom two electrons. One can also consider the possibility of spd-hybridization for the S-atom. 

Experimental confirmation of the metal-nitrogen coordination of thiazole complexes was recently given by Pannell et al. (472), who studied the Cr(0), Mo(0), and W(0) pentacarbonyl complexes of thiazole (Th)M(CO)5. The infrared spectra are quite similar to those of the pyridine analogs the H-NMR resonance associated with 2- and 4-protons are sharper and possess fine structure, in contrast to the broad, featureless resonances of free thiazole ligands. This is expected since removal of electron density from nitrogen upon coordination reduces the N quad-rupole coupling constant that is responsible for the line broadening of the a protons. 

A reliable calculation of polarizabilities requires an adequate description of the outer part of the electron density. For this reason Kassimi and Lin [98JPC(A)9906] used augmented basis sets of triple- quality to study polarizabilities and dipole moments of thiazoles and thiadiazoles. They expect their results to be reliable within 5%. In addition, the authors provide MP2/6-31G geometries for most of their structures. Hyperpolarizabilities for substituted thiazoles obtained from calculations at lower levels are also provided [99MI2]. 

Palladium chemistry involving heterocycles has its unique characteristics stemming from the heterocycles inherently different structural and electronic properties in comparison to the corresponding carbocyclic aryl compounds. One example illustrating the striking difference in reactivity between a heteroarene and a carbocyclic arene is the heteroaryl Heck reaction (vide infra, see Section 1.4). We define a heteroaryl Heck reaction as an intermolecular or an intramolecular Heck reaction occurring onto a heteroaryl recipient. Intermolecular Heck reactions of carbocyclic arenes as the recipients are rare [12a-d], whereas heterocycles including thiophenes, furans, thiazoles, oxazoles, imidazoles, pyrroles and indoles, etc. are excellent substrates. For instance, the heteroaryl Heck reaction of 2-chloro-3,6-diethylpyrazine (1) and benzoxazole occurred at the C(2) position of benzoxazole to elaborate pyrazinylbenzoxazole 2 [12e]. 

The work of Taddei et al.230 on imidazol2,1 -6]thiazole 337 and derivatives has interesting implications on the structure of azapen-talenes, and an important aspect of this study is that the molecular geometry used for calculations on 6-phenylimidazo[2,l-6]thiazole 417 was obtained from X-ray structure determinations130b (Section V,A). The reactivity of this system (Scheme 18, Section IV,C,4,b) is better correlated with Tr-electron densities than with total charges, and 7r-bond orders (by the PPP method) show that the thiazole part of the molecule is more localized than the imidazole part (Section VII). Proton chemical shifts, except that of the H2 proton a to sulfur (Section V,G,2), vary linearly with the total charge carried by the ring carbon atoms. 

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