Pterin dithiolene complex

More recently the Burgmayer group applied the same method to generate a variety of monooxo-mono-pterin-dithiolene Mo complexes on a tris(3,5-di-methylpyrazolyl)hydroborate (Tp ) framework. One example is presented in Scheme 2.23. Notably the Tp Mo-based model system allows synthesis of both sulfido and 0x0 forms of Tp MoO(pterin-dithiolene) complexes in both biologically relevant IV and V oxidation states. ... 

Scheme 2.22 Synthetic strategy for preparing molybdenum pterin dithiolene complex.

Spontaneous Pyran Ring Cyclization Within a Pterin Dithiolene Complex... 

The mechanism leading to thiophene formation is presumed to involve dissociation of one sulfur atom of a dithiolene ehelate followed by sulfur attack on a pyrazine carbon, cyclization and oxidation to form a thiophene ring (A in Scheme 2.21). When the dithiolene-forming reaction in Scheme 2.20 employed a pterinyl alkyne, no tris-pterinyldithiolene complex could be isolated. However, pterinyl dithiolene formation was implied by the isolation of a pterin thiophene whose identity was eonfirmed by X-ray crystallographic determination (B in Scheme 2.21). This observation was an early confirmation of the correctness of the proposed pterin-dithiolene in Moco prior to any available X-ray protein structures. These thiophene products were reminiscent of Moco oxidation products 1 and the natural metabolite of Moeo, urothione 2 (see Figure 2.2), studied by Rajagopalan (C in Seheme 2.21). 

Cyclic voltammetry (CV) is useful to probe the electronic effect of pterin and quinoxaline groups on the Mo reduction potential and this method was applied to a series of I MoO(dithiolene) complexes. The results are graphically summarized in Figure 2.16. Within a series of T MoO(dithio-lenes), it is clear that pterin (or quinoxaline) substitution causes a significant shift in the Mo redox potential to more positive values compared to simpler dithiolenes like benzenedithiolate (bdt) or ethanedithiolate (edt). This conclusion seems to contradict the results from EPR and MCD studies of Tp MoO(pterin-dithiolene vs. T MoO(bdt), which, as noted above, failed to reveal any differences among the dithiolene complexes. 

The final topic addressed in this chapter is the biosynthesis of the dithiolene cofactor ligand and its coordination to molybdenum and tungsten in the enzymes. Nature has clearly devised a synthetic process to overcome the twin difficulties of building a reactive dithiolene unit bearing a complicated and equally reactive pterin substituent. Molecular biology has been the tool to elucidate the steps in this complex process. Although the dithiolene formation step remains mainly a subject of conjecture, definitive information about the reagent molecule that will eventually be converted to a dithiolene is known. 

Alkyne-substituted quinoxalines (10) and pterins have been converted to molybdenum-dithiolenes via Eq. (2) (54) and Eq. (3) (55) by exploiting the reaction of activated acetylenes with molybdenum polysulfide complexes 9, 27, 28). The Mo-S vibration of 11, the final... 

The earliest period of work on pterin models for Moco followed the discovery of the pterin unit within Moco, and occurred prior to the confirmation of the dithiolene chelate. These early studies explored the coordination chemistry between molybdenum and pterins or other structurally related molecules such as pteridines (Figure 2.1, top). The resulting themes of this body of work include the favorable coordination by molybdenum in several oxidation states to the 04, N5 chelate site in pterin (see Figure 2.1 for numbering), a variety of reactivities exhibited by Mo -tetrahydropterin systems and the highly delocalized electronic structures in molybdenum-pterin complexes that defy formal oxidation state assignments to Mo and pterin. 

Second-Generation Dithiolene Pterins and their Complexes... 

From the cofactor structure, we now know that the thiol sulfurs are not directly connected to the pterin ring rather, they are appended from the pyran ring. Therefore, formation of urothione from the molybdenum cofactor must involve a cyclization step, and a similar process may also be involved in the formation of form B (1). Such reactions have been modeled through oxidation of quinoxaline dithiolene ligand (Scheme 2.29) as well as in complexes (see Scheme 2.20 above). ... 

The introduction of pterin- and quinoxaline-substituents on dithiolenes induces reactivity at the pyrazine N atoms. In both Cp2Mo(quinoxalyldithio-lene) (61, in reaction A) and Tp MoO(quinoxalyldithiolene) (62, in reaction B) complexes, electrophilic attack at atom N1 of the pyrazine ring results in an intramolecular cyclization producing a pyrrole-like ring (Scheme 2.40). X-ray structures of both 61 and 62 reveal that the dithiolene chelate is asymmetrically bound, exhibiting asymmetric Mo-S and S-C bond lengths. 

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