Transition metal complexes thiazole

Lappert developed the thermolysis of an electron-rich olefin in the presence of a transition metal complex as another way to synthesise these compounds [4], When, in 1975, Clarke and Taube published their findings on carbon coordinated purine transition metal complexes [5], transition metal NHC complexes with functionalised NHC made their debut in biochemistry. The chemistry of carbenes from natural products became firmly established following the discovery that the catalytic activity of thiamine (vitamin Bl) is based on the intermediate formation of a carbene derived from thiazole [6-9] (see Figure 1.2). 

Novel Thiazole Catalysts and Transition Metal Complexes... 

A photoactive metal center is introduced in these systems. The ruthenium bipyridyl complexes are coordinated to the emeraldine base to form the corresponding polymer complexes as described above." The incorporation of the ruthenium centers to the pyridyl backbone has been also reported to give the ruthenium complexes Conjugated ruthenium bipyridine complexes thus obtained are evaluated to be photorefractive materials. Other transition metal complexes can be employed to form the corresponding polymer complexes. The pyridine unit is replaced by bithienyl, 1,4-diazabutadiene, ethylene, benzimidazole or thiazole. ... 

The recent developments on the metallation chemistry of oxazoles and benzoxazoles, isoxazoles and benzisoxazoles, pyrazoles and indazoles, thiazoles and benzo-thiazoles, and isothiazoles, benzo[c]isothiazoles, and benzoMisothiazoles have been reviewed. The two-decade history of catalytic carbon-carbon bond formation via direct borylation of alkane C-H bonds catalysed by transition metal complexes has been reported. The alkane functionalization via electrophilic activation has been underlined. " Recent advances of transition-metal-catalysed addition reactions of C-H bonds to polar C-X (X=N, O) multiple bonds have been highlighted and their mechanisms have been discussed. The development and applications of the transition metal-catalysed coupling reactions have been also reviewed. - ... 

Despite the weak basicity of isoxazoles, complexes of the parent methyl and phenyl derivatives with numerous metal ions such as copper, zinc, cobalt, etc. have been described (79AHC(25)147). Many transition metal cations form complexes with imidazoles the coordination number is four to six (70AHC(12)103). The chemistry of pyrazole complexes has been especially well studied and coordination compounds are known with thiazoles and 1,2,4-triazoles. Tetrazole anions also form good ligands for heavy metals (77AHC(2l)323). 

There are no examples of isolated and characterised stable benzothiazol-2-ylidenes found in the literature. That is not surprising since benzo-annulation decreases the stability of the free NHC [38] and thus facilitates dimerisation. As the nonannulated thiazole system is already prone to dimerisation, benzothiazol-2-ylidene can be expected to require very considerable steric shielding to make the isolation of the monomeric NHC possible. Thus, benzothiazoUum salts are mainly used to synthesise the respective transition metal carbene complexes rather than the free ligands. 

Given that thiazole is the active centre in vitamin Bl, thiamine has been the centre of intense research of its organocatalytic potential for decades it might be surprising that only very few examples of transition metal NHC complexes are known that use thiazol-2-ylidene or its benzo-annulated analogue benzothiazol-2-ylidene. As we have seen above, one major reason is the instability of the free carbene leading to dimerisation instead. Another major contribution is the apparent inability of thiazol-2-ylidene to coordinate to silver(I) [39] making carbene transfer from silver salts to other transition metals impossible. 

Note The most popular route to transition metal NHC complexes, theAgfO method, is not available for thiazole and benzothiazole. 

Instead, only two reaction pathways are used to generate transition metal NHC complexes with thiazol-2-ylidene orbenzothiazol-2-ylidene (i) thermal cleavage of an electron-rich olefin in the presence of a suitable transition metal (see Figure 6.12) [40,41] and (ii) deprotonation of a thiazolium or benzothiazolium salt in the presence of a transition metal (see Figure 6.13) [40,42-44]. 

Raubenheimer et al. have developed a third route for transition metal NHC complexes with thiazol-2-ylidene, benzothiazol-2-yUdene and even isothiazol-5-ylidene ligands [45 9]. This method uses a thiazolyl transfer from lithium to a transition metal with subsequent protonation [47,49-54] or alkylation [41,45,46,48,51-56] of the nitrogen atom to generate the transition metal NHC complex. In this way, carbene complexes of copper(I)... 

Utilisation of transition metal benzothiazol-2-yMene and especially thiazol-2-ylidene complexes in homogenous catalysis reactions are very rare [42-44], Calo et al. reported the use of bis-(3-methyl-benzothiazol-2-ylidene) palladium diiodide in the Heck reaction of aryl bromides and iodides with a catalyst loading of down to KU mol% and near quantitative yields [42,44],... 

Treatment of a typical CA inhibitor, acetazolamide with la. yields the product as a precipitate, which was identified as a I complex 9. wherein the sulfonamide is deprotonated to coordinate with zinc(II) ions. It is o/ interest to point out that with other transition metal ions (e.g., Ni " ), acetazolamide binds to the thiazole N but not to the deprotonated sulfonamide nitrogen. This different behavior of zinc(II) best illustrates its outstanding (hard) acid nature that favors the anionic N donor over the neutral (soft) N donor. The value of 7.4 for... 

In situations where the rate of assembly demands extremely rapid cure times, or where the surfaces to be bonded are inherently unreac-tive, treatment of substrates with a primer is often necessary. Primers consist of compounds which accelerate the curing reactions. Since they would destabilize the adhesive if added directly to the formulation, they are supplied and used as a separate component. The criteria for an acceptable primer include compatibility with the adhesive, the ability to accelerate the rate of curing, and lack of any adverse effects on bond strengths. Various thiazoles, butyral-dehyde-aniline adducts and thioureas " were found to meet these criteria. Since trace levels of transition metals accelerate anaerobic adhesive cure, primers containing complexed copper have been employed successfully. In another system, acidic primers are used which react with ferrocene in the adhesive to release the required metal ions. ... 

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