Benzothiazol-2-ylidenes

Dimethyl /V-(3-methyl-2(3//)-benzothiazol-2-ylidene)malonate was used as a secondary harmonic generating agent at the production of nonlinear optical materials [89JAP(K)205130]. 

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. 

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],... 

E. Miler-Strenger and R. Guglielmetti. Structure of 6- 2-(3-ethyl-2,3-dihydro-l,3-benzothiazol-2-ylidene)-ethylidene -2-methoxy-4-nitro-2,4-cyclohexadienone A model compound for a photo-merocyanine, Acta Crystallogr., Sect. B 38, 1980-1984 (1982). 

Calo V, Nacd A, MonopoU A (2005) Pd-benzothiazol-2-ylidene complex in ionic Uquids efficient catalyst for carbon-carbon coupling reactions. J Organomet Chem 690 5458-5466... 

Dimethyl-2-[( 3-methyl-2,3-dihydro-1,3-benzothiazol-2-yliden )-methyl]- ,3-thiazolium-bromid1035 38% Schmp. 298c... 

Bei der Umsetzung des 1,3-Benzotbiazols la mit Methyliodid werden die Amino- und die Hydroxy-Gruppe in die Reaktion eiubezogen man erhalt als Hauptprodukt 6-Benzyloxy-2-dimethylamino-5-methoxy-3-me-thyl-l,3-benzothiazolium-iodid (II 32,5% Schmp. 202-204 ) und daneben 12% 6-Benzyloxy-5-methoxy-i-methyl-2-methylamino-l,3-benzothiazolium-iodid (III Schmp. 224- 226° Zers.), 6,5% 6-BenzyIoxy-2-[(6-benzyloxy-5-methoxy-3-methyl-2,3-dihydro-l, 3-benzothiazol-2-yliden)-amino]-5-methoxy-3-methyl-l,3-ben-zothiazolium-iodid (IV Schmp. 265-267°) sowie 11% 6-Benzyloxy-2,5-dimethoxy-l,3-benzothiazol (V Schmp. 167-1690)360 ... 

Arduengo et al. obtained the first stable thiazol-2-ylidene 104 by deprotonation of the corresponding thiazolium salt 103 with potassium hydride in THF (Scheme 1.14). In the presence of protons, NHC 104 dimerized to olefin 104=104 and an equilibrium between monomer and dimer was observed. Both 104 and 104=104 were the first and only carbene/olefin pair where both components could be characterized crystallographically. Thiazol-2-ylidenes with sterically less demanding A-substituents like mesityl or methyl, however, dimerized rapidly at room temperature and only their dimers can be iso-lated. None of the analogous benzothiazol-2-ylidenes could be isolated up to now, although benzothiazolium salts 105 and complexes of benzothiazol-2-ylidenes are readily accessible. ... 

Seddon and co-workers investigated the Heck arylation of 2-methylprop-2-en-l-ol in several solvents including ILs with a trans-[bis(benzothiazol-2-ylidene)Pd] complex as catalyst precursor." The best results were achieved in ILs however, it is possible that nanoparticles are formed during the reaction. 

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