Mesityl imidazole

The preparation of the catalyst starts with the synthesis of 1-mes-ityl-3-(7-octene)-imidazole bromide. This compound is prepared by condensing mesityl imidazole with 8-bromooctene. The resulting salt is deprotonated with (TMS)2NK, where TMS is the tetrameth-ylsilyl radical. This step is performed in tetrahydrofuran at -30°C for 30 min. To this product a solution of the ruthenium complex (PCy3)2Cl2Ru=CHPh is added at 0°C. Bringing the solution slowly to room temperature, after 1 h the ligand displacement was determined to be complete. Afterwards, the reaction mixture is then diluted with n-pentane and heated to reflux for 2 h to induce intramolecular cyclization. 

Through the direct coupling of 2-bromo-4,4-dimethyloxazoline 189 and 1-mesityl imidazole (188) the corresponding imidazolium salt 190 was obtained and used for the preparation of a mono-carbene-palladium complex 191 active as a catalyst in Heck and Suzuki C-C coupling reactions <020M5204>. 

Priihs et al. found an elegant application for a seemingly remote hydroxy group in the sidechain [43]. The remoteness of the hydroxy group implies a mode of introduction to the central imidazole ring different to the epoxide method developed by Arnold (hydroxyethyl) and Thiel (hydroxyhexyl). Indeed, Priihs et al. modilied the classic method of imidazo-lium salt synthesis by using a bromoalkane as carrier for the hydroxy functional group. In this way, they reacted A-mesityl imidazole with l-bromo-ta-hydroxyalkanes to obtain the... 

An example of a more standard, carboxyhc acid amide functionalised imidazolium salt and its apphcations come from the Ghosh group [120]. They reacted chloroacetic acid anilide with A-mesityl imidazole and obtained the respective carboxylic acid amide functionalised imidazolium salt which was reacted with sUver(I) oxide to form the corresponding silver(l) carbene complex featuring two trans-coordinated carbene ligands on the silver(I) ion (see Figure 4.39). 

When the free carbenes are stable enough, as is the case with most unsaturated imidazolylidene derivatives, they can be isolated and engaged in substitution reactions. The first such preparation using this method was reported in 1999 by Nolan [16], who prepared RuCl2(indenylidene)(imidazolylidene)(PR3) complexes 7—10 from Af,A/ -bis((mesityl)imidazol-2-ylidene) (IMes) and A5A/ -bis((2,6-diisopropylphenyl)imidazolyl-2-ylidene) (IPr) carbenes in toluene at room temperature (Scheme 14.5). Other NHCs substituted on the NHC backbone [48, 49] and various Af-aryl groups [50] have also been prepared and coordinated onto ruthenium indenylidene moieties using the free or in sif -generated carbenes with complex 6. 

Fiirstner A, Thiel OR, Ackermann L, Schanz H-J, Nolan SP. Ruthenium Carbene Complexes with N,N -Bis(mesityl)imidazol-2-ylidene Ligands RCM Catalysts of Extended Scope. J Org Chem. 2000 65(7) 2204—2207. 

Figure 1. Key Ru metathesis catalysts converging on RuCl2(L)(CHR) 6 as the active species. IMes = A/ ,A -bis(mesityl)imidazol-2-ylidene.

Selected mono- and di(A,A-diorganyl-imidazol-2-ylidene)gold(i) complexes have been investigated for their antibacterial activity. Positive effects against Staphylococcus and Enterococcus bacteria, Escheria coli, and Pseudomona aeruginosa were encountered in particular with the A-substituents being mesityl or benzyl (Equation (51)).278... 

Scheme 8.4 Grafting of isocyanate-telchelic poly(l, 3-di(l -mesityl)-4- [(bicyclo[2.2.1 ]hept-5-en-2-ylcarbonyl)oxy]methyl -4,5-dihydro-l H-imidazol-3-ium tetrafluoroborate) on silica and generation of the immobilized second generation Grubbs catalyst.

It is interesting to note that the bond lengths C-S in two structurally related compounds l-(mesityl-2-sulfonyl)-3-nitro-l,2,4-triazole [120] and l-(mesitylsulfonyl)-4-nitroimidazole [121] are similar (1.761 and 1.758 A), while the S-N bond in imidazole analog is significantly shorter (1.736 and 1.708 A). 

Initially, a solution of cinnamaldehyde and 4-chlorobenzaldehyde in tetrahydrofuran (THF) was treated with different azolium salts under basic conditions (Scheme 6). The use of thiazolium salt 4 resulted in no formation of the desired y-butyrolactone, only benzoin products were formed. In contrast, using the NHC IMes [l,3-di(2,4,6-trimethyl-phenyl)imidazol-2-ylidene generated in situ from the salt IMesHCl by deprotonation], y-butyrolactone 3a was isolated in 53% yield and a 80 20 cisltrans ratio. This different outcome might be explained by the increased steric demand of IMes compared to 4 (Scheme 7). Most likely, IMes reversibly adds to the aldehyde groups of both substrates resulting in the intermediates la and 2a. Whereas the mesityl groups shield the former aldehyde carbon in both intermediates, the conjugate position of 2a is still accessible and can add to the electrophilic aldehyde. 

It is not strictly necessary to introduce the 1,1 -binaphthyl backbone. For axial chirality, the biphenyl scaffold is sufficient, provided that rotation around the phenyl-phenyl axis is sufficiently hindered. Hoveyda combined this reduced axial chiral motif with additional central chirality in the imidazole backbone (O and C ) [6,7], Synthetically, the task is accomplished by Buchwald-Hartwig amination of enantiomerically pure (H ,21 )-diphenylethylenediamine with 1-methoxy-I -iodo-biphenyl and subsequent reaction with mesityl bromide to introduce the bulky wingtip group on the second amino group of the chiral starting material. Ring closure reaction with triethyl orthoformate and hydrolysis of... 

---