Triazole complexes palladium

Infrared data have been tabulated for benzotriazole and a wide range of its transition metal complexes or adducts (172). Far infrared spectra have been recorded for copper(II) benzotriazole adducts and bands at 270-320 cm-1 have been assigned to Cu-N vibrations (172). Infrared absorptions at approximately 825, 800, and 775 cm-1 in the spectra of cobalt(III)/4,5-disubstituted triazolate complexes have been attributed to triazolate ring vibrations (109). Infrared data have been reported and assignments made for palladium and platinum thiatriazoline-5-thionate complexes (37) and for the parent thione (127). Vibrational spectroscopy has been employed in an attempt to determine coordination sites for a range of 8-azapurine complexes (108). 

Diez-Barra, E., Guerra, J., Hornillos, V. et al. (2003) 1,2,4-Triazole-based palladium pincer complexes. A new type of catalyst for the Heck reaction. Organometallics, 22,4610-2. 

In related work, Elsevier and coworkers have synthesized a family of hybrid palladium NHC-triazole complexes (81, Scheme 12) [175]. Anion exchange to the triflate enabled the X-ray crystal structure of 81 (R — Mes, = Ad) to be determined, and as expected, the NHC-triazole ligand is coordinated to the Pd(II) ion in a bidentate fashion through the N3 nitrogen of the triazole unit and the C2... 

Dimethyl-1,2,4-triazolium iodide with palladium acetate yields the carbene adduct 182 (97JOM(530)259). Under water it undergoes cis-trans isomerization to 183. Some other derivatives were reported in 1981 (81BCSJ800). 1,1 -Methylenebis(4-alkyl-l,2,4-triazolium)diiodides (alkyl = /-Pr, n-Bu, octyl) with palladium(II) acetate give the mononuclear complexes [L Pdl ] (99EJIC1965), where L2= l,l -methylenebis(4-R-l,2,4-triazol-2-ylidene) (R = /-Pr, n-Bu, octyl). Thermolysis of the products in THF gives the rran -dinuclear complexes 184... 

The NHCs have been used as ligands of different metal catalysts (i.e. copper, nickel, gold, cobalt, palladium, rhodium) in a wide range of cycloaddition reactions such as [4-1-2] (see Section 5.6), [3h-2], [2h-2h-2] and others. These NHC-metal catalysts have allowed reactions to occur at lower temperature and pressure. Furthermore, some NHC-TM catalysts even promote previously unknown reactions. One of the most popular reactions to generate 1,2,3-triazoles is the 1,3-dipolar Huisgen cycloaddition (reaction between azides and alkynes) [8]. Lately, this [3h-2] cycloaddition reaction has been aided by different [Cu(NHC)JX complexes [9]. The reactions between electron-rich, electron-poor and/or hindered alkynes 16 and azides 17 in the presence of low NHC-copper 18-20 loadings (in some cases even ppm amounts were used) afforded the 1,2,3-triazoles 21 regioselectively (Scheme 5.5 Table 5.2). 

A variety of triazole-based monophosphines (ClickPhos) 141 have been prepared via efficient 1,3-dipolar cycloaddition of readily available azides and acetylenes and their palladium complexes provided excellent yields in the amination reactions and Suzuki-Miyaura coupling reactions of unactivated aryl chlorides <06JOC3928>. A novel P,N-type ligand family (ClickPhine) is easily accessible using the Cu(I)-catalyzed azide-alkyne cycloaddition reaction and was tested in palladium-catalyzed allylic alkylation reactions <06OL3227>. Novel chiral ligands, (S)-(+)-l-substituted aryl-4-(l-phenyl) ethylformamido-5-amino-1,2,3-triazoles 142,... 

Thiophenes and benzothiophenes were used by Ke, Liu, and coworkers to evaluate the efficacy of a group of a-hydroxyimine palladium complexes in direct cross-coupling reactions with aryl and heteroaryl bromides such as 4-bromobenzonitrile (47), 4-bromoanisole (48), and 5-bromopyrimidine (49) (33 examples 50—97% yield) (2015OM4881). In particular, complex 50 operated most efficiently under aerobic conditions and with low catalyst loading (0.55 mol%). This success prompted further exploration of cross-coupling reactions with other heteroaryls such as furans, thiazoles, imidazo[l,2-a]pyridine, and triazoles (19 examples 57—96% yield). 

As carboxylic acid additives increased the efficiency of palladium catalysts in direct arylations through a cooperative deprotonation/metallation mechanism (see Chapter 11) [45], their application to ruthenium catalysis was tested. Thus, it was found that a ruthenium complex modified with carboxylic acid MesC02H (96) displayed a broad scope and allowed for the efficient directed arylation of triazoles, pyridines, pyrazoles or oxazolines [44, 46). With respect to the electrophile, aryl bromides, chlorides and tosylates, including ortho-substituted derivatives, were found to be viable substrates. It should be noted here that these direct arylations could be performed at a lower reaction temperatures of 80 °C (Scheme 9.34). 

Diisopropyl- and l,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene ligands and complexes of Pd(ii) have been synthesized. The complexes were obtained via an Ag-carbene transfer reaction with PdCl2(NGMe)2 and X-ray structures determined. The complexes were found to be extremely effective in Heck coupling reactions with aryl bromides but much less so with aryl chlorides. Palladium complexes of the triazole-based carbenes, 1,4-dimethyl-l,2,4-triazolin-2-ylidene, and chelating l,T-methylenebis(4-alkyl-l,2,4-triazolin-2-ylidene) have been synthesized by... 

While the copper(II) complex is completely stable in the solid state and in solution, the palladium(II) compound decomposes slowly in dichloromethane solution to give the corresponding complex with an amine (-NH2) function in the position of the azide (-N3). The azide unit of the palladium complex is still capable of undergoing slow [2-i-3]-cycloaddition with EtOOC-C=C-COOEt to yield the corresponding triazole. However, it is not yet clear whether this is due to decoordination of the azide ligand in solution or whether it takes place at the 77 -coordinated ligand. 

Fig. 13 X-ray structure of the hybrid phosphole-1,2,3-triazole palladium(II) complex, 69 [91]. Hydrogen atoms have been omitted for clarity...

Chen and coworkers have synthesized a hybrid NHC-l,2,3-triazole tridentate ligand and its Ag(I), Pd(II), and Pt(II) complexes. Crystallography confirmed the tridentate coordination mode with the triazole unit bound through the N3 nitrogen. The palladium complex, 87, was found to be a highly active catalyst for the... 

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