Nickel aryl fluorides

For nickel(O) complexes prepared from Ni(r -cod)2 and an excess of the free NHC, it was shown that they exhibit outstanding catalytic activity in the Kumada-Corriu reaction at room temperature toward unreactive substrates like aryl chlorides and even aryl fluorides.Again, an essential element of these catalysts is the need for sterically demanding NHC ligands as observed for the palladium catalysts. 

Raney nickel, 265 Aryl halides Aryl fluorides... 

A further advantage is the possible in situ generation of catalysts from simple metal salts or complexes (e. g., Ni(OR)2, PdCU) and azolium salts. The nickel-catalyzed Grignard cross-coupling of aryl chlorides at room temperature [123] and the activation of aryl fluorides [136] are convincing examples. 

Nickel also catalyses the cross-coupling of aryl fluorides with organozinc reagents. To perform partial C-F bond activation of poly-fluoroaromatics, the existence of a directing group is often necessary (Scheme 14.6). However, the combination of Ni(acac)2 and POP can... 

Recently, nickel-based catalysts have featured more prominently in coupUng reactions of aryl fluorides. In 2001, Herrmann was able to demonstrate Kumada coupUngs of aryl fluorides, including electron-rich aryl fluorides, using the tetra-fluoroborate salt of the N-heterocyclic carbene IPr as the supporting ligand for nickel (Equation 2.69) [113]. 

For other examples of nickel-catalyzed Kumada couplings of aryl fluorides, see ... 

Chatani et al. [137] have reported on nickel-catalyzed Suzuki-Miyaura reactions with aryl fluorides as the cyclometalation substrates, as shown in Eqs. (8.27) and (8.28). 

Air-stable sterically congested phosphine oxides such as 48 are also excellent Ug-ands for the nickel-catalyzed cross-coupling of aryl fluorides [43] (Scheme 5.8). The association of nickel salts with these so-caUed heteroatom-substituted secondary phosphine oxide (HASPO) ligands leads to species reactive enough to activate the generally inert arene-fluorine bond (BDE Ph-F = 126kcalmol ). This improved reactivity has been explained by the formation of a bimetallic species 52 (Scheme 5.8), which facilitates the oxidative insertion step that proceeds via transition state TSl (Figure 5.1) [44]. 

A year later, Hermann and co-workers showed that these Corriu-Kumada cross-coupling were efficiently obtained under NHC-Ni catalysis with aryl fluorides as starting materials. They showed that an in situ generated species worked similarly if not better than a pre-formed [(NHC)2Ni] complex. This observation suggested that the catalytic active species would be a zero-valent nickel coordinated with only one N-heterocyclic carbene ligand. The formation of a 12-electrons complex would be evidently favored in an in situ process. Both catalysts were active with electron-rich or electron-poor fluoroarenes as well as with congested organometallic species (Equation (10.12)). 

A mixture of 2-t-butylfuran and 2,5-di-t-butylfuran is obtained by the action of t-butyl chloride on furan in the presence of mesitylene nolybdenum tricarbonyl." The intermediate in the nitration of furan-2-aldehyde in acetic anhydride has been identified as compound (26). Treatment of 5-bromo-2-furoic acid with sulphur tetrafluoride in hydrogen fluoride yields the dihydrofuran (27). Bromo-furans are converted into aryl-furans by crosscoupling with aryl Grignard reagents in the presence of nickel(II)-phosphine complexes. 2-Furoic acid is lithiated at position 5, 3-furoic acid at C-2. 2-Methylfuran yields the 5-methylthio-derivative by lithiation and subsequent treatment with dimethyl disulphide. The corresponding phenylthio-compound (28) has been converted into a series of 4-substituted 2-methyl-furans (29 R = alkyl, MeaSi, CO2H, or RCHOH) by the sequence bromi-nation, lithiation, treatment with the appropriate electrophile, and, finally, desulphurization with Raney nickel. 2-Lithiofuran reacts with copper(II)... 

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