Carbon Negishi coupling

Unlike normal all-carbon aryl halides, regioselective Negishi coupling reaction of 2,3-dibromofuran 27 with 2-furylzinc chloride was achieved at C(2) to afford bisfuran 28 [35]. 

Unlike normal all-carbon aryl halides, regioselective Negishi couplings of dihalofu-rans and trihalobenzofurans are synthetically useful. Bach and coworkers have studied both systems extensively and use a variety of cross-coupling reactions to produce functionalized furans and benzofurans. In one example of a selective Negishi reaction,... 

Zinc Reagents. Negishi coupling of a-zincated vinyltrimethylsilanes with 2-bromopyridine yields 2-vinylpyridine functionalized by a silyl substituent at the a-carbon of the vinyl group 214 (Scheme 81). Similarly, the trifluoroethenyl group has been substituted into the 2-position in pyridine 215 nsing zincated trifluoroethene and 2-iodopyridine. ... 

Thus, in Part III, the Pd-catalyzed cross-coupUng including the carbon-carbon crosscoupling represented by the Negishi, StiUe, and Suzuki protocols as well as the Sono-gashira alkynylation (Sect. III.2) and the more recently developed carbon-heteroatom coupling reactions (Sect. III.3) are presented. In most of these reactions, reductive... 

Chen and Wang described a nickel-catalyzed carbonylative Negishi coupling reactions [82]. In the presence of a catalytic amount of nickel chloride and 4,4 -dimethoxyl-2,2 -bypyridyl under carbon monoxide atmosphere, various enones were produced from enol triflates and diorganozinc reagents (Scheme 4.44). They demonstrate that the rate of CO insertion is increased by the addition of lithium or magnesium halides and the use of polar solvents. Alkenyl iodides can also be used instead of enol triflates. 

Carbon-heteroatom double bonds can also participate in this reaction. These include both carbonyl compounds (Scheme 11.37) and imines (Scheme 11.38). Addition to aldehydes is co-catalysed by tin(II) or indium(III) salts. Under these conditions, tetrahydrofiirans are obtained. The presence or absence of the co-catalyst can also switch the reaction from one mode to another (Scheme 11.39). An indium cocatalysed cycloaddition to a 7-pyrone aldehyde 11.117 was used in a synthesis of aureothin 11.122 and A-acetylaureothamine 11.123 (Scheme 11.40). Cross-metathesis of the exo-cyc ic alkene 11.118 allowed a subsequent Suzuki coupling with a gem-dibromide 11.120 that showed the expected selectivity (Section 2.1.4.2). This reaction required the use of thallium ethoxide as the Lewis base to suppress the formation of side products. A Negishi coupling completed the synthesis of aureothin 11.122. Reduction and acylation of the nitro group yielded A-acetylaureothamine 11.123. The latter compound is active digainst Helicobacter pylori, a bacterium behind stomach ulcers. 

Three isomers of tetraethynylethene (98, C10H4) have been prepared by palladium-catalyzed Negishi coupling of a trimethylsilylbutadiynyl zinc reagent with a bromoalkene, followed by mild deprotection with potassium carbonate in methanol. The unsubstituted enynes, 3-ethynyloct-3-ene-1,5,7-triyne (99), franj-dec-5-ene-l,3,7,9-tetrayne (100), and m-dec-5-ene-1,3,7,9-tetrayne (101), exhibit modest stability at -20 °C but decomposed rapidly at room temperature. 

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