- Replacing conventional carbon nucleophiles with electrophiles: Nickel-catalyzed reductive alkylation of aryl bromides and chlorides
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A general method is presented for the synthesis of alkylated arenes by the chemoselective combination of two electrophilic carbons. Under the optimized conditions, a variety of aryl and vinyl bromides are reductively coupled with alkyl bromides in high yields. Under similar conditions, activated aryl chlorides can also be coupled with bromoalkanes. The protocols are highly functional-group tolerant (-OH, -NHTs, -OAc, -OTs, -OTf, -COMe, -NHBoc, -NHCbz, -CN, -SO2Me), and the reactions are assembled on the benchtop with no special precautions to exclude air or moisture. The reaction displays different chemoselectivity than conventional cross-coupling reactions, such as the Suzuki-Miyaura, Stille, and Hiyama-Denmark reactions. Substrates bearing both an electrophilic and nucleophilic carbon result in selective coupling at the electrophilic carbon (R-X) and no reaction at the nucleophilic carbon (R-[M]) for organoboron (-Bpin), organotin (-SnMe3), and organosilicon (-SiMe2OH) containing organic halides (X-R-[M]). A Hammett study showed a linear correlation of σ and σ(-) parameters with the relative rate of reaction of substituted aryl bromides with bromoalkanes. The small ρ values for these correlations (1.2-1.7) indicate that oxidative addition of the bromoarene is not the turnover-frequency determining step. The rate of reaction has a positive dependence on the concentration of alkyl bromide and catalyst, no dependence upon the amount of zinc (reducing agent), and an inverse dependence upon aryl halide concentration. These results and studies with an organic reductant (TDAE) argue against the intermediacy of organozinc reagents.
- Everson, Daniel A.,Jones, Brittany A.,Weix, Daniel J.
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supporting information; experimental part
p. 6146 - 6159
(2012/05/07)
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- Negishi cross-couplings compatible with unprotected amide functions
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The reaction conditions that allow a general Pd-catalyzed Negishi cross-coupling of a functionalized alkyl, aryl, heteroaryl, and benzylic zinc reagents with aryl acidic hydrogens were investigated. A dry and argon flushed 10 mL Schlenk-tube was charged with N-benzyl-4-bromobenzamide, Pd(OAc) 2, S-PHOS, and THF. 3-pentanoyl-benzylzinc chloride was added slowly over 90 minutes with a syringe pump. The reaction mixture was stirred for 1 hour at 25 °C. The reaction mixture was quenched with a saturated NH 4Cl solution and extracted with diethyl ether. The combined phases were then washed with a aqueous thiourea solution and dried over Na 2SO4. It was observed that the mild conditions considerably increase the ability of the Negishi cross-coupling in the synthesis of complex molecules and natural products.
- Manolikakes, Georg,Dong, Zhibing,Mayr, Herbert,Li, Jinshan,Knochel, Paul
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supporting information; experimental part
p. 1324 - 1328
(2009/09/04)
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