Methods for reduction Nickel(II) precatalysts

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1 methods reduction

1.1 metallic reductants
1.2 organometallic
1.3 hydride donor
1.4 photoredox





methods reduction
metallic reductants

addition of zn, mn, or sodiumamalgam commonly seen in combination these nix2 salts. common additive in these cross-coupling reactions nai, authors have proposed serve promote electron transfer between mn , ni. weix , coworkers found activating zn 2% hcl 1 minute, reaction times decreased little change in overall yield. similarly, tmscl can used activate mn powder. both of these techniques remove metal oxides on surface of dust, may inhibit reactivity. due heterogeneity of reaction mixture, yields contingent on rate of stirring, can pose problem larger scale (i.e. process-scale) reactions. additionally, weix , coworkers found reactivity contingent on source , size of zinc dust.


tetrakis(dimethylamino)ethylene (tdae) can function single-electron reductant. can used rule out intermediacy of in situ formed alkylzinc reagent. below list of redox potentials.



list of redox potentials



organometallic

as shown above, many ni(ii) precatalysts can activated via transmetalation organometallic reagent, i.e. boronic acid, organozinc, grignard. lead lnniarar, can reductive eliminate give biaryl compound ,





l


n







ni




0





{\displaystyle {\ce {l_{\mathit {n}}ni^{0}}}}

, ,





l


n







nix

2








{\displaystyle {\ce {l_{\mathit {n}}nix2}}}

complex. alternatively, in case of doyle’s catalyst above 2 nickel complexes transmetalate, half of precatalyst filtered inactive





l


n







ni




ii





{\displaystyle {\ce {l_{\mathit {n}}ni^{ii}}}}

complex.


several groups have invoked ni(i) active nickel catalyst. in these instances, possible initial transmetalation results in





l


n







ni




ii


xr



{\displaystyle {\ce {l_{\mathit {n}}ni^{ii}xr}}}

intermediate, can undergo disproportionation lnnix2 , lnnir2. subsequent reductive elimination lnnir2 results in ni(0) intermediate can disproportionate





l


n







ni




i


x



{\displaystyle {\ce {l_{\mathit {n}}ni^{i}x}}}

.







if coupling reagent not organometallic, sacrificial organometallic reagent such alme3, et2zn, or memgbr can added reaction reduce ni(ii) ni(0). works through 2 successive transmetalations, yielding dialkylnickel(ii) species, readily undergo reductive elimination release alkane , ni(0) species.


hydride donor

nickel can readily undergo formation of ni-h in presence of hydride source. nix2 precatalyst, 2 successive transmetalations result in nih2 gives off h2 yield ni(0). examples of hydride donors can effect transformation dibal, methanol, isopropanol, , various silanes.


photoredox

nickel has found widespread use in field of photoredox catalysis. commonly, ni(ii) salts (ex. nicl2-glyme) employed precatalysts in these reactions , can reduced via single electron transfer photocatalysts (ir or ru) obtain active ni(0) catalyst.








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