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| '''Nickel(II) precatalysts''' are a type of ] used in ]. Many transformations are catalyzed by ] in ] and in ].<ref>{{Cite journal |last=Tasker |first=Sarah Z. |last2=Standley |first2=Eric A. |last3=Jamison |first3=Timothy F. |title=Recent advances in homogeneous nickel catalysis |journal=Nature |volume=509 |issue=7500 |pages=299–309 |doi=10.1038/nature13274 |pmc=4344729 |pmid=24828188|year=2014 |bibcode=2014Natur.509..299T |url=http://dspace.mit.edu/bitstream/1721.1/103919/1/Jamison_Recent%20advances.pdf }}</ref> Many of these transformations invoke a low valent (generally Ni(0)) species as the active catalyst. Unfortunately, unlike its counterpart, Pd(0), Ni(0) catalysts are predominantly confined to the glovebox due to their high instability to air and water, with the most common Ni(0) catalyst being ]. Additionally, Ni(cod)<sub>2</sub> is more expensive than many Ni(II) salts and the quality varies significantly amongst suppliers.<ref name=":0">{{Cite journal |last=Standley |first=Eric A. |last2=Jamison |first2=Timothy F. |date=2013-01-30 |title=Simplifying Nickel(0) Catalysis: An Air-Stable Nickel Precatalyst for the Internally Selective Benzylation of Terminal Alkenes |journal=Journal of the American Chemical Society |volume=135 |issue=4 |pages=1585–1592 |doi=10.1021/ja3116718 |pmc=3564234 |pmid=23316879}}</ref> To make nickel catalysis more accessible and amenable to synthesis and industrial purposes, the use of air-stable Ni(II) precursors has emerged as an important development in this area of research. This page describes the more commonly employed nickel(II) precatalysts, their synthesis for those not commercially available, and the methods for their reduction to Ni(0) complexes. | |||
| {{R with history}} | |||
| ==Synthesized Air and Moisture-Stable Nickel Precatalysts== | |||
| === Ni(PPh<sub>3</sub>)<sub>2</sub>-(1-naph)Cl === | |||
| Ni(II) precatalyst promote Suzuki cross coupling reactions between chloroarenes and arylboronic acids.<ref>{{Cite journal |last=Chen |first=Chen |last2=Yang |first2=Lian-Ming |date=2007-03-26 |title=Nickel(II)–aryl complexes as catalysts for the Suzuki cross-coupling reaction of chloroarenes and arylboronic acids |journal=Tetrahedron Letters |volume=48 |issue=13 |pages=2427–2430 |doi=10.1016/j.tetlet.2007.01.175}}</ref> This catalyst had previously been used by Brandsma in his cyanation of thiophenes.<ref>{{Cite journal |last=Soolingen |first=J. van |last2=Verkruijsse |first2=H. D. |last3=Keegstra |first3=M. A. |last4=Brandsma |first4=L. |date=1990-11-01 |title=Nickel-Catalyzed Cyanation of 2- and 3- Bromothiophene |journal=Synthetic Communications |volume=20 |issue=20 |pages=3153–3156 |doi=10.1080/00397919008051539 }}</ref> Whereas commercially available Ni(II) catalysts such as NiCl<sub>2</sub>(PPh<sub>3</sub>)<sub>2</sub> and NiCl<sub>2</sub>(dppe) did not catalyze Yang's reaction, the Ni(II) catalyst Ni(PPh<sub>3</sub>)<sub>2</sub>-(1-naph)Cl yielded 24% of the desired product. Further optimization provided the desired products in 87-99% yield at 60 °C. | |||
| ] | |||
| Activation of the Ni(II) complex is proposed to occur through transmetalation of the Ni(II) precatalyst with the boronic acid results in a biaryl Ni(II) intermediate which undergoes reductive elimination to product the biaryl product and Ni(0). | |||
| {{Image frame|align=center|width=572|caption=Reduction of Ni(PPh<sub>3</sub>)<sub>2</sub>-(1-naph)Cl | |||
| |content=<math chem>\begin{matrix}\\ \ce{(PPh3)2Ni^{II}Ar^1Cl -> (PPh3)2Ni^{II}Ar^1Ar^2 -> {Ni^0(PPh3)_\mathit{n}} + Ar^1{-}Ar^2} \\ {} \end{matrix}</math>}} | |||
| === === | |||
| An air and moisture stable Ni(II) precatalyst promotes Suzuki-Miyuara cross-coupling of heteroaryl boronic acids with nitrogen- and sulfur-containing heteroaryl halides.<ref>{{Cite journal |last=Ge |first=Shaozhong |last2=Hartwig |first2=John F. |date=2012-12-14 |title=Highly Reactive, Single-Component Nickel Catalyst Precursor for Suzuki–Miyuara Cross-Coupling of Heteroaryl Boronic Acids with Heteroaryl Halides |journal=Angewandte Chemie International Edition |language=en |volume=51 |issue=51 |pages=12837–12841 |doi=10.1002/anie.201207428 |pmc=3613336 |pmid=23136047}}</ref> The reactions proceeded with 0.5 mol% of without the need for added ligand in 81-97% yield. The catalyst is stable under air in a closed vial at 0 °C for at least two weeks. | |||
| ] | |||
| ]] | |||
| The authors investigated the reduction of and found that was formed quantitatively in 10 minutes at room temperature upon the addition of 2-thienyl boronic acid and K<sub>3</sub>PO<sub>4</sub>. | |||
| to Ni(0)]] | |||
| === ''trans''-(PCy<sub>2</sub>Ph)<sub>2</sub>Ni(''o''-tolyl)Cl === | |||
| Air stable Ni(II) precatalyst have been developed for ] reactions.<ref name=":0" /> After several attempts to develop an air-stable precatalyst, the group came upon ''trans''-(PCy<sub>2</sub>Ph)<sub>2</sub>Ni(''o''-tolyl)Cl, which catalyzes the coupling of benzyl chloride and terminal alkenes in 41-96% yield. | |||
| ] | |||
| The precatalyst can be reduced to Ni(0) ''via'' treatment ] and base at room temperature within minutes. The authors propose that upon chloride abstraction from the Ni(II) precatalyst, transmetalation between the resulting cationic nickel complex and a second Ni(II) precatalyst produces (PCy<sub>2</sub>Ph)<sub>2</sub>Ni(''o''-tolyl)<sub>2</sub> and (PCy<sub>2</sub>Ph)<sub>2</sub>Ni(Cl)(OTf). Reductive elimination from (PCy<sub>2</sub>Ph)<sub>2</sub>Ni(''o''-tolyl)<sub>2</sub> results in the catalytically active Ni(0) complex. This paper documents the first use of silyl triflate to generate Ni(0). | |||
| ] | |||
| === (dppf)Ni(''o''-tolyl)Cl === | |||
| Air-stable Ni(II) precatalysts also promote the amination of aryl chlorides, sulfamates, mesylates, and triflates.<ref>{{Cite journal |last=Park |first=Nathaniel H. |last2=Teverovskiy |first2=Georgiy |last3=Buchwald |first3=Stephen L. |date=2014-01-03 |title=Development of an Air-Stable Nickel Precatalyst for the Amination of Aryl Chlorides, Sulfamates, Mesylates, and Triflates |journal=Organic Letters |volume=16 |issue=1 |pages=220–223 |doi=10.1021/ol403209k |pmc=3926134 |pmid=24283652}}</ref> The nickel catalyst, (dppf)Ni(''o''-tolyl)Cl, can be prepared from ligand exchange with (PPh<sub>3</sub>)<sub>2</sub>Ni(''o''-tolyl)Cl at room temperature in THF. The authors found that additive of MeCN was crucial for high yields of the coupled product. | |||
| ] | |||
| === === | |||
| Interest remains in Ni(II) precatalyst that are air and moisture stable, inexpensive to make, modular (in terms of compatibility with multiple ligand classes), and general (active in numerous reactions).The precatalysts listed above all contain phosphine ligands, limiting their generality for use in other reactions. Simultaneously, the Doyle group <ref name=":1">{{Cite journal |last=Shields |first=Jason D. |last2=Gray |first2=Erin E. |last3=Doyle |first3=Abigail G. |date=2015-05-01 |title=A Modular, Air-Stable Nickel Precatalyst |journal=Organic Letters |volume=17 |issue=9 |pages=2166–2169 |doi=10.1021/acs.orglett.5b00766 |pmc=4719147 |pmid=25886092}}</ref> and a group at Pfizer <ref name=":12">{{Cite journal |last=Magano |first=Javier |last2=Monfette |first2=Sebastien |date=2015-04-17 |title=Development of an Air-Stable, Broadly Applicable Nickel Source for Nickel-Catalyzed Cross-Coupling |journal=ACS Catalysis |volume=5 |pages=3120–3123 |doi=10.1021/acscatal.5b00498}}</ref> speculated that use of N,N,N’,N’-tetramethylethylenediamine (TMEDA) as a dummy ligand would allow for ''in situ'' ligand exchange with the desired ligand for each particular reaction. The complex had the desired properties, showing stability on the bench for at least 3 months. The precatalyst is modular can be used in combination with monodentate and bidentate phosphines, diimine ligands, and NHC ligands. Furthermore, the precatalyst showed comparable results in a wide variety of reactions to other Ni(II) precatalysts or Ni(cod)<sub>2</sub>. | |||
| ] | |||
| The reduction of the precatalyst to Ni(0) can occur through several different pathways contingent on the reaction conditions. In reactions containing a boronic acid, the precatalyst can be reduced via ] between the nickel and boron, resulting in the biary species which can undergo reductive elimination. In cases where no transmetalating agent or amide ion was present for reduction, a nickel-nickel transmetalation event could occur, giving a biaryl nickel complex and L<sub>n</sub>NiCl<sub>2</sub>, which upon reduction of the biaryl, would result in the active Ni(0) complex. | |||
| ] | |||
| === N,N,N-Pincer Complex === | |||
| Ni(II) N,N,N ]es are active in ], ], and ] coupling reactions with unactivated alkyl halides.<ref>{{Cite journal |last=Csok |first=Zsolt |last2=Vechorkin |first2=Oleg |last3=Harkins |first3=Seth B. |last4=Scopelliti |first4=Rosario |last5=Hu |first5=Xile |date=2008-07-01 |title=Nickel Complexes of a Pincer NN2 Ligand: Multiple Carbon−Chloride Activation of CH2Cl2 and CHCl3 Leads to Selective Carbon−Carbon Bond Formation |journal=Journal of the American Chemical Society |volume=130 |issue=26 |pages=8156–8157 |doi=10.1021/ja8025938 |pmid=18528995 }}</ref> In 2016, the group further developed the catalyst to allow for a broader range of coupling partners.<ref>{{Cite journal |last=Di Franco |first=Thomas |last2=Stojanovic |first2=Marko |last3=Keller |first3=Sébastien Carlos |last4=Scopelliti |first4=Rosario |last5=Hu |first5=Xile |date=2016-11-01 |title=A Structure–Activity Study of Nickel NNN Pincer Complexes for Alkyl-Alkyl Kumada and Suzuki–Miyaura Coupling Reactions |journal=Helvetica Chimica Acta |language=en |volume=99 |issue=11 |pages=830–847 |doi=10.1002/hlca.201600165 }}</ref> The catalyst is air stable and made from NiCl<sub>2</sub>-glyme. While the complex is never formally reduced to low valent nickel, the complex first undergoes transmetalation with a Grignard reagent (in the Kumada coupling) resulting in a L<sub>n</sub>Ni(II)-alkyl species which can undergo subsequent oxidative addition with the coupling partner. | |||
| ] | |||
| ==Commonly used commercially available Ni(II) salts== | |||
| Commonly, commercially available Ni(II) salts are employed in cross-coupling reactions. These salts require reduction to low-valent nickel to obtain the catalytically active Ni(0) or Ni(I) species. Below is a list of common Ni(II) salts and a representative publication utilizing these salts. Empirically, these precatalysts can be used interchangeably, with the effect on yield ranging from quite minor to loss of reactivity. | |||
| === ] === | |||
| While NiCl<sub>2</sub> anhydrous is not commonly employed as a precatalyst, the NiCl<sub>2</sub>-6H<sub>2</sub>O has seen some use and can be used when cost is a concern. More commonly, NiCl<sub>2</sub>-dme (or NiCl<sub>2</sub>-glyme) is used due to its increased solubility in comparison to the hexahydrate.<ref>{{Cite journal |last=Cornella |first=Josep |last2=Edwards |first2=Jacob T. |last3=Qin |first3=Tian |last4=Kawamura |first4=Shuhei |last5=Wang |first5=Jie |last6=Pan |first6=Chung-Mao |last7=Gianatassio |first7=Ryan |last8=Schmidt |first8=Michael |last9=Eastgate |first9=Martin D. |date=2016-02-24 |title=Practical Ni-Catalyzed Aryl–Alkyl Cross-Coupling of Secondary Redox-Active Esters |journal=Journal of the American Chemical Society |volume=138 |issue=7 |pages=2174–2177 |doi=10.1021/jacs.6b00250 |doi-access=free |pmc=4768290 |pmid=26835704}}</ref> | |||
| ] | |||
| === NiBr<sub>2</sub> === | |||
| NiBr<sub>2</sub>-glyme, and NiBr<sub>2</sub>-diglyme have been commonly employed by Jarvo and Fu among others, with diglyme showing increased solubility. Empirically, NiBr<sub>2</sub>-glyme has shown increased reactivity compared to that of NiCl<sub>2</sub>-glyme for some transformations.<ref>{{Cite journal |last=Konev |first=Mikhail O. |last2=Hanna |first2=Luke E. |last3=Jarvo |first3=Elizabeth R. |date=2016-06-01 |title=Intra- and Intermolecular Nickel-Catalyzed Reductive Cross-Electrophile Coupling Reactions of Benzylic Esters with Aryl Halides |journal=Angewandte Chemie International Edition |language=en |volume=55 |issue=23 |pages=6730–6733 |doi=10.1002/anie.201601206 |pmid=27099968 }}</ref> | |||
| ] | |||
| === NiI<sub>2</sub> === | |||
| Hydrated ] has been used.<ref>{{Cite journal |last=Everson |first=Daniel A. |last2=Shrestha |first2=Ruja |last3=Weix |first3=Daniel J. |date=2010-01-27 |title=Nickel-Catalyzed Reductive Cross-Coupling of Aryl Halides with Alkyl Halides |journal=Journal of the American Chemical Society |volume=132 |issue=3 |pages=920–921 |doi=10.1021/ja9093956 |pmid=20047282 }}</ref> | |||
| ] | |||
| === Ni(acac)<sub>2</sub> and Ni(OAc)<sub>2</sub> === | |||
| ] | |||
| === Ni(OTf)<sub>2</sub> and Ni(BF4)<sub>2</sub> === | |||
| ] | |||
| === ] === | |||
| ] | |||
| === Ni(dcype)(CO)<sub>2</sub> === | |||
| ] | |||
| ==Methods for reduction== | |||
| === Metallic Reductants === | |||
| Addition of ]<sup>0</sup>, ]<sup>0</sup>, or sodiumamalgam is commonly seen in combination with these NiX<sub>2</sub> salts.<ref name=":0" /> A common additive in these cross-coupling reactions is NaI, which some authors have proposed could serve to help promote the electron transfer between Mn and Ni.<ref>{{Cite journal |last=Cherney |first=Alan H. |last2=Reisman |first2=Sarah E. |date=2014-10-15 |title=Nickel-Catalyzed Asymmetric Reductive Cross-Coupling Between Vinyl and Benzyl Electrophiles |journal=Journal of the American Chemical Society |volume=136 |issue=41 |pages=14365–14368 |doi=10.1021/ja508067c |doi-access=free |pmc=4210114 |pmid=25245492}}</ref> Weix and coworkers found that by activating the Zn with 2% HCl for 1 minute, reaction times were significantly decreased with little change in the overall yield.<ref name=":2">{{Cite journal |last=Everson |first=Daniel A. |last2=Jones |first2=Brittany A. |last3=Weix |first3=Daniel J. |date=2012-04-11 |title=Replacing Conventional Carbon Nucleophiles with Electrophiles: Nickel-Catalyzed Reductive Alkylation of Aryl Bromides and Chlorides |journal=Journal of the American Chemical Society |volume=134 |issue=14 |pages=6146–6159 |doi=10.1021/ja301769r |doi-access=free |pmc=3324882 |pmid=22463689}}</ref> Similarly, ] can be used to activate the Mn powder. Both of these techniques remove metal oxides on the surface of the dust, which may inhibit reactivity. Due to the heterogeneity of the reaction mixture, yields are contingent on the rate of stirring, which can pose a problem for larger scale (i.e. process-scale) reactions. Additionally, Weix and coworkers found that reactivity is also contingent on the source and size of the zinc dust.<ref name=":2" /> | |||
| Tetrakis(dimethylamino)ethylene (TDAE) can also function as a single-electron reductant. It can be used to rule out the intermediacy of an in situ formed alkylzinc reagent.<ref>{{Cite journal |last=Anka-Lufford |first=Lukiana L. |last2=Huihui |first2=Kierra M. M. |last3=Gower |first3=Nicholas J. |last4=Ackerman |first4=Laura K. G. |last5=Weix |first5=Daniel J. |date=2016-08-08 |title=Nickel-Catalyzed Cross-Electrophile Coupling with Organic Reductants in Non-Amide Solvents |journal=Chemistry – A European Journal |language=en |volume=22 |issue=33 |pages=11564–11567 |doi=10.1002/chem.201602668 |pmid=27273457 }}</ref> Below is a list of redox potentials.<ref>{{Cite journal |last=Broggi |first=Julie |last2=Terme |first2=Thierry |last3=Vanelle |first3=Patrice |date=2014-01-07 |title=Organic Electron Donors as Powerful Single-Electron Reducing Agents in Organic Synthesis |journal=Angewandte Chemie International Edition |language=en |volume=53 |issue=2 |pages=384–413 |doi=10.1002/anie.201209060 |pmid=24273111 }}</ref> | |||
| ] | |||
| === Organometallic === | |||
| As shown above, many Ni(II) precatalysts can be activated via ] with an organometallic reagent, i.e. boronic acid, ], Grignard. Often this will lead to a LnNiArAr, which can reductive eliminate to give a biaryl compound and L<sub>n</sub>Ni<sup>0</sup>, and L<sub>n</sub>NiX<sub>2</sub> complex.<ref name=":0" /> Alternatively, in the case of Doyle’s catalyst above where two nickel complexes transmetalate, half of the precatalyst is filtered to an inactive L<sub>n</sub>Ni<sup>II</sup> complex.<ref name=":1" /> | |||
| Several groups have also invoked Ni(I) as the active nickel catalyst. In these instances, it is possible that an initial transmetalation results in a L<sub>n</sub>Ni<sup>II</sup>XR intermediate, which can undergo disproportionation to L<sub>{{mvar|n}}</sub>NiX<sub>2</sub> and L<sub>{{mvar|n}}</sub>NiR<sub>2</sub>. Subsequent reductive elimination from L<sub>{{mvar|n}}</sub>NiR<sub>2</sub> results in a Ni(0) intermediate which can disproportionate into L<sub>n</sub>Ni<sup>I</sup>X.<ref>{{cite journal |last1=Yamamoto |first1=Takakazu |last2=Wakabayashi |first2=Shoichiro |last3=Osakada |first3=Kohtaro |title=Mechanism of C–C coupling reactions of aromatic halides, promoted by Ni(COD)2 in the presence of 2,2′-bipyridine and PPh3, to give biaryls |journal=Journal of Organometallic Chemistry |date=April 1992 |volume=428 |issue=1–2 |pages=223–237 |doi=10.1016/0022-328X(92)83232-7}}</ref> | |||
| {{Image frame|align=center|width=449|caption=Possible pathway for accessing Ni(I) from Ni(0). | |||
| |content=<math chem> | |||
| \begin{align} | |||
| \ce{L}_n\ce{NiCl2} + \color{Grey}\ce{PhMgBr} & \ \ce{-> L}_n\ce{NiClPh} + \color{Grey}\ce{MgBrCl} \\ | |||
| \ce{2L}_n\ce{NiClPh} & \ \ce{-> L}_n\ce{NiCl2} + \ce{L}_n\ce{NiPh2} \\ | |||
| \ce{L}_n\ce{NiCl2} + \ce{L}_n\ce{NiPh2} & \ \ce{-> L}_n\ce{NiCl2} + \ce{L}_n\ce{Ni^0} + \color{Grey}\ce{Ph-Ph} \\ | |||
| \ce{L}_n\ce{NiCl2} + \ce{L}_n\ce{Ni^0} & \ \ce{-> 2L}_n\ce{NiCl} | |||
| \end{align} | |||
| </math>}} | |||
| If the coupling reagent is not an organometallic, a sacrificial organometallic reagent such as AlMe<sub>3</sub>, Et<sub>2</sub>Zn, or MeMgBr can be added to the reaction to reduce the Ni(II) to Ni(0). This works through two successive transmetalations, yielding a dialkylnickel(II) species, which will readily undergo reductive elimination to release an alkane and the Ni(0) species.<ref name=":0" /> | |||
| === Hydride Donor === | |||
| Nickel can readily undergo the formation of a Ni-H in the presence of a hydride source. From a NiX<sub>2</sub> precatalyst, two successive transmetalations result in NiH<sub>2</sub> which quickly gives off H<sub>2</sub> to yield Ni(0). Examples of hydride donors that can effect this transformation are ], methanol, isopropanol, and various silanes.<ref name=":0" /><ref>{{Cite book |title=Organotransition metal chemistry : from bonding to catalysis |last=1964- |first=Hartwig, John Frederick |date=2010-01-01 |publisher=University Science Books |isbn=978-1891389535 |oclc=781082054}}</ref> | |||
| === ] === | |||
| Nickel has found widespread use in the field of photoredox catalysis. Commonly, Ni(II) salts (ex. NiCl<sub>2</sub>-glyme) are employed as precatalysts in these reactions and can be reduced via single electron transfer by the photocatalysts (] or ]) to obtain the active Ni(0) catalyst.<ref>{{Cite journal |last=Zhang |first=Patricia |last2=Le |first2=Chi “Chip” |last3=MacMillan |first3=David W. C. |date=2016-07-06 |title=Silyl Radical Activation of Alkyl Halides in Metallaphotoredox Catalysis: A Unique Pathway for Cross-Electrophile Coupling |journal=Journal of the American Chemical Society |volume=138 |issue=26 |pages=8084–8087 |doi=10.1021/jacs.6b04818 |pmc=5103281 |pmid=27263662}}</ref> | |||
| ==References== | |||
| {{reflist|40em}} | |||
| ] | |||
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