Heterobimetallic catalysis

Complexes of palladium catalyze cross-coupling of electrophiles with organometallic nucleophiles, including those derived from lithium, tin, zinc, and boron.Organic Synthesis using Transition Metals Rod Bates {{ISBN|978-1-84127-107-1}} One example is Sonogashira coupling, where catalytic amount of copper salt (e.g. CuI) reacts with a terminal alkyne (the pronucleophile) under basic conditions to generate a copper acetylide, which transmetalates onto an arylpalladium^II halide, regenerating the copper halide. Reductive elimination from the arylpalladium acetylide yields the cross-coupled product.

File:Reaction-mechanism-v3.png

Other organic pronucleophiles are cross-coupled with arylpalladium halides in the following examples (Scheme 2):

1. Gold-catalyzed cyclization of allenoates followed by cross-coupling with aryl iodides yields 4-arylbutenolides{{cite journal|last=García-Domínguez|first=P.|author2=Nevado, C.|title=Au–Pd Bimetallic Catalysis: The Importance of Anionic Ligands in Catalyst Speciation|journal=J. Am. Chem. Soc.|date=March 2016|volume=138|issue=10|pages=3266–3269|doi=10.1021/jacs.5b10277|pmid=26952216|bibcode=2016JAChS.138.3266G }}

2. Borylcupration of styrenes followed by palladium-catalyzed cross-coupling with aryl halides generates α-aryl-β-boromethyl functionalized arenes.{{cite journal|last=Semba|first=K.|author2=Nakao, Y. |title=Arylboration of Alkenes by Cooperative Palladium/Copper Catalysis|journal=J. Am. Chem. Soc.|date=May 2014|volume=136|issue=21|pages=7567–7570|doi=10.1021/ja5029556|pmid=24810227|bibcode=2014JAChS.136.7567S }}{{cite journal|last=Smith|first=K. B.|author2=Logan, K. M.|author3=You, W.|author4=Brown, M. K. |title=Alkene carboboration enabled by synergistic catalysis|journal=Chem. Eur. J.|date=August 2014|volume=20|issue=38|pages=12032–12036|doi=10.1002/chem.201404310|pmid=25113669}} This reaction has been rendered diastereoselective in the case of cyclic styrenes,{{cite journal|last=Logan|first=K. M.|author2=Smith, K. B.|author3=You, W.|author4=Brown, M. K. |title=Copper/Palladium Synergistic Catalysis for the syn- and anti-Selective Carboboration of Alkenes|journal=Angew. Chem. Int. Ed.|date=April 2015|volume=54|issue=17|pages=5228–5231|doi=10.1002/anie.201500396|pmid=25727074}} and an enantioselective variant has also been developed.{{cite journal|last=Jia|first=T.|author2=Cao, P.|author3=Wang, B.|author4=Lou, Yazhou|author5=Yin, X.|author6=Wang, M.|author7=Liao, J.|title=A Cu/Pd Cooperative Catalysis for Enantioselective Allylboration of Alkenes|journal=J. Am. Chem. Soc.|date=October 2015|volume=137|issue=43|pages=13760–13763|doi=10.1021/jacs.5b09146|pmid=26458555|bibcode=2015JAChS.13713760J }} Enantioselective hydroarylation of styrenes is accomplished similarly via a chiral copper hydride{{cite journal|last=Friis|first=S. D.|author2=Pirnot, M. T.|author3=Buchwald, S. L.|title=Asymmetric Hydroarylation of Vinylarenes Using a Synergistic Combination of CuH and Pd Catalysis|journal=J. Am. Chem. Soc.|date=June 2016|volume=138|issue=27|pages=8372–8375|doi=10.1021/jacs.6b04566|pmid=27346525|pmc=5010014|doi-access=free|bibcode=2016JAChS.138.8372F }}

3. Asymmetric conjugate reduction-allylation of α,β-unsaturated ketones is achieved by Cu-H mediated reduction and subsequent allylation via a chiral PHOX-ligated palladium catalyst{{cite journal|last=Nahra|first=F.|author2=Mac'e, Y.|author3=Riant, O.|title=Copper/Palladium-Catalyzed 1,4 Reduction and Asymmetric Allylic Alkylation of α,β-Unsaturated Ketones: Enantioselective Dual Catalysis|journal=Angew. Chem. Int. Ed.|date=February 2013|volume=52|issue=11|pages=3208–3212|doi=10.1002/anie.201208612|pmid=23382027 }}

File:Pronuc.gif

Also of note is the enantioselective allylation of activated nitriles (Scheme 3).{{cite journal|last=Sawamura|first=M.|author2=Sudoh, M.|author3=Ito, Y.|title=An Enantioselective Two-Component Catalyst System: Rh−Pd-Catalyzed Allylic Alkylation of Activated Nitriles|journal=J. Am. Chem. Soc.|date=April 1996|volume=118|issue=137|pages=3309–3310|doi=10.1021/ja954223e|bibcode=1996JAChS.118.3309S }} A chiral bisphosphine-ligated rhodium catalyst activates the alpha-keto-nitrile component as its corresponding enolate, which is intercepted by a π-allylpalladium complex to yield the α-allylated nitrile in high enantiomeric excess. In the absence of the rhodium catalyst no enantioselectivity is observed, whereas the reaction does not proceed in the absence of palladium.

File:Allylation with Rh-Pd.gif

Catalyst systems in which both metal centers are contained in the same complex are also known (e.g. Shibasaki catalysts); further examples are provided below.

Ion-paired combinations of early and late transition metal complexes can simultaneously interact with a substrate as both Lewis acid and Lewis base. For example, carbonylative ring expansion of epoxides (Scheme 4){{cite journal|last=Schmidt|first=J. A. R.|author2=Lobkovsky, E. B.|author3=Coates, G. W.|title=Chromium (III) octaethylporphyrinato tetracarbonylcobaltate: a highly active, selective, and versatile catalyst for epoxide carbonylation|journal=J. Am. Chem. Soc.|date=July 2005|volume=127|issue=32|pages=11426–11435|doi=10.1021/ja051874u|pmid=16089471|bibcode=2005JAChS.12711426S }}{{cite journal|last=Yutan|first=D. Y. L. Getzler|author2=Mahadevan, V.|author3=Lobkovsky, E. B.|author4=Coates, G. W.|title=Synthesis of β-Lactones: A Highly Active and Selective Catalyst for Epoxide Carbonylation|journal=J. Am. Chem. Soc.|date=January 2002|volume=124|issue=7|pages=1174–1175|doi=10.1021/ja017434u|pmid=11841278|bibcode=2002JAChS.124.1174G }}{{cite journal|last=Mulzer|first=M. |author2=Whiting, B.|author3=Coates, G. W.|title=Regioselective Carbonylation of trans-Disubstituted Epoxides to β-Lactones: A Viable Entry into syn-Aldol-Type Products|journal=J. Am. Chem. Soc.|date=June 2013|volume=135|issue=30|pages=10930–10933|doi=10.1021/ja405151n|pmid=23790074 |doi-access=free|bibcode=2013JAChS.13510930M }} is accomplished by Lewis acid activation by cationic complexes of Cr^III, Ti^III or Al^III with simultaneous ring opening by the [Co(CO)₄]⁻ counterion. Carbonylation of the resultant alkylcobalt followed by lactonization releases the product.

File:Carbonylation IonPair.gif

A heterobimetallic bond-breaking process is also employed in the IPrCuFp-catalyzed C-H borylation system developed by Mankad (Scheme 5).{{cite journal|last=Mankad, N.|title=Non-Precious Metal Catalysts for C-H Borylation Enabled by Metal–Metal Cooperativity|journal=Synlett|date=December 2013|volume=25|issue=9|pages=1197–1201|doi=10.1055/s-0033-1340823|s2cid=196774326 }} Bimetallic cleavage of the B-H bond in pinacolborane generates a copper hydride (IPrCu-H) and an iron boryl [(pin)B-Fp], the latter of which borylates unactivated arenes upon UV irradiation. Bimetallic reductive elimination of H₂ from the combination of H-Fp and IPrCu-H restarts the catalytic cycle. The incorporation of copper into the catalyst is essential; C-H borylation using (pin)B-Fp alone is stoichiometric in iron due to dimerization of the HFp byproduct.

File:Mankad Borylation.gif

Heterobimetallic catalysts containing persistent M¹-M² bonds exhibit altered reactivity due to interaction of the two different metal centers. For example, allylic amination catalyzed by the binuclear complex [Cl₂Ti(N^tBuPPh₂)₂-/Pd(η³-CH₂C(CH₃)CH₂)]⁺ is exceptionally rapid.{{cite journal|last=Tsutsumi, H |author2=Sunada, Y.|author3=Shiota, Y.|author4=Yoshizawa, K.|author5=Nagashima, H.|title=Nickel(II), Palladium(II), and Platinum(II) η3-Allyl Complexes Bearing a Bidentate Titanium(IV) Phosphinoamide Ligand: A Ti←M2 Dative Bond Enhances the Electrophilicity of the π-Allyl Moiety|journal=Organometallics|date=March 2009|volume=28|issue=7|pages=1988–1991|doi=10.1021/om8011085}} DFT studies suggest that a Pd→Ti dative interaction accelerates the typically slow reductive elimination step by withdrawing electron density from Pd in the transition state{{cite journal|last=Walker, W. K.|author2=Kay, B. M.|author3=Michaelis, S.A.|author4=Anderson, D. L.|author5=Smith, S.J.|author6=Ess, D. H.|author7=Michaelis, D.J.|title=Origin of Fast Catalysis in Allylic Amination Reactions Catalyzed by Pd-Ti Heterobimetallic Complexes|journal=Journal of the American Chemical Society|date=2015|volume=137|issue=23|pages=7371–7378|doi=10.1021/jacs.5b02428|pmid=25946518|bibcode=2015JAChS.137.7371W }} (Scheme 6).

File:Pd Ti Allylation.gif

Silica-supported heterobimetallic tantalum iridium catalysts were shown exhibit drastically increased catalytic performances in H/D catalytic exchange reactions with respect to (i) monometallic analogues as well as (ii) homogeneous systems.{{cite journal|last=Lassalle, S. |author2=Jabbour, R.|author3=Schiltz, P.|author4=Berruyer, P.|author5=Todorova, T. K.|author6=Veyre, L.|author7=Gajan, D.|author8=Lesage, A.|author9=Thieuleux, C.|author10=Camp, C.|url=https://pubs.acs.org/doi/10.1021/jacs.9b08311|title=Metal–Metal Synergy in Well-Defined Surface Tantalum–Iridium Heterobimetallic Catalysts for H/D Exchange Reactions|journal=Journal of the American Chemical Society|date=2019|volume=141|issue=49|pages=19321–19335|doi=10.1021/jacs.9b08311|pmid=31710215 |bibcode=2019JAChS.14119321L |s2cid=207944756 }} The key transition state in the C-H activation pathway, computed by DFT, involves (i) donation from the C-H σ orbital to an empty d orbital on the electrophilic early metal (Ta) together with (ii) backdonation from a filled d orbital arising from the late metal (Ir) to the C-H σ* orbital for nucleophilic assistance (Scheme 7). The calculations have shown that steric effects imparted by the ancillary ligands could result in enormous differences in C-H activation energy barriers (ca. 20 kcal/mol-1) in this heterobimetallic cooperative mechanism, indicating that metals accessibility has a drastic impact on the catalytic performances.{{cite journal|last=Del Rosal, I. |author2=Lassalle, S.|author3=Dinoi, C.|author4=Thieuleux, C.|author5=Maron, L.|author6=Camp, C.|title=Mechanistic investigations via DFT support the cooperative heterobimetallic C–H and O–H bond activation across TaIr multiple bonds|journal=Dalton Transactions|date=2021|volume=50|issue=2 |pages=504–510|doi=10.1039/D0DT03818K|pmid=33210676 |s2cid=227064747 |url=https://hal.archives-ouvertes.fr/hal-03009651/file/manuscript%20preprint.pdf }}

File:Camp heterobimetallic tantalum iridium C-H bond activation.gif

The combination of photoredox catalysis with traditional transition metal catalysis enables the use of visible light to drive challenging steps in a catalytic cycle.{{cite journal|author1=Corcoran, E. B.|author2=Pirnot, M. T.|author3=Lin, S.|author4=Dreher, S. D.|author5=DiRocco, D. A.|author6=Davies, I. W.|author7=Buchwald, S. L.|title=Aryl amination using ligand-free Ni(II) salts and photoredox catalysis|journal=Journal of the American Chemical Society|date=July 2016|volume=353|issue=6296|pages=279–283|doi=10.1126/science.aag0209|pmid=27338703|pmc=5027643|bibcode=2016Sci...353..279C |doi-access=free}} For example, nickel-catalyzed aryl amination suffers from a difficult C-N reductive elimination step. Hence instead of nickel, expensive palladium-based precatalysts are often used in combination with sterically encumbered phosphine ligands to facilitate reductive elimination. A more recent approach employs an iridium-based photoredox catalyst to effect single-electron oxidation of the intermediate Ni^II-amido complex. The resulting Ni^III-amido rapidly undergoes reductive elimination, allowing the Ni-catalyzed aryl amination to proceed at room temperature without the use of phosphine ligands.

File:Photoredox Amination.gif

Enzymes containing two or more different metal centers are found in several important biological systems; for example, the Mo-Fe protein of nitrogenase{{Cite journal |vauthors=Burges BK, Lowe DJ | title=Mechanism of Molybdenum Nitrogenase | journal=Chemical Reviews | volume=96 | year=1996 | issue=7 | pages=2983–3011 | doi=10.1021/cr950055x | pmid=11848849}}

catalyzes the conversion of N₂ to NH₃ in nitrogen fixation. Of more relevance to human biology, Cu-Zn superoxide dismutase protects cells from oxidative stress by converting superoxide, O₂⁻, to O₂ and hydrogen peroxide{{cite journal | vauthors = Richardson J, Thomas KA, Rubin BH, Richardson DC | title = Crystal structure of bovine Cu,Zn superoxide dismutase at 3 A resolution: chain tracing and metal ligands | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 72 | issue = 4 | pages = 1349–53 | date = Apr 1975 | pmid = 1055410 | pmc = 432531 | doi = 10.1073/pnas.72.4.1349 | doi-access = free }}.

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Category:Catalysis