Heterobimetallic catalysis

Heterobimetallic catalysis is an approach to catalysis that employs two different metals to promote a chemical reaction. Included in this definition are cases (Scheme 1) where: 1) each metal activates a different substrate (synergistic catalysis, used interchangeably with the terms "cooperative" and "dual" catalysis.[1]), 2) both metals interact with the same substrate, and 3) only one metal directly interacts with the substrate(s), while the second metal interacts with the first.[2]

Scheme 1: Types of heterobimetallic catalysis

In synergistic catalysis

Complexes of palladium catalyze cross-coupling of electrophiles with organometallic nucleophiles, including those derived from lithium, tin, zinc, and boron.[3] 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 arylpalladiumII halide, regenerating the copper halide. Reductive elimination from the arylpalladium acetylide yields the cross-coupled product.[2]

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[4]

2. Borylcupration of styrenes followed by palladium-catalyzed cross-coupling with aryl halides generates α-aryl-β-boromethyl functionalized arenes.[5][6] This reaction has been rendered diastereoselective in the case of cyclic styrenes,[7] and an enantioselective variant has also been developed.[8] Enantioselective hydroarylation of styrenes is accomplished similarly via a chiral copper hydride[9]

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[10]

Alternative pronucleophiles employed in synergistic heterobimetallic catalysis

Also of note is the enantioselective allylation of activated nitriles (Scheme 3).[11] 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.

Scheme 3: Asymmetric allylation of nitrles with a heterobimetallic Rh/Pd catalyst system

With preformed heterobimetallic catalysts

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.[2] For example, carbonylative ring expansion of epoxides (Scheme 4)[12][13][14] is accomplished by Lewis acid activation by cationic complexes of CrIII, TiIII or AlIII with simultaneous ring opening by the [Co(CO)4]− counterion. Carbonylation of the resultant alkylcobalt followed by lactonization releases the product.

Scheme 4: Carbonylation of epoxides catalyzed by a heterobimetallic ion pair

A heterobimetallic bond-breaking process is also employed in the IPrCuFp-catalyzed C-H borylation system developed by Mankad (Scheme 5).[15] 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.

Scheme 5: UV-promoted C-H borylation of arenes catalyzed by IPrCuFp

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(NtBuPPh2)2-/Pd(η3-CH2C(CH3)CH2)]+ is exceptionally rapid.[16] 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[17] (Scheme 6).

Scheme 6: Pd/Ti-catalyzed allylic amination with accelerated reductive elimination due to a Pd-to-Ti dative interaction

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.[18] 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.[19]

Scheme 7: C-H activation promoted by a heterobimetallic tantalum iridium catalyst

In photoredox catalysis

The combination of photoredox catalysis with traditional transition metal catalysis enables the use of visible light to drive challenging steps in a catalytic cycle.[20] For example, nickel-catalyzed aryl amination suffers from a difficult C-N reductive elimination step.[20] Hence instead of nickel, expensive palladium-based precatalysts are often used in combination with sterically encumbered phosphine ligands to facilitate reductive elimination.[20] A more recent approach employs an iridium-based photoredox catalyst to effect single-electron oxidation of the intermediate NiII-amido complex. The resulting NiIII-amido rapidly undergoes reductive elimination,[20] allowing the Ni-catalyzed aryl amination to proceed at room temperature without the use of phosphine ligands.

Scheme 8: Ni-catalyzed aryl amination driven by oxidation of Ni(II) to Ni(III) via photoredox catalysis

Biological significance

Enzymes containing two or more different metal centers are found in several important biological systems; for example, the Mo-Fe protein of nitrogenase[21] 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[22]

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