The Catalyst Review October 2018 - 16

EXPERIMENTAL

Scheme 1. Various Cu-catalyzed approaches in CCl activation.

Scheme 2. (a) The proposed defects formed in the
structure of maghemite anchored AgCuBTC in Ag(I)
positions in approximately 10% of the whole material
that having formula of Ag1.5Cu1.5(BTC)2·xH2O based on
ICP. (b) Plausible mechanism of C-Cl activation on
bimetallic catalytic centers.

The maghemite anchored AgCuBTC nanomaterial efficiently served as a heterogeneous and reusable catalyst for the Ullmann C-O
coupling reaction of various kinds of phenols with iodo- and bromobenzenes. A broad range of substrates including arenes, ethers,
nitroarenes, and bicyclic arenes were tolerated. More importantly, the catalyst showed excellent reactivity in the C-Cl activation of
chlorobenzenes in the coupling with phenols without using any complicated, expensive ligand. Reusability of the catalyst led to a
total TON about 16000 which was gained in C-Cl activation during six runs. Based on their findings, the authors propose the following
mechanism (Scheme 2B) wherein the electron transfer process of Cu(I) to haloarene leading to the formation of aryl radical (the
rate-determining step) is promoted by the synergistic effect of Ag. Deprotonation of acetylacetone ligand generates acetylacetonate
(acac) which promotes the rate-determining step by chelating the Cu(I) and converting neutral Cu(I) catalyst to a cuprate ion. Source:
Ramezani L, Yahyazadeh A, and Sheykhan M. (2108). ChemCatChem, DOI: 10.1002/cctc.201801111.
Electrooxidative Allene Annulations by Mild Cobalt-Catalyzed C−H Activation...
Although allenes are considered to be important intermediates in the field of organic synthesis, their use in preparative strategies
has relied upon lengthy, classical transformations involving prefunctionalized substrates. Alternative approaches have focused
on direct C−H activations with allenes involving the use of precious-transition-metal catalysts. Although some progress has been
made in extending the range of catalysts to include more earth-abundant metals, factors such as cost, toxicity, and product nonselectivity continue to pose challenges. Herein, the authors describe an electooxidative strategy to overcome these issues and which
is characterized by a) oxidative C−H/ N−H transformations for electrochemical allene annulations, (b) cost-effective, earth-abundant
cobalt catalysis, (c) mild reaction conditions, and (d) a user-friendly undivided cell setup.
They began their studies by exploring different parameters for the envisioned C−H/N−H activation for the annulation of allene
2a (Table 1). Preliminary experiments revealed that an operationally simple undivided cell setup proved viable without major
interference arising from cathodic electrodeposition. The authors then studied the robustness of the C−H activation approach and
found that the cobalt catalyst performed with excellent levels of position control. Nor was C−H/N−H activation was limited to arenes
since heterocycles and alkenes were likewise identified as amenable substrates.
Detailed mechanistic studies were conducted, including reactions with isotopically labeled compounds, kinetic investigations, and
in-operando infrared spectroscopic studies. Further, computational studies were supportive of a non-rate-determining C−H cleavage
and gave key insights into the regioselectivity of the allene annulation.
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The Catalyst Review

October 2018

The Catalyst Review October 2018

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