1. Five Slides about Concurrent Tandem Catalysis
Shirley Lin
Department of Chemistry
United States Naval Academy
Created by Shirley Lin, United States Naval Academy. Posted on on July
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2. Catalytic Cycle
Reactants/products enter and leave the cycle via arrows
Intermediates contained within the cycle
Catalyst is reformed
It is common in inorganic chemistry to depict catalysts as working in cycles. In this example, the cycle begins at the top with the active catalyst, the metal-ligand fragment LnM. The catalyst can react with a substrate such as an aryl halide (ArX) in an oxidative addition to break the C-X bond and form a new intermediate in the catalytic cycle that contains a new metal-carbon bond and a new M-X bond. This intermediate can undergo a metathesis reaction where species Y exchanges with X to form a new M-Y bond. This intermediate can then undergo a reductive elimination to form the product that contains a new C-Y bond and regenerate the active catalyst which can then repeat this cycle on other molecules of substrate.

3. Concurrent Tandem Catalysis (CTC)
Two catalytic cycles operating simultaneously
Advantages
Challenges
No isolation of product B
Reduction in waste, cost, energy
Compatibility of catalysts with each other and all species
(reactants, intermediates, products)
Reaction sequence selectivity
Concurrent tandem catalysis (CTC) is when two catalytic cycles are acting simultaneously in the same reaction vessel. In this example, the reaction being conducted is A to P through an intermediate B. In the CTC transformation, A is the substrate for the first catalytic cycle (red) governed by catalyst I, to product product B. Compound B in turn is the substrate for the second catalytic cycle and is turned into product P by catalyst II.
The advantages of CTC are that compound B is not isolated and therefore eliminating an isolation/purification step in the synthetic procedure. The challenges of CTC are ensuring that all the species in the reaction vessel are compatible with each other and that the two desired transformations (A to B and B to P) occur in that order.
CTC can also be categorized as a type of relay cascade catalysis
ACS Catal., 2014, 4 (6) 2086–2087
Chem. Rev. 2005, 105,

4. One-Pot, Sequential Tandem Catalysis
One catalytic cycle synthesizes B from A,
then second catalytic cycle creates P from B
In terms of cascade catalysis, this is referred to as a
sequential cascade
A CTC process becomes similar to sequential tandem catalysis
when the second catalytic step is much slower than the first
Concurrent tandem catalysis can be contrasted with another type of tandem catalysis, one-pot sequential tandem catalysis. Sequential tandem catalysis involves adding substrate A and catalyst I to the reaction vessel. Only after all of A has been transformed into compound B is catalyst II introduced.
Note that the CTC process becomes similar to sequential tandem catalysis if the second catalytic step is much slower than the first such that a significant amount of compound B is formed before catalyst II transforms it into product P.
ACS Catal., 2014, 4 (6) 2086–2087

5. Utilizing Concurrent Tandem Catalysis
When would CTC be especially advantageous?
In what type of synthetic methodology
would CTC be especially advantageous?

6. CTC Advantage
A situation where CTC would be advantageous is mentioned in the 2005 Chem Rev article on CTC. The authors point out that when the transformation of substrate A to intermediate B is an equilibrium that does not favor the formation of B, the presence of the second catalytic cycle where B is consumed to form product P should, through Le Chatelier’s principle, shift the equilibrium of the first step to completely consume substrate A.
“efficient catalysts may allow the coupling of equilibrium-limited reactions with subsequent exothermic ones.”
Chem. Rev. 2005, 105,

7. Example: CTC with Aryl Halides
CTC can render the reactivity of aryl chlorides and aryl bromides equivalent to aryl iodides
Reaction A
Reaction B
An example of a CTC methodology of this type can be found in the published work of Lin and MacArthur. They have developed a process where, in the first catalytic cycle (Reaction A), aryl chlorides and aryl bromides are transformed into a more reactive aryl iodide. This aryl iodide is in turn the substrate for Reaction B, a coupling reaction, to form a new product, ArY. To date, CTC hydrodehalogenation and CTC cyanation to form new C-H and C-C bonds, respectively.
equilibrium-limited
for X = Cl, Br under
some conditions
Cannon, KA; Geuther, ME; Kelly, CK; Lin, S.; MacArthur, AHR. Organometallics 2011, 30,
Coughlin, MM; Kelly, CK; Lin, S; MacArthur, A. Organometallics 2013, 32(12),