The application of cobalt(I) complexes in atom economic carbon-carbon bond formation processes is a major research topic in our group. The never-ending search for new reactivities and selectivities is a constant motivation. Fortunately, cobalt(I) complexes reveal a very broad reactivity profile combined with a high functional group tolerance.
a) Regiodiverse Diels-Alder Reactions[79,75,63,54,45]
The control of the regiochemistry of the arene formed is established by the proper choice of ligands. Phosphine-type ligands predominantly give the para-substituted and pyridine-imine ligands the meta-substituted products.
b) Regiodiverse 1,4-Hydrovinylation Reactions[78,70,65]
The regiochemistry for the carbon-carbon bond formation of terminal alkenes with 1,3-dienes is controlled by the steric hindrance and the electronic demand of the SchmalzPhos ligand to give the linear products while the application of the dppe ligand results in the branched product.
c) 1,4-Hydrobutadienylation Reactions
The unique carbon-carbon bond formation of two different 1,3-dienes to form a single product in the 1,4-hydrobutadienylation reaction is accomplished by phosphine ligands such as the SchmalzPhos ligand to produce a single 1,3,6-triene regio- and stereoisomer (E,E only).
d) Regiodiverse Cyclotrimerisation of Alkynes[57,34]
The regiochemistry of the cyclotrimerisation reaction of terminal and internal alkynes can be controlled by diimine ligands. However, the application of a disulfide ligand results in a remarkable solvent-dependent regioselectivity of the cyclotrimerisation reaction.
e) Regiodiverse Alder-ene Reactions[80,73,53]
Complementary results for the carbon-carbon bond formation of alkynes with alkenes in Alder-ene reactions are obtained either with a ruthenium complex (see B. M. Trost) or with the cobalt-dppe catalyst system.
f) Synthesis of Polycarbonyl-Derivatives[93,77,72,67]
The approach toward polycarbonyl derivatives is accomplished by the cobalt-catalysed carbon-carbon bond formation reaction outlined above in combination with ozonolysis. Thereby, unique polycarbonyl derivatives are accessible.
g) Hydrovinylation/Allylboration Reaction Sequences[87,76]
The combination of regiodiverse 1,4-hydrovinylation reactions (β-1 and α-4) and allylboration reactions led to the synthesis of complex products in a one-pot reaction sequence.
h) Isomerisation Reactions
The application of a tridentate ligand py-imineNMe2 led to an unprecedented isomerization of a 1,3-diene from the E/Z-mixture exclusively to the Z-isomer. On the other hand, the application of the dpppMe2 ligand furnishes the 2Z,4E-isomer via a formal 1,5-H-shift.
Quantification of Lewis Acids and Relation to Reactivities in Organic Transformations
a) Quantification of Simple Boron-, Zinc- and Aluminum-based Lewis acids
2H NMR measurements of Lewis acid adducts are used for the quantification of Lewis acidities. Measurements of rate constants (kDA) give an insight into the reactivity/activity of the Lewis acids in organic transformations such as the Lewis-acid-catalysed Diels-Alder reaction.
The relation of 2H NMR chemical shift differences [ Δδ(2H)] and the rate constants (kDA) reveals that most Lewis acids obey the same order obtained for both measurements. However, deviations are of high interest as they are the key to understand the chemistry behind the scenes.
b) Quantification of Silyl-Triflates as Lewis acid Precursors
Silyl-triflates were evaluated as precursors for Lewis acid-catalysed organic transformations such as the Diels-Alder reaction. The plot of the Δδ(2H) NMR data versus the rate constants kDA shows Me3SiOTf to be outstandingly reactive.