Copper-catalyzed asymmetric conjugate addition reactions are a very powerful and widely applied method for enantioselective carbon-carbon bond formation. However, structural and mechanistic insight into these famous reactions has been very limited so far. In this article, the first direct experimental detection of transmetalation intermediates in copper-catalyzed reactions is presented. Special combinations of (1)H,(31)P HMBC spectra allow for the identification of complexes with chemical bonds between the alkyl groups and the copper complexes. For the structural characterization of these transmetalation intermediates, a special approach is applied, in which samples using enantiopure ligands are compared with samples using enantiomeric mixtures of ligands. It is experimentally proven, for the first time, that the dimeric copper complex structure is retained upon transmetalation, providing an intermediate with mixed trigonal/tetrahedral coordination on the copper atoms. In addition, monomeric intermediates with one ligand, but no intermediates with two ligands, are detected. These experimental results, in combination with the well-known optimal ligand-to-copper ratio of 2:1 in synthetic applications, allow us to propose that a binuclear transmetalation intermediate is the reactive species in copper-catalyzed asymmetric conjugate addition reactions. This first direct experimental insight into the structure of the transmetalation intermediate is expected to support the mechanistic and theoretical understanding of this important class of reactions and to enable their further synthetic development. In addition, the special NMR approach presented here for the identification and characterization of intermediates below the detection limit of (1)H NMR spectra can be applied also to other classes of catalyses.

Conjugated enynes as well as cyclic nitronates are crucial building blocks for numerous natural products and pharmaceuticals. However, so far, no common and metal‐free synthetic route to both conjugated enynes and cyclic nitronates has been reported. Herein, in situ NMR, labelling studies and theoretical calculations were combined to investigate the mechanism of the unusual triple bond formation towards conjugated enynes. Starting from nitroalkene dimers, first an isoxazolidine‐2,5‐diol derivative is formed as central intermediate. From this, enynes were generated by a combination of oxidation, dehydration, and retro 1,3‐dipolar cycloaddition, whereas for nitronates a base induced intramolecular reorganization is proposed. While the product distribution could be controlled and high yields of nitronate were achieved, only medium to good yields for enynes were obtained due to polymerization losses. Nevertheless, we hope that these mechanistic investigations may provide a basis for further developments of organocatalytic or metal‐free preparations of conjugated enynes and nitronates.

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Scheme 1. Proposed Precatalytic Structures of (a) Diphosphine Copper Complexes 11 and (b) Phosphoramidite Copper Complexes 13 Note pubs.acs.org/Organometallics

The Cover Feature shows a machine that generates conjugated enynes and cyclic nitronates within a one‐pot reaction from simple nitroalkenes. The reaction towards both products shares a common pathway that deviates towards enynes or nitronates depending on the reaction conditions. By adjusting additive and base, the selectivity between enynes and nitronates can be controlled. NMR reaction monitoring in combination with theoretical calculations helped to elucidate the mechanism towards both products. This machine stands as a symbol for reaction flasks or NMR‐tubes in which the reactions were carried out. More information can be found in the https://doi.org/10.1002/ejoc.201801153