Manganese-Catalyzed C–C Allylation

3.11 Manganese-Catalyzed C–C Allylation

In contrast to a vast number of reports on manganese-catalyzed C–H activation,^[89] C–C activation strategies largely rely on precious transition metals such as rhodium and palladium, whereas earth-abundant and less toxic manganese catalysis remains underutilized in C–C activation reactions.^[96] Although methods for organometallic C–C activation require the prior installment of a suitable group in the molecule, this strategy allows for the synthesis of 1,2,3-trisubstituted arenes. In contrast, such a substitution pattern can usually not be obtained through C–H activation reactions due to a preferred activation at the sterically less hindered C6 position of meta-substituted arenes.

It is therefore noteworthy that H. Wang, I. Choi, and N. Kaplaneris in the Ackermann group achieved the allylation of arenes 142a by employing a manganese-catalyzed C–C activation strategy with vinyldioxolanones 163a or vinyloxazolidinones 163b as substrates (Scheme 90).^[164]

Commercially available [MnBr(CO)5] proved to be the catalyst of choice and, in addition to commonly employed organic solvents, non-toxic, non-flammable water could be employed as the reaction medium, delivering the desired product in high yield with acetone and CO2 as the sole byproducts.

Scheme 90: Optimized reaction conditions for the manganese-catalyzed C–C activation.

To shed light on the reaction mechanism, DFT calculations were performed in order to investigate the manganese-catalyzed C–C allylation. Geometry optimizations were carried out at the PBE0-D3(BJ)/def2-SVP+SMD(H2O) level of theory[126, 127, 134, 135] and energies were calculated at the

PBE0-3 Results and Discussion

60). Turnover-limiting C–C cleavage with an energy barrier of 25.5 kcal mol^–1 generates five-membered manganacycle 11.C, which is in good agreement with experimental kinetic studies (Figure 61). Ligand exchange of the coordinated acetone with 163a is followed by migratory insertion of the alkene into the Mn–C bond to form seven-membered complex 11.E. Coordination of dioxolanone oxygen and subsequent C–O cleavage via β–carbon elimination with an energy barrier of 15.3 kcal mol^–1 leads to a ring opening and the formation of energetically stable, coordinatively saturated intermediate 11.G. Finally, decarboxylation generates alkoxide complex 11.I, which after proto-demetalation delivers the experimentally obtained compound 164a with a reaction Gibbs free energy of –10.1 kcal mol^–1 in total.

Under the reaction conditions [MnBr(CO)5] was employed as the pre-catalyst, therefore energy values were also calculated relative to the pre-catalyst and the substrate. Salt-metathesis to generate intermediate 11.A formally involves the formation of one molecule of HBr, which in the experiment is stabilized by the formation of HBr∙(H2O)n clusters.^[165] This stabilization can only be incompletely described by the employed gas-phase calculations within the framework of a continuum solvation model, hence resulting in considerably higher relative energies. Already the explicit introduction of a (H2O)5 cluster resulted in a stabilization of HBr by 7.2 kcal mol^–1 and larger clusters should lead to even stronger stabilization effects. With these aspects in mind, the Gibbs free energies relative to the precatalyst need to be considered with caution.

Figure 60: Relative Gibbs free energy profile for the reaction of 142a with vinyldioxolanone 163a. Values in parenthesis correspond to energies relative to [MnBr(CO)5] + 142a + (H2O)n –

HBr∙(H2O)n; superscripts correspond to the number of water molecules.

11.A

3.11 Manganese-Catalyzed C–C Allylation

Figure 61: Structure of key transition state TS11.1. Distances are given in Å and hydrogen atoms are omitted for clarity.

Furthermore, the corresponding high-spin (quintet state) and intermediate spin (triplet state) complexes were investigated regarding the occurrence of spin-crossover (Figure 62). For almost all calculated structures the low-spin, singlet state was found to be favored, while higher spin-states were at least 10 kcal mol^–1 less stable. Although optimization of intermediate 11.F in a triplet state led to a lower energy compared to the singlet state, decoordination of the oxygen atom was observed (dMn–O = 3.12 Å), therefore resulting in a structure similar to complex 11.E. For the quintet state of intermediate 11.H, a decoordination of the alkene and a coordination of two carbonate oxygen atoms in a κ²-fashion was observed. In contrast, the quintet state of transition state TS11.3 as well as intermediate 11.G was calculated to be slightly more stable than the corresponding low-spin complexes without any change in the coordination environment, indicating a possible spin-crossover. Due to the low energy differences between singlet and quintet state (ΔΔG = 1.5 and 0.8 kcal mol^–1, respectively), calculations at a higher level of theory are necessary to confirm a possible spin-crossover event.

3 Results and Discussion

Figure 62: Relative Gibbs free energy profile for low-spin (black line), intermediate spin (red), and high-spin (blue) complexes.

Furthermore, calculations were performed without the SMD solvation model in the geometry optimizations (Figure 63). The obtained results are largely in agreement with results at the PBE0-D3(BJ)/def2-QZVP*+SMD(H2O)//PBE0-D3(BJ)/def2-SVP+SMD(H2O) level of theory and only small energy differences of less than 2 kcal mol^–1 were observed. It is noteworthy that a significantly larger difference was uncovered for intermediate 11.H and omitting the solvation model during optimization resulted in a destabilization by 5.7 kcal mol^–1. Additionally, decarboxylation transition state TS11.4 could be located at this level of theory, which confirmed a facile, not turnover-limiting decarboxylation process (Figure 64).

A comparison with Gibbs free energies obtained in apolar DCE as the solvent, which was shown to be a suitable reaction medium for this transformation, resulted in a decrease of the turnover-limiting C–C cleavage energy barrier by 1.2 kcal mol^–1 and a slightly more exergonic reaction Gibbs free energy of –11.1 kcal mol^–1.

3.11 Manganese-Catalyzed C–C Allylation

Figure 63: Relative Gibbs free energy profile for the reaction of 142a with vinyldioxolanone 163a without solvent model in the optimization in water (black line) and DCE (red).

Figure 64: Structure of decarboxylation transition state TS11.4. Distances are given in Å and non-participating hydrogen atoms are omitted for clarity.

5.5