Computational Study of the Ir-Catalyzed Formation of Allyl Carbamates from CO2

Abstract

We have employed computational methods to investigate the iridium-catalyzed allylic substitution leading to the formation of enantioenriched allyl carbamates from carbon dioxide (CO₂). The reaction occurs in several steps, with initial formation of an iridium-allyl, followed by nucleophilic attack by the carbamate formed in situ from CO₂ and an amine. A detailed isomeric analysis shows that the rate-determining step differs for the (R)- and (S)-pathways. These insights are essential for understanding reactions involving enantioselective formation of allyl carbamates from CO₂.

Scheme 1. Previously Reported Asymmetric Allylic Substitution Reaction Involving an Iridium-Based Catalyst Coordinated by a Chiral Phosphoramidite Ligand L1

DABCO = 1,4-diazabicyclo[2.2.2]octane, 1 (0.24 mmol, 120 mol %), 2 (0.20 mmol, 100 mol %), ratio of products 3:4 = 90:10, yield of product 5 = 13%.

Figure 1

Computational model employed: the Ir-complex with the activated ligand L1, the cinnamyl chloride and propylamine substrates, and the base DABCO.

Scheme 2. (a) Previously Proposed Active Catalyst Species A.^, (b) Proposed Reaction Cycle for the Ir-Catalyzed Allylic Substitution to Furnish Allyl Carbamates Using CO₂ Based on Our Computed Results and Reported Mechanisms^,

Free energies computed at 298 K (values in kcal/mol, PBE0-D3(BJ)/def2-TZVPP,SDD[Ir](PCM)//PBE0-D3(BJ)/def2-SVP,SDD[Ir](PCM) level of theory). The energetic reference state for the mechanistic cycle is complex A plus adduct F.

Figure 2

Optimized transition state geometries for (a) oxidative addition of cinnamyl chloride and (b) nucleophilic attack of carbamate on the allyl for the (COD)(L1)-Ir-catalyzed allylic substitution of 1 (distances are given in Å).

Figure 3

Eight possible isomers of B. The atom labels P and C (red) correspond to the phosphoramidite ligand; C_b and C_t (blue) represent the benzylic and terminal carbon atoms of the allyl, and the black curve denotes the COD ligand.

Figure 4

re and si allyl binding modes of complex B (isomer II). The binding mode with the re face accessible, as seen from the top view, is marked as the re mode. Similarly, the binding mode with the si face accessible is referred to as the si mode.

Figure 5

Gibbs free energy profile (based on isomer II) for the Ir-catalyzed allylic substitution to furnish allyl carbamates using CO₂, including enantioselective and regioselective pathways (298 K), kcal/mol, PBE0-D3(BJ)/def2-TZVPP,SDD[Ir](PCM)//PBE0-D3(BJ)/def2-SVP,SDD[Ir](PCM). The profiles depicted in black and blue represent the formation of the (R)- and (S)-enantiomers of 3, respectively, while the gray profile corresponds to 4. The acronym cb denotes the carbamate nucleophile.