Intramolecular ring-opening from a CO2-derived nucleophile as the origin of selectivity for 5-substituted oxazolidinone from the (salen)Cr-catalyzed [aziridine + CO2] coupling

The (salen)Cr-catalyzed [aziridine + CO2] coupling to form oxazolidinone was found to exhibit excellent selectivity for the 5-substituted oxazolidinone product in the absence of any cocatalyst.


Table of Contents
Page number S1. Computational details S1 S2. General procedures and materials S2 S3. General experimental procedure for obtaining Hammett data S2 S4. Computational evaluation of the binding of aziridine to the (salen)Cr III Cl center S2 S5. Correction factors for the free energy calculations S2 S6. Results of attempts to geometrically optimize intermediates in potential bimetallic mechanisms S4 S7. Characterization data for the oxazolidinone products S4 S8. Coordinates and vibrational frequencies of investigated structures S5 S9. Authors contributions audit S11 S10. References S12

S1. Computational details
All calculations were carried out using Density Functional Theory as implemented in the Jaguar 7.8 suite S1 of ab initio quantum chemistry programs. Geometry optimizations were performed using the M06 functional S2 and the 6-31G** basis set. Chromium was represented using the Los Alamos LACVP basis S3, S4 that includes relativistic effective core potentials. The energies of the optimized structures were reevaluated by additional single-point energy calculations of each optimized geometry using Dunning's correlation consistent triple-ζ basis set, S5 cc-pVTZ(-f) that includes a double set of polarization functions. All stationary points were verified to be minima or transition states by proper vibrational analysis at the double-ζ level.
Solvation calculations were carried out with the 6-31G**/LACVP basis at the optimized gas-phase geometry employing a dielectric constants of ε = 9.08 for dichloromethane. Solvation energies were evaluated using a selfconsistent reaction field (SCRF) approach based on accurate numerical solutions of the Poisson-Boltzmann equation. S6-S8 For all continuum models, the solvation energies are subjected to empirical parameterization of the atomic radii that are used to generate the solute surface. We employed the standard set S9 of optimized radii in Jaguar for H (1.150 Å), C (1.900 Å), O (1.600 Å), N (1.600 Å) and Cr (1.511Å). Analytical vibrational frequencies within the harmonic approximation were computed with the 6-31G**/LACVP basis set to confirm proper convergence to well-defined minima or saddle points on the potential energy surface. The free energy of a molecule in solution phase, G(Sol), is computed as follows: G(Sol) = G(gas) + G solv ( S 2 )

H(gas) = E(SCF) + ZPE ( S 4 )
∆G(Sol) = ∑G(Sol) for products -∑G(Sol) for reactants (S5) where G(gas) is the free energy of the molecule in the gas phase; G solv is the free energy of solvation as computed using the continuum solvation model; H(gas) and S(gas) are the enthalpic and entropic components of the molecule in the gas phase, respectively; T is the temperature (298.15 K); E(SCF) is the self-consistent field energy, i.e., the "raw" electronic energy as computed from the SCF procedure; and ZPE is the zero point energy. The energy of the species in the potential energy surface diagram has been reported after taking account of the concentration effect for excess aziridine and carbon di oxide in the reaction medium.
To locate transition states, the potential energy surface was first explored approximately using the linear synchronous transit (LST) method, S10 followed by a quadratic synchronous transit (QST) S11 search that uses the LST transition state as an initial guess. In QST, the initial part of the transition state search is restricted to a circular Figure S1. The elongation of the C-N bonds in N-n propyl-2-phenylaziridine upon binding to the Lewis-acidic (salen)Cr III center. Shown are the bond lengths (Å) for the free and activated aziridine when bound to (salen)Cr III . The preferential elongation of C 2 -N is quite clear when being compared to C 3 -N.

S5. Correction factors for the free energy calculations
In our experimental conditions, the aziridine concentration was 100 times more than that of the (salen)Cr III Cl catalyst. To account for such a significantly high concentration effect, a correction factor of 2.73 kcal mol -1 (at 25 o C by using the equation ∆G = -RTlnK, where K is the ratio of concentrations of the substrate and the catalyst) has been included in the computation. The dissolved concentration of CO 2 in dichloromethane S16 was also similarly included to give a correction factor of 3.84 kcal mol -1 . Figure S2. The HOMO-21 orbital of 3-TS major showing the π-conjugation of the substituent phenyl group with transient carbocation. Figure S3. The computed structure of the alternative transition state 3-TS' major , where the carbonyl oxygen (O) is behaving as a nucleophile en route to the formation of the 5-substituted oxazolidinone product. This transition state is concerted and asynchronous in nature. For clarity, all of the hydrogens have been removed except for that on the C 2 carbon where the C 2 -O 4 bond formation is taking place.

S6. Results of attempts to geometrically optimize intermediates in potential bimetallic mechanisms
We also attempted to computationally evaluate the possibility of (salen)Cr III Cl being able to promote the ringopening of an activated aziridine in a bimetallic fashion. S17 Unfortunately, multiple attempts of geometry optimization either failed to converge after many iterations or led to chemically unreasonable structures. For example, the Cr-Cl bond of the nucleophilic (salen)Cr III Cl in one of the optimized intermediate structures was stretched to a very long distance of 3.47 Ǻ. When the geometry optimizations were carried out keeping the C-Cl distance (carbon of the Lewis-acid activated aziridine ring which requires opening and Cl from another (salen)Cr III Cl) constrained to a reasonable value to help the aziridine ring-opening, it was observed that that chloride ligand detached completely from the Cr III center. Lastly, when the starting-guess geometry was varied to contain a closed aziridine ring with an appropriately aligned Cr-Cl bond to facilitate the ring-opening, the geometry optimization did not produce a ring-opened product.