One-pot synthesis of oxazolidinones and five-membered cyclic carbonates from epoxides and chlorosulfonyl isocyanate: theoretical evidence for an asynchronous concerted pathway

Abstract

The one-pot reaction of chlorosulfonyl isocyanate (CSI) with epoxides having phenyl, benzyl and fused cyclic alkyl groups in different solvents under mild reaction conditions without additives and catalysts was studied. Oxazolidinones and five-membered cyclic carbonates were obtained in ratios close to 1:1 in the cyclization reactions. The best yields of these compounds were obtained in dichloromethane (DCM). Together with 16 known compounds, two novel oxazolidinone derivatives and two novel cyclic carbonates were synthesized with an efficient and straightforward method. Compared to the existing methods, the synthetic approach presented here provides the following distinct advantageous: being a one-pot reaction with metal-free reagent, having shorter reaction times, good yields and a very simple purification method. Moreover, using the density functional theory (DFT) method at the M06-2X/6-31+G(d,p) level of theory the mechanism of the cycloaddition reactions has been elucidated. The further investigation of the potential energy surfaces associated with two possible channels leading to oxazolidinones and five-membered cyclic carbonates disclosed that the cycloaddition reaction proceeds via an asynchronous concerted mechanism in gas phase and in DCM.

Catalyzed Reaction of Isocyanates (RNCO) with Water

“…We determined that one of the best predictors of barrier heights for the RNCO molecules is the occupation of the in-plane C−O* orbital. relationship with the previously discussed dominant E (2) interaction characteristic of the TS1 structure. The association is so strong because less electron density present in the C−O* orbital allows for more facile delocalization of the water oxygen lone pair.…”

“…M1 is nearly planar, with the NH 2 moiety slightly displaced from the plane of the molecule. The NBO E(2) analysis additionally predicts that significant delocalization of the carbonyl lone pair electron density into the N−C* and O−C* orbitals might be one of the contributing factors to the favorable product energy, affirming the findings of Bharatam and coworkers concerning the importance of delocalization in ureas and related species 112.…”

“…Likewise, the carbon and water oxygen interaction exhibits a bond order of 0.56. The NBO second-order perturbation (E (2) ) analysis indicates that the most significant delocalization corresponds to electron density donated from the water lone pair into one of the isocyanate C−O* orbitals (E (2) =144.2 kcal mol −1 ), which is depicted in M1 can proceed through TS4 with a small barrier of 9.5 kcal mol −1 corresponding to the rotation of the hydroxyl group towards the amino group. The resulting product, M3, 2.…”

“…In the unsubstituted TS2 structure, the dominant E (2) interaction involves the delocalization of the isocyanate oxygen lone pair into the closest water O−H* orbital. As electron density is removed from the C−O* orbital via substituent effects, the C−O bond elongates and the isocyanate oxygen lone pair cannot delocalize as easily.…”

“…In all cases, additional water molecules significantly lower the enthalpy of the transition state, with the first catalyst water having the largest effect and lowering the transition state barrier by at least 20 kcal mol −1 . This is unsurprising and NBO E (2) results indicate significant delocalization between the additional water molecules, lowering the energy of the transition state ring structures. Catalytic water molecules have a greater impact on the TSO barriers, which results in the TSO3 and TSN3 barriers being reasonably competitive in some cases with a difference of less than 4 kcal mol −1 across all substituents.…”