A DFT study of the mechanism of NHC catalysed annulation reactions involving alpha,beta-unsaturated acyl azoliums and beta-naphthol

作者：Jose Aurell M; Domingo Luis R^*; Arno Manuel; Zaragoza Ramon J^*

来源：Organic and Biomolecular Chemistry, 2016, 14(35): 8338-8345.

DOI：10.1039/c6ob01442a

The mechanism of NHC catalysed annulation reactions involving an alpha,beta-unsaturated acyl azolium and beta-naphthol has been studied using DFT methods at the MPWB1K/6-311G(d,p) level in toluene. For the C-C bond formation step, which corresponds to the rate-and stereo-determining step of this NHC catalysed reaction, the two competitive addition modes, i.e. the 1,2- and the 1,4-additions, have been studied. In toluene, acyl azolium forms an ion pair (IP) with the counterion chloride anion. Interestingly, beta-naphthol forms a hydrogen bond with the chloride anion of IP, increasing the nucleophilic character of beta-naphthol and the etectrophilic character of the acyl azolium moiety. For the first time, the transition state (TS) associated with the 1,2-addition is found and characterised. An analysis of the activation Gibbs free energies involved in the two competitive pathways makes it possible to rule out the pathway associated with the 1,2-addition. The relative Gibbs free energy of stereoisomeric TSs present in the 1,4-additions, accounts for the experimentally observed stereoselectivity. Finally, a comparative study of the pathways associated with the 1,2- and the 1,4-addition of beta-naphthalenethiol to the acyl azolium moiety of IP accounts for the low reactivity of beta-naphthatenethiol in these NHC catalysed annulation reactions involving a,p-unsaturated acyl azoliums.

• 出版日期2016-9-21