HF calculations with 6-31G(d) basis set were used to investigate the Michael addition (also called Friedel-Crafts alkylation) reaction mechanism of indole (A) with dimethyl alkylidene malonate (B). The results show that this reaction can proceed through two transition states, A＋B→complex→TS1→ M→TS2→product. At the first stage, the new C—C bond is formed. The calculated energy barriers of this step are 210.5 and 209.6 kJ/mol for the R- and S-configuration product, respectively; and the stage is predicted to be the rate-determining step. Whereas, at the second stage H-transfer from indole to malonate occurs with the formation of the new C—H bond, the energy barriers calculated for this step are 99.2 and 115.1 kJ/mol for the R- and S-configuration product, respectively. During the reaction processes, the transfer and interaction of the π electrons in the reactant molecules may play an important role in the cleavage of the old bond C＝C and the formation of the new ones (C—C and C—H), and it is just because of the electron transfer that makes the reaction proceed. Besides these, the effects of the solvents of alcohol and 1,2-dichloroethane on the title reaction were also tested with the single point energy calculations under the MP2/6-311＋G(d,p) level of theory using the polarized continuum model (PCM) implemented in the Gaussian 03 package. PCM results indicate that the energy barriers of the reaction are reduced in the presence of solvents, and the proton of alcohol may greatly influence and also participate in the H-transfer process, thereafter making this stage be a rate-determining one in alcohol solvent, while the solvent dichloroethane has little influence on the reaction path compared with that taking place in gas phase. The calculated results are helpful to explain explicitly why the deuterated product can be formed in the reported experiment where this addition reaction took place in the CH₃OD solvent.

Key words: Michael addition, Friedel-Crafts alkylation, indole, alkylidene malonate, ab initio