In this work, the mechanism and selectivity of the 1,2-cycloaddition of α-trans-himachalene with dibromocarbene are studied using the DFT method at the B3LYP/6-311G(d,p) computational level. We have shown that α-transhimachalene behaves as a nucleophile, while dibromocarbene behaves as an electrophile. Analysis of the global and local electrophilicity and nucleophilicity indices allows an explanation about the regio- and chemoselectivity of this cycloaddition. The asynchronous concerted mechanism of this reaction is analyzed by the potential energy surface. The product (1R, 2S, 4R, 7S)-3,3-dibromo-8-methylene-4,12,12-trimethyl-tricyclo [5.5.0.02,4] dodecane is more favored by stoichiometric reaction between dibromocarbene and α-trans-himachalene, resulting from the attack of the most substituted double bond at the α side (referred to here as P1(α)), while the product (1R, 2S, 4R, 7S, 8R)- 3,3,13,13-tetrachloro-4,12,12-trimethyl-tricyclo [5.5.0.02,4] -spiro[28] tetradecane is preferred by the reaction between two equivalents of dibromocarbene with α-trans-himachalene, resulting from the attack of the exocyclic double bond at the β side of α-trans-himachalene (referred to here as P2(β)). The stationary points were characterized by frequency calculations in order to verify that the transition states had one and only one imaginary frequency. These results are in good agreement with experimental outcomes.
