The hydroboration of substituted cyclopropanes has been investigated using the B3LYP density functional method employing 6-31G** basis set. Borane moiety approaching the cyclopropane ring has been reported. It is shown that the reaction proceeds via a three-centered, “loose” and “tight,” transition states when boron added to the cyclopropane across a bond to a substituents. Single point calculations at higher levels of theory were also performed at the geometries optimized at the B3LYP level, but only slight changes in the barriers were observed. Structural parameters for the transition state are also reported. 1. Introduction Hydroboration of substituted alkenes has been investigated theoretically and experimentally. Brown and Zweifel [1] have shown that the hydroboration of alkyl substituted olefins yields the anti-Markownikoff addition product predominantly and that addition takes place predominantly at β-carbon atom. For monosubstituted olefins, 93-94% of borane addition takes place at the terminal carbon atom. For di- and trisubstituted olefins the preference for the anti-Markownikoff product is 98-99%. They have also observed steric and electronic effects in the case of trans-2-pentene. When electron withdrawing groups are attached to the alkene preferential formation of the Markownikoff addition products has been reported. Phillips and Stone [2] have shown that borane adds to 1,1,1-trifluoropropene giving the Markownikoff product with 87–92% selectivity in appropriate solvents. Graham et al. [3] have carried out studies on the substituent effect in hydroboration of propylene and cyanoethylene using the partial retention of diatomic differential overlap (PRDDO) method with application of linear synchronous transits (LSTs) and orthogonal optimizations to construct the reaction pathways for the Markownikoff and anti-Markownikoff addition of borane to propylene and cyanoethylene. Villiers and Ephritikhine [4] have carried out the borane-catalysed hydroboration of substituted alkenes by lithium borohydride or sodium borohydride. They have shown the unusual order of decreasing reactivity: tetramethylethylene > 1-methylcyclohexene > cyclohexene. Xu et al. [5] theoretically studied the hydroboration of disilenes with borane. They investigated the reaction mechanism exhaustively and found the mechanism for hydroboration of disilenes to be interestingly different from that proposed for hydroboration of alkenes. We have theoretically investigated the hydroboration of cyclopropane [6] in which the borane moiety was situated along the plane of cyclopropane
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