Simulation of potential energy surface of 2-methyl-1,3,2-dioxaborinane and its oxonium ion

Abstract

The interest in cyclic esters of boric acids with oxygen heteroatoms, 1,3-dioxa-2-boracycloalkanes, is due to the features of their structure, the complex of useful properties, and also to the growing importance of these compounds for the fi ne organic synthesis [1–6]. The oxonium ions of 1,3-dioxa-2-heterocycloalkanes are known to serve as intermediates in numerous acid-catalyzed heterolytic reactions leading to the formation of esters, 1,3-diols, 5,6-dihydro-1,3-oxazines and other valuable products of the organic and petrochemical synthesis [7, 8]. Yet under the experimental conditions these intermediates may be detected only at the temperature below –50°C impeding the application of physicochemical methods to the estimation of fi ne features of their structure. Therefore the study of the structure and conformational behavior of these species by quantum-chemical procedures becomes very urgent [9]. We report here on the investigation of the character of conformational transitions in 2-methyl-1,3,2dioxaborinane (I) and its oxonium ion II by Hartree–Fock method in semiempirical (AΜ1) and also ab initio [STO3G, 3-21G, 6-31G(d) and 6-31G(d,p)] quantum-chemical approximations in the framework of HyperChem software [10] under the conditions simulating the behavior of these molecules in the gas phase. The potential energy surface (PES) of 2-substituted 1,3,2-dioxaborinanes is known to contain a minimum [degenerate by energy forms sofa (S)] and a maximum [2,5-twist-form (2,5-T)] [4–6]. We established that also for ion II degenerate by energy forms sofa correspond to the minimum on PES (in AΜ1 case a distorted sofa), and to the maximum, conformation 2,5-T.