Slow homolytic substitution reactions at selenium: 2-Selenabicyclo[1.1.1]pentane

Ab initio molecular orbital and density functional calculations predict that 3-(alkylseleno)cyclobutyl radicals (12) undergo intramolecular homolytic substitution chemistry to form 2-selenabicyclo[1.1.1 I pentane (14) with energy barriers (El) that depend on both level of theory and nature of the leaving radical. In the absence of electron correlation, HF/6-311G(d,p) calculations provide values of Delta E1 that range from 170.6 (t-Bu) to 206.5 kJ mol(-1) (Me). Inclusion of electron correlation (QCISD, CCSD(T)) serves to reduce these barriers by 30-40 kJ mol(-1). At the highest level (G3(MP2-RAD)), activation energies of 150.0 (Me), 143.3 (Et), 138.0 (i-Pr), 129.7 (t-Bu) and 108.4 (Bn) kJ mol(-1) are calculated. G3(MP2)-RAD also provides rate constants for ring-closure (k(c)) that range from 5 x 10(-1)4 (Me) to 2 x 10(-8) s(-1)(Bn) suggesting that this chemistry is unlikely to be useful for constructing these interesting ring systems. Data for the cyclization for the analogous sulfur-containing system, the 3-(alkylthio)cyclobutyl radicals (15) are also provided. (C) 2015 Elsevier B.V. All rights reserved.