Additivity of Diene Substituent Gibbs Free Energy Contributions for Diels–Alder Reactions between (F₃C)₂B = NMe₂and Substituted Cyclopentadienes

Abstract

allow isolation of substitutional effects on Gibbs free energy barrier heights and reaction Gibbs free energies. The effects appear to be additive in all cases. Substitution at positions 5a and 5b always increases barriers and reaction energies, an effect explained by steric interactions between substituents and the aminoborane moiety. For cases R = CH 3 , regioselectivities differ from those expected from canonical organic chemistry predictions. Frontier molecular orbital calculations suggest this arises from the extreme polarization of the π interaction in 1. For cases R = CF 3 , the 2/3-substitution comparison accords with canon, but the 1/4-substitution comparison does not. This appears to arise from a combination of electronic and steric issues. For cases R = F, many of the reactions are exergonic, in contrast to the cases R = CH 3 , CF 3 . Additionally, fluorine substitution at positions 2 and 4 has a barrier-lowering effect. Frontier molecular orbital calculations support an orbital-based preference for formation of 2-and 4-substituted "meta" products rather than "ortho/para" products.

Additivity of Diene Substituent Gibbs Free Energy Contributions for Diels–Alder Reactions between Me2C═CMe2and Substituted Cyclopentadienes

“…49−51 "Average Substitution Corrections" (denoted ΔG ASC# ‡ and ΔG ASC# for Gibbs free energy barriers and reaction energies, respectively, with # representing the position of interest) were determined by treating the Gibbs free energy data as previously described. 18 Briefly, a substitution correction is the difference between an energy (barrier or overall reaction) determined for a Diels−Alder reaction for a cyclopentadiene with a particular substitution pattern and the energy determined for the reaction involving the cyclopentadiene with that pattern but lacking the substituent in the position of interest. ΔG ASC# ‡ /ΔG ASC# is the average of all such substitution corrections for a particular substituent at a particular position.…”

“…Cyclopentadienes represent common yet fascinating dienes in the Diels–Alder reaction, as they provide bicyclic products containing strained bridges. We recently reported computational studies of Diels–Alder-like reactions between the aminoborane dienophile (F 3 C) 2 BNMe 2 and all permutations of substituted cyclopentadienes c -C 5 R 1 R 2 R 3 R 4 R 5a R 5b (R = H, CH 3 , CF 3 , F) . The data showed that, as in organic Diels–Alder reactions, the likelihood and regiospecificity of product formation were determined far more by transition state issues than thermodynamic ones.…”

“…We recently reported computational studies of Diels−Alder-like reactions between the aminoborane dienophile (F 3 C) 2 B�NMe 2 and all permutations of substituted cyclopentadienes c-C 5 R 1 R 2 R 3 R 4 R 5a R 5b (R = H, CH 3 , CF 3 , F). 18 The data showed that, as in organic Diels−Alder reactions, the likelihood and regiospecificity of product formation were determined far more by transition state issues than thermodynamic ones. Notably, several transition state barriers and relative stabilities of product structures proved inconsistent with the regioselectivity "rules" applicable to organic systems.…”

Strain-Release Promoted Thermal [2 + 2] Cycloaddition and Photochemical [2 + 2] Cycloreversion of a Tetraorganylaminoborane

“…Aminoboranes are also useful in synthesis, e.g., of aldimines and iminium ions . They are also used in aza-Matteson reactions, as dienophiles in Diels–Alder reactions, – and in form of their dimers for alkyne hydroboration . Likewise, they are important in the synthesis of BN-fused aromatics and 1,2-dihydro-1,2-azaborine derivatives .…”