Relative stabilities and conformational ring inversion potentials in heterocyclic decalin systems and stereoelectronic implications

Enthalpies of formation and isomerization of cis‐ and trans‐decalin, and their oxa‐analogs by G3(MP2)//B3LYP calculations

“…For 1-oxadecalin 2, the cis2 form is calculated to be 10.9 kJ mol À1 more stable than its diastereomer cis1, in good agreement with the respective MM3 energy difference, 9.6 kJ mol À1 , of Golender et al, [5] as well as with an experimental Gibbs energy difference of 8.8 kJ mol À1 in a CDCl 3 -CFCl 3 mixture at À708. [12,13] Thus, the cis isomer of 2 favors a structure in which the C9-O bond is projected axially from the cyclohexane ring (the 'O-inside' structure [5] ), contrary to the other structural alternative in which the bond concerned is equatorial.…”

“…For 1-oxadecalin 2, the cis2 form is calculated to be 10.9 kJ mol À1 more stable than its diastereomer cis1, in good agreement with the respective MM3 energy difference, 9.6 kJ mol À1 , of Golender et al, [5] as well as with an experimental Gibbs energy difference of 8.8 kJ mol À1 in a CDCl 3 -CFCl 3 mixture at À708. [12,13] Thus, the cis isomer of 2 favors a structure in which the C9-O bond is projected axially from the cyclohexane ring (the 'O-inside' structure [5] ), contrary to the other structural alternative in which the bond concerned is equatorial. The calculated cis-trans stability difference (Table 2), 4.8 kJ mol À1 (7.1 kJ mol À1 according to MM3 [5] ) in 2, is markedly smaller than that in 1 for obvious reasons: the axial O atom of 2c2 has a more favorable synaxial interaction with the axial H atoms of the cyclohexane ring than an axial CH 2 group in 1c.…”

“…cis reaction enthalpies (Table 2), 11.0 kJ mol À1 (this work) and 12.9 AE 3.3 kJ mol À1 (from experimental enthalpies of formation [1] ), agree favorably. Moreover, MM3 calculations [5] predict a reaction energy of 11.8 kJ mol À1 , and the chemical equilibration studies of Schucker [9] between 272 and 348 8C give a liquid-phase trans ! cis reaction enthalpy of 11.5 AE 0.2 kJ mol À1 for 1t !…”

“…Eleven of the 21 cis-trans pairs of bicyclic compounds treated in this work have been studied previously [5] by molecular mechanics (MM3), but experimental enthalpies of formation, D f H 0 m (g), have been reported only for the decalins (1) and 1,3,5,7-tetraoxadecalins (18). [1,6] In this work, the enthalpies of formation were evaluated from the computational data by both atomization [7] and isodesmic reactions.…”

A third type of alkylamines possessing high nitrogen inversion-rotation barriers

“…11 for the NIR transition state) but the still used MM2 program has no tool to prove whether this structure is a transition state and obviously whether it is the first-order transition state. In contrast, convenient features of the MM3 package, namely full matrix minimization option as well as normal mode vibrational analysis, permit one to find transition states on conformational pathways and also to determine their order 10,16,17 and to establish formal genetic relationships between these transition states and the corresponding conformers. 4,7 Therefore, in order to prove 'the third type of amines' concept and to reveal characteristic features for these amines, we have undertaken an MM3-assisted search and analysis of other systems of this type.…”