Polar Effects and Structural Variation in 4-Substituted 1-Phenylbicyclo[2.2.2]octane Derivatives:  A Quantum Chemical Study

Abstract

The transmission of polar effects through the bicyclo[2.2.2]octane framework has been investigated by ascertaining how the geometry of a phenyl group at a bridgehead position is affected by a variable substituent at the opposite bridgehead position. We have determined the molecular structure of several Ph-C(CH(2)-CH(2))(3)C-X molecules (where X is a charged or dipolar substituent) from HF/6-31G and B3LYP/6-311++G molecular orbital calculations and have progressively replaced each of the three -CH(2)-CH(2)- bridges by a pair of hydrogen atoms. Thus the bicyclo[2.2.2]octane derivatives were changed first into cyclohexane derivatives in the boat conformation, then into n-butane derivatives in the anti-syn-anti conformation, and eventually into assemblies of two molecules, Ph-CH(3) and CH(3)-X, appropriately oriented and kept at a fixed distance. For each variable substituent the deformation of the benzene ring relative to X = H remains substantially the same even when the substituent and the phenyl group are no longer connected by covalent bonds. This provides unequivocal evidence that long-range polar effects in bicyclo[2.2.2]octane derivatives are actually field effects, being transmitted through space rather than through bonds. Varying the substituent X in a series of Ph-C(CH(2)-CH(2))(3)C-X molecules gives rise to geometrical variation (relative to X = H) not only in the benzene ring but also in the bicyclo[2.2.2]octane cage. The two deformations are poorly correlated. The rather small deformation of the benzene ring correlates well with traditional measures of long-range polar effects in bicyclo[2.2.2]octane derivatives, such as sigma(F) or sigma(I) values. The much larger deformation of the bicyclo[2.2.2]octane cage is controlled primarily by the electronegativity of X, similar to deformation of the benzene ring in Ph-X molecules. Thus the field and electronegativity effects of the substituent are well separated and can be studied simultaneously, as they act on different parts of the molecular skeleton.

Switching radical stability by pH-induced orbital conversion

“…Radical stability is influenced by polar effects 12,13 , which, at long range, act primarily through-space rather than through-bond 14 . For example, the stability of the aminoxyl radical is affected by resonance between its two forms, I and II (Fig.…”

Structural variations and electronic substituent effects in phenylcubane derivatives: a quantum chemical study

“…We have imposed C s symmetry to the molecules investigated, with the benzene ring perpendicular to the symmetry plane (orthogonal conformation), as in our previous studies of molecular systems 2-4 [4,5]. Imposing a fixed symmetry has the advantage of keeping any steric effect at the ortho hydrogens of the phenyl group essentially constant within a series of molecules.…”

“…Analysis of the molecular structures of many Ph-X species (1 in Chart 1) has made it possible to derive two linear combinations of the internal ring angles, S E and S R , measuring the electronegativity and resonance effects of the substituent, respectively [1][2][3]. More recently, the possibility of using a phenyl group as a probe for quantifying long-range polar effects (field effects) has been investigated by interposing different spacers between substituent and probe [4,5]. Molecular orbital (MO) calculations on 4-substituted 1-phenylbicyclo [2.2.2]octanes (2) [4], 3-substituted 1-phenylbicyclo[1.1.1]pentanes (3) [5], and x-substituted 1-phenyl [2]staffanes (4) [5] show that the variable substituent X modifies the geometry of the phenyl group at the opposite bridgehead position.…”

“…The geometrical changes are different from those occurring in Ph-X molecules (1) and correlate with traditional indicators of long-range polar effects. A new linear combination of the internal ring angles, termed S F , was introduced to measure this effect [4].…”

“…Cubane, C 8 H 8 , is the common name of pentacyclo[4.2.0.0 2,5 .0 3,8 .0 4,7 ]octane, a saturated hydrocarbon consisting of eight C-H units bound together to make a cube [6][7][8]. The highly symmetrical cage has a spacing of about 2.7 Å between opposite carbon atoms, compared with 1.9 and 2.6 Å for the bridgehead-bridgehead separation in bicyclo [1.1.1] pentane and bicyclo [2.2.2]octane, respectively.…”