Through-bond interaction of two mutually perpendicular .pi. systems. A comparison with spiroconjugation

Orbital-Controlled Stereoselections in Sterically Unbiased Cyclic Systems

“…As shown in the previous examples, the unsymmetric π face of carbonyl groups is postulated to be attributed to orbital interactions between a σ fragment and a π fragment. Possible interactions between two π fragments in a molecule are anticipated, wherein each of the interactive fragments would be subject to efficient reciprocal perturbations in its reactivity (see also section VIII). , As probes to pursue facial selectivities arising from π orbitals rather than from the σ orbitals, spiro[cyclopentane-1,9‘-fluorene]-2-ones 9a − e were designed and synthesized and the facial selectivities of the carbonyl groups of 9a − e are studied in this laboratory. , The carbonyl π orbital can interact with the aromatic π orbital of the fluorene in a manner similar to spiro conjugation. − The ketones 9 were reduced to the alcohols by the action of sodium borohydride in methanol at −43 °C. The anti alcohol, i.e., the syn addition of the reducing reagent with respect to the substituent, is favored in all cases irrespective of the substituent at C-2 or C-4 of the fluorene ring (2-nitro 9b (syn:anti = 68:32), 4-nitro 9c (syn:anti = 71:29), 2-fluoro 9d (syn:anti = 72:28), and 2-methoxyl 9e (syn:anti = 74:26) groups).…”

Infrared and Raman Spectroscopy of 9,9‘-Spirobifluorene, Bis(2,2‘-biphenylene)silane, and Bis(2,2‘-biphenylene)germane. Vibrational Assignment by Depolarization Measurement and HF and Density Functional Theory Studies

“…Spiro compounds consist of two mutually perpendicular π-systems connected via a common insulating tetracoordinate atom. The structural characteristics of spiro compounds have been theoretically − and experimentally − studied. Previous theoretical calculation indicates that the overlap integral of two p π orbitals in the different plane of spiro[4.4]nonatetraene is about 20% of the adjacent p orbitals in planar π electron systems …”

Helical Electronic Transitions of Spiroconjugated Molecules

“…However, there will be additional nodal planes due to the extra s-bonds (Figure S11 and S12). 10,11,42 The electrohelicity effect thus appears to be less pronounced in larger systems. In summary, we have explored how helical p-conjugation between the perpendicular p-systems of allene and spiropentadiene affect their electronic transitions and the helicity thereof.…”

“…In similar fashion, s-orbitals mediate through-bond coupling between the perpendicular p-systems in larger tricyclic analogues of spiroconjugated molecules (Scheme 1) as described by Gleiter and co-workers. 10,11 In chiral allenes, cumulenes, and polyynes the electron delocalization between the two p-systems is expressed in the form of helical MOs. 3,12,13 This helical p-conjugation is also called the electrohelicity effect.…”