ISSN:
0894-3230
Schlagwort(e):
Organic Chemistry
;
Physical Chemistry
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
,
Physik
Notizen:
Ab initio (4-31G) molecular geometry optimizations on cis-3,4-disubstituted cyclobutenes show that the extent and direction of their ground-state pyramidalization depend on the type of substituents and on whether these substituents are part of a ring condensed on to the cyclobutene moiety. Thus, electron-withdrawing substituents such as Cl and OMe bring about an out-of-plane anti bending of the olefinic hydrogens, whereas cyclobutenes condensed on to small carbocyclic rings exhib high syn bending. Intramolecular interactions are analysed in the frame of a new model where the interactions arise from the energy matrix elements between the hybrid atomic orbitals involved in the different bonds. The analysis is carried out by C-INDO calculations on deformed molecules (with the olefinic hydrogens forced out-of-plane by ± 20°). These latter calculations reveal that non-planarity of the double bond occurs in the direction of increasing interactions between the hybrid atomic orbitals involved in the σ-allylic bonds and in the π-bond. The increase in this kind of interaction can also explain non-planarity in norbornene, 7-oxanorbornene, 5,6-dioxanorbornene and 5,6-dioxabicyclo [2.2.2] oct-2-ene. The results strongly suggest that pyramidalization and the related energy asymmetry of out-of-plane bending of olefinic hydrogens are important factors in governing facial selectivity of 1,3-dipolar cycloadditions to cis-3,4-disubstituted cyclobutenes.
Zusätzliches Material:
5 Ill.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1002/poc.610030305
