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The Role of Lone Pairs in Heteroatomic Chemistry. Graphical Analysis and Ab Initio Calculations of Oxirane and Aziridine Ring Opening

Francois Volatron ; Laboratoire de Chimie Theorique, F-91405 Orsay, France
Yves Jean ; Laboratoire de Chimie Theorique, F-91405 Orsay, France
Nguyen Trong Anh ; Laboratoire de Chimie Theorique, F-91405 Orsay, France

Sažetak

The »overlap graph«, where vertices represent atomic orbitals
and edges, their in-phase or out-of-phase overlaps, suggests that, .
in the thermal electrocyclic opening of oxirane, the second oxygen
lone pair has little effect on the allowed conrotatory mode but
lowers the activation energy of the forbidden disrotatory mode.
This effect does not exist in the electrocyclic opening of aziridine,
which is one reason why the energy gap between the allowed and
forbidden processes should remain substantial. In the course of an
aziridine ring opening, the nitrogen's hydrogen may remain in the
plane of the heavy atoms or may move out into either of the two
half-spaces defined by this plane. Three different disrotatory pathways
are thus conceivable. They are compared by a perturbational
treatment using simple Hiickel theory. Localization of the transition
states on these three reaction paths - i. e. determination of
their »early« or »late« character, as measured by the magnitude of
the rotation of the two methylene groups, for example - can be
made with the help of »natural correlation diagrams«. This provides
a simple criterion for predicting the breakdown of the perturbational
treatment, when pathways of similar energies are compared:
if one transition state is much earlier (later) than the other(s), no
safe conclusion can be reached. These qualitative arguments are
fully supported by ab initio calculations (ST0-3G + (3 X 3) CI and
4-31G + extensive CI). Substituent effects and possible cases of
violation of orbital symmetry requirements are discussed.

Ključne riječi

Hrčak ID:

194037

URI

https://hrcak.srce.hr/194037

Datum izdavanja:

21.5.1985.

Posjeta: 760 *