309
10.3 Transformation of Oxiranes in Neutral Environment
Among uncatalyzed reactions of oxiranes in neutral environment theoretical investigations mostly focused on interaction with HF [49], NH 3 [33, 50–52], H 2 O [32, 53],
N-(3-chlorophenyl)piperazine molecules [54]. In early study Alagona and co-workers optimized geometry of transition states for alternative pathways of HF interaction with oxirane with the retention and inversion of carbon atom [49]. According to
their calculations the activation energy for pathway with inversion of configuration
is equal to 136.6 and 141.7 kJ/mol at CI/STO-3G and HF/4-31G levels of theory.
Reaction with retention of configuration is characterized by significantly lower values of activation energy (67.5 and 84.0 kJ/mol). The authors concluded that significant decreasing of activation energy observed at CI level could be explained by
pronounced contribution of excited states to wave function of transition states.
Mechanism of in vacuo oxirane aminolysis has been studied by numerous
quantum-chemical approaches (MINDO/3 [50, 52], B3LYP/6-31G(d) [51], HF/631 + G(d) [33] and MP2/6-31 + G(d) [33]). In all cases transition states are localized
close to typical S N 2-type transition states where nucleophile is located in the plane
of oxirane ring and bond angle between forming and breaking bonds is close to
180 ° (Fig. 10.10).
It should be noted that in spite of some difference in transition states geometries,
all used approached give close values of E act . In contrast to reaction with HF the
alternative reaction with front-side approach of ammonia molecule to oxirane energetically is less favorable (Е act = 200.7 kJ/mol (B3LYP/6-31G(d) [51])).
More complicated aminolysis reaction has been studied theoretically for cyclohexadiene monoepoxide interaction with N-(3-chlorophenyl)piperazine [54]. Both
pathways corresponding to front- and rear-side attacks have been found. The first
pathway is one-step reaction where breaking of C–O bond, forming of C–N bond
and proton transfer between N and O atoms occur simultaneously (Fig. 10.11).
In contrast, at the first step of rear-side attack zwitterionic intermediate is formed
2
1
+ +
+
+
+
+
+
%/<3*G>@
03*G>@
+)*G>@
0,1'2>@
ç&&2
( DFW N-PRO 03*GS%/<3*G>@
%/<3*G>@
0,1'2>@
Fig. 10.10 Structure of transition state for oxirane aminolysis and corresponding values of activation energy (Е act )
10 Quantum-Chemical Investigation of Epoxidic Compounds Transformation
10.3 Transformation of Oxiranes in Neutral Environment
Among uncatalyzed reactions of oxiranes in neutral environment theoretical investigations mostly focused on interaction with HF [49], NH 3 [33, 50–52], H 2 O [32, 53],
N-(3-chlorophenyl)piperazine molecules [54]. In early study Alagona and co-workers optimized geometry of transition states for alternative pathways of HF interaction with oxirane with the retention and inversion of carbon atom [49]. According to
their calculations the activation energy for pathway with inversion of configuration
is equal to 136.6 and 141.7 kJ/mol at CI/STO-3G and HF/4-31G levels of theory.
Reaction with retention of configuration is characterized by significantly lower values of activation energy (67.5 and 84.0 kJ/mol). The authors concluded that significant decreasing of activation energy observed at CI level could be explained by
pronounced contribution of excited states to wave function of transition states.
Mechanism of in vacuo oxirane aminolysis has been studied by numerous
quantum-chemical approaches (MINDO/3 [50, 52], B3LYP/6-31G(d) [51], HF/631 + G(d) [33] and MP2/6-31 + G(d) [33]). In all cases transition states are localized
close to typical S N 2-type transition states where nucleophile is located in the plane
of oxirane ring and bond angle between forming and breaking bonds is close to
180 ° (Fig. 10.10).
It should be noted that in spite of some difference in transition states geometries,
all used approached give close values of E act . In contrast to reaction with HF the
alternative reaction with front-side approach of ammonia molecule to oxirane energetically is less favorable (Е act = 200.7 kJ/mol (B3LYP/6-31G(d) [51])).
More complicated aminolysis reaction has been studied theoretically for cyclohexadiene monoepoxide interaction with N-(3-chlorophenyl)piperazine [54]. Both
pathways corresponding to front- and rear-side attacks have been found. The first
pathway is one-step reaction where breaking of C–O bond, forming of C–N bond
and proton transfer between N and O atoms occur simultaneously (Fig. 10.11).
In contrast, at the first step of rear-side attack zwitterionic intermediate is formed
2
1
+ +
+
+
+
+
+
%/<3*G>@
03*G>@
+)*G>@
0,1'2>@
ç&&2
( DFW N-PRO 03*GS%/<3*G>@
%/<3*G>@
0,1'2>@
Fig. 10.10 Structure of transition state for oxirane aminolysis and corresponding values of activation energy (Е act )
10 Quantum-Chemical Investigation of Epoxidic Compounds Transformation