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Abstract
Density-functional theory is used to model the endo and exo transition states for [2,3]-sigmatropic rearrangement of allylic aryl-selenoxides and -selenimides. The endo transition state is generally preferred for selenoxides if there is no substitution at the 2 position of the allyl group. Based upon the relative energies of the endo and exo transition states, enantioselectivity of rearrangements is expected to be greatest for molecules with substitutions at the 1- or (E)-3- position of the allyl group. Ortho substitution of a nitro group on the ancillary selenoxide phenyl ring reduces the activation barriers, increases the difference between the endo and exo activation barriers and shifts the equilibrium toward products.
Highlights
Introduction[2,3]-Sigmatropic rearrangements of allylic selenoxides and selenimides (Figure 1a) are important tools for the synthesis of primary, secondary and tertiary allylic alcohols and amines [1]
Density-functional theory is used to model the endo and exo transition states for [2,3]-sigmatropic rearrangement of allylic aryl-selenoxides and -selenimides
Calculations were performed on the model allylic phenylselenoxides 1–6, their o-nitrophenyl analogues (1–5 only, i.e., 7–11) and the N-benzenesulfonimide 12 (Figure 2) using the B3PW91 exchange correlation functional
Summary
[2,3]-Sigmatropic rearrangements of allylic selenoxides and selenimides (Figure 1a) are important tools for the synthesis of primary, secondary and tertiary allylic alcohols and amines [1]. Reich and coworkers reported the activation barrier of the selenoxide rearrangement of an allylic o-nitrophenylselenoxide as approximately 12.5 kcal/mol (ΔG‡), with 2 kcal/mol separating distinct transition states where the aryl group is endo or exo to the allyl group (Figure 1c) [14]. The transition states for [2,3]-sigmatropic rearrangements of aryl allyl selenoxides and selenimides are modeled using density-functional theory (DFT) and compared to Reich et al’s experimental data, as well as previously estimated barriers for the sulfoxide rearrangement [15]. The effect of ortho substitution of a nitro group on the activation barriers is examined as computational studies of selenoxide elimination [16,17] have shown that groups capable of intramolecular Se···N,O interactions [18] substantially lower the barrier to elimination
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