Mechanism of [2 + 1] Cycloadditions of Hydrogen Isocyanide to Alkynes: Molecular Orbital and Density Functional Theory Study
详细信息 查看全文
- 作者：Loc Thanh Nguyen ; Trung Ngoc Le ; Frank De Proft ; Asit K. Chandra ; Wilfried Langenaeker ; Minh Tho Nguyen ; and Paul Geerlings
- 刊名：Journal of the American Chemical Society
- 出版年：1999
- 出版时间：June 30, 1999
- 年：1999
- 卷：121
- 期：25
- 页码：5992 - 6001
- 全文大小：212K
- 年卷期：v.121,no.25(June 30, 1999)
- ISSN：1520-5126
文摘
The reactions of hydrogen isocyanide (HN
C) with various simple alkynes (HCC-X, with X =H, CH3, NH2, F), formally [2 + 1] cycloadditions, have been studied by density functional theory (DFT) withthe hybrid exchange correlation B3LYP functional and a 6-311G(d,p) basis set, as well as by MO theory withCCSD(T) calculations. For each reaction, the intrinsic reaction coordinate (IRC) pathway has been constructed.It is shown that each [2 + 1] cycloaddition is nonconcerted but proceeds in two steps: rate-determining additionof HNC to a carbon atom of HCCX, giving rise to a zwitterion intermediate, followed by a ring closureof the latter, yielding finally cyclopropenimine. In all cases, HNC behaves as an electrophile. The activationenergies corresponding to both possible initial attacks of HNC are distinguishable, introducing thus a siteselectivity and an asynchronism of bond formation in the initial step, for which a rationalization using DFT-based reactivity descriptors and the local HSAB principle has been proposed. Except for HCC-F, initialattack on the unsubstituted alkyne carbon is preferred. The hardness and polarizability profiles along the IRCreaction paths of the supersystem have also been constructed. In some cases, there are no clear-cut extrema;in other cases, there is a minimum in the hardness profile and a maximum in the polarizability profile, butthese extrema do not coincide with the energy maximum and are rather shifted toward the side having theclosest value, following apparently a generalized Hammond postulate. While the higher hardness-lowerpolarizability criterion seems to hold true, there is no obvious relationship between hardness and energy. Theactivation energy (Eact) vs hardness difference relationship recently derived by Gázquez turns out to be successfulin the interpretation of the calculated Eact sequences.
C) with various simple alkynes (HCC-X, with X =H, CH3, NH2, F), formally [2 + 1] cycloadditions, have been studied by density functional theory (DFT) withthe hybrid exchange correlation B3LYP functional and a 6-311G(d,p) basis set, as well as by MO theory withCCSD(T) calculations. For each reaction, the intrinsic reaction coordinate (IRC) pathway has been constructed.It is shown that each [2 + 1] cycloaddition is nonconcerted but proceeds in two steps: rate-determining additionof HNC to a carbon atom of HCCX, giving rise to a zwitterion intermediate, followed by a ring closureof the latter, yielding finally cyclopropenimine. In all cases, HNC behaves as an electrophile. The activationenergies corresponding to both possible initial attacks of HNC are distinguishable, introducing thus a siteselectivity and an asynchronism of bond formation in the initial step, for which a rationalization using DFT-based reactivity descriptors and the local HSAB principle has been proposed. Except for HCC-F, initialattack on the unsubstituted alkyne carbon is preferred. The hardness and polarizability profiles along the IRCreaction paths of the supersystem have also been constructed. In some cases, there are no clear-cut extrema;in other cases, there is a minimum in the hardness profile and a maximum in the polarizability profile, butthese extrema do not coincide with the energy maximum and are rather shifted toward the side having theclosest value, following apparently a generalized Hammond postulate. While the higher hardness-lowerpolarizability criterion seems to hold true, there is no obvious relationship between hardness and energy. Theactivation energy (Eact) vs hardness difference relationship recently derived by Gázquez turns out to be successfulin the interpretation of the calculated Eact sequences.