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Preparation and Reactivity of a Triphosphenium Bromide Salt: A Convenient and Stable Source of Phosphorus(I)
Published on: November 22, 2016
Nucleophilic substitution at phosphorus centers (SN2@p)
Marc A van Bochove1, Marcel Swart, F Matthias Bickelhaupt
1Theoretische Chemie, Scheikundig Laboratorium der Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands.
Summary
This study compares bimolecular nucleophilic substitution reactions at phosphorus (SN2@P), carbon (SN2@C), and silicon (SN2@Si) centers. Unlike SN2@C reactions, SN2@P reactions exhibit a single-well potential energy surface, indicating a stable transition complex.
Area of Science:
- Computational Chemistry
- Quantum Chemistry
- Reaction Mechanism Studies
Background:
- Bimolecular nucleophilic substitution (SN2) reactions are fundamental in chemistry.
- Understanding SN2 mechanisms at different centers (carbon, silicon, phosphorus) is crucial for predicting reactivity.
- Previous studies have primarily focused on SN2 reactions at carbon and silicon.
Purpose of the Study:
- To investigate and characterize archetypal model systems for SN2 reactions at phosphorus (SN2@P).
- To compare SN2@P reactions with analogous reactions at carbon (SN2@C) and silicon (SN2@Si).
- To explore the influence of substituents and reaction pathways on SN2@P mechanisms.
Main Methods:
- Application of Density Functional Theory (DFT) using the Generalized Gradient Approximation (GGA) at the OLYP/TZ2P level.
- Modeling of SN2 reactions at tricoordinate (SN2@P3) and tetracoordinate (SN2@P4) phosphorus centers.
- Analysis of potential energy surfaces (PES), including symmetric and asymmetric reactions, backside/frontside pathways, and conformational dependence.
Main Results:
- A key difference observed is the transition from a double-well PES in SN2@C to a single-well PES in SN2@P, featuring a stable transition complex.
- Differences between SN2@P3 and SN2@P4 reactions were found to be minor.
- The study revealed the occurrence of triple-well PES in both symmetric and asymmetric SN2@P4 reactions, particularly with halogen substituents.
Conclusions:
- SN2 reactions at phosphorus centers exhibit distinct mechanistic features compared to carbon and silicon analogs, characterized by a single-well PES.
- Substituent effects, especially halogenation, significantly influence the PES landscape in SN2@P reactions, leading to complex phenomena like triple-well potentials.
- The findings provide valuable insights into the fundamental reactivity and mechanistic pathways of nucleophilic substitution at phosphorus.
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