Mapping the Trajectory of Nucleophilic Substitution at Silicon Using a peri-Substituted Acenaphthyl Scaffold

Hupf, Emanuel; Olaru, Marian; Raţ, Ciprian I.; Fugel, Malte; Hübschle, Christian B.; Lork, Enno; Grabowsky, Simon; Mebs, Stefan; Beckmann, Jens (2017). Mapping the Trajectory of Nucleophilic Substitution at Silicon Using a peri-Substituted Acenaphthyl Scaffold. Chemistry - a European journal, 23(44), pp. 10568-10579. Wiley-VCH 10.1002/chem.201700992

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The nucleophilic substitution reaction of second order (SN2) at a silicon atom is scrutinized using snapshots along a pseudo-reaction coordinate. Phosphine and fluoride represent both attacking and leaving groups in the modeled SN2 reaction. In the experimentally obtained 5-Ph2P-Ace-6-SiMe2F (1) the phosphine moiety and the fluorosilane moiety are forced into immediate proximity through an acenaphthyl scaffold, i.e. they exhibit peri-interactions that serve as the model of the reactant ion-molecule complex and starting point for a theoretical PES scan. Upon dissociation of fluoride, the experimentally obtained silylphosphonium cation [5-Ph2P-Ace-6-SiMe2]+ (2) serves as a model of the product and the end point of the PES scan. The pseudo-reaction pathway is followed by geometric, energetic, spectroscopic, molecular-orbital and topological real-space bonding indicators. It becomes evident that it is crucial to combine such methods to understand the pseudo-reaction since they reveal different aspects based on the different sensitivity to dispersive, electrostatic and polar-covalent contributions to bonding as shown by the reduced density gradient analysis. E.g., AIM theory describes a late topological catastrophe whereas the ELI describes an early concerted reaction and NRT describes a more gradual change of properties. This case study encourages the use of a well-balanced toolbox equipped with complementary methods to emphasize different aspects of bonding.

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