Phosphonium-carbenium and oxophosphonium-carbenium dications in Friedel-Crafts reactions

Holmin, Grant P.; Hood, Jacob C.; Scanlon, Nathan; Klumpp, Douglas A.

doi:10.1016/j.tet.2022.132928

Abstract As a flat trigonal species, the CR₃⁺carbenium ion contains a pair of deep π‐holes above and below its molecular plane. In the case of CH₃⁺a first base will form a covalent bond with the central C, making the combined species tetrahedral. Approach of a second base to the opposite side results in a longer but rather strong noncovalent tetrel bond (TB). While CMe₃⁺can also form a similar asymmetric complex with a pair of bases, it also has the capacity to form a pair of nearly equivalent TBs, such that the resulting symmetric trigonal bipyramid configuration is only slightly higher in energy. When the three substituents on the central C are phenyl rings, the symmetric configuration with two TBs predominates. These tetrel bonds are quite strong, reaching up to 20 kcal/mol. Adding OPH₂or OCH substituents to the phenyl rings permits the formation of intramolecular C⋅⋅O TBs to the central C, very similar in many respects to the case where these TBs are intermolecular.

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