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Dynamic Cross‐Exchange in Halophosphonium Species: Direct Observation of Stereochemical Inversion in the Course of an S N 2 Process
Author(s) -
Nikitin Kirill,
Jennings Elizabeth V.,
Al Sulaimi Sulaiman,
Ortin Yannick,
Gilheany Declan G.
Publication year - 2018
Publication title -
angewandte chemie international edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 1433-7851
DOI - 10.1002/anie.201708649
Subject(s) - steric effects , chemistry , nucleophile , phosphonium , sn2 reaction , crystallography , computational chemistry , medicinal chemistry , stereochemistry , organic chemistry , catalysis
The complex fluxional interconversions between otherwise very similar phosphonium bromides and chlorides R 3 PX + X − (R=Alk, Ar, X=Cl or Br) were studied by NMR techniques. Their energy barriers are typically ca. 11 kcal mol −1 , but rise rapidly as bulky groups are attached to phosphorus, revealing the importance of steric factors. In contrast, electronic effects, as measured by Hammett analysis, are modest ( ρ 1.46) but still clearly indicate negative charge flow towards phosphorus in the transition state. Most significantly, detailed analysis of the exchange pathways unequivocally, and for the first time in any such process, shows that nucleophilic attack of the nucleophilic anion on the tetrahedral centre results in inversion of configuration.