Di‐ and Trifluorobenzenes in Reactions with Me₂EM (E = P, N; M = SiMe₃, SnMe₃, Li) Reagents: Evidence for a Concerted Mechanism of Aromatic Nucleophilic Substitution

Abstract

Abstract Dimethyl(trimethylsilyl)phosphane (Me 3 SiPMe 2 ) has been successfully used for the substitution of fluorine variously in 1,3‐ ( 1 ) or 1,2‐difluorobenzene ( 2 ) or in 1,3,5‐ ( 3 ) or 1,2,3‐trifluorobenzene ( 4 ) by the Me 2 P group at 150–190 °C either in benzene solution or without solvent to give 1‐(dimethylphosphanyl)‐3‐fluorobenzene ( 5 ) from 1 , 1‐(dimethylphosphanyl)‐2‐fluorobenzene ( 6 ) from 2 , 1‐(dimethylphosphanyl)‐3,5‐difluorobenzene ( 7 ) from 3 or a 1:1 mixture of 1‐(dimethylphosphanyl)‐2,3‐difluorobenzene ( 8 ) and 1‐(dimethylphosphanyl)‐2,6‐difluorobenzene ( 9 ) from 4 . The substrate selectivities and regioselectivities exhibited by 4 with Me 3 SiPMe 2 and in competitive reactions between 1 and 2 or 3 and 4 with Me 2 PSiMe 3 , Me 2 PSnMe 3 or Me 2 PLi indicate relative fluorine substituent rate factors f o ‐F > f m ‐F , whereas for reactions that proceed through a two‐step S N Ar mechanism the opposite sequence is typical. High‐level quantum‐chemical DFT and MP2 calculations predict that the gas‐phase reactions should each proceed by a concerted mechanism with a single transition state. These predictions are in good agreement with the experimental observations, especially because the structural features of the Meisenheimer adduct are unfavourable for the S N Ar mechanism. This proposal is consistent with the observation of the opposite sequence (f m ‐F > f o ‐F ) for the reactions between the same substrates and MeONa in DMSO/CH 3 OH solvent mixtures. The novel phosphanes were characterized by spectroscopic (NMR) and spectrometric (MS) investigation, preparation of the thiophosphanes Ar F PSMe 2 10 – 12 , their spectroscopic data and, in the case of 12 , by its X‐ray structure. The phosphanes 5 – 9 were treated with bis(benzonitrile)dichloropalladium(II) to afford the corresponding bis(phosphane)palladium dichloride complexes 17 – 21 and 23 in isolated yields of up to 95 %.