Di‐ and Trifluorobenzenes in Reactions with Me 2 EM (E = P, N; M = SiMe 3 , SnMe 3 , Li) Reagents: Evidence for a Concerted Mechanism of Aromatic Nucleophilic Substitution

• Published in: European journal of organic chemistry Vol. 2010; no. 6; pp. 1111 - 1123
• Main Authors: Goryunov, Leonid I., Grobe, Joseph, Le Van, Duc, Shteingarts, Vitalij D., Mews, Rüdiger, Lork, Enno, Würthwein, Ernst‐Ulrich
• Format: Journal Article
• Language: English
• Published: 01-02-2010
• ISSN: 1434-193X; 1099-0690
• DOI: 10.1002/ejoc.200900880

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 %.