Kinetic investigations of the fluorination of carbonyl compounds with sulfur tetrafluoride

B60

122

KINETIC INVESTIGATIONS OF

THE

FLUORINATION

OF

CARBONYL COMPOUNDS

WITH SULFUR TETRAFLUORIOE

D.Hass,H.Holfter,M.Schonherrand E. Zahm Institut fur Anorganische Chemie der Humboldt-Universitatzu Berlin Hessische Str.l/2, O-1040 Berlin

(F.R.G.)

Carbonyl groups can be transformed into difluoromethylenegroups by SF4. In the past 20 years a great number of such reactions have been discovered. Lewis acids like

HF,

BFg

and

AsF5,

preferably HF

are

used as catalysts.

The mechanism of the fluorination reaction is universally discussed by the formation of SFS+- and carbenium ions independent of the concrete reaction conditions:

SF4

+

HF

--.__--s ___

C=O

+

SF3+

-

:C+O-SF3

:C+F

HF2-

-

+

HF2-

=C+O-SF,

+

+

SF3+

)C+F

+

SOF2

;CF2

+

HF

Our kinetic measurements demonstrate that the "ionic mechanism" can only be maintained under conditions showing a great surplus of phase.

At higher

of HF in

temperatures

in connection

with

only

HF in liquid

Bquimolar

amounts

relation to the ester, especially at temperatures above the

critical temperature of HF at 18B°C, a "molecular mechanism" is to prefer. Steric hindrances are discussed on the basis of the different mechanisms. By choice

of suitable conditions the fluorination reaction can be forced

to the different mechanisms.