Page 255 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

6.12 HIGHER-VALENT SULFUR FLUORIDES 235
The mechanism of fluorination effected by SF or any of the dialkylaminosulfur trifluo-
4
rides is illustrated below for a primary alcohol and DAST. The first step involves coordina-
tion of the alcohol to the tetravalent sulfur, followed by the departure of a fluoride leaving
group:

NEt 2
R
NEt 2 NEt 2 − S F
F + O F − F F
S S O (6.109)
R O F H −
F F + H
F
H
R
The outgoing fluoride then comes back as a nucleophile, forming the alkyl fluoride product
and kicking out an oxygen leaving group, as shown below:

NEt 2
F NEt
S F 2
F + S F
− O
+ H F
F R O
H
R (6.110)
NEt 2
NEt 2
+
F S + HF
S F
F O
−
O
H
Observe that the sulfur remains tetravalent at the end of the reaction; thus, no redox has
taken place.
The “deoxofluorination” step, where oxygen is replaced by fluorine, is shown under
reaction 6.110 as an S 2 displacement. For many alcohols, therefore, deoxofluorination by
N
SF or related reagents involves an inversion of configuration. For other substrates, however,
4
an S 1 pathway appears to operate, based on product stereochemistry as well as rearranged
N
products.

REVIEW PROBLEM 6.16*

The various aminosulfur trifluorides are prepared via the reaction of SF with a
4
∘
dialkylamino(trialkyl)silane at ∼−70 , as shown below for DAST:
Et NSiMe + SF → Et NSF + Me SiF
3
4
3
2
3
2
Suggest a mechanism and comment on the thermodynamic driving forces.