The TASF reagent or tris(dimethylamino)sulfonium difluorotrimethylsilicate is a
reagent
In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...
in
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J.; ...
with
structural formula
The structural formula of a chemical compound is a graphic representation of the molecular structure (determined by structural chemistry methods), showing how the atoms are possibly arranged in the real three-dimensional space. The chemical bondi ...
(CH3)2N)3Ssup>+
2Si(CH3)3sup>−. It is an anhydrous source of
fluoride
Fluoride (). According to this source, is a possible pronunciation in British English. is an inorganic, monatomic anion of fluorine, with the chemical formula (also written ), whose salts are typically white or colorless. Fluoride salts typ ...
and is used to cleave
silyl ether
Silyl ethers are a group of chemical compounds which contain a silicon atom covalently bonded to an alkoxy group. The general structure is R1R2R3Si−O−R4 where R4 is an alkyl group or an aryl group. Silyl ethers are usually used as protecting g ...
protective group
A protecting group or protective group is introduced into a molecule by chemical modification of a functional group to obtain chemoselectivity in a subsequent chemical reaction. It plays an important role in multistep organic synthesis.
In many ...
s. Many other fluoride reagents are known, but few are truly anhydrous, because of the extraordinary basicity of "naked" F
−. In TASF, the fluoride is masked as an adduct with the weak
Lewis acid
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
trimethylsilylfluoride (FSi(CH
3)
3). The
sulfonium
In organic chemistry, a sulfonium ion, also known as sulphonium ion or sulfanium ion, is a positively-charged ion (a " cation") featuring three organic substituents attached to sulfur. These organosulfur compounds have the formula . Together wi ...
cation ((CH
3)
2N)
3S
+ is unusually non-electrophilic due to the electron-donating properties of the three (CH
3)
2N substituents.
This compound is prepared from
sulfur tetrafluoride
Sulfur tetrafluoride is the chemical compound with the formula S F4. It is a colorless corrosive gas that releases dangerous HF upon exposure to water or moisture. Despite these unwelcome characteristics, this compound is a useful reagent for t ...
:
:3 (CH
3)
2NSi(CH
3)
3 + SF
4 → 2 (CH
3)
3SiF +
(CH3)2N)3Ssup>+
2Si(CH3)3sup>−
The colorless salt precipitates from the reaction solvent,
diethyl ether
Diethyl ether, or simply ether, is an organic compound in the ether class with the formula , sometimes abbreviated as (see Pseudoelement symbols). It is a colourless, highly volatile, sweet-smelling ("ethereal odour"), extremely flammable liq ...
.
Structure
The cation
(CH3)2N)3Ssup>+ is a
sulfonium ion. The S-N distances are 1.612 and 1.675 pm. The N-S-N angles are 99.6°. The anion is
2Si(CH3)3sup>−. It is
trigonal bipyramid
In geometry, the triangular bipyramid (or dipyramid) is a type of hexahedron, being the first in the infinite set of face-transitive bipyramids. It is the dual of the triangular prism with 6 isosceles triangle faces.
As the name suggests, i ...
al with mutually trans fluorides. The Si-F distances are 176 picometers. The Si-C distances are 188 pm.
[{{cite journal , doi=10.1002/hc.520040225, title=Structural studies of tris(dialkylamino) sulfonium (TAS) fluorosilicates, year=1993, last1=Dixon, first1=David A., last2=Farnham, first2=William B., last3=Heilemann, first3=W., last4=Mews, first4=R., last5=Noltemeyer, first5=M., journal=Heteroatom Chemistry, volume=4, issue=2–3, pages=287–295]
References
Reagents for organic chemistry
Fluorides
Dimethylamino compounds