Page 231 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

6.4 NUCLEOPHILIC BREAKDOWN OF CYCLOPOLYSULFUR RINGS 211

REVIEW PROBLEM 6.2
Selenium dibromide can be prepared from selenium dichloride via interaction with
trimethylsilyl bromide in an aprotic solvent such as THF:

SeCl + 2Me SiBr → SeBr + 2Me SiCl
2 3 2 3
Suggest a mechanism for this reaction. Also, comment on the overall transformation
vis-à-vis the HSAB principle.
Hint. Observe that Se and Si are essentially swapping ligands. As noted in Section
1.19, such ligand exchanges typically involve bridged intermediates. In this case,
however, there are at least a couple of options. In our view, the most reasonable way
to start the mechanism is to have Cl attack Si, since the latter is the most potent
electrophilic center in the system.

NUCLEOPHILIC BREAKDOWN OF CYCLOPOLYSULFUR RINGS
6.4
The S ring is completely broken down by a variety of nucleophiles. With triphenyl-
8
phosphine, the overall reaction is
S + 8Ph P → 8Ph PS (6.35)
8
3
3
The mechanism might not be obvious, but triphenylphosphine is such a superb nucleophile
that it’s a good bet to have it attack the S ring; the S–S bonds are fairly weak (∼213 kJ/mol)
8
and cleave with relative ease:
+
Ph P Ph P
3 S 3 S
S S S S
S S S S (6.36)
S S
S S S S
−
APh PS unit is now poised as an excellent leaving group, and attack by a second Ph P, as
3
3
shown below, gives us our first molecule of triphenylphosphine sulfide, Ph PS.
3
+ PPh 3 +
Ph 3 P Ph P
S 3 S
S S S − Ph PS S S (6.37)
3
S S S S
S S S S
− −
Additional Ph P molecules continue to chew on the shrinking sulfur chain until we are left
3
with just the following stub:

+ −
Ph 3 P S
S