Page 268 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

GROUP 16 ELEMENTS: THE CHALCOGENS
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The stability of hexavalent tellurium was clearly demonstrated in a reaction of TeCl with
4
an aryllithium, reported by Japanese main-group chemist Kin-ya Akiba and his coworkers
(Tetrahedron 1997, 53, 12195–12202), which led to a disproportionation and produced a
highly stable hexaaryltellurium product:

− 78 °C
2 TeCl 4 + 8 ArLi TeAr 6 + TeAr 2
Ether
(6.138)
Ar = CF 3

We chose this reaction for discussion because, like many disproportionation reactions, the
mechanism is of considerable interest. An interesting clue to the mechanism was that both
TeAr Cl and TeAr Cl were obtained as side products. Nucleophilic attack by ArLi on TeCl 4
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3
produces a whole suite of products, which we may view as intermediates:
TeCl + ArLi ArTeCl 3 Ar TeCl 2
2
4
− (6.139)
Ar TeCl Ar Te Ar Te
3
4
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Now, for disproportionation to occur, two tetravalent Te species somehow need to hook up
into a binuclear species (i.e., a species containing two Te’s), which would then rearrange and
fall apart into hexa- and di-valent Te compounds. A few different pathways are envisioned
below.
Mechanism 1. One possibility involves a chloride-bridged ditellurium intermediate, as
shown below:

Ar
Ar Te
Ar Ar +
Ar Ar
Te Cl − Ar
Cl
Te Ar Te Te + (6.140)
Cl Ar Cl Ar Cl
Ar Ar Ar Ar Ar
Cl
Te
Ar
Ar
Ar
We have thus accounted for both hexa- and di-valent Te products. We only need to arylate
TeAr Cl to produce the TeAr final product:
4
2
6
Cl Ar Cl Ar Ar Ar
Cl ArLi Ar ArLi Ar
Te Te Te (6.141)
Ar − LiCl Ar − LiCl Ar
Ar Ar Ar Ar Ar
Ar Ar Ar
Note that we have also generated the observed by-product TeAr Cl.
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