Page 114 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

GROUP 13 ELEMENTS
94

REVIEW PROBLEM 3.16*
Suggest a mechanism for the following oxidation–rearrangement:

Tl(NO 3 ) 3 • 3 H 2 O O
(1.1 equiv.)
MeOH, AcOH (cat.)
RT, 2 h
H H

3.9 SUMMARY

The following are some highlights of what we discussed:

1. The trivalent state is the normal state for all the elements, except thallium. When the
coordination number is also 3, the compounds act as powerful Lewis acids. A number
of these compounds serve as important reagents in organic synthesis. As a Lewis acid,
boron may even have played a role in the origin of life on Earth (or possibly on Mars).
2. When bonded to strongly basic ligands such as hydride or alkyl, the tetracoordinate
−
states exhibit group transfer reactivity. The [EH ] (E = B, Al, Ga) anions transfer
4
hydride anions and thereby reduce a variety of organic functional groups. In
hydroboration–oxidation, an alkyl group migrates from a negative tetracoordinate
boron to an adjacent oxygen. We will find this to be a common theme in this book: a
negatively charged electropositive atom serves as a launchpad for a migrating alkyl
group.
3. The monovalent state is well established for all the elements except boron, but is
unstable and reactive for Al and Ga. Low-valent and low oxidation-state Al and Ga
chemistry has been an active area of research in recent years.
4. For thallium on the other hand, the monovalent state is the stable state and the trivalent
state is oxidizing one and prone to reductive elimination, thanks to the inert pair effect.
Nevertheless, in spite of their toxicity, trivalent thallium reagents enjoy some unique
applications as oxidants in organic chemistry.

FURTHER READING

1. Stock, A. Hydrides of Boron and Silicon; Cornell: Ithaca, 1933; 250 pp. $2.00. This is the historic
book that inspired H. C. Brown; note the price! The significance of boranes (and silanes) is worth
quoting in the author’s words:

In nature boron’s dominating affinity for oxygen restricts it to the monotonous role of boric
acid and the borates, and prevents it from competing with carbon, its neighbor in the peri-
odic system.