Page 52 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

A COLLECTION OF BASIC CONCEPTS
32
Good examples of reductive eliminations involving main-group elements include the fol-
lowing:
Cl
Cl
Cl Cl
As Cl As + Cl 2
Cl
Cl Cl
(1.58)
I
Tl Ar Ar I + I Tl
I

1.18 MIGRATIONS

Migrations are a somewhat quirky process in which an atom or a group tears itself from
its site of origin and jumps to a neighboring atom along with its pair of bonding electrons.
Thus, typically, the migrating group acts as a nucleophile and the migration terminus as
an electrophile. The process is typically intramolecular, that is, occurring within a single
covalently bound entity.
Some of the best studied examples of migration are provided by carbocation rearrange-
∘
∘
ments. Generally, less stable cations (e.g., 2 ) rearrange to more stable ones (3 ) by under-
going hydride or alkyl shifts, as shown in the example below:
H CH 3 H C CH 3
3
+
C C C C (1.59)
H C + H
3
H 3 C H H 3 C H
Indeed, observation of rearranged products is often seen as “proof” of the involvement of
carbocations and hence of an S 1/E1-type reaction pathway; concerted S 2/E2 pathways
N
N
are not expected to result in a rearrangement.
A typical migration involved in a carbocation rearrangement is called a 1,2-shift because
the origin and terminus of the migration are (typically, but not invariably) adjacent to each
other. We will encounter a few examples 1,2-shifts involving main-group elements. A com-
mon reaction pattern is the following:

R
−
M E M E R + L (1.60)
−
L

The migrating group R is often an alkyl or aryl group; the E–L bond is generally weak,
which makes L a good leaving group and E a good migration terminus. The negative charge
on the migration origin M enhances the migratory aptitude of the R group. Typically, M is a
relatively electropositive p-block element such as boron or silicon, or a higher-valent state
of an electronegative element such as a halogen. A good example of this type of a 1,2-shift
is provided by hydroperoxide-mediated oxidation of organoboranes.