Page 81 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

2.6 DIHYDROGEN ACTIVATION BY FRUSTRATED LEWIS PAIRS (FLPs) 61

REVIEW PROBLEM 2.5
n-Butyllithium is widely used as a polymerization initiator in the synthesis of poly-
butadiene or styrene–butadiene–styrene (SBS), both of which are used in the produ-
ction of tires, hoses, shoe heels and soles, golf balls, and even chewing gum. The first
step may be depicted as follows:

C H Li + CH =CH–CH=CH → C H –CH –CH=CH–CH Li
4 9 2 2 4 9 2 2
Using arrow pushing, show how the polymers might now arise.

2.6 DIHYDROGEN ACTIVATION BY FRUSTRATED LEWIS PAIRS (FLPs)

Recall from Section 1.9 that the Lewis acid and base in a frustrated pair are too bulky or
otherwise sterically constrained to bond with each other but the cleft between them acts like
an enzyme active site and promotes remarkable chemical reactions, perhaps none more
so than the activation of dihydrogen at or below room temperature. Two representative
reactions mediated by P,B-based FLPs are as follows:
F F F F
+ −
P + F B H 2 PH + F BH
3 3 3 3

F F F F

F
F F
F
F
F F
F
H 2
P B F +
F RT, P B − F
minutes F
H H
F
F F
F
F F
F F
(2.38)
Observe that each of these reactions accomplishes a net heterolytic cleavage of an H
2
molecule, an incredibly weak acid (pK = 42), with the P picking up a proton and the
a
B a hydride. How might this occur? Neither a naked proton nor a naked hydride anion is
plausible as an intermediate. A reaction path in which both the P and the B act in concert to
break the H–H bond heterolytically is envisioned below, even though explicit experimental
evidence for such a pathway is still lacking:
Ar Ar Ar′
P B P + − B
Ar Ar′ Ar Ar′ (2.39)
H H Ar′ H H