Page 932 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

916 X X

CHAPTER 10
Concerted Pericyclic X BH 2 CH 3 NH 2
Reactions
B3LYP/6-311++G** 3.72 81.12 69.20
G3 1.52 73.93 67.80
CBS-APNO 0.46 75.82 68.90

The barrier for boron migration is very low, but for X = CH and NH , the barriers
3
2
are close to the bond dissociation energies, indicating that the reaction would be
stepwise. No antarafacial pathways were found. The suprafacial migration is facile for
BH because the empty p orbital is available. Figure 10.34 shows the lowest-energy
2
(QCISD/6-311G ) TS and the NPA bond orders associated with them. We see that the
∗∗
vacant p orbital allows for continuous bonding during the migration of BH , whereas
2
the absence of such an orbital for CH and NH results in electronic repulsion and a
2
3
much higher-energy TS. Note the much greater bond lengths of the allyl fragment. In
the case of X = CH and NH , these bonds are close to single-bond lengths, indicating
2
3
substantial loss of the bond. It is also worth noting that the allylborane system is
isoelectronic with the nonclassical C H carbocation (see p. 453). The computed TS
+
4
7
for BH migration corresponds in structure to the bicyclobutenyl carbocation.
2
10.6.2.2. Examples of Sigmatropic Shifts of Hydrogen and Alkyl Groups With the
generalized selection rules as a unifying theoretical framework (see p. 912), we can
consider specific examples of sigmatropic rearrangements. In accordance with the
orbital symmetry concepts, there are many examples of sigmatropic [1,5]-hydrogen
migrations in molecules that incorporate a pentadienyl fragment. The activation
energies for such reactions are usually in the vicinity of 35 kcal/mol, so the reactions
require moderately elevated temperatures. 220 Two examples are given below. The first
rearrangement is detected by migration of the isotopic label. The second results in
transformation to a more stable diene.

0.500 0.426 0.426
0.532 0.500
0.532
2.388
2.774 2.388
2.774
1.795 0.296 2.795 2.530
1.795 1.742
1.291 1.527 1.527 1.510 1.510
1.291
Fig. 10.34. Transition structures for suprafacial 1,3-migrations of BH 2 (left), CH 3 (center), and NH 2 (right)
at the QCISD/6-311+G ∗∗ level. Bond distances and NPA bond orders are shown. Adapter from J. Phys.
Chem. A, 106, 5709 (2002), by permission of the American Chemical Society.

220
W. R. Roth and J. Koenig, Liebigs Ann. Chem., 699, 24 (1966).