Organic Chemistry

1,3-Hydrogen shifts can take place if the reaction is carried out under photochemi-
cal conditions because the HOMO is symmetric under photochemical conditions,
which means that hydrogen can migrate by a suprafacial pathway (Table 29.4).

Two products are obtained in the previous reaction because two different allylic
hydrogens can undergo a 1,3-hydrogen shift.

[1,5] Sigmatropic migrations of hydrogen are well known. They involve three pairs
of electrons, so they take place by a suprafacial pathway under thermal conditions.

PROBLEM 14

Why was a deuterated compound used in the preceding example?

PROBLEM 15

Account for the difference in the products obtained under photochemical and thermal

conditions.

[1,7] Sigmatropic hydrogen migrations involve four pairs of electrons. They can
take place under thermal conditions because the eight-membered-ring transition state
allows the required antarafacial rearrangement.

CH

H

CH 2

C 4 H 9 C^4 H^9

CH 3

1,7-hydrogen shift

CD 3

∆

h

CD 2

D

CD 3 CD 2

D

H

CH 2 ∆

CH 3

CH 3

CH 3

H

CD 2 ∆ CD 2 H

1,5-hydrogen shifts

h

H

h

H

h +

1,3-hydrogen shifts

Section 29.5 Sigmatropic Rearrangements 121