Page 1165 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

1150 is of lower energy than the exciting radiation. Would you expect the shift in
energy to be more pronounced for fluorescence or phosphorescence? Explain?
CHAPTER 12 12.10. cis-2-Propyl-4-t-butylcyclohexanone undergoes cleavage to 4-t-butyl-
Photochemistry cyclohexanone on photolysis. The trans isomer does not undergo fragmentation
directly, but is converted to the cis isomer, which then fragments. The
trans → cis isomerization is quenched by 1,3-pentadiene, but the photofrag-
mentation is not. Offer an explanation of this pronounced stereochemical
effect.
12.11. The quantum yield for formation of 3-methylcyclobutene from E-1,3-
pentadiene by 254 nm radiation is ten times greater than for cyclization of
the Z-isomer. 1,3-Dimethylcyclopropene is also formed but the difference in
for this reactions is only a factor of two. Offer an explanation for these
differences in terms of energy surfaces that are involved.
12.12. The irradiation of 2-methoxytropone (12-A) leads to methyl 4-oxo-2-
cyclopentenylacetate (12-D). The course of the reaction can be followed
by gas chromatography and two intermediates are observed, which have
structures 12-B and 12-C. Indicate a mechanism for the three successive
reactions. The first two are photochemical, whereas the third is probably an
acid-catalyzed reaction that does not require light.
O O
CH O CH O O CH 3 O O
3
3
CH O CCH 2
2
3
12-A 12-B 12-C 12-D
12.13. When an aryl substituent is placed at C(5) of a 4-substituted cyclohexenone,
a new product type involving formation of a cyclobutanone ring is formed.

O O O
O
Ph
hv
Ph + + Ph H
Ph Ph
Ph Ph C C
Ph Ph Ph H
Ph Ph
13-A 13-E 13-F 13-C
O
O
Ph H
Ph
Ph C C
Ph H
CH 3 13-B CH 3
13-D only product
The reaction products are the same for both direct irradiation and
acetophenone sensitization. When reactant 13-B is used in enantiomerically
pure form, the product 13-D is nearly racemic (6% e.e.). Relate the formation
of the cyclobutanone to the more normal products of 4-substituted cyclo-
hexenones.
12.14. In the rearrangement of 4,4-diphenylcyclocyclohex-2-enone to 5,6-
diphenylbicyclo[3.1.0]hexan-2-one, there is a strong preference for formation
of the endo phenyl stereoisomer. Offer an explanation for this stereo-
selectivity.