- The photochemistry of some analogs of insect growth regulators
(IGR's) was studied. Methyl geranate, TY, was synthesized and the
mechanisms of its photochemical reaction pathways were studied by a
variety of techniques. Direct irradiation of 71 in ether leads to the
formation of five new products: 98, the product of a [1,3] sigmatropic
shift; 72, a bicyclo[2.1.1]hexane derivative; 73, a cylclopentane
derivative; 100, the product of in-chain deconjugation to the Z
isomer; and 97, the product of isomerization about the C-2 bond.
Irradiation in the presence of propiophenone as a triplet sensitizer
lead to the formation of only 72, 73 and 97, identifying them as
arising from the triplet excited state. Irradiation of 71 in ether
with varying amounts of base lead to the formation of (in addition to
products already mentioned) 99 and 100, the other two possible
deconjugation products. Examination of Scheme 29 leads to the
derivation of the kinetic rate expressions in Scheme 31, and plotting
the ratios of 99/73, 100/73 and 101/73 yields values for the relative
rate of [1,5] sigmatropic shift of hydrogen from a photodienol species (119, 121, 123) to regenerate starting esters (71, 97) rather than
going on to deconjugation products (99, 100, 101). It was found that
the relative rates of sigmatropic shift for 119 and 123 were nearly
identical but 121 was about 75 times slower. This decrease in the
relative rate for 121 is ascribed to the extra steric repulsion in
121, causing it to adopt the unreactive s-trans conformation and thus
slowing the reaction.
Insect juvenile hormone, 74, was synthesized and irradiated in
ether and was found to undergo photochemical processes similar to that
of 71. The primary photoprocess is isomerizatrion about the C-2 and
C-6 double bonds. Isomerization about the C-6 bond was unexpected and
indicated the intermediacy of an exciplex.
To study the role of exciplexes in the E/Z isomerizartion of
isolated double bonds in the juvenile hormone, 74, the unsymmetrically
terminated analog 92 and the dihydro analog 93 were synthesized.
Irradiation of 92 and 93 in solvents of varying polarity (ET)
lead to an increase of the quantum yield of E/Z isomerization for both
92 and 93, indicating the intermediacy of an intermolecular
exciplex. to study the role of intramolecular exciplexes 92 and 93
were irradiated in ether at increasing dilution (0.1 M - 0.0008 M).
It was found that the intermolecular exciplex is the major contributor
in the concentration range studied without any evidence for an