5
670
J. Dolenc et al. / Tetrahedron Letters 43 (2002) 5669–5671
The presence of yellow oligomeric products and guaiacol
in the reaction mixture was established by the use of GPC
dissociation energy is 320 kJ/mol. Upon homolytic bond
cleavage, a highly delocalized allylic radical and a phen-
oxyl radical are formed. Whereas subsequent reactions
of the allylic radicals lead to the formation of oligomeric
products, the phenoxyl radical can be further transformed
to guaiacol by the abstraction of a hydrogen atom from
the solvent (Scheme 2). In the presence of oxygen, the
reaction of the allylic radical with oxygen can also
proceed, leading to peroxyl radicals. In solutions of
tetrahydrofuran, which is a good hydrogen atom donor,
abstraction of hydrogen atoms occurs, leading to forma-
tion of hydroperoxide 6, the presence of which was shown
by an iodide test. Under photochemical conditions, the
hydroperoxides are transformed by OꢀO bond cleavage
into alkoxyl and hydroxyl radicals. The latter have a high
oxidation potential and can oxidize guaiacol or the
phenoxyl radical to a mixture of oxidation products.
(
usingpolystyreneasreference), UV–vis spectrophotome-
1 13
try, H and C NMR spectroscopy and FAB mass
spectrometry. GPC analyses (Fig. 1) of the oligomeric
materials showed a broad peak extending from a retention
time of 17 min (corresponding to oligomers with 16 units)
to 21 min (corresponding to trimeric product).
The UV–vis spectra of the oligomers show, beside the
absorption in the UV region, an absorption at about 420
nm, which confers the colour of these materials. To gain
an understanding of the mechanism of photolysis, the rate
of disappearance of quinone methide and the rate of the
formation of guaiacol 5 were determined and the effect
of the quinone methide concentration on the rate of the
reaction was studied. As evident from Table 1, an increase
in quinone methide concentration results in a higher rate
constant for its disappearance.
These suggestions were further confirmed by a series of
observations. Comparative irradiation experiments were
performed in air (Table 1, entry 4) and in solutions
saturated with oxygen (Table 1, entry 5). Although the
rate constants for quinone methide disappearance are
comparable in both series of experiments, the rate
constant for guaiacol formation was much lower in the
case of irradiation of the solution saturated with oxygen.
This can be explained by a much faster oxidation of the
initially formed guaiacol, when the concentrations of
oxygen in solution are higher.
In light of the fact that all of the preceding reactions are
first order, the above observations suggest that the
initially formed intermediates act as photoinitiators. The
rate constants measured for the disappearance of the
starting compound are slightly higher than those of the
guaiacol formation, which can be explained by a further
oxidation of guaiacol. On the basis of the results pre-
sented, the mechanism shown in Scheme 2 is suggested.
Primarily, the homolytic CꢀO bond cleavage between the
allylic carbon atom and the oxygen atom of the aromatic
ether moiety in the excited state of the molecule of the
quinone methide occurs. This is supported by the fact,
that the energy of excitation is 330 kJ/mol, while the bond
Furthermore, in the latter case, the iodide tests showed
a much higher concentration of peroxides than was
observed in experiments run in air (Table 1, entry 4).
Figure 1. GPC chromatogram of an irradiated solution of 1. Polystyrene as reference. tR 20 min represents 1000 g/mol; tR 17 min
represents 3000 g/mol.
Table 1. Rate constants for the disappearance of quinone methide and formation of guaiacol as a function of initial
concentration (c ) of the quinone methide
0
−
7
k (quiacol) 10−7 mol/Ls
Entry
c0 (mmol/L)
k (quinone methide) 10
mol/Ls
1
2
3
4
5
1.2
2.5
6.8
18.3
1.82
2.67
2.93
7.66
8.23
1.45
2.13
2.30
3.97
0.85
18.3 under oxygen
Dolenc, Jozica
