and â-cleavage. The experimental rate constant for the
growth is given by eq 3 in Scheme 3.
Scheme 2. Lactones Examined
The rate constants determined for representative substrates
by using the approach of eq 3 are given in Table 1. Of
Table 1. Rate Constants for the Reactions of tert-Butoxyl
Radicals with Hydrogen Donors and Radical Reactivity toward
Oxygen
rate constant
(106 M-1 s-1
)
radical reacts
with oxygen?a
substrate
toluene10
0.23
0.91
12.4
15.2
18.4
84
yes
yes
no
no
no
no
no
yes
diphenylmethane13
HP-136
1
2
3
4
6
51
5.0
Remarkably, the radical derived from HP-136 (Scheme 2)
is essentially unreactive toward oxygen.8 From a spectro-
scopic point of view, the HP-136 radical is virtually identical
to the diphenylmethyl radical, which reacts with oxygen with
a rate constant of 6.3 × 108 M-1 s-1 in cyclohexane at room
temperature.9
This work was undertaken in an attempt to understand the
structural parameters that make some lactone-derived carbon-
centered radicals unreactive toward oxygen. To this effect
we have examined the lactones in Scheme 2.
As part of this work, we have also examined the reactivity
of various substrates toward alkoxyl radicals. This provides
a comparison of their hydrogen donor ability on the basis of
absolute rate constants using established laser flash photolysis
methods.
The reactivity of the substrates toward alkoxyl radicals
was determined by studying their reaction with tert-butoxyl
radicals using laser flash photolysis techniques.10 The radicals
produced in these reactions have convenient absorptions that
can be used to monitor their formation directly, a somewhat
easier approach than the probe technique that can be used
in the study of these reactions.10 The tert-butoxyl radicals
were produced by laser excitation (308 or 355 nm) of the
peroxide in benzene (reaction 1 in Scheme 3). The growth
a Based of laser flash photolysis work in a 100 µs time scale.
particular interest is the unusually high reactivity of the
2-coumaranone series on comparison with other benzylic
C-H bonds.
It seemed reasonable to assume that the ring substituents
in HP-136 could not play any major role in the lack of
reactivity of the radical toward oxygen. In fact, if one
assumes that part of the role of the lactone moiety relates to
its electron-withdrawing characteristics, then alkyl substit-
uents could only neutralize in part this effect. In any event,
2 contains the basic framework of HP-136, but with no ring
substituents. The same lack of reactivity of the radical toward
oxygen was observed for 2, as already reported for HP-136.
The signal amplitudes were the same under oxygen or
nitrogen, and the spectrum was virtually identical to that
reported for the HP-136 radical. In fact, the absorption
spectrum of the radical from 2 was already available in the
literature.11,12
The photolysis of di-tert-butyl peroxide in the presence
of 2-coumaranone 4 shows the formation of the radical
shown in Figure 1. The visible band (350-450 nm) is far
•
more intense than in the case of PhCH2 (420-470 nm);14
the enhancement due to the presence of heteroatoms is well
documented for a range of related radicals.15 For example,
the ketyl radical from acetophenone and related Norrish type
II biradicals have absorption bands in the 420 nm region,
with extinction coefficients approaching 1000 M-1 cm-1.16
In an oxygen-saturated solution, the rate of decay of the
2-coumaranone radical (7) was essentially the same as under
Scheme 3. H-Abstraction by tert-Butoxyl Radicals
(10) Paul, H.; Small, R. D.; Scaiano, J. C. J. Am. Chem. Soc. 1978, 100,
4520.
(11) Lohray, B. B.; Kumar, C. V.; Das, P. K.; George, M. V. J. Am.
Chem. Soc. 1984, 106, 7352.
(12) Davis, H. F.; Lohray, B. B.; Gopidas, K. R.; Kumar, C. V.; Das, P.
K.; George, M. V. J. Org. Chem. 1985, 50, 3685.
(13) Arends, I. W. C. E.; Mulder, P.; Clark, K. B.; Wayner, D. D. M. J.
Phys. Chem. 1995, 99, 8182.
of the radical signal reflects reaction 2 (k2) and other forms
of decay of tert-butoxyl (k0), such as reaction with the solvent
(8) Scaiano, J. C.; Martin, A.; Yap, G. P. A.; Ingold, K. U. Org. Lett.
2000, 2, 899.
(9) Scaiano, J. C.; Tanner, M.; Weir, D. J. Am. Chem. Soc. 1985, 107,
4396.
(14) Porter, G.; Strachan, E. Spectrochim. Acta 1958, 12, 299.
(15) Chatgilialoglu, C. Electronic Absorption Spectra of Free Radicals;
Scaiano, J. C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol. II, p 3.
(16) Lutz, H.; Bre´he´ret, E.; Lindqvist, L. J. Phys. Chem. 1973, 77, 1758.
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