Bietti et al.
JOCArticle
The factors governing the formation of phosphoranyl
radicals by reaction of alkoxyl radicals with trivalent orga-
nophosphorus compounds as well as their follow-up frag-
mentation reactions have been discussed in detail in a
number of reviews,2,8-10 and useful synthetic applications
based on these processes have been described.11-13 Despite
this interest, limited direct kinetic information is presently
available on the reactions described in Scheme 1, with the
available data being essentially limited to the reactions of
the tert-butoxyl radical,3b,c,4a,14-16 emphasizing in most
cases the decay kinetics of the intermediate phosphoranyl
radicals.2,3b,c,4a,14,16 In particular, these data refer to phos-
phoranyl radicals characterized by significantly different
structures,2 a feature that does not allow a clear comprehen-
sion of the role of structural effects on these processes. Along
this line, it seemed very interesting to obtain additional
information on these processes, in particular, for what
concerns the role of structural effects on both the attacking
radical and the phosphorus compound, and for this pur-
pose we have carried out a time-resolved kinetic study on
the reactions of the cumyloxyl (CumO•) and benzyloxyl
(BnO•) radicals with a series of structurally related trialkyl
phosphites ((RO)3P: R = Me, Et, i-Pr, t-Bu) and on the
β-fragmentation reactions of the intermediate tetraalkoxy-
phosphoranyl radicals thus formed.
Another point of interest is represented by the fact that
triaryl phosphites have found value as antioxidant additives
to polymers17 and provide in particular a convenient method
for the generation of phenoxyl radicals via homolytic
substitution promoted by alkoxyl radicals (Scheme 1, path
a, L = OAr).2,3a,5b,18,19 To our knowledge, however, no
absolute measurement of the rate constants for reaction
of alkoxyl radicals with triaryl phosphites is presently
available. Accordingly, we have considered it of interest
to study also the reactions of CumO• with triphenyl and
tris(2,4-di-tert-butylphenyl) phosphite ((ArO)3P: Ar =
C6H5, 2,4-(t-Bu)2C6H3).20
MeCN solutions at T = 25 °C containing di-tert-butyl,
dicumyl, and dibenzyl peroxide, respectively (eq 1).
In some experiments, CumO• has also been generated by
355 nm LFP of nitrogen-saturated MeCN, benzene, chlor-
obenzene, or CCl4 solutions at T=25 °C containing dicumyl
peroxide.
As described previously, in MeCN solution t-BuO• is
characterized by a relatively weak UV band centered at
280 nm,21,22 while CumO• and BnO• are characterized by
a broad absorption band in the visible region of the spec-
trum centered at 485 and 460 nm, respectively.22,23 Under
these conditions, t-BuO• and CumO• decay mainly by
C-CH3 β-scission,21,22,24 while the decay of BnO• can be
mainly attributed to hydrogen atom abstraction from the
solvent.25,26
Reactions with Trialkyl Phosphites (RO)3P. The reactions
of CumO• and BnO• with trialkyl phosphites have been
studied by LFP. Figure 1 shows the time-resolved absorption
spectra observed after 266 nm LFP of an argon-saturated
MeCN solution (T = 25 °C) containing dicumyl peroxide
10 mM and triisopropyl phosphite ((i-PrO)3P) 1.15 mM.
The time-resolved spectrum recorded 64 ns after the laser
pulse (filled circles) shows the characteristic cumyloxyl radi-
cal absorption band centered at 485 nm.22,23 The decay of
this band (inset b), which is accelerated by the presence of
(i-PrO)3P, is accompanied by the formation of a species
(384 ns, empty circles) characterized by an absorption band
that extends from 340 nm toward the UV region, that, by
comparison with literature data,15 is assigned to the tetra-
alkoxyphosphoranyl radical, CumOP•(Oi-Pr)3, formed fol-
lowing addition of CumO• to the phosphorus center as
described in Scheme 2, path a. This radical in turn decays
(inset a) leading to a stable species assigned to triisopropyl
phosphate OdP(O-i-Pr)3, formed after Cum-O β-scission in
the intermediate tetraalkoxyphosphoranyl radical (Scheme 2,
path b).27
Results and Discussion
t-BuO•, CumO•, and BnO• have been generated by 266 nm
laser flash photolysis (LFP) of argon- or nitrogen-saturated
(8) Marque, S.; Tordo, P. Top. Curr. Chem. 2005, 250, 43–76.
(9) Bentrude, W. G. Acc. Chem. Res. 1982, 15, 117–125.
(10) Roberts, B. P. In Advances in Free Radical Chemistry; Williams,
G. H., Ed.; Heyden: London, 1980; Vol. 6, pp 225-289.
^
(11) Leca, D.; Fensterbank, L.; Lacote, E.; Malacria, M. Chem. Soc. Rev.
2005, 34, 858–865.
(12) Zhang, L.; Koreeda, M. J. Am. Chem. Soc. 2004, 126, 13190–13191.
(13) Kim, S.; Oh, D. H. Synlett 1998, 525–527.
(14) Baban, J. A.; Goddard, J. P.; Roberts, B. P. J. Chem. Soc., Perkin
Trans. 2 1986, 1269–1274.
An analogous behavior has been also observed in the
reactions of CumO• and BnO• with the other trialkyl phos-
phites, where in all cases the decay of the alkoxyl radical
visible absorption band is accompanied by the buildup of the
pertinent tetraalkoxylphosphoranyl radical band in the UV
(21) Tsentalovich, Y. P.; Kulik, L. V.; Gritsan, N. P.; Yurkovskaya, A. V.
J. Phys. Chem. A 1998, 102, 7975–7980.
(15) Roberts, B. P.; Scaiano, J. C. J. Chem. Soc., Perkin Trans. 2 1981,
905–911.
(22) Baciocchi, E.; Bietti, M.; Salamone, M.; Steenken, S. J. Org. Chem.
2002, 67, 2266–2270.
(16) Griller, D.; Ingold, K. U.; Patterson, L. K.; Scaiano, J. C.; Small,
R. D., Jr. J. Am. Chem. Soc. 1979, 101, 3780–3785.
ꢁ
´
(17) See, for example: Habicher, W. D.; Bauer, I.; Pospısil, J. Macromol.
Symp. 2005, 225, 147–164. Naskar, K.; Kokot, D.; Noordermeer, J. W. M.
Polym. Degrad. Stab. 2004, 85, 831–839. Lucarini, M.; Pedulli, G. F.;
Lazzari, D.; Vitali, M.; Andrews, S. M. Macromol. Chem. Phys. 2002, 203,
2239–2244. Haider, N.; Karlsson, S. J. Appl. Polym. Sci. 2002, 85, 974–988.
Ismail, M. N.; Yehia, A. A.; Korium, A. A. J. Appl. Polym. Sci. 2002, 83,
2984–2992.
(18) Arends, I. W. C. E.; Mulder, P.; Clark, K. B.; Wayner, D. D. M.
J. Phys. Chem. 1995, 99, 8182–8189.
(19) Bentrude, W. G. Tetrahedron Lett. 1965, 6, 3543–3548.
(20) Tris(2,4-Di-tert-butylphenyl) phosphite is available under the com-
mercial name Irgafos 168.
(23) (a) Avila, D. V.; Ingold, K. U.; Di Nardo, A. A.; Zerbetto, F.;
Zgierski, M. Z.; Lusztyk, J. J. Am. Chem. Soc. 1995, 117, 2711–2718.
(b) Avila, D. V.; Lusztyk, J.; Ingold, K. U. J. Am. Chem. Soc. 1992, 114,
6576–6577.
(24) Avila, D. V.; Brown, C. E.; Ingold, K. U.; Lusztyk, J. J. Am. Chem.
Soc. 1993, 115, 466–470.
(25) Konya, K. G.; Paul, T.; Lin, S.; Lusztyk, J.; Ingold, K. U. J. Am.
Chem. Soc. 2000, 122, 7518–7527.
(26) BnO• undergoes a rapid 1,2-H-atom shift reaction in water and
alcohols (see ref 25). Accordingly, in MeCN the decay of this radical can
be accelerated by the presence of small amounts of water.
(27) A contribution to the residual absorbance also derives from the
cumyl radical, which accordingly is known to display an absorption at λ e
340 nm (see ref 28).
J. Org. Chem. Vol. 75, No. 13, 2010 4515