RC=O
R
I c=o
CHsO
OCHs
oxidant mlvent
OCH,
CH30
0
OH
l,R=H 2, R = CHs 3, R = C2Ha
I
5 GAUSS
Figure 1. Oxidation of model lignin phenols. Oxidants: (a) 0.3 M K3Fe(CN)6in 2 M aqueous KOH; (b) 0.1 M Ce(HSO& in 1 M aqueous H2S04;(c) Pb02, solid. Solvents: benzene, toluene, and dichloromethane.
lines. The propiosyringone radical (3) yields 17 lines, with intensities closely paralleling those predicted from the constants in Table I ( e . g . , 1:6:2:15:12:21:30:21: 40:21:30:21:12:15:2:6:1); here, the meta proton coupling is 1.5 times that of the methoxyl proton. Theoretical spectra, calculated for each of the above, confirmed these assignments6 The values for the Table 1. Coupling Constants for Syringol Radicals Syringol radical
No. of lines
gauss
1 2 3
9 9 17
1.30 1.30 1.30
AHma**, gauss 1.30 1.45 1.95
methoxyl and meta proton coupling constants are consistent with previously published value^.^^^ The visible absorption spectra of all three radical species in solution are almost identical; principal bands appear at 420, 450,470, and 750 mp. Over a period of time, the 750 mp band height decreases while the 470 mp band height increases. A plot of the log height of the 750 mp band vs. time is parallel to the plot of the log intensity of the e.s.r. signal vs. time, thus indicating that the radical species is represented by the green 750 mp band. A number of conclusions may be drawn from these results: (a) The stability of the phenoxy radical is dependent on the presence of two o-CH30 substituents, as well as a p-C=O function. Guaiacol derivatives, with a free ortho position, apparently dimerize or oxidize rapidly in preference to forming stable phenoxy radicals. (b) The stability of the radical from syringol is remarkable, in light of the fact that the vast majority of previously prepared phenoxyls have been substituted in the 2- and 6-positions by bulky groups, such as tbutyl. Mueller, however, did inferg that lignin model phenoxyls might be stable when he obtained a poor single line e.s.r. spectrum for a syringylacrylonitrile derivative. (c) Apparently, minor changes in the side chains of 1-3 (R = H, CH,, C2HJ do affect the electron density at the two mela positions, giving rise to the different e.s.r. splitting patterns. (d) The slow decay (6) We are indebted to Professor John Harriman of the Department of Chemistry, University of Wisconsin, for these computations. (7) T. J. Stone and W. A. Waters, J . Chem. Soc., 4302 (1964). (8) E. Mueller, H. Kaufmann, and A. Rieker, Ann., 671, 61 (1964). (9) E. Mueller, R. Mayer, H. D . Spanagel, and K. Scheffler, ibid., 645, 61 (1961).
-
5 GAUSS
5 GAUSS
Figure 2. First derivative e.s.r. spectra of radicals 1, 2, and 3.
of these radicals to red species (probably o-quinoneslO) is of significance to lignin chemistry. Oxidation of lignins by biological systems or aqueous chemical systems could produce colored decay products, via intermediates such as 1, 2, and 3. These radical intermediates are stabilized temporarily by solvents such as benzene, or they may be indefinitely stabilized in the solid lignin matrix, thus contributing to the higher spin concentrations observed in solid, oxidized lignins. 3, The a-carbonyl syringyl moiety appears to be an inherent part of native lignin" and oxidized lignin1* structures. Therefore, these well-characterized radicals from syringol constitute excellent models for a study of the course of dehydrogenation reactions of lignins. Such a study is currently in progress. Acknowledgments. We are deeply grateful to Professor John Willard of the Department of Chemistry, University of Wisconsin, for the use of a Varian 4500 spectrometer (100 kc. field modulation), and to Professor John Harriman of the same department for his many helpful discussions of the e.s.r. spectra. We are also indebted to Mr. J. C. Pew of the Forest Products Laboratory for providing some of the model phenols used in this study. (10) E. Adler, Paperi Puu, No. 11, 634 (1961). (11) K. Freudenberg, Holzforsch., 18, 3 (1964). (12) J . Marton, Tappi, 47, 713 (1964). (13) On sabbatical leave from the University of Arizona until Aug. 1965.
Cornelius Steelink' Forest Products Laboratory Forest Service, U.S . Department of Agriculture Madison, Wisconsin Received March 11, 1965
Magnetic Peroxyborates. The Pseudo-Superoxides Sir.
We report that certain peroxyborates are paramagnetic; both static susceptibility and electron spin resonance ( e m . ) measurements show many unpaired spins. To our knowledge there is no previous suggestion that peroxyborates could be paramagnetic. Foerster converted, by careful heating, sodium perborate monohydrate2 to a material which briskly evolved oxygen on being placed in water; hydrogen peroxide is simultaneously formed. Menze13 and others4 prepared similar perborate materials. F.Foerster, Z. angew. Chem., 34, 354 (1921). (2) Names and chemical symbols are those employed by peroxide chemists. They state only stoichiometry; structural implications are absent. (3) H. Menze1.Z. anorg. allgem. Chem., 167, 193 (1927). (4) T. I. Taylor and G.G.Taylor, Ind. Eng. Chem., 27, 672 (1935); (1)
Communications to the Editor
2057
I .
. . . . ...........
-..-
' '-
~
z
. . . .
.
-
42..
..
Figure 1. The derivative curve of the e.s.r. absorption in preparation A-3 is shown as a function of magnetic field for three temperatures: A , 3M)"K.; B, 220°K.: C, 11°K. Instrument parameters were identical for the three temperatures. The left side of the figure is ~ 1 gauss 0 and the right side 5300 gauss, and the field sweep was approximately linear. In A the broad resonance is denoted by X and the narrow one Y. In B and C the additional absorption is labeled the Z resonance.
Two series of preparations have been studied; all samples are off-white powders which effervesce in water. In Table I, data on some preparations are presented. Table I. Some Roperties of the Modified Sodium F'eroxyborales' Prepn. no. A-l A-2 A-3
01,"
Z
2.2 4.1 8.4
A.O.,'
1.1 8. I 6.8
Z
x . ( X I O ~ ) ~ C.F.W.' 1.49 3.22 5.10
88.3 89.4 92.4
References 1, 3, and 4 disclose standard methods for preparing these materials. Calculated from gaseous oxygen released on solution in water. 0 Per cent A.O. (active oxygen) calculated from H a 2 (I- analysis) released on solution of water. * Calculated from data at 25" using the equation xa = X.
