Table 1 Second-order rate constants for the reaction of BTNO radical
with 4-X-C6H4CH2OH in MeCN at 25 uC (followed at 474 nm)
X
K/M21 s21
NO2
CF3
Cl
H
Ph
1.24
1.18
1.93
1.87
4.35
3.80
7.80
CH3
CH3O
Both the r and the kH/kD values obtained here compare very
favourably with those (i.e., 20.64 and 6.4, respectively) obtained in
the oxidation of substituted benzyl alcohols with the laccase/HBT
system (Scheme 1),11 giving clear-cut support to the formation of
the wN–O. radical as the Medox species in those mediated
oxidation procedures. An extensive kinetic study of H-abstraction
reactivity with BTNO, to compare with available data for PINO,5,6
as well as attempts to determine the O–H bond energy of HBT, to
compare with that of HPI (BDEOH 88.1 kcal mol21),4 in view of
the expected relevance of this parameter upon the relative reactivity
of the two aminoxyl radicals, will be reported in a future full paper.
Fig. 2 Absorption spectrum obtained after 355 nm laser flash photolysis of
an Ar-saturated MeCN solution containing 1.0 M dicumyl peroxide and
0.03 M HBT, recorded 1.4 ms after the laser flash.
Notes and references
1 For a recent review, see:, Adv. Synth. Catal., 2004, 346 (Special Issue:
Oxidations), 107–375.
2 Y. Ishii, K. Nakayama, M. Takeno, S. Sakaguchi, T. Iwahama and
Y. Nishiyama, J. Org. Chem., 1995, 60, 3934; Y. Ishii, S. Sakaguchi and
T. Iwahama, Adv. Synth. Catal., 2001, 343, 393.
3 F. Minisci, C. Punta, F. Recupero, F. Fontana and G. F. Pedulli, J. Org.
Chem., 2002, 67, 2671.
4 R. Amorati, M. Lucarini, V. Mugnaini, G. F. Pedulli, F. Minisci,
F. Recupero, F. Fontana, P. Astolfi and L. Greci, J. Org. Chem., 2003,
68, 1747; K. Nobuyoshi, Y. Cai and J. H. Espenson, J. Phys. Chem. A,
2003, 107, 4262; F. Minisci, F. Recupero, A. Cecchetto, C. Gambarotti,
C. Punta, R. Faletti, R. Paganelli and G. F. Pedulli, Eur. J. Org. Chem.,
2004, 109.
5 K. Nobuyoshi, B. Saha and J. H. Espenson, J. Org. Chem., 2003, 68,
9364.
6 C. Annunziatini, M. F. Gerini, O. Lanzalunga and M. Lucarini, J. Org.
Chem., 2004, 69, 3431.
7 L. Eberson, Electron Transfer Reactions in Organic Chemistry, Springer-
Verlag, Berlin, 1987.
Fig. 3 Experimental EPR spectrum of BTNO obtained at room
temperature by reaction of CAN (20 mM) with HBT (20 mM) in
MeCN (a), and its computer simulation (b).
8 T. Iwahama, Y. Yoshima, T. Keitoku, S. Sakaguchi and Y. Ishii, J. Org.
Chem., 2000, 65, 6502; F. Minisci, C. Punta, F. Recupero, F. Fontana
and G. F. Pedulli, Chem. Commun., 2002, 688; A. Cecchetto, F. Minisci,
F. Recupero, F. Fontana and G. F. Pedulli, Tetrahedron Lett., 2002, 43,
3605.
parent alcohols (eqn. (3)).
BTNOzArCH2OH DCCA
ꢀHBT
(3)
O2
ArCHð?ÞOH DCCA ArCHO
9 Y. Ishii, T. Iwahama, S. Sakaguchi, K. Nakayama and Y. Nishiyama,
J. Org. Chem., 1996, 61, 4520.
10 B. J. Sealey, A. J. Ragauskas and T. J. Elder, Holzforschung, 1999, 53,
498.
A Hammett analysis of substituents effect on the reaction rate
gave a better correlation (r ~ 20.55) with the s1 than with the s
parameters, in keeping with the known electrophilic character of
wN–O. radicals.3,6,11 The intramolecular kinetic isotope effect was
also studied for the oxidation of a suitable a-monodeuterated
benzyl alcohol (ArCHDOH), and the kH/kD ratio reckoned as 5.6
from mass spectrometric determination of the relative amount of
the two aldehydes (ArCDO and ArCHO) produced. The small
r value obtained confirms the radical nature of this oxidation
procedure, and the large kH/kD ratio supports a rate determining
H-abstraction step.
11 F. D’Acunzo, P. Baiocco, M. Fabbrini, C. Galli and P. Gentili, New
J. Chem., 2002, 26, 1791; P. Baiocco, A. M. Barreca, M. Fabbrini,
C. Galli and P. Gentili, Org. Biomol Chem., 2003, 1, 191.
12 F. Xu, J. J. Kulys, K. Duke, K. Li, K. Krikstopaitis, H.-J. W. Deussen,
E. Abbate, V. Galinyte and P. Schneider, Appl. Environ. Microbiol.,
2000, 66, 2052.
13 M. Fabbrini, C. Galli and P. Gentili, J. Mol. Catal. B: Enzym, 2002, 16,
231.
14 G. Prabhakar Rao and A. R. Vasudeva Murthy, J. Phys. Chem., 1964,
68, 1573.
C h e m . C o m m u n . , 2 0 0 4 , 2 3 5 6 – 2 3 5 7
2 3 5 7