8366
J . Org. Chem. 1998, 63, 8366-8372
Oxir a n ylca r bin yl Ra d ica ls fr om Allyloxyl Ra d ica l Cycliza tion :
Ch a r a cter iza tion a n d Kin etic In for m a tion via ESR Sp ectr oscop y1
Loris Grossi,*,† Samantha Strazzari,†,2 Bruce C. Gilbert,‡ and Adrian C. Whitwood‡
Dipartimento di Chimica Organica “A. Mangini”, Universita` di Bologna,
Viale Risorgimento 4, I-40136, Bologna, Italy, and Department of Chemistry, University of York,
Heslington, York, YO1O 5DD, England
Received J une 29, 1998
An ESR spin-trapping technique has been used for the detection of oxiranylcarbinyl radicals as
discrete reaction intermediates, formed via cyclization of allyloxyl radicals. The latter were obtained
by photolysis of the corresponding nitrites, which were generated directly via an exchange reaction
between the appropriate allylic alcohols and tert-butyl nitrite (which also acts as the spin-trap).
Experiments have also been conducted with the spin-trap 2,4,6-tribromonitrosobenzene, which has
allowed a study to be made of the competition between ring closure and â-scission of a variety of
allyloxyl species.
Sch em e 1
In tr od u ction
The formation of epoxides via a 1,3 intramolecular ring
closure of the corresponding allyloxyl radical (1) might
be expected to be a facile process,3 but in only a few
instances have the forecast products been character-
ized.4,5 The failure has been usually ascribed to the rapid
fragmentation of the intermediate oxiranylcarbinyl radi-
cal (2)5-8 by C-O bond cleavage to regenerate the
allyloxyl radical (a process that is expected to be preferred
over C-C bond cleavage, except in cases where the
resultant carbon radical is stabilized as a benzyl or an
allyl radical5,6,9-12).
acterize the oxiranylcarbinyl radical (2a ), deriving from
the 3-exo-trig closure of the 2-methylbut-3-en-2-oxyl
radical (1a ), as a discrete intermediate. The allyloxyl
radical (1a ) was conveniently generated by photolysis of
the corresponding nitrite [prepared via the facile ex-
change reaction between tert-butyl nitrite and the parent
alcohol15 (3), reaction 1]
It is perhaps not surprising that the oxiranylcarbinyl
radical itself has never been spectroscopically observed:
a rate of 3.2 × 1010 s-1 has been reported6 for its cleavage
at 25-30 °C (and a lower limit9 of 4 × 108 s-1 at 128 K).
However, Galatsis13,14 has recently described the synthe-
sis, in good yield, of R-iodo epoxides starting from tertiary
and secondary allylic alcohols and using iodine as a trap
for the intermediate oxiranylcarbinyl radical, which
implies that the ring-closed (oxiranyl) species has a
discrete lifetime, under certain circumstances.
R-OH + t-Bu-ONO h R-ONO + t-Bu-OH (1)
and the resulting oxiranylcarbinyl radical trapped16 by
t-BuONO to yield the nitroxide 4a [along with 5a ]: a
possible reaction mechanism for the formation of both
radical species is outlined in Scheme 2. The aim of the
present study was to obtain further and more detailed
evidence on the existence of oxiranylcarbinyl radicals as
intermediates in the 1,3-exo ring closure process involving
tertiary and secondary allyloxyl radicals and to obtain
information on the mechanism and the dependence on
structure of the rates of formation and reaction of these
cyclic species.
As described in a preliminary communication,1 the
spin-trapping ESR technique can be employed to char-
† Universita` di Bologna.
‡ University of York.
(1) For a preliminary account of some of this work, see: Grossi, L.;
Strazzari, S. J . Chem. Soc., Chem. Commun. 1997, 917.
(2) In partial fulfillment of the requirements for the Ph.D. degree
in Chemical Science, University of Bologna.
(3) Baldwin, J . E. J . Chem. Soc., Chem. Commun. 1976, 734.
(4) Nussbaum, A. L.; Millan, S. D.; Yuan, E.; Zagneetko, O.; Oliveto,
E. P. J . J . Am. Chem. Soc. 1962, 84, 1070.
(5) Suginome, H.; Wang, J . B. J . Chem. Soc., Chem. Commun. 1990,
1629.
(6) Krishnamurthy, V.; Rawal, V. H. J . Org. Chem. 1997, 62, 1572.
(7) Ziegler, F. E.; Petersen, A. K. J . Org. Chem. 1994, 59, 2707.
(8) Ziegler, F. E.; Petersen, A. K. J . Org. Chem. 1995, 60, 2666.
(9) Laurie, D.; Nonhebel, D. C.; Suckling, C. J .; Walton, J . C.
Tetrahedron 1993, 49, 5869.
Resu lts a n d Discu ssion
(a ) P h otolysis of Alk en yl Nitr ites in th e P r esen ce
of ter t-Bu tyl Nitr ite. (i) Resu lts. Experiments were
carried out by in situ photolysis (λ > 250 nm) in an ESR
spectrometer of acetonitrile solutions of allylic alcohols
(typically 70 mM) in the presence of an excess of tert-
butyl nitrite (ca. 115 mM), at different temperatures in
the range 233-273 K. Optimum conditions were found
to be at ca. 243 K.
(10) J ohns, A.; Murphy, J . A.; Patterson, C. W.; Wooster, N. F. J .
Chem. Soc., Chem. Commun. 1987, 1238.
(11) Stogryn, E. L.; Gianni, M. H. Tetrahedron Lett. 1970, 34, 3025.
(12) Ayral-Kaloustian, S.; Agosta, W. C. J . Org. Chem. 1983, 48,
1718.
(13) Galatsis, P.; Millan, S. D. Tetrahedron Lett. 1991, 32, 7493.
(14) Galatsis, P.; Millan, S. D.; Faber, T. J . Org. Chem. 1993, 58,
1215.
All allylic alcohols employed led to the detection of ESR
signals typified by a high value of the nitrogen hyperfine
(15) Doyle, M. P.; Terpstra, J . W.; Pickering, R. A.; LePoire, D. M.
J . Org. Chem. 1983, 48, 3379.
(16) Grossi, L. Tetrahedron 1997, 53, 3205. Grossi, L. Tetrahedron
1997, 53, 6401.
10.1021/jo9812678 CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/27/1998