Photochemistry of Acyl-Alkyl Biradicals

Full text of DOI:10.1021/jo990535e

3
802
J . Org. Chem. 1999, 64, 3802-3803
P h otoch em istr y of Acyl-Alk yl Bir a d ica ls
Ta ble 1. P r od u ct Distr ibu tion Obta in ed u p on La ser
Ir r a d ia tion of Keton es 1a -c
Miguel A. Miranda,*,†,‡ Enrique Font-Sanchis,† and
J ulia P e´ rez-Prieto*,§
product distribution (%)
primary secondary Norrish II
two-photon
a
ketone
enals
products
products
Instituto de Tecnolog ´ı a Qu ´ı mica UPV-CSIC/ Departamento de
Qu ı´ mica, Universidad Polit e´ cnica de Valencia, Camino de Vera
s/ n, Valencia, 46071 Spain, and Departamento de Qu ´ı mica
Org a´ nica, Facultad de Farmacia, Universidad de Valencia,
Vic e´ nt Andr e´ s Estell e´ s s/ n, Burjassot, 46100 Valencia, Spain
1a
1b
1c
3a (47)
3b (29)
3c (46)
6 (12), 9 (27), 12 (12)b
7 (34), 10 (4), 13 (4)
8 (20), 11 (8), 14 (8)
4b (27)
4c (18)
a
The structural assignment of photoproducts was confirmed by
unambiguous synthesis, using well-established methods,
comparison with available authentic samples. Part of styrene could
have been evaporated when rotoeavaporating the photolyzate
before analysis.
9
-15
or by
Received March 29, 1999
Acyl-alkyl biradicals are produced upon (n,π*) excitation
of cyclic ketones, followed by R-cleavage.1 - Their transient
absorption spectra are practically coincident with those of
the corresponding alkyl radicals. Thus, for the series of
,2
Sch em e 1
2
-phenylcycloalkanones, the spectra of the resulting biradi-
cals compare very well with those typical of benzyl radi-
cals.3 - In the case of 2,2-dialkylcycloalkanones, the
biradicals possess a tertiary radical center, which leads to
a weak absorption maximum at 320 nm,4,5 similar to that
,4
-
1
of the tert-butyl radical (λmax ) 307 nm, ꢀmax ) 200 M
-
1
cm ). These substitution patterns have facilitated detec-
tion of the intermediates since acyl radicals do not absorb
at 300-320 nm.
Despite the considerable efforts devoted to the study of
alkyl-acyl biradicals, due to their wide occurrence and
general interest, it seems surprising that the photobehavior
of these short-lived species has not been studied yet. Such
species are generated almost “instantaneously” when ir-
radiating the corresponding cyloalkanone at room temper-
ature;2
,4,6
however, two-laser two-color flash photolysis
experiments could be difficult to carry out due to the
relatively short lifetimes (10-100 ns) of these intermedi-
2
ates. In view of the existing interest for establishing the
photochemistry of short-lived intermediates via two-photon
7
processes, we decided to photolyze biradicals 2 (n ) 4-6),
since they possess a benzyl radical terminus strongly
-
1
-1
absorbing at 258 nm (ꢀmax ) 31.400 M cm ) which could
allow their excitation using a 266 nm laser, the same that
could be suitable for their generation from the precursor
cycloalkanones.4
In this Communication we report a comparative study of
the chemistry and photochemistry of acyl-alkyl biradicals
2
in cyclohexane. Thus, the products obtained in the low-
intensity irradiation (medium-pressure mercury, quartz, or
Pyrex filter, 30 min) of deaerated 10 mM cyclohexane
solution of ketones 1a -c were compared with those observed
upon laser excitation [Nd:YAG, 266 nm, 2200 pulses] of 1
mM cyclohexane solution of the ketones. Our results re-
vealed that acyl-alkyl biradicals are photolyzed within the
laser pulse (10 ns) giving rise to new products arising from
†
Universidad Polit e´ cnica de Valencia.
Tel.: 34-6-3877807. Fax: 34-6-3877809. E-mail: mmiranda@qim.upv.es.
Universidad de Valencia. Tel: 34-6-3864934. Fax: 34-6-3864939.
‡
§
E-mail: jperez@uv.es.
dialkyl biradicals.
(1) Turro, N. J . Modern Molecular Photochemistry; Benjamin/Cum-
It has been previously reported2,6,8 that lamp irradiation
mings: Menlo Park, 1978; Chapter 13. Weiss, D. S. Organic Photochemistry;
Padwa, A., Ed.; Marcel Dekker: New York, 1981; Vol. 5, Chapter 4, pp 347-
(λ ) 313 nm) of ketones 1a -c leads to unsaturated alde-
4
20.
hydes 3a -c probably formed from the primary acyl-alkyl
biradicals 2a -c through an H abstraction process. In the
present work we have confirmed these observations; how-
ever, we have observed that under our reaction conditions
(2) J ohnston, L. J .; Scaiano, J . C. Chem. Rev. 1989, 89, 521-547.
Caldwell, R. A.; Sakuragi, H.; Majima, T. J . Am. Chem. Soc. 1984, 106,
2
1
471-2473. Doubleday, C., J r.; Turro, N. J .; Wang, J .-F. Acc. Chem. Res.
989, 22, 199-205.
(
3) Zimmt, M. B.; Doubleday, C., J r.; Turro, N. J . J . Am. Chem. Soc. 1985,
(either Pyrex or quartz filter) 1b,c led to the expected enals
1
07, 6724-6726.
and also to dienes 4b,c, the Norrish type II secondary
products of 3b,c. GC/MS analysis showed that not even
traces of other products were formed.
By contrast, analysis of the photolyzate obtained after
laser irradiation of ketone 1a indicated that significant
(4) Huggenberger, C.; Fischer, H. Helv. Chim. Acta 1981, 64, 338-353.
(5) Weir, D.; Scaiano, J . C. Chem. Phys. Lett. 1985, 118, 526-529.
(6) Wagner, P. J .; Stratton, T. J . Tetrahedron 1981, 37, 3317-3322.
(7) Adam, W.; Denninger, V.; Finzel, R.; Kita, F.; Platsch, H.; Walter,
H.; Zang, G. J . Am. Chem. Soc. 1992, 114, 5027-5035. Ouchi, A.; Koga, Y.;
Adam, W. J . Am. Chem. Soc. 1997, 119, 592-599. Scaiano, J . C.; J ohnston,
L. J .; McGimpsey, W. G.; Weir, D. Acc. Chem. Res. 1988, 21, 22-29.
Redmond, R. W.; Scaiano, J . C.; J ohnston, L. J . J . Am. Chem. Soc. 1990,
1
12, 398-402.
(8) Baum, A. A. Tetrahedron Lett. 1972, 1817-1820.
1
0.1021/jo990535e CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/30/1999

Communications
J . Org. Chem., Vol. 64, No. 11, 1999 3803
amounts of phenylcyclobutane (6), tetraline (9), and styrene
length is the acyl or the benzyl radical site. Taking into
account the much different extinction coefficients, it seems
highly probable that the biradical 5 is generated via light
absorption by the benzyl radical, followed by an intra-
molecular energy transfer process.
(12) were also generated. In the case of ketone 1b (n ) 5),
phenylcyclopentane (7) and minor amounts of 5-phenyl-1-
pentene (10) and 1-phenyl-1-pentene (13) were found in the
irradiated sample (Table 1). Finally, laser irradiation of
ketone 1c (n ) 6) led to cyclohexylbenzene (8) and minor
amounts of 6-phenyl-1-hexene (11) and 1-phenyl-1-hexene
Ack n ow led gm en t. Financial support by the Spanish
DGES (Project PB97-0339) is gratefully acknowledged.
E.F.S. also thanks the Spanish Ministry of Education for
a grant.
(14) as the new products (Table 1).
In all laser experiments the amounts of products arising
from irradiation of (thermally) unreactive photoproducts
were minimized by keeping the conversion of starting ketone
low (less than 25%).
A satisfactory rationalization of the above results can be
found in Scheme 1. Laser excitation of ketones 1 agrees with
the photodecarbonylation of acyl-alkyl biradical 2 during
the 266 nm laser pulse, leading to dialkyl biradical 5. Its
J O990535E
(10) Alcaraz, L.; Harnett, J . J .; Mioskowski, C.; Martel, J . P.; Le Gall,
T.; Shin, D. S.; Falck, J . P. Tetrahedron Lett. 1994, 35, 5453-5456.
(
11) Gamage, S. A.; Smith, R. A. J . Tetrahedron 1990, 46, 2111-2128.
cyclization or disproportionation generates the photoprod-
ucts 6-14,9
-15
(12) Belluci, G.; Chiappe, C.; Cordoni, A. Tetrahedron: Asymmetry 1996,
, 197-202.
(13) Shade, P.; Sch a´ fer, T.; M u´ llen, K.; Bender, D.; Knoll, K.; Bronstert,
- which are the fingerprints for two-photon
7
processes. A final point of interest is whether the absorbing
K. Chem. Ber. 1991, 124, 2833-2841.
chromophore in biradicals 2 at the photolysis laser wave-
(14) Hudson, C. M.; Marzabadi, M. R.; Moeller, K. D.; New, D. G. J . Am.
Chem. Soc. 1991, 113, 7372-7385.
(
9) Holland, H. L.; Kindermann, M.; Kumaresan, S.; Stefanac, T.
(15) Cal o´ , V.; Lopez, L.; Nacci, A.; Mele, G. Tetrahedron 1995, 51, 8935-
Tetrahedron: Asymmetry 1993, 4, 1353-1364.
8940.