Two-photon generation of the 1,4-diphenyl-1,4-butanediyl biradical: Direct detection and product studies

Article abstract of DOI:10.1021/jo990872n

The 1,4-diphenyl-1,4-butanediyl biradical (3) is generated from 1,4- dichloro-1,4-diphenylbutane (1) or 2,5-diphenylcyclopentanone (8) under several irradiation conditions. The products resulting from this intermediate are styrene (4), 1,2-diphenylcyclobutane (5), and 1-phenyl-1,2,3,4- tetrahydronaphthalene (6). The formation of tetrahydronaphthalenes appears to be a fingerprint for the intermediacy of 1,4-biradicals having a phenyl group attached to one of the radical centers as corroborated in the high-intensity irradiation of 2-phenylcyclopentanone (12). The yield of 6 depends on the substrate and irradiation conditions; this is rationalized as due to a conformational memory effect of the nascent biradical.

Full text of DOI:10.1021/jo990872n

7
842
J . Org. Chem. 1999, 64, 7842-7845
Tw o-P h oton Gen er a tion of th e 1,4-Dip h en yl-1,4-bu ta n ed iyl
Bir a d ica l: Dir ect Detection a n d P r od u ct Stu d ies
,
†
†
,‡
,§
Miguel A. Miranda,* Enrique Font-Sanchis, J ulia P e´ rez-Prieto,* and J . C. Scaiano*
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, 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, and
Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa, Ontario K1N6N5, Canada
Received May 28, 1999
The 1,4-diphenyl-1,4-butanediyl biradical (3) is generated from 1,4-dichloro-1,4-diphenylbutane (1)
or 2,5-diphenylcyclopentanone (8) under several irradiation conditions. The products resulting from
this intermediate are styrene (4), 1,2-diphenylcyclobutane (5), and 1-phenyl-1,2,3,4-tetrahydronaph-
thalene (6). The formation of tetrahydronaphthalenes appears to be a fingerprint for the
intermediacy of 1,4-biradicals having a phenyl group attached to one of the radical centers as
corroborated in the high-intensity irradiation of 2-phenylcyclopentanone (12). The yield of 6 depends
on the substrate and irradiation conditions; this is rationalized as due to a conformational memory
effect of the nascent biradical.
In tr od u ction
,4-Diaryl-1,4-butanediyl biradicals have been detected
thermore, minor amounts of 1-aryl-1,2,3,4-tetrahydronaph-
thalene have been detected in the photodimerization of
styrenes, which has been rationalized as due to a second-
1
1
2
in the laser flash photolysis of sulfones or cyclobutanes
2
ary process of the initially obtained cyclobutanes. By
via inter- or intramolecular energy-transfer processes.
Moreover, they have been postulated as intermediates
in a wide range of thermal and photochemical reactions,
contrast, not even traces of this compound appear to be
formed in the photolysis of the cyclic azoalkane 3,6-
diphenyl-3,4,5,6-tetrahydropyrazine, which is thought to
such as the light-induced N
2
and SO
2
loss from cyclic
9
proceed via identical 1,4-biradical intermediates.
azoalkanes3 and sulfones, the geometric isomerization
,4
1
5
2,6-8
Oxygen trapping of biradicals is a well-established
method10 to demonstrate their participation; however, the
only attempts to observe oxygenation products in the
above series of experiments were done in the photolysis
of the cyclic azo compound in solution, under a continuous
stream of oxygen, but they were not successful.
of cyclobutanes and the dimerization of styrenes.
However, studies on the products arising from this type
of biradicals have only been carried out for the dimer-
ization of styrenes, the cycloreversion of cyclobutanes,
and the N
2
extrusion from cyclic azoalkanes. These
studies suggest that 1,4-diaryl-1,4-biradicals undergo two
competitive reactions: fragmentation to styrenes and
cyclization to cyclobutanes. The fact that both types of
photoproducts are also in some cases the starting re-
agents further complicates the interpretation of product
studies. On the other hand, it has been observed that
the photolysis of diarylcyclobutanes leads to 1-aryl-
Altogether, these data show some inconsistencies;
hence, it appeared interesting to generate the 1,4-
diphenylbutanediyl biradical by alternative methods
where the final products could be easily distinguished
from the starting substrates, thus facilitating the unam-
biguous assignment of the products arising from this
intermediate.
1
,2,3,4-tetrahydronaphthalene as an additional photo-
2
,5
product, but it has not been made clear whether this
compound results from direct rearrangement of the
excited diarylcyclobutane or from R to ortho coupling in
the 1,4-biradical, followed by 1,3-hydrogen shift. Fur-
We have previously reported that phenyl-substituted
linear alkanediyls can be generated by a two-photon
1
1
process from acyclic dichlorodiphenylalkanes or by a
1
2
one-photon process from 2,n-diphenylcycloalkanones. In
this context, we report here the detection of the 1,4-
diphenyl-1,4-butanediyl biradical (3) generated from 1,4-
dichloro-1,4-diphenylbutane (1) via a two-photon process
(Scheme 1). The products resulting from this intermedi-
*
To whom correspondence should be addressed. (M.A.M.) Tel.: 34-
-3877807. Fax: 34-6-3877809. E-mail: mmiranda@qim.upv.es. (J .P.-
P.) Tel: 34-6-3864934. Fax: 34-6-3864939. E-mail: jperez@uv.es.
J .C.S.) Tel: (613) 5625896. Fax: (613) 5625170. E-mail: tito@
6
(
photo.chem.uottawa.ca..
†
Universidad Polit e´ cnica de Valencia.
Universidad de Valencia.
University of Ottawa.
1) Zimmt, M. B.; Doubleday, Ch., J r.; Turro, N. J . Chem. Phys. Lett.
987, 134, 549-552.
2) Caldwell, R. A.; Diaz, J . F.; Hrncir, D. C.; Unett, D. J . J . Am.
‡
(9) Kopecky, K. R.; Evani, S. Can. J . Chem. 1969, 47, 4041-4048.
(10) (a) Wilson, R. M. Organic Photochemistry; Padwa, A., Ed.;
Marcel Dekker: New York, 1985; Vol. 7, Chapter 5, pp 339-467. (b)
Adam, W.; Grabowski, S.; Wilson, R. M. Acc. Chem. Res. 1990, 23, 165-
172.
§
(
1
(
Chem. Soc. 1994, 116, 8138-8145.
(11) (a) P e´ rez-Prieto, J .; Miranda, M. A.; Garc ´ı a, H.; K o´ nya, K.;
Scaiano, J . C. J . Org. Chem. 1996, 61, 3773-3777. (b) Miranda, M.
A.; P e´ rez-Prieto, J .; Font-Sanchis, E.; K o´ nya, K.; Scaiano, J . C J . Org.
Chem. 1997, 62, 5713-5719.
(12) (a)Peyman, A.; Beckhaus, H. D.; R u¨ chardt, C. Chem. Ber. 1988,
121, 1027-1031. (b) Zimmt, M. B.; Doubleday, Ch., J r.; Gould, I. R.;
Turro, N. J . J . Am. Chem. Soc. 1985, 107, 6724-6726. (c) Tarasov, V.
F.; Klimenok, B. B.; Askerov, D. B.; Buchachenko, A. L. Bull. Acad.
Sci. USSR, Div. Chem. Sci. 1985, 34, 330-332.
(
3) Kopecky, K. R.; Soler, J . Can. J . Chem. 1974, 52, 2111-2118.
(4) Kohmoto, Sh.; Yamada, K.; J oshi, U.; Kawatuji, T.; Yamamoto,
M.; Yamada, K. J . Chem. Soc., Chem. Commun. 1990, 127-129.
(
(
(
5) J ones, G., II; Chow, V. L. J . Org. Chem. 1974, 39, 1447-1448.
6) Brown, W. G. J . Am. Chem. Soc. 1968, 90, 1916-1817.
7) Li, T.; Padias, A. B.; Hall, H. K., J r. Macromolecules 1990, 23,
3
899-3904.
(8) Kauffmann, K. F. Makromol. Chem. 1979, 180, 2649-2663.
1
0.1021/jo990872n CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/21/1999

Generation of the 1,4-Diphenyl-1,4-butanediyl Biradical
J . Org. Chem., Vol. 64, No. 21, 1999 7843
F igu r e 2. Two-laser-two-color experiment showing excitation
F igu r e 1. Transient absorption spectra recorded following
laser excitation of 1 in cyclohexane under nitrogen 3.12 µs (4)
and 17.0 µs (O) after the laser (266 nm) pulse. The insert shows
the decay as monitored at 320 nm.
of 1,4-dichloro-1,4-diphenylbutane in cyclohexane at 266 nm
(see A), followed by excitation at 308 nm (B) (monitoring
wavelength at 320 nm).
mJ /pulse) led to permanent and irreversible bleaching
of the signal as monitored at 320 nm (Figure 2). The
decay observed following the 308 nm pulse had two
components, one of them coincident with that of the
benzylic radical and a second, faster one corresponding
to a more reactive transient species with lifetime of 200
ns. The absorption spectrum after the 308 nm pulse was
indistinguishable from that of the benzylic radical 2;
however, as its lifetime was in agreement with those
reported for 1,4-diaryl-1,4-butanediyl biradicals, the
structure 3 was assigned to the fast-decaying species.
To establish the nature of the primary photoproducts
arising from biradical 3, laser-drop photolysis was used.
Sch em e 1
1
,2
This technique provides a way of performing high-
intensity photolysis1
4,15
while minimizing the amounts of
products arising from irradiation of (thermally) nonre-
active photoproducts. When drops of 2 mM deareated
solutions of 1 in cyclohexane were irradiated by the
focused output from a 266 nm laser (one cycle) only a
1
0% conversion was observed (Table 1, Scheme 1). The
photoproducts detected were styrene (4, 79%), cis- and
trans-diphenylcyclobutanes (5, 10%, cis/trans ratio: 46/
1
6
5
4), and 1-phenyl-1,2,3,4-tetrahydronaphthalene (6, 11%).
ate are established by laser drop photolysis of 1, and the
results are compared with those obtained in the photoly-
sis of trans-2,5-diphenylcyclopentanone. On the basis of
this information, the previous mechanisms thought to
involve biradical 3 are discussed.
The formation of these compounds can be accounted for
in terms of competition between fragmentation, cycliza-
tion, and R to ortho coupling of the 1,4-diphenyl-1,4-
butanediyl biradical. Laser irradiations were also per-
formed with deaerated and oxygenated 10 mM cyclo-
hexane solutions of 1, using a 266 nm laser, with the
sample contained in a quartz spectrometer cell. Analysis
of the photolyzed deaerated solutions by GC/MS revealed
the formation of the same products as in the case of
the laser-drop experiment (see Table 1, footnote d). When
Resu lts
P h ot olysis of 1,4-Dich lor o-1,4-d ip h en ylb u t a n e.
Laser flash photolysis of deaerated 0.4 mM solutions of
1
,4-dichloro-1,4-diphenylbutane (1) in cyclohexane at 266
nm (Nd:YAG laser, fourth harmonic, <10 ns, 16 mJ /
pulse) yielded a narrow absorption, with maximum at
(14) Banks, J . T.; Scaiano, J . C. J . Am. Chem. Soc. 1993, 115, 6409-
6
413.
3
20 nm (see Figure 1). The lifetime of this species was
(
15) High-intensity photolysis studies of dihalides have also been
around 4 µs under our experimental conditions. This
signal was quenched by oxygen at close to the diffusion-
controlled limit. Hence, it was assigned to be the benzylic
radical 2 obtained through homolytic cleavage of one of
the two C-Cl bonds.13 Photolysis of this transient at 308
nm (HCl/Xe/Ne gas mixtures, excimer laser, ∼6 ns, 50
carried out by means of the laser-jet technique: Adam, W.; Ouchi, A.
Tetrahedron Lett. 1992, 33, 1875-1878.
(
16) By contrast, lamp irradiation (medium-pressure mercury,
quartz filter, 2 h) of deaerated 10 mM cyclohexane solution of 1 led to
complete consumption of the starting material. Analysis of the pho-
tolysate by GC/MS revealed the formation of products arising from
the expected homolytic cleavage of a carbon-chlorine bond followed
by hydrogen abstraction from solvent by chlorine atoms, radical-
radical recombination, and some secondary photolysis of monochlori-
nated products. Not even traces of styrene, 1,2-diphenylcyclobutane,
and 1-phenyl-1,2,3,4-tetrahydronaphthalene were detected in the
photolysate.
(13) Benzylic radicals normally show absorption in this spectral
region: Chatgilialoglu, C. In Handbook of Organic Photochemistry;
Scaiano, J . C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol II, p 3.

7
844 J . Org. Chem., Vol. 64, No. 21, 1999
Miranda et al.
Ta ble 1. Com p a r a tive Stu d ies of th e P h otoch em istr y of
, 8 a n d 12 in Dea er a ted Solu tion s
Sch em e 3
1
relative product distributiona,b (%)
conversn
(%)
5
condns
4c (cis/trans)
6
10 11 14 16 17
1
1
8
8
laser-drop
laser
lamp
10
50
84
20
28
40
79 10 (46/54) 11
d
80 14 (40/60)
50 38 (15/85)
6
4
7
1
laser
40 20 (25/75) 40
1
1
2 lamp
2 laser
>99
47 12 27
12
a
Control experiments showed that styrene did not react sig-
nificantly under the employed irradiation conditions. The cyclobu-
tanes 5 were formed in very low yield (less than 5%), and the cis/
trans ratio was ca. 10:1. On the other hand, compound 6 was
b
detected only as traces. The structural assignment of known
8
8
22
23
18
24
photoproducts 5, 6, 10, 11, 14, and 16 was confirmed by
c
comparison with authentic samples. Part of the styrene could
have been evaporated when rotaevaporating the photolyzate before
d
analysis. Roughly 25% of the reacted material consisted of two-
photon products (4-6); the rest were the typical monophotonic
products mentioned in ref 16.
18
Sch em e 2
the photoproduct distribution was quite different from
that obtained in the low-intensity irradiation. Thus, enal
1
0 and alkene 11 were not formed, which agrees with
the acyl-alkyl biradical 9 being photolyzed within the
laser pulse. However, the most dramatic feature was the
sharp increase of 1-phenyl-1,2,3,4-tetrahydronaphthalene
(40%) under high-intensity irradiation (see Table 1).
In view of the diverging results obtained in the
photolysis of ketone 8 as a function of the light intensity,
the monosubstituted analogue 12 was also studied under
different conditions.
It is known18 that lamp irradiation (λ ) 313 nm) of 12
leads to the unsaturated aldehyde 14, through intramo-
lecular H abstraction in the primary acyl alkyl biradical
(13). Under our irradiation conditions, 14 was also the
only photoproduct. Hence, we wondered whether biradi-
cal 13 can be further photolyzed if generated by means
of a high-intensity laser source. Actually, laser (266 nm)
irradiation of a 1 mM cyclohexane solution of 12 gave
rise to significant amounts of styrene, phenylcyclobutane
oxygenated samples were laser-photolyzed, the oxygen
trapping product 2,5-diphenyltetrahydrofuran (7)17 was
detected.
P h otolysis of tr a n s-2,5-d ip h en ylcyclop en ta n on e.
To the best of our knowledge, the photochemical behavior
of this ketone has not been the subject of any previous
report. Lamp irradiation (Pyrex filter) of 8 in deaerated
cyclohexane for 1 h led to styrene (4, 50%), 1,2-diphe-
nylcyclobutane (5, 38%, cis/trans ratio, 15/85), 1-phenyl-
(
16), and 1,2,3,4-tetrahydronaphthalene (17) (Scheme 3,
Table 1). These results revealed that acyl-alkyl biradical
3 is photolyzed within the laser pulse (10 ns), photode-
1
carbonylating to the dialkyl biradical 15. Its dispropor-
tionation, cyclization, or R to ortho-coupling generates 4,
1
,2,3,4-tetrahydronaphthalene (6, 4%), 2,5-diphenyl-4-
1
6, and 17, which are the fingerprints for two-photon
pentenal (10, 7%), and 1,4-diphenyl-1-butene (11, 1%).
A small amount of the starting ketone (16%) was recov-
ered unchanged (Table 1, Scheme 2).
processes. Finally, laser photolysis of ketone 12 in
oxygenated cyclohexane solution gave rise to the oxygen-
trapping product of the biradical, namely 2-phenyltet-
rahydrofuran (18).19
The formation of enal 10 demonstrated that, in con-
trast with the case of the 2,6-diphenylcyclohexanone,
H-abstraction by the initially generated acyl-alkyl bi-
radical 9 competes with thermal decarbonylation to
biradical 3. This easily explains the formation of com-
pounds 4-6 and 10. To ascertain the origin of 1,4-
diphenyl-1-butene, enal 10 was photolyzed under the
Discu ssion
From the above results, it is clear that in the 1,4-
diphenyl-1,4-butanediyl biradical fragmentation to sty-
rene and cyclization to 1,2-diphenylcyclobutane compete
with R to ortho coupling, leading to 1-phenyl-1,2,3,4-
tetrahydronaphthalene (6). If one takes also into account
the results obtained with the cyclopentanones 8 and 12,
the formation of tetrahydronaphthalenes seems to be a
fingerprint for the intermediacy of 1,4-biradicals having
1
same conditions. Analysis of the photolysate by H NMR
and GC/MS evidenced its partial transformation into 11.
On the other hand, photolysis of ketone 8 in oxygenated
cyclohexane solution gave rise to the oxygen-trapping
product 717 (50/50 cis/trans mixture).
A deaerated 1 mM cyclohexane solution of 8 was also
irradiated at 266 nm (Nd:YAG). Under these conditions,
(
18) Baum, A. A. Tetrahedron Lett. 1972, 1817-1820.
(19) Brown, D. S.; Bruno, M.; Davenport, R. J .; Ley, S. V. Tetrahe-
(17) Neudeck, H.; Schl o´ gl, K. Monatsh. Chem. 1975, 106, 229-259.
dron 1989, 45, 4293-4308.

Generation of the 1,4-Diphenyl-1,4-butanediyl Biradical
J . Org. Chem., Vol. 64, No. 21, 1999 7845
1 (mixture of stereoisomers): 1H NMR (CDCl
, 250 MHz) δ
Sch em e 4
3
1
1
(
.9-2.3 (m, 4 H), 4.7 (m, 2 H), 7.2 (m, 10 H); 13C NMR (CDCl
41.2 (s), 128.7 (d), 128.4 (d), 126.8 (d), 63.0 (d), 62.7 (d), 37.5
278.0629, found
3
,
t), 37.2 (t); HRMS calcd for C16
H
16Cl
2
2
78.0623.
Syn th esis of tr a n s-2,5-Dip h en ylcyclop en ta n on e (8).
Preparation of ketone 8 was accomplished following a proce-
dure described in the literature.26 Thus, a mixture of R,R’-
diphenyladipic acid (3.50 g, 11.74 mmol) and finely powdered
barium hydroxide (0.25 g) was heated under a stream of
nitrogen, at 300-320 °C, until evolution of carbon dioxide
a phenyl group attached to one of the radical centers. This
observation can be a useful tool for the reexamination of
previous mechanisms thought to involve 1,4-diaryl-1,4-
butanediyl biradicals. Thus, the tetrahydronaphthalenes
detected in the sensitized photolysis of styrenes and 1,2-
diarylcyclobutanes would be actually direct products
formed from the biradical intermediates rather than
secondary photoproducts. Hence, their detection consti-
tutes further support for the advanced mechanistic
proposals. By contrast, the lack of formation of 6 in the
photolysis of 3,6-diphenyl-3,4,5,6-tetrahydropyrazine (19)
appears to be against the involvement of biradical 3 as a
key intermediate in this reaction. Instead, it can be
suggested that in this case photolysis of the first C-N
bond could lead to a diazenyl biradical (20), from which
(
about 2 h). Addition of hexane to the crude mixture precipi-
tated 1.5 g (6.35 mmol, 54% yield) of 8.
: 1H NMR (CDCl
, 250 MHz) δ 2.2 (m, 2 H), 2.6 (m, 2 H),
.5 (m, 2 H), 7.2-7.4 (m, 10 H); C NMR (CDCl
δ 215.3 (s), 138.4 (s), 128.6 (d), 128.1 (d), 127.0 (d), 55.8 (d),
8
3
1
3
3
3
, 62.5 MHz)
+
2
9.5 (t); MS m/z 236 (M , 12), 208 (7), 117 (7), 104 (100); HRMS
calcd for C17 16O 236.1201, found 236.1205.
H
Con ven tion a l La m p Ir r a d ia tion of Com p ou n d s 1, 8 10,
a n d 12. A cyclohexane solution of the compound either in a
quartz (1) or Pyrex tube (8, 10, and 12) was irradiated for 1 h
with a 125-W medium-pressure mercury lamp inside a quartz
immersion well, under continuous magnetic stirring. After
evaporation of the solvent, the photomixture was analyzed by
GC/MS.
La ser -Dr op P h otolysis. The beam from a Nd:YAG laser
using the fourth harmonic (266 nm, <10 ns, e16 mJ /pulse)
was focused by means of a quartz lens into a drop of the
photolysis solution suspended from a 2-in. syringe needle (20
gauge). Further details for this experiment have been de-
scribed previously.14
1
,2-diphenylcyclobutane and styrene could be formed
2
0
through nitrogen elimination (Scheme 4). This is also
consistent with the failure to detect oxygen-trapping
products in the photolysis of 19, in contrast with the
observed formation of tetrahydrofuran derivatives in the
two-photon transformation of the dichloride 1 and in the
decarbonylation of the cyclic ketones 8 and 12 in aerated
solutions.
La ser F la sh P h otolysis. These experiments were carried
out using either a Nd:YAG laser using the fourth harmonic
(266 nm, <10 ns, e20 mJ /pulse) or an excimer laser operated
with HCl/Xe/Ne gas mixtures (308 nm, ca. 6 ns, e50 mJ /pulse).
Transient signals were captured with a Tetronix-2440 digital
oscilloscope that was interfaced to a computer that also
controlled the experiment. The system was operated with
software written in the LabVIEW 3.1.1 environment from
National Instruments. Other aspects of this instrument are
similar to those described previously. The two-laser two-color
experiments were performed by sending a trigger pulse to a
delay generator, which then sent TTL pulses which fired the
lasers at the desired sequence. All experiments were carried
out using flow cells constructed from 7 × 7 mm Suprasil quartz
tubing. Samples were contained in a 100 mL resevoir tank
which was purged with a slow stream of either nitrogen or
oxygen, as required.
Finally, the fact that quite different yields of tetrahy-
dronaphthalenes are obtained depending on the starting
substrate and the irradiation conditions deserves a
further comment. Thus, while 6 was a minor product in
the two-photon photolysis of the dichloride 1 or in the
one-photon decarbonylation of the ketone 8, it was
obtained in much higher yield upon high-intensity ir-
radiation of the ketone. This points to differences in the
chemical behavior of biradical 3 linked to the predomi-
nance of different biradical conformations.21 It can be
envisaged that the preferred conformation will be differ-
ent for fragmentation, cyclization, and R to ortho cou-
pling. The nascent biradical could have a lifetime shorter
than that required for rotational equilibration, thus
collapsing by the reaction path associated with the least
motion requirement. The data here obtained suggest that
the formation of 1,2,3,4-tetrahydronaphthalenes is fa-
vored when the biradical is initially generated in a
conformation, such as 3b, with a short distance between
the radical terminii.
Ack n ow led gm en t. Financial support by the Span-
ish DGES (M.A.M., Project No. PB97-0339) and by the
Natural Science and Engineering Council of Canada
through an operating Grant (J .C.S.) is gratefully ac-
knowledged. E.F.S. also thanks the Spanish Ministry
of Education for a grant. Part of this work was per-
formed when J .C.S. was the recipient of a Killam
Fellowship awarded by the Canada Council.
J O990872N
Exp er im en ta l Section
Syn th esis of 1,4-Dich lor o-1,4-diph en ylbu tan e (1). Prepa-
ration of dichloride 1 was accomplished following a procedure
describe in the literature.
(22) Doering, W. E.; Birladeanu, L.; Sarma, K.; Teles, J . H.; Kl a´ rner,
F. G.; Gehrke, J . S. J . Am. Chem. Soc. 1994, 116, 4289-4297.
(23) Snider, B. B.; J ackson, A. C. J . Org. Chem. 1983, 48, 1471-
2
5
1
474.
(
24) Holland, H. L.; Kindermann, M.; Kumaresan, S.; Stefanac, T.
(
20) This mechanism has been found to operate in a number of cases.
Tetrahedron: Asymmetry 1993, 4, 1353-1364
See, for example: Engel, P. S. Chem. Rev. 1980, 80, 99-150. Adam,
W.; Harrer, H. M.; Nau, W. M.; Peteres, K. J . Org. Chem. 1994, 59,
(25) (a) Dodson, R. M.; Zielske, A. J . Org. Chem. 1967, 32, 28-31.
(b) Mikaya, A. I.; Zaikin, V. G.; Finkel’shtein, E. Sh.; Vdovin, V. M.
Izv. Akad. Nauk SSSR, Ser. Khim. 1979, 2221-2226.
(26) Frank, C. E.; Leebrick, J . R.; Homberg, O. J . Org. Chem. 1961,
26, 307-309.
3
786-3797. Simpson, C. J . S. M.; Wilson, G. J .; Adam, W. J . Am. Chem.
Soc. 1991, 113, 4728-4732.
21) Scaiano, J . C. Tetrahedron 1982, 38, 819-824.
(