Primary and Secondary Isotope Effects in the Photooxidation of 2,5-dimethyl-2,4-hexadiene. Elucidation of the Reaction Energy Profile

Full text of DOI:10.1021/jo971994n

6390
J . Org. Chem. 1998, 63, 6390-6393
Sch em e 1. P h otooxygen a tion a n d MeOH
Tr a p p in g P r od u cts of th e Rea ction of DMHD
w ith 1O2
P r im a r y a n d Secon d a r y Isotop e Effects in
th e P h otooxid a tion of
2,5-Dim eth yl-2,4-h exa d ien e. Elu cid a tion of
th e Rea ction En er gy P r ofile
Georgios Vassilikogiannakis, Manolis Stratakis, and
Michael Orfanopoulos*
Department of Chemistry, University of Crete,
Iraklion 71409, Greece
Received October 30, 1997
In tr od u ction
Conjugated dienes react smoothly with singlet oxygen
to form [4 + 2] adducts.1 The reaction is synthetically
useful, especially with dienes bearing a polar stereogenic
center2 at an adjacent position to the double bond. In
this case, a highly diastereoselective formation of en-
doperoxides due to favorable or unfavorable interactions
with the polar substituents was reported. The photo-
oxygenation of 1,3-dienes was initially considered to
proceed via a concerted pathway since it was found to
occur in a suprafacial manner.3 Recent studies, however,
have indicated that the transition state of the [4 + 2]
product formation is preceded by an exciplex intermedi-
ate.4 The photooxygenation of dienes with constraints
of rotation around the single C-C bond to adopt the
reactive s-cis conformation is not stereospecific.5 For
instance, the reaction of 1O2 with the isomeric 2,4-
hexadienes has been studied extensively.6 It was found
that the [4 + 2] pathway was not stereospecific. (E,E)-
2,4-Hexadiene afforded in addition to the endoperoxide
with the cis stereochemistry (suprafacial process) a small
amount of the endoperoxide, where the methyls had a
trans configuration. Also, (E,Z)-2,4-hexadiene gave mix-
tures of the diastereomeric endoperoxides with the major
being the adduct with the “wrong” stereochemistry.
These results were rationalized by the formation of an
open zwitterionic intermediate.
ene product 1a was reported to rearrange to 2,5-dimeth-
ylhexa-3,5-diene-2-hydroperoxide8 via the corresponding
peroxy radical. The rearrangement of the ene hydrop-
eroxides has been studied extensively by Porter and co-
workers.9 The [4 + 2] adduct could be further attributed
3
to reaction of DMHD with O2 by a free-radical process.
Indeed, a sample of DMHD left for prolonged time in air
is gradually contaminated with significant amounts of
endoperoxide.10 The ratio of these adducts depends on
solvent and temperature conditions. Polar protic solvents
and low temperatures favor the dioxetane pathway.
Foote and Manring7b measured the activation param-
eters of the reaction and found them to be identical with
those of the photooxygenation of the simple trisubstituted
alkene, 2-methyl-2-pentene. They concluded that both
reactions proceed through a perepoxide intermediate. The
perepoxide may open to a zwitterionic intermediate that
leads to the dioxetane. A significant feature of the
reaction is that there is a remarkably fast rate of
nonreactive quenching11 in solvents other than methanol.
This was attributed to the transformation of the zwitte-
rionic intermediate to a singlet diradical, which collapses
The reaction of singlet oxygen with the sterically
hindered 2,5-dimethyl-2,4-hexadiene (DMHD, 1) forms
a mixture of ene product 1a , 1,2-dioxetane 1b, endoper-
oxide 1c, and the 1,4-methoxy hydroperoxide adduct 1d
in methanol solvent (Scheme 1).7 The initially formed
3
through intersystem crossing to 1 and O2.
Although the mechanistic details (stereoisotopic stud-
ies) of the ene reaction of alkenes with 1O2 have been
established, there is no analogous study concerning the
(1) Clennan, E. L. Tetrahedron 1991, 47, 1343-1382.
(2) (a) Adam, W.; Prein, M. J . Am. Chem. Soc. 1993, 115, 3766-
3767. (b) Adam, W.; Peters, E. M.; Peters, K.; Prein, M.; Von Schering,
H. G. J . Am. Chem. Soc. 1995, 117, 6686-6690. (c) Adam, W.; Prein,
M. Acc. Chem. Res. 1996, 29, 275-283.
(3) (a) Rio, G.; Berthelot, J . Bull. Soc. Fr. 1969, 1664-1667. (b)
Rigaudy, J .; Capdevielle, J .; Combrisson, S.; Maumy, M. Tetrahedron
Lett. 1974, 2757-2560.
1
reaction of dienes with O2. For example, isotope effect
studies of the ene reaction of singlet oxygen with alkenes
have been reported previously by us12-14 and others.15-17
The primary isotope effects measured in these reactions
(4) (a) Aubry, J .-M.; Mandard-Cazin, B.; Rougee, M.; Bensasson, R.
V. J . Am. Chem. Soc. 1995, 117, 9159-9164. (b) Gorman, A. A.; Gould,
I. R.; Hamblett, I.; Standen, M. C. J . Am. Chem. Soc. 1984, 106, 6956-
6959. (c) Gorman, A. A.; Hamblett, I.; Lambert, C.; Spencer, B.;
Standen, M. C. J . Am. Chem. Soc. 1988, 110, 8053-8059.
(5) For a recent example, see: Delogu, G.; Fabbri, D.; Fabris, F.;
Sbrogio, F.; Valle, G.; De Lucchi, O. J . Chem. Soc., Chem. Commun.
1995, 1887-1888.
(8) (a) Gorman, A. A.; Rodgers, M. A. J . Chem. Phys. Lett. 1978, 55,
52-55. (b) Dussault, P. H.; Woller, K. R. J . Org. Chem. 1997, 62, 1566-
1559. (c) Matsumoto, M.; Dobashi, S.; Kuroda, K.; Kondo, K. Tetrahe-
dron 1985, 41, 2147-2154.
(9) (a) Porter, N. A.; Mills, K. A.; Caldwell, S. E.; Dubay, G. R. J .
Am. Chem. Soc. 1994, 116, 6, 6697-6705. (b) Lowe, J . R.; Porter, N.
A. J . Am. Chem. Soc. 1997, 119, 11534-11535.
(10) Unpublished observations.
(6) (a) Gollnick, K.; Griesbeck, A. Tetrahedron Lett. 1983, 24, 3303-
3306. (b) O’Shea, K. E.; Foote, C. S. J . Am. Chem. Soc. 1988, 110,
7167-7170.
(11) Manring, L. E.; Kanner, R. C.; Foote, C. S. J . Am. Chem. Soc.
1983, 105, 4707-4710.
(12) Orfanopoulos, M.; Foote, C. S. J . Am. Chem. Soc. 1988, 110,
6583-6584.
(7) (a) Hasty, N. M.; Kearns, D. R. J . Am. Chem. Soc. 1973, 95,
3380-3381. (b) Manring, L. E.; Foote, C. S. J . Am. Chem. Soc. 1983,
105, 4710-4717.
(13) Stephenson, L. M.; Grdina, M.; Orfanopoulos, M. Acc. Chem.
Res. 1980, 13, 419-425.
S0022-3263(97)01994-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/07/1998

Notes
J . Org. Chem., Vol. 63, No. 18, 1998 6391
are small and depend on the stereochemistry of the
perepoxide intermediate, whereas the secondary isotope
effects are negligible. For example, the intermolecular
competition12,15-17 of singlet oxygen with alkenes pos-
sesses a very small isotope effect (kH/kD ) 1.08-1.10) and
negligible secondary isotope effect. However, the value
of kH/kD ) 1.38-1.40 was measured for tetramethyleth-
ylenes-d6, when the competing methyl groups are in cis
configuration, and kH/kD ) 1.04-1.10, when the compet-
ing methyl groups are in a trans configuration. These
results (negligible intermolecular and substantial in-
tramolecular isotope effects) were interpreted by the
irreversible formation of a perepoxide intermediate in a
rate-determining step. A perepoxide intermediate was
also recently supported by additional experimental re-
sults.18
Resu lts a n d Discu ssion
In this paper, we report the mechanistic details of the
ene, [2 + 2], and methanol-trapping pathways in the
sensitized photooxygenation of the 2,5-dimethyl-2,4-
hexadiene. To measure the intramolecular primary
isotope effect for the ene reaction, and the â-secondary
isotope effects for the [2 + 2] and methanol-trapping
pathways, we chose the appropriately labeled DMHD
at the geminal methyls, 2,5-dimethyl-2,4-hexadiene-
1,1,1,2′,2′,2′-d6 (DMHD-d6, 2). Diene 2 was prepared by
Wittig coupling of triphenylphosphoranylidene-3-methyl-
2-butene with acetone-d6. Photooxygenations were car-
ried out in CHCl3 where the ene is the major product
and in MeOH to obtain the [2 + 2] and 1,4-trapping
adducts. A Xenon Eimac Cermax 300 W lamp was used
as the light source.
F igu r e 1. 1NMR measurement of kH/kD by the integration of
the proper hydrogen absorptions of products 4a and 4b: Ha
(kH) and Hb (kH) at 5.09 and 4.97 ppm, Hc and He at 3.83 ppm
(kH + kD), Hd and Hf at 2.80 ppm (kH + kD).
Sch em e 2. P r im a r y Isotop e Effects in th e En e
1
Rea ction of O2 w ith DMHD-d 6 in CHCl3
Irradiation of an oxygen-saturated solution of 2 (1 ×
10-2 M) and tetraphenylporphine (TPP) as the sensitizer
(1 × 10-4 M) in CHCl3 containing a drop of pyridine to
prevent dye bleaching, at 0 °C, gave the ene as the major
product, accompanied by a small amount of dioxetane
(∼5%). Since the ene hydroperoxide easily rearranges8
on standing, it was immediately reduced with PPh3 to
the bisallylic alcohol, which was chromatographed with
a silica gel column prewashed with triethylamine and
identified from its spectral properties and by comparison
with literature data.7b No rearranged allylic alcohol was
detected, which was reported recently8b to be the sole ene
ratio 3a /3b or to 4a /4b, (Scheme 2). For instance,
integration of the vinylic hydrogens Ha and Hb of 4a at
5.09 and 4.97 ppm, whose sum is proportional to the
quantity of 2kH, and the hydrogens Hc and He of 4a and
4b next to the hydroxyl group at 3.83 ppm (kH + kD) or
the hydrogens next to the epoxide Hd and Hf at 2.80 ppm
(also kH + kD) measures the isotope effect of kH/kD ) 1.61
( 0.05 (Figure 1). The same value can be obtained by
integrating the single resonance of the allylic methyl
group at 1.72 ppm of 4a and the two diastereotopic
methyls of 4b, which resonate at 1.29 and 1.27 ppm,
respectively. Similar integration of the proper peaks of
1
product in the reaction of 1 with O2 induced by a chiral
phosphite ozonide.
To confirm these results, photooxygenation was carried
out in the presence of equimolar amounts of Ti(i-PrO)4
to form the more stable epoxy alcohols by in situ
epoxidation.19 Also, if Ti(i-PrO)4 is added instead of PPh3
to the hydroperoxide mixture, the same epoxy alcohols
can be isolated in excellent yield. The primary isotope
effect kH/kD of the ene reaction, which is a result of an
intramolecular isotopic competition, is proportional to the
3a and 3b (see Experimental Section) affords a kH/kD
1.58 ( 0.05.
)
(14) Orfanopoulos, M.; Smonou. I.; Foote, C. S. J . Am. Chem. Soc.
1990, 112, 3607-3614.
When the photooxygenation of 2 was run in methanol
(methylene blue as sensitizer) at 0 °C, the dioxetanes 5a
and 5b and the 1,4-methanol-trapping adducts 6a and
6b (Scheme 3) were the major products in a ratio 5/6 )
60/20, along with a 20% of ene and traces of [4 + 2]
adduct. After the end of the photooxygenation of 2,
triphenylphosphine was added as a reducing agent and
the products were fractionated by column chromatogra-
(15) Elemes, Y.; Foote, C. S. J . Am. Chem. Soc. 1992, 114, 6044-
6050.
(16) Gollnick, K.; Hartmann, H.; Paur, H. In Oxygen And Oxy-
radicals In Chemistry And Biology; Rodgers, M. A. J ., Powers, E. L.,
Eds.; Academic Pres: New York, 1981; pp 379-395.
(17) Kopecky, K. R.; van de Sande, J . H. Can. J . Chem. 1972, 50,
4034-4049.
(18) Poon, T. H. W.; Pringle, K.; Foote, C. S. J . Am. Chem. Soc. 1995,
117, 7611-7618.
(19) Adam, W.; Staab, E. Tetrahedron Lett. 1988, 29, 531-534.

6392 J . Org. Chem., Vol. 63, No. 18, 1998
Notes
Sch em e 4. P r op osed En er gy P r ofile in th e
Sch em e 3. Secon d a r y Isotop e Effects for th e
[2 + 2] a n d MeOH Tr a p p in g Ad d u cts in th e
Rea ction of DMHD w ith Sin glet Oxygen
1
Rea ction of O2 w ith DMHD-d 6
DMHD-d6 could be the result of partial reversion of the
perepoxide intermediate to the starting materials, since
the direct isotopic competition is impossible in an inter-
mediate of this geometry (Scheme 4). Subsequent H/D
abstraction in the rate-determining step accounts for the
observed primary isotope effect. This is in contrast to
the energy profile of trisubstituted20 or tetrasubstituted
alkenes,13 where formation of the perepoxide, as we
already mentioned, is the rate-limiting step. However,
phy. Under the experimental conditions, the labile
dioxetanes decomposed to acetone and 3-methylbuten-
2-al. For the 1,4-methanol-trapping adducts, we define
as kH the percent of product where the new C-OOH bond
is formed next to the geminal protonated methyls and
kD the percent of product where the C-OOH bond is
formed next to the geminal deuterated methyls. The
isotope effect was measured by direct integration of the
single peaks at 1.35 ppm corresponding to the two
geminal methyls of 6a and at 1.27 ppm for the two
methyls of 6b and found to be kH/kD ) 1.01 ( 0.05. It is
interesting to note that the two methoxy groups next to
the geminal CH3 methyls in 6b, and next to the geminal
CD3 methyls in 6a , resonate at 3.152 and 3.149 ppm,
respectively. The small but significant chemical shift
change of the methoxy groups is due to a H/D substitu-
tion five bonds away from the resonating methoxy
hydrogens. Thus, the electronegativity of the six hydro-
gens, which is larger than the six deuteria, causes a
higher diamagnetic deshielding. For this reason, the
methoxy group next to the protonated methyl absorbs at
lower field (3.152 ppm).
For the dioxetane pathway, kH is proportional to the
5a product where the new C-O bond is formed next to
the protonated methyls, while kD is proportional to the
5b product where the dioxetane is formed next to the
deuterated methyls. The isotope effect was measured by
GC-MS integration of the two 3-methyl-buten-2-als (5c
and 5d ) resulting from the quantitative thermal cleavage
of the initially formed labile dioxetanes. The capillary
column 50%-50% phenyl methyl silicone separates the
aldehydes 5c and 5d sufficiently to be measured. Mass
spectroscopy showed that the deuterated aldehyde,
3-methyl-buten-2-al-4,4,4,3′,3′,3′-d6, 5c, has a shorter
retention time than the protio aldehyde 5d . The isotope
effect kH/kD is proportional to the product ratio of 5c/5d
and was measured to be kH/kD ) 1.02 ( 0.03, which is
identical with that measured from the 1,4-methanol-
trapping path.
1
in the reaction of O2 with the highly unreactive trans-
2-butene-d3, the unexpected substantial kH/kD ) 1.25 was
rationalized14 in terms of partial reversion of the pere-
poxide intermediate to the starting materials. The
proposed inversion of the intermediate to the products,
in the photooxygenation of DMHD, could be attributed
to the fact that the diene is more nucleophilic than the
simple trisubstituted alkenes and the energy barrier for
the formation of the perepoxide is lower.
When the photooxygenation of 2 was carried out in
methanol, the dioxetanes and the methoxy-trapping
adducts were the major detected products. These prod-
ucts have been proposed to arise probably from an open
zwitterionic intermediate. Significant evidence for the
existence of an open intermediate comes from results
reported by Manring and Foote.7b Formation of the
dipolar zwitterion in polar solvents is expected to have a
higher negative value of entropy, due to the additional
solvation, compared to the more rigid perepoxide. In-
deed, as the temperature decreases, the ratio of the
products arising from the proposed open intermediate in
methanol (dioxetane and trapping) vs the ene product
from the perepoxide, increases; at ambient temperature
(dioxetane + trapping)/ene ) 3/1, whereas at -78 °C
(dioxetane + trapping)/ene ) 15/1. The secondary isotope
effects measured in the photooxygenation of 2 either in
the dioxetane or in the methanol trapping pathways, are
practically unity, kH/kD ) 1.0.
These and previous results11 are consonant with the
formation of a perepoxide as a common intermediate in
the first step for all reaction paths. Dioxetane and
methoxy adducts are probably formed via an open zwit-
terionic intermediate, which is produced by rearrange-
ment of the perepoxide in a faster step (kZI > kper, Scheme
4). The perepoxide is formed reversibly in a fast step
compared to the competing ene step (kper > kene). Al-
The significant primary isotope effect of kH/kD ) 1.6
measured in the ene reaction of singlet oxygen with
(20) Stratakis, M.; Orfanopoulos, M.; Chen, J . S.; Foote, C. S.
Tetrahedron Lett. 1996, 37, 4105- 4108.

Notes
J . Org. Chem., Vol. 63, No. 18, 1998 6393
though the ene pathway is energetically unfavorable (kZI
> kene), it becomes predominant in aprotic solvents, due
to the significant collapse of the zwitterionic intermediate
to starting materials. The zwitterionic intermediate
must involve some biradical character in order to decom-
pose in a spin-allowed proccess. It is well established11
that the rate of disappearance (kr) of DMHD in methanol
is close to the total rate of interaction between DMHD
ther through an open zwitterionic intermediate, which
is formed in a faster step compared to perepoxide
formation or directly from the perepoxide intermediate.
Exp er im en ta l Section
Nuclear magnetic resonance spectra were recorded on 400 and
250 MHz spectrometers. Isomeric purities were determined by
1H NMR and by analytical gas chromatography equipped with
a 50%-50% phenyl methyl silicone capillary column and a
5971A MS detector.
1
and O2 (kr + kq). This indicates that DMHD does not
1
quench O2 by a nonreactive pathway in methanol. The
physical quenching (PQ) is about 4%. In contrast, in
aprotic solvents, kr of DMHD is much less than the total
interaction rate between DMHD and 1O2 (kr + kq). In
2,5-Dim eth yl-2,4-h exa d ien e-1,1,1,2′,2′,2′-d 6 (1) was pre-
pared by Wittig coupling of triphenylphosphoranylidene-3-
methyl-2-butene with acetone-d6 in ether at 0 °C (65% yield).
1H NMR (CDCl3): δ 5.96 (s, 2H), 1.78 (s, 3H), 1.72, (s, 3H). Exact
mass for C8H8D6: calcd 116.1472, found 116.1458. The precur-
sor to ylide phosphonium salt was prepared in 90% yield by
heating neat 4-bromo-2-methyl-2-butene (Aldrich) and an equimo-
lar amount of triphenyl phosphine in a tight tube for 12 h at
100 °C. The phosphonium salt was collected as a white solid
and was washed with hot toluene. 1H NMR: δ 7.90-7.63 (m,
15H), 5.13 (m, 1H), 4.52 (dd, J H-P ) 14.5 Hz, J H-H ) 7.7 Hz,
2H), 1.67 (d, J ) 5.8 Hz, 3H), 1.28 (d, J ) 3.9 Hz, 3H).
En e P r od u cts fr om th e Rea ction of 2 w ith 1O2 in CHCl3.
After reduction of the initially formed hydroperoxides with
excess PPh3 at 0 °C, the bis allylic alcohols were purified by flash
column chromatography over silica gel, prewashed with triethy-
lamine using ether/hexane ) 1:1 as eluent. M+: 132. 1H NMR
(CDCl3): δ 5.16 (d, J ) 8.7 Hz, 1H of 3a + 1H of 3b), 5.00 (br. s,
1H of 3a ), 4.80 (br s, 1H of 3a ), 4.75 (d, J ) 8.7 Hz, 1H of 3a +
1H of 3b), 1.71 (s, 3H of 3a ), 1.70 (s, 3H of 3b), 1.69 (s, 3H of
3b), 1.25 (br s, 1H, hydroxyl).
In a separate experiment, photooxygenation was carried out
in the presence of equimolar amounts of Ti(i-PrO)4. After the
end of the reaction, 5 mL of ether was added and then 1-2 mL
of a saturated solution of Na2SO4. The mixture was extracted
with ether, and the residue was chromatographed with petro-
leum ether/ether ) 2/1 to obtain the pure epoxy alcohols 4a and
4b. The same epoxy alcohols were also obtained in the same
ratio if Ti(i-PrO)4 was added to the mixture of the initially
formed allylic hydroperoxides. 1H NMR (CDCl3): δ 5.09 (s, 1H
of 4a ), 5.00 (s, 1H of 4a ), 3.93 (d, J ) 7.5 Hz, 1H of 4a + 1H of
4b), 2.80 (d, J ) 7.5 Hz, 1H of 4a + 1H of 4b), 1.72 (s, 3H of
4a ), 1.53 (br s, 1H, hydroxyl), 1.29 (s, 3H of 4b), 1.27 (s, 3H of
4b).
Dioxeta n es fr om th e Rea ction of 2 w ith 1O2 in MeOH.
Although the labile dioxetanes were not isolated, the thermally
cleaved products 3-methyl-buten-2-al and 3-methylbuten-2-al-
4,4,4,3′,3′,3′-d6 were analyzed by GC-MS (Tcol ) 40 °C). The
aldehyde-d6 (M+ ) 90) has a retention time of 4.81 min, whereas
the aldehyde-d0 (M+ ) 84) has a retention time of 4.91 min.
1,4-Meth a n ol Ad d u cts fr om th e Rea ction of 2 w ith 1O2
in MeOH. After reduction of the initially formed methoxy
hydroperoxides with excess PPh3, the resulting isomeric (E)-5-
methoxy-2,5-dimethyl-hex-3-en-2-ol-1,1,1,2′,2′,2′-d6, 6a , and (E)-
5-methoxy-2,5-dimethylhex-3-en-2-ol-6,6,6,5′,5′,5′-d6, 6b, were
purified by column chromatography (hexane/ethyl acetate ) 3/1).
M+: 164. 1H NMR (CDCl3): δ 5.74 (d, J ) 16.1 Hz, 1H of 6a +
1H of 6b), 5.62 (d, J ) 16.1 Hz, 1H of 6a + 1H of 6b), 3.15 (s,
3H of 6a + 3H of 6b), 1.50 (br s, 1H, hydroxyl), 1.27 (s, 6H of
6b), 1.35 (s, 6H of 6a ).
1
this case, the major interaction between DMHD and O2
leads to nonreactive quenching (PQ), e.g., 72% in CH3-
CN, 73% in CH2Cl2, 81% in (CH3)2CO, and 93% in C6H6.
Thus, in aprotic solvents, because the zwitterionic inter-
mediate collapses exclusively through the nonreactive
pathway kq to starting materials, the ene product pre-
dominates even if kZI > kene. However, in methanol (kMeOH
+ kdiox > kq), a dramatic increase in dioxetane and
methanol-trapping products was observed. While the
zwitterionic intermediate finds also support from experi-
mental results reported by Foote and Manring,11 the
present negligible secondary isotope effects in the diox-
etane and in the methanol-trapping experiments do not
exclude an alternative mechanistic pathway in which the
[2 + 2] and 1,4-methanol-trapping adducts (Scheme 4)
are formed from the perepoxide without intervention of
a zwitterion. Recently, common perepoxide intermedi-
ates have been also proposed for the ene and [4 + 2]
pathways in the photooxygenation of chiral 1,2-dihy-
dronaphthalenes21 and a cyclic diene.22 Although it is
difficult to distingush between the two mechanistic
pathways, we favor the formation of the zwitterionic
intermediate in a faster step from a preceding perepoxide
intermediate.
In case the dipolar intermediate has been formed in a
slow step, the methoxy adduct next to the geminal CH3
methyls and the [2 + 2] adduct next to the geminal CD3
methyls resulting from ZI1 would be expected to pre-
dominate over the corresponding adducts produced from
ZI2 because hyperconjugation effect23 favors formation of
ZI1 over ZI2.
In conclusion, we have presented an energy profile for
1
the reaction of 2,5-dimethyl-2,4-hexadiene with O2 that
is consistent with kinetic primary and secondary isotope
effects. The reaction proceeds through the formation of
a perepoxide as a common intermediate for the ene and
[2 + 2] pathways. For the ene pathway, the hydrogen
abstraction is the rate-determining step, whereas the
dioxetane and methanol-trapping pathways proceed ei-
Ack n ow led gm en t. We thank professor G. J . Kara-
batsos for valuable comments. This work was supported
by a ΥΠΕΡ-1995 grant to G.V. and NATO (Grant No.
931419). The financial support of M & S Hourdakis SA.
is also acknowledged.
(21) Linker, T.; Rebien, F.; Toth, G. J . Chem. Soc., Chem. Commun.
1996, 2585-2586.
Su p p or tin g In for m a tion Ava ila ble: NMR spectra (9
pages). This material is contained in libraries on microfiche,
immediately follows this article in the microfilm version of the
journal, can be ordered from the ACS, see any current
masthead page for ordering information.
(22) Davis, K. M.; Carpenter, B. K. J . Org. Chem. 1996, 61, 4617-
4622.
(23) Recent experimental results show a significant inverse second-
ary isotope effect of kH/kD ) 0.72 in the [2 + 2] cycloaddition of TCNE
to diene 2, and it is due to the hyperconjugation effect. See: Vassil-
ikogiannakis, G.; Orfanopoulos, M. Tetrahedron Lett. 1996, 37, 3075-
3078.
J O971994N