A new [2 + 2] functionalization of C60 with alkyl-substituted 1,3- butadienes: A mechanistic approach. Stereochemistry and isotope effects

Article abstract of DOI:10.1021/ja981377w

The stereochemistry and secondary isotope effects of the [2 + 2] photocycloaddition of trans,trans- (7), cis,cis- (8), and cis,trans-2,4- hexadiene (9), 2,5-dimethyl-2,4-hexadiene (1), and its deuterated analogues 1-d₁, 1-d₆, and tra

Full text of DOI:10.1021/ja981377w

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J. Am. Chem. Soc. 1998, 120, 9911-9920
9911
A New [2 + 2] Functionalization of C60 with Alkyl-Substituted
1,3-Butadienes: A Mechanistic Approach. Stereochemistry and
Isotope Effects
Georgios Vassilikogiannakis, Nikos Chronakis, and Michael Orfanopoulos*
Contribution from the Department of Chemistry, UniVersity of Crete, Iraklion 71409, Greece
ReceiVed April 22, 1998
Abstract: The stereochemistry and secondary isotope effects of the [2 + 2] photocycloaddition of trans,
trans- (7), cis,cis- (8), and cis,trans-2,4-hexadiene (9), 2,5-dimethyl-2,4-hexadiene (1), and its deuterated
analogues 1-d1, 1-d6, and trans-1-d3 to C60 have been investigated. A loss of stereochemistry in the cyclobutane
ring for photocycloaddition of all three 2,4-hexadiene isomers 7, 8, and 9 to C60 was observed (the trans
stereochemistry in the cyclobutane ring predominates in all cases), while the unreactive double bond retained
its stereochemical integrity in the adducts. The cis double bond of 9 is 1.5 times more reactive than the trans.
The [2 + 2] photocycloaddition of (E)-2,4-dimethyl-2,4-hexadiene (10) to C60 is regiospecific, affording two
diastereomeric adducts, 10a and 10b, by addition on the methyl monosubstituted terminal double bond. These
results, when taken in conjunction with the small inverse intramolecular secondary isotope effect (kH/kD
0.90 ( 0.05) in the [2 + 2] photocycloaddition of 1-d6 to C60, favor the formation of an open biradical
intermediate in the rate-determining step.
)
Introduction
with less emphasis given to reaction mechanisms. The ene, as
well as the [2 + 2] additions to C60 and C70, are less common.
Foote and co-workers have reported [2 + 2] photocycload-
ditions of C60 with electron-rich alkynes (ynamines), such as
N,N-diethylpropynylamine9a,b and N,N-diethyl-4-methylpenten-
3-yn-1-amine.9c They proposed the formation of the triplet
excited state of C60 as the first step in the reaction sequence
(eq 1). Previous results14 had demonstrated that the triplet
excited state of C60 has a reduction potential close to 0.98 V
(36 kcal/mol triplet E) vs SCE and is formed with a quantum
yield of about unity. The second step of the reaction includes
the addition of ynamines to the triplet excited state of C60 via
electron (or at least charge) transfer from the electron-rich
ynamines, followed by rapid collapse of the initial ion pair or
charge-transfer complex to the [2 + 2] cycloadducts (eq 2). In
The discovery of C601,2 (buckminsterfullerene) and its isola-
tion in large quantities3 triggered the investigation of a remark-
able array of its reactions in the last seven years. Buckmin-
sterfullerene is electrophilic,4-6 reacts with alkenes and dienes,
and affords [4 + 2],7,8 [2 + 2],9-12 and ene13 adducts. Most of
the work has focused on adduct isolation and characterization,
(1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R.
E. Nature 1985, 318, 162.
(2) Kratschmer, W.; Fostiropoulos, K.; Huffman, D. R. Chem. Phys. Lett.
1990, 170, 167.
(3) Scrivens, W. A.; Bedworth, P. V.; Tour, J. M. J. Am. Chem. Soc.
1992, 114, 7917.
(4) Hirsch, A. Synthesis 1995, 895-913.
(5) Wudl, F. Acc. Chem. Res. 1992, 25, 157.
(6) Diederich, F.; Isaacs, L.; Philp, D. Chem. Soc. ReV. 1994, 243.
(7) (a) Kra¨utler, B.; Puchberger, M. HelV. Chim. Acta 1993, 76, 1626.
(b) Rubin, Y.; Khan, S.; Freedberg, D. I.; Yeretzian, C. J. Am. Chem. Soc.
1993, 115, 344. (c) An, Y. Z.; Anderson, J. L.; Rubin, Y. J. Org. Chem.
1993, 58, 4799. (d) Linssen, T. G.; Du¨rr, K.; Hirsch, A.; Hanack, M. J.
Chem. Soc., Chem. Commun. 1995, 103. (e) Khan, S. I.; Oliver, A. M.;
Raddon-Row, M. N.; Rubin, Y. J. Am. Chem. Soc. 1993, 115, 4919.
(8) (a) Belik, P.; Gu¨gel, A.; Spikermann, J.; Mu¨llen, K. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 78. (b) Gu¨gel, A.; Kraus, A.; Spikermann, J.; Belik,
P.; Mu¨llen, K. Angew. Chem., Int. Ed. Engl. 1994, 33, 559. (c) Iyoda, M.;
Sultana, F.; Sasaki, S.; Yoshida, M. J. Chem. Soc., Chem. Commun. 1994,
1929. (d) Zhang, X.; Foote, C. S. J. Org. Chem. 1994, 59, 5235.
(9) (a) Zhang, X.; Romero, A.; Foote, C. S. J. Am. Chem. Soc. 1993,
115, 11024. (b) Zhang, X.; Foote, C. S. J. Am. Chem. Soc. 1995, 117, 4271.
(c) Zhang, X.; Fan, A.; Foote, C. S. J. Org. Chem. 1996, 61, 5456.
(10) Hoke, S. H., II; Molstad, J.; Dilattato, D.; Jay, M. J.; Carlson, D.;
Kahr, B.; Cooks, R. G. J. Org. Chem. 1992, 57, 5069.
(11) (a) Wilson, S. R.; Kaprinidis, N.; Wu, Y.; Schuster, D. I. J. Am.
Chem. Soc. 1993, 115, 8495. (b) Wilson, S. R.; Wu, Y.; Kaprinidis, N.;
Schuster, D. I.; Welch, C. J. J. Org. Chem. 1993, 58, 6548. (c) Schuster,
D. I.; Cao, J.; Kaprinidis, N.; Wu, Y.; Jensen, A. W.; Lu, Q.; Wang, H.;
Wilson, S. R. J. Am. Chem. Soc. 1996, 118, 5639. (d) Jensen, A. W.; Khong,
A.; Saunders, M.; Wilson, S. R.; Schuster, D. I. J. Am. Chem. Soc. 1997,
119, 7303.
contrast, with extremely electron rich substrates, such as
ynediamines (e.g., N,N,N′,N′-tetraethylethynediamine), thioyn-
amines (e.g., N,N-diethyl-2-ethylthioethyneamine),9b and tet-
raalkoxyethylenes (e.g., tetraethoxyethylene),9c thermal [2 + 2]
cycloadditions to C60 have been reported. It was suggested that
these reactions with electron-rich alkynes and alkenes proceed
through a charge-transfer mechanism in the absence of light,
because of the high electron affinity of C60 and the high electron-
donating ability of these molecules. The thermal [2 + 2]
cycloaddition of benzyne to C60 has also been reported.10
(13) (a) Wu, S.; Shu, L.; Fan, K. Tetrahedron Lett. 1994, 35, 919. (b)
Komatsu, K.; Murata, Y.; Sugita, N.; Wan, S. M. T. Chem. Lett. 1994,
635. (c) Miles, W. H.; Smiley, P. M.; J. Org. Chem. 1996, 61, 2559.
(14) Arbogast, J. W.; Darmanyan, A. O.; Foote, C. S.; Rubin, Y.;
Diedrich, F. N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J. Phys. Chem.
1991, 95, 11.
(12) (a) Vassilikogiannakis, G.; Orfanopoulos, M. J. Am. Chem. Soc.
1997, 119, 7394. (b) Vassilikogiannakis, G.; Orfanopoulos, M. Tetrahedron
Lett. 1997, 38, 4323.
S0002-7863(98)01377-8 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/15/1998

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9912 J. Am. Chem. Soc., Vol. 120, No. 38, 1998
Vassilikogiannakis et al.
Scheme 1. Self-Sensitized Photooxygenation of 2
Photolysis of the isolated cycloadduct 2 gave complete cyclo-
reversion to C60 and diene in 3 min, consistent with an
equilibrium between C60 and 2. The Keq calculated from the
recovered C60 is approximately 7.7 × 10-2. Similar [2 + 2]
reversible photocyclizations were recently reported by Foote.9c
Along with the major cycloadduct 2, a small amount (∼3%) of
a side product was detected by HPLC. This byproduct, probably
a bisadduct, had a shorter retention time than adduct 2 and was
not further characterized. The adduct 2, which is stable at room
temperature, was purified by flash column chromatography
(toluene:hexane 2:1) and characterized by mass spectrometry,
1
by H NMR spectroscopy (Figure 1), and by its reaction with
1O2. FAB-MS of 2 gave the expected M + 1 ion at 831, which
Figure 1. [2 + 2] photocycloaddition of DMHD to C60.
1
corresponds to the molecular formula C68H14. The H NMR
Schuster and Wilson and co-workers recently reported a new
photochemical [2 + 2] cycloaddition of cyclic enones11a-c and
cyclic 1,3-diones11d to C60. Since this type of photocycload-
dition cannot be achieved by irradiation at 532 nm, where C60
is the only light-absorbing component, this result indicates that
fullerene triplets do not undergo addition to the ground state of
enones. The authors proposed that the photochemical [2 + 2]
cycloadditions of enones to C60 proceed by stepwise addition
of enone triplet excited states to the fullerene via a triplet 1,4-
biradical intermediate. The proposed mechanism was consistent
with the observation that alkenes, such as cyclopentene or
cyclohexene, do not photochemically add to C60.
Recently, we reported a new type of [2 + 2] functionalization
of C60 with moderately electron-rich p-methoxyarylalkenes.12
The stereochemistry and the secondary isotope effects of the
reaction with p-methoxystyrene12a showed that the photochemi-
cal [2 + 2] cycloaddition occurs by a two-step mechanism,
involving the formation of a dipolar or diradical intermediate
in the rate-determining step. The stepwise mechanism was also
supported by results of the [2 + 2] photochemical reaction of
C60 with cis/trans-4-propenylanisole.12b In this case, the
thermodynamically most stable trans cycloadduct was formed,
regardless of the geometry (cis or trans) of the propenylanisole.
In this paper, we report the stereochemistry and the secondary
isotope effects of a new type of photochemical [2 + 2]
cycloaddition of C60 with the less electron-rich alkyl-substituted
1,3-butadienes.
spectrum of 2 shows two signals due to the diastereotopic
methyls (Mea, Meb) at 1.88 and 1.89 ppm with the proper allylic
coupling, J1 ) 1.3 Hz and J2 ) 1.2 Hz, respectively; two
downfield absorptions at 1.94 and 2.07 ppm (lower fields due
to the influence of the C60 currents) which correspond to the
diastereotopic methyls on the cyclobutane ring (Mec, Med); a
doublet at 4.70 ppm which corresponds to the cyclobutane ring
hydrogen (Ha); and a doublet with allylic coupling at 6.20 ppm
for the vinylic hydrogen (Hb) (Figure 1). The UV-vis
absorption spectrum of 2 in toluene shows a strong absorption
at 437 nm, accompanied by a weak absorption at 709 nm. These
absorptions are characteristic15 of the dihydrofullerene structure,
thus indicating that the cycloaddition took place at the junction
of two six-membered rings.
It is interesting to note that, like C60 and C70,16 cycloadduct
2 is also an efficient photosensitizer and produces singlet oxygen
upon irradiation of its oxygenated solutions. Because adduct 2
bears an oxidizable group (alkene moiety), it undegoes facile
self-sensitized photooxygenation and produces, after tri-
phenylphosphine reduction, a mixture of the threo/erythro allylic
alcohols 3a and 3b as the only products, in a 55/45 ratio
(Scheme 1). The two stereoisomers were separated by flash
column chromatography using a mixture of toluene:hexane (2:
1) as eluent. The stereochemistry of adducts 3a and 3b (which
is erythro and which is threo) was not assigned. Similar self-
sensitized photooxygenated products of C60 adducts have been
recently reported.9a,17
The intramolecular secondary product isotope effects of the
[2 + 2] photocycloadditions of C60 with the 2-methyl-1′,1′,1′-
d3-5-methyl-2,4-hexadiene-1,1,1-d3 (DMHD-d6, 1-d6) and the
2,5-dimethyl-2,4-hexadiene-3-d1 (DMHD-d1, 1-d1) were mea-
sured. Substrates 1-d6 and 1-d1 reacted under the same
conditions described above for substrate 1 to yield the [2 + 2]
Results
Addition of 2,5-Dimethyl-2,4-hexadiene and Its Deuterated
Analogues to C60. A mixture of C60 and 20-fold excess of
2,5-dimethyl-2,4-hexadiene (DMHD, 1) in deoxygenated tolu-
ene, in the absence of light, did not react after 24 h at solvent
reflux. However, upon irradiation with a xenon lamp (Variac
Eimac Cermax 300 W), a rapid reaction was observed within
15 min (HPLC). A C60-fused cyclobutane, 6,6-(61,61-dimethyl-
62-(2′-methyl)propenylcyclobutane)dihydro[60]fullerene (2, Fig-
ure 1), was formed in 60% yield, based on recovered C60.
Additional irradiation did not increase the yield, even after
prolonged reaction time (7 h). The yield of the reaction
increased only after an increase of the diene concentration.
(15) (a) Vasella, A.; Uhlman, P.; Waldraff, C. A. A.; Diederich, F.;
Thilgen, C.; Angew. Chem., Int. Ed. Engl. 1992, 31, 1388. (b) Isaacs, L.;
Wehrsing, A.; Diederich, F. HelV. Chim. Acta 1993, 76, 1231. (c) Krautler,
B.; Puchberger, M. HelV. Chim. Acta 1993, 76, 1626.
(16) Orfanopoulos, M.; Kambourakis, S. Tetrahedron Lett. 1994, 35,
1945.
(17) (a) An, Y. Z.; Viado, A. L.; Arce, M. J.; Rubin, Y. J. Org. Chem.
1995, 60, 8330. (b) Torres- Garcia, G.; Mattay, J. Tetrahedron 1996, 52,
5421.

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Functionalization of C60 with Alkyl-1,3-butadienes
J. Am. Chem. Soc., Vol. 120, No. 38, 1998 9913
Scheme 2. [2 + 2] Photocycloaddition of trans-1-d3 to C60
Scheme 3. Photocycloaddition of trans,trans-2,4-Hexadiene
to C60
Figure 2. Intramolecular secondary isotope effects in the [2 + 2]
photocycloaddition of 1-d6 and 1-d1 to C60.
deuterated cycloadducts 4a, 4b and 5a, 5b, respectively (Figure
2). After flash column chromatographic purification (toluene:
hexane 2:1) of the reaction products, the secondary isotope
effects kH/kD, which result from the intramolecular competition
between the two double bonds of 1-d6 and 1-d1, were measured
1
by integration of the relevant H NMR signals of the [2 + 2]
products 4a, 4b and 5a, 5b, respectively. The product ratios
4a/4b and 5a/5b, which are proportional to the secondary isotope
effects kH/kD, are shown in Figure 2. The ratio 4a/4b was
Scheme 4. Photocycloaddition of cis,cis-2,4-Hexadiene to
C60
1
measured by integration of the H NMR signals at 1.94, 2.07
ppm for 4a (Mec, Med) and at 1.88, 1.89 ppm for 4b (Mea,
Meb). Similarly, the 5a/5b ratio was determined by integrating
the two single peaks at 4.70 and 6.17 ppm, which correspond
to 5a (Ha) and 5b (Hb), respectively. The results are summarized
in Figure 2.
To examine the stereochemistry of the [2 + 2] cycloaddition
of the diene to C60, the 2,5-dimethyl-2,4-hexadiene-1,1,1-d3
(DMHD-d3, trans-1-d3) was prepared in high stereochemical
purity by specific deuterium labeling of the trans methyl group
at the diene terminal. Reaction of C60 with trans-1-d3 under
the previously described conditions gave three of the possible
four [2 + 2] adducts (Scheme 2). The ratio 6a/6b/6c was
4-phenyltriazoline-3,5-dione (PTAD),19 tetracyanoethylene (TC-
NE),20 and 1,1-dichloro-2,2-difluoroethylene.21
Addition of the trans,trans-2,4-Hexadiene to C60.
A
1
mixture of C60 and 80-fold excess of the trans,trans-2,4-
hexadiene 7 in deoxygenated toluene reacted rapidly upon
irradiation at 5 °C for 20 min. After solvent evaporation under
reduced pressure, the products were purified by washing many
times with distilled n-hexane. The solid was dried in a vacuum
measured by H NMR integration of the signals of the four
differently shifted methyl groups to be 2:1:1. Under the
experimental conditions, only a small fraction (∼4%) of the
recovered trans-1-d3 was isomerized to the cis analogue.
To obtain further information on the stereochemistry of the
[2 + 2] photocycloaddition reaction of dienes, the addition of
the three stereoisomers of 2,4-hexadiene to C60 was studied.
These substrates have been successfully used in the past to
elucidate the stereochemistry of the [4 + 2] and [2 + 2]
cycloadditions of a number of other electrophiles, such as 1O2,18
1
1
and characterized by H NMR and FAB-MS. The H NMR
spectrum revealed the presence of the [2 + 2] diastereomeric
adducts 7a and 7b (Scheme 3, Figure 3). The coupling constant
between the hydrogens on the cyclobutane ring was 8.5 Hz for
the major adduct and 10.3 Hz for the minor adduct. The first
value is typical for a trans- and the second for a cis-substituted
cyclobutane ring,12a,b indicating that 7a is the major product.
The stereochemistry of the unreacted double bonds of 7a and
7b was determined by homonuclear decoupling experiments.
(18) O’Shea, K. E.; Foote, C. S. J. Am. Chem. Soc. 1988, 110, 7167.
(19) Jensen, F.; Foote, C. S. J. Am. Chem. Soc. 1987, 109, 6376.
(20) O’Shea, K. E.; Foote, C. S. Tetrahedron Lett. 1990, 31, 841.
(21) (a) Montgomery, L. K.; Schueller, K. E.; Bartlett, P. D. J. Am. Chem.
Soc. 1964, 86, 622. In this paper, in eq 2 on page 625, for (x/y)trans read
(y/x)trans. (b) Bartlett, P. D.; Dempster, C. J.; Montgomery, L. K.; Schueller,
K. E.; Wallbillich, G. E. H. J. Am. Chem. Soc. 1969, 91, 405.
(22) Introduction to NMR spectroscopy; Abraham, R. J., Fisher, J., Loftus,
P., Eds.; John Wiley and Sons Ltd.: Chichester, 1988.

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9914 J. Am. Chem. Soc., Vol. 120, No. 38, 1998
Vassilikogiannakis et al.
Figure 3. Photocycloadditions of the three 2,4-hexadienes isomers 7, 8, and 9 to C60.
Upon irradiation of the doublets at 1.83 ppm, which correspond
to the methyls Mea, Mec of 7a and 7b, the multiplets at 5.96
and 5.91 ppm, which correspond to the olefinic hydrogens
geminal to the Mea and Mec, collapsed to doublets, with
coupling constants J ) 15.1 and 14.2 Hz, respectively. These
values are typical for trans-alkyl-substituted double bonds.22 The
the thermal reaction of C60 with 8 is very slow.23 For example,
in the absence of light (thermal conditions), the [4 + 2] adduct
was not detected after 48 h of stirring at room temperature. The
stereochemistry of these products was confirmed by homo-
nuclear decoupling experiments. Decoupling of the hydrogens
Hb and Hd of the cyclobutane rings of 8a and 8b by irradiation
of the methyl groups Meb, Med at 1.98 ppm showed that the
resulting doublet of Hd at 4.46 ppm, which belongs to the less
predominant isomer, had a coupling constant J ) 10.4 Hz. This
value, typical for a cis-substituted cyclobutane ring,12 indicates
a cis stereochemistry for the minor adduct on the cyclobutane
ring. Moreover, the coupling constant between the hydrogens
for the major adduct, J ) 8.7 Hz, is typical of a trans-substituted
cyclobutane ring.12 Irradiation of the vinylic methyl groups Mea,
Mec at 1.88 ppm simplified the multiplet absorptions of Ha and
Hc at 5.84 and 5.91 ppm of 8a and 8b, into two doublets with
coupling constants J ) 10.5 and 10.8 Hz, respectively. These
values are consistent with a cis stereochemistry of the double
1
ratio 7a/7b was measured by H NMR to be 7.8:1.
Along with the [2 + 2] products 7a and 7b, a substantial
amount (21.8%) of the cis [4 + 2] adduct 7c was shown by 1H
NMR. The identical [4 + 2] adduct was isolated by the thermal
addition of trans,trans-2,4-hexadiene to C60 at room temperature.
1
The structure of 7c was confirmed by matching its H NMR
spectrum to that of the adduct produced separately by the
thermal addition of diene 7 to C60.23 Efforts to separate these
cycloadducts by either flash column chromatography or HPLC
were unsuccessful. At the end of irradiation, a small isomer-
ization of the recovered trans,trans-2,4-hexadiene to the cis,
trans-2,4-hexadiene (∼2%) was found by gas chromatography.
Addition of the cis,cis-2,4-Hexadiene to C60. Cycloaddition
of cis,cis-2,4-hexadiene 8 to C60 under photochemical conditions
identical to those described previously produced two diastere-
omeric [2 + 2] products, 8a and 8b (Scheme 4). 1H NMR
analysis of the reaction mixture 8a and 8b showed a spectrum
remarkably different from that of 7a and 7b (Figure 3). In this
case, the thermal [4 + 2] cycloadduct was not detected, because
1
bonds22 in 8a and 8b. The ratio 8a/8b was measured by H
NMR to be 5.3:1. Capillary column gas chromatographic
analysis of the recovered cis,cis diene 8 showed only 4%
isomerization to the cis,trans isomer.
Addition of the cis,trans-2,4-Hexadiene to C60. The
photochemical cycloaddition of the cis,trans-2,4-hexadiene 9
to C60 was also studied under experimental conditions identical
to those described. 1H NMR analysis of the reaction mixture
revealed the formation of the expected four [2 + 2] stereoiso-
(23) A detailed report for the [4 + 2] cycloadditions of the isomeric
2,4-hexadienes to C60 will appear elsewhere.