Photochemical and chemical electron transfer reactions of bicyclo[2.1.0]pentanes (housanes) in solution and in zeolite cavities

Article abstract of DOI:10.1021/ja950397k

Photochemical electron transfer (PET) and chemical electron transfer (CET) studies have been conducted in solution and within zeolite cavities for the bicyclo[2.1.0]pentanes (2a-j), prepared by direct photolysis of the corresponding azoalkanes 1. The adva

Full text of DOI:10.1021/ja950397k

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2380
J. Am. Chem. Soc. 1996, 118, 2380-2386
Photochemical and Chemical Electron Transfer Reactions of
Bicyclo[2.1.0]pentanes (Housanes) in Solution and in Zeolite
Cavities
Waldemar Adam,† Avelino Corma,‡ Miguel A. Miranda,‡
Mar´ıa-Jose´ Sabater-Picot,† and Coskun Sahin*,†
Contribution from the Institut fu¨r Organische Chemie, UniVersita¨t Wu¨rzburg, Am Hubland,
D-97074 Wu¨rzburg, Germany, and Instituto de Tecnolog´ıa Qu´ımica UPV-CSIC,
UniVersidad Polite´cnica de Valencia, Camino de Vera S/n, 46071 Valencia, Spain
ReceiVed February 6, 1995X
Abstract: Photochemical electron transfer (PET) and chemical electron transfer (CET) studies have been conducted
in solution and within zeolite cavities for the bicyclo[2.1.0]pentanes (2a-j), prepared by direct photolysis of the
corresponding azoalkanes 1. The advantage of the CET oxidations is that they proceed catalytically in a clean
manner to afford the rearranged cyclopentenes 3 in excellent yields. A complete reversal in the regioselectivity of
the 1,2 migration has been observed for the unsymmetrical derivatives of bicyclo[2.1.0]pentane, namely 2b (methyl
substitution) Versus 2c,i (phenyl substitution). Both in solution and in the zeolite cavities, the less substituted
cyclopentene 3b′ is obtained for the methyl derivative 2b and the more substituted cyclopentenes 3c,i for the phenyl
cases 2c,i. This unexpected fact is rationalized in terms of delocalization of the positive charge into the aromatic
ring for the phenyl-substituted radical cation, as corroborated by AM1 calculations. Furthermore, the electron transfer
results of stereolabeled housanes demonstrate that for the deuterium-labeled bridgehead dialkyl-substituted housane
2e(D) also the stereochemical memory effect operates. In contrast, for the methyl-labeled housanes anti- and syn-
2h, exclusively hydrogen migration occurs. This differing behavior is interpreted in terms of facile ring inversion
of the syn-2h•+ radical cation to the more stable anti isomer and subsequent preferential migration of the pseudo-
axial hydrogen atom. Moreover, the heterogeneous PET chemistry of the bicyclopentanes 2 in the zeolites establishes
convincingly that tailor-made, encapsulated electron transfer photosensitizers serve as effective electron acceptors
on optical excitation. In spite of the inherent diffusion problems in such solid sensitizers, quite efficient PET activity
is observed compared to that in the homogeneous phase. Unfortunately, the steric confinement imposed by the
zeolite support is not sufficient for the small bicyclopentanes, which penetrate into the zeolite interior, to promote
selective rearrangements of the radical cation intermediates.
Introduction
reactivities of these intermediates differ since some chemical
events occur exclusively at the CIP, SSIP, or FI stage. The
mechanistic complexities in the chemical fate of the resulting
radical ion species are illustrated in the alternative reaction
channels for the PET reaction of bicyclo[2.1.0]pentanes (Scheme
1).3 As a matter of fact, the yield of photoproduct depends to
a high degree on the lifetime of the radical ions, which is fixed
by the rate of electron back-transfer (BET). Therefore, product
formation is only feasible if one can effectively decrease the
rate of BET.2e,4 An effective method for photoinduced electron
transfer to generate more persistent radical ions, is the co-
sensitization technique (eq 2).2c,5 The advantage by employing
Photoinduced electron transfer (PET) is of current interest,
and numerous electron transfer sensitizers have been employed
not only for mechanistic but also for synthetic purposes.1,2 The
dynamics of these PET processes imply the intermediacy of
donor-acceptor (A ) acceptor, D ) donor) entities character-
ized by the intermolecular distance and the degree of charge
separation (eq 1). The formation of molecular species such as
A* + D h [A•-/D•+] h A•- + D•+
(1)
contact ion pairs (CIP), solvent-separated ion pairs (SSIP), and
free ions (FI) have been documented.1,2 It is known that the
(3) (a) Adam, W.; Sahin, C.; Sendelbach, J.; Walter, H.; Chen, G.-F.;
Williams, F. J. Am. Chem. Soc. 1994, 116, 2576-2584. (b) Adam, W.;
Sendelbach, J. J. Org. Chem. 1993, 58, 5316-5322. (c) Adam, W.;
Sendelbach, J. J. Org. Chem. 1993, 58, 5310-5315. (d) Adam, W.;
Denninger, U.; Finzel, R.; Kita, F.; Platsch, H.; Walter, H.; Zang, G. J.
Am. Chem. Soc. 1992, 114, 5027-5035. (e) Adam, W.; Walter, H.; Chen,
G.-F.; Williams, F. J. Am. Chem. Soc. 1992, 114, 3007-3014. (f) Adam,
W.; Do¨rr, M. J. Am. Chem. Soc. 1987, 109, 1570-1572.
†University of Wu¨rzburg.
‡ University of Valencia.
X Abstract published in AdVance ACS Abstracts, March 1, 1996.
(1) (a) Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401-449. (b)
Sakuragi, H.; Nakagaki, R.; Oguchi, T.; Arai, T.; Tokumaru, K.; Nagakura,
S. Chem. Phys. Lett. 1987, 135, 325-329. (c) Oguchi, T.; Arai, T.; Sakuragi,
H.; Tokumaru, K. Bull. Chem. Soc. Jpn. 1987, 60, 2395-2399. (d) Julliard,
M.; Chanon, M. Chem. ReV. 1983, 83, 425-506. (e) Fox, M. A., Chanon,
M., Eds. Photoinduced Electron Transfer; Elsevier: Amsterdam, 1988.
(2) (a) Mattay, J. Angew. Chem., Int. Ed. Engl. 1987, 26, 825-845. (b)
Mattay, J. Synthesis 1989, 223-252. (c) Farid, S.; Mattes, S. L. In Organic
Photochemistry; Padwa, A., Ed.; Marcel Dekker: New York, 1983; Vol.
6, pp 233-326. (d) Fox, M. A. In AdVanced Photochemistry; Volman, D.
H., Hammond, G. S., Gollnick, K., Eds.; J. Wiley & Sons: New York,
1986; Vol. 13, pp 237-327. (e) Kavarnos, G. J. Top. Curr. Chem. 1990,
156, 21-58.
(4) Gould, I. R.; Young, R. H.; Moody, R. E.; Farid, S. J. Phys. Chem.
1991, 95, 2068-2080.
(5) (a) Gould, I. R.; Ege, D.; Mooser, J. E.; Farid, S. J. Am. Chem. Soc.
1990, 112, 4290-4301. (b) Julliard, M. In Photoinduced Electron Transfer;
Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part B, pp
216-313. (c) Schaap, A. P.; Siddiqui, S.; Prasad, G.; Palomino, E.; Lopez,
L. J. Photochem. 1984, 25, 167-181. (d) Arnold, D. R.; Snow, M. S. Can.
J. Chem. 1988, 66, 3012-3026. (e) Majiama, T.; Pac, C.; Nakasone, A.;
Sakurai, H. J. Am. Chem. Soc. 1981, 103, 4499-4508.
0002-7863/96/1518-2380$12.00/0 © 1996 American Chemical Society

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Electron Transfer Reactions of Bicyclo[2.1.0]pentanes
J. Am. Chem. Soc., Vol. 118, No. 10, 1996 2381
Scheme 1
a codonorsin most cases biphenylsis the spatial separation of
the reduced electron acceptor from the desired donor radical
cation.
In the present study, various bicyclo[2.1.0]pentanes 2 were
chosen as substrates for the electron transfer reactions in solution
and within zeolite cavities. The comparison of the CET and
•+
A* + Ph2 h [A•-/Ph2•+] h A•- + Ph2
(2)
Ph2•+ + D h [Ph2/D•+] h Ph2 + D•+
We have previously shown that chemical electron transfer
(CET) with tris(aryl)aminium salts constitutes a useful method
to minimize BET to 1,3-cyclopentanediyl radical cations derived
from bicyclo[2.1.0]pentanes.6 These tris(aryl)aminium salts are
convenient one-electron oxidants, whose oxidation potential can
be conveniently tuned by the number of bromine substituents
contained in the three aryl rings.7 The catalytically generated
1,3-diyl radical cations rearrange regioselectively to the corre-
sponding cyclopentenes. Since intermediary D-A radical ion
pairs are not formed in the CET compared to the PET mode
(eq 3), alternative reaction channels for the 1,3-radical cation
are minimized and excellent yields of defined rearrangement
products are obtained.6 Therefore, this oxidative rearrangement
methodology may constitute a useful synthetic tool for tailor-
made substrates.
PET modes provides valuable mechanistic insight on the
rearrangement of the intermediary 1,3-diyl radical cations. The
convenience and scope of CET by tris(aryl)aminium salts is
demonstrated in its application to the bicyclo[2.1.0]pentanes 2.
-
-
SbCl6 Ar3N•+ + D h [SbCl6 /Ar3N/D•+] h
SbCl6- + Ar3N + D•+ (3)
Results
Synthesis of Starting Materials and Structure Assign-
ments. Synthesis of the Azoalkanes. 2,3-Diazabicyclo[2.2.1]-
hept-2-ene (1a),11 1-methyl-2,3-diazabicyclo[2.2.1]hept-2-ene
(1b),12 1-phenyl-2,3-diazabicyclo[2.2.1]hept-2-ene (1c),13 1,4-
dimethylbicyclo[2.2.1]hept-2-ene (1d)12 and (1R,4R,4aR,5â,
8â,8aR)-1,4,4a,5,8,8a-hexahydro-9,9-dimethyl-1,4-diphenyl-1,4:
5,8-dimethanophthalazine (1j)14a were prepared as reported. The
synthesis of the 1,4-dialkylated 2,3-diazabicyclo[2.2.1]hept-2-
enes 1e-g with stereolabels at the C-7 position has already been
published.15 The procedure requires syn carbometalation of the
protected 1-alkynones by dialkylcuprates, followed by hydrolysis
of the ketal functionality to afford the γ,δ-unsaturated ketones.
Conversion to the tosylhydrazones followed by BF3-catalyzed
Recently, the 2,4,6-triphenylpyrylium cation (Y[TP+]),8 a
well-established electron acceptor in its excited state,9 has been
entrapped in Y zeolite for heterogeneous PET studies. It was
discovered, that the zeolite framework stabilizes the resulting
ion pairs and regulates decisively the rate of BET.10 Thus, such
novel heterogeneous sensitizers offer the opportunity to explore
the chemical behavior of radical cations in confined spaces.
(6) Adam, W.; Sahin, C. Tetrahedron Lett. 1994, 35, 9027-9030.
(7) Schmidt, W.; Steckhan, E. Chem. Ber. 1980, 113, 577-585.
(8) (a) Corma, A.; Forne´s, V.; Garc´ıa, H.; Miranda, M. A.; Primo, J.;
Sabater, M. J. J. Am. Chem. Soc. 1994, 116, 2276-2280. (b) Corma, A.;
Forne´s, V.; Garc´ıa, H.; Miranda, M. A.; Sabater, M. J. J. Am. Chem. Soc.
1994, 116, 9767-9768. (c) Corma, A.; Garc´ıa, H.; Iborra, S.; Mart´ı, V.;
Miranda, M. A.; Primo, J. J. Am. Chem. Soc. 1993, 115, 2177-2180. (d)
Alvaro, M.; Corma, A.; Garc´ıa, H.; Miranda, M. A.; Primo, J. J. Chem.
Soc., Chem. Commun. 1993, 1041-1042. (e) Corma, A.; Forne´s, V.; Rey,
F. Appl. Catal. 1990, 59, 267-274.
(9) Miranda, M. A.; Garc´ıa, H. Chem. ReV. 1994, 94, 1063-1089.
(10) (a) Ramamurthy, V., Ed. Photochemistry in Organized and Con-
strained Media; Verlag Chemie: New York, 1991. (b) Ramamurthy, V.;
Caspar, J. V.; Corbin, D. R. J. Am. Chem. Soc. 1991, 113, 594-600. (c)
Gessner, F.; Scaiano, J. C. J. Photochem. Photobiol., A 1992, 67, 91-100.
(d) Sankararaman, S.; Yoon, K. B.; Yobe, T.; Kochi, J. K. J. Am. Chem.
Soc. 1991, 113, 1419-1421.
(11) (a) Gassman, P. G.; Mansfield, K. T. Org. Synth. 1969, 49, 1-6.
(b) Criegee, R.; Rimmelin, A. Chem. Ber. 1957, 90, 414-417.
(12) Wilson, R. M.; Rekers, J. W.; Packard, A. B.; Elder, R. C. J. Am.
Chem. Soc. 1980, 102, 1633-1641.
(13) Adam, W.; Grabowski, S.; Platsch, H.; Hannemann, K.; Wirz, J.;
Wilson, R. M. J. Am. Chem. Soc. 1989, 111, 751-753.
(14) (a) Beck, K.; Hu¨nig, S. Chem. Ber. 1987, 120, 477-483. (b) Beck,
K.; Ho¨hn, A.; Hu¨nig, S.; Prokschy, F. Chem. Ber. 1984, 117, 517-533.
(15) Adam, W.; Sahin, C.; Schneider, M. J. Am. Chem. Soc. 1995, 117,
1695-1702.

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2382 J. Am. Chem. Soc., Vol. 118, No. 10, 1996
Adam et al.
Table 1. Product Studies of the Chemical and Photochemical
Electron Transfer Reactions for the Bicyclopentanes 2 in Solution
and within Zeolite Cavities
cyclization to the azoalkanes completes the sequence. The
hitherto unknown 1,4-dialkylated azoalkane 1h with an ad-
ditional methyl stereolabel at the C-7 position was synthesized
by following this protocol. Contrary to the deuterium-labeled
cases, the intermediary vinylcuprate had to be first activated
prod
dist (%)b,c
t
convb mbb
entry substrate modea (min) solvent (%) (%)
16
3
3′
with P(OEt)3 before the electrophile (CH3I) was added. For
the deuterium-labeled tosylhydrazones, as well as for the methyl-
labeled one, the BF3-catalyzed cyclization process is highly
diastereoselective in that the initial syn/anti diastereomeric ratio
of the tosylhydrazone dictates the stereochemical outcome in
the final azoalkane. The methyl group at δ 0.43 for the major
isomer (83%) of azoalkane 1h possesses the syn configuration
(shielding by the proximate azo group17), while the correspond-
ing methyl resonance for the minor anti isomer is located at δ
0.69. NOE experiments corroborated the proposed stereo-
chemistry. Whereas irradiation of the methyl group of the major
isomer at δ 0.43 gave an enhanced intensity only for 7-Ha
(2.4%), irradiation of the methyl group of the minor isomer at
δ 0.69 increased the 7-Hs (2.5%) and 5,6-Hx (1.5%) resonances.
The tricyclic azoalkane 1i was synthesized according to the
Hu¨nig route.14 By comparison with the literature spectral data
of the two symmetrically substituted derivatives, for the
unsaturated precursors, the major regioisomer was assigned the
1-phenyl-4,8,8-trimethyl and the minor the 4-phenyl-1,8,8-
trimethyl substitution pattern.
Synthesis of the Bicyclopentanes. The bicyclo[2.1.0]-
pentanes 2 were obtained by direct photolysis of the azoalkanes
1 and separated by preparative gas or column chromatography.
As for simple DBH-type azoalkanes,18 the photolysis of azo-
alkanes 1 afforded exclusively housanes 2 and only traces of
olefins 3. Accordingly, bicyclo[2.1.0]pentane (2a),11b,19 1-meth-
ylbicyclo[2.1.0]pentane (2b),20 1-phenylbicyclo[2.1.0]pentane
(2c),21 1,4-dimethylbicyclo[2.1.0]pentane (2d),20 and 4,4-di-
methyl-3,5-diphenyl-endo-tetracyclo[5.2.1.02,6.03,5]dec-8-ene (2j)14
were prepared and their spectral and physical data found to be
in accord with those reported. The stereochemical assignment
in the deuterium-labeled bicyclo[2.1.0]pentanes 2e-g(D) was
confirmed by comparison with the literature spectral data of
the parent 1,4-dimethylbicyclo[2.1.0]pentane (2d) and on the
basis of the characteristic W coupling (J ) 1.80-2.04 Hz) only
for 5-Ha.20 Thus, the high-field proton in 2d-f (δ 0.13-0.22)
was assigned to 5-Ha and the low-field one (δ 0.61-0.69) to
5-Hs. In contrast, for housane 2g, the high-field proton (δ 0.39)
belongs to 5-Hs and the low-field one (δ 0.56) to 5-Ha. The
different chemical shifts presumably derive from the proximity
of the bulky tert-butyl group. NOE experiments on the bicyclo-
[2.1.0]pentanes 2d,g corroborated the proposed stereochemistry.
For housane 2h, the methyl groups at δ 0.86 and 1.00 were
assigned the anti and syn configurations as confirmed also by
NOE experiments.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
2a
2a
2a
2b
2b
2b
2c
2c
2c
2c
2d
2d
2e
2e
TPT
30 CH2Cl2
60 CH2Cl2
11
1
100
Y[TP]
TDA•+
TPT
12 90 100
1
5
CH2Cl2 100 75 100
CH2Cl2 23 26
41
CH2Cl2 100 89
100
67
100
23
22
20
22
d
Y[TP]
15 CH2Cl2
1
7
33
TBA•+
DCA/Ph2 10 CH3CN
TPT
31 60
36 57
18 40
100 90
15 60
100 95 100
17 58 100
77
78
80
78
95
20 CH2Cl2
30 CH2Cl2
Y[TP]
TBA•+
1
CDCl3
DCA/Ph2 10 CH3CN
TBA•+
TPT
1
5
CDCl3
CH2Cl2
Y[TP]
30 CH2Cl2
4
50 100
15
2e(D)e TBA•+
1
1
C6D6
C6D6
100 92 100e
100 95 100f
2e(D)f TBA•+
2f
2f
2f
TPT
10 CH2Cl2
30 CH2Cl2
18 51
17 43
61
57
61
60
61
50
62
62
68
39
43
39
40
39
50
38
38
32
Y[TP]
TBA•+
TBA•+
TPT
1
1
5
CH2Cl2 100 91
CH3CN 100 85
CH2Cl2
2g
2hg
2hg
2hg
17 73
62
Y[TP]
30 CH2Cl2
7
TBA•+
1
1
1
CH2Cl2 100 89
CH2Cl2 100 99
CH2Cl2 100 85
anti-2h TBA•+
syn-2h TBA•+
2i
2i
2i
2j
2j
2j
TPT
10 CDCl3
30 CDCl3
20 70 100
12 60 100
100 97 100
70 85
0
Y[TP]
TBA•+
DCA/Ph2
Y[TP]
TBA•+
1
3
CDCl3
CH3CN
27
h
h
60 CDCl3
CDCl3
0
1
100 97
27
a TPT ) 2,4,6-triphenylpyrylium tetrafluoroborate, Y[TP] ) 2,4,6-
triphenylpyrylium cation entrapped in the Y zeolite, TDA•+ ) tris(2,4-
bromophenyl)aminium hexachloroantimonate, and TBA•+ ) tris(4-
bromophenyl)aminium hexachloroantimonate. b Conversion, mass balance
(mb), and product distribution were determined by quantitative capillary
GC analysis (error ca. 5% of stated value) or by 1H-NMR spectroscopy
(error ca. 10% of stated values). c Relative yields normalized to 100%,
error ca. 2-5% of the stated values. d The remainder 5% is dimethyl-
cyclopentadiene. e syn/anti-2e(D) ) 42:58 gives a ratio of 3e(2-D):
1
3e(3-D) ) 58:42 as determined by H-NMR spectrosccopy, error ca.
3% of stated value. f syn/anti-2e(D) ) 48:52 gives a ratio of 3e(2-D):
1
3e(3-D) ) 52:48 as determined by H-NMR spectroscopy, error ca.
3% of stated value. g syn/anti isomers, ratio ) 60:40. h 73% 5,5-
dimethyl-1,4-diphenylcyclopentadiene (4j) and 73% cyclopentadiene
were formed.
product by GC (entry 1, Table 1); the remainder was undefined
higher-molecular-weight material. The highest mass balance
was achieved in the photosensitized reaction with encapsulated
Y[TP] (entry 2). While complete conversion (entry 3) was
observed when housane 2a was treated with catalytic amounts
(2-10 mol %) of the strong oxidant (Eox ) 1.50 V Vs SCE)7
tris(2,4-dibromophenyl)aminium hexachloroantimonate (TDA•+),
the less powerful (Eox ) 1.06 V Vs SCE)7 tris(4-bromophenyl)-
aminium hexachloroantimonate (TBA•+) failed to oxidize this
substrate (not shown in Table 1).
Contrary to the parent housane 2a, the unsymmetrically
bridgehead-substituted housanes 2b,c were consumed com-
pletely by catalytic amounts of TBA•+ (entries 6 and 10).
Housane 2b yielded in all cases the less substituted cyclopentene
3b′ 22 as the major (entry 5) or as the exclusive (entries 4 and
6) oxidation product. The related phenyl-substituted housane
2c afforded always (entries 7-10) the cyclopentenes 3c21 and
Electron Transfer Reactions of Bicyclopentanes 2. The
product data are summarized in Table 1. Oxidation of the parent
bicyclo[2.1.0]pentane 2a under PET conditions gave a very low
mass balance (1%) of cyclopentene 3a as the only detectable
(16) Usually the cancerogenic hexamethylphosphoric triamide is used
in activating vinylcuprates, cf.: Alexakis, A.; Normant, J. J. Organomet.
Chem. 1975, 96, 471-485. Alexakis, A.; Cahiez, G.; Normant, J. F.
Synthesis 1979, 826-830.
(17) For related systems see ref. 3e and also the following Paquette, L.
A.; Leichter, L. M. J. Am. Chem. Soc. 1971, 93, 5128-5136.
(18) (a) Adam, W.; DeLucchi, O. Angew. Chem. 1980, 92, 815-832;
Angew. Chem., Int. Ed. Engl. 1980, 762. (b) Engel, P. S. Chem. ReV. 1980,
80, 99-150.
(19) Adam, W.; Oppenla¨nder, T.; Zang, G. J. Org. Chem. 1985, 50,
3303-3312.
(20) Wiberg, K. B.; Kass, S. R.; Bishop, K. C., III J. Am. Chem. Soc.
1985, 107, 996-1002.
(22) (a) Descoins, C.; Julia, M.; Sang, H. V. Bull. Soc. Chim. Fr. 1971,
4087-4093. (b) Ohbe, Y.; Matsuda, T. Bull. Chem. Soc. Jpn. 1975, 48,
2389-2390.
(21) McKinney, M. A.; Chou, S. K. Tetrahedron Lett. 1974, 1145-1148.

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Electron Transfer Reactions of Bicyclo[2.1.0]pentanes
J. Am. Chem. Soc., Vol. 118, No. 10, 1996 2383
3c′ 23 as rearrangement products, with the higher substituted 3c
as the major regioisomer. The tricyclic housane 2i gave for all
three electron transfer modes (entries 26-28) cyclopentene 3i
as the only rearrangement product.
Y[TP] mode even in the presence of the hindered base. Only
for this extremely acid-sensitive housane does the zeolite
framework8 seem to be acidic enough to catalyze rearrangement
in competition with electron transfer.
Control experiments on the cyclopentenes 3b,b′ 3e and 3c,c′
confirmed that these olefins did not interconvert into each other
under the employed electron transfer conditions. Furthermore,
by monitoring the product distribution of the photolyses as a
function of time, it has been determined that no secondary
oxidation of the cyclopentenes 3 took place up to the conversions
stated in Table 1.
Location of the 2,4,6-Triphenylpyrylium Cation (TP+) in
the Y-Type Faujasite. This question was assessed by absorp-
tion studies with DBH derivatives of different size on protonated
Y zeolite (HY). For the small azoalkanes 1a,h, a high
percentage (91-100%) of material was absorbed in contrast to
the large and bulky azoalkane 1j. Thus, the azoalkanes 1a,h
have the proper size to penetrate into the interior of the zeolite,
whereas the 1,4-diphenyl-substituted azoalkane 1j remains
outside, possibly absorbed on the external surface. Indeed, when
the PET reaction of azoalkane 1j was performed in the
heterogeneous phase, hardly any oxidation was observed. This
contrasts sharply with the extensive oxidation observed in
solution.3c Thus, these results suggest that most TP+ cations
are located internally in the cavities.
For the symmetrical 1,4-dialkylated housanes 2d,e, the CET
mode (TBA•+) proved to be again more suitable than the
photochemical ones in regard to conversion and mass balance
(compare entries 12, 15, and 16 Versus 11, 13, and 14). Thus,
complete conversion of the substrates to cyclopentenes 3d24
(entry 12) and 3e25 (entries 15 and 16) was achieved with
catalytic amounts of TBA•+. Monitoring of the reaction
1
progress by H NMR spectroscopy revealed that the initial
deuterium distribution in the housane 2e(D) was retained (eq
4) quantitatively (entries 15 and 16) in the cyclopentene product
Discussion
Chemical Electron Transfer of Bicyclopentanes 2. Table
1 reveals convincingly that the CET mode is advantageous for
the oxidative rearrangement of the bicyclopentanes 2. The same
chemistry as that for the PET mode is observed, except that
the oxidations proceed catalytically in a clean manner to afford
the rearranged cyclopentenes 3 in high yields. The lack of
alternative reaction channels (see Scheme 1) for the 1,3-radical
cation (and the 1,2-radical cation) presumably accounts for the
observed clean reactions and the high yields (75-99%) of the
CET mode, while in the PET mode, reaction with the reduced
photooxidant occurs (low mass balance, side products, etc.).3
This underscores the advantage of the chemical electron transfer
(CET) over the photosensitized (PET) modes.
However, as shown in the Results, at least one bridgehead
alkyl or aryl substituent on the bicyclopentane framework is
necessary for oxidation by TBA•+. Its oxidation power is not
sufficient to convert the parent unsubstituted housane 2a (Eox
) 1.91 V).29 Although the stronger oxidant TDA•+ smoothly
oxidizes even housane 2a, its application as electron transfer
oxidant is restricted to substrates for which the rearrangement
product, in this case the cyclopentene 3, has an unsubstituted
double bond, e.g. the cyclopentenes 3a,b′,c′, which are not
oxidized further to an intractable higher-molecular-weight
material. Fortunately, tri- and tetrasubstituted olefins persist
toward TBA•+. For this reason, only TDA•+ was used to oxidize
bicyclopentane 2a (entry 3).
In all cases examined, catalytic amounts (2-10 mol %) of
Ar3N•+ oxidant were enough to achieve complete conversion
of the substrate within a few seconds, although the aminium
salts possess lower oxidation potentials than the housanes 2.
The driving force presumably derives from the irreversible
rearrangement step which follows the endothermic electron
transfer process.30 Therefore, a catalytic cycle is proposed
(Scheme 2) in which electron transfer from housane 2 to the
aminium salt Ar3N•+ serves for initiation, followed by subse-
quent 1,2 Wagner-Meerwein rearrangement of the housane
radical cation 2•+. The cycle is completed by BET from the
3e(D). The well-separated 5-Ha,s resonances for housane 2e(D)
and the 2,3-H resonances for 3e(D) provided reproducible results
in the NMR analysis.
For the unsymmetrical 1,4-dialkylated housanes 2f-h, the
expected regioisomeric cyclopentenes 3f-h25,26 and 3f-h′ 27
were obtained in all cases (entries 17-25), with 3f-h as the
major regioisomers. Unexpectedly, oxidation of the trialkylated
housane 2h yielded only the two cyclopentenes 3h,h′ (entries
21-23) from H shifts, instead of the four possible ones derived
from H as well as methyl shifts.
The superiority of the chemical electron transfer mode
(complete conversion and higher mass balance) over the
photochemical ones was confirmed also for housane 2j (entries
29 and 31). Both PET and CET yielded olefin 3j3c and the
cycloreversion products 4j3c,28 and cyclopentadiene. The lack
of rearrangement products for the Y[TP] mode (entry 30)
confirmed that this bulky housane 2j did not enter the interior
of the zeolite and that outside the zeolite framework no electron
transfer process occurs.
The possible involvement of acid-catalyzed rearrangement
of the housanes 2 was suppressed by the addition of the hindered
base 2,6-di-tert-butylpyridine (not shown in Table 1). The
unaltered product distributions confirmed that this hindered
pyridine interferes with neither the PET nor CET reaction.
Contrary to the PET and CET reactions in solution, acid-
catalyzed rearrangement was observed with housane 2b for the
(23) (a) von Braun, J.; Ku¨hn, M. Ber. Dtsch. Chem. Ges. 1927, 60, 2551-
2557. (b) von Braun, J.; Kamp, E.; Kopp, J. Ibid. 1937, 70, 1750-1760.
(24) Rei, M. H. J. Org. Chem. 1978, 43, 2173-2178.
(25) Sisido, K.; Kurozumi, S.; Utimoto, K.; Isida, T. J. Org. Chem. 1966,
31, 2795-2802.
(26) (a) Vaveck, D.; Jaques, J. Bull. Soc. Chim. Fr. 1969, 10, 3505-
3515. (b) Richer, J. C.; Belanger, P. Can. J . Chem. 1966, 44, 2057-2066.
(27) Gajewski, J. J.; Squicciarini, M. P. J. Am. Chem. Soc. 1989, 111,
6717-6728.
(28) (a) Adam, W.; Reinhard, G.; Platsch, H.; Wirz, J. J. Am. Chem.
Soc. 1990, 112, 4570-4571. (b) Paquette, L. A.; Leichter, L. M. J. Org.
Chem. 1974, 39, 461-467.
(29) (a) Gassman, P. G.; Yamaguchi, R. Tetrahedron 1982, 38, 1113-
1122. (b) Gassman, P. G.; Yamaguchi, R.; Koser, G. F. J. Org. Chem. 1978,
43, 4392-4393.
(30) (a) The (irreversible) oxidation potentials for 2a and 2i are 1.91
and 1.42 V and the (reversible) ones for TDA and TBA are 1.50 and 1.06
V. The cyclovoltammetric measurements were performed in Prof. M.
Schmittel’s research group at the University of Wu¨rzburg, and we are
grateful to H. Trenkle for technical assistance. (b) The (irreversible)
oxidation potentials for 3a and 3i are 2.03 and 1.58 V.