Effect of supramolecular inclusion on the 1,2-rearrangement of free radicals

Article abstract of DOI:10.1002/1522-2675(200202)85:2<552::AID-HLCA552>3.0.CO;2-1

The free radicals 3-ethoxy-2-(ethoxycarbonyl)-3-oxopropyl (1′) and 3-ethoxy-2-(ethoxycarbonyl)-2-methyl-3-oxopropyl (2′) were generated by photolysis of perester precursors in i) hexane solution, ii) in the presence of β-cyclodextrin, and iii) in NaY zeolite. While free radicals in solution are reluctant to rearrange, they do so when encapsulated in β-cyclodextrin or NaY zeolite. The coenzyme-B₁₂-dependent enzymic rearrangement of methylmalonyl-CoA to succinyl-CoA could be mimicked by photochemical generation of an analogue of the putative intermediate radical in a molecular container.

Full text of DOI:10.1002/1522-2675(200202)85:2<552::AID-HLCA552>3.0.CO;2-1

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Helvetica Chimica Acta Vol. 85 (2002)
Effect of Supramolecular Inclusion on the 1,2-Rearrangement
of Free Radicals
by Ying Chen, Eric Kervio, and Ja¬nos Re¬tey*
Institut f¸r Organische Chemie, Universit‰t Karlsruhe, Richard-Willst‰tter-Allee, Postfach 6980,
D-76128 Karlsruhe (e-mail: biochem@ochhades.chemie.uni-karlsruhe.de)
¬
Dedicated to Professor Andre M. Braun on the occasion of his 60th birthday
.
The free radicals 3-ethoxy-2-(ethoxycarbonyl)-3-oxopropyl (1 ) and 3-ethoxy-2-(ethoxycarbonyl)-2-
.
methyl-3-oxopropyl (2 ) were generated by photolysis of perester precursors in i) hexane solution, ii) in the
presence of b-cyclodextrin, and iii) in NaY zeolite. While free radicals in solution are reluctant to rearrange, they
dosowhen encapsulated in b-cyclodextrin or NaY zeolite. The coenzyme-B12-dependent enzymic rearrange-
ment of methylmalonyl-CoA to succinyl-CoA could be mimicked by photochemical generation of an analogue
of the putative intermediate radical in a molecular container.
Introduction.
Coenzyme-B12-dependent enzymic rearrangements occur via
radical intermediates that would be highly unstable and reactive in solution or gas-
phase chemistry. It has been postulated that, at the enzyme×s active site, these reactive
species are protected from possible reaction partners thereby preventing undesired side
reactions [1]. Such a device prolongs the lifetime of the radical intermediate, thus
allowing its rearrangement, which is not a typical reaction of the radicals in solution.
For the selective prevention of reactions of highly unstable intermediates by enzymes,
the term −negative catalysis× has been coined [1].
One of the most thoroughly investigated and physiologically important coenzyme-
B12-dependent rearrangements is that of methylmalonyl-CoA to succinyl-CoA
(Scheme 1).
Scheme 1. The MMCoA Mutase Reaction
Homolysis of the CoÀC bond of coenzyme B12 leads toa shrot-lived 5 '-
deoxyadenosyl radical, which abstracts an H-atom from the Me group of methylmal-
onyl-CoA to yield the substrate radical. The rearrangement of the substrate radical
produces the product radical [2]. Evidence for these chemically unprecedented events
came from isotope labeling [3] and EPR experiments [4]. More recently, the protective
function of the enzyme has been shown by the X-ray structure of a mutant of
methylmalonyl-CoA mutase in which the channel to the active site has been partially

Helvetica Chimica Acta Vol. 85 (2002)
553
opened. The oxygen sensitivity of the mutant was 80 times higher than that of the wild-
type [5].
Chemical devices tostabilize highly unstable species are alsoknown. When such
species like cyclobutadiene [6], o-benzyne [7], or cycloheptatetraene [8] are generated
in a molecular container, called hemicarcerand, they show a prolonged lifetime and
different chemical behavior than they do in free solution. Naturally occurring materials
like cyclodextrin and zeolite have also been investigated as molecular containers
[9][10]. Here we examine the behavior of the photochemically generated free radicals
.
3-ethoxy-2-(ethoxycarbonyl)-3-oxopropyl (1 ), and 3-ethoxy-2-(ethoxycarbonyl)-2-
.
methyl-3-oxopropyl (2 ) (Figure) i) in hexane solution, ii) in the presence of b-
cyclodextrin, and iii) in zeolite NaY.
Figure. Studied 3-Ethoxy-2-(ethoxycarbonyl)-3-oxopropyl Radicals
.
.
Results. As precursors for the radicals 1 and 2 , the peresters tert-butyl 4-ethoxy-
3-(ethoxycarbonyl)-4-oxobutaneperoxoate (3) [11] and its 3-methyl derivative 4 were
prepared (Scheme 2). The expected products were in part commercially available and
characterized by NMR spectroscopy and elemental analysis. All were identified by GC/
MS. Photoirradiation of the peresters 3 [11] and 4 was carried out in a quartz vessel at
room temperature with a low-pressure Hg lamp. Upon irradiation, 3 and 4 underwent
homolytic cleavage with concomitant decarboxylation, generating a tert-butoxy radical
.
.
and radicals 1 and 2 , respectively (Scheme 2) [11 13].
Scheme 2. Generation and Rearrangement of the 3-Ethoxy-2-(ethoxycarbonyl)-3-oxopropyl Radicals
The main products of the irradiation of 3 and 4 in hexane consisted of both
recombination products with the tert-butoxy radical and H-abstraction products
(Scheme 3), i.e., 2-[(tert-butoxy)methyl]malonic acid diethyl ester (5) and methyl-

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Helvetica Chimica Acta Vol. 85 (2002)
malonic acid diethyl ester (1) fo r3, and 2-[(tert-butoxy)methyl]-2-methylmalonic acid
diethyl ester (7) and dimethylmalonic acid diethyl ester (2) fo r4. The ratios of the two
kinds of products 1/5 was ca. 1:1, whereas it was 0.7:1 for the products 2/7. Only very
small amounts of rearranged product 6 (2.2%) were detected in the irradiation
experiment with 3, while none was found in the corresponding experiment with 4.
These results are in agreement with the work of Keese×s group [11] who found 1.7% of
rearrangement product upon photolysis of 3 in cyclohexane.
Scheme 3. Products of the Photolysis in Hexane
Photolysis of the peresters 3 and 4 in cyclodextrin and in zeolite afforded
dramatically different results (see Schemes 4 and 5 and Table). Supramolecular
encapsulation in b-cyclodextrin favored the 1,2-rearrangement of the generated
radicals. Moreover, recombination with the simultaneously generated tert-butoxy
radical was also favored with respect to H-atom capture. The corresponding product
.
.
ratios were 5.3 :1 and 9.6 :1 for 1 and 2 , respectively. Obviously, the −cage effect× in the
cyclodextrin cavity prevents the escape of the tert-butoxy radical.
In contrast, during irradiation in the presence of the zeolite NaY, the proportion of
the recombination products decreased (ca. 10%) and that of the products arising from
.
.
.
H-abstraction increased (56% for 1 and 61% for 2 ) (Scheme 5). The rearrangement
.
products 10 and 11 (14 and 9%, resp.) arising from 1 and 2 were formed exclusively by
H-atom abstraction. Photolysis of 3 afforded 21% of methylidenemalonic acid diethyl
ester (9), while that of 4 gave 20% of methylmalonic acid diethyl ester (1), implying
.
the loss of a Me group from 2 . These Me groups must have been lost as methyl radicals
since a minor amount (4.3%) of 2-ethyl-2-methylmalonic acid diethyl ester (12) was
detected in the product mixture (Scheme 5); 12 is supposed to be the recombination
.
product of 2 with a methyl radical.

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555
Scheme 4. Products of the Photolysis in the Presence of b-Cyclodextrin
Table. Yields of Rearrangement Products under Different Conditions
Substrate
Yields of rearrangement products (GC % of products)
hexane
b-cyclodextrin
zeolite NaY
3
4
2.2
0
8.1
7.2
14
9
Discussion. The present results are relevant for the rearrangement reactions
catalyzed by coenzyme-B12-dependent enzymes. It has been postulated that the radicals
generated at the active site of the corresponding enzymes are protected from
bimolecular reactions by a −cage effect× [1]. Photochemical generation and inclosure of
.
.
putative radical intermediates of a model reaction, namely 1 and 2 in b-cyclodextrin or
zeolite NaY, can be regarded as models for the events at the enzyme active site. Indeed,
the percentage of rearrangement products was increased by factors of 4 and 7,
respectively. The simultaneous generation of the tert-butoxy radical favored, however,
recombination with the rearranged and unrearranged radicals in the b-cyclodextrin
cage. Interestingly, such a recombination was suppressed when the same radicals were
generated in the zeolite cage. A possible explanation for this may be the presence of
easily abstractable H-atoms, so that H-abstraction can successfully compete with the
recombination of the radicals generated in the cage.
Experimental Part
1. General. Column chromatography (CC): Merck silica gel 60 column; solvents distilled before use. NMR
Spectra: Bruker AC-250 or DRX-500 instruments; d in ppm, residual nondeuteriated solvent peak as an internal
standard, J values in Hz. GC/MS: Hewlett-Packard 5890 series II gas chromatograph with a Hewlett-Packard

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Helvetica Chimica Acta Vol. 85 (2002)
Scheme 5. Products of the Photolysis in the Presence of Zeolite NaY
5972 mass selective detector; Hewlett-Packard HP-5 trace analysis capillary column (25 m  0.2 mm); m/z (rel.
%). Elemental analyses were performed with a CHN-O-Rapid instrument from Heraeus.
2. Precursors. tert-Butyl 4-Ethoxy-3-(ethoxycarbonyl)-4-oxobutaneperoxoate (3) was synthesized as
1
described [11]: 90% yield. H-NMR (500 MHz, CDCl3): 1.29 (t, J ˆ 7.3, MeCH2); 1.33 (s, tBu); 2.94 (d, J ˆ 7.3,
t
CH2CO2 Bu); 3.88 (t, J ˆ 7.3, CH(CO2Et)2]; 4.24 (q, J ˆ 7.3, MeCH2). 13C-NMR (126 MHz, CDCl3): 13.9
t
(MeCH2); 26.0 (Me3C); 30.3 (CH2CO3 Bu); 47.5 (CH(CO2Et)2); 62.0 (MeCH2); 83.9 (Me3C); 167.9 (CˆO).
Anal. calc. for C13H22O7; C 53.78, H 7.64; found: C 53.70, H 7.62.
tert-Butyl 4-Ethoxy-3-(ethoxycarbonyl)-3-methyl-4-oxobutaneperoxoate (4). Dry benzene (6 ml) was
added to a 60% suspension of NaH in oil (0.87 g, 20 mmol), followed by dropwise addition of methylmalonic
acid diethyl ester (3.48 g, 20 mmol) to the stirred mixture. Then tert-butyl bromoacetate (3.90 g, 20 mmol) was
added dropwise, and the mixture was stirred overnight at r.t. Filtration and evaporation gave the crude product,
which was purified by CC (hexane/tBuOMe 4 :1): propane-1,2,2-tricarboxylic acid 1-(tert-butyl) 2,2-diethyl ester
(4.5 g, 78%). 1H-NMR (250 MHz, CDCl3): 1.25 (2t, J ˆ 7.3, 2 MeCH2); 1.42 (s, tBu); 1.52 (s, CH2C(Me)(-
t
CO2Et)2); 2.85 (s, CH2CO2 Bu); 4.19 (q, J ˆ 7.3, 2 MeCH2).
The tert-butyl diethyl ester (2.02 g, 7 mmol) was cleaved as described [11] above giving propane-1,2,2-
tricarboxylic acid 2,2-diethyl ester (1.45 g, 89%). The acid (0.80 g, 3.4 mmol) was reacted with tert-butyl
hydroperoxide to give 4 (0.82 g, 78%). 1H-NMR (500 MHz, CDCl3): 1.24 (t, J ˆ 7.2, 2 MeCH2); 1.30 (s, tBu); 1.55
(s, CH2C(Me)(CO2Et)2); 2.93 (s, CH2CO3 Bu); 4.19 (q, J ˆ 7.2, 2 MeCH2). 13C-NMR (126 MHz, CDCl3): 13.9
t
t
(MeCH2); 20.2 (CH2C(Me)(CO2Et)2); 26.0 (Me3C), 37.1 (CH2CO3 Bu); 51.5 (C(CO2Et)2); 61.9 (MeCH2); 83.6
(Me3C); 167.9, 170.6 (CˆO). Anal. calc. for C14H24O7: C 55.25, H 7.95; found: C 55.34, H 7.77.
3. Inclusion Compounds from Perester 3 or 4 and b-Cyclodextrin. A 1:1 (molar ratio) mixture of 3 or 4 and
b-cyclodextrin (b-CD) was dissolved in H2O by stirring overnight. The resulting soln. was lyophilized. The

Helvetica Chimica Acta Vol. 85 (2002)
557
complex formation in the aq. soln. was confirmed by 1H-NMR the inner protons of b-CD (HÀC(3), HÀC(5),
and HÀC(6) being shifted upfield to various extents, thus providing evidence for the inclusion of 3 and 4 into
the hydrophobic cavities of b-CD.
t
3/b-CD: 1H-NMR (500 MHz, D2O): 1.30 (t, J ˆ 7.2, 6 H, MeCH2); 1.35 (s, 9 H, Bu); 2.98 (t, J ˆ 7.0, 2 H,
t
CH2CO3 Bu); 3.62 (t, J ˆ 9.5, 7 H, HÀC(4) of b-CD); 3.66 (dd, J ˆ 9.9, 3.6, 7 H, HÀC(2) of b-CD); 3.83 (m, 7 H,
HÀC(5) of b-CD); 3.87 (m, 14 H, HÀC(6) of b-CD); 3.93 (t, J ˆ 9.5, 7 H, HÀC(3) of b-CD); 4.00 (t, J ˆ 7.1,
1 H, CH(CO2Et)2); 4.29 (m, 4 H, MeCH2); 5.08 (d, J ˆ 3.7, 7 H, HÀC(1) of b-CD).
1
t
4/b-CD: H-NMR (500 MHz, D2O): 1.29 1.32 (m, 15 H, MeCH2, Bu); 1.55 (s, 3 H, MeC(CO2Et)2); 2.99
t
(s, 2 H, CH2CO3 Bu); 3.63 (t, J ˆ 9.5, 7 H, HÀC(4) of b-CD); 3.67 (dd, J ˆ 9.9, 3.6, 7 H, HÀC(2) of b-CD); 3.80
(m, 7 H, HÀC(5) of b-CD); 3.86 (m, 14 H, HÀC(6) of b-CD); 3.92 (t, J ˆ 9.6, 7 H, HÀC(3) of b-CD); 4.30
(m, 4 H, MeCH2); 5.08 (d, J ˆ 3.6, 7 H, HÀC(1) of b-CD).
Pure b-CD: 1H-NMR (500 MHz, D2O): 3.60 (t, J ˆ 9.2, 7 H, HÀC(4)); 3.67 (dd, J ˆ 9.9, 3.7, 7 H, HÀC(2));
3.88 (m, 7 H, HÀC(5)); 3.90 (m, 14 H, HÀC(6)); 3.99 (t, J ˆ 9.5, 7 H, HÀC(3)); 5.09 (d, J ˆ 3.7, 7 H, HÀC(1)).
4. Inclusion of Perester 3 or 4 with Zeolites. Weighed amounts of substrates 3 or 4 and activated NaY
zeolite were stirred in hexane for ca. 10 h. In a typical experiment, 400 mg of zeolite and 5 mg of the perester
were added to 20 ml of hexane. The solid was isolated by filtration, washed with hexane twice, and dried in
vacuo.
5. Reference Compounds for the Rearrangement Products. 2-(tert-Butoxy)butanedioic Acid Diethyl Ester
(6) was prepared by the reaction of diethyl malate (ˆdiethyl hydroxybutanedioate) and 2-methylpropene in the
presence of Amberlyst 15 [7]. 1H-NMR (250 MHz, CDCl3): 1.20 (s, tBu); 1.26 (t, J ˆ 7.3, 1 MeCH2); 1.27 (t, J ˆ
7.0, 1 MeCH2); 2.63 2.67 (m, CH2CHOtBu); 4.08 4.24 (m, 2 MeCH2); 4.45 (m, CHOtBu). GC/MS: 231(2),
173(12), 145(5), 118(6), 117(100), 101(3), 99(2), 89(7), 87(3), 75(4), 73(6), 71(13), 59(6), 58(4), 57(88), 56(7),
55(7), 45(2), 43(10), 42(2), 41(13).
2-[(tert-Butoxy)methyl]-2-methylpropanedioic Acid Diethyl Ester (7) was prepared by the reaction of 2-
(hydroxymethyl)-2-methylpropanedioic acid diethyl ester [8] and 2-methylpropene in the presence of
Amberlyst 15 [7]. 1H-NMR (250 MHz, CDCl3): 1.14 (s, tBuO); 1.24 (t, J ˆ 7.3, 2 MeCH2); 1.47 (s, MeÀC(2));
3.69 (s, CH2ÀC(2)); 4.17 (q, J ˆ 7.0, 2 MeCH2). GC/MS: 204(2), 203(18), 187(13), 174(6)), 159(16), 157(2),
141(2), 131(7), 130(5), 129(34), 128(8), 115(8), 114(3), 113(5), 103(6), 102(3), 101(14), 100(7), 99(2), 87(15),
86(9), 85(17), 84(3), 83(6), 75(3), 73(2), 72(3), 70(2), 69(27), 68(2), 59(28), 58(6), 57(100), 56(9), 55(6),
47(3), 45(9), 44(2), 43(27), 42(6), 41(73).
2-(tert-Butoxy)-2-methylbutanedioic Acid Diethyl Ester (8) could not be synthesized but was identified by
GC/MS: 245 (2), 215 (2), 201(11), 187(15), 174(5), 173(5), 159(7), 141(6), 132(4), 131(94), 129(7), 128(12),
127(4), 114(3), 113(9), 112(7), 103(5), 101(4), 100(6), 99(3), 87(8), 86(5), 85(16), 83(2), 75(3), 74(3), 73(4),
69(13), 59(7), 58(7), 57(100), 56(8), 55(6), 45(5), 44(6), 43(10), 41(23).
2-(tert-Butoxy)-3-methylbutanedioic Acid Diethyl Ester was prepared by the reaction of 2-hydroxy-3-
methylbutanedioic acid diethyl ester and 2-methylpropene in the presence of Amberlyst 15 [7]. 1H-NMR
(250 MHz, CDCl3): 1.10 (d, J ˆ 7.0, MeCH); 1.17 (s, tBuO); 1.14 (t, J ˆ 7.0, 1 MeCH2); 1.16 (t, J ˆ 7.0, 1 MeCH2);
2.78 (m, MeCH); 4.11 4.25 (m, 2 MeCH2, tBuOCH). GC/MS: 159(2), 141(2), 132(3), 131(40), 113(2), 103(3),
102(2), 87(5), 86(2), 85(15), 75(3), 74(3), 73(2), 69(7), 59(7), 58(9), 57(100), 56(8), 55(3), 45(4), 43(9), 43(2),
41(33).
2-Methylbutanedioic Acid Diethyl Ester (11) was obtained from the corresponding commercially available
carboxylic acid (1 g, 8 mmol) by refluxing it in EtOH containing 5% conc. HCl soln.: 1.1 g (77%) of 11.
1H-NMR (250 MHz, CDCl3): 1.24 (m, 2 MeCH2, MeÀC(2)); 2.56 (m, CH2); 2.90 (m, CH); 4.14 (m, 2 MeCH2).
GC/MS: 144(8), 143(100), 142(34), 117(2), 116(15), 115(93), 114(23), 113(3), 101(17), 99(3), 89(3), 88(16),
87(36), 86(7), 85(2), 74(2), 73(40), 71(8), 70(6), 69(16), 60(4), 59(2), 56(2), 55(6), 45(23), 44(2), 43(34),
42(31), 41(26), 40(4).
2-Hydroxy-2-methylbutanedioic Acid Diethyl Ester was obtained from the corresponding carboxylic acid
(1 g, 8 mmol) by refluxing in EtOH soln. containing 5% conc. HCl soln.: 1.0 g (73%) of ester. 1H-NMR
(250 MHz, CDCl3): 1.25 (t, J ˆ 7.1, 1 MeCH2); 1.30 (t, J ˆ 7.2, 1 MeCH2); 1.44 (s, MeC(OH)); 2.81 (JAB ˆ 16.3,
CH2COOEt); 3.785 (s, OH); 4.14 (q, J ˆ 7.9, 1 MeCH2); 4.26 (q, J ˆ 7.0, 1 MeCH2). GC/MS: 159(3), 132(7),
131(100), 115(2), 113(2), 103 (18), 89(4), 88(3), 86(4), 85(74), 61(5), 60(3), 58(8), 57(2), 45(3), 44(2), 43(77),
42(5), 41(3).
6. Photolysis Experiments and Product Analysis. Photoirradiation of perester 3 or 4 in hexane soln. was
carried out in a quartz vessel at r.t. by irradiation with a low-pressure Hg lamp for 2 h. The irradiation products
were analyzed by GC/MS.

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Aq. solns. of the perester/b-CD inclusion complexes were prepared by dissolving the complexes and an
excess of b-CD in H2O. The solns. were first purged with Ar for 30 min and then irradiated in quartz tubes with a
low-pressure Hg lamp. After irradiation, the products were extracted with CHCl3 and analyzed by GC/MS.
The precursors included in the dry NaY zeolite were irradiated as solids with a low-pressure Hg lamp. After
irradiation, the products were extracted with CHCl3 after dissolving the zeolite with 1n HCl at r.t. and analyzed
by GC/MS. The structures of the products were assigned either by comparison of the retention times and MS
with the standard or by the fragmentation pattern in their MS. For the estimation of the product ratios, the peak
areas of the GC signals were considered. Only those esters that were formed from the irradiation of perester
were considered. The percentage of the rearrangement product was quite reproducible in at least three
independent experiments (errors for irradiation in hexane and b-CD aq. soln, Æ0.5%; for irradiation in NaY,
Æ2%).
This work was supported by the Deutsche Forschungsgemeinschaft, the European Union and the Fonds der
Chemischen Industrie. Dr. Ying Chen thanks the Alexander von Humboldt Foundation for a postdoctoral
scholarship and Dr. Stefan H. Bossmann for his advice and help with the irradiation equipment.
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Received September 28, 2001