Palladium-catalyzed cyclopropanation of unsaturated endoperoxides. A new peroxide-preserving reaction

Article abstract of DOI:10.1002/adsc.200800804

Unsaturated bicyclic endoperoxides are efficiently cyclopropanated with excess diazomethane in the presence of catalytic palladium(II) acetate [Pd(OAc)₂] in a stereoselective manner. This method represents a new peroxide-preserving transformati

Full text of DOI:10.1002/adsc.200800804

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DOI: 10.1002/adsc.200800804
Palladium-Catalyzed Cyclopropanation of Unsaturated
Endoperoxides. A New Peroxide-Preserving Reaction
Michael A. Emerzian,a William Davenport,a Jiangao Song,a Jim Li,a
and Ihsan Erdena,*
a
San Francisco State University, Department of Chemistry and Biochemistry, 1600 Holloway Avenue, San Francisco, CA
94132, USA
Fax : (+1)-415-338-2384; phone: (+1)-415-338-1627; e-mail: ierden@sfsu.edu
Received: December 22, 2008; Revised: March 9, 2009; Published online: April 29, 2009
This paper is dedicated to Professor Armin de Meijere on the occasion of his 70th birthday.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800804.
Abstract: Unsaturated bicyclic endoperoxides are
efficiently cyclopropanated with excess diazome-
thane in the presence of catalytic palladium(II) ace-
dioxabicyclo
ACHTUNGTRENN[UNG 2.2.1]heptane series (7-carbo- and
heterocyclic analogs; Figure 1) such as the naturally
AHCTUNGTRENNUNG
occurring and physiologically active prostaglandin en-
doperoxides are, as expected, among the least stable
bicyclic endoperoxides. To date, except for the selec-
tive diazene reduction method, there does not appear
to be any other method whereby the etheno bridge in
the 2,3-dioxabicyclic system combines with a reagent
or reagents under conditions mild and selective
enough to preserve the thermally as well as reagent-
labile peroxide linkage.[3] Herein, we report the
second peroxide-preserving reaction that is applicable
to most endoperoxides with a few exceptions.
The requisite unsaturated endoperoxides used in
this study were readily obtained by photooxygenation
of the respective cyclic 1,3-dienes at low tempera-
tures.[4] In the cases where the unsaturated endoper-
oxide was unstable at 08C or room temperature (5
and 9), they were generated in situ. In all the other
cases the endoperoxides were stable at room tempera-
ture and were isolated before cyclopropanation. The
cyclopropanations were carried out by slow addition
of an ether solution of diazomethane (generated from
tate [PdACHTUNGTRENNUNG(OAc)2] in a stereoselective manner. This
method represents a new peroxide-preserving trans-
formation. Whereas the unsaturated endoperoxides
in the [2.2.1]series are attacked by the carbene
from the exo face, the analogs with larger bridges
are preferentially attacked from the face syn to the
peroxo bridge. Only in the case of the benzannelat-
ed [2.2.2]system does the attack occur exclusively
from the face proximal to the benzene ring. Certain
strained cyclopropanated endoperoxides are re-
duced by diazomethane to give cis-diols. 2-Methyl-
furan endoperoxide gives rise to cis-1-formyl-2-ace-
tylcyclopropane in excellent yield.
Keywords: cyclopropanation; diazomethane; palla-
dium; singlet oxygen; stereoselectivity
The selective diazene reduction of unsaturated endo- Diazaldꢀ) at 08C in diethyl ether in the presence of
peroxides still constitutes the single most important catalytic amounts of Pd
(OAc)2 at ꢀ788C to a stirred
breakthrough in the synthesis and characterization of solution of endoperoxide in CH2Cl2 either at ꢀ788C
stable saturated endoperoxides.[1] Many peroxides in the case of 5 and 9, or at 08C in all the other cases,
previously postulated as intermediates in the singlet
oxygenation of cyclic dienes have thus been rendered
stable enough for further exploration of chemical,
physical and spectroscopic properties.[2] The notorious
instability of unsaturated endoperoxides is doubt-
less a consequence of the weak peroxide bond
(~36 kcalmolꢀ1) that is compounded by the additional
strain brought on by the double bond in the bicyclic
Figure 1. PdACTHGNUTREN(NUNG OAc)2-catalyzed cyclopropanation of unsaturat-
framework. In particular, endoperoxides in the 2,3- ed endoperoxides.
Adv. Synth. Catal. 2009, 351, 999 – 1004
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Michael A. Emerzian et al.
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Table 1. Cyclopropanation of unsaturated endoperoxides.
followed by allowing the mixtures to warm up to
room temperature.[5] In most cases, the cyclopropanat-
ed endoperoxide was obtained in good to excellent
yield. The crude product mixtures were practically
free of any by-products as well as starting materials.
For spectroscopic and analytical purposes, the cyclo-
propanation products were purified at low tempera-
ture (ꢀ308C) by column chromatography (or prepara-
tive TLC) on SiO2.[6] Although all cyclopropanated
products exhibited greater thermal stability than the
unsaturated counterparts, their purity was determined
by means of their NMR spectra as well as by iodo-
metric titration (>98% in each case) since they are
notoriously unstable and potentially explosive when
neat and did not survive shipment. The starting mate-
rials, products, product ratios and isolated yields are
depicted in Table 1.
In cases where stereoisomers were formed, the
endo-exo ratios were determined by NMR, in particu-
lar by inspecting the absorptions of the cyclopropane
protons. Compound 13 was independently synthesized
from the known norcaradiene-singlet oxygen adduct
by diazene reduction.[7] Once the structures of 12 and
13 were established, the assignments of the other
endo and exo isomers were straightforward. Also the
spectra of the known carbocyclic analogs were used
to confirm the stereochemical assignments.[8]
The stereoisomeric ratios in the cyclopropanation
of the unsaturated peroxides suggest that there is ste-
reochemical bias in the carbene additions. In the
carbo- and heterobicyclic [2.2.1]systems, the facial se-
lectivity is clearly that favoring the exo attack. It is a
well known fact that norbornene and its derivatives
are attacked by a variety of cycloaddends on the exo
face of the bicyclic system. This exo preference, owing
to its uncertain origin, has been called factor X by
Huisgen.[9] Houk et al. later offered a consistent ra-
tionalization of this effect invoking greater eclipsing
of the newly formed bonds during the attack at the
Entry Diene-1O2-Adduct[a] Product(s)[b]
(ratios)
Yield
[%][c]
AHCTUNGTRENNUNG
1
2
58
82
3
4
73
78
5
6
7
69
57
85
8
92
[a]
[b]
[c]
Prepared from the respective cyclic 1,3-dienes by photo-
oxygenation.
NMR spectra of all products are available in the Sup-
porting Information.
Isolated yields after low-temperature column chromatog-
raphy.
ꢀ
ꢀ
endo face with the C C and C H bonds on the
bridgehead carbon, whereas the exo attack can occur
with nearly ideal staggering with respect to the afore-
mentioned bonds.[10] In the case of 1,4-diphenyl
system 6, however, the ortho hydrogens in the phenyl
rings are interfering with the approach of the carbene posed upon warming the reaction mixture to room
at the exo face to the extent that the overwhelming temperature to give cis-1-formyl-2-acetylcyclopropane
exo preference is somewhat diminished to allow for 24[11] in 65% overall yield from 2-methylfuran. This
the endo attack in 10% relative yield. In the case of reaction constitutes an excellent method for the syn-
the 2,5-dimethylfuran endoperoxide 9, only the exo thesis of cis-1,2-diacylcyclopropanes from a furan pre-
product was formed as well. The substitution at both cursor in a one-pot procedure in two steps.
bridgehead positions of the endoperoxide, as is the
The formation of the diacylcyclopropane suggests
case with other analogs such as 6, 7 and 8, imparts the that diazomethane might have served as a reducing
molecule increased stability; thus compound 10 was agent for the endoperoxide, since furan endoperox-
stable enough at room temperature as well as on ides (ozonides) are converted to 1,4-dicarbonyl com-
silica gel at low temperature to allow purification and pounds upon reduction with an oxygen atom acceptor
characterization. On the other hand, the monometh- such as PPh3 or (CH3)2S (DMS).[12] The mechanism
yl-substituted analog 22 was not stable and decom- depicted in Figure 2 is consistent with our results. The
1000
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Palladium-Catalyzed Cyclopropanation of Unsaturated Endoperoxides
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Figure 2. Deoxygenation of cycloprotonated ozonide 22 with
diazomethane.
postulate that diazomethane acted as a reducing
agent in the case of 22 was strenghtened by the fact
1
that the cyclopropanation of the O2-adducts of 6,6-di- Figure 4. Mechanism of diazomethane reduction of 26 and
ACTHNUTRGNEUNG
phenylfulvene[13] and spiro[3.4]hepta-4,6-diene,[14] 25
29 to diols 27 and 30.
and 28 did not yield the expected endoperoxides; in-
stead, the cis-diols 27 and 30 were isolated in high
yields in each case by reduction with diazomethane face proximal to the benzene ring (endo) to give 33 as
(Figure 3).
the sole product.[15] These results corroborate the
The mechanism that satisfactorily rationalizes the findings of Anciaux, Hubert et al. that Pd-catalyzed
formation of 27 and 30 from 26 and 29, respectively, cyclopropanations are exceedingly sensitive to the
is analogous to the one depicted in Figure 2. After di- steric effects of the substrate.[16]
azomethane attack at the peroxide oxygen and subse-
The product ratios presented in Table 1 also suggest
quent protonation and loss of nitrogen, the resulting that the approach of Pd-carbene complex from the
hemiacetal expels formaldehyde to give the diols 27 face proximal to the peroxo bridge in the case of
and 30. The mechanism for this transformation is dioxabicyclo
shown in Figure 4. This preference is more pronounced when the n-
It is noteworthy that ascaridole (31) failed to react bridge is larger than 2. Whereas 11 gives rise to only
with diazomethane in the presence of Pd(OAc)2, even a slight excess of 12 over 13, compounds 14, 16 and
ACHTUNGTRENN[UNG n.2.2]systems (n=2, 3, 4) is favored.
AHCTUNGTRENNUNG
when a very large excess of CH2N2 was used, presum- 18 give almost exclusively (by NMR) the syn products
ably due to the presence of the bridgehead isopropyl 15, 17, 19, respectively. This stereochemical prefer-
group that presents steric hindrance in the approach ence may be due to greater steric hindrance presented
of the palladium-carbene complex to the etheno by the larger bridges in 14, 16 and 18. Transition state
bridge. Also the stereoselective cyclopropanation of calculations of PdACTHUNRGTNEUNG(OAc)2-catalyzed cyclopropanations
the 1,4-dimethylnaphthalene-1O2 adduct deserves with diazomethane are hampered by the fact that the
some comment: in this case, the cyclopropanation mechanistic details of these reactions have not been
occurs exclusively anti to the peroxo bridge, by con- elucidated; experimental evidence and theoretical
trast to the non-benzannelated counterpart. Obvious- studies suggest that the cyclopropanations with metal-
ly, the interaction of the Pd catalyst with the benzene locarbenes are exothermic.[17–20] A recent DFT study
ring is a factor that favors the approach of the palladi- by Straub[21] on the Pd-catalyzed cyclopropanation of
um carbene complex from the face proximal to the alkenes renders Pd(II) a precatalyst; the actual cata-
benzene ring. In order to test this hypothesis we re- lytic resting state is Pd
acted the carbocyclic analog 32 with Pd(OAc)2-CH2N2 undergoes reversible ligand exchange reactions. Also
and found that the benzobicyclo
[2.2.2]system in 32 in accord with Anciaux et al.ꢂs study,[16] the Pd-coordi-
(h2-C2H4)n (n=2 or 3) which
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(Figure 5) was cyclopropanated exclusively from the nated carbene reacts with the olefin that is coordinat-
Figure 3. Reduction of endoperoxides 26 and 29 by diazomethane.
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Michael A. Emerzian et al.
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Figure 5. Reactivities of ascaridole (31) and benzobicycloACHTUNTRGENN[GU 2.2.2]octadiene (32) toward diazomethane-PdCAHTUNGTREN(NUNG OAc)2.
ed to the same metal via a palladacyclobutane before drance presented by the peroxo bridge than the
it undergoes reductive elimination to the cyclopro- ethano bridge. This fact manifests itself in the slightly
pane. Whatever the exact mechanism, it is obvious higher proportion of 12 vs. 13 (60:40). Of the two con-
that the facial selectivity of carbene transfer to the formers of 14, 14a is more stable than 14b by
double bond in a bicyclic system is controlled to a 2.1 kcalmolꢀ1, thus the syn approach of the metallo-
great extent by steric factors. Despite the exothermic- carbene is favored. After the formation of the cyclo-
ity of these reactions, and the likelihood of early tran- propanated products (in both cases, the most stable
sition states, a comparison of the enthalpy differences conformer was considered), the syn endoperoxide 15a
of the stereoisomeric products should reflect to a sig- is substantially more stable (by 5.9 kcalmolꢀ1) than
nificant extent the stereochemical issues encountered the anti isomer 15b. Finally, in the 2,3-dioxa-
in the transition states of these cyclopropanations.
ACHTUNGTREN[NGNU 4.2.2]series, the syn isomer 19a is considerably more
Based on these considerations we carried out ab initio stable than the anti isomer 19b. In both cases the
calculations on the products in the [n.2.2] series. Elec- methylene groups in the propano and butano bridges,
tronic energy optimizations were carried out at the respectively, appear to present considerably more
MP2/6-31G* level with ZPVE corrections calculated steric hindrance than the peroxo bridge. Also 17a
at HF/6-31G*, and scaled by 0.9153 as recommended turned out to be more stable than 17b by
by Scott and Radom.[22] Thermal corrections were not 2.3 kcalmolꢀ1 and was isolated as the sole product.
made. Stationary points were found for all com- This result is also in line with a report by de Meijere
pounds. The results, given as DHisom, are listed in et al. that the cyclopropanation of bicyclo
Table 2, and are in good qualitative agreement with 2,6,8-triene, the carbocyclic analog of 16, by the Furu-
the observed experimental stereoisomer ratios. kawa method (Et2Zn/CH2I2) likewise gave exclusively
ACHTUNGTREN[NUNG 3.2.2]nona-
The syn isomer 12 is more stable than 13 by only the exo-product stemming from carbene addition
1.3 kcalmolꢀ1, reflecting the slightly greater steric hin- from the face distal to the propeno bridge.[23]
The stereochemical assignments of the cyclopropa-
nation products deserve some comment. The endo
and exo 5,6-dioxabicycloACTHNUTRGENUGN[3.2.1]nonanes 12 and 13 are
Table 2. Calculated enthalpies of cyclopropanation of
dioxabicyclo
ACHTUNGTRENNUNG
representative of the other endo and exo isomers. In
all cases where the cyclopropane ring is syn to the
peroxo bridge, the secondary proton syn to the
peroxo bridge absorbs downfield (1.2 ppm) whereas it
absorbs upfield (0.5 ppm) when it is syn to the ethano
bridge, in analogy to the carbocyclic system.[8] More-
over, the tertiary cyclopropane protons syn to the
ethano bridge in 12 absorb at 1.17 ppm, the same pro-
tons syn to the peroxo bridge absorb at 1.65 ppm in
13 (Figure 6). The endo stereochemistry in the case of
21 was readily established by the fact that the secon-
dary cyclopropane proton syn to the benzene ring ab-
sorbs upfield, at ꢀ0.9 ppm due to the diamagnetic ani-
sotropy of the aromatic ring. In the carbocyclic
analog 33, the same proton absorbs at ꢀ1.0 ppm.
Entry
DHisom [kcalmolꢀ1][a]
1.3 (12!13)
1
2
3
4
2.1 (14a!14b)
5.9 (15a!15b)
2.3 (17a!17b)
13.7 (19a!19b)
Finally, it is noteworthy that Rh2ACTHUNTRGNE(UGN OAc)4 did not
prove to be an effective catalyst for the cyclopropana-
tion of unsaturated bicyclic endoperoxides with diazo-
methane: neither 4 nor 11 underwent cyclopropana-
5
tion under the same conditions using Rh2ACTHNUTRGNEN(UG OAc)4 as
[a]
catalyst; in fact, no traces of 5, or 12/13, respectively,
were detected in the NMR spectra of the reaction
mixtures. In the case of 4, only cis-4,5-epoxy-2-pent-
Optimized at the MP2/6-31G* level with frequencies
computed at the HF/6-31G* level using the Spartan06
software package. ZPVEs were scaled by 0.9153 as rec-
ommended by Scott and Radom.[22]
AHCTUNGTREGeNNUN nal and cis-1,2,3,4-diepoxycyclopentane were ob-
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Palladium-Catalyzed Cyclopropanation of Unsaturated Endoperoxides
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fied by low-temperature column chromatography at ꢀ308C
on SiO2 (petroleum ether/CH2Cl2, 1:1). In the case of 27 and
30, CH2Cl2/MeOH (10:1) was used for chromatography. The
products were identified by means of spectral data (see Sup-
porting Information).
Supporting Information
Figure 6. Chemical shifts of the cyclopropane protons in 12
and 13.
Spectral data for all cyclopropanation products are available
as Supporting Information.
tained, as described by Ohloff et al.[24] whereas only
unchanged starting material was recovered from 11.
These results are in line with the findings of Dzhemi-
lev et al., who investigated the efficiency of a variety
of transition metal catalysts in cyclopropanations of
alkenes with diazomethane, and found that, for in-
stance, norbornene did not react with CH2N2/
Acknowledgements
This work was supported by funds from the National Insti-
tutes of Health, MBRS-SCORE Program-NIGMS (Grant
No. GM52588). We also acknowledge funding from the Na-
tional Science Foundation (Grants No. DBI-0521342 and
DUE-9451624) for the purchase of 500 MHz and 300 MHz
NMR spectrometers. We thank Mr. Wee Tam for recording
the NMR spectra.
Rh
2ACHTUNGTRENNUNG(OAc)4, whereas Pd-based catalysts [PdAHCUTNGTREN(NUGN OAc)2 or
Pd
ACHTUNGTRENNUNG
tempts to cyclopropanate endoperoxide 11 with ethyl
diazocarboxylate (EDA) in the presence of catalytic
amounts of Rh2ACHTUNGTRENNUNG(OAc)4 failed, leading to an intracta-
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In summary, we have uncovered a new peroxide-
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Experimental Section
Typical Procedure For Singlet Oxygenations-
Cyclopropanations
A solution of cyclic diene (5 mmol) in 15 mL of CH2Cl2 was
photooxygenated at 08C (cyclopentadiene, spiroACTHNUTRGNE[UNG 3.4]hepta-
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Adv. Synth. Catal. 2009, 351, 999 – 1004
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Michael A. Emerzian et al.
COMMUNICATIONS
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