Ruthenium(II) porphyrin catalyzed cyclopropanation of alkenes with tosylhydrazones

Article abstract of DOI:10.1016/j.tetlet.2003.09.157

The diastereoselective ruthenium(II) porphyrin catalyzed cyclopropanation of a variety of alkenes with aryl diazomethanes generated in situ from stable tosylhydrazone derivatives, was achieved in good to excellent yields (up to 92%) and product turnovers.

Full text of DOI:10.1016/j.tetlet.2003.09.157

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 8733–8737
Ruthenium(II) porphyrin catalyzed cyclopropanation of alkenes
with tosylhydrazones
Jun-Long Zhang, Philip Wai Hong Chan and Chi-Ming Che*
Department of Chemistry and Open Laboratory of Chemical Biology of the Institute of Molecular Technology for Drug
Discovery and Synthesis, The University of Hong Kong, Pokfulam Road, Hong Kong, PR China
Received 30 June 2003; revised 29 August 2003; accepted 10 September 2003
Abstract—The diastereoselective ruthenium(II) porphyrin catalyzed cyclopropanation of a variety of alkenes with aryl dia-
zomethanes generated in situ from stable tosylhydrazone derivatives, was achieved in good to excellent yields (up to 92%) and
product turnovers. The practical utility of [RuII(p-Cl-TPP)(CO)] (H2(p-Cl-TPP)=meso-tetrakis(p-chlorophenyl)porphyrin) 3 was
illustrated in the synthesis of the potent HIV-1 reverse transcriptase inhibitor 10. Preferential formation of sulfone products for
reactions involving o- and m-monosubstituted aryl tosylhydrazones demonstrated a hitherto unknown ruthenium(II) porphyrin
catalyzed sulfonation reaction.
© 2003 Elsevier Ltd. All rights reserved.
Transition metal-catalyzed carbenoid transfer reactions
to CꢀC bonds and insertion to CꢁH bonds have
demonstrated important applications in drug synthe-
sis.1,2 Aggarwal et al. recently reported one such strat-
egy involving the Rh- or Fe-catalyzed cyclopropanation
of N-vinylphthalimide with tosylhydrazone derivatives2
in the synthesis of urea-PETT analogues 1,2a a potent
class of HIV-1 reverse transcriptase inhibitors (Scheme
1).3
for reactions involving o- and m-monosubstituted aryl
tosylhydrazones, a hitherto unknown ruthenium(II)
porphyrin catalyzed sulfonation reaction, is also
discussed.
Ruthenium(II) porphyrins catalyzed carbenoid transfer
reactions using diazo compounds have received much
attention in recent years (Fig. 1).1,4–6 We and others
recently demonstrated high product turnovers and reac-
tivity exhibited by this class of catalysts in reactions
involving CꢁC,4 CꢁO4i,5 and CꢁN4i,6 bond formations.
Work in our laboratory found that tosylhydrazone
derivatives could be used as the carbene source in
stereoselective Ru-catalyzed intramolecular carbenoid
insertion to CꢁH bonds.1i,4m In light of this work, we
wondered whether the same carbene source could be
applied to the ruthenium(II) porphyrin catalyzed cyclo-
propanation of alkenes. Herein we describe the realiza-
Scheme 1. Rh- and Fe-catalyzed strategies to the synthesis of
urea-PETT analogues 1.
tion of
a
Ru-catalyzed protocol for effecting
intermolecular alkene cyclopropanation with aryl tosyl-
hydrazones. The competitive formation of aryl sulfones
Figure 1. Ruthenium(II) porphyrin catalysts 2–8 used in
this work. H2(TTP)=meso-tetrakis(tolyl)porphyrin, H2(2,6-
Cl2TPP)=meso-tetrakis(2,6-dichlorophenyl)porphyrin,
H2-
(TMP)=meso-tetrakis(2,4,6-trimethylphenyl)porphyrin, H2-
(F20-TPP)=meso-tetrakis(pentafluorophenyl)porphyrin.
* Corresponding author. Tel.: +(852) 2859-2154; fax: +(852) 2857-
1586; e-mail: cmche@hku.hk
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.09.157

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J.-L. Zhang et al. / Tetrahedron Letters 44 (2003) 8733–8737
Table 2. Cyclopropanation of alkenes with tosylhydrazone
derivatives catalyzed by 2 and 3a
The cyclopropanation of styrene with benzaldehyde
tosylhydrazone was chosen as the model system to
establish the reaction conditions. This revealed treat-
ment of the benzaldehyde tosylhydrazone sodium salt
with [RuII(TTP)(CO)] 2 (0.1 mol%) and BnEt3N+Cl−
(phase transfer catalyst, 5.0 mol%) in C6H6 giving the
best result. Under these conditions the trans-product
was preferentially furnished (trans:cis=95:5) in excel-
lent yield (91%) and with a product turnover of 910
(Table 1, entry 1). The analogous Rh- and Fe-catalyzed
reactions gave slightly lower trans-selectivity.2a Exami-
nation of other different ruthenium(II) porphyrins
revealed the performance of 3 was similar to that
observed for 2 in terms of yield, stereoselectivity and
product turnover (entry 2). In contrast, ruthenium(II)
porphyrins 4–6 containing either electron-withdrawing
or electron-donating groups substituted at the o-posi-
tion were found to exhibit lower reactivity and selectiv-
ity (entries 3–5). In every case, the main product was
identified as stilbene. The use of Merrifield resin sup-
ported ruthenium porphyrins 7 and 85i gave product
yields and selectivity lower than that of their homoge-
nous counterparts 2 and 3 (entries 6 and 7).
a All reactions were performed in C6H6 at 40°C for 24 h with catalyst:
BnEt3N+Cl−: tosylhydrazone: alkene ratio of 1:50:1000:5000.
b Isolated yield based on tosylhydrazone consumed.
c Determined by GC comparison with authentic sample.
d Determined by 1H NMR compared with known compounds.1h,8
Under similar conditions, the reactions of a variety of
alkenes with either benzaldehyde- or naphthalene-2-carb-
aldehyde tosylhydrazone were examined (Table 2).
These reactions gave the corresponding trans-cyclo-
propanes in good to excellent yields (up to 90%) and
turnovers (up to 800) (entries 1–2, 4–6 and 8). Even in
one instance where it was envisaged that the use of the
sterically hindered alkene trans-b-methyl styrene would
not be favorable under the present conditions, moder-
ate product yield and selectivity was still obtained
(entry 3). Notably, cyclopropanation of buta-1,3-
dienyl-benzene occurred at the terminal CꢀC bond,
suggested the present protocol to be regioselective
(entry 7). Furthermore, the reaction of styrene with
trans-cinnamaldehyde tosylhydrazone giving [2-(2-
phenyl-cyclopropyl)-vinyl]-benzene as the sole product
in 68% yield implied the present procedure was also
chemioselective. No byproducts that could be
attributed to competitive dimerization were detected
(entry 9). It is worth mentioning that the analogous
[Rh2(CH3CO2)4]-catalyzed reaction gave both the cyclo-
propane and 2-(6-phenyl-hexa-1,3,5-trienyl)-benzene as
an inseparable mixture of products in 38% yield.7 The
chemioselectivity of the present procedure was further
highlighted by the cyclopropanation of styrene with
3-phenyl-but-2-enal tosylhydrazone which afforded the
corresponding cyclopropane as the sole product in 78%
yield (entry 10). Albeit not unexpected, 3,4-dihydro-
2H-pyran and tosylhydrazones derived from aryl
ketones were the only examples found not to undergo
reaction.2a
Table 1. Cyclopropanation of styrene with benzaldehyde
tosylhydrazone catalyzed by ruthenium porphyrins 2–8a
The cyclopropanation of N-vinylphthlimide with a vari-
ety of substituted tosylhydrazones was found to be
somewhat intriguing (Table 3). Reaction of N-vinyl-
phthlimide with benzaldehyde tosylhydrazone gave the
desired product with preferential cis-selectivity in yields
up to 72% (entries 1 and 2). However, with o-
fluorobenzaldehyde tosylhydrazone the corresponding
cis-cyclopropane was obtained as the minor product in
moderate yields and turnovers (up to 350). The major
isolated product was the corresponding arylsulfone
(entries 3 and 4).9 In this work, the analogous
[Fe(TPP)Cl]-(H2TPP=meso-tetrakis(phenyl)porphyrin)
and [Rh2(CH3CO2)4]-catalyzed reactions afforded either
the desired cis-cyclopropane adduct in yields up to 35%
(entries 5 and 6). Similarly, the reactions of both o-
bromo- and o-chlorobenzaldehyde tosylhydrazone gave
Entry
Catalyst
Yield (%)b (TON)
trans:cisc
1
2
3
4
5
6
7
2
3
4
5
6
7
8
91 (910)
92 (920)
42 (420)
36 (360)
25 (250)
68 (680)
66 (660)
95:5
96:4
34:66
70:30
74:26
72:28
75:25
a All reactions were performed in C6H6 at 40°C for 24 h with catalyst:
BnEt3N+Cl−: benzaldehyde tosylhydrazone:styrene ratio of
1:50:1000:5000.
b Isolated yield based on benzaldehyde tosylhydrazone consumed.
c Determined by GC compared with authentic sample.

J.-L. Zhang et al. / Tetrahedron Letters 44 (2003) 8733–8737
8735
Table 3. Cyclopropanation of N-vinylphthalimide with arylaldehyde tosylhydrazones catalyzed by 2 and 3a
Entry
R
Catalyst
Yield of I (%)d (TON)
cis:transe
Yield of II (%)d
1
2
3
4
5
6
7
8
H
H
2-F
2-F
2-F
2-F
2-Cl
2-Br
3-OMe
4-OMe
2-Cl,6-F
2-Cl,6-F
2-Cl,6-F
2
3
2
66 (660)
72 (720)
20 (100)
35 (350)
20 (10)
35 (18)
–
60:40
66:34
cis only
cis only
cis only
cis only
–
–
–
65
50
–
3
b
c
–
3
3
3
3
83
86
20
–
–
–
–
–
9
70 (700)
43 (430)
46 (460)
30 (30)
40 (40)
cis only
75:25
65:35
65:35
90:10
10f
11
12
13
3
b
c
–
a All reactions were performed in C6H6 at 40°C for 48 h with catalyst: BnEt3N+Cl−: tosylhydrazone: N-vinylthphalimide ratio of 1:50:1000:5000.
b [Fe(TPP)Cl], base: LiHMDS, in PhCH3 at 40°C for 48 h with catalyst: BnEt3N+Cl−: tosylhydrazone: N-vinylthphalimide ratio of 1:5:100:500.
c [Rh2(CH3CO2)4], base: LiHMDS, in dioxane at 40°C for 48 h with catalyst: BnEt3N+Cl−: tosylhydrazone: N-vinylthphalimide ratio of
1:10:100:500.
d Isolated yield based on tosylhydrazone consumed.
e Determined by 1H NMR analysis compared with known literature.1h,8
f Base: LiHMDS.10
product turnovers (up to 880) (entries 5 and 6). In other
instances, the sulfone was furnished as the minor
product in moderate yields along with the correspond-
ing cis-stilbene12 (entries 1, 2 and 8). The analogous
Rh- and Fe-catalyzed reactions of o-fluorobenzalde-
hyde tosylhydrazone, in comparison, gave only the
corresponding cis-stilbene (entries 3 and 4).
the corresponding arylsulfones as the sole products
(entries 7 and 8).9 Reactions with m- and p-methoxy-
benzaldehyde tosylhydrazones, on the other hand,
afforded the desired cyclopropanes in moderate to good
yields (up to 70%).10 Note that for m-methoxybenzalde-
hyde tosylhydrazone, cyclopropanation was observed
to occur with exclusive cis-selectivity (entries 9 and
10).8 Furthermore, cyclopropanation with 2-chloro-6-
fluorobenzaldehyde tosylhydrazone gave 2-[2-(6-chloro-
2-fluoro-phenyl)-cyclopropyl]-1,3-isoindoledione in 46%
yield (entry 11). This compared favorably to the
analogous reactions catalyzed by [Rh2(CH3CO2)4] or
[Fe(TPP)Cl] (entries 12 and 13). In this work, the
moderate turnover number achieved for the Ru-cata-
lyzed reaction (460) has not been attained for the same
reaction catalyzed by the latter two catalysts studied in
this laboratory (entries 11–13).
To highlight the practical applicability of the present
procedure, attention was turned to its deployment in
the synthesis of the potent HIV-1 reverse transcriptase
inhibitor precursor 10 previously studied by Aggarwal2a
and Doyle1h using [Rh2(CH3CO2)4] catalyst. Aryl tosyl-
hydrazone 9 was prepared from commercially available
4-chloro-2-fluorophenol in three steps in 78% overall
yield.2 Deprotonation of 9 with NaOMe (1 M) and
Whilst it is currently unclear as to why the formation of
the sulfone byproduct is preferentially favored, the
mechanistics of this reaction is speculated to involve
initial formation of a ruthenium-carbene intermediate.11
Nucleophilic attack of this reactive species by the tosyl-
ate anion generated in situ, a hitherto unknown com-
petitive reaction pathway in ruthenium-carbene chem-
istry, is subsequently reasoned to be responsible for the
formation of the sulfone product (Scheme 2).
This is supported by studies examining the effect of
metal-catalyzed decomposition of tosylhydrazones in
the absence of alkene substrate (Table 4). These reac-
tions furnished sulfone adduct in a number of cases as
the major product in excellent yields (up to 88%) and
Scheme 2. Proposed mechanism for aryl sulfone II formation.

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J.-L. Zhang et al. / Tetrahedron Letters 44 (2003) 8733–8737
Table 4. Sulfonation of aryl tosylhydrazones catalyzed by ruthenium(II) porphyrins 2 and 3a
Entry
R
Catalyst
Yield of II (%) (TON)d
Yield of III (%) (TON)d
1
2
3
4
5
6
7
8
9
2-F
2-F
2-F
2-F
2-Cl
2-Br
3-Br
3-MeO
4-MeO
2-Cl,6-F
2
30 (300)
32 (320)
–
–
84 (840)
88 (880)
–
22 (220)
–
–
60 (600)
61 (610)
90 (90)
90 (90)
10 (100)
9 (90)
80 (800)
74 (740)
90 (900)
82 (820)
3
b
c
3
3
3
3
3
3
10
a All reactions were performed in C6H6 at 40°C for 24 h with catalyst: BnEt3N+Cl−: sodium tosylhydrazone salt ratio of 1:50:1000.
b [Fe(TPP)Cl] with catalyst: BnEt3N+Cl−: lithium tosylhydrazone salt ratio of 1:50:100.
c [Rh2(CH3CO2)4] with catalyst: BnEt3N+Cl−: lithium tosylhydrazone salt ratio of 1:50:100.
d Isolated yield based on aryl tosylhydrazone consumed.
Table 5. Cyclopropanation of N-vinylphthalimide with
tosylhydrazone 9 catalyzed by 2, 3 and 8a
Council (HKSAR), the Hong Kong Research Grants
Council (No. 7077/01P) and the University Development
Fund of the University of Hong Kong.
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1
2
3
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69 (690)
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