Cyclopropanation of strained alkenes by palladium-catalyzed reaction of 3-trimethylsilyl- or 3-pinacolatoboryl-1-arylallyl acetates

Article abstract of DOI:10.1002/adsc.201100806

The palladium-catalyzed cyclopropanation of strained alkenes with 3-trimethylsilyl- or 3-pinacolatoboryl-1-arylallyl acetate derivatives is described. This reaction gives cyclopropanation products in good to high yields with a single diastereomer, and the key step is likely to involve the formation of a palladacyclobutene complex from the α-trimethylsilyl- or α-pinacolatoboryl-σ-allylpalladium complex. Copyright

Full text of DOI:10.1002/adsc.201100806

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DOI: 10.1002/adsc.201100806
Cyclopropanation of Strained Alkenes by Palladium-Catalyzed
Reaction of 3-Trimethylsilyl- or 3-Pinacolatoboryl-1-arylallyl
Acetates
Yoshikazu Horino,a,* Naoki Homura,a Kana Inoue,a and Saori Yoshikawaa
a
Department of Applied Chemistry, Graduate School of Science and Engineering, University of Toyama, Toyama 930-855,
Japan
Fax : (+81)-76-445-6819; e-mail: horino@eng.u-toyama.ac.jp
Received: October 18, 2011; Revised: November 23, 2011; Published online: March 13, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100806.
Abstract: The palladium-catalyzed cyclopropana-
tion of strained alkenes with 3-trimethylsilyl- or 3-
pinacolatoboryl-1-arylallyl acetate derivatives is de-
scribed. This reaction gives cyclopropanation prod-
ucts in good to high yields with a single diastereo-
mer, and the key step is likely to involve the forma-
tion of a palladacyclobutene complex from the a-
trimethylsilyl- or a-pinacolatoboryl-s-allylpalladium
complex.
activity pattern. Bis-p-allylpalladium complexes,[1] tri-
methylenemethane-palladium
complexes[2]
and
pincer-type p-allylpalladium complexes[3] are repre-
sentative examples. In 1983, Trost reported the gener-
ation of Pd-complexed vinylcarbene species B via the
a-elimination of acetoxytrimethylsilane from a-trime-
thylsilyl-s-allylpalladium
intermediate
A
(Scheme 1).[4] Although the reactivity of metal carbe-
noids mainly depends on the functionality of the car-
bene and the nature of the metal that donates the
electron into the empty p orbital of the carbene
carbon atom,[5] the Pd-complexed vinylcarbene spe-
cies B generated from 1 and a palladium complex ap-
parently exhibits a different reactivity from typical
palladium carbenoids.[6,7]
Keywords: allylsilanes; cyclopropanation; pallada-
cyclobutenes; palladium carbenoids; p-allylpalladi-
um species
Recently, Fillion demonstrated the ambiphilic vinyl-
carbenoid nature of an a-tributylstannyl-p-allylpalla-
p-Allylpalladium intermediates have been widely dium complex.[7b] In light of these results, it is evident
studied due to their possible involvement in a large that the carbon atom between the palladium and the
number of important organic transformations involv- trimethysilyl group in Trostꢀs gem-dimetallic inter-
À
À
ing the formation of C C and C heteroatom bonds. mediate A has a Brønsted base character, and can
Changing the behavior of such allyl species on palla- therefore subsequently undergo protodesilylation to
dium complexes has the potential of altering their re- produce p-allylpalladium intermediate C in the pres-
Scheme 1. Working hypothesis.
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Cyclopropanation of Strained Alkenes by Palladium-Catalyzed Reaction
Table 1. Optimization of the reaction conditions.[a]
Entry
X
1
Additive
Time [h]
3a [%][b]
4 [%][b]
1
OCOMe
OCOMe
OCOMe
OCOMe
OCOMe
OCOMe
OCOMe
OCO2Et
OCONHTs
OCONHPh
OCON(Me)Ph
OCSNHPh
Br
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
1e
1f
1g
1h
CsF
CsF
CsF
KF
3
6
78
47
27
35
68
33
20
74
74
54
58
0
0
2[c]
3[d]
4
24
14
0
0
0
0
0
0
0
0
0
0
0
10
24
6
20
36
4
3
2
4
5
TBAT[e]
Cs2CO3
–
6
7[f]
8
CsF
CsF
CsF
CsF
CsF
CsF
CsF
9
10
11
12
13
14
12
12
12
0
0
OEt
[a]
Reaction was performed with 1 (0.5 mmol), 2a (1.5 mmol), Pd2CATHUNGTRENN(NUG dba)3CHCl3 (0.025 mmol) and additive (1.0 mmol) in
DMF (2.5 mL) at 608C.
Isolated yield.
2 equiv. of 2a to 1a were used.
1 equiv. of 2a to 1a was used.
TBAT=tetrabutylammonium difluorotriphenylsilicate.
Reaction was carried out at 808C.
[b]
[c]
[d]
[e]
[f]
ence of nucleophiles such as b-keto esters or b-di- tries 1–3). Among the additives tested, cesium fluo-
ACHTUNGTRENNUNGesters that possess acidic protons. In this case, the ride was found to provide 3a in high yield (entries 1
À
Tsuji–Trost reaction is induced rather than C H inser- and 4–6). Furthermore, addition of a fluoride source
tion of the palladium carbenoid. as an additive was found to be essential for the pres-
On the basis of these reactivity principles, we envi- ent reaction (entry 7). Importantly, intermediate A
sioned that palladacyclobutene intermediate E might must allow the intramolecular coordination of a car-
be generated from a-trimethylsilyl-s-allylpalladium bonyl oxygen to the silicon atom in order to generate
intermediate A via an SE2’-type reaction in which the palladacyclobutene intermediate E (entries 8–14). The
allylsilane moiety acts as an intramolecular hard nu- use of p-allylpalladium chloride dimer as a catalyst af-
cleophile that can attack the palladium complex if the forded similar results to Pd2ACHTNUTRGNE(NUG dba)3CHCl3, whereas
silicon atom can be activated appropriately.[8] To the other palladium complexes gave diminished yields of
best of our knowledge, there are very few previous re- 3a.[10] Furthermore, addition of a phosphine ligand
ports regarding the reactivity of palladacyclobutene was found to enhance the Brønsted base character of
intermediates.[9]
the carbon atom between the palladium and trime-
After several attempts, we finally found that the re- thylsilyl group, which resulted in protodesilylation to
action of 1a (1 equiv.) with 2,5-norbornadiene 2a produce cinnamyl acetate.[11]
(3 equiv.) in the presence of cesium fluoride (2 equiv.)
Under the optimized reaction conditions, the sub-
and Pd2A(dba)3CHCl3 (5 mol%) in DMF under an strate scope of the present Pd-catalyzed cyclopropa-
CHTUNGTRENNUNG
argon atmosphere at 608C provided the cyclopropa- nation of 2a is summarized in Table 2. The present re-
nation product 3a in 78% isolated yield as a single action was found to proceed independently of the
diastereomer (Table 1, entry 1). Cyclopropanation oc- electronic nature of the substituent at the aromatic
curred exclusively on the exo face of 2a, and the ste- position. For example, the aryl-containing substrates
reochemistry was determined to be anti by NOE ex- bearing electron-donating groups 1i–1l and electron-
periments. Three equivalents of 2a were needed to withdrawing groups 1m–1r afforded the desired prod-
obtain 3a in high yield. The yield of 3a was found to ucts 3b–3k in good to high yields (entries 1–10). Nota-
decrease significantly and the double cyclopropana- bly, bromo-containing substrate 1q afforded product
À
tion product 4 was formed in considerable yield when 3j in good yield while leaving the C Br bond intact
2a was reduced to less than three equivalents (en- (entry 9). Moreover, the present cyclopropanation
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Table 2. Pd-catalyzed cyclopropanation of 2a with 1.[a]
Entry
R
1
Time [h]
3 [%][b]
1
2
3
4
5
6
7
8
2-MeC6H4
4-MeOC6H4
2,6-Me2C6H3
2-PhC6H4
3-MeOC6H4
3,5-(MeO)2C6H3
4-FC6H4
4-ClC6H4
4-BrC6H4
4-CF3C6H4
2-thiofuryl
2-furyl
1i
1j
1k
1l
1m
1n
1o
1p
1q
1r
4
3
6
5
4
4
4
4
4
6
8
8
3b (81)
3c (78)
3d (55)
3e (60)
3f (77)
3g (50)
3h (75)
3i (70)
3j (52)
3k (55)
3l (53)
3m (50)
9[c]
10
11
12
1s
1t
[a]
Reaction was performed with 1 (0.5 mmol), 2a (1.5 mmol), Pd
DMF (2.5 mL) at 608C.
Isolated yield.
2ACHTUNGTRNEN(UNG dba)3CHCl3 (0.025 mmol) and additive (1.0 mmol) in
[b]
[c]
4ꢂ molecular sieves (250 mg) was used.
Scheme 2. Pd-catalyzed cyclopropanation of 2a with 1u and 1v.
also proceeded in the presence of heteroaromatic propanes (E)-3n and (Z)-3n were obtained respective-
groups such as 2-thiofuryl 1s and 2-furyl 1t to give the ly as a single diastereomer.
corresponding cyclopropanation products 3l and 3m,
We then explored the scope of this Pd-catalyzed in-
respectively, in good yield (entries 11 and 12). Inter- termolecular cyclopropanation with a variety of alk-
estingly, all the cyclopropanation products were iso-
ACHTUNGTRENeNUGN nes (Table 3). In sharp contrast to the typical reactiv-
lated as a single diastereomer, which suggests that the ity of palladium carbenoids,[6] the present reaction
present reaction would proceed via a different mecha- generally proceeds with strained alkenes.
nism from Fillionꢀs palladium vinylcarbenoid pathway.
For example, no cyclopropanation occurred when
Unfortunately, the use of substrates bearing an alkyl the reaction of 1a with styrene was tested.[10] On the
group in place of the aryl group resulted in complex other hand, use of strained alkenes 2b–2e led to the
mixtures, possibly due to lack of an aryl effect[12] or desired products 3o–3r, respectively, in good to high
a b-hydrogen elimination in intermediate E. Similarly, yields (entries 1-4). In all cases, the cyclopropanation
the sterically demanding substrate 1u proved to be product was isolated as a single diastereomer. It is
unreactive (Scheme 2). On the other hand, the use of worth noting that 3r was obtained with excellent che-
1v produced the desired product in good yield as moselectivity (entry 4).
a 4.2:1 mixture of (E)-3n and (Z)-3n isomers. It
To gain further insight into the nature of the reac-
should be noted that cyclopropanation occurred ex- tive species involved in this process, the reaction of 2a
clusively on the exo face of 2a, and that the anti-cyclo- with cinnamaldehyde tosylhydrazone sodium salt 5
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Cyclopropanation of Strained Alkenes by Palladium-Catalyzed Reaction
Table 3. Reaction of 1a with various alkenes 2.[a]
Entry
Alkene
2
Product
3 [%][b]
3o (80)
3p (40)
3q (65)
1
2
3
2b
2c
2d
4
2e
3r (58)
[a]
Reaction was performed with 1a (1.0 mmol), 2 (3.0 mmol), [(allyl)PdCl]2 (0.1 mmol), CsF (2.0 mmol) in DMF (5.0 mL) at
808C.
Isolated yield.
[b]
Scheme 3. Cyclopropanation of 2a using diazo compound precursors 5 and d-5.
Scheme 4. Deuterium labelling experiments.
that is known to generate styryldiazomethane in the shown in Scheme 1. To further verify our working hy-
presence of a phase-transfer catalyst was examined pothesis, the palladium-catalyzed dimerization of 1a
(Scheme 3).[13] We were surprised to find that cyclo- in the absence of 2a was examined because transition
propanation of 2a with styryldiazomethane afforded metal complexed vinylcarbenes generated from a tran-
3a as a mixture of diastereomers (dr=1.2:1), and sition metal complex with styryldiazomethane are
anti-cyclopropane was the major product. Moreover, prone to dimerization.[13a] However, no dimerization
3s was obtained without scrambling of the deuterium product was observed and instead a trace amount of
atom.[14] Similarly, when the reaction of deuterium-la- cinnamyl acetate was obtained although metallacyclo-
belled 3-d-1a and 1-d-1a with 2a was examined, no butene intermediates are known to rearrange to form
scrambling of the deuterium atom between C-1 and metal vinylalkylidene complexes,[15] This result sug-
C-3 was also observed in the respective products 3s gests that 2a is not only the substrate of cyclopropa-
and 3t in either case (Scheme 4). These mechanistic nation but also may play an important role as a ligand
experiments strongly support our working hypothesis to generate palladacyclobutene intermediate E.[16]
Adv. Synth. Catal. 2012, 354, 828 – 834
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Yoshikazu Horino et al.
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Scheme 5. Reactions of 2a with 1 bearing a pinacolatoboryl group.
Scheme 6. A plausible reaction mechanism.
In light of the above findings, we considered that if alternative pathway involving Pd-complexed vinylcar-
the present cyclopropanation proceeds dominantly bene intermediate H generated from G should be
through palladacyclobutene intermediate E and this also taken into consideration. However, this process
formation step would be the rate-limiting step, then usually affords the 3a as a mixture of diastereomers
the allylborane species produced upon complexation (Scheme 3).
of 1w to the palladium complex would be more reac-
In summary, we have developed a palladium-cata-
tive than the corresponding allylsilane species. In lyzed cyclopropanation of strained alkenes with 3-tri-
order to prove this hypothesis, the reaction of 1w with methylsilyl- or 3-pinacolatoboryl-1-arylallyl acetates,
2a was examined (Scheme 5). We were pleased to which gives cyclopropanation product as a single dia-
find that the reaction proceeded smoothly to give 3a stereomer. The key step in the present reaction is
in moderate yield after 1 h at room temperature. likely to involve the formation of a palladacyclobutene
Likewise, the reaction of 1x with 2a also afforded 3a complex generated from the a-trimethylsilyl- or a-pi-
in moderate yield at room temperature (Table 1, nacolatoboryl-s-allylpalladium complex. Further ef-
entry 9 versus Scheme 5). Thus, a mechanism involv- forts to expand the synthetic utility of the present
ing a palladacyclobutene intermediate E may account methodology are underway.
for the observed different reactivities.
Scheme 6 depicts a plausible catalytic cycle for the
present cyclopropanation of 2a. Dimetallic species F
bearing an allylsilane moiety, which is formed initially
Experimental Section
via the p-allylpalladium intermediate, is transformed
into palladacyclobutene intermediate G in the pres-
ence of cesium fluoride. Compound 2a then inserts
into G to produce palladacylobutane intermediate I,
which subsequently undergoes reductive elimination
upon coordination of another molecule of 2a to the
palladium complex to furnish product 3a. This is
likely to be a facile process since reductive elimina-
Typical Procedure
In a 20-mL Schlenk tube were charged Pd2ACTHNUTRGNEG(NU dba)3CHCl3
(5 mol%), CsF (1 mmol), 2,5-norbornadiene 2a (1.5 mmol)
and dry DMF (2.5 mL) under an argon atmosphere. The re-
sulting suspension was stirred at room temperature for
15 min, and the substrate 1a (0.5 mmol) was then added.
The reaction mixture was heated at 608C, and the reaction
mixture was allowed to stir for the time specified. After re-
tion is generally accelerated by p-acidic ligands.[17] An action completion, as monitored by TLC, the reaction mix-
832
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Cyclopropanation of Strained Alkenes by Palladium-Catalyzed Reaction
ture was quenched with saturated aqueous NH4Cl (10 mL).
The aqueous phase was extracted with diethyl ether (2ꢃ
20 mL), and the combined organic extracts were washed
with brine (2ꢃ20 mL). After the organic layer had been
dried over MgSO4, the solvent was removed under reduced
pressure. The residue was purified by silica gel chromatogra-
phy to give 3a as a colorless oil.
[7] For recent developments in palladium carbenoid medi-
ated reactions, see: a) E. Fillion, N. J. Taylor, J Am.
Chem. Soc. 2003, 125, 12700–12701; b) V. ꢈ. Trꢉpanier,
E. Fillion, Organometallics 2007, 26, 30–32; c) J. M.
Goll, E. Fillion, Organometallics 2008, 27, 3622–3625;
d) R. Kudirka, S. K. Devine, C. S. Adams, D. L. Van V-
ranken, Angew. Chem. 2009, 121, 3731–3734; Angew.
Chem. Int. Ed. 2009, 48, 3677–3680; e) S. K. Devine,
D. L. Van Vranken, Org. Lett. 2008, 10, 1909–1911;
f) Y. Zhang, J. Wang, Eur. J. Org. Chem. 2011, 1015–
1026; g) Z. Zhang, Y. Liu, L. Ling, Y. Li, Y. Dong, M.
Gong, X. Zhao, Y. Zhang, J. Wang, J. Am. Chem. Soc.
2011, 133, 4330–4341; h) Q. Xiao, J. Ma, Y. Yang, Y.
Zhang, J. Wang, Org. Lett. 2009, 11, 4732–4735.
The Supporting Information contains the experimental
details, product characterization and NMR spectra.
Acknowledgements
[8] For recent examples of allylation of alkynes using allyl-
silanes involves SE2’-type reaction, see: a) Y. Horino,
M. R. Luzung, F. D. Toste, J. Am. Chem. Soc. 2006, 128,
11364–11365; b) S. Park, D. Lee, J. Am. Chem. Soc.
2006, 128, 10664–10665; c) Y. Horino, Y. Nakashima,
K. Hashimoto, S. Kuroda, Synlett 2010, 2879–2882.
[9] Palladacyclobutene complex mediated reactions, see:
a) I. Nakamura, G. B. Bajracharya, Y. Yamamoto, J.
Org. Chem. 2003, 68, 2297–2299; b) M. Yoshida, M. Hi-
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This work was partially supported by a Grant-in-Aid for
Young Scientists (B) from the Ministry of Education, Culture,
Sports, Sciences, and Technology of Japan.
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[16] The use of 2,2’-bipyridine instead of 2a as a ligand
(1 equiv. to Pd) afforded 1,6-diphenyl-1,3,5-hexatriene
in 88% as a mixture of (1E,3E,5E) and (1E,3Z,5E) iso-
mers (2.8:1). This shows the reactivity of the palladium
carbenoid, which will be reported in due course.
[17] An alternative mechanism involves insertion of 2a to
a p-allylpalladium intermediate that would be pro-
duced by palladium complexing with cinnamyl acetate
generated from protodesilylation of 1a. However, this
process preferentially produces a [2+2]cycloadduct
rather than a cyclopropanation product, thus making
this possibility rather unlikely. For details, see: N. Tsu-
kada, T. Sato, Y. Inoue, Tetrahedron Lett. 2000, 41,
4181–4184.
834
asc.wiley-vch.de
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 828 – 834