Platinum-catalyzed cyclization of N-allyl carbamates for the synthesis of 5-vinyloxazolidinones

Article abstract of DOI:10.1002/ejoc.201101799

Platinum-catalyzed addition of the oxygen nucleophile of a carbamate to an allyl bromide was carried out to afford a range of biologically active 5-vinyl-substituted oxazolidinones in good yields. Over the course of the reaction, a platinum complex, SnCl<

Full text of DOI:10.1002/ejoc.201101799

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201101799
Platinum-Catalyzed Cyclization of N-Allyl Carbamates for the Synthesis of
5-Vinyloxazolidinones
Hyo-Sang Yoon,[a] Ju-Hye Kim,[a] Eun Joo Kang,[b] and Hye-Young Jang*[a]
Keywords: Vinyloxazolidinones / N-Allyl carbamates / Platinum / Cyclization / Heterogeneous catalysis
Platinum-catalyzed addition of the oxygen nucleophile of a
carbamate to an allyl bromide was carried out to afford a
generate an electrophilic allyl-platinum intermediate in the
presence of carbamate oxygen nucleophiles. This method
range of biologically active 5-vinyl-substituted oxazolidin- provides a wide range of 5-vinyloxazolidinones regardless of
ones in good yields. Over the course of the reaction, a plati-
num complex, SnCl2, and the allyl bromide are assumed to
the substitution at the nitrogen and oxygen atoms.
platinum complex,[9b,10] and the use of these complexes in
the synthesis of synthetically and biologically useful 5-vinyl-
oxazolidinones.[11,12] Although numerous examples of plati-
num-catalyzed reactions of allyl alcohols with amines and
allyl acetates with carbon nucleophiles are known,[10] plati-
num-catalyzed reactions of allyl halides with fragmented
carbamate oxygen nucleophiles are being reported here for
the first time.
Introduction
Oxazolidinone derivatives have been widely used as chi-
ral building blocks for asymmetric reactions and as biolo-
gically active units; thus, selective and efficient synthesis of
oxazolidinones has been intensively studied.[1] For example,
reactions of 1,2-amino alcohols with phosgene, diethyl car-
bonate, isocyanates, and CO2,[1,2] epoxide and aziridine ring
opening reactions,[3] and electrolytic reactions of a proparg-
[4]
ylamine with CO2 have been reported for the synthesis of
oxazolidinones. In addition, the transition-metal-catalyzed
reaction of amine compounds with CO2,[5] Pd-catalyzed all-
ylic substitution with amines,[6,7] and Au- and Cu-catalyzed
cyclization of alkynyl carbamates[8] have been employed to
prepare a range of oxazolidinone derivatives. By using dif-
ferent methods, different positions can be substituted and
specific functional groups can be introduced.
Among the above-mentioned methods, reactions cata-
lyzed by allyl-metal complexes have attracted our atten-
tion,[3f,6a,6c] as we have studied the catalytic generation and
reactions of allyl-platinum complexes.[9] During a study of
the reactivity of allyl-platinum intermediates, it was not
clear whether the electronic properties of the allyl-platinum
complex could be switched in the presence of a nucleophile
and an electrophile. Furthermore, to extend the use of allyl-
platinum-mediated reactions to the synthesis of useful
building blocks, the platinum-catalyzed reaction of an allyl
bromide with a nucleophile was attempted. In this study, we
present the expanded reaction scope of electrophilic allyl-
platinum complexes derived from an allyl bromide and a
Results and Discussion
Table 1 lists the cyclization results for allyl carbamate 1a.
This allyl carbamate was subjected to the allyl-platinum
forming conditions involving PtCl2 (5 mol-%), phosphanes
(10 mol-%), and SnCl2 (25 mol-%) in dichloroethane (DCE)
at 80 °C,[9b] and 5-vinyloxazolidinone 1b was obtained in
38% yield with P(Ph-pCF3)3 and 32% yield with P(furyl)3
(entries 1 and 2). The yield significantly increased to 89%
when electron-rich phosphanes were used (entries 3–5). The
stoichiometry of the phosphane ligand appears to be an
important factor influencing the product yield (entry 6).
Reduced loadings of SnCl2 afforded a reasonable yield of
1b; however, no reaction occurred in the absence of SnCl2
(entries 7 and 8). As a platinum(II) source, PtBr2 showed
good catalytic activity, but Pt(acac)2 did not induce cycliza-
tion at all (entries 9 and 10). In addition to platinum(II)
complexes, Pd(PPh3)4, Pd(PPh3)2Cl2, and Ru(PPh3)3Cl2
were added to a solution of 1a, and it was observed that
only Ru(PPh3)3Cl2 catalyzed the reaction to provide 1b in
11% yield (entry 11).[13] When gold complexes AuCl3 and
Au(PPh3)Cl, which were employed for the cyclization of
alkynyl carbamates, were treated with 1a, they afforded only
N-Boc-deprotected compounds. This cyclization was also
attempted in the absence of platinum catalysts, assuming
that the cyclization of 1a might occur by SnCl2-assisted
cleavage of the C–O bond in the carbamate followed by the
[a] Division of Energy Systems Research, Ajou University,
Suwon 443-749, Korea
Fax: +82-31-219-1615
E-mail: hyjang2@ajou.ac.kr
[b] Department of Applied Chemistry, Kyung Hee University,
Yongin 449-701, Korea
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101799.
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H.-S. Yoon, J.-H. Kim, E. J. Kang, H.-Y. Jang
SHORT COMMUNICATION
addition of the oxygen nucleophile to allyl bromide (SN2Ј 1:1 and 1.7:1). The diastereomers are separable by silica gel
type reaction). In the presence of SnCl2 (25 mol-% and chromatography. Presumably, the chirality of the amine in
100 mol-%), most of 1a underwent N-Boc deprotection ini- the tether of 8a and 9a was not transferred to the newly
tiated by SnCl2-assisted C–O bond cleavage, and 1b was formed C–O bond. Substrate 10a, which possesses a chiral
formed with low yields (18% and 21%). Relative to the unit bound to the carbamate oxygen, participated in the
platinum–stannane catalysis conditions (entries 5 and 9), reaction to afford a cyclized product 1b in 90% yield with
Lewis acid SnCl2-mediated SN2Ј type reaction and Ru-allyl- 0% enantiomeric excess (ee) (entry 10, Table 2). When the
mediated cyclization (entry 11) provided 1b in low yield.
Thus, the combination of a platinum complex and SnCl2
was chosen as the optimum catalyst for the formation of 5-
vinyl oxazolidinones with high yield.
Table 2. Substrate scope.
Table 1. Optimization of the cyclization of 1a.
A range of substrates were evaluated under the standard
cyclization conditions by using PtCl2 (5 mol-%), P(Ph-
OMe3)3 (10 mol-%), and SnX2 (X = Cl, Br) in dichloro-
ethane (DCE) at 80 °C (Table 2). To assess the leaving group,
an allyl chloride derivative was used in the reaction to af-
ford 1b in lower yield than that in the reaction of 1a (entries
1 and 2). Instead of allyl halides, an allyl-acetate-substituted
compound was subjected to the standard cyclization condi-
tions, and there was no product formation. The cyclization
yield varied dramatically with a change in the allyl substitu-
ent. Next, to check the effect of substituents on the carb-
amate oxygen atom, allyl- and benzyl-substituted carb-
amates were tested (3a and 4a). Surprisingly, in the presence
of Pt catalysts, the allyl group on 3a and the benzyl group
on 4a underwent fragmentation and cyclization to afford 1a
in good yields (entries 3 and 4). Next, the p-toluenesulfonyl
group on nitrogen was replaced by benzyl and allyl groups
in compounds 5a and 6a (entries 5 and 6). In the presence
of 10 mol-% SnCl2, substrates 5a and 6a underwent cycliza-
tion to afford products in good yields (93% and 66%). On
the basis of the cyclization results for compound 6b, it was
inferred that the carbamate oxygen reacted with the allyl
bromide selectively over the alkene. The competition be-
tween the allyl bromide and the alkyne in 7a led to the
preference of the addition of the carbamate to the allyl
bromide over the reaction with the alkyne (entry 7). In the
case of chiral compounds 8a and 9a (entries 8 and 9,
Table 2), cyclization occurred successfully, but, disappoint-
ingly, a low diastereomeric ratio was observed (trans/cis = [a] SnCl2 (25 mol-%). [b] SnBr2 (25 mol-%). [c] SnCl2 (10 mol-%).
1902 Eur. J. Org. Chem. 2012, 1901–1905
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Platinum-Catalyzed Cyclization of N-Allyl Carbamates
cyclization occurs, the chiral unit of 10a is too far from the complexes are formed from platinum(II) complexes during
reaction center, or the cleavage of the carbamate C–O bond the oxidation of allyl alcohols by PtII complexes at the ini-
takes place prior to the ring closing.
On the basis of the results in Table 2, plausible catalytic undergo oxidation to form platinum(0) species.
cycles are proposed in Schemes 1 and 2. Both catalytic cy- As an alternative mechanism, platinum-catalyzed SN2Ј
tial stage; however, allyl bromide in our reaction cannot
cles begin with the formation of the platinum–stannane allyl bromide substitution with an adjacent carbamate was
complex from PtCl2, phosphane, and SnCl2, which is as- considered (Scheme 2). The platinum complex may coordi-
sumed to react with 1a.[9b,14] The role of SnCl2 in both nate to allyl bromide, and the neighboring carbamate at-
mechanisms is speculated as a promoter for the cleavage tacks the coordinated olefin moiety (III). At the end of the
of the C–O bond in the carbamate as well as a catalyst cycle, product 1b is formed along with the release of tert-
activator.[15]
butyl cations. The formation of isobutylene and H+ from
tert-butyl cations was not checked; however, in the case of
benzyl-substituted 4a, benzyl bromide was observed, which
was derived from the benzyl cation and the bromide ion.
To probe the mechanism, the following experiments were
conducted (Scheme 3): (Z)-allyl carbamate 11a was sub-
jected to standard conditions to provide a mixture of (E)
and (Z) alkenes without ring formation. In the presence of
a non-fragmented rigid carbamate group, the platinum
complex promotes alkene isomerization via the allyl-plati-
num intermediate.[17] The isomerization of 11a via a zwitter-
ionic intermediate formed by activation of the alkene by
PtII may not occur, according to the results of 12a. (Z)-
Allyl acetate 12a was exposed to the reaction conditions to
recover 12a without isomerization and cyclization. Accord-
ingly, the platinum catalyst in this study does not promote
simple isomerization via a zwitterionic intermediate, and
the formation of the allyl-platinum intermediate is crucial
for the cyclization. Under the hydrogenation atmosphere,
the cyclization was attempted to trap the allyl intermediate.
Gratifyingly, in the presence of hydrogen, the desired 5-
vinyl oxazolidinone 1b was obtained in good yield (78%)
along with 1c and 1d (18% yield). Compounds 1c and 1d
are formed from reductive elimination of the allyl-platinum
intermediate in the presence of the hydride ligand around
the platinum ion.[9b] On the basis of the above-mentioned
Scheme 1. The catalytic cycle mediated by the allyl-platinum com-
plex.
Scheme 2. The catalytic cycle including a platinum-coordinated
alkene intermediate.
Once the platinum–stannane complex is formed, oxidat-
ive insertion of the platinum complex into allyl bromide
takes place along with the release of a bromide ion
(Scheme 1).[16] Subsequently, the oxygen nucleophile of the
carbamate is added to the allyl-platinum intermediate, and
the removal of the tert-butyl group from II provides the
desired product 1b. Alternatively, the tert-butyl group might
be released before the ring is closed, as shown in intermedi-
ate IIЈ. Although the proposed catalytic cycle involving
allyl-platinum(IV) I is inconclusive, the cyclization mediated
by allyl-platinum(II) complexes derived from allyl bromide
and in situ generated platinum(0) complexes is less favor-
able under our reaction conditions. In platinum-catalyzed
coupling of allyl alcohols with nucleophiles,[10] platinum(0) Scheme 3. Control experiments.
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H.-S. Yoon, J.-H. Kim, E. J. Kang, H.-Y. Jang
SHORT COMMUNICATION
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mediate is formed via allylic oxidative insertion of the plati-
num–stannane complex, leading to the desired cyclization.
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Conclusions
We have discovered that a platinum complex can be uti-
lized for the synthesis of synthetically and biologically im-
portant 5-vinyl oxazolidinones by cyclization of allyl carb-
amates. A variety of substrates possessing tosyl-, benzyl-,
allyl-, and propargyl-substituted amines participate in the
reaction to afford 5-vinyloxazolidinones. As a substituent
at the carbamate oxygen, benzyl and allyl groups as well as
the tert-butyl group undergo fragmentation to promote the
desired cyclization. Regarding the working mechanism, the
formation of the allyl-platinum intermediate and interac-
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Platinum-Catalyzed Cyclization of N-Allyl Carbamates
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Received: December 14, 2011
Published Online: March 1, 2012
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