Synthesis of naphthalenyl acetate by platinum-catalyzed [1,5]-sigmatropic hydrogen shift of propargylic esters

Article abstract of DOI:10.1002/chem.200801591

A study was conducted to demonstrate the synthesis of naphthjalenyl acetate by platinum-catalyzed [1,5]-sigmatropic hydrogen shift of propargylic esters. Optimization investigations of the synthesis started with propargylic ester, with various catalysts. It was demonstrated that platinum-promoted rearrangement of propargylic esters provides two isomers of allene complex. It was also demonstrated that the [1,5]-sigmatropic hydrogen shift product was isolated, when the methyl group was introduced in the benzyl position. It was observed that the configurational orientation of the acyloxy group away from the large substituents led to the formation of a platinum-coordinated six-membered ring opposite the R group, which prevented the H-shift process.

Full text of DOI:10.1002/chem.200801591

DOI: 10.1002/chem.200801591
Synthesis of Naphthalenyl Acetate by Platinum-Catalyzed [1,5]-Sigmatropic
Hydrogen Shift of Propargylic Esters
Xing-Zhong Shu, Ke-Gong Ji, Shu-Chun Zhao, Zhao-Jing Zheng, Jing Chen, Li Lu,
Xue-Yuan Liu, and Yong-Min Liang*[a]
The [1,5]-sigmatropic hydrogen shift is a useful tool in or-
ganic synthesis[1] which has stimulated many mechanistic
studies[2] and has found numerous applications in complex
molecules synthesis.[3] In cyclic systems,[1c,4] this process has
also been proved efficiently by using acyclic dienes and its
derivates,[1c,5] such as cis-1,3-dienes,[5a,b] cis-1-alkyl-2-vinylcy-
clopropanes[5c,d] and cis-1-allen-4-enes[5e,f] [Eq. (1–3)]. How-
ever, in the alkyne area this reaction mechanism has not
been observed until Liu and co-workers recently reported
that cis-3-en-1-ynes did undergo the [1,5]-sigmatropic hydro-
gen shift via ruthenium–vinylidene intermediates [Eq. (4)].[6]
Consequently, reactions of the [1,5]-hydrogen shift in the
alkyne chemistry are still largely unexplored.
As a special of alkynes, readily available propargylic
esters have continued to attract the interest of different re-
search groups.[7] The metal–allene complexes, which are
formed during the reaction, are considered common inter-
mediates, that further react with various functional groups
resulting in astonishingly diverse products under platinum
and gold catalysis.[7,8] We envisioned that this type of allenyl
esters B could realize the [1,5]-hydrogen shift process, al-
though the nucleophilic attack on the C2 position still re-
mains virtually unknown[9] compared with C1[8h,i] and C3[8j–l]
positions [Eq. (5)]. Herein, we report the first examples of
[1,5]-sigmatropic hydrogen shift reaction of propargylic
esters.
Optimization studies of this transformation began with
propargylic ester 1a (0.3 mmol) with various catalysts.
Among the platinum catalysts used (Table 1, entries 1–4),
PtCl2 (10 mol%) with CO (1 atm) gave the best result
(Table 1, entry 4). Low-catalyst loading led to the poor yield
of 2a, whereas a similar result was obtained when 20 mol%
of PtCl2 were added (Table 1, entries 5 and 6). The changes
in solvent and temperature did not give better results
(Table 1, entries 7–10). Gold catalysts could also catalyze
this cyclization, however, no superior results were obtained
(entries 11–14). Thus, the use of PtCl2 (10 mol%) and CO
(1 atm) in toluene (2 mL) at 858C was found to be most effi-
cient, which was subsequently used as the standard reaction
condition.
[a] Dr. X.-Z. Shu, K.-G. Ji, S.-C. Zhao, Z.-J. Zheng, J. Chen, L. Lu,
X.-Y. Liu, Prof. Dr. Y.-M. Liang
State Key Laboratory of Applied Organic Chemistry
Lanzhou University
Lanzhou 730000 (China)
Fax : (+86)931-8912582
E-mail: liangym@lzu.edu.cn
Under these optimal conditions, we studied the scope of
this cyclization as shown in Table 2. Besides the phenyl
group, various aryl substituents were tolerated at the propar-
gylic position (entries 1–4). The vinyl group also gave mod-
erate yields of 2e (entry 5). When the furyl substituent was
used, uneliminated product 3a[10] was isolated in 81% yield
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801591.
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COMMUNICATION
Table 1. Optimization of reaction conditions for the cyclization of 1a.[a]
Furthermore, various protective groups of benzyl alcohols
were investigated, which always resulted in moderate to
high yields of desired products (Table 3). Among these, the
best attractive one might belong to the group of TBS, as the
singly uneliminated product 3c and [1,7]-sigmatropic hydro-
gen shift product 4a were obtained in high yields from the
respective substrates (entries 5 and 6).
Interestingly, when the benzyl position was substituted by
N-containing groups such as lactams, this tandem process
also proceeded smoothly under optimal conditions to afford
3d[10] in 43% yield. A similar result was obtained by using
Entry Catalyst [mol%]
Conditions
Yield [%][b]
1
2
3
4
5
6
7
8
PtCl2 (10)
toluene, 858C, 24 h trace
toluene, 858C, 24 h 52
toluene, 858C, 24 h trace
PtCl2 (10), COD (40)
PtCl2 (10), O2 (1 atm)
PtCl2 (10), CO (1 atm)
PtCl2 (5), CO (1 atm)
PtCl2 (20), CO (1 atm)
PtCl2 (10), CO (1 atm)
PtCl2 (10), CO (1 atm)
PtCl2 (10), CO (1 atm)
PtCl2 (10), CO (1 atm)
AuCl3 (2)
toluene, 858C, 6 h
toluene, 858C, 24 h 78
toluene, 858C, 5 h 85
CH3CN, 858C, 24 h trace
DCE, 858C, 4 h 26
87
[AuACTHNUTGRNEUNG(PPh3)BF4] as the catalyst [Eq. (6)].
9
toluene, 608C, 24 h 63
toluene, 1008C, 6 h 72
toluene, 858C, 24 h trace
toluene, 858C, 24 h 37
toluene, 858C, 24 h 43
10
11
12
13
14
AuCl (2)
AuACHTUNGTRENNUNG(PPh3)Cl (2), AgBF4 (2)
AuN
trace
[a] Unless noted, all reactions were carried out using 1 (0.3 mmol).
[b] Isolated yield.
The deuterium-labeling ex-
Table 2. PtII-catalyzed [1,5]-H shift reactions of propargylic esters.[a]
periments (Scheme 1) showed
that the deuterium on the
benzyl position transferred into
the b-position of the ester after
reaction ([D1]-2i and [D1]-4a).
However, the shift of deuterium
on the methyl group was only
observed in the product [D3]-
4a[11]. This observation is con-
sistent with our proposed mech-
anism, as depicted in Scheme 1.
The PtII-promoted [1,3]-OAc
shift lead to the formation of al-
lenyl esters A,[7] which under-
goes a [1,5]-H shift process[5] to
form intermediates B. Subse-
quent 6p cyclization[12] of com-
Entry
Substrate
t [h]
Product (yield [%])[b]
1
2
R=Ph, X=R1 =H (1a)
R=p-ClC6H4, X=R1 =H (1b)
R=p-CH3C6H4, X=R1 =H (1c)
R=2,3-methylenedioxy,
X=R1 =H (1d)
6
6
4
2a (87)
2b (88)
2c (68)
2d (96)
3
4[c]
18
5
6
R=vinyl, X=R1 =H (1e)
R=2-furanyl, X=R1 =H (1 f)
7
1
4
4
5
6
2e (56)
3a (81)
2 f (52)[d]
2g (93)
2h (87)
2i (55)
4a (32)
7
8
9
R=Ph, X=Cl, R1 =H (1g)
R=Ph, X=H, R1 =Ph (1h)
R=Ph, X=H, R1 =CH3 (1i)
pounds
B
afford cyclization
[a] Unless noted, all reactions were carried out using 1 (0.3 mmol) with 10 mol% of PtCl2 under CO (1 atm) in
toluene (2 mL) at 858C. [b] Isolated yield. [c] 20 mol% of PtCl2 was used. [d] H2O (1 equiv) was added.
Table 3. Affects of different protective groups.[a]
with high cis-selectivity, where-
as 1 equiv of H2O led to a com-
plete transformation (entry 6).
However, for R=H or alkyl
groups, the desired products
were not detected. Substituents
on the benzene as well as
benzyl positions did not affect
the reactivity (entries 7–9).
However, when the methyl
group was introduced in the
benzyl position, the [1,7]-sigma-
tropic hydrogen shift product
4a was also isolated (entry 9).
Entry
Substrate
t [h]
Product (yield [%])[b]
1
2
3
4
R=allyl, R1 =H (1j)
R=Ph, R1 =H (1k)
R=THP, R1 =H (1l)
R=Bn, R1 =H (1m)
3
5
4
2a (91)
2a (65)
2a (53)
2a (14) 3b (84)
2a (94)
3
13
12
5
5[c]
6
R=TBS, R1 =H (1n)
3c (93)
4a (81)
R=TBS, R1 =CH3 (1o)
[a] Unless noted, 0.3 mmol of 1 and 10 mol% of PtCl2 was used. [b] Isolated yield. [c] DCE was used.
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Y.-M. Liang et al.
products C, which further eliminated to give products [D1]-
2i. At the same time, the intermediates B might also under-
go a [1,7]-H shift[13] to afford compound D, which could be
easily transformed into ketone [D1]-4a by enolization.
In conclusion, we have reported a novel platinum-cata-
lyzed transformation of 3-(2-alkyl)phenyl-propynyl acetate
to naphthalenyl acetate. The deuterium-labeling experi-
ments show that the reaction might include the [1,3]-OAc
shift, [1,5]-sigmatropic hydrogen shift and 6p cyclization
processes. The high stereoselectivity of this tandem reaction
was also disclosed.
Experimental Section
General produce for the platinum catalyzed tandem reactions of propar-
gylic esters 1: PtCl2 (8.0 mg, 10 mol%) was added under CO atmosphere
(1 atm) at 858C to a stirred solution of propargylic esters 1 (0.30 mmol)
in toluene (2.0 mL). When the reaction was considered complete as de-
termined by TLC analysis, the reaction mixture was diluted with ethyl
acetate (20 mL) and evaporated under reduced pressure. The residue was
purified by column/flash chromatography on silica gel to afford corre-
sponding products.
Acknowledgements
We thank the NSF (NSF-20621091, NSF-20672049) and the “Hundred
Scientist Program” from the Chinese Academy of Sciences for financial
support.
Keywords: Hydrogen shift
·
homogeneous catalysis
·
naphthalenyl acetate · platinum · propargylic ester
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Platinum-Catalyzed [1,5]-Sigmatropic Hydrogen Shift
COMMUNICATION
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Received: August 2, 2008
Published online: October 17, 2008
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