Highly stereoselective facile synthesisl of 2-acetoxy-1,3(E)-alkadienes via a Rh(I)-catalyzed isomerization of 2,3- allenyl carboxylates

Article abstract of DOI:10.1021/ol200198z

A highly stereoselective Rh(I)-catalyzed 1,3-acetoxyl rearrangement of 1,2-allen-3-yl carboxylates leading to 2-acetoxy-1,3(E)-alkadienes has been developed. In addition to the high catalytic efficiency and the scope, the excellent E-selectivity of the do

Full text of DOI:10.1021/ol200198z

ORGANIC
LETTERS
2011
Vol. 13, No. 8
1920–1923
Highly Stereoselective Facile Synthesis of
2-Acetoxy-1,3(E)-alkadienes via a
Rh(I)-Catalyzed Isomerization of 2,3-
Allenyl Carboxylates
Xiaobing Zhang, Chunling Fu, and Shengming Ma*
Laboratory of Molecular Recognition and Synthesis, Department of Chemistry,
Zhejiang University, Hangzhou 310027, Zhejiang, People’s Republic of China
masm@sioc.ac.cn
Received January 22, 2011
ABSTRACT
A highly stereoselective Rh(I)-catalyzed 1,3-acetoxyl rearrangement of 1,2-allen-3-yl carboxylates leading to 2-acetoxy-1,3(E)-alkadienes has been
developed. In addition to the high catalytic efficiency and the scope, the excellent E-selectivity of the double bond is remarkable.
1,3-Alkadien-2-yl carboxylates are important synthetic
building blocks in organic synthesis since they may readily
undergo the Diels-Alder reaction,1 asymmetric hydroge-
nation,2 and hydrolysis to afford the functionalized R,β-
unsaturated enones.3 In the classical synthesis of 1,3-alka-
dien-2-yl carboxylate, 1-buten-3-yne4 and 2-enones1c,3b,5
have been used as starting materials; however, Hg(II)
(25 mol %)4a or H2SO4 (40 mol %)1c is required. Recently,
transition metals such as Ag(I) (5 mol %),6 Au(I)
(1-5 mol %),6a,b,7 Cu(I) (5 mol %),8 Hg(II) (5 mol %),9
(7) (a) Wang, S.; Zhang, L. J. Am. Chem. Soc. 2006, 128, 8414. (b)
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r
10.1021/ol200198z
Published on Web 03/10/2011
2011 American Chemical Society

Pt(II) (5-10 mol %),10 Pd(II) (5-10 mol %),11 Ru(I) (5
mol %),12 and Rh(I) (5 mol %)13 to catalyze the rear-
rangement of 2-alkynyl carboxylates have been exten-
sively studied, some of which afforded 1,3-alkadien-2-yl
carboxylates as the major products.14 For example
Zhang et al. reported rearrangement-elimination of
TMS-substituted propargylic carboxylates affording
1,3-alkadien-2-yl carboxylates as major products (84%)
with the formation of minor products such as 3-alken-2-
ones (4%) and propargylic carboxylates (3%) (Scheme 1,
eq 1);14a Zhang et al. also reported that 1,2-migration of
propargylic pivalates gave the (1Z,3E)-2-pivaloxy-1,3-
dienes (86%) together with the R,β-unsaturated enones
(8%) (Scheme 1, eq 2).14b As an alternative method for
the synthesis of 1,3-alkadienes, allenols are regarded as
effective starting materials.15 For the generation of 1,3-
alkadien-2-yl carboxylates, the isomerization of 2,3-alle-
nyl carboxylates catalyzed by Au(I) was recently re-
ported (Scheme 1, eq 3).16 Although the reaction
employed a set of a mild conditions and excellent yields,
there are several issues such as unsatisfactory stereose-
lectivity and the fact that substitutions can only be
introduced to the 4-position of 1,3-alkadien-2-yl carboxy-
lates. Thus, highly chemo- and stereoselective synthesis
of 1,3-alkadien-2-yl carboxylates from 2,3-allenyl car-
boxylates is highly desirable.
β-elimination to yield 1,3-butadien-2-yl carboxylates E-2.
Predominant E-stereoselectivity would be realized by con-
sideration of the more favorable cyclic intermediatetrans-4
since there is steric interaction betwþeen the R1 and the Rh
atom in cis-4 (Scheme 2). RhL3 may have a more
remarkable steric effect by coordinating with three ligands
and may be able to afford the product with a much better
stereoselectivity than AuLþ, which always coordinated
with a single ligand. Here, we wish to present the realiza-
tion of such a concept: Rh-catalyzed highly chemo- and
stereoselective preparation of 3,4-disubstituted-1,3-(E)-al-
kadien-2-yl acetates from readily available 2,3-allenyl
acetates, carbonate, or benzoates.
Scheme 2. Rationale for a Higher Stereoselectivity
At the beginning, 3-butyl-3,4-pentadien-2-yl acetate 1a
wasselectedasthe model substrate. A systematicstudy was
undertaken to optimize the reaction conditions (Table 1).
When 5 mol % of Rh(CO)(PPh3)2Cl were used as the
catalyst the reaction in toluene under reflux afforded
3-butyl-1,3(E)-pentadien-2-yl acetate E-2a in 74% yield with
14% recovery of starting material 1a. However, we were happy
to note that an excellent E/Z ratio (g98:2) was observed
(Table 1, entry 1), which encouraged us to conduct the
reaction with other rhodium complexes: [Rh(COD)Cl]2 did
not promote the reaction at all (Table 1, entry 2) while
Rh(PPh3)3Cl gave a better result with a higher yield
and stereoselectivity (Table 1, entry 3); although a ratio
of >99/1 was observed for the reaction in 1,4-dioxane, the
yield was quite low (Table 1, entry 4); other solvents such as
DMF or DMSO led to a low yield or no reaction at all
(Table 1, entries 5 and 6). The loading of Rh(PPh3)3Cl
couldbereducedfrom 5mol %to2mol%orevenaslow as
1 mol % with a higher concentration (Table 1, entries 8 and
9). To our delight, the addition of 3 mol % of PPh3 would
provide a little bit higher yield (81%) and a remarkable
excellent stereoselectivity (E/Z ratio >99/1) (Table 1, entry
10). We then defined the isomerization of 1 catalyzed by
1 mol % of Rh(PPh3)3Cl and 3 mol % of PPh3 in toluene
under reflux as conditions A. Additionally, Rh(PMe3)3Cl
gave a similer yield and slightly lower stereoselectivity as
compared to Rh(PPh3)3Cl (Table 1, entry 11).
Scheme 1
By analogy with the proposed plausible mechanism for
this 1,3-acetoxyl rearrangement of 2,3-allenyl carboxy-
lates,6a,7a,10b,16 we believe that Rhþ may undergo the
following process: coordination with allenyl carboxylates
would afford complex 3. Subsequent oxametala-
tion affords cyclic intermediate 4, which undergoes
(11) Caruana, P. A.; Frontier, A. J. Tetrahedron 2007, 63, 10646.
ꢀ
(12) Bray, C. V.-L.; Derien, S.; Dixneuf, P. H. Angew. Chem., Int. Ed.
2009, 48, 1439.
(13) Shibata, Y.; Noguchi, K.; Tanaka, K. J. Am. Chem. Soc. 2010,
132, 7896.
(14) (a) Wang, S.; Zhang, L. Org. Lett. 2006, 8, 4585. (b) Li, G.;
Zhang, G.; Zhang, L. J. Am. Chem. Soc. 2008, 130, 3740.
(15) For examples of stereoselectivivty synthesis of 1,3-alkadienes
from allenols, see: (a) Shen, Q.; Hammond, G. B. Org. Lett. 2001, 3,
2213. (b) Horvath, A.; Backvall, J.-E. J. Org. Chem. 2001, 66, 8120. (c)
Ma, S.; Wang, G. Tetrahedron Lett. 2002, 43, 5723. (d) Alcaide, B.;
Almendros, P.; Aragoncillo, C.; Redondo, M. C. Eur. J. Org. Chem.
2005, 98. (e) Deng, Y.; Jin, X.; Ma, S. J. Org. Chem. 2007, 72, 5901. (f)
Deng, Y.; Yu, Y.; Ma, S. J. Org. Chem. 2008, 73, 585. (g) Alcaide, B.;
Almendros, P.; Campo, T. M.; Carrascosa, R. Chem.;Asian J. 2008, 3,
1140. (h) Deng, Y.; Li, J.; Ma, S. Chem.;Eur. J. 2008, 14, 4263. (i) Deng,
ꢀ
€
Under this set of optimized reaction conditions, the
scope and the limitations for this reaction were explored.
When R2 is n-butyl and R1 is n-hexyl, i-butyl, or n-hexynyl,
the reaction provided the corresponding products
ꢀ
ꢀ
Y.; Jin, X.; Fu, C.; Ma, S. Org. Lett. 2009, 11, 2169. (j) Duran-Galvan,
M.; Connell, B. T. Eur. J. Org. Chem. 2010, 2445.
(16) Buzas, A. K.; Istrate, F. M.; Gagosz, F. Org. Lett. 2007, 9, 985.
Org. Lett., Vol. 13, No. 8, 2011
1921

Table 1. Effects of Catalyst, Solvent, Temperature, and Ligand
on the Rh-Catalyzed 1,3-Rerrangement Reaction of 1aa
Table 2. Substrate Scope of the Rh-Catalyzed 1,3-Rerrange-
ment Reaction of 2,3-Allenyl Carboxylates 1
solvent/temp
t
NMR yield
t
yield of E-2
(E/Z)b,c
entry
cat. (mol %)
(°C)b
(h) of 2ac (E/Z) 1ad
entry
R1/R2
conditionsa (h)
1
2
3
4
5
6
7
8
9e
Rh(CO)(PPh3)2Cl (5) toluene/120
8
6
4
6
6
6
6
6
74 (>98/2)
0
14
99
0
1
Me/n-C4H9 (1a)
n-C6H13/n-C4H9 (1b)
i-C4H9/n-C4H9 (1c)
n-hexynyl/n-C4H9 (1d)
Ph/n-C4H9 (1e)
Ph/n-C4H9 (1e)
Ph/n-C4H9 (1e)
Ph/Allyl (1f)
A
A
A
A
A
B
B
B
B
B
B
B
10
10
81 (2a) (>99/1)
89 (2b) (99/1)
[Rh(COD)Cl]2 (2.5)
Rh(PPh3)3Cl (5)
Rh(PPh3)3Cl (5)
Rh(PPh3)3Cl (5)
Rh(PPh3)3Cl (5)
Rh(PPh3)3Cl (5)
Rh(PPh3)3Cl (2)
Rh(PPh3)3Cl (1)
toluene/120
toluene/120
dioxane/120
DMF/120
2
84 (>98/2)
44 (>99/1)
0
3
4.5 88 (2c) (>99/1)
0
4
6
9
8
9
76 (2d) (97/3)
74 (2e) (92/8)
92 (2e) (>99/1)
88 (2e) (>99/1)
93
66
75
0
5
DMSO/120
toluene/80
toluene/120
2
6
7d
8
18 (99/1)
88 (98/2)
4.5 65 (2f) (99/1)
toluene/120 10 85 (98/2)
toluene/120 10 89 (>99/1)g
toluene/120 10 84 (>98/2)
0
9
Me/Ph (1g)
12
12
75 (2g) (98/2)
80 (2h) (96/4)
10e,f Rh(PPh3)3Cl (1)
0
10
11
12
p-MeOC6H4/Ph (1h)
Ph/H (1i)
11e,h Rh(PMe3)3Cl (1)
0
2.5 53 (2i) (>99/1)
1.5 49 (2j) (>99/1)
p-ClC6H4/H (1j)
a The reaction was conducted by using 0.25 mmol of 1a and
corresponding catalyst in 6 mL of solvent. b Temperature of the oil bath.
c NMR yield determined by using 1,3,5-trimethylbenzene as the internal
standard. d 1a recovered after the reaction. e 0.5 mmol of 1a in 3 mL of
solvent was used. f 3 mol % of PPh3 were added. g Isolated yield was
81%. h 3 mol % of PMe3 were added.
a Conditions A: The reaction was conducted by using 0.5 mmol of 1,
1 mol % of Rh(PPh3)3Cl, and 3 mol % of PPh3 in 3 mL of toluene under
120 °C. Conditions B: The reaction was conducted by using 1 mmol of 1
and 2 mol % of Rh(CO)(PPh3)2Cl in 6 mL of toluene under 120 °C.
b Isolated yields. c E/Z ratio was confirmed by 1H NMR analysis before
chromatography on silica gel. d 4 mmol of 1e and 1 mol % of Rh(CO)
(PPh3)2Cl were applied.
E-2b-2din76%-89% yieldsunder conditionsA (Table 2,
entries 2-4); the formation of the Z-isomer was not
serious. For 2-butyl-1-phenyl-2,3-butadien-1-yl acetate
1e, an unsatisfactory E/Z ratio (92/8) was observed, but
the Rh(CO)(PPh3)2Cl-catalyzed reaction showed an ex-
cellent stereoselectivity with 2 mol % of the catalyst
(Table 2, entries 5 and 6). We then defined 1 catalyzed by
2 mol % of Rh(CO)(PPh3)2Cl in toluene under reflux as
conditions B. Furthermore, we may easily conduct the
reaction of 0.9701 g (4 mmol) of 1e to provide 0.8535 g of
E-2e (88%), and the loading of catalyst may also be
reduced to 1 mol % (Table 2, entry 7). Under conditions
B, R1 may be alkyl and aryl groups; R2 may be allyl and
aryl groups generating the products in 65-80% yields with
E/Z ratios ranging from 96/4 to 99/1 (Table 2, entries
8-10). In addition, an excellent stereoselectivity (E/Z ratio
>99/1) with moderate yields was observed when the
reaction of the substrates with R1 being aryl groups and
R2 being H was performed (Table 2, entries 11 and 12).
Compared to the reaction catalyzed by Au(I) (condi-
tions: 0.5 mmol of 1, 1 mol % of Au(X-Phos)NTf2 in 2 mL
of CH2Cl2),16 Rh(I) provided a notably higher stereoselec-
tivity and higher yield with an R2 substituent (Scheme 2, 1a
and 1e), while Au(I) gave a higher yield but Rh(I) provided
a better stereoselectivity with R2 being H (Scheme 3, 1i). A
slightly higher E-2e/Z-2e ratio was observed after heating
the E/Z mixture (59:41) at 120 °C in toluene with 2 mol %
of Rh(CO)(PPh3)2Cl for 5 h, indicating that the thermo-
dynamic process of isomerizing the Z-isomer into the
E-isomer is not obvious (Scheme 3, eq 4).
Scheme 3
reaction to afford the corresponding products in good
yields with an excellent stereoselectivity (Scheme 4, eqs 5
and 6) and the structure including the configuration of
the CdC bond in E-2k was unambiguously determined
by single crystal X-ray diffraction analysis (Figure 1).17
(17) Crystal data for E-2k: C21H21NO4, MW = 351.39, monoclinic,
space group P2(1)/c, final R indices [I > 2σ(I)], R1 = 0.0608, wR2 =
0.1451, R indices (all data) R1 = 0.0694, wR2 = 0.1500, a = 7.7254(7)
o
˚
˚
˚
A, b = 8.2025(7) A, c = 29.583(3) A, R = 90°, β = 96.638(3) , γ = 90°,
V = 1862(3) A , T = 173 K, Z = 4, reflections collected/unique: 20256/
3
˚
3269 (Rint = 0.0364), number of observations [>2σ(I)] 2783, para-
meters: 243. Supplementary crystallographic data have been deposited
at the Cambridge Crystallographic Data Center (CCDC 805894).
Further studies showed that not only acetate but also
p-nitrobenzoate and carbonate could be used in the
1922
Org. Lett., Vol. 13, No. 8, 2011

Scheme 4
Scheme 5
Scheme 6. Synthetic Application of the 1,3(E)-Alkadien-2-ol
Acetates
5-acetoxy-3-butyl-4-phenylcyclohexa-1,3-diene- 1,2-dicar-
boxylate 7 was formed directly from a Diels-Alder reac-
tion between E-2e and DMAD in xylene (Scheme 6).20
In summary, we have achieved a highly stereoselective
Rh(I)-catalyzed 1,3-rearrangement of 2,3-allenyl carboxy-
lates affording 3,4-disubstituted-1,3(E)-butadien-2-yl car-
boxylates in good to excellent yields. The products readily
undergo the hydrolysis, iodination, and Diels-Alder reac-
tions to afford synthetically useful compounds. Due to the
easy availability of the starting materials, excellent stereo-
selectivity, and potential of the products, this reaction will
be useful in organic chemistry. Further studies in this area
are ongoing in our laboratory.
Figure 1. ORTEP drawing of E-2k.
However, due to the steric effect, remarkably more catalyst
was needed with a longer reaction time when 2-methyl-3-
phenylpenta-2-yl acetate 1m was used in the reaction
(Scheme 4, eq 7).
It is interesting to note that 2,4-disubstituted 2,3-allenyl
carboxylates can also be used in the reaction: 6-propyl-6,7-
pentadecadien-5-yl acetate 1n undergoes the Rh-catalyzed
1,3-rearrangement to afford 6-propyl-5(E),7(Z)-pentade-
cadien-7-yl acetate 5(E),7(Z)-2n and 6-propyl-5(E),7(E)-
pentadecadien-7-yl acetate 5(E),7(E)-2n in 69% yield with
an EZ/EE ratio of 68/32; a better EZ/EE ratio of 87.5/12.5
was observed, but a higher temperature in 1,3,5-trimethyl-
benzene was needed when 3-butyl-1-phenyl-1,2-nonadien-
4-yl acetate 1o was applied (Scheme 5).
Acknowledgment. We greatly acknowledge the finan-
cial support from the NSF of China (20732005) and the
National Basic Research Program of China (2011CB-
808700). S.M. is a Qiu Shi Adjunct Professor at Zhejiang
University. We thank Mr. Jiajia Chen in this group for
reproducing the results presented in entries 4 and 11 in
Table 2 and eq 6 in Scheme 4.
Treatment of 3-butyl-4-phenyl-1,3(E)-butadien-2-yl
acetate E-2e with LiOH H2O or NIS generated the hydro-
3
lysis product R,β-unsaturated enone E-5 and iodination
product 3-butyl-1-iodo-4-phenyl-3(E)-buten-2-one E-6 in
75% and 77% yields, respectivily.18,19 Moreover, dimethyl
Supporting Information Available. General procedure
and spectroscopic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
(18) Bestmann, H.-J.; Hartung, H. Chem. Ber. 1966, 99, 1198.
(19) Romo, J.; Rosenkranz, G.; Sondheimer, F. J. Am. Chem. Soc.
1954, 79, 5034.
(20) Ma, S.; Gu, Z.; Deng, Y. Chem. Commun. 2006, 94.
Org. Lett., Vol. 13, No. 8, 2011
1923