Palladium-catalyzed divergent reactions of α-diazocarbonyl compounds with allylic esters: Construction of quaternary carbon centers

Article abstract of DOI:10.1002/anie.201106619

Take two: α-Diazocarbonyl compounds display a diverse pattern of reactivity upon palladium-catalyzed reaction with esters. Esters bearing an alkynyl group on the carbonyl carbon atom lead to two different C-C bonds at the same carbon atom in a single operation through decarboxylation and migratory insertion, whereas aromatic and benzylic acid derivatives afford aromatic and benzylic esters bearing an O-substituted quaternary carbon center. Copyright

Full text of DOI:10.1002/anie.201106619

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DOI: 10.1002/anie.201106619
Palladium Catalysis
Palladium-Catalyzed Divergent Reactions of a-Diazocarbonyl
Compounds with Allylic Esters: Construction of Quaternary Carbon
Centers**
Zi-Sheng Chen, Xin-Hua Duan, Ping-Xin Zhou, Shaukat Ali, Jian-Yi Luo, and
Yong-Min Liang*
The efficient construction of quaternary carbon centers
remains an important challenge for organic chemists.[1]
Quaternary carbon centers are prevalent throughout most
classes of naturally occurring, biologically active compounds
and pharmaceutical agents.[2] Synthetic studies on these
special units have been extensively carried out in recent
years.[1] Although many methods are available for the syn-
thesis of quaternary carbon centers,[1] it is highly desirable to
develop alternative methods that could be advantageous in
terms of functional-group tolerance, operational simplicity,
and the use of readily available and stable starting materials.
Scheme 1. Palladium-catalyzed cross-coupling reactions of diazo com-
pounds.
Palladium is a versatile transition metal with intriguing
reactivity with various functional groups.[3] In particular,
palladium-catalyzed cross-couplings are nowadays recog-
nized as one of the most powerful and reliable methods for
new methods for the cross-coupling of diazo compounds with
organopalladium species generated from other precursors will
facilitate the synthesis of more valuable compounds.
the formation of C C bonds.[4] Recently, palladium-catalyzed
À
cross-coupling with diazo compounds as nucleophilic cou-
pling partners has been introduced as a new method for the
Palladium-catalyzed decarboxylative coupling is a new
strategy for the generation of organopalladium intermediates
that utilizes readily available carboxylic acids[14,15] or esters[16]
and produces CO2 as its only byproduct. On the basis of our
study of palladium-catalyzed migratory insertion reactions,[12]
decarboxylative couplings,[12,17,18] and others,[19] it occurred to
us that the transmetalation step could be circumvented by
decarboxylative metalation of carboxylic acid derivatives
(Scheme 2), where the palladium carbene species (E)[9] might
be obtained through a palladium allyl acetylide (D).[19]
Herein, we present the ready decarboxylation of propiolic
acid derivative 1a in the presence of catalytic palladium, and
the coupling of palladium allyl acetylide (D) with methyl-
phenyldiazoacetate (2a), in which an all-carbon quaternary
center is generated to afford 1,5-enyne 3a (Scheme 2). Five-
membered ring frameworks, which widely occur in natural
products and biologically active molecules, can be efficiently
constructed by the transition-metal-catalyzed cycloisomeriza-
tion of these products.[20]
To put the above hypothesis to the test, allylic alkynoate
1a was treated with a-diazocarbonyl compound 2a in the
presence of [Pd(PPh3)4]. The desired product, 1,5-enyne 3a
was obtained in 43% yield (Table 1, entry 1). Encouraged by
this initial result, we proceeded to optimize the reaction
conditions (for details, see the Supporting Information).
Changing the substrate ratio of 1a/2a from 1.0:1.2 to 1.0:2.0
resulted in a lower yield (entry 2). Under these conditions, we
detected the formation of a 1,4-enyne byproduct, which was
attributed to the palladium-catalyzed, decarboxylative cou-
pling of allylic alkynoate 1a.[19] So, the yield of the expected
[5]
À
formation of C C bonds. The characteristic steps of the
mechanism that differentiate it from traditional cross-cou-
plings are shown in Scheme 1. Generation of a palladium
carbene species (A), followed by the migratory insertion of an
aryl,[6] benzyl,[7] vinyl,[8] allyl,[9] acyl,[10] alkynyl,[11] or allenyl[12]
group, gives rise to the alkyl-palladium complex (B). In most
cases, this complex undergoes b-hydride elimination to afford
an olefin[5] (Scheme 1, pathwaya). Alternatively, intermediate
B can undergo transmetalation followed by reductive elim-
À
ination to form two different C C bonds at the same carbon
atom in a single reaction[6g,13](Scheme 1, pathwayb). How-
ever, this transmetalation step either involves the use of
reagents that are toxic (tributylphenyltin)[13] and/or reagents
that necessarily produce stoichiometric quantities of
unwanted byproducts.[6g,13] Thus, the development of effective
[*] Dr. Z. S. Chen, P. X. Zhou, S. Ali, J. Y. Luo, Prof. Dr. Y. M. Liang
State Key Laboratory of Applied Organic Chemistry
Lanzhou University, Lanzhou 730000 (China)
E-mail: liangym@lzu.edu.cn
Dr. X. H. Duan
Department of Applied Chemistry, Faculty of Science
Xi’an Jiaotong University, Xi’an 710049 (China)
[**] We thank the National Science Foundation (NSF 21072080), the
National Basic Research Program of China (973 Program)
2010CB833203, and the “111” program of the Ministry of Education
for financial support. Xin-Hua Duan thanks the Fundamental
Research Funds for the Central Universities for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106619.
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Scheme 2. Construction of quaternary carbon centers by decarboxyla-
tion and migratory insertion.
Table 1: Screening conditions for the palladium-catalyzed reaction of 1a
and 2a.[a]
Entry
Pd catalysis [b]
Base [b]
t [h]
Yield [%][c]
1
[Pd(PPh3)4] (5)
[Pd(PPh3)4] (5)
[Pd(PPh3)4] (5)
[Pd(PPh3)4] (5)
[Pd(PPh3)4] (5)
–
–
–
–
4.5
3.0
4.5
5.5
4.5
43
41
55
52
62
2[d]
3[e]
4[f]
5[e]
K2CO3 (300)
[a] Reaction conditions: 1a (0.1m, 1.0 equiv), 2a (0.12m, 1.2 equiv) in
toluene, at 908C. [b] Number given in parenthesis is mol% used.
[c] Yield of isolated product. [d] 1a/2a=1.0:2.0. [e] 1a/2a=2.0:1.0.
[f] 1a/2a=3.0:1.0.
product was improved by simply changing the substrate ratio
of 1a/2a from 1.0:1.2 to 2.0:1.0 (entry 3). Finally, the yield
could be further improved by adding K2CO3 (entry 5), which
may suppress the formation of the protonation byproduct.[16]
After having optimized the conditions, the scope and
generality of the reaction was studied. Examples of the
reactions between allylic alkynoates and a-diazocarbonyl
compounds are shown in Scheme 3. Reaction of allylic
alkynoates 1 and a-diazocarbonyl compounds 2 in the
presence of a palladium catalyst affords the corresponding
products 3 in moderate yields. The reaction was not signifi-
cantly affected by the substituents on the aromatic ring of the
allylic alkynoates. Both electron-donating and electron-with-
drawing groups were tolerated under the reaction conditions
(3b–l, Scheme 3), although the reaction rate decreased in the
case of electron-withdrawing groups. The reaction was also
not noticeably affected by the position of the substituents on
the aromatic ring of the allylic alkynoates. However, sub-
stituents on the aromatic ring of the a-diazocarbonyl com-
pounds did significantly affect the reaction. Electron-with-
Scheme 3. Palladium-catalyzed reaction of allylic alkynoates 1 and a-
diazocarbonyl compounds 2. Reaction conditions: 1 (0.2m, 2.0 equiv),
2 (0.1m, 1.0 equiv), K2CO3 (0.3m, 3.0 equiv), and [Pd(PPh3)4]
(5 mol%) in toluene at 908C. All yields refer to product isolated after
chromatography on silica gel.
drawing groups were tolerated under the same reaction
conditions (3n and 3o, Scheme 3), but the expected product
(3m) could not be obtained from a-diazocarbonyl compound
2b, which bears an electron-donating group (Scheme 3). The
reaction also did not give the corresponding product (3p)
from 1,1-disubstituted allylic alkynoate 1m (Scheme 3). This
result indicates that steric hindrance significantly inhibits the
reaction.
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Inspired by these results, we wished to further investigate
the scope of the reaction by using aromatic and benzylic acid
derivatives, which could be synthesized from commercially
available aromatic and benzylic acids through esterfication.
However, the corresponding product 2,2-diphenylpent-4-
enoic acid methyl ester could not be obtained by decarbox-
ylation under the above optimized conditions (Table 2,
entry 1). Surprisingly, a product with an O-substituted qua-
Table 2: Screening conditions for the palladium-catalyzed reaction of 4a
with 2a.[a]
Entry Catalyst [b]
Ligand [b]
–
Solvent t [h] Yield [%][c]
1
[Pd(PPh3)4] (5)/K2CO3
toluene 4.0
toluene 2.0
CH3CN 0.5
CH3CN 0.5
CH3CN 0.5
CH3CN 0.5
17
53
66
70
40
88
(300)
2
[Pd2(dba)3] (2.5)
P(2-furyl)3
(10)
P(2-furyl)3
(10)
P(2-furyl)3
(10)
P(2-furyl)3
(10)
3
[Pd2(dba)3] (2.5)
[Pd2(dba)3] (2.5)
[Pd2(dba)3] (2.5)
[Pd2(dba)3] (2.5)
4[d]
5[e]
6[d]
P(2-furyl)3
(20)
[a] Reaction conditions: 4a (0.1m, 1.0 equiv), 2a (0.12m, 1.2 equiv), Pd
(5 mol%), and the ligand (10 mol%) in solvent, at 908C. [b] Number
given in parenthesis is mol % used. [c] Yield of isolated product.
[d] 4a/2a=1.0:1.4. [e] 1a/2a=1.0:1.8. dba=dibenzylideneacetone.
ternary carbon center was obtained in 17% yield, a result that
we attribute to direct reductive elimination instead of
decarboxylation. Encouraged by this result, we further
optimized these reaction conditions (for details, see the
Supporting Information). To our delight, it was found that
using [Pd2(dba)3]/P(2-furyl)3 (dba = dibenzylideneacetone)
could significantly improve the yield (entry 2); so, we
proceeded to screen other reaction parameters with this
complex. The reaction was found to be more efficient in
CH3CN (entry 3). When the ratio of substrates was examined,
it was found that a 4a/2a ratio of 1.0:1.4 afforded a higher
yield of 5a (entry 4). Finally, the yield could be further
improved by increasing the loading of P(2-furyl)3 to 20%
(entry 6).
Scheme 4. Palladium-catalyzed reaction of aromatic acid derivatives 4
with a-diazocarbonyl compounds 2. Reaction conditions: 4 (0.1m,
1.0 equiv), 2 (0.14m, 1.4 equiv), P(2-furyl)3 (20 mol%), and [Pd2(dba)3]
(2.5 mol%) in CH3CN at 908C. All yields refer to isolated product after
chromatography on silica gel. dba=dibenzylideneacetone.
To demonstrate the generality of this reaction, a series of
aromatic and benzylic acid derivatives 4 were reacted with a-
diazocarbonyl compounds 2 under the optimized reaction
conditions. As presented in Scheme 4, this reaction afforded
the corresponding products in moderate to excellent yields.
Both electron-rich and electron-poor aromatic acid deriva-
tives can be used. The functional group tolerance of the
process is remarkable: chloro, fluoro, and nitro groups on the
aromatic ring of aromatic acid derivatives had no effect on the
reaction (5e–g, Scheme 4). However, a bromo group on the
aromatic ring of a-diazocarbonyl compounds was not toler-
ated (5m; Scheme 4). We were delighted to find that allyl
cinnamate 4m and allyl 2-phenylacetate 4n were also suitable
substrates for the reaction ([Pd2(dba)3] (2.5%), P(2-furyl)3
(20%), CH3CN (2 mL), 90 8C), affording 5r and 5s in 86%
and 57% yields, respectively [Eqs (2) and (3)]. However,
decarboxylation did not occur when using allyl 2-oxo-2-
phenylacetate 4l [Eq (1)].[21] Finally, the molecular structure
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of 5n was unambiguously confirmed through X-ray crystal-
lography.[22]
Although the precise mechanism of our reaction remains
unclear at this moment, we believe that the reaction may
intermediate B might also be converted into product 4
through reductive elimination.[19] In route II, p-allyl palla-
dium carbene intermediate G may be produced by the
reaction of A with a-diazocarbonyl compound 2.[9] Isomer-
ization leads to s-allyl palladium carbene intermediate H,
from which migratory insertion of the allyl group occurs to
afford the alkyl palladium intermediate I.[9] Decarboxylation
of I gives palladium intermediate J,[19,25] which undergoes
reductive elimination to afford product 3 and regenerate the
Pd0 catalyst. Product 5, with an O-substituted quaternary
carbon center, could also be obtained from I if it directly
undergoes reductive elimination.
In conclusion, we have developed a new method for the
À
formation of two different C C bonds on the same carbon
atom in a single reaction involving a palladium-catalyzed
decarboxylation and migratory insertion process. This reac-
tion provides a novel method for the generation of all-carbon
quaternary centers. Furthermore, the palladium-catalyzed
coupling of aromatic and benzylic acid derivatives with
a-diazocarbonyl compounds to prepare aromatic and ben-
zylic ester derivatives with an O-substituted quaternary
carbon center in moderate to excellent yields was also
developed. This new approach is complementary to prior
methods for the construction of quaternary carbon centers.
The reaction involves the use of stable, readily available
starting materials and is operationally simple.
involve two different routes. As shown in Scheme 5, the
palladium catalyst initially promotes the oxidative addition of
allyl esters 1 to generate p-allyl palladium intermediate A.[23]
In route I, p-allyl palladium carbene intermediate C, gener-
ated from the interaction of palladium intermediate
B
[19](obtained by decarboxylation of p-allyl palladium inter-
mediate A) with a-diazocarbonyl compound 2, isomerizes to
s-allyl palladium carbene intermediate D.[9] Then, migratory
insertion of the allyl[9,24] group of D most likely occurs to
afford the alkyl palladium intermediate E, although the alkyl
palladium intermediate F may also be obtained from migra-
tory insertion of the alkynyl[11] group of D. Subsequent
reductive elimination of intermediate E or F then gives
product 3, with an all-carbon quaternary center, and regen-
erates the Pd0 catalyst. It should be noted that palladium
Experimental Section
General procedure for the preparation of 5: Allyl ester 4 (0.20 mmol,
1.0 equiv), diazoester (2a–f) (0.28 mmol, 1.4 equiv), P(2-furyl)3
(0.04 mmol, 20 mol%), and [Pd2(dba)3] (0.005 mmol, 2.5 mol%)
were added to an oven-dried Schlenk tube under an argon atmos-
phere. Anhydrous CH3CN (2.0 mL) was then introduced by syringe
and the mixture was stirred at 908C. When the reaction was
Scheme 5. Proposed reaction mechanism.
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considered complete (determined by TLC analysis), the reaction
mixture was cooled to room temperature. The solvent was removed
under reduced pressure and the corresponding product (5) was
purified by chromatography on silica gel using a mixture of 40:1
petroleum ether/ethyl acetate as eluent.
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Received: September 18, 2011
Published online: December 23, 2011
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Keywords: cross-coupling · decarboxylation ·
homogeneous catalysis · palladium · quaternary carbon
.
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