Dual platinum and pyrrolidine catalysis in the direct alkylation of allylic alcohols: Selective synthesis of monoallylation products

Article abstract of DOI:10.1002/anie.201311200

A dual platinum- and pyrrolidine-catalyzed direct allylic alkylation of allylic alcohols with various active methylene compounds to produce products with high monoallylation selectivity was developed. The use of pyrrolidine and acetic acid was essential, not only for preventing undesirable side reactions, but also for obtaining high monoallylation selectivity. Two cats are better than one: The combined use of platinum and pyrrolidine catalysts enabled the direct alkylation of allylic alcohols with reactive methylene compounds. Pyrrolidine was essential for obtaining high selectivity of the monoallylation products, which were produced without the use of excess nucleophiles. cod=1,5- cyclooctadiene, EWG=electron-withdrawing group.

Full text of DOI:10.1002/anie.201311200

Angewandte
Chemie
DOI: 10.1002/anie.201311200
Synthetic Methods
Dual Platinum and Pyrrolidine Catalysis in the Direct Alkylation of
Allylic Alcohols: Selective Synthesis of Monoallylation Products**
Ryozo Shibuya, Lu Lin, Yasuhito Nakahara, Kazushi Mashima,* and Takashi Ohshima*
Abstract: A dual platinum- and pyrrolidine-catalyzed direct
allylic alkylation of allylic alcohols with various active
methylene compounds to produce products with high mono-
allylation selectivity was developed. The use of pyrrolidine and
acetic acid was essential, not only for preventing undesirable
side reactions, but also for obtaining high monoallylation
selectivity.
carbonates, via a p-allylmetal intermediate, the so-called
Tsuji–Trost reaction.[2] In these reactions, however, undesir-
able overreactions may occur to give the diallylated[3] com-
pound 5, and thus making it difficult to separate the desired
monoallylation products from the reaction mixture. Although
several highly mono-selective stoichiometric[4] and catalytic[5]
reactions with some specific substrates have been reported,
further development of mono-selective allylation with
a broader substrate generality is in high demand.
Direct catalytic substitution of underivatized allylic alco-
hols with carbon and heteroatom (N, O, and S) nucleophiles,
which generated the desired allylated products together with
water as the sole co-product, is a more straightforward and
desirable method in terms of atom-economy and environ-
mental concerns (Scheme 1, 1!4).[6] Despite the poor leaving
ability of the hydroxyl group, several palladium-[7,3c] and
platinum-catalyzed[8] direct substitutions of allylic alcohols
without the use of an activator were recently developed by
various research groups, providing a highly atom-economical
synthetic method for linear allylation products. In the case of
carbon nucleophiles, however, Ozawa and Yoshifuji devel-
oped the only successful monoselective allylation by using
unique p-allyl palladium complexes bearing bulky diphosphi-
nidenecyclobutene (DPCB) ligands,[7a] but the substrate scope
was limited.
T
he allylation of activated methylene compounds, such as
1,3-dicarbonyl compounds, is a very important carbon–carbon
bond forming reaction because of the usefulness of the
corresponding allylation products for the synthesis of natural
and unnatural bioactive compounds.[1] As outlined in
Scheme 1 (1!2!4), these transformations are generally
We have previously demonstrated that a Pt catalyst
system of [Pt(cod)Cl2] and a ligand with a large bite angle,
2,2’-bis(diphenylphosphino)diphenyl ether (DPEphos) or 4,5-
bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos),
under microwave irradiation conditions promotes the direct
catalytic amination of allylic alcohols with arylamines, alkyl-
amines, and ammonia with high selectivity of the monoally-
lation products, where a sterically congested p-allyl Pt
intermediate created by the large-bite-angle ligand allows
for a preferential selective reaction with the substrate over the
monoallylation compound.[8a–c] Herein, we report a dual
platinum- and pyrrolidine-catalyzed allylic alkylation of
both aryl- and alkyl-substituted allylic alcohols (18 examples)
with various active methylene compounds (15 examples) and
high monoallylation selectivity (92:8 to > 99:1). The addition
of a catalytic amount of pyrrolidine and acetic acid is essential
not only for preventing undesirable side reactions, but also for
obtaining high monoallylation selectivity. Microwave irradi-
ation efficiently accelerated the reaction without a loss of
selectivity, making it possible to lower the reaction temper-
ature to 608C and reduce the reaction time to 5 h. Based on
several mechanistic investigations, we propose that pyrroli-
dine reacts with b-keto carbonyl compounds to generate
enamines, which act as sterically more congested, but more
reactive, nucleophiles.
Scheme 1. Conventional two-step synthesis (1!2!4) and direct syn-
thesis (1!4).
achieved using allylic halides with more than equimolar
amounts of base. Another synthetic method is the transition-
metal-catalyzed substitution reaction of activated allylic
alcohol derivatives, such as allylic halides, carboxylates, and
[*] R. Shibuya, L. Lin, Prof. Dr. T. Ohshima
Graduate School of Pharmaceutical Sciences, Kyushu University,
CREST, JST, Maidashi 3-1-1, Higashi-ku, Fukuoka, 812-8582 (Japan)
E-mail: ohshima@phar.kyushu-u.ac.jp
Homepage: http://green.phar.kyushu-u.ac.jp/
http://www.organomet.chem.es.osaka-u.ac.jp/
Y. Nakahara, Prof. Dr. K. Mashima
Department of Chemistry, Graduate School of Engineering Science,
Osaka University, CREST, JST, Toyonaka, Osaka, 560-8531 (Japan)
E-mail: mashima@chem.es.osaka-u.ac.jp
[**] This work was financially supported by Grant-in-Aid for Scientific
Research (B) (No. 24390004), Scientific Research on Innovative
Areas (No. 24106733) and Platform for Drug Discovery, Informatics,
and Structural Life Science from MEXT, and CREST from JST. R.S.
and Y.N. are the grateful recipient of a scholarship from JSPS.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201311200.
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We began our study by evaluating efficient catalytic
conditions using cinnamyl alcohol (1a) and ethyl acetoacetate
(3a) as representative substrates. The reaction was first
performed under the optimized reaction conditions for aryl-
amine (Pt-Xantphos (1 mol%), DMF, 808C, microwave
irradiation), but the corresponding linear monoallylation
product 4aa was obtained in only 37% yield, along with
several by-products, such as diallylation product 5aa, branch
monoallylation product 6aa, and unexpected ketone product
7aa; this last product could have been generated through
transesterification and subsequent Pt-catalyzed decarboxyla-
tive allylation[9] (Table 1, entry 1). Because this undesirable
transesterification could be catalyzed by hydrochloric acid
generated in situ, we examined the addition of bases. As
expected, the use of tertiary amines (entries 2 and 3), pyridine
(entry 4), DBU (entry 5), and inorganic bases (entries 6 and
7) suppressed the formation of ketone 7aa, but the yield of
the monoallylation product remained lower than 40%, and
a significant amount of the undesired diallylation product 5aa
and branch product 6aa were obtained. To our surprise, the
addition of pyrrolidine (50 mol%) not only completely
suppressed the formation of 7aa, but also greatly increased
the monoallylation selectivity and regioselectivity, affording
the desired product 4aa in 82% yield (entry 8). Nearly
identical results were obtained when proline was used instead
of pyrrolidine (entry 9), but significant differences between
them were observed when the amount of these additives was
reduced to 10 mol%; the effect of pyrrolidine was remarkably
diminished (entry 10), whereas the effect of proline was
maintained (entry 11).
Identical results were achieved with pyrrolidine and
proline when acetic acid (10 mol%) was added to the
pyrrolidine system (entry 12). Without pyrrolidine, acetic
acid exhibited no effect (entry 13). These results clearly
indicate that both pyrrolidine and acetic acid are essential for
the highly selective synthesis of 4aa, and that these additives
work independently. Under this dual co-catalyst system, the
reaction temperature was reduced to 608C (entry 14), and this
was selected as the best condition for further investigation.
Even when using just one equivalent of nucleophile 3a, the
reaction proceeded with high monoallylation selectivity
(entry 15). Furthermore, conventional oil-bath heating was
also applicable to this reaction (entry 16). Moreover, a gram-
scale reaction using only 0.5 mol% of the Pt catalyst also
proceeded smoothly (entry 17). Although proline and pyrro-
lidine have been used previously in reactions of the p-allyl
palladium complex with ketone or aldehyde,[10] they have not
been applied to reactions of activated methylene compounds,
except for one example[10e] in which the use of 10 equivalents
of nucleophile was required to obtain the monoallylation
products.
Table 1: Additive effects on Pt-catalyzed direct alkylation with a b-
ketoester.[a]
Entry Additive
Yield of
4aa/5aa[b] Yield of
6aa [%][b] 7aa [%][b]
Yield of
(x mol%)
4aa [%][b]
1
2
3
4
5
6
7
8
none
Et3N (50)
37
38
39
32
30
28
37
70:30
61:39
62:38
60:40
55:45
55:45
60:40
92:8
12
11
9
9
7
13
n.d.
n.d.
7
n.d.
8
n.d.
n.d.
n.d.
trace
n.d.
n.d.
With the optimized conditions in hand, we examined the
scope of the active methylene compounds 3 (Table 2). Direct
alkylation with 1,3-diketones (3b–3d) proceeded smoothly
with high yield and high selectivity (entries 1–3). Notably, the
reaction with cyclic 1,3-diketone 3e, which has a highly acidic
Cy2NEt (50)
pyridine (50)
DBU (50)
K2CO3 (50)
NaHCO3 (50)
pyrrolidine (50) 82
dl-proline (50) 83
pyrrolidine (10) 50
dl-proline (10) 84
pyrrolidine (10) 85
CH3COOH (10)
4
6
ꢀ
C H group, also proceeded smoothly (entry 4). Several
trace
trace
8
trace
trace
different b-keto esters were also applicable. When tert-butyl
3-oxobutanoate (3 f) was used, the reaction proceeded with-
out the decomposition of the tert-butyl ester (entry 5). The
reaction with ethyl 3-oxo-3-phenylpropanoate (3g) and its
derivatives with electron-donating (entries 7 and 8) and
electron-withdrawing (entry 9) substituents proceeded
smoothly to afford the corresponding monoallylation prod-
ucts 4ag–4aj in good yield. The a-substituted active methyl-
ene compounds 3k and 3l were also applicable to the present
catalysis (entries 10 and 11). The reaction with 2-hydrox-
yethyl 3-oxobutanoate (3m) proceeded with high yield and
high selectivity (entry 12). Furthermore, reactions with b-
ketoamides 3n and 3o (entries 13 and 14) and b-ketosulphone
3p (entry 15) also proceeded to afford the corresponding
monoallylation product 4an–4ap in good to high yields.
We next examined the direct alkylation of various allylic
alcohols 1 (Table 3). Direct alkylation of a series of cinnamyl
alcohol derivatives, 1b–1i, including the substrate with
a highly acid-sensitive THP group 1g, afforded the corre-
sponding monoallylation products 4ba–4ia with high yield
9
92:8
72:28
93:7
10
11
12
93:7
13
CH3COOH (10) 38
pyrrolidine (10) 89
CH3COOH (10) 85 [d]
65:35
95:5
13
trace
8
n.d.
14[c]
15[c,e] pyrrolidine (10) 83
CH3COOH (10)
91:9
95:5
93:7
trace
trace
trace
n.d.
n.d.
n.d.
16[f]
pyrrolidine (10) 87
CH3COOH (10)
17[g]
pyrrolidine (10) 82 [d]
CH3COOH (10)
[a] 2.0 mmol scale, 0.4 mL of DMF was used. [b] Determined by 1H NMR
analysis of the crude reaction mixture. [c] 2 mol% of [Pt(cod)Cl2] and
Xantphos was used. Reaction was performed at 608C and 5 h microwave
irradiation. [d] Yield of isolated product. [e] 1.0 equiv of 3a was used.
[f] 2 mol% of [Pt(cod)Cl2] and Xantphos was used. Reaction was
performed by conventional heating at 708C for 24 h. [g] 50 mmol (6.7 g
of 1a) scale using 0.5 mol% of [Pt(cod)Cl2] and Xantphos. DBU=1,8-
diazabicyclo[5.4.0]undec-7-ene, Cy=cyclohexyl, n.d.=not detected.
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Table 2: Pt-catalyzed direct alkylation of various active methyene com-
pounds 3.[a]
Table 3: Pt-catalyzed direct alkylation of various allylic alcohols 1.[a]
Entry
Nucleophile
3
Yield of
4/5[c]
4 [%][b]
Entry
Allylic alcohol
1
Yield of
4/5[c]
4 [%][b]
1
3b
3c
3d
3e
3 f
82[d]
91
93:7
>99:1
>99:1
92:8
1
2
3
4
5
6
7
1a (R=H)
1b (R=Me)
1c (R=Br)
1d (R=OMe)
1e (R=OH)
1 f (R=OTBS)
1g (R=OTHP)
85
81
87
82
83
80
84
95:5
92:8
94:6
93:7
97:3
95:5
96:4
2
3[e]
4
87
86
8
9
1h
1i
84
82
95:5
95:5
5[f]
83
95:5
6
7
8
9
3g (R=H)
88
91
88
82
97:3
97:3
97:3
93:7
10
11
12
1j
1k
1l
85[d]
86
95:5
93:7
97:3
3h (R=OMe)
3i (R=NMe2)
3j (R=CN)
91
10[g]
3k
85
–
13[e]
1m
80
97:3
11[g]
3l
87
–
14[e]
15[e]
16[e]
1n
1o
1p
73
97:3
96:4
94:6
78[f]
78
12
3m
3n
3o
3p
83
87
93
91
95:5
93:7
13
17[e,g]
18[e]
1q
1r
77
92:8
96:4
14
97:3
86[h]
15[e]
>99:1
[a] 2.0 mmol scale, 0.4 mL of DMF was used. [b] Yield of isolated
product. [c] Determined by 1H NMR analysis of the crude reaction
mixture. [d] Linear product 4aa was obtained. [e] 50 mol% each of
pyrrolidine and acetic acid was used. [f] A 2.4:1 E/Z mixture was
obtained. [g] Reaction was performed at 808C. [h] A 2.8:1 E/Z mixture
was obtained. TBS=tert-butylsilyl, THP=tetrahydropyranyl.
[a] 2.0 mmol scale, 0.4 mL of DMF was used. [b] Yield of isolated
product. [c] Determined by 1H NMR analysis of the crude reaction
mixture. [d] Product was a 1:1 mixture of keto and enol forms.
[e] Reaction was performed at 808C. [f] 20 mol% of pyrrolidine was used.
[g] Reaction was performed at 708C.
and high selectivity (entries 2–9). Notably, the branched
allylic alcohol 1j was efficiently converted into the linear
monoallylation product 4aa with the same 4/5 ratio (entry 10)
as in the reaction of 1a (entry 1). The reaction of allyl alcohol
(1k) also proceeded (entry 11) similarly to the reactions of
cinnamyl alcohol derivatives 1a–1g.
Furthermore, highly unstable thiophene-substituted
allylic alcohol 1l was successfully converted into the corre-
sponding monoallylation product 4la without decomposition
(entry 12). Alkyl-substituted allylic alcohols have the poten-
tial risk of b-hydride elimination. For reactions with 1m–1r,
we obtained good yield and high selectivity by using 50 mol%
of both pyrrolidine and acetic acid to enhance the reactivity
by increasing the enamine intermediates. Even when using
the less-reactive b-substituted allylic alcohols 1q and 1r, good
yields were obtained under these conditions. To the best of
our knowledge, this is the first report of such broad substrate
generality (both aryl- and alkyl-substituted allylic alcohols
and several active methylene compounds) and such high
monoallylation selectivity in this type of direct reaction.
Development of the present highly monoselective allyla-
tion allowed for a one-pot sequential allylation of two
different allylic alcohols, 1a and 1k, with active methylene
compound 3a to provide dissymmetric diallylated compound
8 containing different allyl moieties in 78% yield (Scheme 2).
Thus, this new method could provide increased flexibility for
designing a synthetic route.
To gain insight into the effects of pyrrolidine and acetic
acid, we performed the following controlled reactions. Direct
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tigation of the scope of the syntheses of highly functionalized
bioactive compounds, and application to enantioselective
variants are ongoing in our group.
Experimental Section
Cinnamyl alcohol 1a (268 mg, 2.0 mmol, 1.0 equiv), ethyl acetoace-
tate 3a (390 mg, 3.0 mmol, 1.5 equiv), acetic acid (12 mg, 0.2 mmol,
10 mol%) and pyrrolidine (14 mg, 0.2 mmol, 10 mol%) were added
to a solution of [Pt(cod)Cl2] (15.0 mg, 0.04 mmol, 2 mol%) and
Xantphos (23.2 mg, 0.04 mmol, 2 mol%) in DMF (0.4 mL). The
reaction was heated at 608C using a CEM Discover single-mode
microwave reactor (standard mode) for 5 h. The selectivity was
determined by the 1H NMR spectrum of the crude reaction mixture.
The mixture was purified using n-hexane/ethyl acetate (20:1–3:1) as
eluent to give 4aa as a colorless oil (419.4 mg, 85% yield).
Scheme 2. One-pot synthesis of diallylated compound 8 with two
different allyl moieties from active methylene compound 3a.
alkylation of 1a with enamine 9a produced results identical to
those of the reaction of b-keto ester 3a in the presence of
catalytic amounts of pyrrolidine and acetic acid.[11] Moreover,
direct alkylation with the mixture of 3a and a catalytic
amount of 9a and acetic acid also produced almost the same
result. On the other hand, the use of N-methylpyrrolidine
instead of pyrrolidine gave a mixture of 4aa, 5aa, and 6aa.
These results strongly suggest that pyrrolidine acts as an
organocatalyst to form the enamine intermediate 9, which
then reacts with the p-allyl platinum complex generated
in situ (Scheme 3). The above-mentioned byproducts were
Received: December 25, 2013
Revised: February 6, 2014
Published online: March 6, 2014
Keywords: allylic alkylation · homogeneous catalysis ·
.
organocatalysis · platinum · pyrrolidine
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not formed due to the steric effects of the bulkier enamine
nucleophile 9 (compared with b-keto ester 3). In the absence
of acetic acid, both the yield of the product and the
monoallylation selectivity were highly dependant on the
amount of pyrrolidine present. In addition, when enamine 9
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for easy synthesis of diallylation compounds with two differ-
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