Palladium/Bronsted acid-catalyzed α-allylation of aldehydes with allylic alcohols

Article abstract of DOI:10.1002/adsc.201100260

A simple, highly efficient, and readily scalable direct α-allylation of aldehydes with allylic alcohols that is co-catalyzed by palladium and a Bronsted acid has been developed. Copyright

Full text of DOI:10.1002/adsc.201100260

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DOI: 10.1002/adsc.201100260
Palladium/Brønsted Acid-Catalyzed a-Allylation of Aldehydes
with Allylic Alcohols
Gaoxi Jianga and Benjamin Lista,*
a
Max-Planck-Institut fꢀr Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mꢀlheim an der Ruhr, Germany
Fax : (+49)-208-306-2999; e-mail: list@mpi-muelheim.mpg.de
Received: April 6, 2011; Published online: July 7, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100260.
Abstract: A simple, highly efficient, and readily
scalable direct a-allylation of aldehydes with allylic
alcohols that is co-catalyzed by palladium and a
Brønsted acid has been developed.
not require any stoichiometric activating reagents and
is co-catalyzed by Pd(0) and a simple Brønsted acid.
The desired products are furnished smoothly in near
quantitative yields on a molar scale under mild reac-
tion conditions.
Keywords: aldehydes; allylation; allylic substitu-
tion; Brønsted acids; homogeneous catalysis; palla-
dium
Recently, we have developed a highly enantioselec-
tive a-allylation of aldehydes with N-benzhydrylallyl-
amine, co-catalyzed by Pd(0) and the chiral Brønsted
acid TRIP.[5] In continuation of these studies, we
became interested in further simplifying and poten-
tially improving our protocol by directly utilizing al-
lylic alcohols rather than the amine reagent, which
Transition metal-catalyzed Tsuji–Trost-type a-allyla- has to be synthesized in a separate step. We were en-
tions of carbonyl compounds are versatile and widely couraged by previous studies by Yamamoto et al.,[6]
[1]
À
used methodologies for constructing C C bonds.
A
and Kobayashi et al.,[7] who have illustrated the gener-
large range of nucleophiles including ketones, malo- al suitability of combining Brønsted acid and Pd catal-
nates, and aldehydes has been used previously, al- ysis for the activation of allylic alcohols in Tsuji–
though typically in an activated form such as an eno- Trost-type processes.
late, enamine, or enol silane.[2] Similarly, the electro-
Indeed, initial experiments showed that while
(PPh3)4 alone (5 mol%) proved to be rather inef-
phile in these reactions is quite commonly an activat- PdACHTUNGTRENNUNG
ed allylic species, for example, an acetate, halide, fective in mediating the desired direct allylation of
phosphate, or carbonate. The more practical and hydratopic aldehyde (1a) with allylic alcohol (2a)
atom-economic direct a-allylation of an unactivated (Table 1, entry 1), the reaction could be accelerated in
aldehyde with an allylic alcohol itself [Eq. (1)], gener- the presence of a Brønsted acid. Pd
ACHTUNGTREN(UNNG PPh3)4 combined
ating only water as by-product has rarely been ap- with trifluoroacetic acid or benzoic acid gave product
proached.[3–4]
3a in 46% and 70% NMR conversions, respectively
(entries 2 and 3). A brief examination of solvents re-
vealed that toluene is optimal, providing aldehyde 3a
in almost quantitative yield (entries 4–6). Remarka-
bly, reducing the Pd loading to 2.5 mol% and that of
This may be due to (i) the instability of aldehyde the acid co-catalyst to 3.75 mol%, still gave product
enols, (ii) their tendency to undergo aldolizations, and 3a 98% isolated yield (entry 7). As expected, benzoic
(iii) the poor leaving capability of the hydroxy group. acid alone is entirely ineffective in catalyzing the re-
A breakthrough in this area was achieved when action (entry 8).
Tamaru and co-workers found that (over)stoichiomet-
With these optimized reaction conditions, we
ric amounts of Et3B, NEt3, and LiCl in combination became interested in determining the substrate scope
with 5–10 mol% of a Pd catalyst readily mediate a of our process. Indeed, the reaction is quite general
“strikingly simple direct a-allylation of aldehydes with and tolerates aromatic aldehydes bearing electron-do-
allyl alcohols: remarkable advance in the Tsuji–Trost nating and electron-withdrawing groups, aliphatic al-
reaction”.[3a] Herein, we describe a method that does dehydes, and an a-unbranched aldehyde (Table 2).
Adv. Synth. Catal. 2011, 353, 1667 – 1670
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1667

Gaoxi Jiang and Benjamin List
COMMUNICATIONS
Table 1. Identification of suitable reaction conditions.[a]
Table 2. Direct a-allylation of aldehydes with allylic alco-
hol.[a]
Entry R1
R2
Product Yield [%]
Entry Pd (mol%)
Acid (mol%) Solvent Conv.
[%][b]
1
C6H5
Me
Me
Me
Me
Me
Me
Me
Me
3a
3b
3c
3d
3e
3f
3g
3h
3i
98
97
98
97
99
99
98
97
99
94
60
70
81[d]
93
40
1
2
3
4
Pd
Pd
Pd
Pd
Pd
Pd
Pd
G
–
MTBE 20
2
3
4
5
6
7
4-MeO-C6H4
4-Me-C6H4
3-Me-C6H4
4-Ph-C6H4
4-Cl-C6H4
2-F-C6H4
61
76
5
6
7[d]
PhCO2H
(3.75)
toluene >99 (98)[c]
8
3-F-C6H4
(2.5)
–
9
6-MeO-2-naphthyl Me
C6H5
C6H5
4-t-Bu-C6H4CH2
n-C2H5
C6H5
8
PhCO2H (10) toluene n.r.[e]
10[b]
11[c]
12[c]
13[c]
14[e]
15[e,g]
Et
3j
[a]
c-C5H9 3k
Reaction conditions: 1a (0.2 mmol), 2a (0.8 mmol), 4ꢂ
Me
Me
H
3l
MS (50 mg), solvent (1.0 mL).
Determined by GC-MS or H NMR.
Yield of isolated product in parenthesis.
0.4 mmol of 2a used.
[b]
[c]
[d]
[e]
1
3m
3n[f]
3n’[h]
C6H5
H
[a]
No reaction.
Reaction conditions:
1
(0.2 mmol), 2a (0.4 mmol),
Pd(PPh3)4 (2.5 mol%), PhCO2H (3.75 mol%), 4ꢂ MS
AHCTUNGTRENNUNG
(50 mg), toluene (1.0 mL), 608C, 12 h. All yields refer to
isolated pure products.
Treating different aromatic aldehydes 1a–1j with allyl-
ic alcohol (2a, 2 equiv.) in the presence of Pd(PPh3)4
[b]
[c]
[d]
[e]
[f]
Pd
Pd
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(2.5–5.0 mol%), PhCO2H (3.75–10 mol%), and 4ꢂ
molecular sieves at 608C in toluene for 12 h facilely
furnished the corresponding a-allylated aldehydes 3a–
3j in 94–99% yields (entries 1–10). With the sterically
more demanding substrate 1k and with aliphatic alde-
hydes 1l and 1m, a slightly higher catalyst loadings
and temperature gaves the corresponding products
3k–3m in 60–81%. Interestingly, the reaction of a-un-
branched aldehyde 1n with 3.0 equiv. of 2a smoothly
afforded bis-allylation product 3n in 93% yield. Sub-
jecting aldehyde 1n to only 0.5 equiv. of alcohol 2a
gave the corresponding mono-allylation product 3n’ in
40% yield.
GC yield.
Reaction performed on a 0.4 mmol scale.
Bis-allylation product 3n (R1 =C6H5, R2 =CH2CH=CH2).
0.1 mmol of 2a was slowly added by syringe pump over
11 h; isolated yield based on 2a.
[g]
Mono-allylation product 3n’ (R1 =C6H5, R2 =H).
[h]
Importantly, the reaction is not restricted to the use Scheme 1. 1 mol-scale direct allylation.
of allylic alcohol itself (2a), but can be extended to a
range of substituted allylic alcohols (Table 3). Accord-
ingly the allylation of three different aldehydes (1a,
In summary, we have developed a simple, mild,
1e, and 1f) was studied with three substituted allylic highly efficient, and practical direct a-allylation of al-
alcohols (2b, 2c, and 2d) and shown to readily furnish dehydes with allylic alcohols that is co-catalyzed by
the desired products 3o–3w in excellent 92 to >99% Pd(0) and a Brønsted acid. The protocol can be easily
yields.
Our direct a-allylation reaction can be easily scaled
extended to a molar scale in almost quantitative yield.
up to a one mol scale without appreciable decrease in
product yields. Accordingly, treating aldehyde 1a
(134 g, 1.0 mol) with 2.0 equiv. of allylic alcohol (2a)
Experimental Section
in the presence of 2.5 mol% of PdACHTNUTRGNE(UNG PPh3)4, 5.0 mol%
General Procedure
of PhCO2H, and 500 g of 4ꢂ molecular sieves in 2 L
of toluene at 608C for 12 h, smoothly provided
161.3 g of pure product 3a in 93% yield after distilla-
tion (Scheme 1).
To a flame-dried Schlenck tube charged with 4ꢂ molecular
sieves (50 mg/0.1 mmol) and magnetic stir bar, was added al-
dehyde 1 (0.2–12 mmol), allylic alcohol 2 (0.21–12.2 mmol),
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Adv. Synth. Catal. 2011, 353, 1667 – 1670

Palladium/Brønsted Acid-Catalyzed a-Allylation of Aldehydes with Allylic Alcohols
Table 3. Direct allylation of aldehydes with diverse allylic alcohols.[a]
Entry
Substrates
Product
R1 =Ph, R2 =H
Yield [%]
1[b]
2[c]
3[c,d]
4
1a/2b
1a/2c
1a/2d
1e/2b
1e/2c
1e/2d
1f/2b
1f/2c
1f/2d
3o
3p
3q
3r
3s
3t
3u
3v
3w
>99
95
92
97
>99
96
96
>99
98
R1 =H, R2 =Ph
R1 =H, R2 =Me
R1 =Ph, R2 =H
R1 =H, R2 =Ph
R1 =H, R2 =Me
R1 =Ph, R2 =H
R1 =H, R2 =Ph
R1 =H, R2 =Me
5[c]
6[c,d]
7
8[c]
9[c,d]
[a]
Reaction conditions: 1 (0.2 mmol), 2 (0.21 mmol), PdACTHNUGTRENUNG(PPh3)4 (5.0 mol%), PhCO2H (10 mol%), 4ꢂ MS (50 mg), toluene
(1.0 mL), 608C, 12 h. All yields refer to isolated pure products.
Reaction performed on a 12 mmol scale.
At 808C.
[b]
[c]
[d]
With 2.0 equiv. of 2d.
Pd
N
1H); 13C NMR (125 MHz, CDCl3): d=18.8, 40.5, 53.6, 53.7,
55.3, 105.4, 105.7, 118.6, 119.0, 119.2, 125.5, 125.7, 126.1,
126.3, 127.2, 127.4, 128.9, 129.5, 133.3, 133.6, 134.3, 158.0,
202.0; HR-MS (EI): m/z=254.130914, calcd. for [C17H18O2]
([M]+): 254.130677.
in 1.0–6.0 mL of dried toluene. The resulting suspension was
stirred at the specified temperature under argon for 12 h.
Upon completion of the reaction, the mixture was concen-
trated under vacuum and purified by flash chromatography
to afford pure products 3. For the one mol scale reaction,
product 3a was collected by distillation at 62–648C/1.2ꢃ
10À1 mbar.
2-Cyclopentyl-2-phenylpent-4-enal
(3k):
1H NMR
(500 MHz, CDCl3): d=1.11–1.55 (m, 7H), 1.78–1.82 (m,
2H), 2.59–2.67 (m, 2H), 4.98–5.04 (m, 2H), 5.51–5.56 (m,
1H), 7.16–7.40 (m, 5H), 9.78 (s, 1H); 13C NMR (125 MHz,
CDCl3): d=25.2, 25.4, 27.3, 27.9, 39.0, 42.7, 59.7, 118.2,
127.1, 128.4, 128.6, 133.4, 138.4, 204.2; HR-MS (EI): m/z=
228.151389, calcd. for [C16H20O] ([M]+): 228.151412.
2-Allyl-2-phenylpent-4-enal (3n): 1H NMR (500 MHz,
CDCl3): d=2.70–2.72 (m, 4H), 5.05–5.10 (m, 4H), 5.51–5.59
(m, 2H), 7.21–7.40 (m, 5H), 9.52 (s, 1H); 13C NMR
(125 MHz, CDCl3): d=36.7, 56.8, 118.9, 127.5, 127.6, 128.8,
132.6, 138.0, 201.8; HR-MS (EI): m/z=200.119998, calcd.
for [C14H16O] ([M]+): 200.120112.
(E)-2-(Biphenyl-4-yl)-2-methyl-5-phenylpent-4-enal (3r):
1H NMR (500 MHz, CDCl3): d=1.53 (s, 3H), 2.80–2.90 (m,
2H), 5.95–6.01 (m, 1H), 6.42 (d, J=16.0 Hz, 1H), 7.20 (m,
1H), 7.26 (m, 4H), 7.36 (m, 3H), 7.45 (m, 2H), 7.60–7.64
(m, 4H), 9.60 (s, 1H); 13C NMR (125 MHz, CDCl3): d=
19.0, 39.9, 53.9, 124.8, 126.1, 127.1, 127.3, 127.5, 127.57,
127.63, 128.5, 128.8, 133.7, 137.2, 138.4, 140.3, 140.4, 201.9;
HR-MS (EI): m/z=326.167329, calcd. for [C24H22O] ([M]+):
326.167062.
2-(4-Methoxyphenyl)-2-methylpent-4-enal (3b): 1H NMR
(500 MHz, CDCl3): d=1.42 (s, 3H), 2.57–2.69 (m, 2H), 3.80
(s, 3H), 5.02–5.07 (m, 2H), 5.51–5.58 (m, 1H), 6.91 (d, J=
8.5 Hz, 2H), 7.16 (d, J=9.0 Hz, 2H), 9.46 (s, 1H); 13C NMR
(125 MHz, CDCl3): d=18.7, 40.6, 52.9, 55.2, 114.2, 118.5,
128.4, 131.2, 133.3, 158.8, 201.9; HR-MS (EI): m/z=
204.115230, calcd. for [C13H16O2] ([M]+): 204.115031.
2-(Biphenyl-4-yl)-2-methylpent-4-enal
(500 MHz, CDCl3): d=1.47 (s, 3H), 2.64–2.75 (m, 2H),
5.04–5.10 (m, 2H), 5.54–5.63 (m, 1H), 7.31–7.36 (m, 3H),
7.42–7.45 (t, 2H), 7.58–7.61 (m, 4H), 9.54 (s, 1H); 13C NMR
(125 MHz, CDCl3): d=18.9, 40.6, 53.5, 188.8, 127.1, 127.5,
127.6, 127.7, 128.9, 133.2, 138.5, 140.2, 140.4, 201.9; HR-MS
(EI): m/z=250.135860, calcd. for [C18H18O] ([M]+):
250.135768.
2-(4-Chlorophenyl)-2-methylpent-4-enal (3f): 1H NMR
(500 MHz, CDCl3): d=1.43 (s, 3H), 2.58–2.69 (m, 2H),
5.03–5.07 (m, 2H), 5.48–5.56 (m, 1H), 7.18 (d, J=8.5 Hz,
2H), 7.35 (d, J=8.5 Hz, 2H), 9.49 (s, 1H); 13C NMR
(125 MHz, CDCl3): d=18.8, 40.6, 53.3, 119.0, 128.6, 129.0,
132.7, 133.4, 137.9, 201.4; HR-MS (EI): m/z=208.065262,
calcd. for [C12H13OCl] ([M]+): 208.065491.
2-(Biphenyl-4-yl)-2-methyl-4-phenylpent-4-enal
(3s):
1H NMR (500 MHz, CDCl3): d=1.36 (s, 3H), 3.13–3.27 (q,
2H), 4.94 (s, 1H), 5.20 (s, 1H), 7.17–7.21 (m, 7H), 7.32–7.35
(m, 1H), 7.41–7.53 (m, 6H), 9.45 (s, 1H); 13C NMR
(125 MHz, CDCl3): d=18.9, 41.9, 54.2, 117.8, 126.7, 127.1,
127.28, 127.30, 127.5, 127.7, 127.8, 128.2, 128.9, 138.4, 140.1,
140.6, 142.2, 145.1, 201.3; HR-MS (EI): m/z=326.167177,
calcd. for [C24H22O] ([M]+): 326.167063.
2-(6-Methoxynaphthalen-2-yl)-2-methylpent-4-enal
1H NMR (500 MHz, CDCl3): d=1.53 (s, 3H), 2.67–2.82 (m,
2H), 3.91 (s, 3H), 5.01–5.09 (m, 2H), 5.52–5.60 (m, 1H),
7.11–7.17 (m, 2H), 7.30–7.33 (dd, J=2.0 Hz, 8.5 Hz, 1H),
7.62 (d, J=1.0 Hz, 1H), 7.73 (t, J=9.5 Hz, 2H), 9.49 (s,
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Gaoxi Jiang and Benjamin List
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2-(Biphenyl-4-yl)-2,4-dimethylpent-4-enal (3t): 1H NMR
(500 MHz, CDCl3): d=1.45 (s, 3H), 1.50 (m, 3H), 2.67–2.78
(q, 2H), 4.65 (s, 1H), 4.83 (s, 1H), 7.33–7.36 (m, 3H), 7.42–
7.45 (m, 2H), 7.58–7.61 (m, 4H), 9.57 (s, 1H); 13C NMR
(125 MHz, CDCl3): d=18.6, 24.2, 44.2, 53.3, 115.5, 127.0,
127.4, 127.5, 127.7, 128.8, 138.8, 140.1, 140.3, 141.4, 201.8;
HR-MS (EI): m/z=264.151476, calcd. for [C19H20O] ([M]+):
264.151415.
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2-(4-chlorophenyl)-2-methyl-4-phenylpent-4-enal
(3v):
1H NMR (500 MHz, CDCl3): d=1.30 (s, 3H), 3.06–3.20 (q,
2H), 4.88 (s, 1H), 5.17 (d, J=1.5 Hz, 1H), 7.04–7.07 (m,
2H), 7.13–7.15 (m, 2H), 7.18–7.22 (m, 5H), 9.39 (s, 1H);
13C NMR (125 MHz, CDCl3): d=18.9, 41.9, 54.0, 118.0,
126.6, 127.4, 128.2, 128.7, 128.8, 133.3, 137.9, 142.1, 144.8,
200.9; HR-MS (EI): m/z=284.096722, calcd. for
[C18H17OCl] ([M]+): 284.096791.
2-(4-chlorophenyl)-2,4-dimethylpent-4-enal
(3w):
1H NMR (500 MHz, CDCl3): d=1.14 (s, 3H), 1.45 (s, 3H),
2.61–2.70 (q, 2H), 4.61 (s, 1H), 4.81 (t, J=1.5 Hz, 1H), 7.21
(d, J=8.5 Hz, 2H), 7.34 (d, J=9.0 Hz, 2H), 9.51 (s, 1H);
13C NMR (125 MHz, CDCl3): d=18.7, 24.2, 44.2, 53.2, 115.7,
128.7, 128.9, 133.4, 138.3, 141.0, 201.4; HR-MS (EI): m/z=
222.080917, calcd. for [C13H15OCl] ([M]+): 222.081146.
Acknowledgements
We thank Anna Lee for kindly donating several racemic al-
dehydes 1. Generous funding by the Max Planck Society is
gratefully acknowledged.
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Adv. Synth. Catal. 2011, 353, 1667 – 1670