Communication
p–allyl Pd intermediates causes
the observed regioselectivity.
When H2O was used as the nu-
cleophile, a primary allyl alcohol
(2,7-octadien-1-ol) was prepared
from 1,3-butadiene, under CO2
pressure, by the addition of car-
bonic acid (H2CO3) to a bis-p–
allyl Pd intermediate, followed
Table 2. The substrate scope of Pd-catalyzed transformation of terminal alkenes into primary allylic alcohols.[a]
Entry
1
Substrate
Product
Yield [%][b]
90
by
decarboxylation.[8]
This
method has been successfully
applied in industry; however, the
substrate scope of the butadiene
telomerization is limited, proba-
bly owing to the facile formation
and stability of the bis-p–allyl Pd
intermediates.
2
3
4
91
84
81
Regarding the direct conver-
sion of terminal alkenes to pri-
mary allylic alcohols, a gas-phase
reaction of propylene catalyzed
by heterogeneous mixed-metal
oxides afforded allyl alcohol,
however, the reaction required
2808C.[9]
27
18
5[c]
6
0
[a] 1 (1 mmol), [Pd(PPh3)4] (5 mol%), 2,5-tBu2BQ (1.5 equiv), and H2O (10 eq) in dioxane, under CO2 (4 MPa) was
stirred at 408C for 24 h. [b] GC yield. [c] 72h.
Moreover, selective hydrofor-
mylation and hydroformylation/
hydrogenation sequences have
been developed to convert ter-
minal alkenes into aldehydes and primary alcohols.[10]
dination ability to the Pd center, which could enhance the co-
ordination of substrate and result in high yield. Electron-with-
drawing groups increased the oxidation ability of BQ and the
use of conditions by Jiang et al. (PdCl2, 2,3-dichloro-5,6-dicya-
no-1,4-benzoquinone (DDQ), and H2O) afforded enones.[13]
However, chlorobenzoquinones failed to afford the desired
product (entries 8 and 9), probably because they form stable
Pd complexes, which inhibit the catalytic cycle. The reaction in
the presence of Cu salts and O2 failed under these conditions.
The substrate scope was also examined (Table 2). Allylben-
zene derivatives with electron-donating and electron-with-
drawing substituents selectively afforded (E)-allylic alcohols
2b–e in good yields (entries 1–4). Aliphatic substrate 1 f
showed lower reactivity and selectivity, affording 2 f and 3 f in
27 and 18% yields, respectively. b-Methyl styrenes and homo-
allylbenzene failed to afford the desired product. At this stage,
the substrate scope is limited to allylbenzenes.
The hydration of terminal alkynes is a facile process com-
pared to that of alkenes, and an anti-Markovnikov version of
this reaction was developed and studied extensively by our
group.[11] Simultaneous reduction to alcohols has also been de-
veloped.[11j,k] We also studied the homogeneous Pd-catalyzed
oxygen functionalization of the terminal position of an epox-
yalkene (butadiene monoxide),[12a] and the heterogeneous Co-
catalyzed direct synthesis of primary alcohols from syngas (CO/
H2) based on Fischer–Tropsch synthesis.[12b] Herein, we report
the homogeneous Pd-catalyzed direct conversion of terminal
alkenes into primary allylic alcohols.
The initial screening of catalysts is summarized in Table S1
(see the Supporting Information). The reaction was catalyzed
by various Pd complexes in the presence of 1,4-benzoquinone
(1,4-BQ), and the use of [Pd(PPh3)4] afforded the highest activi-
ties. A small amount (up to 24%) of a byproduct (4-hydroxy-
phenyl cinnamyl ether) was detected by the addition of 4-hy-
droquinone (4-HQ). The application of CO2 pressure enhanced
the reactivity.
The reaction probably proceeds via p–allyl intermediates. Al-
lylic acetate 5, which easily affords a p–allyl complex, was sub-
jected to the reaction conditions in the absence of oxidants
(Scheme 1). The same product, 2a, was obtained in 61% yield,
along with a small amount of an isomeric ester. In contrast, al-
lylic alcohol 3a afforded only 2% yield of product owing to
sluggish oxidative addition.
The structure of the BQ significantly affected the yield and
selectivity of the reaction (Table 1). The introduction of methyl
and tert-butyl groups improved the yield of 2a to 52 and 76%,
respectively (entries 2 and 3). Performing the reaction at 408C
afforded a slightly better yield (entry 4). The use of 2,5-di-tert-
butyl-1,4-benzoquinone (2,5-tBu2BQ) increased the yield to
95% (entry 5), and no HQ was observed. The steric hindrance
of 2,5-tBu2BQ suppressed the addition and decreased the coor-
A proposed reaction mechanism is shown in Scheme 2. The
allylic CÀH bond could be activated to form PdII-p–allyl inter-
mediate B.[5d] Prior to this step, Pd0 is oxidized, by BQ, to PdII
and the benzylic proton is removed, as H2CO3, on reaction
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Chem. Eur. J. 2014, 20, 1 – 5
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ÝÝ These are not the final page numbers!