Nickel-Catalyzed Homo- And Cross-Coupling of Allyl Alcohols via Allyl Boronates

Article abstract of DOI:10.1021/acs.orglett.0c01424

A nickel-catalyzed homo- and cross-coupling of allylic alcohols to 1,5-dienes in the presence of B2pin2 with excellent regioselectivity has been developed. Mechanistic studies indicate that the reaction proceeds via sequential nickel-catalyzed borylation of allyl alcohols followed by cross-coupling of the resulting allyl boronates with allyl alcohols. The method was effectively applied to nickel-catalyzed allylation of aldehydes using allylic alcohols directly.

Full text of DOI:10.1021/acs.orglett.0c01424

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Letter
Nickel-Catalyzed Homo- and Cross-Coupling of Allyl Alcohols via
Allyl Boronates
Yi Gan, Hui Hu, and Yuanhong Liu*
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ABSTRACT: A nickel-catalyzed homo- and cross-coupling of allylic alcohols to 1,5-dienes in the presence of B2pin2 with excellent
regioselectivity has been developed. Mechanistic studies indicate that the reaction proceeds via sequential nickel-catalyzed borylation
of allyl alcohols followed by cross-coupling of the resulting allyl boronates with allyl alcohols. The method was effectively applied to
nickel-catalyzed allylation of aldehydes using allylic alcohols directly.
ransition-metal-catalyzed allyl−allyl cross-coupling reac-
T
tions between allylic electrophiles and allylmetal reagents
such as allyl Grignard reagents,1 allyl boronates,2 allyl-silanes,3
or allyl-stannanes4 represent some of the most efficient
methods for the synthesis of 1,5-dienes (Scheme 1), which
Scheme 1. Metal-Catalyzed Allyl−Allyl Coupling Reactions
Figure 1. 1,5-Diene-containing terpenes.
to catalyze allyl−allyl cross-couplings.1−4 Notwithstanding the
advances, these reactions require pre-prepared stoichiometric
allyl organometallic reagents, which are usually sensitive to air
and moisture and suffer from low stability and low functional-
group tolerance. In addition, most of these studies use
activated derivatives such as allylic halides, esters, and
phosphates as the electrophiles.1−4 These substrates are often
prepared from allylic alcohols. The direct employment of allylic
alcohols as the allylic electrophiles is much more attractive
are valuable building blocks in organic synthesis5 and also
found in many biologically active substances and naturally
occurring terpenes6 (Figure 1). Such allyl−allyl couplings are
significantly challenging because both homo- and cross-
couplings might be involved in these reactions; therefore, it
is hard to obtain the desired products with high chemo- and
regioselectivity and good yields. To date, transition metals such
as palladium, nickel, copper, iridium, and gold have been used
Received: April 24, 2020
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© XXXX American Chemical Society
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because of its easy availability, high stability, and atom
economy.7 There are very few examples using allylic alcohols
in the allyl−allyl cross-couplings due to the poor leaving ability
of the hydroxyl group. In 2011, Kobayashi et al.2d reported a
Ni-catalyzed regioselective cross-coupling between allylic
alcohols and terminal allyl boronates leading to linear products
(type III). Later, Carreira3b and Yang2h reported Ir-catalyzed
asymmetric coupling of branched allylic alcohols with allyl-
silanes or allyl boronates, which afforded branched products
(type II). Herein, we report a nickel-catalyzed homo- and
cross-coupling of allylic alcohols leading to linear 1,5-dienes.
The nucleophilic allyl boronate is generated in situ by a Ni-
catalyzed process, thus obviating the use of presynthesized
stoichiometric allyl metals. In addition, the method can be
extended successfully to highly regioselective coupling of allylic
alcohols with aldehydes (Scheme 1).
B2nep2 instead of B2pin2 provided 2a in a lower yield (entry 4).
We speculated that the allyl boronate intermediate derived
from a diboron reagent might be formed, which facilitated the
homocoupling of allyl alcohols. The effects of other ligands
were also examined. Monodentate ligands such as P(p-
MeC6H4)3, P(p-CF3C6H4)3, PMePh2, and PMe2Ph provided
2a in good yields of 68−81%, while PnBu3 gave 2a in lower
yield (entries 5−9). Bidentate ligands such as dppb and 2,2′-
bipy resulted in lower yields (entries 10 and 11, respectively).
Ni(PPh3)4, which has been shown to be an active catalyst in
Ni-catalyzed cross-couplings between allyl alcohols and allyl
boronates,2d failed to give the desired product (entry 12). The
use of a Ni(II) complex such as NiBr2(DME) or Ni(acac)2 was
ineffective in this reaction (entry 13). In the absence of PPh3,
2a was formed in 23% yield (entry 14). In the absence of a
nickel catalyst, no desired product was formed (entry 15).
Next, the scope of the nickel-catalyzed homocoupling of
allylic alcohols was examined (Table 2). For cinnamyl alcohol
derivatives, the phenyl ring bearing a 4-OMe or 4-CF3
substituent worked well to afford product 2b or 2c,
respectively, in 50−83% yields (Table 2, entry 2 or 3,
respectively). However, treatment of 1d possessing a 3-Cl
During our study of nickel-catalyzed reductive coupling of
allylic alcohols with other electrophiles, we found that 1,5-
dienes derived from dimerization of allylic alcohols were
always observed as the side products under various reaction
conditions. According to the literature, Wurtz-type dimeriza-
̈
tion of allylic halides and esters has been reported; however,
the regioselectivity was often unsatisfactory.8 A mild and
regioselective dimerization by direct use of allylic alcohols
remains to be developed. We then deeply evaluated this
reaction. After extensive optimization studies, including the
effects of the nickel source, ligands, reducing agents, reaction
temperature, etc., we found that the additive B2pin2 played a
key role in the effective transformation. Thus, 1,5-diene 2a was
obtained in 86% yield with excellent linear selectivity (>30:1)
using 5 mol % Ni(COD)2 and 10 mol % PPh3 in the presence
of 1.0 equiv of B2pin2 in CH3CN at 50 °C (Table 1, entry 1).
No apparent formation of other regioisomers was observed.
Without B2pin2, only 4% of the desired product was obtained
(entry 2). Other reducing agents such as Zn and Mn, which
were widely used in Ni-catalyzed reductive coupling reactions,9
failed to give the dimerization product (entry 3). The use of
Table 2. Scope of the Ni-Catalyzed Homocoupling of Allyl
Alcohols
Table 1. Optimization of the Reaction Conditions
a
entry
derivation from standard conditions
yield (%)
b
1
none
86 (82 )
2
3
4
5
6
7
8
9
10
11
12
13
14
15
no B2pin2
Zn or Mn instead of B2pin2
B2nep2 instead of B2pin2
P(p-MeC6H4)3 instead of PPh3
P(p-CF3C6H4)3 instead of PPh3
PMePh2 instead of PPh3
4
3−4
56
81
72
81
68
46
24
27
0
PMe2Ph instead of PPh3
PnBu3 instead of PPh3
5 mol % dppb instead of 10 mol % PPh3
5 mol % 2,2′-bipy instead of 10 mol % PPh3
Ni(PPh3)4 instead of Ni(COD)2, without PPh3
NiBr2(DME) or Ni(acac)2 instead of Ni(COD)2
no PPh3
0
23
0
no Ni(COD)2
a
a
b
c
NMR yields using 1,3,5-trimethoxybenzene as the internal standard.
Isolated yields.
Isolated yields. On a 6 mmol scale. With 10 mol % Ni(COD)2 and
d
b
20 mol % PPh3. At 40 °C.
B
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substitutent resulted in a trace of product (determined by
TLC), possibly due to the fact that the C−Cl bond is
susceptible to oxidative cleavage in nickel catalysis. Gratify-
ingly, a moderate yield of 2d could be achieved by doubling
the amounts of Ni(COD)2 and PPh3 and decreasing the
reaction temperature to 40 °C (entry 4). A high yield of 2e
with a 4-F substitutent was also achieved (entry 5). Allyl
alcohol 1f, a regioisomer of 1a, worked well to afford the same
product 2a in 83% yield (entry 6), indicating that a common π-
allyl nickel intermediate was involved in the reaction. A range
of branched alcohols bearing either electron-donating (4-Me,
4-tBu, 4-NMe2, and 4-OPiv) or electron-withdrawing (4-CN
and 4-CO2Me) groups on the phenyl ring were converted to
the corresponding linear 1,5-dienes 2g−2l in 76−86% yields
(entries 7−12, respectively). It was noted that increasing the
amounts of the nickel catalyst and ligand was also required to
achieve a better yield of 2j bearing a pivalate group (entry 10).
The results indicated that the electronic nature of aryl
substituents had little influence on the reaction. The 3-
OTBS and 3,5-diOMe groups on the aryl ring were tolerated
well (entries 13 and 14, respectively). Moreover, when the
sterically congested allyl alcohols bearing 2-methyl and 2-
methoxy groups on the phenyl ring were employed, the
corresponding 2o and 2p, respectively, were formed in high
yields, indicating that the reaction was not sensitive to steric
effects (entries 15 and 16, respectively). A silyl group on the
phenyl ring was also suitable (2q). Substrate 1r, which
included a 4-Bpin-phenyl moiety, gave 2r in a lower yield of
42%. Allyl alcohol 1s bearing an internal alkynyl group
provided 2s in 23% yield (entry 19). The gram-scale synthesis
of 1,5-diene 2a was also performed (entry 1), which
demonstrated the practicality of this method.
Table 3. Scope of the Ni-Catalyzed Cross-Coupling of Allyl
Alcohols
a
b
c
Isolated yields. Containing a trace of byproduct. Containing 9%
isomer, possibly a Z-isomer.
We next tried to isolate the possible reaction intermediate.
To our delight, without the ligand, allyl boronate 5 could be
formed in 78% yield, along with dimer 2a in 10% yield
(Scheme 3, eq 1). The yield of 5 could be dramatically
improved to 90% upon addition of 5 mol % PhCO2H to the
reaction mixture, and the dimerization could be completely
When different allyl alcohols 1a and 1b were subjected to
this catalytic system, cross-coupling product 3 was obtained in
40% yield, together with homocoupling products 2a and 2b in
26−27% yields (Scheme 2). To our delight, no apparent
branched isomers were observed in this reaction.
Scheme 3. Mechanistic Studies
Scheme 2. Nickel-Catalyzed Reaction of Allyl Alcohols 1a
and 1b
The cross-coupling of branched alcohol 1h with terminal
allyl alcohol 1u proceeded efficiently to give 4a in 55% yield
using excess 1u and 2.0 equiv of B2pin2 (Table 3, entry 1). The
homocoupling products of 2h (33% yield) and 2u (∼55%
NMR yield, based on 1u) could also be observed. 2u could
easily be removed by evaporation. This method was applied to
various allyl alcohols, and in all cases, the desired 1,5-dienes
were formed in moderate yields with excellent linear selectivity.
The allyl alcohols with either electron-poor or electron-rich
aryl rings coupled well with 1u (4b and 4c). Reactions of
thienyl- or pyridyl-substituted allyl alcohols were also suitable
(4e and 4f). Linear alcohols such as cinnamyl alcohol 1a and
2-furanyl-substituted alcohol 1z transformed smoothly into 4g
and 4h, respectively.
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inhibited (Scheme 4, eq 2). In addition, addition of 5 mol %
PhCO2H to the reaction mixture under the standard
of the products. For example, the allylation of aliphatic
aldehydes such as 3-phenylpropanal (6j), octanal (6k), and 3-
(5-methylfuran-2-yl)propanal (6l) proceeded well to provide
7j−7l, respectively, in good yields. The reaction of aliphatic
allyl alcohol and aldehyde also took place smoothly (7p).
Control experiments indicated that benzaldehydes 6a could
react with allyl boronate 5 without the need for a nickel
catalyst in the presence or absence of PhCO2H, leading to 7a
in 56−60% yields, indicating that allyl boronate 5 served as the
intermediate for this reaction.
Scheme 4. Scope of the Ni-Catalyzed Allylation of
a
Aldehydes
On the basis of the previous reports and the results
presented above,13 a plausible reaction mechanism is depicted
in Scheme 5. Oxidative addition of allylic alcohol to Ni(0)
Scheme 5. Possible Reaction Mechanism
a
b
Isolated yieds with >20:1 dr. Containing small amounts of an
impurity.
generates a π-allylnickel(II) complex 8. In this process, B2pin2
may function as a Lewis acid to facilitate the elimination of the
OH group. Transmetalation of 8 with B2pin2 occurs to give
complex 9, which delivers allyl boronate 10 by reductive
elimination (cycle A). In cycle B, 10 reacts with π-
allylnickel(II) complex 11 to give a diallylnickel intermediate
12. Reductive elimination of 12 via a bis(η1-allyl)nickel
complex affords linear product 2 or 4 (cycle B). In the
presence of aldehyde 6, allyl boronate 10 reacts with 6 via
transition state 1314 to give homoallylic alcohols 7 stereo-
selectively.
In summary, we have developed a nickel-catalyzed homo-
and cross-coupling of allylic alcohols to 1,5-dienes with
excellent regioselectivity. The reaction proceeds via sequential
nickel-catalyzed borylation of allylic alcohols followed by cross-
coupling of the resulting allyl boronates with allyl alcohols. The
key intermediate of allyl boronates was found to be formed in
high NMR yields under very simple conditions without the
need for a ligand. The method was effectively applied to nickel-
catalyzed allylation of aldehydes using allylic alcohols directly.
conditions resulted in a trace of dimer 2a; thus, PhCO2H
plays a role in inhibiting the dimerization (Scheme 4, eq 3). It
was noted that boronate 5 was unstable during isolation; thus,
an only 42% isolated yield was obtained. To the best of our
knowledge, there has been no report of boroylation of allylic
alcohols using economic nickel as the catalyst,10,11 and our
reaction provided a simple route for aryl allyl boronates. The
reaction of allylic alcohol 1a with allyl boronate 5 under
standard reaction conditions afforded the desired 2a in 86%
yield (Scheme 4, eq 4). It should be noted that most of the
metal-catalyzed allyl−allyl coupling involved boranes limited to
terminal allyl boronates, and there have been few reports using
substituted allyl boronates;2a,e,f in particular, coupling with aryl
allyl boronates such as 5 has not been reported. The reaction
shown in eq 4 may provide an efficient way to selectively
prepare unsymmetrically substituted 1,5-dienes. We also
monitored the reaction by 1H NMR spectroscopy. These
studies showed that 4−23% of allyl boronate 5 was present
throughout the reaction. The amounts of allyl boronate 5
reached a maximum value after 2 h and then steadly decreased.
These results strongly suggest that the reactions proceed via an
allyl boronate intermediate.
The results presented above also suggest that the current
process could be extended to other catalytic transformations
involving allyl boronate. We then performed the allylation of
aldehydes using allylic alcohols.12 After briefly screening the
conditions, we found that the desired allylation occurred
efficiently with excellent regio- and stereoselectivity using 5
mol % PhCO2H as the additive (Scheme 4). Without
PhCO2H, an only moderate yield of 7a was formed. The
reaction proved to be quite general with respect to substitution
of allylic alcohols and aldehydes, because aryl or alkyl groups
were all suitable for these substrates, showing a broad diversity
ASSOCIATED CONTENT
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The Supporting Information is available free of charge at
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Experimental details and spectroscopic characterization
of all products and new substrates (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Yuanhong Liu − State Key Laboratory of Organometallic
Chemistry, Center for Excellence in Molecular Synthesis,
D
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(b) Hamilton, J. Y.; Hauser, N.; Sarlah, D.; Carreira, E. M. Iridium-
Catalyzed Enantioselective Allyl−Allylsilane Cross-Coupling. Angew.
Chem., Int. Ed. 2014, 53, 10759−10762.
Shanghai Institute of Organic Chemistry, University of Chinese
Academy of Sciences, Chinese Academy of Sciences, Shanghai
200032, People’s Republic of China; orcid.org/0000-0003-
1153-5695; Email: yhliu@sioc.ac.cn
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́
(b) Porcel, S.; Lopez-Carrillo, V.; García-Yebra, C.; Echavarren, A. M.
Authors
Gold-Catalyzed Allyl−Allyl Coupling. Angew. Chem., Int. Ed. 2008, 47,
Yi Gan − State Key Laboratory of Organometallic Chemistry,
Center for Excellence in Molecular Synthesis, Shanghai Institute
of Organic Chemistry, University of Chinese Academy of
Sciences, Chinese Academy of Sciences, Shanghai 200032,
People’s Republic of China
Hui Hu − State Key Laboratory of Organometallic Chemistry,
Center for Excellence in Molecular Synthesis, Shanghai Institute
of Organic Chemistry, University of Chinese Academy of
Sciences, Chinese Academy of Sciences, Shanghai 200032,
People’s Republic of China
1883−1886.
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Complete contact information is available at:
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National Key R&D Program of China
(2016YFA0202900), the Strategic Priority Research Program
of the Chinese Academy of Sciences (XDB20000000), and the
Science and Technology Commission of Shanghai Municipal-
ity (18XD1405000) for financial support.
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