Direct arylation/alkylation/magnesiation of benzyl alcohols in the presence of grignard reagents via Ni-, Fe-, or Co-catalyzed sp3 C-O Bond activation

Article abstract of DOI:10.1021/ja307045r

Direct application of benzyl alcohols (or their magnesium salts) as electrophiles in various reactions with Grignard reagents has been developed via transition metal-catalyzed sp³ C-O bond activation. Ni complex was found to be an efficient catalyst for the first direct cross coupling of benzyl alcohols with aryl/alkyl Grignard reagents, while Fe, Co, or Ni catalysts could promote the unprecedented conversion of benzyl alcohols to benzyl Grignard reagents in the presence of ⁿhexylMgCl. These methods offer straightforward pathways to transform benzyl alcohols into a variety of functionalities.

Full text of DOI:10.1021/ja307045r

Communication
pubs.acs.org/JACS
Direct Arylation/Alkylation/Magnesiation of Benzyl Alcohols in the
Presence of Grignard Reagents via Ni‑, Fe‑, or Co-Catalyzed sp3 C−O
Bond Activation
Da-Gang Yu,† Xin Wang,†,‡ Ru-Yi Zhu,† Shuang Luo,† Xiao-Bo Zhang,§ Bi-Qin Wang,§ Lei Wang,‡
and Zhang-Jie Shi*,†,⊥
†Beijing National Laboratory of Molecular Sciences and Key Laboratory of Bioorganic Chemistry and Molecular Engineering of
Ministry of Education, College of Chemistry, Peking University, Beijing 10087, China
‡Department of Chemistry, Huaibei Normal University, Anhui 235000, China
§College of Chemistry and Material Science, Sichuan Normal University, Sichuan 400061, China
⊥State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China
S
* Supporting Information
Transition metal-catalyzed cross-coupling reactions are
ABSTRACT: Direct application of benzyl alcohols (or
important and powerful tools to construct carbon−carbon
bonds.5 Compared with the reactions of aryl/alkenyl electro-
their magnesium salts) as electrophiles in various reactions
philes,6,7 application of alkyl electrophiles in cross couplings has
with Grignard reagents has been developed via transition
metal-catalyzed sp3 C−O bond activation. Ni complex was
been less investigated, which may arise from their difficult
found to be an efficient catalyst for the first direct cross
coupling of benzyl alcohols with aryl/alkyl Grignard
reagents, while Fe, Co, or Ni catalysts could promote
the unprecedented conversion of benzyl alcohols to benzyl
oxidative addition and reductive elimination as well as other
competitive side reactions (e.g., β-H elimination).8 Compared
with alkyl halides, alcohols are much more desirable because of
their low toxicity and easy availability. However, due to the high
bond dissociation energy of the C−O bond,9 good coordinative
ability and low leaving ability of the OH− group, and easy
oxidation through β-H elimination, protection of the hydroxyl
group and simultaneous activation of the C−O bond are
commonly required. For example, alkyl sulfonates have been
applied in various cross-coupling reactions.10
Obviously, direct transformation of alcohols is much more
step- and atom-economic as well as environmentally benign.
With the efforts to achieve this goal, highly active alcohols, such
as allylic, allenic, and propargylic alcohols, have been directly
employed in cross-coupling reactions by several groups.11
Recently, in a significant contribution, Yi reported the Ru-
catalyzed C−H alkylation of alkenes and phenols with general
alcohols.12 To date, direct cross coupling of benzyl alcohols has
never been achieved, although benzyl carbonates, carboxylates,
phosphates, and ethers have been applied in various cross
couplings by several groups, including ours.13
n
Grignard reagents in the presence of hexylMgCl. These
methods offer straightforward pathways to transform
benzyl alcohols into a variety of functionalities.
lcohols broadly exist in nature and can also be easily
Asynthesized from carbonyl compounds or alkenes. Owing
to their low toxicity and cost, direct transformations of alcohols
to valuable chemicals has attracted much attention in organic
synthesis.1 In classical organic chemistry, alcohols (or their salts)
always act as the nucleophiles and reductants. Recent advances
have demonstrated that alcohols could be applied as efficient
electrophiles in not only Brønsted/Lewis acid-promoted
Friedel−Crafts reaction2 but also transition metal-catalyzed
hydrogen-borrowing reactions.3 Meanwhile, Brønsted/Lewis
acid-promoted and/or transition metal-catalyzed reductions of
alcohols have also been reported.4 Herein, we demonstrate that
benzyl alcohols (or their magnesium salts) can not only act as
electrophiles in Ni-catalyzed cross coupling but also undergo Fe-,
Ni-, or Co-catalyzed direct magnesiation with different Grignard
reagents via sp3 C−O bond activation (Scheme 1).
With the success of activation of sp2 C−O bond in
naphthoxides via Ni catalysis,14 we planned to apply this idea
to sp3 C−O bond cleavage of alkoxides. The choice of benzyl
alcohol at the initial stage was inspired by the relatively higher
reactivity of such an sp3 C−O bond9 and the importance of
diarylmethanes, which are a common structural motif in
biologically active drugs15 and supramolecular structures,16 as
desired products.
Scheme 1. Catalytic Transformations of Benzyl Alcohols with
Different Grignard Reagents via Fe-, Ni-, or Co-Catalyzed sp3
C−O Bond Activation
2-Naphthylmethanol 1a was chosen as the standard substrate,
and relatively inexpensive MeMgBr was used as the base to
Received: January 20, 2012
Published: August 25, 2012
© 2012 American Chemical Society
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Communication
a
generate magnesium naphthylmethoxide for the subsequent
coupling. Recent studies indicated that Ni catalysts had a great
potential to activate both sp2 and sp3 C−O bonds.7,13,14,17−19
After much screening, we found that NiCl2(PCy3)2 was efficient
for cross coupling 1a with PhMgX (X = Br, Cl) to afford 2-
benzylnaphthalene 3aa (Table S1).
Table 2. Cross Coupling of Benzyl Alcohols 1 with 2a′
With the optimized conditions, we investigated the scope of
Grignard reagents (Table 1). Various arylmagnesium bromides,
a
Table 1. Cross Coupling of 1a with Grignard Reagents 2
a
b
c
d
Reaction conditions same as in Table 1. 30 °C. 48 h. Both of the
e
C−O bonds were phenylated. 10 mol% of DCyPE was used instead
of 20 mol% of PCy3. GC yield with n-decane as an internal standard.
f
additional PCy3. Under this modified condition, 4-biphenylme-
thanols 1f−h with different substituents and even simple benzyl
alcohol 1i showed credible reactivity. Notably, quinolin-2-
ylmethanol (1j) is also applicable, albeit with relatively low
conversion and yield. α-Arylated benzylic alcohols also worked
but showed lower reactivity. For example, 1k could only afford
3ka in 40% yield (eq 2).
a
Reaction conditions: 0.2 mmol of 1a, 2.0 equiv of 2, 1.2 equiv of
MeMgBr, 10 mol% of NiCl2(PCy3)2, 20 mol% of PCy3 in a mixture of
0.75 mL of PhMe and 0.25 mL of nBu2O at 60 °C under N2 for 24 h.
b
c
d
30 °C. 22 h. 48 h.
including those with steric hindrance (2b, 2c, 2h), showed good
efficiency. Moreover, Grignard reagents with electron-donating
groups (2f, 2g) showed good reactivity even at 30 °C. It is
important to note that both sp2 C−OMe (2g) and C−F bonds
(2h) can survive in the reaction and can further undergo
orthogonal cross coupling with different organometallic
reagents.19,20 Heterocycles such as pyrrole (2i) can also be
tolerated. Besides aryl Grignard reagents, benzylmagnesium
chloride (2j) also showed good reactivity. Unfortunately, alkyl
and alkenyl Grignard reagents were not suitable for this
transformation. In some cases, some coupling products were
observed, accompanied by a small amount of reductive product
2-methylnaphthalene (4a). To our delight, in the presence of a
bidentate phosphine ligand, 1,1′-bis(dicyclohexylphosphino)-
ferrocene (DCyPF),18a Ni(acac)2 showed good activity in
methylation of 1a under similar reaction conditions (eq 1).
The product 3ak was obtained in 68% yield, along with 9% of 4a
(please refer to Table 3).
As mentioned above, reductive products were observed when
alkyl Grignard reagents were used. Inspired by this observation,
we searched for efficient methods to reduce benzyl alcohols.
Previously, Pd, Ru, Au, Rh, and other metals showed activity in
catalytic reduction of benzyl alcohols.2,4 However, iron catalysts,
which are much less expensive and nontoxic, have never been
applied in this reaction.21 After systematic screening, we were
pleased to find that various iron catalysts exhibited good
reactivity to afford the reductive product in the presence of
nhexylMgCl (2k) (Table S2). Moreover, several cobalt and nickel
salts, such as CoCl2 and Ni(acac)2 (Table 3, entries 2 and 3), also
showed good activity, while copper salts gave a very low
efficiency (Table S2), which indicated that the contaminant from
copper might not be the active catalyst in this reaction.22
Under the optimized conditions, reduction of different benzyl
alcohols was conducted (Table 3). Beside benzylic alcohols
involving fused rings (1a, 1c, and 1e), biphenylmethanol
derivatives 1f−m and even simple benzyl alcohol 1n (Table 3,
entries 6−10) could be reduced well. It is noteworthy that many
functional groups, such as amino group and sp2 C−OMe and C−
F bonds, are well tolerated (Table 3, entries 5−10).
The substrate scope of benzylic alcohols was further surveyed
with PhMgCl (2a′) as the coupling partner (Table 2). Benzylic
alcohols involving fused rings, such as 1b−1d, afforded the
desired products in moderate to good yields. When 1e was
applied in the reaction, both C−O bonds were cleaved and the
diphenylated product was obtained with good efficiency.
Although the simple benzylic alcohol showed low reactivity
under identical conditions, the conversion and yield could be
significantly promoted by using a bidentate ligand, 1,2-
bis(dicyclohexylphosphino)ethane (DCyPE),17b to replace the
In the initial mechanistic investigation, we hypothesized that
reduction arose from β-H elimination of an alkyliron complex
and reductive elimination. Two possible catalytic cycles with
different active catalysts were proposed (Scheme S1). As we
know, the alkene would be generated in the process of β-H
elimination. To prove these hypotheses, we applied 1-
tetradecylmagnesium chloride in the reaction. As expected, the
tetradecene was detected as a byproduct (Scheme S2).
Moreover, several deuterium-labeled alkyl Grignard reagents
were synthesized to confirm the source of “H” in final products
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disclosed,24 to our knowledge, this is the first example of
transition metal-catalyzed magnesiation of a C−O bond in
alcohols.25
a
Table 3. “Reduction” of Different Benzyl Alcohols 1
Based on our results and previous mechanistic studies of iron
catalysis,21 we put forward a new possible mechanism (Scheme
2). An inorganic Grignard reagent [(Ln)Fe(MgCl)2] (6), formed
Scheme 2. Proposed Catalytic Cycle for Magnesiation of
Benzylic Alcohols in the Presence of 2k
a
Reaction conditions: 0.2 mmol of 1, 3.5 equiv of 2k, 10 mol% of
FeBr2, 40 mol% of PCy3 in a mixture of 0.75 mL of PhMe and 0.25 mL
b
of iPr2O at 120 °C under N2 for 24 h. GC yields with n-decane as an
c
d
internal standard. 10 mol% of CoCl2 as the catalyst. 10 mol% of
e
Ni(acac)2 as the catalyst. 48 h.
in situ in the presence of 2k and PCy3, was the active catalytic
species and underwent the following catalytic cycle.21e Oxidative
addition of benzylic C−O bond and release of magnesium salts
generated 7, which underwent transmetalation with 2k to
generate a more stable benzyl Grignard reagent 5 and iron
complex 9.24 The latter was attacked by 2k to afford 10, which
further underwent β-H elimination and/or reductive elimination
to regenerate the active catalyst 6.21e Transmetalation of 7 with
2k can proceed through σ-metathesis (Path a), or attack of 2k
followed by release of 5 (Path b), both of which are hypothesized
to be reversible.
(Scheme S3). To our surprise, no D-incorporation was observed
in 4e when the hexa-deuterium-labeled alkyl Grignard reagent
was used (eq 3), which indicated that the “H” did not come from
Grignard reagent and excluded these two proposed mechanisms
(Scheme S1).
In conclusion, we have developed the first nickel-catalyzed
cross coupling and an unprecedented iron-, nickel-, or cobalt-
catalyzed magnesiation of benzylic alcohols in the presence of
different Grignard reagents via sp3 C−O bond activation. The
coupling affords an atom- and step-economic transformation of
benzylic alcohols to construct important diarylmethanes under
mild conditions. The new pathway to directly generate benzyl
Grignard reagent from alcohols is theoretically important,
offering the potential for generation of organometallic reagents
from alcohols via transition metal catalysis. Further investigations
to understand the reaction mechanism, expand the substrate
scope, and carry out other novel transformations of alcohols are
underway in our laboratory.
To unveil the real source of “H”, we did several isotopic
tracking experiments. We tested 1e-d2 as the substrate, toluene-
d8 as solvent, and exchange with THF-d8, but no D-incorporation
was observed (Scheme S3). In a pleasant surprise, we found that
mono-deuterium-labeled product 4a-d1 (92% D) was obtained in
84% yield when the reaction of 1a was quenched with CD3OD
after 8 h (eq 4), in strong support of benzylic C−M bond
formation. Due to the catalytic amount of FeBr2, we concluded
that benzyl Grignard reagent 5a was formed. Beside protons,
carbon-based electrophiles were used to trap the generated 5a to
construct C−C bonds.23 For example, when BnBr was used as
the electrophile, the desired product 3aj was obtained, albeit with
relatively low efficiency (eq 5). Although formations of new
ASSOCIATED CONTENT
■
S
* Supporting Information
Brief experimental details; spectral data for products. This
material is available free of charge via the Internet at http://pubs.
acs.org.
AUTHOR INFORMATION
Corresponding Author
zshi@pku.edu.cn
■
Grignard reagents via transition metal-catalyzed hydromagne-
siation or carbomagnesiation of alkynes and alkenes have been
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Support of this work by NSFC (No. 20925207, 21072010,
20821062) and the “973” Project from the MOST of China
(2009CB825300) is gratefully acknowledged.
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