Direct cross-coupling of benzyl alcohols to construct diarylmethanes via palladium catalysis

Article abstract of DOI:10.1039/c4cc10084k

A direct arylation to furnish diarylmethanes from benzyl alcohols was realized through Pd(PPh₃)₄-catalyzed Suzuki-Miyaura coupling via benzylic C-O activation in the absence of any additives. The arylation is compatible with various functional groups. This development provides an atom- and step-economic way to approach a diarylmethane scaffold under mild and environmentally benign conditions. This journal is

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DOI: 10.1039/C4CC10084K
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Direct Cross Coupling of Benzyl Alcohols to Construct Diarylmethanes
via Palladium Catalysis
Zhi-Chao Cao,a Da-Gang Yu,a Ru-Yi Zhu,a and Jiang-Bo Wei,a Zhang-Jie Shi,*a,b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Direct arylation to furnish diarylmethanes from benzyl alcohols was realized through Pd(PPh3)4-catalyzed Suzuki-Miyaura coupling via
benzylic C-O activation in the absence of any additives. The arylation is compatible with various functional groups. This development
provides an atom- and step- economic way to approach diarylmethane scaffold under mild and environmentally benign conditions.
Diarymethane is an important structural motif in 50 completely failed (eq. 1). To our delight, if sodium carbonate
10 supramolecules1 and pharmaceuticals.2 Conventionally they
are prepared by electrophilic aromatic substitution with
carbon electrophiles (Friedel–Crafts benzylation).3
This
method showed great power while to some extents the
was used as the base in the absence of other additives, the
cross coupling of 2-naphthylmethanol 1a with phenylboroxine
2a took place and the desired product 1ac was obtained in the
presence of Ni(COD)2 and PCy3 as catalysts, albeit in a low
disadvantages, such as low regio-selectivity and requirement 55 yield. Different parameters of the reaction conditions were
15 of large amounts of acids/Lewis acids, hampered its
applications (scheme 1a). Recent development of transition
metal-catalyzed cross-coupling has been one of the most
powerful tools to achieve such a goal since 1970s.4 However,
tested and we found that the highest GC yield of 60% of 1ac
was reached (eq. 2). Unfortunately, any other trials we have
done could not promote the efficiency, thus we had to give up
such a catalytic system. Therefore we turned out to search for
at this stage, the use of benzyl halides and their equivalents5 60 the Pd catalytic system to precede such a desirable arylation
20 limited its application due to the sluggish preparation and
undesirable byproducts. Recently, more readily available and
environmentally benign chemicals, such as benzylic
carboxylates,6 ethers,7 and other derivatives of benzyl
alcohol5a,5b,8 have been successfully applied in cross couplings
25 to partially solve this problem.
reaction.
Scheme 1. Design of direct cross coupling of benzyl alcohols
with arylboronic acid derivatives.
In comparison, alcohol is one of the most abundant organic
compounds in nature and synthetic world. However, direct
application of alcohols in cross coupling to construct carbon-
carbon bonds is rarely touched, although it exhibited the great
30 step-, redox- as well as atom- economy. To the best of our
knowledge, only relatively active allylic,9 propargylic,10 and
allenylic alcohols11 have been directly applied as the
electrophiles in cross couplings up to date. As one of broadly
existing alcohols, benzyl alcohols have never been directly
35 used as coupling partner until the only example of direct
cross-coupling of benzylic alcohol catalyzed by nickel
catalyst with using excess Grignard reagents (Scheme 1b).12
Due to the high reactivity of Grignard reagents as partners, the
poor functional group tolerance highly limited the potential
40 applications. According to the importance of diarylmethane
scaffolds, we conceived that direct arylation of benzyl
alcohols through cross coupling with other stable
organometallic reagents. Therein, we reported direct Suzuki-
Miyaura coupling of benzylic alcohols with Pd(PPh3)4 as
45 catalyst in the absence of any bases or additives (Scheme 1c).
65
In this context, we also conceived that the Lewis acidity of
boronic acid derivatives and the Lewis basicity of benzyl
alcohol themselves might be strong enough to activate each
other by the formation of the borates (or their interaction
strong enough) to facilitate the desired cross coupling. Based
70 on this assumption, we first tested the interaction between
naphthylmethanol 1a and phenylboroxine 2a. We compared
1
the H and 13C NMR of naphthylmethanol, phenylboroxine
and the mixture of these two compounds in d8-THF (Figure 1).
To our interest, the very clean NMR spectroscopy indicated
75 that, both chemical shift of H NMR and 13C NMR signal of
1
We initiated our studies based on the reported arylation of
naphtholates with aryl boroxines via potential "mutual
activation".13 However the test of the same conditions as the
cross coupling of naphtholates into the desired coupling
benzylic-CH2 moved downfield in the presence of the
phenylboroxine. These data strongly support the interaction
between naphthylmethanol and Lewis acidic phenylboroxine
(details refers to supporting information). With the additional
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reactivity investigation of boranes-alcohol complex (eq. 3)
and HRMS studies, we ruled out the formation of
benzylboronic ester and confirmed the activation of C-O bond 35 generate Pd(PPh3)4 in-situ from other kinds of VpieawlAlartdicileuOmnline
to be the most efficient, giving the desired product 1ac in 67%
isolated yield (Entry 1). Additionally, we considered to
DOI: 10.1039/C4CC10084K
via intermolecular coordination.
catalysts and the corresponding
5
Scheme 2. Nickel-catalyzed Suzuki-Miyaura coupling based
Table 1. Screening of phenylboronic acid derivatives.a,b
on mutual activation.
Table 2. Exploration of reaction conditions to carry out arylation
40 of 2-naphthylmethanol.a
Figure 1. 1H NMR spectra: (1) 2-naphthalenemethanol, δ= 4.66
10 ppm; (2) 2-naphthalenemethanol + phenylboroxine, δ= 4.66 and
5.22 ppm; 13C NMR spectra: (3) 2-naphthalenemethanol, δ=
64.8 ppm; (4) 2-naphthalenemethanol + phenylboroxine, δ=
64.8 and 65.1 ppm.
15 To our interest, the cross coupling between naphthylmethanol
and phenylboronic acid only in the presence of catalytic
Pd(PPh3)4 with THF as solvent gave the desired product 3a in
42% NMR yield (Table 1). Stimulated by this result, we
further tested the different derivatives of phenylboronic acid.
20 The phenylboroxine showed the best reactivity to produce the
desired product in 67% NMR yield while the corresponding
phenylboronic ester gave a poor yield. Obviously, the
prepared borates, such as potassium phenyl trifluoroborate
and sodium tetraphenylborate, failed in this cross coupling by
25 losing the Lewis acidity of boron atom, further supporting our
suspension.
ligands. To our disappointment, only Pd(OAc)2 and Pd(TFA)2
afforded the desired product while in lower yields (Entries 2
and 4). Particularly, when Pd(dba)2 or K2PdCl4 (Entries 3, 5, 6
45 and 7) were used, no desired product was obtained at all. The
combination of Pd(OAc)2 and other more electron-rich
phosphines also failed (Entries 8, 9, 10 and 11). The
investigation of catalyst loading indicated that the
concentration of catalyst was very important for the reaction.
50 The yield decreased dramatically when the catalyst loading
was reduced. For example, only a trace amount of desired
product was achieved when 1 mol% Pd(PPh3)4 was used
(Entry 9). As we expected, solvents also played a significant
role in this transformation. Compared to non-polar toluene,
55 polar DMF, and protic solvent methanol (Entries 15, 16 and
Based on this observation, we screened different parameters
of the reaction conditions (Table 2). We systematically
examined the influences of different kinds of palladium
30 catalysts, solvents, temperature, catalyst loading and reaction
time with 1a and 2a as the model substrates. Among the
palladium catalysts that were screened, Pd(PPh3)4 was proved
2
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17), etheric solvents were more effective (Entries 1 and 14),
supplemental method to construct complicated diarylmethanes
probably arising from the stabilization of the intermediate by
weak interaction. Notably, an accurate control of temperature
is critical and highest yield was obtained at 80 C (Entries 18,
toward the Friedel-Craft reaction (2oc and 2pc).
View Article Online
Table 4. Scope of arylboroxines.a,b
DOI: 10.1039/C4CC10084K
o
5
19 and 20). The influence of time was also investigated and it
was found that the reaction gave a complete conversion in 24
h (Entry 18).
Table 3. Scope of various benzylic alcohols.a,b
Ph
Ph
Ph
HO
MeO
1ac (74%)
1bc (78%)
Ph
1cc (75%)
Ph
H3CCOO
MOMO
1dc (67%)
1ec (80%)
Ph
Ph
Ph
Ph
BocHN
1hc (32%)d
(34%)d,e
1fc (71%)
Ph
1gc (66%)
Ph
Ph
X
45
Me2N
1ic (40%)
(61%)c
X = O, 1jc (85%)
X = S, 1kc (90%)
1lc, 41%
a All reactions were carried out on a 0.2 mmol scale with 10 mol% Pd(PPh3)4 in
THF (1 mL); b Isolated yield; c 1.2 equiv. (PhBO)3; d 2.0 equiv. (PhBO)3;
e 20 mol% Pd(PPh3)4
10
With the optimized conditions in hand, we tested a variety
of arylmethanols (Table 3). We found that, various reactive
functional groups survived very well under the standard
conditions, such as ether (1cc), acetal (1dc) and ester (1ec),
which can undergo subsequential transformation via
15 transition-metal catalysis.14 It is noteworthy that phenolic
hydroxyl group was tolerated in this catalytic system and the
synthesis of product (1bc) was first reported via cross-
coupling, which can be further coupled with different
organometallic reagents via Ni catalysis.13,15 Products (1hc)
20 and (1ic) can be furnished in synthetically useful yields,
although with a little variation from the standard conditions.
This methodology was also applied to fused ring (1gc) and
heterocycles (1jc and 1kc). We also investigated the simple
benzylic alcohols in this system. Product 4'-benzyl-N,N-
25 dimethylbiphenyl-4-amine (1lc) can be obtained albeit in
lower yield, which is concord with the precedent works. 7e,16
Scheme 3. Proposed mechanism
We proposed the catalytic cycle as scheme 3. Benzylic
alcohols 1a reacted with the phenylboroxine 2a to form the
50 key intermediate (I), which has two important effects to
facilitate the cross coupling: Firstly, coordination of boranes-
alcohols weakened the benzylic C-O bond in benzylic
alcohols. Secondly, the alcohols act as an inner-base to
activate the C-B bond. I subsequently underwent oxidative
55 addition to Pd(0) to generate the Pd(II) species (II), which
further proceeded the intramolecular (path a) or
intermolecular (path b) transmetallation to form key
intermediate (III). The reductive elimination of (III) gave the
desired product 1ac to fulfill the catalytic cycle by
60 regenerating the active catalytic Pd(0) species.
Further investigation also indicated that the broad scope of
boroxines highly expanded its application (Table 4).
Substituted arylboroxines, such as para-, meta-, ortho-
30 substituted boroxines, are suitable, offering the desired
diarylmethane derivatives in moderate to good yields. Both
electron-rich and electron-deficient arylboroxines were
suitable nucleophiles while electron-rich ones need harsher
conditions to give satisfactory yields (2ac, 2cc and 2ec).
35 Notably, the strong electron-withdrawing group dramatically
decreased the yield (2dc). It is important to note that C(sp2)-Cl
and C(sp2)-S bond survived, providing the opportunity to
furnish more complex compounds by orthogonal cross-
coupling.17 For the multi-substituted phenylboroxines, the
40 desired diarylmethane derivatives also can be furnished in
high yields with predictable selectivity, thus providing a
In summary, we for the first time developed a simple and
efficient cross-coupling of benzylic alcohol with arylboronic acid
derivatives through Pd-catalyzed C-O bond activation. Such a
transformation is atom- and step- economic to construct
65 diarylmethanes from benzyl alcohols in the absence of any
additives and bases under mild conditions. This method is a
privileged alternative to build up diarylmethane scaffold, thereby
complementing earlier catalytic methods typically from benzyl
halides and benzylic alcohol derivatives and Friedel-Crafts
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Chem. Soc., 2013, 135, 3307; (l) A. Correa and R. Martin, J.
Am. Chem. Soc., 2014, 136, 7253; (m) C. Zarate and R. Martin,
benzylation. Further investigations to expend the substrate scope,
to promote the efficiency and detailed mechanism are underway.
Acknowledgment
Support of this work by the “973” Project from the MOST of
China (2013CB228102, 2015CB856600) and NSFC (No.
21332001) is gratefully acknowledged.
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