Gold(I)-catalyzed Benzylation of (Hetero)aryl Boronic Acids with (Hetero)benzyl Bromides by the Strategy of a SN2-type Reaction

Article abstract of DOI:10.1002/asia.201800923

Herein, the first example of gold-catalyzed benzylation of (hetero)aryl boronic acids with (hetero)benzyl bromides to give the corresponding cross-coupling products in moderate to good yields is reported. The reaction proceeds through a possible intermolecular S_N2-type reaction pathway to give a wide variety of di(hetero)arylmethanes as the desired products. An intriguing reaction mechanism has been proposed on the basis of control experiments, ³¹P-NMR spectroscopic detection and DFT calculations.

Full text of DOI:10.1002/asia.201800923

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Gold(I)-catalyzed benzylation of (hetero)aryl boronic acids with
(hetero)benzyl bromides by the strategy of a SN2-type reaction
Authors: Wenqing Zang, Yin Wei, and Min Shi
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To be cited as: Chem. Asian J. 10.1002/asia.201800923
Link to VoR: http://dx.doi.org/10.1002/asia.201800923
A Journal of
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Chemistry - An Asian Journal
DOI: 10.1002/chem.201((will be filled in by the editorial staff))
Gold Catalysis
Gold(I)-catalyzed benzylation of (hetero)aryl boronic acids with (hetero)benzyl
bromides by the strategy of a SN2-type reaction
Wenqing Zang,† Yin Wei, †* Min Shi†*
produce diarylmethanes has never been reported before.
Abstract: The first example of gold-catalyzed benzylation of
(hetero)aryl boronic acids with (hetero)benzyl bromides to give the
corresponding cross coupling products in moderate to good yields is
reported in this paper. The reaction proceeded through a possible
intermolecular SN2-type reaction pathway to give a wide variety of
di(hetero)arylmethanes as the desired products. The intriguing
reaction mechanism has been proposed on the basis of control
experiments, 31P-NMR spectroscopic detection and DFT
calculations.
Herein, we wish to report the first example of gold-catalyzed
benzylation of (hetero)aryl boronic acids with (hetero)benzyl
bromides without the assistance of any oxidants. Unlike allyl
bromides, the more electrophilic (hetero)benzyl bromides react with
a wide range of in situ generated PPh3Au(aryl) species through a
possible SN2-type reaction, which avoids using any external
oxidants and produces the desired cross coupling products in high
yields. This reaction expands the scope of gold-catalyzed cross-
coupling reactions in a fundamental sense and offers effectively
synthetic methods of di(hetero)arylmethanes bearing different
functional groups (Scheme 1c).
The construction of carbon-carbon bonds is fundamental to
organic synthesis, while transition metal catalyzed cross-coupling
reaction has proven an efficient way to construct C-C bond.[1]
Compared with Negishi[2] and Kumada’s coupling reactions,[3] the
Suzuki–Miyaura cross coupling reaction,[4] using organoboron
compounds as reagents, is widely applied due to its superior features
of easy manipulation, ready availability and low toxicity. Extensive
studies on this coupling reaction mediated/catalyzed by Pd or Cu
have become the most established options[5] (Scheme 1a).
Nevertheless, the discovery of new methodologies involving other
transition metals with the possibility of providing better substrate
tolerance is still highly desirable.
During the last decade, the gold catalyst has attracted
considerable attention due to its unique performance as Lewis acid
catalyst, π–activator or carbenoid intermediate.[6] There are also a
wide variety of Au-catalyzed cross-coupling reactions have been
developed through redox process in this period.[7] However, because
of the high oxidation potential of the Au(I)/Au(III) redox couple (E0
= +1.41 V in water[8]), using strong external oxidants such as
Selectfluor or PhI(OAc)2 is hard to be avoided. In recent years,
many efforts have been devoted to solve this problem, and some
Scheme 1. Gold catalyzed reactions of alkyl or aryl electrophiles
with arylboron reagents
We decided to investigate the reaction condition by applying 1a
and 2a as model substrates, and the results are summarized in Table
1. Firstly, using PPh3AuCl (5 mol%) as the catalyst, 3a was given in
improvements have been achieved by using substrates or catalysts
o
9]
with specific structures.[7i,
In 2014, Toste and co-workers
only 33% yield in CD3CN at 60 C (C = 0.2 M) (Table 1, entry 1).
This result indicated that the previously reported conditions for the
coupling of primary allyl electrophiles might not be suitable for
benzyl bromides. After examining the effects of different
equivalents of 2a, we decided to use 4.0 eq. of 2a to further
investigating the reaction conditions, and the highest yield of 3a was
70% under these standard conditions if using CDCl3 as the solvent
described a C(sp2)–C(sp3) cross-coupling reaction of allylation of aryl
boronic acid, catalyzed by a special bimetallic gold catalyst
[DPPA(AuCl)2] (DPPA = N-(diphenylphosphanyl)-N-isopropyl-1,1-
diphenylphosphanamine) through an Au(I)/Au(III) redox pattern
without the existence of external oxidants[10] (Scheme 1b). To our
surprise, gold-catalyzed Suzuki–Miyaura coupling reaction to
o
(Table 1, entries 2-4). Raising the temperature to 80 C afforded 3a
in slightly lower yield (66%), while none of the desired product was
detected if the reaction was run at room temperature (Table 1,
entries 5 and 6). Next, we found that the yield of 3a slightly
increased to 72% when the concentration of the reaction mixture
was increased to 0.25 M (Table 1, entry 7). The solvent effect was
also investigated thoroughly. The examination of several other
solvents such as DCM, 1,2-DCE and THF revealed that 1,2-DCE
was the suitable one, affording 3a in 89% and 85% yield at 60 oC or
80oC respectively (Table 1, entries 9-11). Furthermore, using K2CO3
as base provided no product while CsF only gave 23% yield of 3a
(Table 1, entries 12, 13) When we changed BnBr to BnI, the yield of
3a experienced a sharp decrease to 39% and no desired product was
given when 2a was switched to benzyl alcohol or benzyl choride
(Table 1, entries 14, 15). The control experiment indicated that no
W.-Q. Zang, Dr. Y. Wei, Prof. Dr. M. Shi
†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, 345 Ling-Ling Lu, Shanghai 200032, China.
Email: weiyin@sioc.ac.cn, mshi@sioc.ac.cn. Fax: 86-21-64166128
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.2015xxxxx.
1
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10.1002/asia.201800923
Chemistry - An Asian Journal
reaction occurred in the absence of gold catalyst (Table 1, entry 16)
As a consequence, we identified that using PPh3AuCl (5 mol%) as
the catalyst and carrying out the reaction in 1,2-DCE at 60 oC or 80
oC as the two standard conditions for this transformation.
Table 1. Optimization of the reaction condition for the
transformation of 1a and 2a to 3a
After optimizing the conditions, we next surveyed the substrate
scope of this reaction, and the results are summarized in Table 2.
We first examined the reaction outcomes using differently
substituted (hetero)benzyl bromide as substrates. As can be seen
from Table 2, for substrates 2a-2k, all of the reactions proceeded
smoothly in 1,2-DCE to furnish the corresponding products 3a-3k in
58-87% yields. Especially, the substrates bearing functional groups
such as CN (2h) or NO2 (2i) and vinyl group (2j) afforded the
desired products in good yields, ranging from 68% to 75%,
respectively. The reaction also performed well when 2-
(bromomethyl)thiophene or 2-(bromomethyl)furan was used as
substrates, affording the corresponding products 3l in 77% yield and
3m in 72% yield, respectively. Then we investigated a wide range of
aryl boronic acids, the more electron-rich aryl boronic acids
produced the corresponding diarylmethane products in higher yields
such as products 3o and 3p, while the less electron-rich ones gave
the desired products in relatively lower yields from product 3q to
product 3t. In addition, this coupling reaction also tolerated a variety
of heteroaryl boronic acids, giving the desired products 3u to 3y in
good yields, fluctuating from 56% to 72%. The structure of 3w has
been further confirmed by X-ray diffraction. The corresponding
ORTEP drawing is shown in Table 2 and the CIF data are
summarized in the Supporting Information.
To clarify the reaction mechanism,
a couple of control
experiments had been carried out as shown in Scheme 2. We firstly
used stoichiometric amount of PPh3Au(naphthyl) (for the
preparation, see Supporting Information for the more synthetic
details) to react with benzyl bromide in 1,2-DCE at 60 oC, affording
3a in 83% yield, in which PPh3AuBr was detected by 31P-NMR
spectrum during this transformation (Scheme 2a). Moreover, we
used catalytic amount PPh3Au(naphthyl) as catalyst to carry out this
reaction under the standard condition, still giving 3a in high yield
(84%) (Scheme 2b). In this control experiment, only one signal of
PPh3Au(naphthyl) was identified by 31P-NMR spectrum after the
reaction completed, suggesting that PPh3Au(naphthyl) is the only
catalytic species in this reaction. Furthermore, none of 3a was
detected in the absence of PPh3Au(naphthyl) or PPh3AuCl catalyst
(Scheme 2c). These control experiments suggest that
PPh3Au(naphthyl) is not only the actively catalytic species, but also
the key intermediate in this catalytic reaction system.
Table 2. Reaction scope of (hetero)aryl boronic acid 1 and
(hetero)benzyl bromide 2.a,b
2
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10.1002/asia.201800923
Chemistry - An Asian Journal
propose that this step is the rate-determining step of this pattern.
Int2 was then given as an intermediate and a following TSred for the
reductive elimination takes place at 22.6 kcal/mol. DFT calculation
shows that PPh3Au(naphthyl) reacts with benzyl bromide through a
SN2-type pathway in 1,2-DCE is more reasonable due to the gap
between the peak points of energy barriers of both patterns is more
than 3.5 kcal/mol. However, the possibility of a redox pathway
could not be completely excluded at this stage if the reaction was
carried out at higher temperature.
Scheme 2. Control experiments.
Next, we embarked on a DFT study to gain further insights into
the reaction mechanism, and specifically we focused on the step of
in situ formed PPh3Au(naphthyl) (Int1) over benzyl bromide
(2a).[10-11] All calculations have been performed at SMD/M06/6-
311+G(d,p)/SDD//M06/6-31G(d)/SDD level with Gaussian 09
program.[12] The solvation Gibbs free energy profile in 1,2-DCE for
the suggested reaction pathway is shown in Scheme 3 (the ∆G298
(kcal/mol) (see Supporting Information for the details). We
proposed two possible reaction patterns of this step. In the first
reaction pattern, the naphthyl moiety of Int1 acting as a nucleophile,
whose C1 position was activated by gold catalyst, attacks 2a through
a SN2 pathway that is similar to the reported mechanism of copper-
catalyzed allylation of arylboronates.[5a, 5b] The transition state of
this process is TS1 with an energy barrier of 24.6 kcal/mol. Then,
the product 3a is formed with a large ∆G298 of -35.9 kcal/mol in 1,2-
DCE, indicating that this reaction is thermodynamically favorable.
Another possibility is through Au(I)/Au(III) redox pathway. In this
pattern, BnBr reacts with Int1 through either ordinary or SN2-like
oxidative addition [11a, 13]via transition states TSOX1 (31.9 kcal/mol)
or TSOX2 (28.1 kcal/mol), respectively. These energy barriers are the
two largest ones among the whole reaction pathway, and thus we
Scheme 4. Proposed mechanism.
According to these studies, we proposed the most possible
reaction mechanism that the PPh3AuCl firstly reacts with 1-
naphthylboronic acid through a transmetallation with the assistance
of Cs2CO3.[14] Then, the in situ formed PPh3Au(naphthyl) attacks
electrophilic benzyl bromide through a possible SN2 process and
finally affords diarylmethane as the corresponding product (Scheme
4).
Scheme 3. DFT calculations on the possible reaction pathways
3
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10.1002/asia.201800923
Chemistry - An Asian Journal
Research Program of China ((973)-2015CB856603), the
Strategic Priority Research Program of the Chinese Academy of
Sciences, Grant no. XDB20000000 and sioczz201808, the
National Natural Science Foundation of China (20472096,
21372241, 21572052, 20672127, 21421091, 21372250,
21121062, 21302203, 20732008, 21772037 and 21772226), and
the Fundamental Research Funds for the Central Universities
(222201717003).
In conclusion, we have discovered the first example of gold-
catalyzed arylation of benzyl bromides with aryl boronic acids
under mild condition, giving diarylmethane in moderate to good
yields. The intriguing reaction mechanism is probably the
activated naphthyl attack BnBr directly through an unreported
SN2 pathway and has been thoroughly investigated by control
experiments and DFT calculations. Further investigations on the
mechanistic details and exploration of new methodology based
on this kind of reaction pattern are currently underway in our
laboratory.
Conflict of interest
The authors declare no conflict of interest.
Received: ((will be filled in by the editorial staff))
Acknowledgements
Published online on ((will be filled in by the editorial staff))
We are grateful for the financial support from the National Basic
Keywords: gold catalysis, cross coupling reaction, arylboronic acids, benzyl bromides
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Gold Catalysis
Wenqing Zang,†, Yin Wei,†* Min Shi†*
__________ Page – Page
Gold(I)-catalyzed benzylation of
(hetero)aryl boronic acids with
(hetero)benzyl bromides by the
strategy of a SN2-type reaction
The first example of gold-catalyzed benzylation of (hetero)aryl boronic acids
with (hetero)benzyl bromides through an intermolecular SN2-type reaction is
reported.
6
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