Ruthenium(ii)-catalyzed electrooxidative [4+2] annulation of benzylic alcohols with internal alkynes: Entry to isocoumarins

Article abstract of DOI:10.1039/c8cc08759h

A new ruthenium(ii)-catalyzed electrooxidative [4+2] annulation of primary benzylic alcohols with internal alkynes is described, which enables benzylic alcohols as weakly directing group precursors to access isocoumarins via multiple C-H functionalization. The reaction works well with a broad substrate scope, tolerates a wide range of functional groups, and incorporates practically the isocoumarin unit into diverse bioactive molecules. Mechanistic studies indicate that activation of aryl C(sp²)-H bonds is achieved through the generation of the active benzoyloxy-Ru(ii) intermediates via oxidation of benzylic alcohols using an electrooxidative Ru(ii) catalyst.

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Ruthenium(II)-catalyzed electrooxidative [4+2]
annulation of benzylic alcohols with internal
alkynes: entry to isocoumarins†
Cite this:DOI: 10.1039/c8cc08759h
Received 2nd November 2018,
Accepted 19th December 2018
Mu-Jia Luo,a Ming Hu,b Ren-Jie Song,*b De-Liang He*a and Jin-Heng Li
*
abc
DOI: 10.1039/c8cc08759h
rsc.li/chemcomm
A new ruthenium(II)-catalyzed electrooxidative [4+2] annulation of concise site-selective C–H activation to construct valuable
primary benzylic alcohols with internal alkynes is described, which O-heterocycles. However, such successful examples are rare
enables benzylic alcohols as weakly directing group precursors to and are all limited to tertiary alcohols presumably due to the
access isocoumarins via multiple C–H functionalization. The reaction challenge in the competitive oxidation of primary and secondary
works well with a broad substrate scope, tolerates a wide range of alcohols to carbonyl compounds and their relatively weaker
functional groups, and incorporates practically the isocoumarin unit coordination ability. Typically, the Satoh/Miura group5d and
into diverse bioactive molecules. Mechanistic studies indicate that the Ackermann group5e have independently reported oxidative
activation of aryl C(sp2)–H bonds is achieved through the generation C–H activation and annulation of tertiary benzylic and allyl
of the active benzoyloxy–Ru(II) intermediates via oxidation of benzylic alcohols with alkynes using the Rh or Ru catalysts, which is
alcohols using an electrooxidative Ru(II) catalyst.
driven by the use of the weakly coordinating aliphatic hydroxyl
groups. We envisioned that by using the transition-metal oxidative
A transition-metal-catalyzed annulation reaction involving inter- catalysis, benzylic alcohols would be transformed in situ into
molecular C–H functionalization has become a powerful and higher reactive intermediates containing an effective weakly
atom-economic tool for building complex cyclic systems from readily coordinating directing group, thereby offering a conceptually
available starting materials,1,2 which avoids prefunctionalization of novel approach for C–H functionalization and annulation.
the C–H bonds through the generation of organometallic C-[M]
Herein, we report a new ruthenium(II)-catalyzed electro-
species at the ipso position. However, the development of such a oxidative3w,4f,6 [4+2] annulation of primary benzylic alcohols
method is generally hindered by the requirement of strongly ortho- with internal alkynes that access isocoumarins via the formation
coordinating directing groups (often nitrogen-based groups), of multiple chemical bonds (e.g., C(sp2)–C(sp2) bond, and C(sp2)–
thereby limiting the cyclic structure choice partly because these O bonds) (Scheme 1b). Mechanistic studies show that benzylic
directing group elements are generally integrated into the resulting alcohols are converted into effective benzoyloxy–Ru(II) inter-
cyclic frameworks.1 Recently, strategies for annulation of sub- mediates, not aryl acids, for achieving C–H functionalization.
strates, especially acid derivatives2–4 and alcohols,5 containing a Notably, isocoumarins are frequently found in natural products
weakly ortho-coordinating directing group with 2p components and pharmaceuticals with strong bioactivities.7
using various transition-metal (e.g., Pd, Rh, Ir, Co, Ru) catalysts
have been successfully established, which enabled the formation
of five-membered to six-membered cyclic systems. Attractive
methods include transition-metal-catalyzed annulation with
aryl-containing alcohols through C–H activation,5 and offer a
a State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University,
Changsha 410082, China. E-mail: delianghe@hnu.edu.cn, jhli@hnu.edu.cn
b Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources
Recycle, Nanchang Hangkong University, Nanchang 330063, China.
E-mail: srj0731@hnu.edu.cn
c State Key Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou 730000, China
† Electronic supplementary information (ESI) available. CCDC 1874275. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c8cc08759h
Scheme 1 Ru-catalyzed [4+2] annulation of benzylic alcohols.
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Initial experiments began with annulation of benzyl alcohol and MeCN, were less reactive (entries 14–16). The reaction is
1a with 1,2-diphenylethyne 2a under the reported reaction specific to the Ru catalyst, as two Ru catalysts, [RuCl2(COD)] and
conditions5d,e (eqn (3); Scheme 1). The annulation reaction could [Ru(bpy)3]Cl2, showed no catalytic activity (entry 17) and the
not proceed when using the reported Rh5d or Ru5e catalyst com- reaction could not proceed without Ru catalysts (entry 18).
bined with the conventional oxidants, such as AgSbF6/Cu(OAc)2/air, Reducing the loading of [RuCl2(p-cymene)]2 to 5 mol% led to
Cu(OAc)2 and O2 (eqn (3)). As shown in Table 1, the reaction could diminishing yield (entry 19), and raising the loading to 15 mol%
occur to enable access to the desired isocoumarin 3aa in 64% yield gave the similar results to that obtained at 10 mol% (entry 20).
under the electrocatalysis conditions ([RuCl2(p-cymene)]2, NaOPiv, a However, the yield of 3aa decreased in either argon (entry 21) or
graphite rod anode and a platinum plate cathode at 4 mA constant O2 (entry 22) replaced by air, and O2 showed the lowest
t
current in AmOH/H2O at 100 1C) (entry 1). Raising the reaction efficiency. Use of a 1 : 2 alcohol 1a/alkyne 2a ratio decreased
temperature to 110 1C caused no further improvement of the yield the yield (entry 23).
(entry 2), but lowering the temperature to 80 1C dramatically
With the optimal reaction conditions in hand, the substrate
decreased the yield (entry 3). A higher (8 mA) and a lower (2 mA) scope of the Ru(II)-catalyzed electrooxidative [4+2] annulation
constant current both had a negative effect (entries 4 and 5). In protocol was explored (Table 2 and Scheme 2). This reaction
the absence of electric current (entry 6), the reaction could not worked well with a wide range of internal alkynes 2b–m and
form 3aa in the presence of 10 mol% [RuCl2(p-cymene)]2, but it 2p–t (3ab–am and 3ap–at), but was unsuitable for terminal
afforded 51% yield of 3aa at 100 mol% [RuCl2(p-cymene)]2 (entry alkynes 2n–o (3an–ao). For diaryl alkynes, several substituents,
7). Brief screening of bases indicated that 2 equiv. NaOPiv was such as Me, t-Bu, MeO, Cl and F, on the aryl ring were tolerated
optimal (entries 1 and 8–12), as other bases, including NaOAc, well, and the substitution electron effect had an obvious influence
KOAc, Na2CO3 and K2CO3, were less effective. Using a graphite on the yield (3ab–ag). While alkynes 2b–d and 2g possessing an
rod cathode instead of a platinum plate cathode exhibited lower electron-donating substituent (e.g. Me, t-Bu, and MeO) formed
efficiency (entry 13). Other solvents, such as tBuOH, HFIP 3ab–ad and 3ag, respectively, in high yields, alkyne 2f having a F
group delivered 3af with diminishing yield (48%). 1,2-Di(thiophen-
2-yl)ethyne 2h was also suitable for accessing 3ah. In the case of
Table 1 Screening of optimal reaction conditionsa
Table 2 Variation of benzylic alcohols (1) and alkynes (2)a
Entry [Ru] (mol%) I (mA) Base (equiv.) Solvent T (1C) Yieldb (%)
1
2
10
10
10
10
10
10
100
10
10
10
10
10
10
10
10
10
10
0
4
4
4
8
2
0
0
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
NaOPiv (2)
NaOPiv (2)
NaOPiv (2)
NaOPiv (2)
NaOPiv (2)
NaOPiv (2)
NaOPiv (2)
NaOPiv (1)
NaOAc (2)
KOAc (2)
Na2CO3 (2)
K2CO3 (2)
NaOPiv (2)
NaOPiv (2)
NaOPiv (2)
NaOPiv (2)
NaOPiv (2)
NaOPiv (2)
NaOPiv (2)
NaOPiv (2)
NaOPiv (2)
NaOPiv (2)
NaOPiv (2)
tAmOH 100
tAmOH 110
tAmOH 80
tAmOH 100
tAmOH 100
tAmOH 100
tAmOH 100
tAmOH 100
tAmOH 100
tAmOH 100
tAmOH 100
tAmOH 100
tAmOH 100
tBuOH 100
64
60
9
47
38
0
51
53
51
54
52
53
5
44
38
28
Trace
0
3
4c
5
6
7
8
9
10
11
12
13d
14
15
16
17e
18
19
20
21f
22g
23h
HFIP
100
MeCN 100
tAmOH 100
tAmOH 100
tAmOH 100
tAmOH 100
tAmOH 100
tAmOH 100
tAmOH 100
5
35
63
56
31
52
15
10
10
10
a
Reaction conditions: undivided cell, graphite rod anode, platinum
plate cathode, constant current = 4 mA, 1a (0.6 mmol), 2a (0.3 mmol),
[RuCl2(p-cymene)]2, base, solvent/H2O (3 : 1; 6 mL), and 24 h. Some by-
products, including benzil (4a), benzaldehyde (5a) and benzoic acid
b
(6a), were observed as determined by GC-MS analysis. Isolated yield.
c
a
Benzil (4a) was isolated in 41% yield that relied on the amount of 2a.
Reaction conditions: undivided cell, graphite rod anode, platinum
d
e
Graphite rod cathode instead of a Pt cathode. Using [RuCl2(COD)] or plate cathode, constant current = 4 mA, 1 (0.6 mmol), 2 (0.3 mmol),
f
t
[Ru(bpy)3]Cl2 instead of [RuCl2(p-cymene)]2. Using argon balloon. [RuCl2(p-cymene)]2 (10 mol%), NaOPiv (2 equiv.), AmOH/H2O (3:1; 6 mL),
g
h
Using O2 balloon. 1a (0.3 mmol) and 2a (0.6 mmol).
100 1C, and 24 h.
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experiments with alcohol 1j alone were performed under the
optimal conditions, affording only 17% yield of aldehyde 5j and
a trace amount of acid 6j (eqn (5)). However, no electric current
led to a trace amount of aldehyde 5j and acid 6j even in the
presence of 100 mol% Ru catalyst (eqn (5)). Transformation of
benzaldehyde 1n under the optimal conditions afforded only
48% yield of 3aa, whereas benzoic acid 1o was efficiently
converted into 3aa in 70% yield (eqn (6)). These results suggest
that the annulation reaction proceeds not via the direct formation
of aldehyde or acid intermediates, and electricity is needed (entry
6; Table 1). Surprisingly, two oxygen atoms in 3aa are main the 18
O
isotope when using H218O (eqn (7)), but no 18O isotope exchange of
H218O with product 3aa occurs (eqn (8)). We found that 18O isotope
exchange of H218O with benzyl alcohol 1a also took place (eqn (9)).
Experiments using 18O2 further support that two oxygen atoms are
from H2O, and not from air (eqn (7)). The results of intermolecular
competition experiments show that the reaction of electron-rich
arenes 1 or arylalkynes 2 is preferential (eqn (10) and (11)). The
reactions with isotopically labelled D2O suggest a reversible C–H
cleavage via organometallic C–Ru formation, but the alcohol group
is not enough to activate the ortho-C–H bond with the active RuII
species (eqn (12)). These results support the fact that coordination
of a benzylic alcohol with the active RuII species first occurs to
form an alcohol–Ru(II) intermediate, which sequentially undergoes
oxidation to afford an effective benzoyloxy–Ru(II) intermediate (not
the acid monomer) and then generates the organometallic C–Ru(II)
species via C–H activation.
The electrochemical activation of the Ru catalyst and the
C–H bond by means of cyclovoltammetric analysis was examined
(Fig. S1; ESI†). The oxidation potential peak of [RuCl2(p-cymene)]2
Scheme 2 Control experiments.
unsymmetrical alkyl aryl alkynes 2i–j the reaction proceeded left shifts from 0.142 VAg/AgCl to À0.021 VAg/AgCl in the presence of
selectively (3ai–aj).8 Dialkyl alkynes 2k–m also succeeded in NaOPiv, suggesting the coordination of [RuCl2(p-cymene)]2 with
assembling 3ak–am.
the PivOÀ group. A combination of [RuCl2(p-cymene)]2 and alcohol
This electrooxidative [4+2] annulation was applicable to an 1a gives an oxidation peak at 0.237 VAg/AgCl, verifying the formation
array of primary benzylic alcohols 1b–k. Tolerated on the aryl of alcohol–Ru(II) species.
ring moiety were a number of substituents, including Me, MeO,
The possible mechanism for this electrooxidative [4+2]
MeS, Br and Cl, which make the corresponding benzylic alcohols annulation protocol is proposed in Scheme 3.2–6 Initially, the
1b–h highly effective four-atom annulation units (3ba–ha), albeit alcohol–Ru(II) species A is formed via complexation of alcohol
affecting the yields by their electronic properties and positions. 1a with the active PivO-based Ru(II) species that is generated
For Me-substituted benzylic alcohols, the reactivity order in situ from [RuCl2(p-cymene)]2 coordinated with the PivOÀ
increased from ortho (3ba) to meta (3ca) to para substitution (3da) group.3p–w,5d,e Anodic single electron oxidation of the O–Ru
in terms of yields. Importantly, halogen atoms, including Br and Cl,
were tolerated, leaving ample room for further modification. Using
diMeO-substituted benzylic alcohol 1j enabled the formation of 3ja
in 63% yield. The reaction could be competent to furnish valuable
thieno[2,3-c]pyran-7-one 3ka. Synthetic utility of this electrooxidative
[4+2] annulation reaction in modified bioactive molecule derivatives
(3ap–at) was also assessed.9 Alkynes 2p–s built on the backbones of
useful species9a,b and amino esters9c,d were converted efficiently
to the desired complex 1H-isochromen-1-ones 3ap–3as in
54–70% yields. The estrone unit9e was also readily incorporated
into the resulting 1H-isochromen-1-one 3at by annulation of
estrone-containing alkyne 2t with benzyl alcohol 1a.
As shown in Scheme 2, two secondary benzylic alcohols 1l–m
were converted into ketones 5l–m, not the expected annulation
Scheme 3 Possible mechanism.
products 3 (eqn (4)). To understand the mechanism, control
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C, which sequentially undergoes a nucleophilic reaction with
H2O, oxidation and C–H activation cascades to afford the
organometallic C–Ru(II) intermediate E. Coordination of inter-
mediate E with alkyne 2a gives intermediate F, which rapidly
executes migratory alkyne insertion to form the seven-membered
ruthena(II)cycle G.3p–w,5d,e The electronic properties of the aryl
groups might be more efficiently stabilized by the vinyl–Ru
intermediate G, thus resulting in a high regioselectivity in the
case of unsymmetrical alkynes. Finally, reductive elimination of
intermediate G delivers the desired product 3aa and the Ru(0)
complex, followed by reoxidation through anodic oxidation of the
Ru(0) complex to regenerate the active PivO-based Ru(II) species.
In summary, we have developed the first Ru(II)-catalyzed
electrooxidative [4+2] annulation of benzylic alcohols with
internal alkynes, where benzylic alcohols act as weakly directing
group precursors to enable the formation of isocoumarins through
multiple C–H functionalization. The reaction is distinguished by its
success achieved through the use of a Ru(II)-catalyzed electro-
oxidative strategy with exquisite regio- and site-selectivity, as the
conventional terminal oxidants, including Cu(II) salts, Ag(I) salts
and O2, had no effect on this event. Importantly, the reaction
features facile incorporation of an isocoumarin unit into bio-
active structural systems.
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We thank the National Natural Science Foundation of China
(No. 21625203 and 21762030) for financial support.
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Conflicts of interest
There are no conflicts to declare.
Notes and references
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