Ruthenium(II)-catalyzed synthesis of isochromenes by C-H activation with weakly coordinating aliphatic hydroxyl groups

Article abstract of DOI:10.1002/chem.201400161

Cationic ruthenium(II) complexes have been employed for the highly effective oxidative annulation of alkynes with benzyl alcohols to deliver diversely decorated isochromenes. The hydroxyl-directed C-H/O-H functionalization process proceeded efficiently under an atmosphere of air. Detailed mechanistic studies were indicative of a kinetically relevant C-H metalation. Cationic ruthenium(II) complexes enabled highly efficient oxidative annulation of alkynes with benzylic alcohols to deliver isochromenes (see scheme). This aliphatic hydroxyl-directed C-H/O-H activation proceeded efficiently under an air atmosphere. Mechanistic studies were indicative of an irreversible C-H metalation as the rate-limiting step.

Full text of DOI:10.1002/chem.201400161

DOI: 10.1002/chem.201400161
Full Paper
&
CÀH Activation
Ruthenium(II)-Catalyzed Synthesis of Isochromenes by CÀH
Activation with Weakly Coordinating Aliphatic Hydroxyl Groups
Sachiyo Nakanowatari and Lutz Ackermann*[a]
Abstract: Cationic ruthenium(II) complexes have been em-
ployed for the highly effective oxidative annulation of al-
kynes with benzyl alcohols to deliver diversely decorated iso-
chromenes. The hydroxyl-directed CÀH/OÀH functionaliza-
tion process proceeded efficiently under an atmosphere of
air. Detailed mechanistic studies were indicative of a kineti-
cally relevant CÀH metalation.
Introduction
rhodium-catalyzed oxidative annulations of alkynes with a,a-
disubstituted benzylic alcohols.[4] On the contrary, the use of
less expensive[5] ruthenium catalysts[6] for direct CÀH function-
alization[7,8] through aliphatic hydroxyl-group-assisted[9] CÀH
bond activation has, thus far, proven elusive. Indeed, challeng-
ing aliphatic hydroxyl-directed CÀH activation, followed by CÀ
O bond formation, is limited to palladium[10] or rhodium[4] cat-
alysis, whereas activated phenols,[11,12] naphthols,[9a,13,14] and
enols[9b,15] have been actively utilized for metal-catalyzed CÀH
activation. Within our research program on sustainable CÀH
functionalization for organic synthesis,[16] we have now estab-
lished the first ruthenium(II)-catalyzed oxidative alkyne annula-
tions with challenging benzylic alcohols by hydroxyl-directed
oxidative CÀH activation.
Isochromene derivatives are important structural motifs in vari-
ous biologically active natural products (Figure 1).[1] Among
the established methods for the construction of isochro-
menes,[2] one of the most generally applicable strategies in-
Results and Discussion
Optimization studies
We commenced our studies by exploring the effect of repre-
sentative co-catalytic additives and solvents for the proposed
ruthenium-catalyzed alkyne annulation (Table 1 and Table S1 in
the Supporting Information). Among various co-catalytic addi-
tives, AgPF6 gave rise to the most satisfactory results (en-
tries 1–8). It is noteworthy that the cationic ruthenium(II) cata-
lysts derived from AgPF6 proved to be more effective than the
corresponding complexes generated with co-catalytic amounts
of AgBF4, AgOTf, AgSbF6, KPF6, NaPF6, NH4PF6, CsOAc, HOPiv
(pivalic acid), or KOAc. Among the representative solvents, t-
AmOH (2-methyl-2-butanol) was found to be optimal in terms
of catalytic activity (entry 8–10). The oxidative annulation pro-
ceeded efficiently under an atmosphere of air, and the amount
of Cu(OAc)2·H2O could be significantly reduced (entry 11).
Moreover, a test reaction confirmed that the transformation
did not proceed in the absence of the ruthenium catalyst
(entry 12).
Figure 1. Selected bioactive isochromene derivatives.
volves palladium-catalyzed annulations of alkynes with ortho-
halo-substituted benzyl alcohol derivatives.[3] Although this ap-
proach proved to be highly useful, it inherently requires ortho-
prefunctionalized halogenated benzyl alcohols as the sub-
strates. A more atom- and step-economical approach was ele-
gantly devised by Miura and Satoh and co-workers through
[a] S. Nakanowatari, Prof. Dr. L. Ackermann
Institut fꢀr Organische und Biomolekulare Chemie
Georg-August Universitꢁt
Tammanstrasse 2, 37077 Gçttingen (Germany)
Fax: (+49)551-39-6777
E-mail: lutz.ackermann@chemie.uni-goettingen.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400161.
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substituted isochromenes 3ba–3ia, bearing valuable electro-
philic functional groups, such as fluoro-, chloro-, bromo-,
nitro-, or ester substituents, were chemoselectively accessed.
Electron-rich alkyl-substituted alcohols were also found to be
appropriate substrates for producing annulation products 3ba
and 3ca. Substituents in the meta- or ortho-position of the aro-
matic moiety were well tolerated by the catalytic system. The
oxidative functionalization of meta-substituted alcohol 1j pro-
ceeded with excellent site selectivity at the less sterically en-
cumbered CÀH bond. In contrast, the use of the substrate 1k,
which has an electronegative fluorine atom in meta-position,
gave exclusively product 3ka,[17] corresponding to the activa-
tion of the kinetically more acidic CÀH bond. a,a-Dimethyl-1,3-
benzodioxole-5-methanol (1m) was converted to give tricyclic
product 3ma as a single isomer with 72% yield of isolated
product. 1-Phenylcyclopentanol (1n) was likewise reacted to
give spirocyclic isochromene 3na, albeit in a slightly dimin-
ished yield. The reaction was also found to be applicable to
heteroaromatic substrate 2-(thiophen-3-yl)propan-2-ol (1o).[18]
In this case, the CÀH activation occurred again at the more ki-
netically acidic site with excellent selectivity. In contrast with
tertiary alcohols, substrates with primary or secondary benzylic
alcohols unfortunately furnished unsatisfactory results.[19]
Table 1. Optimization of oxidative annulation of alkyne 2a with alcohol
1a.[a]
Entry
Additive
Solvent
Yield [%]
1
2
3
4
5
6
7
8
–
t-AmOH
t-AmOH
t-AmOH
t-AmOH
t-AmOH
t-AmOH
t-AmOH
t-AmOH
1,4-dioxane
m-xylene
t-AmOH
t-AmOH
7[b]
4[b]
31[b]
51
21
71
64
85
68
KOAc
AgBF4
AgOTf
AgSbF6
NaPF6
NH4PF6
AgPF6
AgPF6
AgPF6
AgPF6
AgPF6
9
10
11
12
60
85[c]
[d]
–
[a] Reaction conditions: 1a (1.00 mmol), 2a (0.50 mmol), [RuCl2(p-
cymene)]2 (5.0 mol%), additive (30 mol%), Cu(OAc)2·H2O (1.0 mmol), sol-
vent (3.0 mL), 1108C, under N2, yield of isolated product is shown. [b] GC-
MS conversion. [c] AgPF6 (20 mol%), Cu(OAc)2·H2O (50 mol%), under air (1
atm). [d] In the absence of [RuCl2(p-cymene)]2.
Next, we examined the scope of the reaction with a repre-
sentative set of alkynes 2 (Scheme 2). Under the optimized re-
action conditions a range of functionalized aryl- or alkyl-substi-
Reaction scope
With an optimized catalytic system in hand, we examined its
versatility in the oxidative annulation of alkyne 2a, utilizing di-
versely decorated alcohols 1 (Scheme 1). A variety of differently
Scheme 2. Ruthenium(II)-catalyzed oxidative annulation with alkynes 2.
tuted alkynes reacted efficiently. We observed that tolane de-
rivatives 2b–j, featuring either electron-donating or electron-
withdrawing groups, were converted by the cationic rutheniu-
m(II) complex with high chemoselectivity. Annulations of dia-
lkyl-substituted alkynes 2k–m with alcohol 1a produced the
desired dialkylisochromenes 3ak–am. Unfortunately, terminal
Scheme 1. Scope of ruthenium(II)-catalyzed oxidative annulation with alco-
hols 1.
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Table 2. Ruthenium(II)-catalyzed oxidative annulation of unsymmetrical al-
kynes 2.[a]
Entry
Alk
3/3’[b]
Isolated Yield [%]
3
3’
1
2
3
4
5
Me
Et
cPr
nBu
nHex
78:22
86:14
70:30
80:20
84:16
3an:64
3ao:68
3ap:58
3aq:64
3ar:71
3an’:10
3ao’:16
3ap’:20
3aq’:11
3ar’:12
[a] Reaction conditions: 1a (1.00 mmol), 2 (0.50 mmol), [RuCl2(p-cymene)]2
(5.0 mol%), AgPF6 (20 mol%), Cu(OAc)2·H2O (1.0 mmol), t-AmOH (3.0 mL),
110 8C, under N2, yield of isolated product is shown. [b] Determined by GC-
MS.
alkynes could not be employed for the proposed oxidative an-
nulations. To study the regioselectivity of this transformation
a set of unsymmetrically substituted alkynes were subjected to
the optimized reaction conditions (Table 2). The oxidative cou-
pling reaction occurred with moderate to high regioselectivity,
furnishing predominantly the isomers 3an–ar. The major iso-
mers, 3, displayed their aromatic moiety proximal to the
oxygen atom on the isochromene ring. The lower regioselec-
tivity observed with the cyclopropyl-substituted alkyne 2p[20]
may be attributed to its unique electronic properties.[21]
Scheme 4. Intermolecular competition experiments between alkynes 2.
Moreover, the in situ generated cationic ruthenium(II) cata-
lyst allowed the oxidative annulation of tolane 2a with a,a-di-
methylallyl alcohol (1p) to give the corresponding 2H-pyran
derivative 3pa in 23% yield. Notably, the efficacy of this alke-
nylic CÀH activation could be significantly improved when
AgOTf was used as the co-catalytic additive (Scheme 3).
periments revealed that electron-rich derivatives were more re-
active than electron-poor derivatives (Scheme 5). Finally, we
performed studies with the isotopically labeled substrate [D]5-
Scheme 3. Ruthenium(II)-catalyzed oxidative annulation with allylic alcohol
1p.
Mechanistic studies
Scheme 5. Intermolecular competition experiment between alcohols 1.
Given the high catalytic activity of the optimized ruthenium(II)
catalyst, we became interested in delineating its mode of
action. To this end, we performed intermolecular competition
experiments between differently substituted acetylenes 2.
When alkyl- and phenyl-substituted alkynes were employed,
the electron-deficient substrate 2a reacted preferentially
(Scheme 4a). On using differently substituted tolane deriva-
tives the electron-rich alkyne 2b reacted preferentially
(Scheme 4b). For alcohols 1, intermolecular competition ex-
1a (Scheme 6a), and a D/H exchange reaction could not be
detected. Additional kinetic studies were suggestive of an irre-
versible CÀH bond metalation with a kinetic isotope effect
(KIE) of kH/kD ꢀ5.3 (Scheme 6b). A KIE of this magnitude is in
good agreement with a concerted acetate-assisted metalation
transition state as the model for the CÀH activation.[22]
Based on these mechanistic studies, we propose the ruthe-
nium(II)-catalyzed oxidative annulation to begin with an ace-
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Experimental Section
[RuCl2(p-cymene)]2
5.0 mol%), AgPF6
(15.3 mg,
(25.3 mg,
20 mol%), alkyne 2 (0.50 mmol),
Cu(OAc)2·H2O (50 mg, 50 mol%), t-
AmOH (3.0 mL), and alcohol
1 (1.00 mmol) were placed into
a 25 mL Schlenk tube equipped
with a septum, under air. The reac-
tion mixture was stirred at 1108C
for 22 h under an atmosphere of
air. At ambient temperature, the
reaction mixture was diluted with
saturated aqueous NH4Cl/NH3 (1:1,
10 mL) and extracted with EtOAc
(100 mL). The organic layer was
washed with brine (15 mL) and
dried over Na2SO4. After filtration
and evaporation of the solvents in
vacuo, the crude product was puri-
fied by column chromatography
on silica gel (n-hexane/EtOAc:
1000/3) to afford the desired prod-
uct 3.
Scheme 6. Experiments with isotopically labeled compound 1a-[D5].
tate-assisted irreversible cycloruthenation, followed by coordi-
nation of alkyne 2 (Scheme 7). Subsequent regioselective mi-
gratory insertion delivers the seven-membered intermediate 5,
which upon reductive elimination furnishes the desired iso-
chromene 3.
Acknowledgements
Support by the European Research Council under the Europe-
an Community’s Seventh Framework Program (FP7 2007–
2013)/ERC Grant agreement 30753 and the JASSO (fellowship
to S.N.) is gratefully acknowledged. We further thank Ms. Mar-
gherita Donati and Dr. Vedhagiri S. Thirunavukkarasu for pre-
liminary studies.
Keywords: annulations
catalysis · reaction mechanisms · ruthenium
·
CÀH activation
· homogeneous
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Scheme 7. Proposed catalytic cycle.
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Received: January 14, 2014
Published online on March 27, 2014
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