Synthesis of isoquinolines from α-aryl vinyl azides and internal alkynes by Rh-Cu bimetallic cooperation

Article abstract of DOI:10.1002/anie.201101009

Catalysts in a relay: A synthetic method for delivering highly substituted isoquinolines has been developed (see scheme; Cp =C₅Me₅, DMF=N,N-dimethylformamide, TEMPO=2,2,6,6-tetramethylpiperidine-1-oxyl). A preliminary mechanistic study showed that the rhodium and copper cooperate synergistically in the multistep sequence.

Full text of DOI:10.1002/anie.201101009

DOI: 10.1002/anie.201101009
Synthetic Methods
Synthesis of Isoquinolines from a-Aryl Vinyl Azides and Internal
Alkynes by Rh–Cu Bimetallic Cooperation**
Yi-Feng Wang, Kah Kah Toh, Jian-Yuan Lee, and Shunsuke Chiba*
Multimetallic catalytic systems including their synergystic
cooperation can potentially achieve chemical transformations
that are unprecedented with monometallic catalysts. In spite
of the recent and significant development of the multimetallic
catalytic reactions in organic synthesis,[1,2] it is still a challenge
to achieve the rational design of artificial multimetallic
catalytic systems and their application to organic transforma-
tions that possess distinct functionality and are highly
efficient. A hetero-bimetallic system that performs sequential
reactions is of great interest, in which one catalyst achieves
the initial step to give an intermediate that is relayed to
another for the next transformation to produce the final
product.[3]
We have recently been interested in the application of
vinyl azides for the synthesis of aza-heterocyles.[4] One of the
intriguing chemical features of vinyl azides is their thermal
decomposition into highly strained three-membered cyclic
imines, 2H-azirines, which could be regarded as an equivalent
of a vinyl nitrene (Scheme 1a).[5] Our current study focuses on
the use of these nitrogen atoms derived from a-aryl vinyl
Scheme 1. a) Thermal decomposition of vinyl azides into 2H-azirines.
b) This work. Cp*=C5Me5. TEMPO=2,2,6,6-tetramethylpiperidine-1-
oxyl.
[6]
À
azides to direct a metal complex for ortho C H metallation,
Davies and co-workers showed that the combined use of
[{Cp*RhCl2}2] and NaOAc generates [Cp*Rh(OAc)n] species
À
À
which might be followed by a C C and C N bond-formation
sequence to construct aza-heterocyclic frameworks. Herein,
we report the synthesis of highly substituted isoquinolines
from readily available a-aryl vinyl azides and internal alkynes
À
and results in fission of certain C H bonds with the aid of an
intramolecular directing group such as the imino group to
afford rhodacycle complexes.[7] This chemistry has been
thoroughly investigated in terms of the reactivity of the
rhodacycles as well as the reaction mechanism of the cyclo-
metallation reported by Jones et al.[8] Recently, this method
was successfully applied to various kinds of heterocycle
syntheses involving the insertion of alkynes.[9,10] Based on
under
(Scheme 1b).
a
rhodium/copper bimetallic catalytic system
preliminary mechanistic investigation
A
revealed that both the rhodium and copper are prerequisites
for achieving the catalytic cycle, and play their particular roles
with synergistically during the multistep sequence. The
present transformation is carried out in the following steps:
1) CuI-mediated denitrogenative reductive formation of
imine derivatives from a-aryl vinyl azides probably through
Table 1: Optimization of the reaction conditions.[a]
À
the C N bond cleavage of putative 2H-azirine intermediates,
2) formation of an iminyl rhodium(III) species and their ortho
À
À
C H rhodation, alkyne insertion, and C N bond reductive
elimination to afford isoquinolines.
Entry [{Cp*RhCl2}2] Additive 1
Additive 2
(mol%)
Conditions 3aa
[mol%]
(mol%)
[%][b]
[*] Y.-F. Wang, K. K. Toh, J.-Y. Lee, Dr. S. Chiba
Division of Chemistry and Biological Chemistry
School of Physical and Mathematical Sciences
Nanyang Technological University
Singapore 637371 (Singapore)
1
2
3
4
5
6
7
5
5
5
5
5
2.5
2.5
NaOAc (30)
CsOPiv (30)
Cu(OAc)2 (20)
Cu(OAc)2 (20) H2O (100) 908C, 1 h
Cu(OAc)2 (20) AcOH (100) 908C, 0.6 h 80
Cu(OAc)2 (20) AcOH (100) 908C, 2 h 84
CuOAc (20) AcOH (100) 908C, 0.5 h 84
–
–
–
1108C, 12 h
1108C, 12 h
1108C, 0.3 h 70
0
0
67[c]
Fax: (+65)6791-1961
E-mail: shunsuke@ntu.edu.sg
[**] This work was supported by funding from Nanyang Technological
University and Singapore Ministry of Education (Academic
Research Fund Tier 2: MOE2010-T2-1-009).
[a] All reactions were carried out in the scale of 0.5 mmol of alkyne 2a
with 1.2 equiv of vinyl azide 1a under a N2 atmosphere. [b] Yields of
isolated products are based on alkyne 2a. [c] Yields as determined by
NMR analysis of the crude reaction mixture.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201101009.
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Communications
these studies, we embarked on our investigation of the
methyl moiety could be introduced not through a radical
pathway but in an ionic manner.
reaction of a-azido styrene (1a) and diphenylacetylene (2a)
using [{Cp*RhCl2}2] as a catalyst with carboxylate sources to
target isoquinoline derivatives (Table 1). Although the uti-
lization of NaOAc or CsOPiv (30 mol%) as carboxylate
À
sources did not afford any ortho C H functionalization
products (entries 1 and 2), the reaction with Cu(OAc)2
(20 mol%) at 1108C in DMF gave 1-methyl-3,4-diphenyliso-
quinoline (3aa) in 70% yield (entry 3). The addition of acetic
acid (1 equiv) proved to be optimal for the isoquinoline
formation, allowing the use of a lower temperature (908C)
and a catalytic amount of [{Cp*RhCl2}2] (2.5 mol%; entries 5
and 6). Notably, an acceleration of the reaction rate was
observed when utilizing CuOAc instead of Cu(OAc)2
(entry 7).[11] It was also confirmed that the reaction with
Cu(OAc)2 in the absence of [{Cp*RhCl2}2] did not afford
isoquiniline 3aa at all.
By utilizing the [{Cp*RhCl2}2]/Cu(OAc)2 (5/20 mol%)
catalytic system,[12] we examined the generality of this
catalytic method for the synthesis of substituted isoquinolines
(Table 2). The present process showed wide substrate toler-
ance for internal alkynes (entries 1–8). Diarylacetylenes
reacted smoothly with vinyl azide 1a, giving isoquinolines 3
in good yields (entries 1–3). The reactions with dialkyl-
substituted alkynes also proceeded smoothly (entries 4 and
5). Insertion of an unsymmetrical alkyne, 1-phenyl-1-propyne
(2g), occurred regioselectively to provide 4-methyl-3-phenyl-
isoquinoline (3ag) as a sole product (entry 6). Similarly,
methyl 3-phenylpropiolate (2h) and thienylacetylene 2i
afforded isoquinoline 3ah and 3ai, respectively, in a regiose-
lective manner albeit in moderate yields (entries 7 and 8).
Electron-withdrawing groups could be installed as substitu-
ents on the benzene ring of vinyl azides 1[13] to result in
isoquinoline formation in good yields, although the vinyl
azide 1c bearing an electron-donating moiety (OMe:
entry 10) as well as 1-naphthyl vinyl azide 1g (entry 14)
Next, we thermally decomposed 1a in toluene at 1008C to
prepare 2H-azirine 4a, which was then subjected to the
reaction with alkyne 2a in the presence of [{Cp*RhCl2}2] and
metal acetates as catalysts. The reaction with Cu(OAc)2 or
CuOAc as a metal acetate afforded isoquinoline 3aa, whereas
utilization of NaOAc did not form 3aa [Eq. (2)]. The reaction
with CuOAc was completed within 10 minutes whereas that
of Cu(OAc)2 needed 2 hours, which was consistent with the
reactions of vinyl azides (Table 1, entries 6 and 7). The
reaction of vinyl azide 1a with 2 equivalents of CuOAc in the
presence of AcOH provided acetophenone (6a) in 48% yield,
À
probably by the hydrolysis of the putative N H imine
intermediate 5a [Eq. (3)]. Notably, the reaction with
[{Cp*RhCl2}2]/Cu(OAc)2 under an oxygen atmosphere did
not afford isoquinoline 3aa at all, whereas a CO atmosphere
gave isoquinoline 3aa in 82% yield within 0.5 hours [Eq. (4)].
These results suggested that both rhodium and copper are
À
required to induce ortho-C H functionalization from 2H-
azirine 4a. Lower-valent copper(I) species might play an
indispensable role of reductive ring opening of 2H-azirines to
give imine derivatives[15] that could be used to initiate the
III
À
Rh -catalyzed ortho-C H rhodation with subsequent inser-
tion of alkynes. The UV/Vis spectra for the treatment of
Cu(OAc)2 in DMF at 908C showed the disappearance of the
band in the visible region corresponding to Cu(OAc)2
(=700 nm) within 30 minutes (see the Supporting Informa-
tion), thereby implying that DMF might reduce Cu(OAc)2 to
form a CuI species.[16,17]
À
were sluggish. This process tolerated C Br bonds (entries 3,
12, and 15). In the case of meta-substituted substrates,
regioisomeric mixtures were obtained where the less sterically
À
hindered C H bond was preferentially cleaved (marked in
blue) (entries 15 and 16). This method allowed the construc-
tion of g-carboline and 1H-pyrrolo[2,3-c]pyridine structures
(entries 17 and 18). Similarly, benzofuranyl and benzothio-
furanyl vinyl azides 1l and 1m could be utilized for this
transformation albeit in moderate yield in the case of
benzofuranyl derivative 3la (entries 19 and 20). At the
b position of vinyl azides 1, methyl, hydroxymethyl, and
aminomethyl functional groups could be introduced, thus
leading to the corresponding isoquinolines 3 in good to
moderate yields (entries 21–26).
To probe the reaction mechanism especially with regard
to the function of both catalysts in the present isoquinoline
formation, a series of experiments were examined using vinyl
azide 1a and alkyne 2a. The reaction in the presence of D2O
(5 equiv) led to incorporation of deuterium at the methyl
group of isoquinoline [D]-3aa [Eq. (1); DMF = N,N-dime-
thylformamide], whereas the utilization of [D7]DMF as a
solvent did not provide deuterated isoquinoline at all.[14]
These observations indicated that a hydrogen atom at the
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Table 2: Substrate scope.[a]
Entry Vinyl azides 12 Alkynes 2
Yield
Entry Vinyl azides 1
Alkynes 2 Yield
17
1
1a
1a
1a
1a
1a
1a
1a
2b (R3, R4 =4-MeOC6H4)
2c (R3, R4 =4-ClC6H4)
2d (R3, R4 =4-BrC6H4)
2e (R3, R4 =nPr)
3ab: 77%
3ac: 70%
3ad: 83%
3ae: 71%
3af: 54%
3ag: 82%
3ah: 27%
1j
2a
2a
2a
2a
3ja: 82%
2
18
3[c]
4
1k
19[f]
1l
3ka: 77%
3la: 45%
3ma: 75%
5
2 f (R3, R4 =CH2OTBS)
2g (R3 =CH3, R4 =Ph)
2h (R3 =CO2Me, R4 =Ph)
6
7
20
1m
2i (R3 =n-hexyl, R4 =2-
thienyl)
8
1a
3ai: 52%[d]
21[e] 1n
2a
3na: 85%
9
1b (R1 =Me)
2a
3ba: 80%
22[e] 1n
23[e] 1n
2e
2g
3ne: 54%
3 ng: 80%
10
1c (R1 =OMe) 2a
3ca: 45%[d]
1d
11[e]
12
2a
3da: 86%
3ea: 80%
(R1 =CO2Me)
1e (R1 =Br)
2a
24
25
1o (R5 =TBDPS)
2a
2a
3oa: 46%
3pa: 40%
1p (R5 =allyl)
13
14
1 f
2a
3 fa: 70%
26
1q
2a
3qa: 85%
1g
2a
3ga: 38%
15
16
1h (R1 =Br)
2a
2a
3ha: 74%
3ia: 66%
3ha’: 12%
3ia’: 5%
1i (R1 =NO2)
[a] The reactions were carried out by treatment of a mixture of vinyl azides 1 (1.2 equiv) and alkyne 2 (0.5 mmol) with [{Cp*RhCl2}2] (5 mol%) and
Cu(OAc)2 (20 mol%) in the presence of AcOH (1 equiv) in DMF (2.5 mL) at 908C under N2 atmosphere for 1–2 h. [b] Yields of isolated products.
[c] 1.5 equiv of vinyl azides 1a was used. [d] NMR yields. [e] 2.5 mol% of [{Cp*RhCl2}2] was utilized. [f] 10 mol% of [{Cp*RhCl2}2] was utilized. TBS=
tert-butyldimethylsilyl. TBDPS=tert-butyldiphenylsilyl. Ts=p-toluenesulfonyl.
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Communications
Based on these experimental data, a proposed mechanism
for the reaction using the [{Cp*RhCl2}2]/Cu(OAc)2 catalytic
system was outlined in Scheme 2. First, Cu(OAc)2 might be
radical E (path b).[18] Formation of the rhodacycle G from 5a
or iminyl copper species such as D with RhIII via iminyl
À
rhodium F, subsequent insertion of alkyne 2a, and C N
reductive elimination from H could provide isoquinoline 3aa
with generation of a RhI species (step C). Finally, a redox
reaction between RhI and CuII species would lead to
regeneration of RhIII and CuI (step D).
It could be speculated that the reductive formation of
imine derivatives from vinyl azides proceeds by the proto-
nation of the CuII aza-enolates such as C (Scheme 2, step B).
We envisioned trapping such putative aza-enolates with the
other electrophiles for further functionalization of isoquino-
line derivatives. Recent literature precedents have shown that
CuII/TEMPO complexes work as an ionic electrophile.[19]
When TEMPO (2 equiv) was added instead of AcOH in the
reaction of vinyl azides 1n and 1q with alkynes 2 under the
[{Cp*RhCl2}2]/Cu(OAc)2 catalytic system, isoquinoline/
TEMPO adducts 7 and alcohols 7ꢀ were isolated in good
combined yields (Table 3).[20–23] From vinyl azide 1q, the 1,2-
aminoalcohol unit could be installed in the isoquinoline
framework (entry 4).
In summary, we have developed a synthetic method to
deliver highly substituted isoquinolines from readily available
a-aryl vinyl azides and internal alkynes in the presence of a
[{Cp*RhCl2}2]/Cu(OAc)2 bimetallic catalyst system. Further
exploitation of the other types of multimetallic cooperation,
which achieve unprecedented organic transformations, is
currently underway.
Scheme 2. The proposed reaction pathway.
reduced by DMF to form a CuI species (step A). Thermal
denitrogenative decomposition of the vinyl azide 1a gave 2H-
azirines 4a, which could be reduced by the CuI species to
Received: February 9, 2011
Revised: March 24, 2011
Published online: May 17, 2011
À
afford the anion radical A (step B, path a). Consecutive C N
bond cleavage of A formed the iminyl copper(II) radical
Keywords: alkynes · azides · copper · heterocycles · rhodium
.
intermediate B, which could be additionally reduced with CuI
and protonated to deriver N H imine 5a along with a Cu
species. Alternatively, the direct reduction of the vinyl azide
1a by a CuI species could also be proposed to form the
putative radical intermediates B through a vinyl azide anion
II
À
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Vinyl azides 1
Alkynes 2
Yield[b]
Yield[b]
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1
2
1n (R2 =CH3)
1n (R2 =CH3)
1n (R2 =CH3)
1q (R2 =CH2NPI)
2a (R3, R4 =Ph)
7na: 68%
7ne: 39%
7 ng: 55%
7qa: 48%
7na’: 15%
7ne’: 42%
7 ng’: 18%
7qa’: 29%
2e (R3, R4 =nPr)
2g (R3 =CH3, R4 =Ph)
2a (R3, R4 =Ph)
3
4[c]
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[a] All reactions were carried out with alkyne 2 (0.5 mmol), vinyl azide 1 (1.5 equiv),
and of TEMPO (2 equiv) in DMF (2.5 mL) under a N2 atmosphere for 1 h. [b] Yields
of isolated products based on alkynes 2. [c] 5 mol% of [{Cp*RhCl2}2] was utilized.
NPI=N-phthalimidoyl.
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À
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À
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À
À
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[20] It was confirmed that treatment of isoquinoline 3na with
TEMPO under the present catalytic reaction conditions does
not form 7na and 7na’ at all.
[21] The structure of 7 ng was confirmed by the X-ray crystallo-
graphic analysis. CCDC 805444 (7 ng) contains the supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif. See the
Supporting Information.
[11] The other solvents such as MeOH, DMSO, and toluene were not
viable for this transformation. For the reactions in the presence
of other Cu salts; see the Supporting Information.
[12] Cu(OAc)2 was mainly utilized as the catalyst since CuOAc is
very sensitive to moisture, air, and light.
[22] The formation of alcohols 7’ might occur through the reduction
of the TEMPO adduct 7 with CuI species that is generated in situ.
À
[23] Oxidative and reductive cleavages of the N O bond of 7 were
examined; see the Supporting Information.
[13] All vinyl azides 1 were prepared from the corresponding
alkenes; see the Supporting Information.
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