(Pentamethylcyclopentadienyl)cobalt(III)-Catalyzed Oxidative [4+2] Annulation of N H Imines with Alkynes: Straightforward Synthesis of Multisubstituted Isoquinolines

Article abstract of DOI:10.1002/adsc.201600025

A synthetic method for isoquinoline synthesis via a [4+2] annulation of N H imines with alkynes using the high-valent (pentamethylcyclopentadienyl)cobalt(III) [Cp*Co(III)] catalyst is described. Cerium(IV) sulfate was found to be an efficient oxidant in lieu of the commonly used copper or silver salts. Broad substrate scope, high functional group tolerance, and generally good yields were observed. (Figure presented.).

Full text of DOI:10.1002/adsc.201600025

UPDATES
DOI: 10.1002/adsc.201600025
(Pentamethylcyclopentadienyl)cobalt(III)-Catalyzed Oxidative
À
[4+2] Annulation of N H Imines with Alkynes: Straightforward
Synthesis of Multisubstituted Isoquinolines
Shang-Shi Zhang,a Xu-Ge Liu,a Shi-Yong Chen,a Dong-Hang Tan,a
Chun-Yong Jiang,a Jia-Qiang Wu,a Qingjiang Li,a,* and Honggen Wanga,*
a
School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou 510006, Peopleꢀs Republic of China
Fax: (+86)-20-3994-3000; e-mail: liqingj3@mail.sysu.edu.cn or wanghg3@mail.sysu.edu.cn
Received: January 11, 2016; Revised: March 13, 2016; Published online: April 15, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600025.
Abstract: A synthetic method for isoquinoline syn-
thesis via a [4+2] annulation of N H imines with
the sustainable chemistry point of view, there is an in-
tensive need for cheap metal catalysts in lieu of the
often used but precious late transition metals. In this
regard, great advances have been realized on earth-
abundant first-row transition metals catalysis (for in-
stance, iron, cobalt, nickel, and copper), which effects
À
alkynes using the high-valent (pentamethylcyclo-
pentadienyl)cobalt(III) [Cp*Co(III)] catalyst is de-
scribed. Cerium(IV) sulfate was found to be an effi-
cient oxidant in lieu of the commonly used copper
or silver salts. Broad substrate scope, high function-
al group tolerance, and generally good yields were
observed.
similar, and more importantly, complementary reac-
[11]
À
tivities for C H activation reaction. For isoquino-
line synthesis, Wang recently reported an elegant
[4+2] annulation of imines with alkynes under the
catalysis of manganese (Scheme 1c).[12]
À
Keywords: annulation; cerium salts; C H activa-
tion; cobalt; isoquinolines
Isoquinolines are frequently encountered in a diverse
array of functional molecules, such as phosphorescent
materials,[1] fluorosensors,[2] and ligands for metal cat-
alysis.[3] In particular, a number of biologically impor-
tant compounds contain the isoquinoline motif.[4] The
classical protocols for isoquinoline synthesis, as repre-
sented by the Bischler–Napieralski, Pomeranz–
Fritsch, and Pictet–Gams reactions, suffer from disad-
vantages such as lack of easy availability of the sub-
strate/reagent, harsh reaction conditions, limited prod-
uct diversity, and/or low yields.[5] Recently, with the
À
advent of transition metal-catalyzed C H activation
reactions,[6] several synthetic methodologies toward
À
isoquinolines based on straightforward oxidative C H
annulations were elegantly developed (Scheme 1).[7–10]
The majority of these methods take the advantage of
the high reactivities of late transition metals, such as
rhodium,[8] ruthenium[9] and palladium,[10] with the ex-
istence of an external oxidant or by using an oxidizing
directing group (Scheme 1a). While these methods
are effective, their synthetic application may be com-
À
promised by the high cost of the metal catalyst. From Scheme 1. Isoquinoline synthesis via direct C H annulation.
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Shang-Shi Zhang et al.
Pioneered by Kanai and Matsunaga, high-valent idizing oxime or its ester as directing group
Cp*Co(III) complexes have recently been recognized (Scheme 1b).
À
as efficient catalysts for a number of C H activation
We commenced our studies by reacting diphenyl-
reactions.[13,14] It is intriguing that in contrast to its methanimine (1a) with 1,2-diphenylethyne (2a) using
congener Cp*Rh(III), Cp*Co(III) seems to favour Cp*Co(CO)I2 as catalyst (Table 1). It should be noted
À
redox-neutral C H addition reactions, probably that the starting materials – ketimines – are easily ac-
À
driven by the relatively higher basicity of the Co C cessible via organometallic addition to benzonitriles.
À
bond. In fact, only three examples of oxidative C H To our delight, after extensive exploration, we were
functionalization promoted by Cp*Co(III) were able to obtain the desired product 1,3,4-triphenyliso-
known,[14w–y] wherein silver and/or copper salts were quinoline (3a) in 83% isolated yield under the reac-
used as terminal oxidants. In continuation of our on- tion conditions of Cp*Co(CO)I2 (10 mol%), Ce(SO4)2
going exploration of cheap metal catalysis, we herein (2.0 equiv.), AgOTf (20 mol%) and KOAc (40 mol%)
À
report a Cp*Co(III)-catalyzed oxidative C H annula- in nitromethane (0.2M) at 1008C under an N2 atmos-
tion of imines with various alkynes (Scheme 1c).[14p,y] phere (Table 1, entry 1). Control experiments indicat-
We identified in this reaction that Ce(SO4)2 is an effi- ed that Cp*Co(CO)I2 is essential for the reactivity
cient oxidant for Cp*Co(III) regeneration. We antici- (entry 2). The omission of iodide scavenger AgOTf
pate that the discovery of this new oxidant Ce(SO4)2 gave a lower yield of 44% (entry 3). In addition, the
À
might extend the scope of Cp*Co(III)-catalyzed C H absence of oxidant Ce(SO4)2 led to a much lower
activation reactions. In addition, various substituted yield of 30% (entry 4). The attempts to use copper or
alkynes including terminal alkynes were well tolerat- silver salts, which were effective in other Cp*Co(III)-
[14w–y]
À
ed in this reaction. During the preparation of this catalyzed oxidative C H activation reactions,
manuscript, the groups of Kanai and Matsunaga,[15a] were proven to be unsuccessful in the current trans-
Ackermann,[15b] and Sundararaju[15c] reported that formation (entries 5–8). Another cerium salt CAN
multisubstituted isoquinolines could be accessed via [Ce(NH4)2(NO3)6] shut down the reactivity complete-
Cp*Co(III)-catalyzed annulation reaction using an ox- ly (entry 9). Interestingly, the replacement of KOAc
Table 1. Optimization of the reaction conditions.[a]
Entry
Variation from the standard conditions
Yield [%][b]
1
none
89 (83)[c]
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
without Cp*Co(CO)I2
without AgOTf
without Ce(SO4)2
0
44
30
24
0
13
7
Cu(OAc)2 instead of Ce(SO4)2
AgOAc instead of Ce(SO4)2
Cu(OAc)2·H2O/Ag2CO3 instead of Ce(SO4)2
Ag2O instead of Ce(SO4)2
CAN instead of Ce(SO4)2
without KOAc
NaOAc instead of KOAc
PivOH instead of KOAc
5 mol% of Cp*Co(CO)I2
DCE instead of CH3NO2
MeOH instead of CH3NO2
DMF instead of CH3NO2
808C instead of 1008C
0
13
70
76
68
53
65
trace
75
[a]
Reaction conditions: 1a (0.2 mmol), Cp*Co(CO)I2 (10 mol%), AgOTf (20 mol%), Ce(SO4)2 (2.0 equiv.), KOAc
(40 mol%), 3Š MS (300 mg) in CH3NO2 (1.0 mL) at 1008C for 32 h.
Yield determined by 1H NMR using 1-iodo-4-methoxybenzene as internal standard.
Isolated yield.
[b]
[c]
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(Pentamethylcyclopentadienyl)cobalt(III)-Catalyzed Oxidative [4+2] Annulation
with either basic NaOAc or acidic PivOH gave slight- ing the reaction at the less sterically hindered position
ly lower yields (entries 11 and 12); the omission of (3d). Importantly, we were pleased to find that aryl
KOAc, however, led to a significantly lower efficiency alkyl ketimines are also compatible with the current
(entry 10). The importance of carboxylate for this reaction conditions even though lower yields were ob-
transformation might be rationalized by a carboxylate tained, thus greatly broadening the scope of this reac-
[16]
À
assisted C H activation mechanism. When 5 mol% tion (3f–h).
of Cp*Co(CO)I2 were employed, a lower yield (68%)
The scope of the alkynes was also examined
was obtained (entry 13). Other solvents, such as DCE, (Scheme 3). It was found that not only diarylacety-
MeOH and DMF, gave inferior results (entries 14– lenes (3i–u), but also dialkylacetylenes (3v–z) were
16). A lower temperature of 808C led to the incom- suitable coupling partners for this transformation,
plete conversion of 1a (entry 17).
forming the corresponding multisubstituted isoquino-
With the optimized reaction conditions established, lines in generally good yields. Numerous commonly
we first investigated the scope and limitation of this encountered functional groups were well tolerated. In
transformation by reacting 1,2-diphenylethyne (2a) addition, diethyl acetylenedicarboxylate also gave the
with a variety of imines. As shown in Scheme 2, it desired product in 25% yield (3aa). Terminal alkynes
turned out that diaryl ketimines bearing different sub- are often problematic coupling partners in oxidative
À
stituents regardless of their electronic properties such C H activation reactions, especially when copper or
as methoxy (3b), fluoro (3c), trifluoromethyl (3d) and silver are used as oxidant due to the unproductive
methyl (3e) groups underwent the reactions smoothly. Glaser–Hay coupling reaction. We were delighted to
An ortho-substitutent did not hamper the efficiency find that terminal alkynes including aromatic, hetero-
(3e). In addition, a meta-trifluomethylated aryl keti- aromatic and aliphatic alkynes reacted smoothly to
mine provided a single regioisomeric product, favour- give the annulation products with the substituents
positioned exclusively next to the nitrogen atom.
A kinetic isotope effect (KIE) study was carried
out to give clues on the mechanism. A small KIE
À
value of 1.2 was observed, indicating that C H bond
cleavage is not involved in the turnover-limiting step
(see the Supporting Information for details). In addi-
tion, no cis-stilbene derived from reduction of the
alkyne was found, indicating a different reaction path-
way from the manganese-catalyzed version.[12] Based
on this observation and other relevant reports on
Cp*Co(III)- and Cp*Rh(III)-catalyzed[8,14,15] oxidation
annulation reactions, a plausible mechanism is out-
lined in Scheme 4. Firstly, the proposed active cationic
catalyst I [Cp*Co(III)OAc]+ was formed via halogen
abstraction and ligand exchanges. Coordination of the
À
ketimine is followed by a C H activation to form a cy-
clocobalt species III (path A). The cleavage of the C
À
H bond might be assisted by the acetate and is not
turnover-limiting. A subsequent migratory insertion
of alkyne delivers a seven-membered ring intermedi-
ate VI. The isoquinoline product was formed upon
C N reductive elimination. The reduced Cp*Co(I)
species is then oxidized by Ce(SO4)2 to complete the
catalytic cycle.
À
Another mechanistic scenario featuring an intramo-
lecular single electron transfer (SET) might be also
evoked based on a recent theoretical study (path
B).[14x] After the coordination of imine to Cp*Co(III),
an intramolecular SET might occur to deliver a radical
cation IV. A radical addition of IV to alkyne 2a is fol-
lowed by the radical combination with Cp*Co(II) to
regenerate a Cp*Co(III) species. This Wheland inter-
mediate V is then deprotonated to give the same spe-
cies VI as path A. Radical scavenger experiments
Scheme 2. Scope of imines.
using TEMPO (2,2,6,6-tetramethylpiperidinyl 1-oxy)
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Shang-Shi Zhang et al.
Scheme 3. Scope of the alkynes.
or BHT (butylated hydroxytoluene) as radical inhibit- Experimental Section
er did not hamper the reactivity. These results might
suggest that pathway B is less likely.
General Procedure
In summary, we have successfully developed
a cheap metal-catalyzed [4+2] annulation reaction of
N H imines with alkynes for the construction of mul-
To an oven-dried Schlenk tube, under a stream of argon,
were added imine (0.2 mmol, 1.0 equiv.), alkyne
1
2
À
(0.5 mmol, 2.5 equiv.), Cp*Co(CO)I2 (9.5 mg, 0.02 mmol,
10.0 mol%), AgOTf (10.3 mg, 0.04 mmol, 0.2 equiv.), KOAc
(7.8 mg, 0.08 mmol, 0.4 equiv.), Ce(SO4)2 (133.0 mg,
0.4 mmol, 2.0 equiv.), 3Š molecular sieves (300.0 mg) and
anhydrous CH3NO2 (1.0 mL). The mixture was stirred at
1008C for 36 h. The reaction mixture was then diluted with
DCM (10 mL) and washed with brine. The aqueous phase
was extracted with DCM again. The organic layers were
combined, washed with brine and dried over Na2SO4. The
crude product was purified by flash column chromatography
on silica gel with an appropriate solvent to afford the pure
product 3.
tisubstituted isoquinolines. The reaction is catalyzed
by a high valent [Cp*Co(III)] complex. Ce(SO4)2 was
identified as an efficient oxidant for this transforma-
tion. This finding is important given that the common-
ly used copper or silver salts may cause undesired
chemical processes due to their diverse reactivies in
synthetic chemistry. Broad substrate scope, high func-
tional group tolerance, and generally good yields
were found.
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(Pentamethylcyclopentadienyl)cobalt(III)-Catalyzed Oxidative [4+2] Annulation
Scheme 4. Proposed mechanism.
[5] J. J. Li, E. J. Corey, Name Reactions in Heterocyclic
Chemistry, John Wiley & Sons, Hoboken, NJ, 2005,
pp 375–494.
Acknowledgements
We are grateful for the support of this work by “1000-Youth
Talents Plan”, a Start-up Grant from Sun Yat-sen University
and National Natural Science Foundation of China
(81402794 and 21472250).
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Adv. Synth. Catal. 2016, 358, 1705 – 1710