Nickel(0)-catalyzed [2 + 2 + 1] carbonylative cycloaddition of imines and alkynes or norbornene leading to γ-lactams

Article abstract of DOI:10.1021/ja509171a

The first nickel(0)-catalyzed [2 + 2 + 1] carbonylative cycloaddition reaction of imines and alkynes or norbornene has been achieved by employing phenyl formate as a CO source. With this method, a variety of N-benzenesulfonyl, -tosyl, and -phosphoryl-substituted γ-lactams can be prepared in good to high yields.

Full text of DOI:10.1021/ja509171a

Communication
pubs.acs.org/JACS
Nickel(0)-Catalyzed [2 + 2 + 1] Carbonylative Cycloaddition of Imines
and Alkynes or Norbornene Leading to γ‑Lactams
Yoichi Hoshimoto,†,‡ Tomoya Ohata,† Yukari Sasaoka,† Masato Ohashi,† and Sensuke Ogoshi*,†,§
†Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
‡Frontier Research Base for Global Young Researchers, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871,
Japan
§JST, ACT-C, Suita, Osaka 565-0871, Japan
S
* Supporting Information
Scheme 1. Nickel(0)-Catalyzed [2 + 2 + 1] Carbonylative
Cycloaddition of an Imine and an Alkyne or Alkene
ABSTRACT: The first nickel(0)-catalyzed [2 + 2 + 1]
carbonylative cycloaddition reaction of imines and alkynes
or norbornene has been achieved by employing phenyl
formate as a CO source. With this method, a variety of N-
benzenesulfonyl, -tosyl, and -phosphoryl-substituted γ-
lactams can be prepared in good to high yields.
γ-Lactams are an important synthetic intermediate of a variety of
natural products and biologically active compounds in drug
discovery.1 One of the straightforward approaches for the
construction of the γ-lactam structure is the transition-metal-
catalyzed or -mediated carbonylative cycloaddition, which has
been referred to as the hetero-Pauson−Khand (or aza-Pauson−
Khand) reaction, shown by eq 1:2
lactams. The key to the present system is the use of phenyl
formate as a CO source that enables both carbonylation of the
heteronickelacycle intermediate and the resultant regeneration of
the nickel(0) catalyst without the formation of Ni(CO)3L
(Scheme 1).
In order to achieve the nickel(0)-catalyzed carbonylative
cycloaddition, the concentration of CO should be high enough to
react with the heteronickelacycle intermediate, yet simulta-
neously low enough for the formation of a catalytically unreactive
nickel tricarbonyl complex. Among the reported procedures used
to generate CO in situ from a variety of carbonyl compounds,7
the use of phenyl formate was interesting, because Tsuji et al.7c
and Manabe et al.7b had already used it to develop a procedure
whereby CO could be generated through a reaction with
organic/inorganic bases such as NEt3 or NaHCO3 in the absence
of transition metal reagents. First, we examined the reaction of
the isolated heteronickelacycle A5a with phenyl formate (5 equiv)
and NEt3 (7.5 equiv) in DMF-d7, CD3CN, or C6D6 at 60 °C
(Scheme 2a). The transformation of A into α,β-unsaturated γ-
lactam 3aa took place almost quantitatively in DMF-d7 and
CD3CN, both of which were reported as suitable solvents to
generate CO from phenyl formate.7b,c The formation of PhOH
The synthesis of γ-lactams, particularly the α,β-unsaturated
ones, via transition-metal-catalyzed carbonylative cycloaddition
has historically been somewhat limited in the well-established
Pauson−Khand reaction.2,3 This might be due to inadequate
development of the procedures needed in order to generate the
heterometalacycle compounds that would be a key reaction
intermediate in the hetero-Pauson−Khand reaction. For the
development of an efficient and a straightforward synthetic
method of γ-lactams by carbonylation, it would be critical to
employ an appropriate transition metal that can efficiently form a
heterometalacycle. Therefore, nickel(0) is a promising candidate
since many studies on the formation of heteronickelacycles have
been reported.4,5 We reported the formation of α,β-unsaturated
γ-lactams by the reaction of CO gas with heteronickelacycle
compounds generated through the oxidative cyclization of an
imine and an alkyne on nickel(0) (Scheme 1).5a,b,d Thus far,
however, the expansion of this carbonylation to the catalytic
process has been totally hampered under the CO atmosphere by
the formation of catalytically unreactive nickel carbonyl
complexes such as Ni(CO)3L.6 Herein, we report the
nickel(0)-catalyzed [2 + 2 + 1] carbonylative cycloaddition of
imines and alkynes to give a variety of α,β-unsaturated γ-lactams.
Furthermore, norbornene was also utilized instead of alkynes
under the optimized conditions, which afforded bicyclic γ-
1
and Ni(CO)3(PCy3) was also observed by H and 31P NMR
analyses. On the other hand, the reaction in C6D6, with an
efficiency of CO generation that is only moderate,7b,c gave a
rather complicated mixture that includes 3aa (37%), PhOH, and
a trace amount of Ni(CO)3(PCy3).8 Next, we examined the
carbonylation of A generated in situ through the oxidative
cyclization of N-benzylidene benzenesulfonamide (1a) and
Received: September 5, 2014
© XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
reaction efficiency (entries 4 and 5), and 3ab was formed in 74%
yield for 24 h at 70 °C, which was chosen as our optimal
conditions. The reaction also took place to give 3ab in 63% yield
in THF-d8 at 70 °C (entry 6). DBU, DMAP, and quinuclidine
were not suitable as a base under the presented conditions.10 In
entries 3−6, the concomitant formation of hexapropylbenzene
and/or 1,2-dihydropyridine was also detected by NMR, the
former of which was formed by the nickel-catalyzed trimerization
of 2b11 and the latter by the [2 + 2 + 2] cycloaddition reaction of
1a and 2 equiv of 2b.5a,b,d
Scheme 2. Reaction of A with Phenyl Formate in the Presence
of NEt3
a
In order to gain insight into the rate of the in situ generation of
CO, the reaction of phenyl formate with NEt3 (1.3 equiv) was
monitored in C6D6 or THF-d8 by 1H NMR at 70 °C. The rate of
the generation of CO is described as d[CO]/dt = k[HCOOPh]
and k was estimated as 2.23(1) × 10−6 and 5.00(1) × 10−6 s−1 in
C6D6 and THF-d8, respectively. Thus, both the carbonylation of
the heteronickelacycle intermediate and the regeneration of an
active nickel(0) species could be compatible under these reaction
conditions (entries 4 and 6 in Table 1).12 In addition, this
information would help in the practice and/or reproduction of
the present catalytic system.
a
NMR yields determined by using 2-methoxynaphthalene as an
internal standard.
diphenylacetylene (2a) with Ni(cod)2/PCy3 (Scheme 2b).
Although CD3CN did not afford a positive result due to the
poor solubility of Ni(cod)2, 3aa was again formed in excellent
yield in DMF-d7. These results showed the possibility that phenyl
formate can be used as a CO source in the nickel(0)-catalyzed [2
+ 2 + 1] carbonylative cycloaddition of alkynes and imines.
In the presence of 10 mol % Ni(cod)2 and 20 mol % PCy3, the
reaction of 1a, 4-octyne (2b) (1.0 equiv), phenyl formate (1.5
equiv), and NEt3 (2.0 equiv) was examined in DMF-d7; however,
the desired γ-lactam 3ab was not obtained at all because of the
rapid and quantitative formation of Ni(CO)3(PCy3) based on
the amount of Ni(0), which was confirmed by 31P NMR (entry 1,
Table 1). In order to efficiently prepare 3ab with the nickel(0)
A variety of α,β-unsaturated γ-lactams (3) were prepared by
the nickel(0)-catalyzed [2 + 2 + 1] carbonylative cycloaddition of
imines (1) and alkynes (2) with phenyl formate (Table 2). The
aryl- and alkyl-substituted symmetrical alkynes (2a, 2b, 2e, and
2f) gave the corresponding γ-lactams (3aa, 3ab, 3ae, and 3af) in
moderate to good isolated yields; however, bis(trimethylsilyl)-
acetylene (2c) and dimethyl acetylenedicarboxylate (2d) did not
afford the targets. This is probably due to a difficulty in the
simultaneous coordination of the alkyne (2c or 2d) and 1a to
nickel(0).13 Asymmetric alkyne 2g gave 3ag in 83% yield as a
mixture of regioisomers with a ratio of 89/11, and the major
isomer is shown in Table 2. On the other hand, 3ah and 3ai were
formed as single regioisomers from 2h and 2i, respectively. We
reported that oxidative cyclization of (p-CF3C6H4)NC(H)Ph
and 2i with a nickel(0) complex proceeded in a highly
regioselective manner to give a sole heteronickelacycle with an
η3-butadienyl moiety.5d The scope of imines (1b−1p) was
conducted with 2a under the optimized conditions. A variety of
N-benzylidene-toluenesulfonamide derivatives (1b−1k) includ-
ing chloro-, cyano-, and ester-substituted ones were applicable to
the presented catalytic system giving the γ-lactams (3ba−3ka) in
good to high yields while a significant decrease in the yield was
found in the case of 1f having an electron-rich arene ring. The
thienyl- and furyl-substituted imines (1l and 1m) also afforded
3la and 3ma in 79% and 76% yields, respectively. Employing
alkyl-tosylimines was more challenging under the presented
conditions; in the presence of 20 mol % Ni(cod)2 and 40 mol %
PCy3, N-Cy- and N-tBu-γ-lactams (3na and 3oa) were obtained
in 46% and 68% yields, respectively. We reported that an N-
diphenylphosphinic group on imine can accelerate the formation
of a heteronickelacycle as an electron-withdrawing group.5d In
fact, the catalytic carbonylation proceeded to give 3pa in good
yield (69%).
a
Table 1. Optimization of Reaction Conditions
entry
2b (equiv)
solv
temp (°C)
time (h)
yield (%)
1
2
3
4
5
6
1.0
1.0
2.0
2.0
2.0
2.0
DMF
C6D6
C6D6
C6D6
C6D6
THF-d8
60
60
60
70
80
70
48
48
−
44
83
74
69
63
110
24
32
24
a
Reaction conditions: 1a (0.20 mmol), 2b (0.20−0.40 mmol), phenyl
formate (0.30 mmol), NEt3 (0.40 mmol), Ni(cod)2 (0.020 mmol),
PCy3 (0.040 mmol), solvent (0.50 mL). The reaction was monitored,
and the yield of 3ab was determined by H NMR by using 1,3,5-
1
trioxane or 2-methoxynaphthalene as an internal standard.
catalyst, the rate of the in situ generation of CO should be
lowered. Thus, C6D6 was employed as a solvent (entry 2). As a
result, 3ab was formed in 44% yield at 60 °C over a period of 48
h.8 It is noteworthy that PhOH did not react with the
heteronickelacycle intermediate while PhOH was incorporated
into the product in the reported works.7b,c Employing other
phosphine ligands such as PPh3, P(o-tol)3, PnBu3, PtBu3, and
PMetBu2, or IPr9 dramatically lowered the yield of 3ab.10 In these
reactions, only trace amounts of 1a and 2b were consumed.
Employing 2 equiv of 2b gave a better result over a period of 110
h (entry 3). Elevating the reaction temperature promoted the
Next, we employed norbornene instead of alkynes as a
preliminary scope of alkene substrates (Table 3). Under the
optimized conditions in Table 2, the carbonylative [2 + 2 + 1]
cycloaddition of imines (1a, b, l, and m) and norbornene (1.5
equiv) occurred to furnish bicyclic γ-lactams (4−7) in good to
high yields.14 The structure of the products was unambiguously
determined by NMR spectroscopy. In addition, the structure of 5
was also confirmed by X-ray analysis, and its crystal structure is
B
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Journal of the American Chemical Society
Communication
Table 2. Nickel(0)-Catalyzed Carbonylative [2 + 2 + 1]
Cycloaddition of Imines (1) and Alkynes (2) with Phenyl
Formate
Table 3. Ni(0)-Catalyzed Carbonylative [2 + 2 + 1]
Cycloaddition of Imines (1) and Norbornene with Phenyl
a
a
Formate
a
Reaction conditions: 1 (0.40 mmol), norbornene (0.6 mmol), phenyl
formate (0.60 mmol), NEt3 (0.80 mmol), Ni(cod)2 (0.040 mmol),
PCy3 (0.080 mmol), toluene (1.0 mL). Isolated yields are given. The
crystal structure of 5 is shown with ellipsoids set at 30% probability.
shown in Table 3. This result is consistent with the result from
NMR.
Removal of the N-SO2Ar groups in 3aa (Ar = Ph) or 3ba (Ar =
p-tolyl) via nucleophilic substitution reaction by using a
phosphide anion15 gave a synthetically valuable N-protonated
γ-lactam 8 in excellent yields (Scheme 3). Treatment of 8 with
Scheme 3. Synthesis of N-Protonated and N-Boc-Substituted
a
γ-Lactams 8 and 9
a
Isolated yields are given.
Boc2O (1.5 equiv) and DMAP (0.1 equiv) afforded N-Boc-γ-
lactam 9 in 78% yield, the structural motif of which is an
important synthetic intermediate.1p−s,16 Combined with these
derivatizations, the present catalytic system would afford a wide
range of γ-lactams without employing toxic CO gas and
expensive transition metals under the harsh reaction conditions,
which were often found in the reported works.3
In summary, a nickel(0)-catalyzed [2 + 2 + 1] carbonylative
cycloaddition of imines and alkynes or norbornene was achieved
for the first time. A variety of γ-lactams with N-benzenesulfonyl
and tosyl groups were prepared in good to high yields by
employing phenyl formate as a CO source. In addition, these N-
substituent groups were easily removed, which would give
diversity for an accessible γ-lactam. A key to the success of this
method was to control the concentration of CO in situ; i.e., the
concentration had to be high enough to react with the
heteronickelacycle intermediate, yet simultaneously low enough
not to quench the catalytic activity of the nickel(0) complex. The
present methodology will contribute to the development of a
straightforward, simple, and versatile synthetic method to give
valuable heterocyclic compounds.
a
Reaction conditions: 1 (0.4 mmol), 2 (0.8 mmol), phenyl formate
(0.6 mmol), NEt3 (0.8 mmol), Ni(cod)2 (0.04 mmol), PCy3 (0.08
mmol), toluene (1 mL). Isolated yields are given. The structure of
the minor regioisomer was shown in parentheses. The ratio of
regioisomers is 89/11. 20 mol % Ni(cod)2/40 mol % PCy3 was used.
b
c
C
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Journal of the American Chemical Society
Communication
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ASSOCIATED CONTENT
* Supporting Information
Experimental details and spectral data. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
ogoshi@chem.eng.osaka-u.ac.jp
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(5) For our recent works on heteronickelacycle chemistry, see:
(a) Ogoshi, S.; Ikeda, H.; Kurosawa, H. Angew. Chem., Int. Ed. 2007, 46,
4930. (b) Ogoshi, S.; Ikeda, H.; Kurosawa, H. Pure Appl. Chem. 2008, 80,
1115. (c) Ohashi, M.; Kishizaki, O.; Ikeda, H.; Ogoshi, S. J. Am. Chem.
Soc. 2009, 131, 9160. (d) Hoshimoto, Y.; Ohata, T.; Ohashi, M.; Ogoshi,
S. Chem.Eur. J. 2014, 20, 4105.
■
This work was supported by Grants-in-Aid for Scientific
Research (A) (21245028), Scientific Research on Innovative
Area “Molecular Activation Directed toward Straightforward
Synthesis” (23105546), Grants-in-Aid for Young Scientists (A)
(25708018) from MEXT, ACT-C from JST, and the Asahi Glass
Foundation. Y.H. acknowledges support from the Frontier
Research Base for Global Young Researchers, Osaka University,
on the program of MEXT.
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