Oxidative cleavage of the carbon-carbon σ-bond using reusable copper on iron

Article abstract of DOI:10.1002/adsc.201100225

An efficient and resuble copper on iron catalyst has been prepared for the cleavage of the carbon-carbon σ-bond in α-aminocarbonyl compounds leading to the corresponding formylamides and acids with a maximal TON of up to 51,000. It is noteworthy that the copper on iron catalyst can be easily separated from the reaction mixture, and retains its activity after several reuses. Copyright

Full text of DOI:10.1002/adsc.201100225

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DOI: 10.1002/adsc.201100225
À
Oxidative Cleavage of the Carbon Carbon s-Bond Using
Reusable Copper on Iron
Ren-Jie Song,a,b Yu Liu,b Rui-Xiang Hu,b Yan-Yun Liu,b Ji-Cheng Wu,b
Xu-Heng Yang,b and Jin-Heng Lia,*
a
College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, Peopleꢀs Republic of China
Fax : (+86)-731-8887-2531; phone: (+86)-731-8887-2531; e-mail: jhli@hunnu.edu.cn
Key Laboratory of Chemical Biology & Traditional Chinese Medicine Research (Ministry of Education), Hunan Normal
b
University, Changsha 410081, Peopleꢀs Republic of China
Received: March 29, 2011; Published online: June 14, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100225.
Abstract: An efficient and resuble copper on iron
catalyst has been prepared for the cleavage of the
carbon-carbon s-bond in a-aminocarbonyl com-
pounds leading to the corresponding formylamides
and acids with a maximal TON of up to 51,000. It is
noteworthy that the copper on iron catalyst can be
easily separated from the reaction mixture, and re-
tains its activity after several reuses.
Here, we report a new, reusable copper on iron heter-
ogeneous catalyst for cleaving the carbon-carbon s-
bond in a-aminocarbonyl compounds with the aid of
TEMPO and O2, providing the corresponding formyl-
AHCTUNGTRENNGaUN mides and acids in moderate to good yields
(Scheme 2).[13] It is noteworthy that the carbon-
carbon single bond cleavage can be carried out under
conditions of relatively lower loading of Cu with a
maximal TON (turnover number) of up to 51,000.
Our investigation began with the reaction of 2-
Keywords: a-aminocarbonyl compounds; copper;
formylamides; iron; oxidative cleavage; reusable
catalysts
[methylACTHNUTRGNEUG(N phenyl)amino]-1-phenylethanone (1a) in
MeCN at 508C for 12 h under an O2 atmosphere;
however, the reaction did not take place (entry 1 in
Table 1). Interestingly, the reaction proceeded
smoothly when both 2 mol% Cu (Cu on Fe) and O2
were added, providing the corresponding N-methyl-
Transition metal-catalyzed cleavage of the carbon- N-phenylformamide (2a) in 61% yield along with a
carbon bond,[1] particularly the inert carbon-carbon s-
bond, is one of the most challenging tasks in organic
chemistry. Until now, numerous unique and useful
carbon-carbon bond cleavage procedures have been
developed using stoichiometric or catalytic amounts
of transition metals, such as rhodium,[2] ruthenium,[3]
gold,[4] palladium,[5] silver,[6] nickel,[7] copper[8] or
iron[9] catalysts. Among these procedures, two general
routes to the carbon-carbon s-bond cleavage include
À
(i) oxidative addition to the C C bonds with low-
valent metals through an insertion process,[7,10] which
often employs the strained ring as the reaction part-
ner, and (ii) b-carbon elimination of the carbon-metal
Scheme 1. Two routes to the carbon-carbon s-bond cleav-
age.
[3b,11]
À À À
species (M C C C)
(M Y C C, Y=O, N) (Scheme 1).
or heteroatom-metal species
[2i,5a–d,12]
À À À
However, all these procedures are restricted in
both the cost and loading of the catalysts. To the best
of our knowledge, examples on the use of lower-load-
ing and reusable catalysts for the purpose of carbon-
Scheme 2. Cu-catalyzed oxidative carbon-carbon s-bond
carbon s-bond cleavage have not been described. cleavage.
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Table 1. Screening for the reaction conditions.[a]
(entry 5), the yield was enhanced to 76% using a
purity of 99.5 wt% for Cu and to 82% yield with a
purity of 99.999 wt% Cu (entries 6 and 7). On the
contrary, the catalytic activities of Fe powders were
decreased with increasing purity (entries 8–10). Com-
mercially available common Fe powders (purity: wt%
Fe =88.2667%) afforded product 2a in 41% yield
(entry 8), whereas use of 99.5% purity Fe lowered the
yield of 2a to 18% and to 10% yield when using
99.998% purity Fe (entries 9 and 10). These observa-
tions suggest that Cu is the real catalyst, and Fe may
play a part as ligand to improve the reaction to some
extent.
The screening results demonstrated that the solvent
effect has some influence on the reaction, other sol-
vents, such as CH2Cl2, THF, DMF and DMSO, were
effective for the reaction, but they were inferior to
MeCN (entries 11–14). It is noteworthy that oxygen is
necessary for the reaction: the yield of 2a was re-
duced to 63% in air and to 22% in argon (entries 15
and 16). From a reaction temperature screen, it
turned out that the reaction at 508C gave the best re-
sults (entries 3 and 17–19). Notably, the reaction con-
ditions are compatible with a large scale: 10 mmol of
Entry [M] (weight%) Additive Solvent Yield [%][b]
2a
3a
1
2
3
4
–
–
–
MeCN
MeCN
0
0
50
65
Cu/Fe
Cu/Fe
–
61
85
9
69
76
82
41
18
10
TEMPO MeCN
TEMPO MeCN
trace
ND
ND
65
ND
ND
ND
ND
ND
ND
ND
ND
ND
5
Cu (98.5674%) TEMPO MeCN
6
7
Cu (99.5%)
Cu (99.999%)
TEMPO MeCN
TEMPO MeCN
8
Fe (88.2667%) TEMPO MeCN
9
Fe (99.5%)
Fe (99.998%)
Cu on Fe
Cu on Fe
Cu on Fe
Cu on Fe
Cu on Fe
Cu on Fe
Cu on Fe
Cu on Fe
Cu on Fe
Cu on Fe
Cu on Fe
TEMPO MeCN
TEMPO MeCN
TEMPO CH2Cl2 76
TEMPO THF
TEMPO DMF
TEMPO DMSO 62
TEMPO MeCN
TEMPO MeCN
TEMPO MeCN
TEMPO MeCN
TEMPO MeCN
TEMPO MeCN
TEMPO MeCN
10
11
12
13
14
15[c]
16[d]
17[e]
18[f]
19[g]
20[h]
21[i]
68
60
63
22
trace ND
70
80
86
38
ND
62
71
2-[methylACTHNUGRTNEUNG(phenyl)amino]-1-phenylethanone (1a) were
treated with 2 mol% Cu on Fe, TEMPO and O2 and
recated smoothly to furnish an 86% yield of 2a and a
71% yield 3a (entry 20). To our delight, the reaction
could be carried out at a relatively lower loading of
27
[a]
Reaction conditions: 1a (0.2 mmol), additive (1.2 equiv.),
[M] (2 mol%), O2 (1 atm) and solvent (2 mL) at 508C for
12 h. Cu/Fe (wt%): 37.5% Cu and 31.3% Fe. ND=not Cu: 0.075 mol% Cu (Cu on Fe) which furnished the
determined.
[b]
[c]
[d]
[e]
[f]
Yield of isolated products.
Air (1 atm) instead of O2.
Under the argon atmosphere.
At room temperature.
At 808C.
At 1008C.
1a (10 mmol).
[g]
[h]
[i]
1a (10 mmol) and Cu (0.075 mol%, Cu on Fe) for 12 h,
and the TON was 51000.
50% yield of benzoic acid (3a) (entry 2). We were
pleased to find that TEMPO could facilitate the reac-
tion: the yield was enhanced to 85% together with
benzoic acid (3a) in 65% yield in the presence of
1.2 equivalents of TEMPO (entry 3). To our surprise,
the reaction could run in 9% yield of 2a after 12 h in
the presence of TEMPO alone (entry 4). The reason
may be that there is some copper leftover in TEMPO
as determined by ICP-MS analysis.[14] Subsequently,
the catalytic activities of Cu or Fe powders were in-
vestigated to better understand of the reaction. Inter-
estingly, the results demonstrated that the catalytic ac-
tivities of Cu or Fe powders are affected by their
purity (entries 5–10). While commercially available
common Cu powders (purity: wt% Cu=98.5674%)
Figure 1. Reuse of the Cu on Fe catalyst during the reaction
of 1a: (left) before the reaction, (right) after the reaction.
displayed lower activity leading to 69% yield of 2a The Cu on Fe catalyst was absorbed on the stirred bar.
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Oxidative Cleavage of the Carbon Carbon s-Bond Using Reusable Copper on Iron
desired product 2a in 38% yield in 12 h along with activity of the Cu on Fe catalyst was not obviously de-
product 3a in 30% yield (TON=51,000, entry 21).
creased during the recycling processes. However, the
As shown in Figure 1, the recycling of the copper loss of some Cu species during the reuse experiments
on iron catalyst during the reaction of substrate 1a resulted in a slight reduction of the yields.
was tested using an external magnetic field. The ex-
The scope of a-aminocarbonyl compounds was
periments indicated that the copper on iron catalyst then explored in the presence of Cu on Fe, TEMPO
could be reused at least 5 times without losing catalyt- and O2, and the results are summarized in Table 2.
ic activity. After initial experimentation, the reaction The results demonstrate that numerous substituents,
mixture was removed, the catalyst was washed, dried, such as MeO, Me, Ph, Cl or NO2, on aryl ring of the
and subjected to a second run of the oxidative cleav- 1-arylethanone moiety were perfectly tolerated under
age by charging with the same substrate 1a, oxidizing the standard conditions (entries 1–10). Substrate 1b
reagents and solvent for 12 h. Gratifyingly, five runs with a methoxy group, for instance, was treated with
were almost consistent in yields, suggesting that the Cu on Fe, TEMPO and O2 to smoothly afford the de-
Table 2. The carbon-carbon bond cleavage reactions of a-aminocarbonyl compounds (1) using the copper on iron catalyst.[a]
Entry
a-Aminocarbonyl (1)
Time [h]
Yield [%][b]
2
3
1
2
(1b)
(1c)
10
8
78 (2a)
62 (3b)
75 (2c)
72 (2d)
60 (3b)
64 (3b)
3
(1d)
8
4[c]
(1e)
(1f)
(1g)
12
12
9
40 (2e)
81 (2a)
63 (2a)
55 (3b)
66 (3f)
43 (3g)
5
6
7
(1h)
8
83 (2a)
50 (3h)
8
(1i)
12
12
16
69 (2a)
71 (2a)
58 (2a)
61 (3i)
69 (3j)
45 (3k)
9
(1j)
(1k)
10
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Table 2. (Continued)
Entry
a-Aminocarbonyl (1)
Time [h]
Yield [%][b]
2
3
11
(1l)
17
30
13
68 (2a)
59 (3l)
60 (3m)
72 (3b)
12[d]
13
(1m)
(1n)
63 (2a)
85 (2n)
68 (2o)
71 (2p)
65 (2q)
71 (2r)
50 (2s)
14
15
16
17
(1o)
(1p)
(1q)
(1r)
(1s)
13
13
9
50 (3b)
58 (3b)
55 (3b)
57 (3b)
73 (3b)
12
38
18
[a]
Reaction conditions: 1a (0.2 mmol), Cu (2 mol%, Cu on Fe), TEMPO (1.2 equiv.), O2 (1 atm) and MeCN (2 mL) at 508C.
Yield of isolated product.
Some unidentified by-products were observed.
[b]
[c]
[d]
GC yield of 3m using nitrobenzene as the internal standard.
sired products 2a and 3b in 78% and 62% yields, re- tions were also compatible with aliphatic ketone 1m
spectively (entry 1). We were delighted to discover (entry 12).
that the analogous ketones 1c and 1d with the N-
Subsequently, the substitutents on the N-aryl ring
methyl group replaced by a benzyl group or an allyl were investigated (entries 13–17). The results showed
group were compatible with the standard reaction that substrates 1n–1r, having either alkyl or chloro
conditions (entries 2 and 3). Notably, 1-phenyl-2-(phe- groups, were successfully reacted with Cu on Fe,
nylamino)ethanone (1e), a secondary amine, was also TEMPO and O2 in good yields. For example, sub-
suitable for the cleavage reaction (entry 4). Substrates strate 1r with an N-2,3-dihydro-1H-inden-5-yl group
1i and 1j bearing chloro groups successfully under- provided the target products 2r and 3b in 71% and
went the reaction to furnish the corresponding prod- 57% yields, respectively (entry 17). It was interesting
ucts in good yields (entries 8 and 9). Moderate yields to observe that 1-(4-methoxyphenyl)-2-morpholinoe-
were still achieved from NO2-substituted substrate 1k thanone (1s) was effectively cleaved to the desired
(entry 10). Gratifyingly, a-amino enone 1l could be products under the standard conditions (entry 18).
cleaved by Cu on Fe, TEMPO and O2 leading to 68%
As shown in Scheme 3, this present methodology
yield of N-methyl-N-phenylformamide (2a) and 59% was applied to the synthesis of 6H-benzo[c]chromen-
yield of cinnamic acid (3l) (entry 11). It is noteworthy 6-one (5a), which displays great potential utilizations
that the latter, cinnamic acid (3l), is widely used in in organic synthesis and for the preparation of phar-
flavors and fragrances, food additives, and the phar- maceuticals, such as estrogen receptor modulators.[15]
maceutical industry. Interestingly, the standard condi- In the presence of Cu on Fe, TEMPO and O2, (6H-
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Oxidative Cleavage of the Carbon Carbon s-Bond Using Reusable Copper on Iron
Scheme 3. Application of this carbon-carbon bond cleavage reaction.
Scheme 4. Control experiments.
benzo[c]chromen-6-yl)
(phenyl)methanone (4a) selec-
The data of in situ FT-IR analysis indicated that
tively underwent the cleavage reaction to afford the substrate 1a has two peaks: 1701 cmÀ1 (the C=O
desired product 5a in 68% yield along with benzoic stretching vibration) and 1562 cmÀ1 (the N H bending
acid (3a) in 63% yield.
À
vibration). Interestingly, a new peak, 1567 cmÀ1, is in-
To understand the mechanism, some control experi- creasing and then decreasing over 1000–2000 s besides
ments were carried out (Scheme 4). The cleavage re- another peak at 1681 cmÀ1 (the C=O stretching vibra-
action of 1a was tested in the presence of 2 equiva- tion of 2a) (Scheme 5). These results suggest that an
lents of H218O under the standard conditions [Eq. imine intermediate is generated.[14,16]
(1)]. However, only a content of 2.1% of the 18O-la-
Therefore, a possible mechanism as outlined in
beled product 2a was determined by GC-MS analysis, Scheme 5 was proposed.[13,16–18] Intermediate A is gen-
suggesting the two new oxygen atoms in product 2a erated from the reaction of Cu with O2,[13] followed
are from O2. The deuterium-labeled substrate 1m-D5 by addition to the imine intermediate B to afford in-
gave a monodeuterium-labeled product 2a-D1 [Eq. termediate C.[17,18] The imine intermediate B is readily
(2)]. The results showed that the reaction of N- formed from substrate 1a with the aid of Cu and oxi-
methyl-2-oxo-N,2-diphenylacetamide (6a) did not dants, which is supported by in situ FT-IR analy-
take place in the presence of Cu on Fe, TEMPO and sis.[13,16] The cleavage of the C C s-bond and reduc-
O2 [Eq. (3)], which implies the reaction does not pro- tive elimination in intermediate C takes place leading
À
ceed via the dicarbonyl intermediate. The value of the to intermediate D. Intermediate D sequentially under-
À
intermolecular kinetic isotope effect (kH/kD =1.5) im- goes the O O bond cleavage and reductive elimina-
plies that the C H bond functionalization is the rate- tion to furnish the desired products and generate the
limiting step.
À
CuL species.
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Ren-Jie Song et al.
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dried over Na2SO4 and concentrated under vacuum, and the
resulting residue was purified by silica gel column chroma-
tography (hexane/ethyl acetate) to afford the desired prod-
uct.
N-Methyl-N-phenylformamide (2a):[19] Colorless oil;
1H NMR (500 MHz, CDCl3): d=8.41 (s, 1H), 7.34 (t, J=
8.0 Hz, 2H), 7.21 (t, J=7.5 Hz, 1H), 7.10 (d, J=9.0 Hz,
2H), 3.25 (s, 3H); 13C NMR (125 MHz, CDCl3): d=162.4,
142.2, 129.6, 126.4, 122.4, 32.1; IR (KBr): n=1671 cmÀ1; LR-
MS (EI 70 eV): m/z (%)=135 (M+, 68), 106 (100).
Acknowledgements
We thank the Scientific Research Fund of Hunan Provincial
Education Department (No. 08A037), Natural Science Foun-
dation of China (Nos. 20872112 and 20871046) and Hunan
Provincial Innovation Foundation for Postgraduates (No
CX2009B104) for financial support.
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In summary, we have prepared and successfully ap-
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