Catalytic alkylation of benzylic C-H bonds with 1,3-dicarbonyl compounds utilizing oxygen as terminal oxidant

Article abstract of DOI:10.1016/j.tetlet.2009.12.080

The oxidative alkylation of benzylic C-H bonds with 1,3-dicarbonyl compounds was developed using oxygen as the terminal oxidant in the presence of catalytic amounts of FeCl₂, CuCl and NHPI.

Full text of DOI:10.1016/j.tetlet.2009.12.080

Tetrahedron Letters 51 (2010) 1172–1175
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Tetrahedron Letters
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Catalytic alkylation of benzylic C–H bonds with 1,3-dicarbonyl compounds
utilizing oxygen as terminal oxidant
*
Camille A. Correia, Chao-Jun Li
Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6
a r t i c l e i n f o
a b s t r a c t
Article history:
The oxidative alkylation of benzylic C–H bonds with 1,3-dicarbonyl compounds was developed using
oxygen as the terminal oxidant in the presence of catalytic amounts of FeCl2, CuCl and NHPI.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 29 October 2009
Revised 12 December 2009
Accepted 14 December 2009
Available online 22 December 2009
The recent emphasis on green and sustainable chemistry has
fed a growing interest towards direct C–C bond formation through
functionalization of C–H bonds.1 One particular class of reactions
in this respect is the transition metal-catalyzed functionalization
of sp3 C–H bonds.2 Due to their usefulness, there is a wide interest
in the benzylation of 1,3-dicarbonyl compounds and much of the
research in this area centers on the metal-catalyzed3 or Bron-
sted-acid catalyzed4 alkylation of the benzylic alcohol. However,
all these reactions use functionalized alcohols as substrates. In
our effort to seek novel reactions of C–H bonds, we envisioned
the alkylation through the direct oxidative reactions of both the
1,3-dicarbonyl and benzylic C–H bonds.
Previous reports by us and Powell have described the metal-cat-
alyzed oxidative alkylation of benzylic C–H bonds in the presence
of excess peroxide.5 As part of our interest in finding more environ-
mentally friendly and economical alternatives we turned our
attention to utilizing oxygen as the terminal oxidant. Previous
work reported by us and others have described the use of molecu-
lar oxygen for reactions with the C–H bond adjacent to nitrogen.6
More recently we have shown that, with the use of N-hydroxyph-
thalimide (NHPI) and metal catalysts, cyclic benzylic ethers could
be oxidatively alkylated.7 In the presence of oxygen, NHPI is capa-
ble of oxidizing benzylic C–H bonds through a radical mecha-
nism.8,9 To the best of our knowledge, oxidative C–C formation
involving benzylic C–H bonds using oxygen as terminal oxidant
has not yet been reported.
conditions provided the expected product (3a) in 75% NMR yield
(Table 1, entry 1). From our previous work we are aware that FeCl2
mediates the alkylation in the presence of peroxide,5a we therefore
replaced the indium catalyst with FeCl2 and increased the temper-
ature and were pleased to obtain the product in 84% yield (entry 2).
Rare earth metals10 are known to act as Lewis acids and are able to
generate benzylic cations from benzylic alcohols, therefore we
speculated that if benzhydrol was produced in situ, Sc(OTf)3 would
be able to aid in the alkylation; however its addition affected the
Table 1
Optimization of the reaction conditionsa
NHPI,
O
O
O
O
[M1], [M2]
+
Ph
OEt
Ph Ph
Ph
OEt
O2, heat
1a
2a
Ph Ph 3a
Entry
[M1] (mol %)
[M2] (mol %)
T (°C)
Yieldb (%)
1
2
3
Cu(OTf)2 (5)
Cu(OTf)2 (5)
Sc(OTf)3 (5)
CuCl (5)
CuCl (5)
—
CuCl (5)
CuCl (5)
CuCl (10)
CuCl (5)
CuCl (5)
CuCl (5)
InCl3 (5)
FeCl2 (5)
FeCl2 (5)
FeCl2 (5)
FeCl2 (5)
FeCl2 (5)
FeCl2 (5)
FeCl2 (5)
FeCl2 (5)
FeCl2 (5)
—
75
75
84
73
94
42
68
82
78
85
18
17
72
105
105
105
105
105
105
105
105
105
105
105
4
5c
6
7d
8e
9
10f
11
12g
Herein, we describe an efficient oxidative alkylation of benzylic
C–H bonds with 1,3-dicarbonyls in the presence of catalytic
amounts of NHPI, FeCl2, and CuCl, utilizing oxygen as the terminal
oxidant.
FeCl2 (5)
a
Reaction conditions: 2a (0.2 mmol, 35 lL), 1a (5 equiv, 0.17 mL), and 20 mol %
We first started our investigation using the reaction conditions
reported in our previous publication for the oxidative alkylation of
benzylic ethers and ketones.7 We were happy to note that these
NHPI under oxygen at 105 °C overnight unless otherwise noted.
b
H NMR yields obtained using mesitylene as the internal standard.
Reaction carried out in air.
10 mol % NHPI.
30 mol % NHPI.
c
d
e
f
No NHPI added.
* Corresponding author. Tel.: +1 514 398 8457; fax: +1 514 398 3797.
E-mail address: cj.li@mcgill.ca (C.-J. Li).
g
2.5 equiv 1a used.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.12.080

C. A. Correia, C.-J. Li / Tetrahedron Letters 51 (2010) 1172–1175
1173
yield negatively (entry 3). We were able to increase the yield even
further by replacing Cu(OTf)2 with CuCl (entry 4). A dramatic de-
crease in yield was observed when either NHPI or FeCl2 was re-
moved which is indicative of their importance in the reaction
(entries 10 and 11). The removal of CuCl or running the reaction
in air also caused a decrease in yield although not as dramatic com-
pared to NHPI and FeCl2. Neither increasing the amount of CuCl
(entry 9) nor changing the amount of NHPI (entries 7 and 8) in
the system was favorable. An excess of diarylmethane is needed
in this reaction due to the oxidation of 1a to benzophenone and
5 equiv of the diarylmethane was found to be optimal.
With our optimized conditions in hand (Table 1, entry 4) we
then turned to the scope of the alkylation reaction (Table 2). Both
aromatic b-ketoesters and 1,3-diketones were good substrates
affording moderate to high yields. 1,3-Dicarbonyl substrates with
aliphatic groups such as benzoyl acetone (entry 9) and methyl ace-
toacetate (entry 7) are more difficult to enolize and thus gave low
to moderate yields. Diethyl malonate also reacted with diphenyl
methane to afford the product albeit in low yield which was diffi-
cult to isolate. Electron donation on the phenyl ring of the 4-meth-
oxybenzylanisole better activated the C–H bond and afforded the
product in high yield (entry 12). Although 2-methoxybenzylanisole
(entry 11) is more activated than diphenylmethane, the lower yield
can be attributed to the steric bulk of the ortho-methoxy group.
The reaction also proceeded smoothly for the less activated cyclic
benzylic substrate (entry 10) although in low yield.
We propose two possible pathways that the reaction may be
occurring through (Scheme 1). In oxygen, NHPI generates the
phthalimide N-oxyl (PINO) radical which can then abstract a hydro-
gen radical from diphenylmethane producing a benzyl radical (4).
The benzyl radical can either directly add to the iron-enolate inter-
mediate (7) via path a, or it can be oxidized to benzhydrol (5). A third
possibility exists where the benzyl radical can be trapped by dioxy-
gen to form the peroxide radical. This peroxide radical can abstract
hydrogen from diphenylmethane effectively in a chain propagation
step which produces more benzyl radical (4). The diphenylmethyl
peroxide radical in the presence of iron can then disproportionate
into alcohol (5) and ketone (6) similar to the mechanism we
proposed for isochroman.7,11 Benzhydrol[3d,e] also has the ability to
add to the iron-enolate (path b). We do not know the exact role of
the copper in the reaction but it may aid in the formation of the
diphenylmethane radical. Recently Chang reported a CuCl facilitated
C–O formation between NHPI and benzylic compounds where they
proposed copper was involved in theformation of the PINO radical.12
The presence of benzophenone (6) in the crude reaction mixture is
due to the oxidation of benzhydrol or disproportionation of the
diphenylmethyl peroxide. We further investigated the mechanism
(Scheme 2) by subjecting benzhydrol to the optimized reaction con-
Table 2
Scope for the oxidative alkylation reactiona
Entry
1
Benzylic substrate
1,3-Dicarbonyl substrate
Product
Yieldb (%)
79
O
O
3a
OEt
1a
2a
O
O
2
3
4
5
1a
3b
3c
3d
3e
79
69
66
60
2b
O
O
OMe
1a
1a
1a
2c
Cl
O
O
OEt
2d
O
O
OEt
2e
F3C
O
O
6
7
8
1a
1a
1a
3f
60
55
84
2f
MeO
O
OMe
O
O
3g
3h
OMe
O
2g
O
OEt
2h
O2N
O
9
1a
3i
3j
41
27
2i
10
2b
1b
11
12
2b
2b
3k
3l
71
83
OMe
1c
OMe
1d
a
Reaction conditions: 1a (1.0 mmol), 2a (0.2 mmol), 5 mol % CuCl, 5 mol % FeCl2, and 20 mol % NHPI under O2 at 105 °C overnight.
Isolated yields.
b

1174
C. A. Correia, C.-J. Li / Tetrahedron Letters 51 (2010) 1172–1175
Pfeffer, M.. Chem. Rev. 2002, 102, 1731; (d) Dyker, G. Handbook of C–H
O2
Transformations; Wiley-VCH: Weinheim, 2005; (e) Godula, K.; Sames, D. Science
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Alberico, D.; Scott, M. E.; Lautens, M. Chem. Rev. 2007, 107, 174; (h) Herrerias,
C.; Yao, X.; Li, Z.; Li, C.-J. Chem. Rev. 2007, 107, 2546; (i) Li, C.-J. Acc. Chem. Res.
2009, 42, 335.
NHPI
PINO
O
OH
O2
O2
Ph
Ph
Ph Ph
2. For recent example see: (a) intramolecular reactions: (i) Watanabe, T.
Oishi, T. S.; Fujii, S. N.; Ohno, H. Org. Lett. 2008, 10, 1759; (ii) Zhang, C.; Zhao,
Y.; Li, B.; Song, H.; Xu, S.; Wang, B. Dalton Trans. 2009, 26, 5182; (iii) Xu, K.; Li,
B.; Xu, S.; Song, H.; Wang, B. Organometallics 2009, 28, 4438; (b) Examples of C–
H bonds near nitrogen: (i) Zhao, L.; Basle, O.; Li, C.-J. PNAS 2009, 106, 4106; (ii)
Li, Z.; Li, C.-J. J. Am. Chem. Soc. 2006, 128, 56; (iii) Li, C.-J.; Li, Z. Pure Appl. Chem.
2006, 78, 935; (iv) Zhang, Y.; Li, C.-J. J. Am. Chem. Soc. 2006, 128, 4242; (c) For
recent examples of alkane functionalization: (i) Deng, G.; Chen, W.; Li, C.-J. Adv.
Synth. Catal. 2009, 351, 353; (ii) Deng, G.; Li, C.-J. Org. Lett. 2009, 11, 1171; (iii)
Deng, G.; Zhao, L.; Li, C.-J. Angew. Chem., Int. Ed. 2008, 47, 6278; (d) Examples of
sp3 C–H activation by Ru and Rh: (i) Zhang, S.-Y.; Tu, Y.-Q.; Fan, C.-A.; Jiang, Y.-
J.; Shi, L.; Cao, K.; Zhang, E. Chem. Eur. J. 2008, 14, 10201; (ii) Pastine, S. J.;
Gribkov, D. V.; Sames, D. J. Am. Chem. Soc. 2006, 128, 14220; (iii) Shi, L.; Tu, Y.-
Q.; Wang, M.; Zhang, F.-M.; Fan, C.-A.; Zhao, Y.-M.; Xia, W.-J. J. Am. Chem. Soc.
2005, 127, 10836.
Ph Ph
Ph Ph
1a
[Cu]
4
5
6
path a
path b
FeIII
O
[Fe]
O
O
O
Ph
OEt
Ph
OEt
7
2a
O
O
Ph
OEt
3. For examples of metal catalyzed alkylation of benzylic C–H bonds, see: (a)
Yasuda, M.; Somyo, T.; Baba, A. Angew. Chem., Int. Ed. 2006, 45, 793; (b) Kischel,
J.; Mertins, K.; Michalik, D.; Zapf, A.; Beller, M. Adv. Synth. Catal. 2007, 349, 865;
(c) Babu, S. A.; Yasuda, M.; Tsukahara, Y.; Yamauchi, T.; Wada, Y.; Baba, A.
Synthesis 2008, 1717; (d) Yuan, Y.; Shi, Z.; Feng, X.; Liu, X. Appl. Organomet.
Chem. 2007, 21, 958; (e) Jana, U.; Biswas, S.; Maiti, S. Tetrahedron Lett. 2007, 48,
4065.
4. For recent examples of Bronsted acid catalyzed alkylation of benzylic C–H
bonds see: (a) Sanz, R.; Miguel, D.; Martinez, A.; Alvarez-Gutierrez, J. M.;
Rodriguez, F. Org. Lett. 2007, 9, 2027; (b) Yadav, J. S.; Subba Reddy, B. V.;
Pandurangam, T.; Raghavendra Rao, K. V.; Praneeth, K.; Narayana Kumar, G. G.
K. S.; Madavi, C.; Kunwar, A. C. Tetrahedron Lett. 2008, 49, 4296.
Ph Ph
3a
Scheme 1. Proposed reaction pathways.
20 mol% NHPI,
5mol% FeCl2, 5 mol% CuCl
O
O
OH
Ph Ph
O
O
+
Ph
OEt
Ph
OEt
105oC, O2,
Ph Ph
[90%]
5. (a) Li, Z.; Cao, L.; Li, C.-J. Angew. Chem., Int. Ed. 2007, 46, 6505; (b) Borduas, N.;
Powell, D. A. J. Org. Chem. 2008, 73, 7822.
Scheme 2. Reaction of benzhydrol with ethyl benzoyl acetate.
6. (a) Murahashi, S.-I.; Komiya, N.; Terai, H.; Nakae, T. J. Am. Chem. Soc. 2003, 125,
15312–15313; (b) Basle, O.; Li, C.-J. Green Chem. 2007, 9, 1047; (c) Basle, O.; Li,
C.-J. Org. Lett. 2008, 10, 3661; (d) Murahashi, S.-I.; Nakae, T.; Terai, H.; Komiya,
N. J. Am. Chem. Soc. 2008, 130, 11005; (e) Shen, Y.; Li, M.; Wang, S.; Zhan, T.;
Tan, Z.; Guo, C. C. Chem. Commun. 2009, 953; (f) Singhal, S.; Jain, S.; Suman, L.;
Sain, B. Chem. Commun. 2009, 2371.
ditions and found that the reaction gave the C–C coupling product in
90% (NMR yield). Furthermore, this reaction also gives the product in
83% NMR yield when run in the absence of NHPI and under nitrogen,
both of which supports path b.13
In summary, we have developed an efficient method for the
construction of new C–C bonds through the alkylation of benzylic
C–H bonds with 1,3-dicarbonyl compounds. Furthermore, with
the addition of catalytic amounts of FeCl2, CuCl, and NHPI we were
able to utilize oxygen as the terminal oxidant. Further investiga-
tion into the mechanism, scope, and application of this chemistry
is in progress.
7. Yoo, W.-J.; Correia, C. A.; Zhang, Y.; Li, C.-J. Synlett 2009, 138.
8. For reviews, see: (a) Ishii, Y.; Sakaguchi, S.; Iwahama, T. Adv. Synth. Catal. 2001,
343, 393; (b) Sheldon, R. A.; Arends, I. W. C. E. Adv. Synth. Catal. 2004, 346, 1051;
(c) Recupero, F.; Punta, C. Chem. Rev. 2007, 107, 3800; (d) Galli, C.; Gentili, P.;
Lanzalunga, O. Angew. Chem., Int. Ed. 2008, 47, 4790.
9. For the oxidation of C–H bonds using NHPI in oxygen, see: (a) Ishii, Y.;
Nakayama, K.; Takeno, M.; Sakaguchi, S.; Iwahama, T.; Nishiyama, Y. J. Org.
Chem. 1995, 60, 3934; (b) Ishii, Y.; Iwahama, T.; Sakaguchi, S.; Nakayama, K.;
Nishiyama, Y. J. Org. Chem. 1996, 61, 4520; (c) Kato, S.; Iwahama, T.; Sakaguchi,
S.; Ishii, Y. J. Org. Chem. 1998, 63, 222; (d) Sakaguchi, S.; Nishiwaki, Y.;
Kitamura, T.; Ishii, Y. Angew. Chem., Int. Ed. 2001, 40, 222; (e) Hara, T.; Iwahama,
T.; Sakaguchi, S.; Ishii, Y. J. Org. Chem. 2001, 66, 6425; (f) Hirabayashi, T.;
Sakaguchi, S.; Ishii, Y. Angew. Chem., Int. Ed. 2004, 43, 1120; (g) Sheldon, R. A.;
Arends, I. W. C. E. J. Mol. Catal. A: Chem. 2006, 251, 200; (h) Amorati, R.; Lucarini,
M.; Mugnaini, V.; Pedulli, G. F.; Minisci, F.; Recupero, F.; Fontana, F.; Astolfi, P.;
Greci, L. J. Org. Chem. 2003, 68, 1747.
Procedural information: 2-benzyl anisole 1c and 4-benzyl ani-
sole 1d were synthesized according to earlier methods.5a Com-
pounds 3a–3c, 3g, and 3i–3l are known compounds and their 1H
NMR and 13C NMR spectra corresponded to the literature.5a,3d 1a
10. (a) Rueping, M.; Nachtsheim, B. J.; Kuenkel, A. Org. Lett. 2007, 9, 825; (b)
Rueping, M.; Nachtsheim, B. J.; Ieawsuwan, W. Adv. Synth. Catal. 2006, 348,
1033; Noji, M.; Konno, Y.; Ishii, K. J. Org. Chem. 2007, 72, 5161.
(1.0 mmol, 0.17 mL) and 2a (0.2 mmol, 35 lL) were placed in a
sealable tube with a magnetic stir bar, to this CuCl (5 mol %,
1.0 mg), FeCl2 (5 mol %, 1.3 mg) and NHPI (20 mol %, 6.5 mg) were
added. The tube was flushed with oxygen and a balloon of oxygen
was attached, after which it was placed in an oil bath at 105 °C
overnight. The mixture was allowed to cool then flushed through
a short column of silica gel in a pasture pipette with ethyl acetate
and, after rotary evaporation, the compound was isolated by flash
column chromatography on silica gel (hexane/ethyl acetate 5:1,
Rf = 0.4). Off-white solid 3a was obtained (56.3 mg, 79%).14
11. Jones, C. W. Application of Hydrogen Peroxide and Derivatives; The Royal Society
of Chemistry: Cambridge, 1999.
12. Lee, J. M.; Park, E. J.; Cho, S. H.; Chang, S. J. Am. Chem. Soc. 2008, 130, 7824.
13. The production of 3a in the absence of oxygen and NHPI suggest
a
diphenylmethane carbocation intermediate and subsequent attack by the
iron enolate. This carbocation formation could be assisted by the Lewis acids in
the mixture.
14. (a) Characterization of unknown compounds: Ethyl 2-(4-methylbenzoyl)-3,3-
diphenylpropanoate (3d): Mp 148.5–151.8 °C; 1H NMR (500 MHz, CDCl3, ppm) d
7.95 (d, J = 8.3 Hz, 2H), 7.39 (d, J = 7.3 Hz, 2H), 7.31–7.21 (m, 6H), 7.20–7.15 (m,
3H), 7.07 (t, J = 7.3 Hz, 1H), 5.41 (d, J = 11.8 Hz, 1H), 5.10 (d, J = 11.7 Hz, 1H),
3.98–3.87 (m, 2H), 2.41 (s, 3H), 0.94 (t, J = 7.3 Hz, 3H); 13C NMR (125 MHz,
CDCl3, ppm) d 192.2, 167.9, 144.6, 141.8, 134.1, 129.4, 128.9, 128.6, 128.5,
128.3, 127.7, 126.8, 126.5, 61.5, 59.2, 50.8, 21.7, 12.7; HRMS (ESI): m/z:
[M+Na]+ calcd for C25H24O3Na: 395.1623; found: 395.1614. Ethyl 3,3-diphenyl-
2-(4-(trifluoromethyl)benzoyl)propanoate (3e): Mp 118.0–120.2 °C; 1H NMR
(500 MHz, CDCl3, ppm) d 8.11 (d, J = 8.4 Hz, 2H), 7.71 (d, J = 8.3 Hz, 2H), 7.40 (d,
J = 7.5 Hz, 2H), 7.31 (t, J = 7.4 Hz, 2H), 7.27–7.16 (m, 5H), 7.1–7.07 (t, J = 7.4 Hz,
1H), 5.42 (d, J = 11.8 Hz, 1H), 5.09 (d, J = 11.8 Hz, 1H), 3.99–3.92 (m, 2H), 0.96 (t,
J = 7.4 Hz, 3H); 13C NMR (125 MHz, CDCl3, ppm) d 192.3, 167.3, 141.4, 141.3,
139.2, 134.9, 134.6, 129.0, 128.7, 128.6, 128.1, 127.7, 127.0, 126.8, 125.8, 125.8,
125.7, 125.7, 61.9, 59.9, 50.9, 13.7; HRMS (ESI): m/z: [M+Na]+ calcd for
C25H21O3F3Na: 449.1340; found: 449.1332. 2-Benzhydryl-1,3-bis(4-methoxy-
phenyl)- propane-1,3-dione (3f): Mp 182.0–183.5 °C; 1H NMR (500 MHz, CDCl3,
ppm) d 7.88 (d, J = 8.8 Hz, 4H), 7.26 (d, J = 7.6 Hz, 4H), 7.16 (t, J = 7.6 Hz, 4H),
7.06 (t, J = 7.4 Hz, 2H), 6.82 (d, J = 9 Hz, 4H), 6.22 (d, J = 11.5 Hz, 1H), 5.33 (d,
Acknowledgments
We thank the Canada Research Chair (Tier 1) foundation (to CJL)
and NSERC for their support to this research. C.A.C. would also like
to thank McGill University for post-graduate fellowships (Bill and
Christina Chan Fellowship and Principle’s Graduate Fellowship).
References and notes
1. For recent reviews, see: (a) Bergman, R. G. Nature 2007, 446, 391; (b) Jia, C.;
Kitamura, T.; Fujiwara, Y. Acc. Chem. Res. 2001, 34, 633; (c) Ritleng, V.; Sirlin, C.;

C. A. Correia, C.-J. Li / Tetrahedron Letters 51 (2010) 1172–1175
1175
J = 11.8 Hz, 1H), 3.81 (s, 6H); 13C NMR (125 MHz, CDCl3, ppm) d 192.6, 163.6,
142.0, 131.1, 130.0, 128.5, 128.2, 126.5, 113.8, 62.2, 55.4, 52.2; HRMS (ESI):
m/z: [M+Na]+ calcd for C30H26O4Na: 473.1729; found: 473.1719. Ethyl 2-(4-
nitrobenzoyl)-3,3-diphenylpropanoate (3h): Mp 104.5–107.2 °C; 1H NMR
(500 MHz, CDCl3, ppm) d 8.27 (d, J = 8.5 Hz, 2H), 8.13 (d, J = 8.5 Hz, 2H), 7.40
(d, J = 7.6 Hz, 2H), 7.31 (t, J = 7.6 Hz, 2H), 7.27–7,22 (m, 3H), 7.17 (t, J = 7.6 Hz,
2H), 7.08 (t, J = 7.3 Hz, 1H) 5.42 (d, J = 11.9 Hz, 1H), 5.08 (d, J = 11.8 Hz, 1H),
4.12–3.93 (m, 2H) 0.974 (t, J = 7.1 Hz, 3H); 13C NMR (125 MHz, CDCl3, ppm)
d 192.9, 167.1, 150.4, 141.2, 141.1, 141.0, 129.6, 128.8, 128.7, 128.0, 127.7,
127.1, 126.9, 123.9, 62.1, 60.2, 51.1, 13.7; HRMS (ESI): m/z: [M+Na]+ calcd for
C24H21O5NNa: 426.1317; found: 426.1310.