An efficient palladium-catalyzed synthesis of benzils from aryl bromides: Vinylene carbonate as a synthetic equivalent of glyoxal

Article abstract of DOI:10.1016/j.tetlet.2011.04.108

An expedient synthetic procedure of benzil derivatives from aryl bromides was developed using vinylene carbonate as a glyoxal equivalent in a palladium-catalyzed reaction. The reaction involved a sequential diarylation of vinylene carbonate to form 4,5-diaryl-1,3-dioxol-2-one, ring-opening to benzoin derivative, and an oxidation process.

Full text of DOI:10.1016/j.tetlet.2011.04.108

Tetrahedron Letters 52 (2011) 3463–3466
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An efficient palladium-catalyzed synthesis of benzils from aryl bromides:
vinylene carbonate as a synthetic equivalent of glyoxal
⇑
Ko Hoon Kim, Bo Ram Park, Jin Woo Lim, Jae Nyoung Kim
Department of Chemistry and Institute of Basic Science, Chonnam National University, Gwangju 500-757, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
An expedient synthetic procedure of benzil derivatives from aryl bromides was developed using vinylene
carbonate as a glyoxal equivalent in a palladium-catalyzed reaction. The reaction involved a sequential
diarylation of vinylene carbonate to form 4,5-diaryl-1,3-dioxol-2-one, ring-opening to benzoin derivative,
and an oxidation process.
Received 15 April 2011
Revised 26 April 2011
Accepted 26 April 2011
Available online 5 May 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Benzils
Vinylene carbonate
Palladium
Glyoxal
Aerobic oxidation
1,2-Diketones are very important structural moieties in numer-
ous biologically interesting compounds and are broadly utilized for
construction of complex structures in organic synthesis.1 Among
the numerous 1,2-diketones, benzil derivatives have received a
special attention. Usually benzil derivatives have been synthesized
by oxidation of alkynes,2 acyloin condensation of aldehydes and a
subsequent oxidation,3 coupling of acyl cyanide,4 Pd-catalyzed ary-
lation of arylglyoxals with arylboronic acids,5a and other meth-
ods.5b–f To the best of our knowledge, there is no precedent
synthetic method of benzils from haloarenes.
We reasoned that vinylene carbonate (2) could be used as a
two-carbon unit, a glyoxal equivalent, in a Pd-catalyzed Heck type
reaction with bromobenzene (1a) to form 4,5-diphenyl-1,3-dioxol-
2-one (4a) or benzil (5a), as shown in Scheme 1. The most impor-
tant point in our rationale is that a Heck reaction between PhPdBr
and vinylene carbonate could occur via a formal anti-elimination of
HPdBr presumably via a facile formation of an oxonium ion inter-
mediate II (vide infra).6,7
K2CO3 in DMF at 90 °C (entry 1); however, benzil (5a) was produced
in low yield (5%). The use of Cs2CO3 in refluxing CH3CN increased
the yield of 5a to 15% (entry 2). The reaction under DMF/Cs2CO3
conditions raised the yield of 5a to 46% (entry 3). Increasing the
amount of a palladium catalyst (entry 4) provided a good yield
(74%) of 5a. Finally, the yield of 5a was improved to 81% at 120 °C
(entry 5) within short time (30 min).8 Reducing the amount of bro-
mobenzene (entries 6 and 7) decreased the yield of 5a. The yield of
5a was similar with that of entry 5 when we used excess amounts of
bromobenzene (entry 8). Thus we chose entry 5 as the optimum
conditions. Interestingly, 4-phenyl-1,3-dioxol-2-one (3a) was iso-
lated in low yield (entry 9) when we used Et3N in DMF (vide infra).
The use of iodobenzene produced low yield of 5a (entry 10). In this
case, reductive dimerization to biphenyl was the major pathway.
The reaction with chlorobenzene failed completely (entry 11).
Encouraged by the successful results, we examined the reaction
with various aryl bromides 1a–j and the results are summarized in
Table 2. The reactions with p-bromotoluene (1b), o-bromotoluene
(1c), p-bromoanisole (1d), m-bromoanisole (1e), 1-bromo-4-chlo-
robenzene (1f), 2-bromonaphthalene (1g), 1-bromonaphthalene
Thus, we examined the reaction of 1a and 2 under various con-
ditions as shown in Table 1. At the outset of our studies we used
Ph
Ph
O
O
Ph
Ph
Pd0
Pd0
Br
O
O
O
O
and/or
+
O
O
O
1a
O
O
Ph
4a
5a
1a
2
3a
Scheme 1.
⇑
Corresponding author. Tel.: +82 62 530 3381; fax: +82 62 530 3389.
E-mail address: kimjn@chonnam.ac.kr (J.N. Kim).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.04.108

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K. H. Kim et al. / Tetrahedron Letters 52 (2011) 3463–3466
Table 1
Optimization of reaction conditions for the synthesis of benzil (5a) from bromobenzene (1a)
Entry
Conditionsa
5ab (%)
1
2
3
4
5
6
7
8
9
1a (3.0 equiv), Pd(OAc)2 (5 mol %), PPh3 (10 mol %), K2CO3 (2.2 equiv), DMF, 90 °C, 1 h
1a (3.0 equiv), Pd(OAc)2 (5 mol %), PPh3 (10 mol %), Cs2CO3 (2.2 equiv), CH3CN, reflux, 2 h
1a (3.0 equiv), Pd(OAc)2 (5 mol %), PPh3 (10 mol %), Cs2CO3 (2.2 equiv), DMF, 90 °C, 1 h
1a (3.0 equiv), Pd(OAc)2 (10 mol %), PPh3 (20 mol %), Cs2CO3 (2.2 equiv), DMF, 90 °C, 1 h
1a (3.0 equiv), Pd(OAc)2 (10 mol %), PPh3 (20 mol %), Cs2CO3 (2.2 equiv), DMF, 120 °C, 30 min
1a (2.2 equiv), Pd(OAc)2 (10 mol %), PPh3 (20 mol %), Cs2CO3 (2.2 equiv), DMF, 120 °C, 30 min
1a (1.0 equiv), Pd(OAc)2 (10 mol %), PPh3 (20 mol %), Cs2CO3 (1.0 equiv), DMF, 120 °C, 30 min
1a (4.0 equiv), Pd(OAc)2 (10 mol %), PPh3 (20 mol %), Cs2CO3 (3.0 equiv), DMF, 120 °C, 30 min
1a (3.0 equiv), Pd(OAc)2 (10 mol %), Et3N (3.0 equiv), DMF, 120 °C, 30 min
5
15
46
74
81
55
60c
79
0d
10
11
Iodobenzene (3.0 equiv), Pd(OAc)2 (10 mol %), PPh3 (20 mol %), Cs2CO3 (2.2 equiv), DMF, 120 °C, 30 min
Chlorobenzene (3.0 equiv), Pd(OAc)2 (10 mol %), PPh3 (20 mol %), Cs2CO3 (2.2 equiv), DMF, 120 °C, 1 h
36
0
a
b
c
Vinylene carbonate (2, 1.0 equiv) is common.
Isolated yield based on 2, and appreciable amounts of biphenyl were observed.
Yield based on 1a.
d
4-Phenyl-1,3-dioxol-2-one (3a) was isolated in 27%.
Table 2
(41%). In all entries, the corresponding biaryl derivatives (24–46%
based on 1a–j) and arenes (0–7%) were formed as side products.
The reactions with aryl bromides having an electron-withdrawing
substituent showed a sluggish reactivity in the reaction. As exam-
ples, we could not obtain the corresponding benzils in appreciable
amounts for the reactions of 2-bromobenzaldehyde and 2-
bromopyridine.
Synthesis of benzil derivatives 5a-j
O
Conditionsa
O
O
Ar-Br
+
O
Ar
Ar
O
2
1a-j
5a-j
It is interesting to note that appreciable amounts of aryl–aryl
interchange reaction of ArPdL2Br (L = PPh3) complexes9 has been
observed when we used aryl bromides having a methoxy group
such as 1d and 1e. As an example, mixed benzil 7 (12%) and mixed
biaryl 8 (6%) were formed together when we used p-bromoanisole
(1d), as shown in Scheme 2.10
O
O
O
O
Me
Me
5a (81)b
5b (78)c
In order to prepare an unsymmetrical benzil derivative such as
9 (Scheme 3), we examined the synthesis of 4-phenyl-1,3-dioxol-
2-one (3a). As stated above (entry 9 in Table 1), compound 3a
was isolated in 27% when Et3N was used as a base. After a few trials
we could increase the yield of 3a to 45% when we used DABCO as a
base.11 With this 3a in our hands, compound 9 was synthesized in
high yield (90%) by the reaction with 2-bromonaphthalene (1g).
The mechanism for the formation of benzil (5a) could be postu-
lated as shown in Scheme 4. Syn-carbopalladation of PhPdBr to
vinylene carbonate (2) produced an intermediate I. The intermedi-
ate has no syn-b-hydrogen atom with respect to -PdBr moiety,
thus compound 3a cannot be formed by usual b-H elimination.
Instead, 3a could be formed via a facile deprotonation of the oxo-
nium ion intermediate II that produced via an SN1 type solvolysis
process.6,12 A subsequent arylation of 3a produced 4a by following
the same mechanism. Compound 4a might be ring-opened to pro-
duce benzoin III presumably by the moisture in the basic reaction
mixture.11 The oxidation of III to 5a could be conducted, in part, by
a base-mediated aerobic oxidation.3a,13 In another part, a Pd0/1a-
mediated redox process14 could convert III to 5a as well as 1a to
biphenyl (6a) at the same time. Biphenyl (6a) was isolated in
28% (based on 1a) along with 5a (81%, based on 2) under the typ-
ical reaction conditions (entry 5 in Table 1), while the reaction of
1a without vinylene carbonate (2) produced 6a in only 5%. The re-
sults stated that the presence of III increased the yield of 6a, and a
redox process is obviously contributing to the formation of 5a and
6a.
O
O
O
O
Me
Me
O
MeO
Cl
OMe
5c (74)
5d (59)d
O
O
O
MeO
OMe
Cl
5f (59)b
O
5e (71)c
O
O
O
5g (76)
5h (67)
O
O
O
O
MeO2C
CO2Me
MeO2C
CO2Me
5j (41)
5i (55)
aArBr (1a–j, 3.0 equiv), vinlyene carbonate (2, 1.0 equiv), Pd(OAc)2 (10 mol %), PPh3
(20 mol %), Cs2CO3 (2.2 equiv), DMF, 120 °C, 1 h.
bReaction time is 30 min.
cMixed benzil derivative was formed in trace amounts (<5%).
dMixed benzil 7 was isolated in 12% (see Scheme 2).
In summary, a facile synthetic procedure of benzil derivatives
from aryl bromides was developed using vinylene carbonate as a
glyoxal equivalent in a palladium-catalyzed reaction. The reaction
involved a sequential diarylation of vinylene carbonate to form
4,5-diaryl-1,3-dioxol-2-ones, ring-opening to benzoin derivatives,
and an oxidation process.
(1h), methyl 3-bromobenzoate (1i), and methyl 4-bromobenzoate
(1j) were examined. The corresponding benzil derivatives 5b–i
were obtained in good to moderate yields (55–78%) except 5j

K. H. Kim et al. / Tetrahedron Letters 52 (2011) 3463–3466
3465
O
O
Br
Ph PPh2
Ar Pd Br
PPh3
Ar PPh2
PhPd Br
PPh3
Table 2
5d (59%)
+
(Ar = p-methoxyphenyl)
Aryl-aryl interchange reaction of
ArPdL2Br (L = PPh3) complexes
MeO
OMe
7 (12%)
from PPh3
1d
MeO
OMe
MeO
+
Yields of 5d and 7 (based on 2)
Yields of 6d and 8 (based on 1d)
6d (46%)
8 (6%)
Scheme 2.
1a (3.0 equiv)
1g (1.5 equiv)
O
O
O
O
Pd(OAc)2 / PPh3
Pd(OAc)2 / PPh3
O
O
Cs2CO3 (1.5 equiv)
DMF, 120 oC, 2 h
DABCO (2.2 equiv)
DMF, 120 oC, 30 min
Ph
O
O
3a (45%)
Scheme 3.
2
9 (90%)
2 / Pd(OAc)2 / PPh3 / Cs2CO3, DMF, 120 oC, 30 min
Pd0
1a
PhPdBr
5a
2
(i) base-mediated aerobic oxidation
and/or (ii) Pd-catalyzed redox process
Br
Pd
H
Ph
Ph
- Pd0
- Br
H
Ph
Ph
O
O
O
O
O
PhPdBr
O
- H
O
O
O
O
- Pd0, - HBr
Ph
O
O
O
4a
Ph
OH
Ph
H
I
H
3a
III
II
Pd2+
PhPh
(6a)
III
5a + H+
Pd0
1a
Scheme 4.
2011, 13, 1556–1559; (i) Mori, S.; Takubo, M.; Yanase, T.; Maegawa, T.;
Monguchi, Y.; Sajiki, H. Adv. Synth. Catal. 2010, 352, 1630–1634; (j)
Chandrasekhar, S.; Reddy, N. K.; Kumar, V. P. Tetrahedron Lett. 2010, 51,
3623–3625.
Acknowledgments
This work was supported by the National Research Foundation
of Korea Grant funded by the Korean Government (2010-0015675).
Spectroscopic data were obtained from the Korea Basic Science
Institute, Gwangju branch.
3. For the synthesis of benzil derivatives by benzoin condensation and
subsequent oxidation process, see: (a) Shimakawa, Y.; Morikawa, T.;
Sakaguchi, S. Tetrahedron Lett. 2010, 51, 1786–1789; (b) Jing, X.; Pan, X.; Li,
Z.; Shi, Y.; Yan, C. Synth. Commun. 2009, 39, 492–496.
4. For the synthesis of benzils from acyl cyanides, see: (a) Saikia, P.; Laskar, D. D.;
Prajapati, D.; Sandhu, J. S. Tetrahedron Lett. 2002, 43, 7525–7526; (b) Baruah, B.;
Boruah, A.; Prajapati, D.; Sandhu, J. S. Tetrahedron Lett. 1997, 38, 7603–7604; (c)
Okimoto, M.; Itoh, T.; Chiba, T. J. Org. Chem. 1996, 61, 4835–4837.
References and notes
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2. For the synthesis of benzil derivatives by oxidation of alkynes, see: (a) Santoro,
S.; Battistelli, B.; Gjoka, B.; Si, C.-W. S.; Testaferri, L.; Tiecco, M.; Santi, C. Synlett
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Chem. 2006, 71, 826–828; (f) Giraud, A.; Provot, O.; Peyrat, J.-F.; Alami, M.;
Brion, J.-D. Tetrahedron 2006, 62, 7667–7673; (g) Chu, J.-H.; Chen, Y.-J.; Wu, M.-
J. Synthesis 2009, 2155–2162; (h) Xu, C.-F.; Xu, M.; Jia, Y.-X.; Li, C.-Y. Org. Lett.
5. For the other synthetic routes of benzils, see: (a) Zhang, Q.; Xu, C.-M.; Chen, J.-
X.; Xu, X.-L.; Ding, J.-C.; Wu, H.-Y. Appl. Organomet. Chem. 2009, 23, 524–526;
(b) Jain, S. L.; Sharma, V. B.; Sain, B. Tetrahedron Lett. 2004, 45, 1233–1235; (c)
Moorthy, J. N.; Singhal, N.; Senapati, K. Tetrahedron Lett. 2006, 47, 1757–1761;
(d) Kashiwabara, T.; Tanaka, M. J. Org. Chem. 2009, 74, 3958–3961; (e) Nowak,
P.; Malwitz, D.; Cole, D. C. Synth. Commun. 2010, 40, 2164–2171; (f) Tada, N.;
Shomura, M.; Nakayama, H.; Miura, T.; Itoh, A. Synlett 2010, 1979–1983.
6. For the solvolysis in an SN1 fashion of the palladium intermediate, see: (a)
Martins, S.; Branco, P. S.; de la Torre, M. C.; Sierra, M. A.; Pereira, A. Synlett 2010,
2918–2922; (b) Emrich, D. E.; Larock, R. C. J. Organomet. Chem. 2004, 689, 3756–
3766; (c) Pletnev, A. A.; Tian, Q.; Larock, R. C. J. Org. Chem. 2002, 67, 9276–9287.
7. For the anti-hydride elimination of the palladium intermediate, see: (a)
Lautens, M.; Fang, Y.-Q. Org. Lett. 2003, 5, 3679–3682; For the other
examples of formal anti-elimination in a cyclic alkene moiety, see: (b) Lu, J.;
Tan, X.; Chen, C. J. Am. Chem. Soc. 2007, 129, 7768–7769; (c) Dandepally, S. R.;
Williams, A. L. Tetrahedron Lett. 2009, 50, 1395–1398; (d) Ikeda, M.; El Bialy, S.

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K. H. Kim et al. / Tetrahedron Letters 52 (2011) 3463–3466
A. A.; Yakura, T. Heterocycles 1999, 51, 1957–1970. and further references cited
therein; For the only trial on the palladium-catalyzed arylation of vinylene
carbonate, see: (e) Samizu, K.; Ogasawara, K. Heterocycles 1995, 41, 1627–1629.
2H), 7.85–7.95 (m, 3H), 7.98–8.11 (m, 3H), 8.40 (s, 1H); 13C NMR (CDCl3,
75 MHz) d 123.56, 127.14, 127.89, 129.00, 129.13, 129.51, 129.87, 129.94,
130.23, 132.26, 133.03, 133.51, 134.87, 136.32, 194.62 (2C); ESIMS m/z 283
[M+Na]+.
8. Typical procedure for the synthesis of benzil (5a):
A stirred mixture of
bromobenzene (471 mg, 3.0 mmol), vinylene carbonate (86 mg, 1.0 mmol),
Pd(OAc)2 (22 mg, 10 mol %), PPh3 (52 mg, 20 mol %), and Cs2CO3 (715 mg,
2.2 mmol) in DMF (1.5 mL) was heated to 120 °C for 30 min under nitrogen
atmosphere. After aqueous extractive workup and column chromatographic
9. For the aryl-aryl interchange reaction of ArPdL2X (L = PPh3) complexes, see: (a)
Goodson, F. E.; Wallow, T. I.; Novak, B. M. J. Am. Chem. Soc. 1997, 119, 12441–
12453; (b) Sakamoto, M.; Shimizu, I.; Yamamoto, A. Chem. Lett. 1995, 1101–
1102.
purification process (hexanes/EtOAc, 20:1) benzil was obtained as
a
pale
10. In order to prohibit an aryl–aryl interchange reaction, we examined the
reaction of 1d without PPh3. However, the reaction failed to obtain 5d. The
reaction of 1d under Jeffery ligandless conditions (TBAC, Cs2CO3, DMF, 120 °C,
2 h) in the presence of Pd(OAc)2 did not produce 5d also.
11. 4,5-Diphenyl-1,3-dioxol-2-one (4a) was also isolated albeit in low yield (8%)
during the synthesis of 3a in Scheme 3. Compound 4a was converted
quantitatively to benzil under the influence of Cs2CO3 in DMF within 10 min
(120 °C).
12. The reaction of 1a and 2 in toluene, xylene, and CH3CN (entry 2 in Table 1)
showed a sluggish reactivity, while the reaction in a polar solvent such as DMA,
NMP and sulfolane showed very similar results with that in DMF. The results
are well coincidence with an involvement of solvolysis mechanism.
13. For the base-mediated aerobic oxidation of benzoin to benzil, see: (a)
Muthupandi, P.; Sekar, G. Tetrahedron Lett. 2011, 52, 692–695; (b) Joo, C.;
Kang, S.; Kim, S. M.; Han, H.; Yang, J. W. Tetrahedron Lett. 2010, 51, 6006–6007;
(c) Kang, S.; Joo, C.; Kim, S. M.; Han, H.; Yang, J. W. Tetrahedron Lett. 2011, 52,
502–504.
14. For the oxidation of alcohol with Pd(II) during Pd(0)-catalyzed reductive
homocoupling of aryl halides, see: (a) Shao, L.; Du, Y.; Zeng, M.; Li, X.; Shen, W.;
Zuo, S.; Lu, Y.; Zhang, X.-M.; Qi, C. Appl. Organometal. Chem. 2010, 24, 421–425;
(b) Zeng, M.; Du, Y.; Shao, L.; Qi, C.; Zhang, X.-M. J. Org. Chem. 2010, 75, 2556–
2563. further references cited therein.
yellow solid, 170 mg (81% based on vinylene carbonate), along with biphenyl
(65 mg, 28% based on bromobenzene) as a white solid. Other compounds were
synthesized similarly and the representative spectroscopic data of 3a, 4a, 5i, 7
and 9 are as follows.
Compound 3a:15a white solid, mp 81–82 °C (Lit.15a 82–83 °C); IR (KBr) 3140,
1868, 1807, 1077 cmÀ1 1H NMR (CDCl3, 300 MHz) d 7.34 (s, 1H), 7.40–7.50 (m,
;
5H); 13C NMR (CDCl3, 75 MHz)
d 123.82, 123.41, 124.60, 129.04, 129.91,
143.54, 152.61; ESIMS m/z 185 [M+Na]+.
Compound 4a:15b white solid, mp 73–74 °C (Lit.15b 75–76 °C); IR (KBr) 2930,
1819, 1735, 1211 cmÀ1 1H NMR (CDCl3, 300 MHz) d 7.41–7.44 (m, 6H), 7.56–
;
7.59 (m, 4H); 13C NMR (CDCl3, 75 MHz) d 125.51, 126.46, 128.95, 130.07,
137.04, 151.74; ESIMS m/z 261 [M+Na]+.
Compound 5i: 55%; yellow solid, mp 166–167 °C; IR (KBr) 2929, 1728, 1668,
1280 cmÀ1 1H NMR (CDCl3, 300 MHz) d 3.96 (s, 6H), 7.66 (t, J = 7.8 Hz, 2H),
;
8.23 (d, J = 7.8 Hz, 2H), 8.36 (d, J = 7.8 Hz, 2H), 8.63 (s, 2H); 13C NMR (CDCl3,
75 MHz)
d 52.49, 129.33, 131.03, 131.20, 132.96, 133.78, 135.66, 165.64,
192.50; ESIMS m/z 349 [M+Na]+. Anal. Calcd for C18H14O6: C, 66.26; H, 4.32.
Found: C, 66.43; H, 4.25.
Compound 7:2f,5a pale yellow solid, mp 63–64 °C (Lit.2f 65 °C); IR (KBr) 2934,
1672, 1597, 1265, 1166 cmÀ1 1H NMR (CDCl3, 300 MHz) d 3.89 (s, 3H), 6.95–
;
7.01 (m, 2H), 7.48–7.53 (m, 2H), 7.62–7.68 (m, 1H), 7.92–8.00 (m, 4H); 13C
NMR (CDCl3, 75 MHz) d 55.63, 114.34, 126.04, 128.94, 129.89, 132.38, 133.15,
134.71, 164.97, 193.17, 194.86; ESIMS m/z 263 [M+Na]+.
15. (a) Morris, L. R.; Hubbard, D. J. J. Org. Chem. 1962, 27, 1451–1453; (b) Sheehan,
J. C.; Guziec, F. S., Jr. J. Org. Chem. 1973, 38, 3034–3040; (c) Hoyos, P.;
Sansottera, G.; Fernandez, M.; Molinari, F.; Sinisterra, J. V.; Alcantara, A. R.
Tetrahedron 2008, 64, 7929–7936.
Compound 9:15c pale yellow solid, mp 85–86 °C; IR (KBr) 3060, 1670, 1626,
1594, 1174 cmÀ1 1H NMR (CDCl3, 300 MHz) d 7.46–7.55 (m, 3H), 7.59–7.67 (m,
;