Direct Formation of Benzoins from Diarylmethanones via a Rearrangement Reaction Promoted by Samarium Metal in DMF

Article abstract of DOI:10.1055/s-2004-815408

Diarylmethanones react readily with DMF when promoted by samarium metal in DMF with TMSCl or iodine as an activator, to afford benzoins in good to excellent yields via rearrangement of aryl groups. As for asymmetric diarylmethanones, products resulting from the migration of either aryl group were obtained, where the migration of aryl groups shows certain priority.

Full text of DOI:10.1055/s-2004-815408

LETTER
445
Direct Formation of Benzoins from Diarylmethanones via a Rearrangement
Reaction Promoted by Samarium Metal in DMF
a
a
a,b
D
Y
irec
t
Formation o
o
f
Benzoins
f
n
rom
D
iarylm
g
e
thanones jun Liu, Xiaoliang Xu, Yongmin Zhang*
a
Department of Chemistry, Zhejiang University, Xi-xi Campus, Hangzhou, 310028, P. R. China
b
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai,
200032, P. R. China
Fax +86(571)88807077; E-mail: yminzhang@mail.hz.zj.cn
Received 13 November 2003
O
C
O
C
OH
CH
Abstract: Diarylmethanones react readily with DMF when pro-
moted by samarium metal in DMF with TMSCl or iodine as an
activator, to afford benzoins in good to excellent yields via rear-
rangement of aryl groups. As for asymmetric diarylmethanones,
products resulting from the migration of either aryl group were ob-
tained, where the migration of aryl groups shows certain priority.
Sm/TMSCl
DMF
Scheme 1
Key words: samarium, DMF, diarylmethanone, rearrangement re- The unexpected formation of benzoin indicates that the
action, benzoin, benzil
additional carbonyl group comes from the dimethyl-
formamide and dimethylamino group acts as a leaving
group in this reaction. Despite the fact that the carbonyl
Although samarium diiodide (SmI ) is a very useful groups of amides are unreactive toward many carbon nu-
2
1
reducing reagent, however, its application in organic cleophiles and are employed consequently as solvents in
8
carbon-carbon bond-formation reactions, DMF as a good
formylation reagent is well known (Vilsmeier–Hack reac-
synthesis is limited to some extent. Its storage is difficult
because of its sensitivity to air oxidation. Besides, Sm2
can only donate one electron in the reaction and the lack
of atom economy seriously restricts its application on a
large scale. Therefore, direct use of metallic samarium as
+
9
10a
tion). Besides, such necleophiles as Grignard reagents,
organolithium1 and the radical anions resulting from
the reduction by sodium in THF can attack the carbonyl
group in DMF to afford the corresponding aldehydes. In
0a,b
10c
a reducing agent in organic transformations has attracted
2
the attention of many organic chemists. Usually, reac- the reactions mentioned above, the formoyl group in DMF
tions promoted directly by samarium metal are carried out was introduced into the products with dimethylamino
3
in THF and metallic samarium has to be activated by oth- anion acting as a leaving group.
er reagents such as iodine, hydrochloric acid, alkyl ha-
lides, TMSCl, etc.1,3,4 so as to ensure the reactions proceed
smoothly. We have recently found that metallic samarium
exhibits some interesting properties in organic synthesis
To have some idea about the mechanism of the present re-
action, we undertook an extensive literature survey and
were fortunate to find some clues to this reaction in the
electron spin resonance study of the radicals obtained
5
when DMF is used as a solvent instead of THF.
11
from acetophenone and benzaldehyde. At high reduc-
6
The diarylmethanone dianion is a known species whose tion potentials and by an irreversible electrode process,
reactions with carbonyl compounds or benzonitrile have the conversion of acetophenone to 1-phenyl-propane-1,2-
7
been observed long time before. Reduced by SmI , dia- semidione in DMF was proposed to undergo a carbene
2
rearrangement.1
1b
rylmethanone forms ketyl readily, which has considerable
stability and constitutes the important intermediate in
cross-coupling reactions with ketones, aldehydes, imines,
aroyl chlorides, conjugated carbon-carbon double bonds,
To have an in-depth understanding of the reaction, a series
of diarylketones were subjected to the same conditions
and the desired benzoins were formed smoothly. In some
cases, benzils were also obtained as the unexpected
byproducts (Scheme 2). The results of the reactions are
described in Table 1. The reaction of diarylketones with
electron-donating group attached to the aromatic rings
proceeds efficiently. In contrast, diarylketones with elec-
tron-withdrawing group require relatively harsh reaction
conditions and give lower total yields. As can be seen,
bis(p-methylphenyl)-methanone (1b) and bis(p-methoxy-
phenyl)-methanone (1c) can give 4,4¢-dimethylbenzoin
1
and so on.
However, when diphenylmethanone was treated with sa-
marium metal activated with TMSCl in DMF, surprising-
ly, benzoin was obtained in excellent yield (Scheme 1).
SYNLETT 2004, No. 3, pp 0445–0448
1
8.
0
2.
2
0
0
4
Advanced online publication: 12.01.2004
DOI: 10.1055/s-2004-815408; Art ID: U24103ST.pdf
©
Georg Thieme Verlag Stuttgart · New York

4
46
Y. Liu et al.
LETTER
O
C
OH
CH
R1
R2
2
O
C
O
C
O
C
Sm/TMSCl
DMF
+
+
R2
R1
R2
R1
OH O
CH C
1
4
R1
R2
3
Scheme 2
Table 1 Reactions of Diarylmethanones with DMF Promoted by
Sm/TMSCl in DMF
Table 2 Reaction of Diarylmethanones with DMF Promoted by
Sm/I in DMF
2
1
2
1
2
En- R , R in substrate Temp Time Total yield
try 1
Ratio Yield of
En- R , R in substrate Temp Time Total yield of Ratio Yieldof4
(°C) (h) of 2, 3 (%)a of 2/3 4 (%)
b
try 1
(°C) (h) 2, 3 (%)
of 2/3 (%)
1
2
3
4
H, H (1a)
CH , CH (1b)
80
70
2
1
1
1
90 (2a)12b
79 (2b)12b
78 (2c)12b
63 (2d)12d
0
1
2
3
4
H, H (1a)
CH , CH (1b)
90 10 91 (2a) 0 (4a)
15 (4b)12f
80
CH O, CH O (1c) 80
6
4
74 (2b)
53 (2c)
–
18 (4b)
35 (4c)
–
3
3
3
3
CH O, CH O (1c) 50
16 (4c)12f
21 (4d)12f
3
3
3
3
4-(CH ) N,
120 36
4-(CH ) N,
80
3
2
3 2
4
-(CH ) N (1d)
4- (CH ) N (1d)
3
2
3 2
5
6
7
8
9
4-Cl, 4-Cl (1e)
120 12
–
–
5
6
7
8
9
4-Cl, 4-Cl (1e)
120
70
5
1
1
5
5
–
–
3
4-CH , H (1f)
3
82 (2f, 3f)12c 68/32 11 (4f)12g
87 (2g, 3g)12e 81/19 9 (4g)12h
41 (2h, 3h)12c 0/100 13 (4h)12f
4-CH , H (1f)
90
80
8
5
77 (2f, 3f)
68/32 14 (4f)
c
4-Ph, H (1g)
4-Cl, H (1h)
4-Br, H (1i)
47 (2g, 3g) 19/81 16 (4g)
4-Ph, H (1g)
4-Cl, H (1h)
4-Br, H (1i)
50
d
120 24 52 (2a, 3h) 0/100
0
0
100
100
e
120 24 53 (2a)
58 (2a)
0
a
b
c
Products 2 and 3 were inseparable and the overall yields were given.
The ratio was determined by HPLC and H NMR.
A considerable amount of reductive product biphenyl-4-yl-phenyl-
methanol was detected.
Dechlorination reaction affording 26% of 2a occurred simultaneous-
1
reaction proceeds to such an extent that benzoin (2a) was
isolated as the only product. An attempt was also made
but with a negative result in investigating if acetophenone
and benzaldehyde could undergo such a reaction.
d
ly in the formation of the corresponding benzoin.
e
Exclusive debromination product 2a was formed.
Considering TMSCl is an acidic activator, we explored
neutral activator iodine in a further investigation. To our
delight, better results were obtained in the presence of I2,
as shown in Table 2.
and anisoin respectively (entries 2 and 3) in good yields,
while bis(p-chlorophenyl)-methanone (1e) failed to afford
the corresponding types of products 2 and 3. It is surpris-
ing to find bis(p-dimethylaminophenyl)-methanone (1d)
did not undergo the reaction under the same conditions.
The amino group may form a salt with TMSCl and thus
bear an electron-withdrawing character, which makes the
reaction difficult to occur just like the case of bis(p-chlo-
rophenyl)-methanone (1e). As for asymmetric diaryl-
methanones, two kinds of benzoins can be obtained, with
the carbonyl groups adjacent to the electron-richer aryl
groups predominant. This may indicate the reaction fol-
lows a certain rule in the rearrangement.
With iodine as an activator, the reaction proceeded more
easily with milder reaction conditions and higher total
yields than that activated with TMSCl, except that
substrate bis-(p-chlorophenyl)-methanone (1e) still failed
to give encouraging result (entry 5). Bis-(p-dimethylami-
nophenyl)-methanone, to our expectation, can undergo
the same kind of reaction under this conditions and afford
the corresponding benzoin and benzil in high total yield
(
entry 4). The benzoins with the carbonyl groups adjacent
to the electron-richer aryl groups still predominate in the
products when asymmetric diarylmethanones are used as
It is worth noting that besides the 4¢-chlorobenzoin (3h),
benzoin (2a) was also obtained. The formation of 2a
obviously results from a dechlorination process under the
reaction conditions accompanying the rearrangement. As
for phenyl p-bromophenyl methanone, the debromination
substrates under the Sm/DMF/I conditions. An exception
2
is biphenyl-4-yl-phenyl-methanone (1g) gives the ben-
zoin with the carbonyl group adjacent to the biphenyl
group as a major product (entry 7).
Synlett 2004, No. 3, 445–448 © Thieme Stuttgart · New York

LETTER
Direct Formation of Benzoins from Diarylmethanones
447
O
CH3
CH3
2
HC N
R1
R1
O
C
Sm
+ e
Sm
+ e
O
O
R1
R2
1
R2
R2
O
C
O
O
C
OH
C:
CH3
CH3
O
C
OH
CH3
CH3
[N(CH3)2] -
C N
C N
R1
R1
H
R1
R2
R2
R2
A
O
C
OH
OH O
CH C
1
, tautomerization
C
R2
R1
R2
R1
2
, protonation
2
(major)
B
C
OH O
CH C
O
C
OH
C
rearrangement
1, tautomerization
2, protonation
R1
R2
R1
R2
3
(minor)
1
2
Electron donating ability R > R
Scheme 3
The formation of benzils may be explained as a result of In conclusion, with TMSCl or iodine as the activator, the
the auto-oxidation of benzoins. The reports that some reaction between diarylketones and DMF via a rearrange-
1
3a
13b
metal salts such as copper(I) and Ce(IV) can easily ment process promoted by samarium metal can afford
catalyze the autoxidation of benzoins to the corresponding benzoins and benzils in good overall yields. The complex-
benzils make us deduce that the presence of samarium ing of DMF with samarium ion15 may play an important
salts here may accelerate the formation of benzils from role in promoting the reaction to occur, and studies on the
benzoins during the treatment process. Further investiga- exact mechanism are currently in progress.
tion showed that the benzil could also be obtained when
benzoin was treated under the same reaction conditions.
This validates our suggestion.
Acknowledgment
We are grateful to the National Natural Science Foundation of Chi-
na (Project No.20072033) and Specialized Research Fund for the
Doctoral Program of Higher Education of China.
We attempted the reaction without activator (TMSCl or
I2) and found that the reaction did not occur at all, and fur-
ther attempts to expand the reaction to other metals such
as zinc failed. We also tried other solvents such as THF,
DMSO, and toluene instead of DMF but did not find any
benzoin formed. This result may indicate the additional
carbonyl group comes from DMF. On the other hand, the
result that no obvious reaction between Sm/activator and
DMF was observed without diarylmethanone in the reac-
tion system shows the reaction must be initiated by ketyl
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Synlett 2004, No. 3, 445–448 © Thieme Stuttgart · New York