Reductions of benzene derivatives whose benzylic positions bear oxygen atoms under mild conditions

Article abstract of DOI:10.1002/hlca.200890250

Reductions of compounds whose benzylic positions bear O-atoms, such as benzyl alcohol, dibenzyl ether, styrene oxide, benzaldehyde, acetophenone, and benzophenone, to the corresponding non-conjugated dienes were performed by using t-BuOH, Li, and gaseous

Full text of DOI:10.1002/hlca.200890250

Helvetica Chimica Acta – Vol. 91 (2008)
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Reductions of Benzene Derivatives Whose Benzylic Positions Bear Oxygen
Atoms under Mild Conditions
by Abdullah Menzek*, Melek Gçkmen Karakaya, and Afs¸in Ahmet Kaya
Department of Chemistry, Faculty of Science, Atatꢀrk University, TR-25240 Erzurum
(phone: þ 904422314423; fax: þ 904422360948; e-mail: amenzek@atauni.edu.tr)
Reductions of compounds whose benzylic positions bear O-atoms, such as benzyl alcohol, dibenzyl
ether, styrene oxide, benzaldehyde, acetophenone, and benzophenone, to the corresponding non-
conjugated dienes were performed by using t-BuOH, Li, and gaseous NH3 in THF at room temperature.
In these reductions, it was observed that the functional groups at benzylic positions were reduced first.
Introduction. – As is well known, the reduction of aromatic compounds is very
important in organic chemistry and therefore these reactions have been investigated by
many chemists [1]. The Birch reactions are very commonly used for the reduction of
aromatic compounds. However, they generally require harsh conditions such as high
temperatures or liquid NH3 [1][2].
The formation of Birch reduction products of naphthalene (1) depends on the
reaction conditions [1a]. High temperatures (60 – 1458) have been used in the reduction
of naphthalene to 1,4-dihydronaphthalene (2) since 1942 [3]. We performed the
reduction of naphthalene and naphthalene derivatives with Na and t-BuOH to give the
corresponding 1,4-dihydronaphthalene and naphthalene derivatives in high yields at
room temperature as the sole product (Scheme 1) [4].
Scheme 1
Benzene (3) and especially benzene rings with electron-donating groups are
reduced by alkali metal in liquid NH3 to the corresponding products [1][2][5]. The
temperature is usually below ꢀ 338 in these reactions. We also performed the reduction
of benzene and its derivatives with lithium and t-BuOH to the corresponding non-
conjugated dienes 4 as the sole product in high yield at room temperature (Scheme 1)
[6].
ꢁ 2008 Verlag Helvetica Chimica Acta AG, Zꢀrich

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Reduction reactions performed at room temperature have certain advantages
[4][6]. Reduction reactions of compounds whose benzylic positions bear heteroatoms
such as O- and N-atoms may be important under mild conditions. In these compounds,
two centres, the benzylic C-atom and the benzene ring, will be reduced, and their
reactivities will be different. Therefore, reductions of compounds whose benzylic
positions bear O-atoms, such as benzyl alcohol (BnOH), benzaldehyde, benzophenone,
styrene oxide, and Bn2O, were investigated at room temperature with gaseous NH3 and
Li.
Results and Discussion. – Solutions of benzyl alcohol (5), dibenzyl ether (6), and
styrene oxide (7) in THF and t-BuOH were reacted with metallic Li and in an NH3
atmosphere at room temperature (Scheme 2, Table). In these reduction reactions, NH3
is found in atmospheres and solutions of the reactions as gaseous and dissolved,
respectively. 1-Methylcyclohexa-1,4-diene (8) was obtained from the reduction of
BnOH in 54% yield and is a reduction product of toluene [6]. The 1H-NMR spectrum
of the mixture formed in the reduction of Bn2O (6) showed that it contains toluene (9)
and 1-methylcyclohexa-1,4-diene (8) [6][7] in an 8 :92 ratio (Table). In the same way, it
was observed that 2-phenylethanol (10) and 2-(cyclohexa-1,4-diene-1-yl)ethanol (11)
were formed from styrene oxide (7). Compound 11 [8] is a reduction product of alcohol
10. To test the formation of 11 from alcohol 10, we studied the reduction of 10 under
the same conditions. It was observed that the reduction products 11 [8] and 12 [9] in a
31:7 ratio were present in the reaction mixture (Scheme 2). When the reduction of
Scheme 2

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Table. Reduction Reactions of Benzene Derivatives Whose Benzylic Positions Bear O-Atoms
Li (equiv.) t-BuOH (equiv.) Time [h] Ratio of the compounds in the mixture (yield [%])a)
5
6
7
10
13
20
21
25
25
7
11
2.9
8
8
10
2.8
7
20
12
24
10
5/9/8b) 0 : 0 : 100
6/9/8 0 : 8 : 92
7/10/11 23 : 71 : 6
10/11/12 24 : 62 : 14
No reaction
20/5/9/8 26 : 31 : 19 : 24
21/23/24/25 27: 8 : 56 : 9
25/26/27/28/29 7: 1 : 2 : 0 : 0
25/26/27/28/29 0 : 0 : 62 : 14 : 24
25/26/27/28/29 0 : 0 : 0 : 59 : 41
4.5
7
3.5
8
4
7
3
7
15
22
7
24
24
24
25 17
a) Equivalents of benzene derivatives were taken as one, and yields of the products in the mixtures [%]
were determined from their 1H-NMR spectra. b) Obtained by work-up procedure A as described in the
Exper. Part.
(dimethoxymethyl)benzene (13) was attempted at room temperature with gaseous
NH3 atmosphere with different equivalents of t-BuOH and Li, in all cases only starting
material was found. In the reductions of 5 – 7, functional groups such as hydroxide,
ether, and epoxide in benzylic positions were removed. However, ether and OH groups
in b-position (with respect to Ph) of 7 and 10 were not affected by the reduction.
Similar reactions are known (Scheme 3) [10]. Compounds 14 and 15 bear O-atoms
in benzylic positions as in the form of an epoxide and an ether group, respectively, and
they were reduced to the corresponding products 16 and 17, respectively. Only the
aromatic ring was reduced in the case of 18 [10a]. In 15 and 18, the OH groups in b-
position of the aromatic rings were not removed by reduction. These results support our
observations.
Solutions of benzaldehyde (20), acetophenone (21), and benzophenone (22) in
THF and t-BuOH were reacted with metallic Li at room temperature in an NH3
atmosphere (Scheme 4, Table). From these reduction reactions, benzylic alcohols, alkyl
benzenes, and 1-alkylcyclohexa-1,4-dienes were obtained as the reduction products.
Therefore, the products of benzaldehyde (20) were 5, 8 [6][7], and 9, while the prod-
ucts of acetophenone (21) were 23, 24, and 25 [11]. The reduction of benzophenone
(22) gave 1,1’-methanediyldibenzene (26) and the cyclohexadiene derivatives 27, 28,
and 29.
According to reduction reactions of benzene derivatives with C¼O groups in the
benzylic position, e.g., 20, 21, and 22, the sequence of reduction products formed should
be benzylic alcohols, alkyl benzenes, and 1-alkylcyclohexa-1,4-dienes. However, the
ratio of the products should depend on equivalents of reagents. Therefore, we studied
the reduction reactions of benzophenone (22) with different equivalents of t-BuOH
and Li (Table). 1,1’-Methanediyldibenzene (26) and (cyclohexa-1,4-dien-1-ylmethyl)-
benzene (27) were present in the reaction mixture of benzophenone (1 equiv.) with t-
BuOH (3 equiv.), and Li (3.5 equiv.), while products 28 and 29 were absent. The
products 28 and 29 were only present under reaction conditions with t-BuOH
(15 equiv.) and Li (17 equiv.). The reduction products 26 – 29 should be formed in the

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Helvetica Chimica Acta – Vol. 91 (2008)
Scheme 3
Scheme 4

Helvetica Chimica Acta – Vol. 91 (2008)
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reduction of benzophenone via diphenylmethanol, which was not observed in these
reactions. It has been reported that diphenylmethanol was formed in 3% yield in the
conversion of benzophenone with Na into 1,1’-methanediyldibenzene [12].
We have not observed any diene or alkene with C¼O or OH functional groups
formed in the reaction mixtures of benzyl alcohol (5), dibenzyl ether (6), styrene oxide
(7), benzaldehyde (20), acetophenone (21), and benzophenone (22) with t-BuOH,
metallic Li and in an NH3 atmosphere at room temperature. It has been reported that
acetophenone (21) was converted into dienone 30 [13] by reductive alkylation and that
the indanone derivative 31 was reduced to dienol 32 [1e] (Scheme 5). The reason why
dienes or alkenes with CO or OH functional groups were not observed in our reactions
may be that intermediates such as anions or radical anions in benzene rings are not
more stable than those in benzylic position.
Scheme 5
To rationalize the formation of the reduction products, the following reaction
mechanism was proposed (Scheme 6). Li attacks the C¼O group in aldehyde or ketone
33 to give the radical lithium alkoxide 34. Intermediate 34 reacts with Li/R’OH to
convert into intermediate 35 via the dianion. Intermediate 35 can react with both a
proton (from R’OH), to give benzyl alcohol derivative 36, and Li/R’OH, to give radical
37. However, a benzyl alcohol derivative 36 can also be converted into intermediate 37
under the reaction conditions. Intermediate 37 can react with Li and R’OH (as proton
source), respectively, to give, via lithium phenylmethanides 38, alkyl benzene 39. As
earlier described [1][6], alkyl benzenes 39 were reduced with Li/R’OH to the
corresponding cyclohexa-1,4-dienes 40 via intermediate 41. Alkene 42 is produced from
cyclohexa-1,4-dienes 40 by reduction of one C¼C bond.
In the reduction of epoxide 7, Li attacks the benzylic C-atom of the epoxide in 7 to
give radical lithium alkoxide 43 which is a stabilized intermediate [14]. The latter also
reacts with Li to give a dianion, and then this dianion is converted with alcohol (R’OH)
into intermediate 44. Intermediate 44 can react with R’OH (as proton source) to give 2-
phenylethanol (10), which is converted into alcohols 11 and 12 [7] by reduction of the
aromatic ring. While OH groups at benzylic C-atoms were reduced, the OH groups at
other positions were not reduced. However, radicals, radical anions, or dianions such as
34, 37, and 43 may resonate with their aromatic rings.

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Helvetica Chimica Acta – Vol. 91 (2008)
Scheme 6
As reported in the literature [13], it was expected that 1,4-diene derivatives with
C¼O, OR, and OH groups positioned at the C-atom neighboring the ring would be
formed in these reactions. They were not observed, however.
Conclusions. – We have performed reductions of compounds whose benzylic
positions bear O-atoms, such as BnOH, Bn2O, styrene oxide, benzaldehyde, acetophe-
none, and benzophenone, with gaseous NH3 atmosphere and Li to yield the
corresponding nonconjugated dienes at room temperature. As reported in [13], it
was expected that 1,4-diene derivatives with C¼O, OR, and OH groups neighboring
the ring would be formed in these reactions, but they were not observed. The reason for
this may be that intermediates such as anions or radical anions in benzene rings are not
more stable than those in benzylic position. Therefore, it was observed that functional
groups at benzylic positions were reduced first, and only then the benzene rings were
reduced. Ratios of reduction products depend on equivalents of reactants. A reduction
product(s) may be obtained as the major or sole product if the equivalents of reactants
were adjusted.

Helvetica Chimica Acta – Vol. 91 (2008)
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However, 2-phenylethanol (10) and a cyclohexadiene derivative with a OH group,
11, were obtained in the reduction of styrene oxide (7) as a mixture. In order to test the
formation of 11 [8] from alcohol 10, the reduction of alcohol 10 was studied under the
same conditions, and alcohol 11 was obtained from this reaction. It was seen that the
OH group at non-benzylic C-atoms was not reduced, while those at benzylic position
were reduced.
Experimental Part
All chemicals and solvents are commercially available and were used after distillation or treatment
with drying agents except for NH3 gas. There is no need to purify the commercial NH3 gas by distilling.
Free oil on lithium wire was removed with hexane, and lithium was freshly cut into pieces in hexane
before use. All reactants (BnOH, Bn2O, styrene oxide, benzaldehyde, acetophenone, benzophenone,
(dimethoxymethyl)benzene and benzophenone) and some products (5, 9, 10, 22, 23, and 26) are
commercial products. The other products (8 [6][7], 11 [8], 12 [9], and 24 [11]) are known. Column
chromatography (CC) was performed on silica gel (SiO2; 60 mesh, Merck). Preparative thick-layer
chromatography (PLC): 1 mm of silica gel 60 PF (Merck) on glass plates. 1H- and 13C-NMR Spectra: 200
(50)-MHz Varian spectrometer; d in ppm, with Me4Si as the internal standard. Elemental analyses were
performed on a Leco CHNS-932 apparatus.
General Procedure. In a two-necked, round-bottomed flask fitted with stirring bar were placed dry
THF, t-BuOH, and compounds whose benzylic positions bear O-atoms. The flask was attached to a
balloon filled with NH3 gas whose pressure was ca. 1 atm, and the resulting soln. was stirred and cooled in
an ice – water bath. After the mixture was stirred in NH3 atmosphere for 15 – 20 min, freshly cut small
pieces of wire lithium were added within 3 – 5 min. The temp. of the bath was allowed to rise gradually to
r.t. Then, the mixture was stirred for the appropriate time and then cooled in an ice – water bath again.
Then, the NH3-gas tube was closed, the balloon was removed, and cold H2O or HCl (1%) was added
carefully drop by drop to the flask until all the Li was consumed. The mixture was poured into a mixture
of H2O and ice. The workup was carried out in two different ways: A) The org. layer was separated and
washed several times with ice – water to remove t-BuOH and THF. Then, it was dried (Na2SO4) to give
the reduction product(s). B) The mixture was extracted with a solvent (CHCl3, CH2Cl2, or others), and
the extracts were washed several times with ice – water to remove t-BuOH and THF. The soln. was dried
(Na2SO4) and the solvent removed to yield the reduction product(s). Ratios and yields of the products in
1
the mixtures were determined by H-NMR spectroscopy.
Reduction of BnOH (¼ Phenylmethanol; 5). According to the General Procedure, in a 250 ml-flask,
with t-BuOH (27.45 g, 370 mmol, 8 equiv.), dry THF (150 ml), and benzyl alcohol (5.0 g, 46 mmol, 1
equiv.), and Li (3.24 g, 460 mmol, 10 equiv.); reaction time 20 h. Workup procedure A (2 ꢁ 50 ml of H2O
for wash); 1-methylcyclohexa-1,4-diene (8) [6][7] (2.5 g, 54%).
Reduction of 2-Phenyloxirane (7). According to the General Procedure, in a 250 ml-flask, with t-
BuOH (17.27 g, 233 mmol), dry THF (50 ml), and 2-phenyloxirane (10 g, 83.3 mmol), and Li (1.74 g,
240 mmol); reaction time 24 h. Workup procedure B; product mixture (7.74 g) of 2-phenyloxirane (7), 2-
phenylethanol (10), and a reduction product of 2-phenylethanol, 2-(cyclohexa-1,4-dien-1-yl)ethanol (11)
[8] with a ratio of 23 :71:6.
Reduction of 2-Phenylethanol (10). According to the General Procedure, in a 250 ml-flask, with t-
BuOH (12.74 g, 172 mmol, 7 equiv.), dry THF (50 ml), and 2-phenylethanol (3.3 g, 24.6 mmol, 1 equiv.),
and Li (1.38 g, 196 mmol, 8 equiv.); reaction time 10 h. Workup procedure B; product mixture (2.94 g) of
2-phenylethanol (10), 2-(cyclohexa-1,4-dien-1-yl)ethanol (11) [8] (60%), and 2-(cyclohex-1-en-1-
yl)ethanol (12) [9] (13%) with a ratio of 24 :62 :14.
Reduction of Bn2O (6). According to the General Procedure, in a 100 ml-flask, with t-BuOH (7.5 g,
100.9 mmol, 10 equiv.), dry THF (40 ml), and dibenzyl ether (2.0 g, 10.1 mmol, 1 equiv.), and Li (777 mg,
111 mmol, 11 equiv.); reaction time 24 h. Workup procedure B; product mixture (643.44 mg) of toluene
(9) (3.0%) and 1-methylcyclohexa-1,4-diene (8) [6][7] (33.0%), in a ratio of 8 :92.

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Helvetica Chimica Acta – Vol. 91 (2008)
Reduction of Benzaldehyde (20). According to the General Procedure, in a 250 ml-flask, with t-
BuOH (44.47 g, 0.6 mol), dry THF (110 ml), and benzaldehyde (15.9 g, 0.15 mol), and Li (4.725 g,
0.675 mol); reaction time 22 h. Workup procedure B; product mixture (5.20 g) of benzaldehyde (20)
(9.1%), phenylmethanol (5) (10.9%), toluene (9) (6.7%), and 1-methylcyclohexa-1,4-diene (8) [6][7]
(8.4%), in a ratio of 26 :31:19 :24.
Reduction of Acetophenone (¼1-Phenylethanone; 21). According to the General Procedure, in a
250 ml-flask, with t-BuOH (62.16 g, 0.84 mol, 7 equiv.), dry THF (50 ml), acetophenone (24 g, 0.12 mol,
1 equiv.), and Li (5.88 g, 0,84 mol, 7 equiv.); reaction time 7 h. Workup procedure B; product mixture
(7.64 g) of acetophenone (21) (9.3%), ethylbenzene (24) (19.2%), 1-phenylethanol (23) (2.8%), and 1-
ethylcyclohexa-1,4-diene (25) [10] (3.1%), in a ratio of 27:56 :8 :9.
Reduction of Benzophenone (¼ Diphenylmethanone; 25). Benzophenone was reduced under three
different conditions (see below). Workup procedure Awas used for these reactions. Crude mixtures were
submitted to PLC or CC (SiO2) with hexane. The reduced products diphenylmethane (26), (cyclohexa-
1,4-dien-1-ylmethyl)benzene (27), 1,1’-methanediylbis(cyclohexa-1,4-diene) (28) and 1-(cyclohexen-1-
ylmethyl)cyclohexa-1,4-diene (29) were obtained. However, ratios of the products in the mixtures were
determined on the basis of their 1H-NMR spectra.
1) In a 250-ml flask, with t-BuOH (2.444 g, 32.967 mmol, 3 equiv.), dry THF (80 ml), and
benzophenone (2.0 g, 10.989 mmol, 1 equiv.), and Li (267.2 mg, 38.462 mmol, 3.5 equiv.); reaction time
24 h. The residue (2.145 g) was obtained, and the ratio of the products 25, 26, and 27 was 7:1:2.
2) In a 250-ml flask, with t-BuOH (5.702 g, 76.923 mmol, 7 equiv.), dry THF (80 ml), and
benzophenone (2.0 g, 10.989 mmol, 1 equiv.), and Li (615.4 mg, 87.912 mmol, 8 equiv.); reaction time
24 h. The residue (1.550 g) was obtained, and ratios of the products 27, 28, and 29 was 62 :14 :24.
3) In a 250-ml flask, with t-BuOH (12.217 g, 164.8 mmol, 15 equiv.), dry THF (80 ml), and
benzophenone (2.0 g, 10.989 mmol, 1 equiv.), and Li (1.308 g, 186.8 mmol, 17 equiv.); reaction time 24 h.
The residue (1.636 g) was obtained, and ratio of the products 28 and 29 was 59 :41.
(Cyclohexa-1,4-dien-1-ylmethyl)benzene (27). Colorless liquid. IR (CHCl3): 3051, 2897, 2871, 2820,
1
2382, 1661, 1610, 1507, 1455, 1429, 1095, 1043, 966. H-NMR (200 MHz, CDCl3): 7.30 – 7.19 (m, 5 arom.
H); 5.70 (br. s, 2 olefinic H); 5.60 (br. s, 1 olefinic H); 3.30 (br. s, 2 aliphatic H); 2.73 – 2.50 (m, 4 aliphatic
H). 13C-NMR (50 MHz, CDCl3): 141.7 (C); 136.5 (C); 131.0 (CH); 130.6 (CH); 130.3 (CH); 128.0 (CH);
126.3 (CH); 126.1 (CH); 122.3 (CH); 46.3; 30.8; 29.9. Anal. calc. for C13H14: C 91.71, H 8.29; found: C
91.44, H 8.27.
1
1,1’-Methanediylbis(cyclohexa-1,4-diene) (28). Colorless liquid. H-NMR (200 MHz, CDCl3): 5.70
(br. s, 4 olefinic H); 5.48 (br. s, 2 olefinic H); 2.72 – 2.54 (m, 10 aliphatic H). 13C-NMR (50 MHz, CDCl3):
134.8 (CH); 126.5 (CH); 126.1 (CH); 122.2 (CH); 48.2; 30.6; 28.9. Anal. calc. for C13H16: C 90.64, H 9.36;
found: C 90.41, H 9.38.
1
1-(Cyclohexen-1-ylmethyl)cyclohexa-1,4-diene (29). Colorless liquid. H-NMR (200 MHz, CDCl3):
5.71 (br. s, 2 olef. H); 5.45 (br. s, 2 olef. H); 2.72 – 2.51 (m, 6 aliph. H); 2.00 – 1.87 (m, 4 aliph. H); 1.67 –
1.52 (m, 4 aliph. H). 13C-NMR (50 MHz, CDCl3): 137.5 (C); 135.4 (C); 126.6 (CH); 126.1 (CH); 124.9
(CH); 121.8 (CH); 48.8; 30.6; 29.8; 28.9; 27.4; 25.0; 24.5. Anal. calc. for C13H18: C 89.59, H 10.41; found: C
89.94, H 10.37.
:
The authors are indebted to the Department of Chemistry (Atatꢀrk University) and TꢂBITAK
(Project No: 104T354) for financial support and to associate Prof. Dr. Cavit Kazaz for technical
assistance.
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Received May 19, 2008