Bull. Chem. Soc. Ethiop. 2025, 39(1), 131-139.

ISSN 1011-3924
 2025 Chemical Society of Ethiopia and The Authors Printed in Ethiopia
DOI: https://dx.doi.org/10.4314/bcse.v39i1.11 Online ISSN 1726-801X

CHEMOSELECTIVE METAL FREE DEALLYLATION OF α-ALLYL-PHENYL-

CARBOXYLIC ESTERS UNDER REDUCTION CONDITION

Beena R. Nawghare1,2*, Rekha R. Joshi1 and Pradeep D. Lokhande1

1
The Centre for Advanced Studies, Department of Chemistry, Savitribai Phule Pune University,
Pune 411007, India
2
Dr. D. Y. Patil Institute of Technology, Pimpri, Pune 411018, India

(Received April 5, 2024; Revised September 27, 2024; Accepted October 3, 2024)

ABSTRACT. A simple and efficient method for chemoselective deallylation of –COO-allyl group in
presence of C-allyl group has been developed. C-allyl cleavage of α-methylene compounds was
successfully completed by refluxing with excess sodium borohydride in methanol. The reagent's
stability, ready availability and ease of handling encourage its usage for deallylation.

KEY WORDS: C-allyl cleavage, Sodium borohydride, Chemoselectivity, Reduction

INTRODUCTION

The allyl groups for protecting carboxylic acid are now generally used for the synthesis of peptides
[1-3] and the liquid–solid phase synthesis [4-8]. Although several methods have been developed
for the deprotection of allyl groups, Pd° complex in presence of nucleophiles were highly effective
[9, 10]. Palladium-catalyzed hydrosilylation leads to allylsilanes effectively but further
investigation for deallylation not been reported [11]. The variety of nucleophiles have been
reported to use as allyl scavenger that includes NaBH4 [10], LiBH4 [12], NaBH3CN [13, 14],
Bu3SnH, DIBAH [15-17], HCOOH, LiBHEt3 [13-14], dimidinebarbutaric acid [18], phthallimide,
LiAlH4 [19], potassium hexanoate, HCOO- etc. The major drawback of these methods is
availability and side reactions limit its usage in organic synthesis. The use of excess nucleophile
however decreases the atom efficiently and sometimes causes problems during purification of the
products. Berkefeld and co-workers reported cleavage of allyl ethers by Ni-H precatalyst and
excess Brønsted acid [20]. Although O-allyl and N-allyl functional groups are deallylated by this
method, the synthesis of complex nickel hydride required two steps is need to think of its use.
Recently, O-deallylation was promoted by cobalt hydride catalyst, oxone, silane (TMDSO) and
CoSalen complex [21]. We aspire to use less number of catalysts and reduce number of steps in
deallylation of allyl esters. So, cleavage of allyl esters by using sodium borohydride in
dimethylsulphoxide was explored [22]. The reaction was performed at room temperature and
progress well in all weather conditions. Most importantly the activation by transition metal
complexes was not required. Reaction of α-allyl carboxylic esters was also carried out in
dimethylsulphoxide and iodine catalyst. It resulted in the cyclization of α-allyl esters to γ-
butyrolactones [23].
Chemoselectivity allows chemists to bring out targeted product by allowing reaction at
specific position and keeping other reactive sites in the molecule intact [24]. It also simplifies
reactions of complex molecules by attacking specific functional groups while leaving others
unchanged [25]. So, we wanted to examine sodium borohydride for chemoselective deallylation
of α-allyl-esters and accomplished it successfully. In present work, we report the application of

__________
*Corresponding authors. E-mail: brnawghare@gmail.com
This work is licensed under the Creative Commons Attribution 4.0 International License
132 Beena R. Nawghare et al.

sodium borohydride in selective cleavage of allyl group in various α-allyl allyl phenyl acetates
and α-allyl-esters in methanol.
RESULTS AND DISCUSSION

The use of sodium borohydride as a reducing agent in organic chemistry has been known over 60
years [26, 27]. Mild reducing properties of it allow for considerable selectivity in the reduction of
organic compounds. Aldehydes and ketones are in general readily converted into their
corresponding alcohol in the presence of variety of functional groups. The mild reaction condition,
ease of work up and high yields contribute to the widespread use of sodium borohydride as hydride
transfer reagent for reduction of aldehydes and ketones. A hydroxylic solvent is required for
reduction. We previously used dimethylsulphoxide as a solvent in the deallylation process with
sodium borohydride. Now, O-deallylation process is investigated by using hydroxylic solvent.
The deallylation simply proceed by the treatment of allyl carboxylic ester with sodium
borohydride (5 mmol) in methanol at room temperature for 48 hours. The cleavage was
remarkably accelerated under reflux condition furnishing α-allyl carboxylic acid in a very high
yield in 5 hours. There is margined difference in the yield when methanol was used as solvent.
While on increase in molecular weight of alcohol, yield decreases. In butanol yield of the product
was lowest. Encouraged by double benefit of increased yield and substantial reduction on reaction
time we proceeded to conduct deprotection of several α-allylallyl phenyl acetate derivatives. The
results are summarized in Table 1. It is noteworthy that nitro, methoxy, ester, chloro group remain
unaffected. In all substrates (1a-p) that have C-allyl or O-allyl group, only O-allyl group of ester
was selectively cleaved to give corresponding α-allyl phenyl acetic acid under reflux in methanol.
We found that NaBH4/MeOH might interact with the C-allyl group if excess of sodium
borohydride (8-10 mmol) was used for 12 hours. The product mixture was analyzed by NMR.
The phenyl acetic acid (30%) was one of the product identified showing that α-allyl group also
undergo reductive deallylation.
The rate of deallylation of substrate 1a, 1b, 1c were studied and observed that rate of reaction
for these substrates are 1a > 1b > 1c. Fields of deprotection for O-allyl ester bearing electron
donating methoxy group was higher as compared to electron withdrawing group like nitro. Some
α-allylallyl carboxylic esters of aliphatic acids also undergo deallylation but at longer time under
reflux.

R1 R1
O OH
NaBH4 (5 mmol)

O Methanol, Reflux, 5h O
R R
1 2
Scheme 1. Deallylation of α-allyl allyl phenyl acetate.

While no detailed mechanistic study has yet undertaken, we propose that the reaction
proceeded via the formation of trimethoxyborane which form a π-complex with alkene which on
hydrolysis afforded deallylated product acids as in Scheme 2.

B(OMe)2
O O O B(OMe)3 O
MeO H
O Ar O Ar O Ar OH
Ar

Scheme 2. A proposed mechanism for deallylation of α-allyl allyl carboxylic esters.

Bull. Chem. Soc. Ethiop. 2025, 39(1)

 Chemoselective metal free deallylation of α-allyl-phenyl-carboxylic esters 133

Table 1. Result of deallylation of α-allyl allyl phenyl acetate.

Entry R R1 Time (h) Yield % (a-p)

1a H H 6 79
1b OMe H 6 84
1c NO2 H 4 70
1d Cl H 5 82
1e H PHCH2 4 75
1f OMe PHCH2 7 82
1g NO2 PHCH2 3.5 72
1h Cl PHCH2 4 81
1i H OEt 5 80

O
1j OMe OEt 5 85

O
1k NO2 OEt 5.5 73

O
1l Cl OEt 5 82

O
1m H 4 70
OEt

O
1n OMe 6 80
OEt

O
1o NO2 4.5 78
OEt

O
1p Cl 5 80
OEt

Amine 2-borane complex has been used for the deallylation process and formation of
trimethoxyborane is a very well-known reaction. We demonstrated the metal free deallylation of
diallyl carboxylic esters by using sodium borohydride in methanol. The reaction could be
conducted simply by refluxing allyl ester with the inexpensive, handy reducing reagent in alcohol.
Sodium borohydride also completed C-deallylation in presence of methanol. The α-allyl
carboxylic ester (3c) was subjected to deallylation by treating it with NaBH4/MeOH. The product
was characterised by their signals in 1H NMR. In 1H NMR spectrum of product, the signals at
1.19 (d, 3H, CH3), 1.27 (t, 3H, CH3), 2.28 (q, 1H, CH) was observed. Allyl protons were observed
at 2.45 (m, 4H, CH2), 5.07 (dd, 4H, CH2) and 5.68 (m, 2H). These results have confirmed that
under the given condition, instead of deallylation there was a reduction of compound. Finally,
refluxing the compound (3c) with excess quantity of sodium borohydride resulted in deallylation
reaction (Scheme 3).

Bull. Chem. Soc. Ethiop. 2025, 39(1)

134 Beena R. Nawghare et al.

R3
R1 R2 NaBH4 (excess), R1 R2
MeOH
R3 O O
O Reflux, 5h OH

4
a
-
m
3a-m
R1=CH3, OC2H5
R2=CH3, OC2H5
R3=H, PhCH2, CH2CH=CH2,
C2H4COOCH3, C3H6COOCH3

Scheme 3. C-deallylation in active methylene compounds by using NaBH4/MeOH.

Table 2. C-deallylation in active methylene compounds by using NaBH4/MeOH.

Entry 3 Starting 4 Product Time Yield (4)

(3) (4) (h) %

O O
1 3a 4a OH O 5 58
O
O

O O OH O

2 3b O 4b O
5 61

O O
OH O
3 3c 4c 8 69
O O

O O

4 3d 4d O O 5 57
O O

O O

O O O O

5 3e O O 4e O O 5 65

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 Chemoselective metal free deallylation of α-allyl-phenyl-carboxylic esters 135

O O O O

6 3f 4f 5 63
O O O O

O O
O O
7 3g O O 4g 9 59
O O

O O O O
8 3h 4h 5 61
O O O O
C2H4COOCH3 C2H4COOCH3

O O O O
9 3i 4i 5 60
O O O O
C3H6COOCH3 C3H6COOCH3

O O
OH OH
10 3j 4j 5 56

OH OH
O O

11 3k 4k 5 62

OH OH
O O
12 3l 4l 5 68

O O OH OH
13 3m 4m 9 62

Bull. Chem. Soc. Ethiop. 2025, 39(1)

136 Beena R. Nawghare et al.

Figure 1. Transallylation shift of C-allyl group to O-allyl. A) 1H NMR spectra of compound 3c,
b) 1H NMR spectra of compound 5 obtained by stirring 3c with NaBH4 (3 equ) at r.t. c)
1
H NMR spectra of compound 6 obtained by refluxing 3c with NaBH4 (excess).

Bull. Chem. Soc. Ethiop. 2025, 39(1)

 Chemoselective metal free deallylation of α-allyl-phenyl-carboxylic esters 137

We have tried C-allyl cleavage in various active methylene compounds by NaBH4/MeOH

reagent. During these reactions it was observed that first allylic compound shows reduction of
C=O group to C-OH. Then some of the C-allylic groups get converted to O-allyl group. In 1H
NMR spectrum of diallylic ethyl acetoacetate (3c), allylic CH proton appears at δ 5.51 as a
multiplet. But after reduction of compound two multiplets are observed at δ 5.66 and 5.95. These
results enable us to think about transallylation shift of C-allyl group to O-allyl in the same
compound.
EXPERIMENTAL

General procedure for deallylation of α-allyl allyl phenyl acetate (1a-p)

To a solution of α-allyl allyl phenyl acetate (1 mmol), 5 equivalent of sodium borohydride was
added in presence of methanol. The reaction mixture was refluxed for 5 hours. After cooling, the
reaction mixture was acidified by 2N HCl and the residue was extracted with ethyl acetate (10
mL) and dried over Na2SO4. The product was purified by column chromatography (hexane/ethyl
acetate, 9:1). C-deallylation (3a-m) was also completed by following the same procedure.

Ethyl 2-acetylpent-4-enoate (3a). IR: 1643, 1735 cm-1; 1H NMR: δ 1.26 (t, 3H), 2.25 (s, 3H), 2.58
(dd, J = 6.8, 6.8 Hz, 2H), 3.50 (t, 3H), 4.17 (q, 2H), 5.04 (m, 2H), 5.68 (m, 1H).

Ethyl 2-acetyl-2-benzylpent-4-enoate (3b). IR: 1644, 1735 cm-1; 1H NMR: δ 1.26 (t, 3H), 1.94 (t,
3H), 2.54 (dd, J = 7.1, 7.4 Hz, 2H), 3.19 (s, 2H), 4.05 (q, 2H), 5.10 (dd, J = 3.3, 3.8 Hz, 2H), 5.63
(m, 1H), 7.01 (Ar, 5H).

Ethyl 2-acetyl-2-allylpent-4-enoate (3c). IR: 1643, 1734 cm-1; 1H NMR: δ 1.23 (t, 3H), 2.13 (s,
3H), 2.53 (m, 4H), 4.15 (q, 2H), 5.06 (dd, J = 4.8, 4.9 Hz, 4H), 5.51 (m, 2H).

Diethyl 2-allylmalonate (3d). IR: 1643, 1732 cm-1; 1H NMR: δ 1.22 (t, 6H), 2.57 (dd, J = 7.4, 7.1
Hz, 2H), 3.38 (t, 1H), 4.13 (q, 4H), 5.03 (dd, J = 11, 6.3 Hz, 2H), 5.57 (m, 1H).

Diethyl 2-allyl-2-benzylmalonate (3e). IR: 1644, 1734 cm-1; 1H NMR: δ 1.20 (t, 6H), 2.54 (dd, J
= 6.3, 5.5 Hz, 2H), 3.21 (s, 2H), 4.14 (q, 4H), 5.11 (dd, J = 10.4, 3.3 Hz, 2H), 5.71 (m, 1H), 7.07
(Ar, 5H).

Diethyl 2,2-diallylmalonate (3f). IR: 1643, 1732 cm-1; 1H NMR: δ 1.22 (t, 6H), 2.62 (dd, J = 6.8,
7.1 Hz, 4H), 4.17 (q, 4H), 5.03 (dd, J = 11.2, 4.1 Hz, 4H), 5.63 (dm, 2H).

Diethyl 2-allyl-2-methyl acrylic malonate (3h). IR: 1643, 1734 cm-1; 1H NMR: δ 1.23 (t, 6H),
2.16 (dd, J = 7.4, 7.1 Hz, 2H), 2.29 (dd, J=7.1, 7.4 Hz, 2H), 2.61 (d, J=7.4 Hz, 2H), 3.65 (s, 3H),
4.16 (q, 4H), 5.07 (dd, J = 9.9, 7.1 Hz, 2H), 5.57 (m, 1H).

Diethyl 2-allyl-2-methylmethacrylic malonate (3i). IR: 1643, 1735 cm-1; 1H NMR: δ 1.20 (d, J =
3.5 Hz, 3H), 1.22 (t, 6H), 1.89 (dd, J = 1.6, 1.6 Hz, 2H), 2.36 (m, 1H), 2.55 (m, 2H), 3.61 (s, 3H),
4.07 (q, 4H), 5.04 (dd, J = 9.9, 7.1 Hz, 2H), 5.55 (m, 1H).

3-Allylpentane-2,4-dione (3j). IR: 1639, 1697 cm-1; 1H NMR: δ 2.10 (s, 3H), 2.18 (s, 3H), 2.57
(m, 2H), 3.70 (t, 1H), 4.97 (m, 2H), 5.06 (dd, J=4.8, 4.9 Hz, 2H), 5.53 (m, 1H).

3-Allyl-3-benzylpentane-2,4-dione (3k). IR: 1638, 1697 cm-1; 1H NMR: δ 2.10 (s, 3H), 2.12 (s,
3H), 2.6 (d, J = 7.1 Hz, 2H), 3.22 (d, J = 15.6 Hz, 1H), 5.13 (dd, J = 1.6, 4.9 Hz, 2H), 5.56 (m,
1H), 6.99 (m, 2H), 7.19 (m, 3H).
Bull. Chem. Soc. Ethiop. 2025, 39(1)
138 Beena R. Nawghare et al.

3,3-Diallylpentane-2,4-dione (3m). IR: 1639, 1697 cm-1; 1H NMR: δ 2.10 (s, 6H), 2.63 (d, J = 7.4
Hz, 4H), 5.07 (dd, J = 6.3, 9.9 Hz, 4H), 5.44 (m, 2H).

Ethyl 2-(1-hydroxyethy)pent-4-enoate (4a). 1H NMR: δ 1.19 (d, J=6.4 Hz, 3H), 1.27 (t, 3H), 2.27
(m, 1H), 3.89 (m, 2H), 4.16 (q, 2H).

Ethyl 2-acetyl-2- benzylpent-4-enoate (4b). 1H NMR: δ 1.13 (d, J = 6.6 Hz, 3H), 1.25 (t, 3H), 2.13
(m, 1H), 3.19 (d, 2H), 3.88 (t, 2H), 4.06 (q, 2H), 7.06 (Ar, 5H).

Diethyl malonate (4d). IR: 1186, 1371, 1732 cm-1; 1H NMR: δ 1.22 (s, 6H), 3.35 (s, 2H), 4.16 (q,
4H).

Diethyl-2-benzylmalonate (4e). IR: 1644, 1734 cm-1; 1H NMR: δ 1.14 (t, 6H), 2.23 (d, J = 11.2
Hz, 2H), 3.19 (t, 1H), 4.07 (q, 4H), 7.09 (Ar, 5H).

Diethyl-2-methyl acrylic malonate (4h). 1H NMR: δ 1.26 (t, 6H), 2.18 (m, 2H), 2.29 (m, 2H), 3.41
(t, 1H), 3.67 (s, 3H), 4.11 (q, 4H).

Pentane-2,4-diol (4m). 1H NMR: δ 1.19 (d, J = 6.4 Hz, 3H), 1.24 (d, J = 6.3 Hz, 3H), 2.08 (m,
2H), 3.71 (m, 1H).
CONCLUSION

Under the reflux condition, sodium borohydride provides excellent chemoselectivity in the
deallylation of –COO-allyl group in presence of C-allyl group. The selectivity observed is
comparable to the previously reported methods. C-deallylation of α-methylene compounds was
successfully completed by refluxing with excess NaBH4 in methanol. It is observed that using this
protocol, reduction reaction is preferred over deallylation. Also, intramolecular transallylation
shift was detected from C-allyl group to O-allyl group in presence of NaBH4/MeOH. The
experimental simplicity, easy work-up procedure and low cost catalyst as compare to palladium
mediated methods is noteworthy. Therefore, this new approach could be a beneficial addition to
the current processes.
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