1
786
S. K. Prajapti et al. / Tetrahedron Letters 55 (2014) 1784–1787
glycosylation,1
hols with silane, and hydrogenation of imines,
reactions between activated arenes or heteroarenes and
6b–d
hydrosilylation of imines,16e reduction of alco-
Table 2
1
6f
16g
B(C F ) catalyzed acylation, benzoylation, and propionylation of alcohol, phenol,
6
5 3
Friedel–Crafts
-amido-
has
been utilized to catalyze the regio- and stereoselective cyclizations
a
amine, and thiophenol
a
1
6h
16i
Time (min) Yieldsb
sulfones,
Sakurai allylation.
In this perception, B(C
6
F
5
)
3
Entry
Substrate
Acylating agent
of unsaturated alkoxysilanes.1 B(C
7a
OH
6 5 3
F ) is also an efficient activa-
114f
Propionic anhydride
Benzonic anhydride
5
96
94
15
tor for polymethylhydrosiloxane in the reduction of different func-
1
7b,c
tional groups.
Herein, we wish to report a mild and efficient method for the
acylation of alcohols, phenols, amines, and thiophenols with acetic
25b,7b
Propionic anhydride
Benzonic anhydride
1
10
98
95
OH
anhydride, using a catalytic amount of B(C
conditions (Scheme 1).
F
6 5
)
3
under solvent-free
In a test reaction, 1 mmol of benzyl alcohol was treated with
.2 mmol of Ac O in the presence of B(C (0.5 mol %) at room
1
2
6 5 3
F )
temperature. The completion of reaction was monitored by TLC,
in which the complete disappearance of starting material was ob-
served in 2 min and yielded 98% of acylated product (Table 1, entry
320
Propionic anhydride
Benzonic anhydride
4
15
97c
95
HO
O
O
c
1
). Encouraged by the success of this reaction various primary, sec-
ondary, and cyclic alcohols were subjected to acylation in excellent
yields under similar reaction conditions.18 To explore the simplifi-
410e,7c
521,7b
622
Propionic anhydride
Benzonic anhydride
4
10
98
96
cation and scope further, B(C
6
F
5
)
3
catalyzed acylation was exam-
OH
ined using other structurally diverse phenols, amines, and thiols
and the results are summarized in Table 1.
Phenolic compounds were efficiently acetylated (entries 2–5)
but required prolonged reaction time than alcohols due to their
relatively poor nucleophilicity. Phenolic compounds containing
both electron donating (entry 3) and electron withdrawing (entry
Propionic anhydride
Benzonic anhydride
2
15
96
94
NH2
SH
4
) reacted equally efficiently under present reaction conditions.
Similarly, b-naphthol (entry 5) and 7-hydroxy coumarin (entry 6)
were also converted into corresponding acetate, in excellent yields
without any side products.
Propionic anhydride 25
Benzonic anhydride 45
92
90
Furthermore, sugars were also subjected to O-acylation using
6 5 3
an excess of acetic anhydride and 0.5 mol % of B(C F ) to afford
fully acetylated products in quantitative yields (entries 7–10).
Moreover, racemization or epimerization was not observed to
any extent in the acetylated products, when stereogenic centers
were present in the substrates. Cyclohexanol (entry11), and cyclo-
pentanol (entry 12), were all promptly acylated to afford corre-
sponding acetates under similar reaction conditions.
a
b
c
All reactions were performed by using 1.2 equiv of acylating agent.
Isolated yields.
Excess of acylating agent used.
Similarly, acylationof l-mentholwas carried out withoutany det-
rimental effect on optical purity. It is noteworthy that acid sensitive
functional groups such as TBDMS, THP, and Boc can survive in the
present method, demonstrating the mildness of the acylation pro-
cess (entries 14–16). Interestingly, extension of the present method-
ology to various amines and thiophenols furnished corresponding
acylated product in excellent yields. The reaction of amines with
acetic anhydride was so fast in comparison to that of alcohols (en-
tries 17–22). In case of aromatic amines, there was no significant
influence of electron donating or electron withdrawing substituents
and reacted equally efficiently under the standard reaction condi-
In summary, we have demonstrated that tris(pentafluorophenyl)
borane is an extremely facile and efficient catalyst for acylation, benzo-
ylation, and propionylation of alcohols, phenols, amines as well as thi-
ophenols. The advantages of this method are environmental benign,
low catalyst loading, non toxic catalyst, solvent-free condition, rapid
reaction, high yields of the desired products, and simple experimental
procedure. In addition, chirality retention, and tolerance of acid labile
protecting groups such as TBDMS, THP, and Boc are an added advantage
of the present method.
An established ‘Lewis acid assistance’ mechanism holds good
for this strategy considering that coordination of carbonyl oxygen
tions. Consequently, the reaction of methyl ester of
phenyl alanine with acetic anhydride in the presence of 0.5 mol %
of B(C afforded the desired products in 91% and 94% yields
respectively without any disturbance on chirality (entries 23–24).
Finally, the reactions of thiophenol and mercapto cyclohexane were
slow under present reaction conditions. However, acetates of thio-
phenol and mercaptocyclohexane were achieved in 92% and 90%
yields respectively (entries 25–26).
L
-proline and
L
-
6 5 3
with B(C F ) results in partial charge-transfer character, which
makes the lone-pair donor effectively more electronegative and
enhances the electrophilicity of acylating agents toward nucleo-
philic attack.
6 5 3
F )
Acknowledgments
We thank the Department of Pharmaceuticals (Ministry of
Chemicals and Fertilizers) for providing funds and also CSIR-Indian
Institute of Chemical Technology, Hyderabad for providing the
facilities.
As a logical extension of this methodology, we further investi-
gated the efficiency of B(C F ) as a catalyst for propionylation
6 5 3
and benzoylation using propionic anhydride and benzoic anhy-
dride respectively (Table 2). To our delight all the studied sub-
strates such as benzyl alcohol, phenols, cyclohexanol, aniline, and
thiophenol were successfully provided corresponding propionylat-
ed and benzoylated products in excellent yields as tabulated in
Table 2.
References and notes
1.
Prajapti, Santosh Kumar
