Facile and fast pinacol rearrangement by AlCl3 in the solid state

Article abstract of DOI:10.3390/60500442

A facile and efficient synthetic procedure for effecting the pinacol rearrangement catalyzed by AlCl₃ in the absence of solvent is developed. The rearrangement product is obtained at room temperature in a few minutes and in almost quantitative yield. Benzylic pinacols rearrange under these conditions, while aliphatic pinacols do not react.

Full text of DOI:10.3390/60500442

Molecules 2001, 6, 442-447
molecules
ISSN 1420-3049
http://www.mdpi.org
Facile and Fast Pinacol Rearrangement by AlCl in the Solid
3
State
Parviz Rashidi-Ranjbar* and Ebrahim Kianmehr
Department of Chemistry Tehran University, Box 14155-6455, Tehran, Iran. Tel. (+98)-021-6113301,
Fax (+98)-021-6405141.
*
Author to whom correspondence should be addressed; e-mail Ranjbar@khayam.ut.ac.ir
Received: 12 February 2000; in revised form 9 April 2001 / Accepted: 10 April 2001 / Published: 30
April 2001
Abstract: A facile and efficient synthetic procedure for effecting the pinacol rearrangement
catalyzed by AlCl in the absence of solvent is developed. The rearrangement product is
3
obtained at room temperature in a few minutes and in almost quantitative yield. Benzylic
pinacols rearrange under these conditions, while aliphatic pinacols do not react.
Keywords: Pinacol rearrangement, Solid state reactions, AlCl , Lewis acid
3
Introduction
Organic reactions are found to occur efficiently in the solid state and many examples have been
reported so far [1-5]. The pinacol rearrangement is usually carried out under drastic conditions, like
heating in H SO . Toda found that the reaction proceeds faster and more selectively in the solid state
2
4
by using organic acids like CCl CO H and p-TsOH [6]. It is possible to use a strong Lewis acid for the
3
2
pinacol rearrangement and to the best of our knowledge, there has been no report on pinacol
rearrangement by using a Lewis acid like AlCl . This reagent has previously been used for Friedel-
3
Crafts alkylation and acylation in the solid state [7] and also in Beckman rearrangements [8] under
similar conditions. The novel utility of aluminium chloride as an efficient reagent for pinacol
rearrangement is reported in this paper.

Molecules 2001, 6
443
Results and Discussion
Benzylic pinacols react with AlCl to give the corresponding rearrangement product in almost
3
quantitative yield after only a few minutes. The reaction is easily carried out in an agate mortar and the
reaction conditions and the subsequent workup are simple and quick. The pinacol compound is mixed
with AlCl in a ratio of 1:3 respectively and is then thoroughly ground at room temperature. A fast
3
reaction happens, HCl evolves and a colored mass is immediately obtained. Workup of the reaction
mixture by adding dilute H SO and extracting with chloroform affords the rearrangement product in
2
4
good yield. By adopting the above method, benzylic pinacol compounds have been rearranged to the
corresponding pinacolones and it is found that the method is much better than all the methods known
so far for its simplicity and high conversion. The list of substrates that have been rearranged by this
method is given in Table 1. For anthrapinacol, entry 7, the only product detected was bianthryl which
shows that this procedure can be used in systems that aromatize by dehydration. For tetralone pinacol,
entry 5, the product of rearrangement was obtained in fairly good yield compared to the procedures
described previously [9]. In series of compounds 2-6 it was found that the selectivity is affected by
AlCl and for entry 6 the rearranged product, 8, undergoes keto-enol tautomerization to give 10-
3
phenyl-9-phenanthrol, 6 (Scheme 1).
O
Ph
OH Ph
HO
Ph
OH
AlCl
3
8
6
Scheme 1
Entry 8 shows that for rearrangement under these conditions, the existence of an aryl group
adjacent to carbon which bears the OH group is necessary. The reaction is expected to proceed via the
formation of an aluminium adduct 9 which is formed by reaction of aluminium chloride with the OH
group followed by liberation of HCl gas. In the second step the rearrangement takes place by the
migration of alkyl, aryl or hydride groups (Scheme 2).
-
AlCl
2
+
OH OH
O
C
OH
AlCl
3
Ar
C
C
R
Ar
C
R
Product
4
4
-HCl
R
R
3
R
R
2
2
3
9
Scheme 2

Molecules 2001, 6
444
Table 1. Rearrangements of pinacols to the corresponding products. All the reactions
were carried out at room temperature.
Entry Pinacol
product
Time (min)
Yield
HO
OH
1
2
1
98
1
2
O
Ph
CH CH Ph
OH OH
Ph
Ph
C
O
CH
Ph
2
2
2
87
71
Ph
3
4
Ph CH
C
Ph
C
O
CH
Ph
Ph
3
4
OH OH
Ph
X
CH
C
Ph
Ph
CH
Ph
2
2
94
C
X
O
OH OH
X=OCH
3
5
5
5
6
OH
5
HO
O
HO
Ph
HO
Ph
OH
6
7
2
1
84
92
HO
OH
7
8
HO OH
no reaction

Molecules 2001, 6
445
Acknowledgments
Parviz Rashidi Ranjbar is grateful to the Research Council of Tehran University for financial
support. Professor Tony Durst is highly acknowledged for providing us access to NMR and HRMS
facilities.
Experimental
General
Solvents and reagents for the preparation of starting materials were used as received from Aldrich.
H- and C-NMR spectra were recorded on Varian Unity Plus instruments (500 and 200 MHz). FT-IR
1
13
spectra was obtained using a Shimadzu 4300 spectrophotometer.
General synthetic procedure
A mixture of the appropriate pinacol compound and anhydrous aluminium chloride (Aldrich, 99.99
) was thoroughly ground in an agate mortar for a few minutes under an efficient hood. Hydrogen
%
chloride is evolved violently. The reaction mixture was worked up by adding H SO (2N) and
2
4
extracting with chloroform. The organic layer was dried over Na SO and the solvent was removed in
2
4
vacuo to give the products. Pinacol starting materials were prepared by the methods of Toda [10] and
Khurana [11].
Spectral Data
1
Compound 1, H-NMR (500 MHz, CDCl ) δ: 6.61(dd, J=7.8 and 1.3 Hz, 1H), 7.03(d, J=7.6Hz, 2H),
3
7
.06(dt, J=7.7 and 1.2Hz, 1H), 7.16 (dt, J=7.5 and 1.1 Hz, 2H), 7.34(dd, J=8.4 and 1.4Hz, 1H), 7.37(dt,
J=7.5 and 1.0Hz, 2H), 7.43(dt, J=7.6 and 1.0Hz, 1H), 7.79(d, J=7.6Hz, 2H), 7.8(m, 1H), 7.98(dd, J=7.7
and 1.5Hz, 1H), 8.08(dd, J=8.2 and 1.0Hz, 1H), 8.18(dd, J=8.1 and 0.3 Hz, 1H); C-NMR (125 MHz,
1
3
CDCl ) δ: 68.68, 120.51, 123.17, 124.06, 124.70, 127.899, 127.984, 128.076, 128.258, 128.278,
3
1
28.529, 129.175, 130.09, 130.50, 134.81, 138.10, 139.32, 141.60, 147.08, 197.12, HRMS calcd for
C H O 344.12012, found 344.12163.
Compound 2, H-NMR (200 MHz, CDCl ) δ: 4.30(s, 2H), 7.29-7.50(m, 8H), 8.03-8.08(m, 2H); C-
26
16
1
13
3
NMR (50 MHz, CDCl ) δ: 45.22, 126.67, 128.38, 128.44, 129.29, 132.96, 134.36, 136.33, 197.36,
3
HRMS calcd for C H O 196.08882, found 196.08696.
1
4
12
1
Compound 3, H-NMR (200 MHz, CDCl ) δ: 6.05(s, 1H), 7.22-7.51(m, 13H), 7.99-8.03(d, J=7.5Hz,
3
1
3
2
H); C-NMR (50 MHz, CDCl ) δ: 59.39, 127.14, 128.61, 128.71, 128.96, 129.13, 133.05, 136.75,
3
139.05, 198.178, HRMS calcd for C H O 272.12012, found 272.12148.
2
0
16

Molecules 2001, 6
446
1
Compound 4, H-NMR (500 MHz, CDCl ) δ: 3.75 (s, 3H), 5.97(s, 1H), 6.84 (m, 2H), 7.18-7.26(m,
5
3
1
3
H), 7.29-7.32(m, 2H), 7.37-7.40(m, 2H), 7.48-7.50(m, 1H), 7.99-7.99(m, 2H); C-NMR (125 MHz,
CDCl ) δ: 55.17, 58.55, 126.99, 128.53, 128.62, 128.89, 129.00, 130.12, 131.11, 132.90, 136.87,
3
1
39.45, 158.66, 198.41, HRMS calcd for C H O 302.14191, found 302.13247.
2
1
18
2
1
Compound 5, H-NMR (500 MHz, CDCl ) δ: 1.70-2.08(m, 6H), 2.15-2.21(m, 1H), 2.27-2.33(m, 1H),
3
1
3
2.73-2.93(m, 4H), 6.97-7.01(m, 1H), 7.04-7.18(m, 4H), 7.30-7.37(m, 1H), 7.37-7.44(m, 2H); C-
NMR (125 MHz, CDCl ) δ: 19.17, 22.33, 29.44, 29.85, 32.31, 36.25, 52.82, 125.91, 126.16, 126.94,
3
-1
1
27.70, 127.78, 128.45, 129.51, 131.09, 136.15, 137.17, 141.93, 142.47, 214.86, FTIR (1680 cm ,
CO), HRMS calcd for C H O 276.15142, found 276.15170.
2
0
20
1
Compound 6, H-NMR (500 MHz, CDCl ) δ: 5.45(s, OH), 7.24-7.44(m, 2H), 7.46-7.55(m, 4H), 7.59-
3
7
.72(m, 4H), 8.38(ddd, J=8.2, 1.5 and 0.5Hz, 1H), 8.66(ddd, J=8.7, 1.2 and 0.5Hz, 1H), 8.71(ddd,
13
J=8.7, 1.2 and 0.5Hz, 1H); C-NMR (125 MHz, CDCl ) δ: 117.25, 122.51, 122.57, 123.07, 123.99,
3
1
1
24.98, 125.36, 126.41, 126.63, 126.81, 127.22, 127.85, 128.63, 129.80, 131.03, 131.47, 132.47,
34.49, HRMS calcd for C H O 270.10447, found 270.10628.
2
0
14
1
Compound 7, H-NMR (500 MHz, CDCl ) δ: 7.06-7.14(m, 8H), 7.41(dd, J=6.3 and 1.2Hz, 2H),
3
1
3
7
1
3
.43(dd, 6.3 and 1.2Hz, 2H), 8.13(d, 8.5Hz, 4H), 8.66(s, 2H); C-NMR (125 MHz, CDCl ) δ: 125.24,
25.75, 126.78, 127.16, 128.47, 131.492, 131.570, 133.024, HRMS calcd for C H 354.14085, found
54.13923.
3
2
8
18
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1
2
3
4
5
6
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8
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