Pinacol-Pinacolone Rearrangement on FeCl3 Modified Montmorillonite K10

Article abstract of DOI:10.1081/SCC-120027268

Pinacol-pinacolone rearrangement was investigated over Fe-modified montmorillonite K10 catalysts in liquid phase. Exceptional activity was found to be associated with FeCl₃ impregnated on K10 through its acetonitrile solution. 100% conversion of benzopinacol was observed in 10 minutes. The catalysts are recyclable.

Full text of DOI:10.1081/SCC-120027268

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Pinacol-Pinacolone Rearrangement on FeCl3 Modified
Montmorillonite K10
Ajit B. Shinde a , Nilesh B. Shrigadi a , Ramakrishna P. Bhat a & Shriniwas D. Samant a
a Organic Chemistry Research Laboratory, Institute of Chemical Technology , University
of Mumbai , N. M. Parekh Road, Matunga, Mumbai, 400 019, India
Published online: 18 Oct 2011.
To cite this article: Ajit B. Shinde , Nilesh B. Shrigadi , Ramakrishna P. Bhat & Shriniwas D. Samant (2004) Pinacol-
Pinacolone Rearrangement on FeCl3 Modified Montmorillonite K10, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 34:2, 309-314, DOI: 10.1081/SCC-120027268
To link to this article: http://dx.doi.org/10.1081/SCC-120027268
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 2, pp. 309–314, 2004
Pinacol-Pinacolone Rearrangement on
FeCl3 Modified Montmorillonite K10
Ajit B. Shinde, Nilesh B. Shrigadi, Ramakrishna P. Bhat,
*
and Shriniwas D. Samant
Organic Chemistry Research Laboratory, Institute of Chemical
Technology, University of Mumbai, Matunga, Mumbai, India
ABSTRACT
Pinacol-pinacolone rearrangement was investigated over Fe-modified
montmorillonite K10 catalysts in liquid phase. Exceptional activity was
found to be associated with FeCl3 impregnated on K10 through its
acetonitrile solution. 100% conversion of benzopinacol was observed in
10 minutes. The catalysts are recyclable.
Key Words: Pinacol-pinacolone rearrangement; Montmorillonite K10;
FeCl3.
*
Correspondence: Shriniwas D. Samant, Organic Chemistry Research Laboratory,
Institute of Chemical Technology, University of Mumbai, N. M. Parekh Road,
Matunga, Mumbai-400 019, India; Fax: 91 22 414 5614; E-mail: ssd@udct.ernet.in.
309
DOI: 10.1081/SCC-120027268
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Shinde et al.
INTRODUCTION
The pinacol-pinacolone rearrangement is an acid catalyzed rearrange-
ment of vicinal diols, which involves internal rearrangement with transforma-
tion and reduction in functionality.[1] Besides producing crowded ketones, the
reaction poses mechanistic challenges. To effect the rearrangement strong
¨
Bronsted acids such as H2SO4, H3PO4 or Lewis acids like AlCl3, are
commonly used; generally in large quantities.[2] In a recent drive to replace
hazardous stoichiometric catalysts by cleaner and environmentally benign
catalysts, various heterogeneous solid acidic catalysts viz. alumina, clays,
zeolites, ion-exchange resins, heteropoly acids have been tested for this
transformation.[3] Among the ion-exchanged clays Cr3þ and Al3þ exchanged
clays were found to be active.[4] Recently, Hsien has observed that Fe-
substituted molecular sieves to be very active catalyst in this rearrangement.[5]
In our previous study we noticed that Fe modified K10 catalysts activated at a
¨
low temperature have significant Bronsted acidity, which coupled with
reducible nature of Fe3þ cation could serve as a promising catalyst for
Friedel–Crafts benzylation[6] and Beckmann rearrangement.[7] Beckmann
rearrangement involves protonation of hydroxyl group of oximes in which
¨
Bronsted acids have a special role to play. This prompted us to employ FeCl3
modified montmorillonite K10 for pinacol-pinacolone rearrangement (Sch. 1).
RESULTS AND DISCUSSION
Montmorillonite K10 was treated with anhydrous FeCl3 in its acetonitrile
(K10-Fe-O) and aqueous (K10-Fe-A) solutions. These two catalysts were used
after drying at 808 and 1208C (K10-Fe-O-80/120 and K10-Fe-A-80/120).
The preparation and characterization of these catalysts have already been
reported.[6] We have already noticed that as the temperature of calcination was
¨
increased beyond 1208C the Bronsted acidity decreased. Hence catalysts were
activated at 808C and 1208C.
Scheme 1.

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Pinacol-Pinacolone Rearrangement
311
Table 1. The rearrangement of benzopinacol (1a) in
the presence of different K10 catalysts.
Catalyst
Yield of benzopinacolone (2a) (%)
K10
K10-120
58
43
97
62
70
53
K10-Fe-O-80
K10-Fe-O-120
K10-Fe-A-80
K10-Fe-A-120
Notes: Benzopinacol ¼ 1 mmol, Catalyst ¼ 0.1 g,
Time ¼ 1 h, Temp. ¼ 808C, Toluene ¼ 5 mL.
The rearrangement of benzopinacol (1a) was attempted using different
K10 catalysts (Table 1). With K10-Fe-O-80, 97% of benzopinacolone (2a)
was obtained in 10 minutes. With all other catalysts, we could achieve
maximum of 70% product even after continuing for 1 h. Among the ion-
exchanged catalysts also, the activity of catalyst activated at 808C was found
be higher than that activated at 1208C. This observation is in consonance with
¨
the fact that the higher the activation temperature lower is the Bronsted
acidity. Moreover, in the Beckmann rearrangement, we have noticed that
impregnated catalysts are more active than the ion-exchanged catalysts.
Using K10-Fe-O-80 catalyst, a series of symmetrical benzopinacols
(1a–e) were rearranged (Table 2). As the reaction can lead to the formation of
two pinacolones 2 and 3 (Scheme 2), a modified Bachmann procedure was
employed to find out the composition of the product mixture.[8] Thus, a
Table 2. The Pinacol rearrangement of substituted benzopinacols in
the presence of K10-Fe-O-80.
Reactant
Ar
Selectivity (%)
2(a–e) 3(a–e)
Entry
1
1
2
3
4
5
1a
1b
1c
1d
1e
Ph-
(100)a
4-Me-Ph-
4-MeO-Ph-
4-Cl-Ph-
4-Br-Ph-
56
70
26
21
44
30
74
79
Notes: Benzopinacol ¼ 1 mmol, Catalyst ¼ 0.1 g, Time ¼ 10 min,
Temp. ¼ 808C, Toluene ¼ 5 mL, Coversion of 1 ¼ 100%.
aSingle product.

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Shinde et al.
Scheme 2.
mixture of the pinacolones was cleaved into triarylmethanes (5 and 7) and
acids (4 and 6) (Sch. 2). The acids formed were then esterified (8 and 9) and
from the ratio of the esters (GC), the relative amount of pinacolones formed
was calculated.
From Table 2, we can see that the presence of electron-donating substitu-
ents at the para position increases the migratory aptitude of the aryl group.
The catalyst K10-Fe-O-80 was found to be recyclable. There was no
significant change in the activity of the catalyst even after 5 cycles.
EXPERIMENTAL
Montmorillonite K10 was obtained from Aldrich (Manufactured by M/s
Sud-Chemie AG). Prior to any further treatment, the clay was dried overnight

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Pinacol-Pinacolone Rearrangement
313
at 1208C. The catalysts were prepared as per the reported procedure.[6]
Benzopinacols 1,2,4,5[9] and 3[10] were prepared by following the reported
procedures.
Pinacol Rearrangement
The rearrangement was carried out in a 25 mL round bottom flask fitted
with a reflux condenser. A solution of benzopinacol (1) (1 mmol) in toluene
(5 mL) along with catalyst (0.1 g) was heated at 808C. Reaction was monitored
by TLC. After completion of the reaction, toluene was removed by distillation
to get a mixture of benzopinacolones (2 and 3).
General Procedure for the Analysis of the Mixture of 2 and 3
To the pinacolone mixture (2.0 g), a solution of KOH (10 g) in methanol
(40 mL) was added. Benzene (10 mL) was used to aid the dissolution of the
pinacolones. The solution was refluxed till the splitting of the pinacolone was
complete (about 3 days). The alcohol and benzene were distilled and water
(ca. 30 mL) was added to dissolve the potassium salt of the acids, while the
triarylmethanes precipitated. The aqueous solution was acidified with
hydrochloric acid and extracted with solvent ether in order to get the acids.
The mixture of acids was esterified by the reported procedure[11] and analysed
by gas chromatograph.
CONCLUSIONS
Pinacol-pinacolone rearrangement of benzopinacols can be effected using
FeCl3 modified montmorillonite K10 catalysts prepared from acetonitrile
solution and activated at a low temperature 808C (K10-Fe-O-80). 100%
conversion of benzopinacol was achieved in 10 minutes. A normal migratory
aptitude was observed in all the cases. The catalyst is recyclable.
ACKNOWLEDGMENT
We are thankful to G. D. Gokhale Charitable Trust, Mumbai, for the
financial support.

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Shinde et al.
REFERENCES
1. Coldham, I. Comprehensive Organic Functional Group Transformations;
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3. Molnar, A. The pinacol rearrangement. In Fine Chemicals through
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4. Gutierrez, E.; Loupy, A.; Bram, G.; Ruiz-Hitzky, E. Inorganic solids in
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5. Hsien, M.; Sheu, H.T.; Lee, T.; Cheng, S.; Lee, J.F. Fe-substituted
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8. Bachmann, W.E.; Moser, F.H. The pinacol-pinacolin rearrangement. The
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Received in the UK March 18, 2003

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