Synergism of low energy microwave irradiation and solid-liquid phase transfer catalysis for selective alkylation of phenols to phenolic ethers

Article abstract of DOI:10.1081/SCC-200026623

A 100% selectivity with an order of magnitude rate enhancement is obtained in the synthesis of phenolic ethers when synergistic combination of solid-liquid phase transfer catalysis and low energy microwave irradiation (MISL-PTC) is employed. As against conventional microwave heating with 600 W power input, the current work demonstrates that a low input of 40 W leads to remarkable enhancement in rates without any destruction of the catalyst.

Full text of DOI:10.1081/SCC-200026623

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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 16, pp. 2885–2891, 2004
Synergism of Low Energy Microwave
Irradiation and Solid–Liquid Phase
Transfer Catalysis for Selective Alkylation
of Phenols to Phenolic Ethers
*
Ganapati D. Yadav and Priyal M. Bisht
Department of Chemical Engineering, University Institute of Chemical
Technology (UICT), University of Mumbai, Matunga, Mumbai, India
ABSTRACT
A 100% selectivity with an order of magnitude rate enhancement is
obtained in the synthesis of phenolic ethers when synergistic combination
of solid–liquid phase transfer catalysis and low energy microwave
irradiation (MISL-PTC) is employed. As against conventional microwave
heating with 600 W power input, the current work demonstrates that a low
input of 40 W leads to remarkable enhancement in rates without any
destruction of the catalyst.
Key Words: Substituted phenols; Microwave irradiation; Solid–liquid
phase transfer catalysis; Synergism; Enhancement of rates and selecti-
vities; MISL-PTC.
*
Correspondence: Ganapati D. Yadav, Department ofChemicalEngineering, University
Institute of Chemical Technology (UICT), University of Mumbai, Matunga, Mumbai 400
019, India; Fax: 91-22-2414 5614; E-mail: gdyadav@yahoo.com or gdyadav@udct.org.
2885
DOI: 10.1081/SCC-200026623
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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INTRODUCTION
Phase transfer catalysis (PTC) is used in pharmaceuticals, agrochemicals,
perfumes, flavors, dyes, specialty polymers, pollution control, etc.[1–3] Aro-
matic ethers used in a variety of industries, are produced by PTC routes.[4,5]
Selectivity of ethers by PTC is greatly improved under solid–liquid (S–L)
`
vis-a-vis liquid–liquid (L–L) conditions due to suppression of side reactions
promoted by water.[6–8] Microwave irradiation is found to be useful for rapid
organic synthesis including solvent-free solid–organic liquid PTC using very
high energy inputs.[9–17] S–L PTC is inherently faster due to greater mass
transfer rates provided by fine solid particles than liquid droplets.[18]
All publications on microwave assisted PTC reactions deal with S–L
PTC at very high input energy (of the order of 600 W), leading to a final temp-
erature of over 140–1808C depending on type of nature and polarity of the
medium in which solubility of the solid increases substantially thereby
masking the effect of PT catalyst. Furthermore, most of the quaternary salts
used as catalysts are unstable at high temperatures, particularly beyond
120–1308C may be ineffective. Synthesis of aromatic ethers using microwave
irradiated L–L PTC (MILL-PTC) reactions at enhanced rate was reported for
the first time by us.[19] However, product selectivity was not found to be
100%. It was hence thought worthwhile to study the effect of S–L PTC on
the rate intensification and the selectivity profile of the products under low
power microwaves. Now we report here a general synthesis of phenolic
ethers using microwave irradiated S–L PTC (MISL-PTC) under low energy
input using tetrabutylammonium bromide (TBAB) as catalyst in presence of
NaOH at 908C (Fig. 1) with a mechanism (Fig. 2). Benzyl chloride was
used as an alkylating agent. In each alkylation process, phenyl ethers were
`
produced with 100% selectivity in MISL-PTC vis-a-vis MILL-PTC. The
rate of the reaction increased significantly under microwave irradiation as
compared with the conventional heating under otherwise similar conditions
(Table 1). The rate enhancement was in the following manner:
MISL-PTC ðmaxÞ . MILL-PTC . LꢀL PTC ðconventional heatingÞ
Figure 1. Synthesis of O-alkylated phenols using under MI–PTC.

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Alkylation of Phenols to Phenolic Ethers
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Figure 2. Mechanism for the alkylation of phenols under S–L PTC conditions.
Since a very mild energy input was provided, it was necessary to inves-
tigate whether the extraction of the ion-pair QþOR2 was enhanced with the
microwave irradiation leading to higher reaction rates or whether the enhance-
ment was due to the ion-pairs being highly polar species get activated under
microwave irradiation. It was found that neither the extraction constants
were affected, nor the activation energy of the reaction, but the collision
frequency and entropy of the system increase resulting into orders of
magnitude enhancement under microwave irradiation and the details are a
matter of a separate publication.[20a]
EXPERIMENTAL
The experimental set-up for the conventional heating consisted of a 5 cm
i.d. fully baffled mechanically agitated reactor of 100 cm3 capacity, which was
equipped with four baffles and a six-bladed pitched-turbine impeller. The
entire reactor assembly was immersed in a thermostatic oil bath maintained
at the desired temperature with an accuracy of +18C. Typical runs were con-
ducted by taking 0.01 mol substituted phenol and 0.012 mol of NaOH in water
and making up to 30 cm3. To this 0.002 mol of TBAB catalyst was added. The
organic phase comprised of 0.01 mol benzyl chloride and 2.0 cm3 diphenyl
ether as an internal standard (IS) and the total organic volume was made to
30 cm3 with toluene. In the case of nonpolar solvents (toluene, xylene, hydro-
carbons, etc.), there exists a very weak solvent–microwave interaction. Thus
specific absorption by reactants is possible. This enables energy transfer from
the reactants to the solvent unlike in the case when the polar solvents, either
aprotic (e.g. DMF) or protic (e.g. alcohols), were used.[21] All the reactions
were carried out at 908C and 500 rpm.
The experimental set-up for the microwave experiment was the commer-
cially available “Discover” unit (Model CEM-SP 1245, CEM Corp., USA). It
consists of a circular single-mode cavity design which directs the microwave
energy into a defined area, resulting in a homogenous field pattern surrounding

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Yadav and Bisht
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