Organic Process Research & Development 2010, 14, 722–727
Selective O-Alkylation of 2-Naphthol using Phosphonium-Based Ionic Liquid as the
Phase Transfer Catalyst†
Ganapati D. Yadav* and Smruti P. Tekale
Department of Chemical Engineering, Institute of Chemical Technology (ICT), Matunga, Mumbai-400019, India
Abstract:
organic compounds and organometallic catalysts.11 The ease
of separation of the products from reactions conducted in ionic
liquids by using various organic solvents adds to their advantages.
Because of the near absence of any vapor pressure, distil-
lation of unreacted reagents and products facilitates product
separation by distillation. This is a great advantage from a
process engineering viewpoint, since separation by distillation
is an effective means for product isolation.11
A new promising class of ionic liquids is based on the
phosphonium ion, such as the halide ions Cl-, Br-, etc., as well
as more complex counterions such as PF6-, etc.12 These
phosphonium-based ionic liquids, when compared with some
other routine ionic liquids such as imidazolium-based salts13
exhibit a relatively wide electrochemically stable window, high
ion mobility, a broad range of room temperature liquid
composition, and negligible vapor pressure.13 They can be used
both as green solvents and phase transfer catalysts in multiphase
reactions.6 The hydrophobic nature of certain ionic liquids
allows for formation of two-phase systems with water, due to
their immiscibility with polar phases.6 This work is concerned
with the catalytic properties of phosphonium-based ionic liquids
in biphasic systems in O-alkylations.
The majority of reported phase transfer-catalysed reactions,
such as nucleophilic substitution, elimination, and polymerisa-
tion reactions are reported under liquid-liquid phase transfer
catalysis (L-L PTC) conditions. One of the drawbacks of the
L-L PTC processes from environmental aspects is that the
catalyst distributed between the two phases is normally not
recovered and reused. The organic phase is washed with copious
quantities of water to remove the catalyst, and the aqueous phase
is discarded. This is because the cost of the catalyst per kilogram
of product is very low, and many PTC processes involve high-
value low-volume products. This is where the phosphonium-
based ionic liquids will promote ‘green’ practices in L-L PTC
reactions.15
The selective preparation of O-alkylated aromatic products from
substituted phenol and naphthols is challenging. The O-alkylation
of 2-naphthol with benzyl chloride has been studied in this work
using phosphonium-based ionic liquids as catalysts such as tri-
hexyl(tetradecyl)phosphonium chloride (THTDPC), trihexyl(tet-
radecyl)phosphonium bromide (THTDPB), trihexyl(tetradecyl)phos-
phonium decanoate (THTDPD), and trihexyl(tetradecyl)phosphon-
ium hexafluorophosphate (THTDPH). This is a liquid-liquid
phase-transfer-catalysed reaction with reuse of catalyst. The effects
of various parameters such as agitation speed, various phospho-
nium-based ionic liquids, phase volume ratio, catalyst concentra-
tion, NaOH concentration, mole ratio of starting materials, and
temperature were studied systematically to understand the conver-
sion patterns and the selectivity of the desired product. A
mechanism of the reaction and a kinetic model are proposed.
1. Introduction
Ionic liquids have added to the repertoire of chemists for
the development of benign routes to synthetic targets. Ionic
liquids have been used in a variety of areas such electrochem-
istry (electrolytes for batteries and fuel cells),1 heavy metal ion
extraction,2-4 phase transfer catalysis, polymerization,5 and as
green solvents, mainly as a replacement for conventional volatile
organic compounds (VOC).6,7 Ambient temperature ionic liquids
encompassing 1,3-dialkylimidazolium cations have shown great
promise as attractive alternatives to conventional solvents.8-10
The important properties of these ionic liquids are low volatility,
negligible vapor pressure, ease of handling, accelerated reaction
rates, potential for recycling, and compatibility with various
† Institute of Chemical Technology (ICT) is formerly the University of
Mumbai Institute of Chemical Technology (UICT), now a separate university.
* Author to whom correspondence may be addressed. E-mails: gdyadav@
yahoo.com, gd.yadav@ictmumbai.edu.in. Telephone: 91-22-3361-1111. Fax:
+91-22-3361-1002/1020.
Synthesis of aromatic ethers is an industrially important
process because of their extensive use in the dyestuff, perfume,
flavor, agriculture, and pharmaceutical industries, and we have
reported the synthesis of aromatic ethers by phase transfer
catalysis (PTC).16-25 Benzyl-2-naphthyl ether is one of the
(1) Wilkes, J. S.; Levinsky, J. A.; Wilson, R. A.; Hussey, C. L. Inorg.
Chem. 1982, 21, 1263.
(2) Dai, S.; Ju, Y. H.; Barnes, C. E. J. Chem. Soc., Dalton Trans. 1999,
1201.
(3) Visser, A. E.; Swatloski, R. P.; Rogers, R. D. Green Chem. 2000,
2, 1.
(4) Visser, A. E.; Swatloski, R. P.; Reichert, W. M.; Rogers, R. D.;
Mayton, R.; Sheff, S.; Wierzbicki, A.; Davis, J. H. Chem. Commun
2001, 135.
(11) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772.
(12) Bradaric, C. J.; Downard, A.; Kennedy, C.; Allan, J. R.; Yuehui, Z.
Green Chem. 2003, 5, 143.
(5) Carmichael, A. J.; Haddleton, D. M.; Bon, S. A. F.; Seddon, K. R.
Chem. Commun. 2000, 1237.
(6) Welton, T. Chem. ReV. 1999, 99, 2701.
(13) Dupont, J.; DeSouza, R. F.; Paulo Suarez, A. Z. Chem. ReV. 2002,
102, 3667.
(7) Wilkes, J. S.; Zaworotko, M. J. J. Chem. Soc., Chem. Commun. 1992,
965.
(14) Yadav, G. D. Top. Catal. 2004, 29 (3-4), 145.
(15) Yadav, G. D.; Jadhav, Y. B. Clean Technol. EnViron. Policy 2003, 6
(1), 32.
(8) Holbrey, J. D.; Seddon, K. R. Clean Prod. Processes 1999, 1, 223.
(9) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.;
Rogers, R. D. Chem. Commun. 1998, 1765.
(16) Yadav, G. D.; Naik, S. S. Org. Process Res. DeV. 1999, 3, 83.
(17) Yadav, G. D.; Jadhav, Y. B. Langmuir 2002, 18, 5995.
(10) Varma, R. S.; Namboodiri, V. V. Chem. Commun. 2001, 643.
722
•
Vol. 14, No. 3, 2010 / Organic Process Research & Development
10.1021/op100052g 2010 American Chemical Society
Published on Web 05/06/2010