Quaternary Ammonium Salts

Quaternary Ammonium Salts
Their Use in Phase-Transfer Catalysis (Best Synthetic Methods)
Author: R. Alan Jones
Elsevier, 2001
IN: 978-0-12-389171-6

The quaternary ammonium cations have the ability to form essentially nonsolvated
electrically neutral lipophilic ion-pairs with the anionic species, which are
soluble in polar and non-polar organic solvents. The general basic experimental
procedures were established independently by three groups working in Sweden [5],
Poland [6] and the USA [7], although Triton B (benzyltrimethylammonium hydroxide)
was well established as a base for use in non-aqueous media well before the
advent of phase-transfer catalysed reactions as we now know them, and several isolated
examples of the use of quaternary ammonium salts to accelerate two-phase
reactions were known prior to the 1960s [e.g. 8] and with patents dating to the early
1900s [e.g. 9]. The fundamental technique was initially referred to as 'extractive
alkylation' or 'ion-pair extraction', but the term 'phase-transfer catalysis', which
was first used by Starks in 1971 [10] is now generally accepted.
For most phase-transfer catalysed reactions, the rate-determining step is the
interaction of the reactive substrate with the anionic species in the organic phase
and, compared with the corresponding interfacial reaction in the absence of the
catalyst, rate enhancements of 10 7 are not uncommon. The virtual absence of
water from the organic phase under strongly basic liquid:liquid or solid:liquid
two-phase conditions allows for the formation of water-sensitive anions, such as
carbanions (Chapter 6), and obviates the need for strictly anhydrous conditions
and the use of bases such as sodium hydride or sodamide, etc. The phase-transfer
catalytic process consequently has lower safety risks and is environmentally more
friendly.
The ammonium catalyst can also influence the reaction path and higher yields of
the desired product may result, as the side reactions are eliminated. In some cases,
the structure of the quaternary ammonium cation may control the product ratio
with potentially tautomeric systems as, for example, with the alkylation of 2-naphthol
under basic conditions. The use of tetramethylammonium bromide leads to
predominant C-alkylation at the 1-position, as a result of the strong ion-pair binding
of the hard quaternary ammonium cation with the hard oxy anion, whereas
with the more bulky tetra-n-butylammonium bromide O-alkylation occurs, as the
binding between the cation and the oxygen centre is weaker [11]. Similar effects
have been observed in the alkylation of methylene ketones [e.g. 12, 13]. The
stereochemistry of the Darzen's reaction and of the base-initiated formation of
cyclopropanes under two-phase conditions is influenced by the presence or
absence of quaternary ammonium salts [e.g. 14], whereas chiral quaternary ammonium
salts are capable of influencing the enantioselectivity of several nucleophilic
reactions (Chapter 12).

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