Methylation with scCH OH over Solid Catalysts
A R T I C L E S
3
Scheme 1
removal of the reaction products from the catalyst surface. An
increase in the solubility of the gaseous reactants such as alkenes
or H2 in scCO2 causes a significant improvement in the
selectivity of heterogeneously catalyzed reactions.5,6 Further-
more, the favorable heat-transfer properties of SCFs can remove
heat generated by highly exothermic hydrogenation, leading to
facile manipulation of the reactivity and selectivity in the
reaction.
In fact, Poliakoff has demonstrated a highly efficient Friedel-
Craft alkylation of aromatic compounds with propene or
2-propanol over polysiloxane-based solid acid catalysts under
supercritical conditions5c,d or hydrogenation of unsaturated
compounds over the heterogeneous Pd and Pt catalysts supported
on polysiloxane in scCO25a,b leading to the desired products in
a selective manner. Additionally, Baiker has reported that control
of the pressure of scNH3 caused a marked increase in the
selectivity of the amination of diols or amino alcohols to
diamines with scNH3 over a solid Co-Fe catalyst.4 Thus,
heterogeneous catalysis in SCFs has attracted considerable
attention by attaining highly chemoselective molecular trans-
formations using unique properties of SCFs. We have recently
reported preliminary results on the selective N-methylation of
2-aminoethanol (1) with scCH3OH over a solid acid-base
catalyst, where scCH3OH acts as a methylating agent and a
reaction medium.7 Changing the pressure of scCH3OH resulted
in a marked increase in product selectivity.
predominantly intermolecular dehydration products, piperazine
(5) or triethylenediamine (6), in addition to N-methylated
products, N-methylaminoethanol (2) and N,N-dimethylamino-
ethanol (3). At higher temperatures, the alkaline earth metal-
Si or alkali metal-P-Si oxides in the gas phase selectively
afford the intramolecular dehydration product, ethyleneimine
(4), instead of the intermolecular dehydration products, 2 or
3.8,9 Thus, the N-alkylation of 1 with methanol under gas-phase
conditions over solid catalysts proceeds in a nonselective
manner, leading to various side reactions such as cyclization,
oligomerization, and decomposition through C-N bond cleav-
age. Therefore, the N-methylation of functionalized amines is
carried out by means of reductive amination under the pres-
surized hydrogen gas through imine formation using formal-
dehyde or selective alkylations using alkyl halides or diazo-
methane in the solution phase.10 These conventional methods,
however, are far from the ideal greener synthetic routes because
of the formation of undesired byproducts derived from dehy-
drogenated products, aldehydes, or a large amount of inorganic
salts as wastes. Therefore, the development of environmentally
more benign direct N-methylation processes by means of
dehydrative methylation of functionalized amines with methanol
is highly desirable.
For the transformation of 1 with CH3OH to industrially useful
common chemicals, the product distribution is highly influenced
by the reaction phases as well as the catalysts used, as illustrated
in Scheme 1. For example, the vapor-phase reaction of 1 over
the conventional solid acid catalyst, H-â zeolite, provides
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N-Methylation of aromatic amines with methanol over solid
catalysts has been studied in a great detail and has been proved
to proceed by two possible mechanisms: (i) dehydrogenation
of methanol to formaldehyde, leading to an imine which is
hydrogenated with pressurized H2 and (ii) direct methylation
via methyl cation generated on the acid catalyst surface. In
contrast to the reaction of aromatic amines, alkylation of
functionalized amines has been limited to reductive alkylation
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