DOI: 10.1002/cctc.201702037
Communications
N-Alkylation of Aqueous Ammonia with Alcohols Leading
to Primary Amines Catalyzed by Water-Soluble
N-Heterocyclic Carbene Complexes of Iridium
Ryohei Yamaguchi[a]
A new catalytic system for the N-monoalkylation of aqueous
ammonia with a variety of alcohols was developed. Water-solu-
ble dicationic complexes of iridium bearing N-heterocyclic car-
bene and diammine ligands exhibited high catalytic activity for
this type of reaction on the basis of hydrogen-transfer process-
es without generating harmful or wasteful byproducts. Various
primary amines were efficiently synthesized by using safe, inex-
pensive, and easily handled aqueous ammonia as a nitrogen
source. For example, the reaction of 1-(4-methylphenyl)ethanol
with aqueous ammonia in the presence of a water-soluble N-
heterocyclic carbene complex of iridium at 1508C for 40 h
gave 1-(4-methylphenyl)ethylamine in 83% yield.
Recently, much attention has been directed to the use of am-
monia as a nitrogen source for organic synthesis, because of
its abundance and low price.[1] To date, a number of reports on
Scheme 1. Selective catalytic synthesis of primary amines by N-alkylation of
ammonia with alcohols.
transition-metal-catalyzed reactions for the synthesis of organic
amines by using gaseous (or liquid) ammonia or its solution in
an organic solvent have appeared.[2] Because the selective cata-
lytic synthesis of primary amine derivatives by the reaction of
alcohol substrates with ammonia generates no harmful or
wasteful byproducts (generating only H2O as a coproduct),
such a synthesis is considered important and desirable meth-
odology in the field of synthetic organic chemistry.[3,4] The re-
action is based on hydrogen-transfer processes, called “bor-
rowing hydrogen” or “hydrogen autotransfer”.[5] For example,
Gunanathan and Milstein reported the synthesis of primary
amines from primary alcohols and pressurized gaseous ammo-
nia under the catalysis of an acridine-based pincer complex of
ruthenium (Scheme 1a).[3a] Additionally, Beller et al. and Vogt
et al. independently reported the synthesis of primary amines
from secondary alcohols and liquid ammonia under the cataly-
sis of the Ru3(CO)12/2-(dicyclohexylphosphino)-1-phenyl-1H-pyr-
role system (Scheme 1b).[3b,c]
tems involving the use of aqueous ammonia for the synthesis
of organic amines have been reported. However, most of them
required the employment of harmful organic halides as sub-
strates.[6] On the other hand, during the course of our studies
on hydrogen-transfer reactions catalyzed by iridium com-
plexes,[7] we reported the synthesis of a water-soluble dication-
ic iridium–ammine complex, [Cp*Ir(NH3)3][I]2 (1) (Cp*=h5-pen-
tamethylcyclopentadienyl), and its high catalytic activity for
the multialkylation of aqueous ammonia with alcohols to give
tertiary and secondary amines.[8] However, monoalkylation of
aqueous ammonia with alcohols leading to primary amines
has so far not been achieved.[9,10] Herein, we report the synthe-
sis of new water-soluble iridium N-heterocyclic carbene (NHC)
complexes and their high catalytic performance for the N-alky-
lation of aqueous ammonia with alcohols to give primary
amines (Scheme 1c).[11]
Aqueous ammonia is an attractive nitrogen source consider-
ing its advantages in terms of safety and handling. Some sys-
First, iridium NHC complexes 2–7 illustrated in Figure 1 were
synthesized.[12] Details of the procedures for their synthesis and
characterization (including single-crystal X-ray analysis of 4 and
5) are described in the Supporting Information. All of these
complexes were found to be stable in air, and dicationic com-
plexes 1 and 4–7 were soluble in water.
The catalytic activities of these iridium NHC complexes for
the monoalkylation of aqueous ammonia leading to primary
amines were evaluated. Initially, the reaction of 1-(4-methyl-
phenyl)ethanol with aqueous ammonia was investigated as a
[a] Prof. Dr. K. Fujita, S. Furukawa, N. Morishima, M. Shimizu,
Prof. Dr. R. Yamaguchi
Graduate School of Human and Environmental Studies
Kyoto University
Sakyo-ku Kyoto 606-8501 (Japan)
Supporting Information and the ORCID identification number(s) for the
ChemCatChem 2018, 10, 1 – 6
1
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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