Angewandte
Communications
Chemie
Homogeneous Catalysis
Iron-Catalyzed gem-Specific Dimerization of Terminal Alkynes
Qiuming Liang, Kimberly M. Osten, and Datong Song*
Abstract: We report a gem-specific homo- and cross-dimeri-
zation of terminal alkynes catalyzed by a well-defined iron(II)
complex containing Cp* and picolyl N-heterocyclic carbene
(NHC) ligands, and featuring a piano-stool structure. This
catalytic system requires no additives and is compatible with
a broad range of substrates, including those with polar
functional groups such as NH and OH.
kynes under mild conditions at 1–3 mol% catalyst loadings in
the absence of any additive.[7] Later, Mandal and co-workers
reported a cyclic (alkyl)amino carbene iron(0) catalyst that
can catalyze the homodimerization of arylalkynes in the
presence of a large excess of KOtBu at high temperatures with
moderate to high yields and poor to high E selectivity.[6c]
Similar to Z and E 1,3-enynes, geminal (gem) 1,3-enynes are
useful synthons for complex organic architectures.[1c,d,i–l]
Nevertheless, to the best of our knowledge, iron-catalyzed
gem-selective alkyne dimerization has not been reported.[7]
Gem-selective alkyne dimerization has been reported with
palladium, rhodium, early metal, and aluminum cata-
lysts.[4g,8,9] While the late-metal catalysts are compatible with
a broad scope of substrates,[4g,8] the early metal and alumi-
num-based catalysts show limited substrate scope, for exam-
ple, NH and OH groups cannot be tolerated.[9] Herein we
report the first iron-catalyzed[10,11] gem-specific homo- and
cross-dimerization of terminal alkynes in the absence of
additives. Interestingly, this catalytic system is compatible
with a broad scope of substrates, including those with OH and
NH functional groups. Such gem specificity applies to the
cross-dimerization of terminal alkynes as well.
Conjugated 1,3-enynes are not only a common structural
motif in natural products, biologically active compounds,
organic materials, and other complex molecules, but also
useful precursors to these compounds.[1] They are generally
prepared through the transition-metal-catalyzed cross-cou-
pling of terminal alkynes and pre-activated alkenes, Wittig
olefination of conjugated alkynals, dehydration of propargylic
alcohols, or reactions involving metal carbenes.[2] The selec-
tive dimerization of alkynes through direct hydroalkynylation
across carbon–carbon triple bonds is a more desirable route
with the highest atom economy.[3] One of the challenges in
alkyne dimerization is the selectivity. Homodimerization of
terminal alkynes may yield three possible products: head-to-
tail (gem) products or one of two head-to-head (E/Z)
products (Scheme 1), while cross-dimerization of two differ-
ent terminal alkynes may give 12 possible products.[1a]
The well-defined catalyst 2 can be synthesized in two steps
(Scheme 2). The reaction between in situ generated 3-mesi-
Scheme 1. Isomeric 1,3-enynes resulting from terminal alkyne homodi-
merization.
Scheme 2. Syntheses of 1 and 2. a) 1 equiv of KOtBu, Et2O, RT, 1 h,
then 1 equiv of Cp*FeCl(TMEDA), RT, 3 h, 86%. b) 1 equiv of
LiHMDS, THF, RT, overnight, 89%. TMEDA=tetramethylethylenedi-
amine, THF=tetrahydrofuran.
A large number of metal-based catalysts have been
developed for the dimerization of terminal alkynes, mostly
based on precious metals and f-block elements.[1a,4,5] In
contrast, only a few examples of iron-based catalysts have
been reported.[6,7] The first reported iron-catalyzed alkyne
dimerization uses FeCl3 as the catalyst in the presence of
a ligand and large excess of KOtBu; this reaction requires an
extremely high catalyst loading (30 mol%) and high temper-
atures, and gives E enyne products.[6a] Recently, Milstein and
co-workers reported a well-defined iron(II)-pincer catalyst
that can achieve Z-selective alkyne dimerization of arylal-
tyl-1-(2-picolyl)-imidazol-2-ylidene (HL)[12] and Cp*FeCl-
(TMEDA) leads to the formation of the paramagnetic
piano-stool complex Cp*Fe(HL)Cl (1). The addition of
1 equiv of LiHMDS to 1 deprotonates the o-Me of the
mesityl group instead of the methylene bridge between the
two heterocycles, yielding the dark purple diamagnetic
complex Cp*Fe(l-k3C,C’,N) (2). Similar cyclometallation
processes on Cp*Fe(NHC)Cl (NHC = N-heterocyclic car-
bene = IiPr or IMes) have been reported and presumably
À
proceed through the deprotonation of the agostic C H of
a methyl group.[13] In the 1H NMR spectrum of 2 in C6D6, the
pyridylic CH2 and Fe-CH2 groups give rise to two diagnostic
AB-pattern signals at d = 4.4 (J = 14 Hz) and 1.9/0.9 (J =
9 Hz) ppm, respectively. The molecular structure of 2 is
further confirmed by X-ray crystallography (see the Support-
ing Information).
[*] Q. Liang, Dr. K. M. Osten, Prof. Dr. D. Song
Department of Chemistry, University of Toronto
80 St. George Street, Toronto, Ontario, M5S 3H6 (Canada)
E-mail: dsong@chem.utoronto.ca
Supporting information for this article can be found under:
Angew. Chem. Int. Ed. 2017, 56, 1 – 5
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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