Angewandte
Communications
Chemie
Homogeneous Catalysis
An Agostic Iridium Pincer Complex as a Highly Efficient and Selective
Catalyst for Monoisomerization of 1-Alkenes to trans-2-Alkenes
Yulei Wang, Chuan Qin, Xiangqing Jia, Xuebing Leng, and Zheng Huang*
Abstract: A unique Ir complex (tBuNCCP)Ir with the pyridine–
phosphine pincer as the sole ligand, featuring a dual agostic
However, in the hydrocarbon case (for example, 4-methyl-1-
pentene), overisomerization occurred to give 3-alkene
(9%).[14] More recently, the same group reported an improved
system using a mixture of (C5Me5)Ru complexes (1 mol%)
that enables selective isomerization of linear a-olefins to
trans-2-alkenes at 408C. The reactions could be performed at
RT, albeit with higher catalyst loadings (5 mol%) and longer
reaction times (97 h). Alcohols and silyl ethers proved
compatible with the catalyst, but other substrate types were
not tested.[15] High E-selectivity (40:1 E/Z) was also achieved
in the isomerization of linear 1-alkenes by a Co carbene
complex (5 mol%). However, the reactions employ
a Grignard reagent as an additive (0.5–1 equiv), which may
pose a challenge in the isomerization of substrates bearing
reactive functional groups.[16] Thus, despite the significant
advances in this field, it is of great interest to develop highly
efficient and widely functional-group-tolerant catalysts for
selective alkene isomerizations.
À
interaction between the Ir and two s C H bonds from a tBu
substituent, has been prepared. This complex exhibits excep-
tionally high activity and excellent regio- and stereoselectivity
for monoisomerization of 1-alkenes to trans-2-alkenes with
wide functional-group tolerance. Reactions can be performed
in neat reactant on a more than 100 gram scale using
0.005 mol% catalyst loadings with turnover numbers up to
19000.
A
lkenes are versatile intermediates for the synthesis of
value-added chemicals and materials, including pharmaceut-
ical intermediates and polymers.[1] Among them, terminal
alkenes can be readily prepared with high regioselectivity by
several methods, such as olefin oligomerization,[2] elimination
reactions,[3] and Wittig reactions.[4] The stereoselective for-
mation of 2-alkenes,[5] however, is much more difficult
relative to 1-alkene synthesis. In this regard, the monoisome-
rization of widely accessible 1-alkenes is attractive as an
atom-economic route to 2-alkenes.
Our group recently reported that NCOP-type Ir pincer
complexes of pyridine–phosphinite ligands (1; Figure 1) are
very active for alkene isomerizations at 608C, but give low
regio- and stereoselectivity.[17] We envisioned that a pyridine-
based pincer Ir catalyst with a more sterically hindered ligand
The ultimate challenge for the monoisomerization process
is the simultaneous control of stereo- and regioselectivity
because both trans and cis isomers can be formed, and
overisomerization often occurs for linear hydrocarbon sub-
strates (for example, linear a-olefins) that lack heteroatoms
or branches to prevent from producing 3-alkenes or higher
homologues.[6] For example, RajanBabu reported regioselec-
tive isomerizations of 1-alkenes to 2-alkenes using a Pd or Ni
complex of triarylphosphine (5 mol%), but the processes
gave moderate stereoselectivity (ca. 3.5:1 E/Z) when using
linear 1-alkenes.[7] Skrydstrup applied an in situ generated
bulky Pd hydride (0.5–1 mol%) for selective isomerization of
various 1-alkenes. Note that in some cases of linear 1-alkenes,
the E/Z selectivities (2.3:1–4:1) are close to the thermody-
namic ratio.[8] Similarly, a number of transition-metal catalysts
(for example, Fe,[9] Ru,[10] Co,[11] Mo,[12] and Ir[13]) offer
regioselectivities for 2-alkenes; unfortunately the stereose-
lectivities demonstrated in these works are moderate or low.
In 2012, Grotjahn disclosed an imidazolyl-phosphine Ru
complex for E-selective isomerizations of linear allyl sub-
strates (for example, O-, N-allyl substrates and allylarenes).
Figure 1. RNCOP and RNCCP iridium complexes.
might offer the combination of efficiency and selectivity in the
isomerization. Early studies by Goldman and Brookhart on
PCP-type pincer Ir complex-catalyzed olefin isomerization
established a p-allyl methanism,[18] and revealed that the
nature of the linkers connecting the P atoms and the aryl
backbone have large impact on the steric properties.[18,19] In
this context, we sought to develop more bulky pyridine-
phosphine Ir complexes (RNCCP)IrHCl (2) with a methene
(CH2) linker in place of the O-linker in 1. A novel tBu-
substituted complex (tBuNCCP)Ir (3; Figure 1) that possesses
[*] Y. Wang, C. Qin, X. Jia, X. Leng, Prof. Dr. Z. Huang
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
345 Lingling Rd, Shanghai 200032 (China)
E-mail: huangzh@sioc.ac.cn
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a dual agostic interaction between the metal and two s C H
bonds in the tBu(py) group (tBu on the pyridine ring) is further
generated. This complex, which requires no activator, is
extremely active for isomerization of 1-alkenes, including
challenging linear a-olefins, producing trans-2-alkenes with
high positional and stereochemical selectivity.
Supporting information for this article can be found under:
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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