CATALYSIS
Elongation and branching ofa-olefins by two
ethylene molecules
Thomas Dietel, Fabian Lukas, Winfried P. Kretschmer, Rhett Kempe*
a-Olefins are important starting materials for the production of plastics, pharmaceuticals, and fine and
bulk chemicals. However, the selective synthesis ofa-olefins from ethylene, a highly abundant and
inexpensive feedstock, is restricted, and thus a broadly applicable selectivea-olefin synthesis using
ethylene is highly desirable. Here, we report the catalytic reaction of ana-olefin with two ethylene
molecules. The first ethylene molecule forms a 4-ethyl branch and the second a new terminal carbon-
carbon double bond (C2 elongation). The key to this reaction is the development of a highly active and
stable molecular titanium catalyst that undergoes extremely fastb-hydride elimination and transfer.
a
-Olefins are important starting ma-
terials for the synthesis of plastics,
pharmaceuticals, and bulk and fine
chemicals ( 1 , 2 ). The synthesis of
a-olefins from ethylene, an abun-
dantly available and inexpensive feedstock,
is carried out in megaton scale annually and
represents one of the most important appli-
cations of homogeneous catalysis ( 1 , 2 ). The
selective synthesis ofa-olefins from this attrac-
tive feedstock is restricted to threea-olefins:
1-butene, 1-hexene, and 1-octene ( 3 , 4 ). Other
synthetic protocols give rise to distributions
ofa-olefins that must be separated, and the
demand for a specifica-olefin rarely matches
its proportion in the product distribution. A
broadly applicablea-olefin elongation re-
action in whicha-olefins become selectively
elongated by ethylene would be highly de-
sirable. If the reaction were successively ap-
plicable, meaning that the elongateda-olefin
could undergo a further elongation of the
same kind, a very large number ofa-olefins
would be accessible. Moreover, the selective
introduction of branches is attractive for the
synthesis of synthetic lubricants ( 5 ). Fur-
thermore, brancheda-olefins are applied as
monomers for the synthesis of highly trans-
parent plastics and as membranes for gas
separation ( 6 ). Selectively brancheda-olefins
could also be of interest for functionalization
chemistry such as hydroformylation, a large-
scale reaction ( 1 , 2 ).
We recently introduced a broadly tunable
synthesis of lineara-olefins ( 7 ) and report here
on the catalytic reaction of ana-olefin with
two ethylene molecules. One of the two eth-
ylene molecules forms a 4-ethyl branch and
the other a new terminal C–C double bond
rendering the initial olefin C2-elongated. Many
lineara-olefins, including long-chain examples
such as 1-hexadecene, can undergo this re-
action. Elongation of 4-ethyldec-1-ene, the
elongation and branching product of 1-octene,
indicates that branched aliphatica-olefins
can be elongated and that successive elongation
is possible. Additionally, aromatic and cyclic
olefins, such as styrene and bicyclo[2.2.1]hept-
2-ene, have been modified successfully. The
keytoourolefinelongationreactionisthe
development of a highly active, long-term
stable and selective catalyst. The catalyst can
mediate C–C coupling at rates observed for
highly active enzymes (catalyzing other re-
actions). Ethylene consumption studies in-
dicate the stability of this catalyst without
substantial loss of its high activity for at least
3 hours. At that point, our reactor is com-
pletely filled, and we have to stop the reaction.
The catalyst has a higha-olefin incorporation
rate, undergoes extremely fastb-hydride elim-
ination and transfer, and is based on titanium,
the second-most abundant transition metal of
Earth’s crust.
Co-trimerization of lineara-olefins and eth-
ylene with catalysts that induce selective tri-merization of ethylene via metallacyclopentane
intermediates ( 4 , 8 ) has been investigated ex-
perimentally ( 9 , 10 ) and theoretically ( 11 ).
These investigations indicate that selective
co-trimerization is difficult to accomplish in
such a mechanistic scenario. We concluded
that a different mechanism, for example, a
Cossee-Arlman mechanism ( 12 , 13 ), could be
an alternative and surmised that a catalyst that
produces Schulz-Flory distributions ( 14 – 16 )
with a very small chain-growth probability
(a-value) might be suitable if a higha-olefin
incorporation rate is observed. Then,a-olefin
insertion could be followed by ethylene inser-
tion and, subsequently,b-H elimination and
transfer to ethylene. Such a reaction mecha-
nism is also advantageous with regard to by-
product formation. The dominant by-product,
1-butene, is a gas under normal conditions and
can be separated easily from the liquida-olefin
elongation products.
We have a long-standing interest in the coor-
dination chemistry of aminopyridinato ligands
( 17 ) and previously introduced imidazolidi-
niminato ligands into olefin polymerization
( 18 ). Here, we observed that coordinating both
ligands to titanium can result in a highly
active catalyst that is able to produce Schulz-
Flory distributions with a very smalla-value
(Fig. 1). The organotitanium precursor (com-
pound 1 ;Fig.1,AandB)isthekeytosynthe-
sizing aminopyridinato-imidazolidiniminato
titanium dialkyl complexes. In addition, the
order of introducing the two ligands and the
distinct synthetic protocols, toluene and salt
elimination, proved crucial (Fig. 1A). Nuclear
magnetic resonance (NMR) tube experiments
revealed that 3 reacts with anilinium borate
to eliminate toluene and cleanly form the cat-
ionic species 4 (Fig. 1A and figs. S7 to S9).
Investigation of the ethylene dimerizationSCIENCEscience.org 4 MARCH 2022•VOL 375 ISSUE 6584 1021
Lehrstuhl für Anorganische Chemie IIÐKatalysatordesign,
Sustainable Chemistry Centre, Universität Bayreuth, 95440
Bayreuth, Germany.
*Corresponding author. Email: [email protected]
Table 1. Elongation and branching of lineara-olefins.Reaction conditions are as follows:T= 15°C;
t= 15 min;Peth= 1.0 bar; precatalyst isn 3 = 0.05mmol;na-olefin= 50 mmol; ammonium borate
activator is [R 2 N(CH 3 )H]+[B(C 6 F 5 ) 4 ]−(R = C 16 H 33 to C 18 H 37 ); Ti/B = 1/1.1; scavenger is 300mmol
TIBA; solvent is cumene or toluene; andV= 15 ml. SDs are given in parentheses. Three independent
runs were carried out and averaged for eacha-olefin. The TOFs and the selectivity were calculated
based on co-oligomerization products.Entry a-Olefin Veth(liters)Selectivity (mol %)
TOF
(×10^6 h−^1 )
R R R(^1) .....................................................................................................................................................................................................................1-hexene 1.2 (0.2) 19 (0.3) 73 (0.2) 5 (0.1) 1.4 (0.009)
(^2) .....................................................................................................................................................................................................................1-octene 1.0 (0.1) 19 (0.4) 73 (0.1) 5 (0.2) 1.4 (0.001)
(^3) .....................................................................................................................................................................................................................1-decene 1.0 (0.1) 19 (0.0) 73 (0.2) 4 (0.2) 1.3 (0.014)
(^4) .....................................................................................................................................................................................................................1-dodecene 0.8 (0.0) 20 (0.6) 73 (0.2) 4 (0.2) 1.2 (0.011)
(^5) .....................................................................................................................................................................................................................1-tetradecene 0.5 (0.1) 21 (0.2) 72 (0.1) 4 (0.1) 0.9 (0.009)
6 1-hexadecene 0.2 (0.0) 20 (0.0) 73 (0.2) 4 (0.1) 0.8 (0.006)
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