isshowninFig.1D.Afterasmallinitialde-
crease of the ethylene consumption, a linear
profile indicates almost no catalyst decay. We
had to stop the experiment after 3 hours be-
cause our 300-ml reactor was completely filled
(fig.S20andsafetynotethere).TheSchulz-
Flory olefin product distributions obtained
in a run at 40°C with 7-bar ethylene pressure
indicate a Cossee-Arlman–type mechanism
(Fig. 1E). Activity, selectivity, and long-term
stability is only observed if both N-ligands are
present. Variation of substituents of the two
N-ligands did not result in catalysts with a lower
a-value. With a highly active, selective, and long-
term stable catalyst in hand, we explored the
scope ofa-olefin incorporation (Fig. 2).
a-Olefin elongation and branching was
investigated using 1-hexene (Fig. 2A). The liquid-
product analysis revealed formation of a co-
trimerization product (4-ethyloct-1-ene) with a
selectivity of 74 mol % at 15°C. Furthermore, two
main by-products have been identified: 3-
methyleneheptane (co-dimerization product,
18 mol %) and 6-ethyldec-1-ene (co-tetramerization
product, 5 mol %). The sum of the main product
and the two by-products gave 97 mol %, indi-
cating that other liquid by-products are of low
relevance. Repeating the experiments indi-
cated good reproducibility. Investigation of
the temperature dependence of the selectiv-
ity (Fig. 2A) revealed an increase of the co-
tetramerization product yield with increasing
temperature, similar to the increase of the
a-value in the oligomerization of ethylene (Fig.
1C). Assuming a Cossee-Arlman mechanism
( 27 ) for the catalyst system based on 3 (Fig. 1,
C and E), the proposed reaction sequence
that could give rise to the co-trimerization
product and the main by-products is shown
in Fig. 2B.b-H elimination and transfer to
the monomer ethylene is the starting point
(intermediate I; Fig. 2B). Our catalyst is es-
sentially inactive in the presence ofa-olefins
only, and we do not see products indicating
other chain-termination reactions. 1,2-Insertion
of thea-olefin takes place next, giving rise to
only one H atom for theb-H elimination and
transfer, which is attached to a tertiary carbon
atom and is, therefore, more protected steri-
cally (intermediate III; Fig. 2B) than the two
b-H atoms attached to the secondary carbon
atom, which is formed after ethylene insertion
(intermediate IV; Fig. 2B). The relatively slow
b-H elimination and transfer of III forms the
co-dimerization product (3-methyleneheptane
for 1-hexene and ethylene). The steric protec-
tion of theb-H atom and the presence of only
oneb-Hatomcouldleadtoslowerb-H elimi-
nation and transfer and seems to be the key to
reduce co-dimer formation. Further ethylene
insertion provides easily accessibleb-H atoms
(intermediate IV; Fig. 2B) and very fast chain
termination can take place to form the elon-
gation and branching product. Further ethylene
insertions give rise to the co-tetramerization
product and even longer ethyl branched oligo-
mers at higher temperatures and/or ethylene
pressures. Notably, thea-value of this process
and that of the lineara-olefin by-product dis-
tribution are similar (figs. S30 and S44).
The 1-GHz^1 H NMR spectrum of the co-
trimerization product of 1-hexene is shown
in Fig. 2C, including the assignments of key
resonances. The shifts, multiplicity, and in-
tegrals of the signals are in agreement with
the structure of the main product, 4-ethyloct-
1-ene.
We next investigated the lineara-olefin
scope of the elongation and branching reac-
tion. A variety of lineara-olefins (Table 1) were
modified. Alla-olefins, including long-chain
examples such as 1-hexadecene, were elongated
and branched with a selectivity of 72 mol %
or higher. We carried out the corresponding
experiments three times to demonstrate good
reproducibility. Multigram-scale synthesis of
4-ethyldec-1-ene, the elongation and branch-
ingproductof1-octene,isshowninFig.3A.
Ethylene consumption studies revealed a
slow decrease of the uptake within the first
15 min and a nearly linear uptake for the re-
maining 45 min (Fig. 3A). A reaction time
of 1 hour seemed optimal for a high activity
and product selectivity of 76 mol %, as deter-
mined by gas chromatography. Three runs
were carried out to demonstrate good re-
producibility. We combined two of the three
runs and obtained 18 g of product with a
purity greater than 96% after a single dis-
tillation. The isolated olefin was used again
for a second elongation and branching to
4,6-diethyldodec-1-ene with a selectivity of
66 mol % (Fig. 3B). This experiment indicates
the possibility of the elongation and branch-
ing of brancheda-olefins. Furthermore, suc-
cessive elongation and branching is feasible,
which offers access to a large number of
a-olefins from the existinga-olefin feed-
stock. The successful elongation and branch-
ing of styrene indicates that the reaction is
not limited to purely aliphatica-olefins ( 28 ).
Using norbornene, we demonstrated that
cyclic olefins can also be modified ( 29 ). A se-
lectivity of elongation and branching products
of greater than 80 mol % (Fig. 3, C and D) was
observed for these latter reactions. Consid-
eringthatonlythreea-olefins could previ-
ously be synthesized selectively from ethylene,
our elongation and branching reaction subs-
tantially extends the scope of selectively acces-
siblea-olefins from ethylene. Related catalysts
might have potential for the selective syn-
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ACKNOWLEDGMENTS
We thank K. Schweimer and the North Bavarian NMR Centre for
recording the 1-GHz NMR spectra and A. Goller for support in
the lab.Funding:We acknowledge financial support from the
University of Bayreuth.Author contributions:R.K. conceived
the concept. W.P.K., T.D., F.L., and R.K. jointly devised the
experimental program. W.P.K. supervised the experimental
program. W.P.K., T.D., and F.L. synthesized and characterized
the catalysts. T.D. and F.L. performed the oligomerization
experiments and analyzed the products. All authors jointly wrote
the manuscript.Competing interests:The authors declare
no competing interests.Data and materials availability:
Crystallographic data for compounds 1 to 3 are available free
of charge from the Cambridge Crystallographic Data Centre under
references CCDC 2111472, CCDC 2111473, and CCDC 2111475,
respectively. All other data are available in the manuscript or the
supplementary materials. Correspondence and requests for
materials should be addressed to R.K.SUPPLEMENTARY MATERIALS
science.org/doi/10.1126/science.abm5281
Materials and Methods
Figs. S1 to S62
Tables S1 to S15
References ( 30 – 44 )23 September 2021; accepted 14 January 2022
10.1126/science.abm52811024 4 MARCH 2022•VOL 375 ISSUE 6584 science.orgSCIENCE
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