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\DOI{10.5802/crchim.434}
\datereceived{2025-09-01}
\daterevised{2025-11-21}
\dateaccepted{2025-11-28}
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\section*{Declaration of interests}
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\COI{The authors do not work for, advise, own shares in, or receive
funds from any organization that could benefit from this article, and
have declared no affiliation other than their research organizations.}

\dateposted{2026-04-23}
\begin{document}

\begin{noXML}

\editornote{Article submitted by invitation}
\alteditornote{Article soumis sur invitation}

\CDRsetmeta{articletype}{research-article}

\title{Synthesis of mono- and dibranched \tralicstex{α}{${\upalpha}$}-olefins via
elongation and branching by ethylene}

\alttitle{Synth\`{e}se d'\tralicstex{α}{${\upalpha}$}-ol\'{e}fines mono- et
di-ramifi\'{e}es par allongement et ramification avec de
l'\'{e}thyl\`{e}ne}

\author{\firstname{Christian} \lastname{Heber}} 
\address{Lehrstuhl Anorganische Chemie II Catalyst Design,
Sustainable Chemistry Center, University of Bayreuth, 95440 Bayreuth, Germany}

\author{\firstname{Andre} \lastname{Dickert}} 
\addressSameAs{1}{Lehrstuhl Anorganische Chemie II Catalyst Design,
Sustainable Chemistry Center, University of Bayreuth, 95440 Bayreuth, Germany}

\author{\firstname{Fabian} \lastname{Lukas}}
\addressSameAs{1}{Lehrstuhl Anorganische Chemie II Catalyst Design,
Sustainable Chemistry Center, University of Bayreuth, 95440 Bayreuth, Germany}

\author{\firstname{Winfried P.} \lastname{Kretschmer}} 
\addressSameAs{1}{Lehrstuhl Anorganische Chemie II Catalyst Design,
Sustainable Chemistry Center, University of Bayreuth, 95440 Bayreuth, Germany}

\author{\firstname{Rhett}\nobreakauthor\lastname{Kempe}\CDRorcid{0000-0002-9138-4155}\IsCorresp}
\addressSameAs{1}{Lehrstuhl Anorganische Chemie II Catalyst Design,
Sustainable Chemistry Center, University of Bayreuth, 95440 Bayreuth, Germany}
\email[R. Kempe]{Rhett.Kempe@uni-bayreuth.de}

\shortrunauthors

\thanks{Deutsche Forschungsgemeinschaft (DFG KE756/35-1).}

\keywords{\kwd{\tralicstex{α}{${\upalpha}$}-olefins}\kwd{Branched
olefins}\kwd{Ethylene}\kwd{Oligomerization}\kwd{Titanium}}

\altkeywords{\kwd{\tralicstex{α}{${\upalpha}$}-ol\'{e}fines}\kwd{Ol\'{e}fines
ramifi\'{e}es}\kwd{\'{E}thyl\`{e}ne}\kwd{Oligom\'{e}risation}\kwd{Titane}}

\begin{abstract}
The synthesis of branched \tralicstex{α}{${\upalpha}$}-olefins employing the diversely
available and inexpensive feedstock ethylene is 
challenging.
The recently reported elongation and branching reaction of
\mbox{\tralicstex{α}{${\upalpha}$}-olefins} by two ethylene molecules and the
identification of more selective catalysts for this reaction permit the
multigram synthesis of mono- and dibranched \tralicstex{α}{${\upalpha}$}-olefins.
We report here the synthesis of 4-ethyl-1-octene, 4-ethyl-1-decene and
4-ethyl-6-methyl-1-heptene. The key is the long-term stability of the
catalyst employed.
\end{abstract}

\begin{altabstract}
La synth\`{e}se d'\tralicstex{α}{${\upalpha}$}-ol\'{e}fines ramifi\'{e}es \`{a} partir
d'\'{e}thyl\`{e}ne, une mati\`{e}re premi\`{e}re largement disponible
et peu co\^{u}teuse, est un d\'{e}fi. La r\'{e}action d'allongement et
de ramification des \tralicstex{α}{${\upalpha}$}-ol\'{e}fines par deux mol\'{e}cules
d'\'{e}thyl\`{e}ne r\'{e}cemment rapport\'{e}e et l'identification de
catalyseurs plus s\'{e}lectifs pour cette r\'{e}action ont permis la
synth\`{e}se de plusieurs grammes d'\tralicstex{α}{${\upalpha}$}-ol\'{e}fines mono- et
di-ramifi\'{e}es. Nous rapportons ici la synth\`{e}se du
4-\'{e}thyl-1-oct\`{e}ne, du 4-\'{e}thyl-1-d\'{e}c\`{e}ne et du
4-\'{e}thyl-6-m\'{e}thyl-1-hept\`{e}ne. La cl\'{e} r\'{e}side dans la
stabilit\'{e} \`{a} long terme du catalyseur utilis\'{e}.
\end{altabstract}

%\input{CR-pagedemetas}

\maketitle

\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1}
\vspace{-3pt}
${\upalpha}$-Olefins synthesized from the diversely available and
inexpensive feedstock ethylene are of high importance in chemical
industry and academic research. More than 7.14 million tons of
${\upalpha}$-olefins will have been produced in 2025 from ethylene, and
essentially all of them are linear in structure~\cite{1}. The most
important applications of these linear ${\upalpha}$-olefins are co-feed
for plastic production, and surfactant, softener, and lubricant
production. Unfortunately, the synthesis of branched
${\upalpha}$-olefins employing ethylene is challenging, but recent
progress in selective co-oligomerization reactions of
${\upalpha}$-olefins and ethylene or ethylene only and related catalyst
development work (Figures~\ref{fig1} and~\ref{fig2})
seem to enable the synthesis of numerous mono- and
dibranched olefins, permitting the exploration of their advantage in
{comparison} to their linear relatives~\cite{2,3,4}. {\mbox{Regarding}
\mbox{advantages} or \mbox{applications} of the polymerizable branched
${\upalpha}$-olefins, the synthesis of isotactic poly(4-ethyl-1-hexene)
was reported~\cite{5} and gave rise to an ethylene-based, high-melting
(220~\textdegree C) and highly transparent plastic.} {Furthermore,
copolymerization of ethylene and 4-ethyl-1-octene via coordinative
chain transfer polymerization~\cite{6} is the basis of the synthesis
of a multiply and long-chain branched polyolefin with low density
polyethylene (LDPE)-like properties containing two types of functional
groups~\cite{7}. We report here the synthesis of 4-ethyl-1-octene,
4-ethyl-1-decene, and 4-ethyl-6-methyl-1-heptene 
(Figure~\ref{fig3}). The key is the
long-term stability of the catalyst employed. 


\begin{figure}
\includegraphics{fig01}
\caption{\label{fig1}Synthesis of new mono- or multiply-branched
${\upalpha}$-olefins on a multigram scale ($R = n$-butyl, ${n}$-hexyl,
isobutyl)~\cite{2,3}.}
\end{figure}

\begin{figure}
{\vspace*{-3pt}}
\includegraphics{fig02}
\caption{\label{fig2}Synthesis of branched ${\upalpha}$-olefins.}
{\vspace*{-5pt}}
\end{figure}

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}Synthesis of mono- or dibranched ${\upalpha}$-olefins
\textbf{a}\textendash{}\textbf{c} from an ethylene and
${\upalpha}$-olefin base stock of 1{-}hexene, 1-octene, or
4-methyl-1-pentene.}
\end{figure*}


\section{Experimental section}\label{secII}

All manipulations were carried out under the rigorous exclusion of air
and moisture using {standard} Schlenk techniques and
{glovebox} procedures (mBraun 120-G) with high-capacity
\mbox{recirculation} ($\mathrm{O}_{2}  < 0.1$ ppm) under an atmosphere of
argon or {nitrogen}. The oligomerization
precatalyst
[1,3-bis(2,6-dimethylphenyl)-imidazolidin-2-{\ubreak}imido]-[N-(2,6-diiso-propylphenyl)-6-(2,4,6-triisopropylphenyl)-pyridin-2-amido{]}-di-{\ubreak}(phenylmethanido)titanium(IV)  
\textbf{1} was synthesized according to the literature~\cite{2}.
Depleted methylaluminoxane (d-MAO) was prepared by removing all
volatiles under reduced pressure from commercial methylaluminoxane
(MAO) in toluene.


\subsection{General procedure for the synthesis of branched
${\alpha}$-olefins}\label{sec2.1}

In order to synthesize the branched ${\upalpha}$-olefins, 300 mL of the
respective dried and degassed linear ${\upalpha}$-olefin was placed in
the evacuated autoclave and the temperature adjusted to 30~\textdegree C.
Subsequently, an ethylene pressure of 2.5 bar was applied in the
autoclave. Stock solutions of precatalyst \textbf{1}
{(0.001 M)}{,} cocatalyst
{(0.035 M)}, and scavenger {(0.400
M)} in cumene were injected while stirring. {d-MAO}
was employed as the cocatalyst, while triisobutylaluminum (TIBA) was
used as the scavenger. Further parameters are listed in the
corresponding tables. After ethylene consumptions of 5, 10, 20, 30, 50,
100, 150, and 200~L$_{\mathrm{n}}$, samples were taken from the
ongoing reaction and analyzed by gas chromatography (GC). Upon
completion of the oligomerization, the product mixture was quenched
using 2 mL of deionized H$_{2}$O and washed using 50~mL
H$_{2}$O/HCl. The \mbox{organic} phase was distilled to separate
the individual ${\upalpha}$-olefin \mbox{fractions.} The branched
${\upalpha}$-olefin desired was isolated and characterized by $^{1}$H
and $^{13}$C NMR spectroscopy as well as GC and mass
spectrometry. For the detailed methodology and equipment used for the
synthesis, purification procedure, and analysis of the branched
${\upalpha}$-olefins, see the Electronic Supporting Information.

\section{Results and discussion}\label{secIII}

\subsection{Multigram synthesis of mono- or dibranched
${\alpha}$-olefins}\label{sec3.1}


\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}Catalytic cycle for the synthesis of branched
${\upalpha}$-olefins {\textbf{A}} {and byproducts 1-butene}
{\textbf{B}} {and 4-ethyl-1-hexene} {\textbf{C}}. The coordination
site is shown as a square (R ${=}$ butyl, hexyl, isobutyl).}
\end{figure*}

To the best of our knowledge, no synthesis of 4-ethyl-1-octene
\textbf{a}, 4-ethyl-1-decene \textbf{b}, or
4{-}ethyl-6-methyl-1-heptene \textbf{c} has been reported in the
literature. Compound \textbf{a} was observed by GC as one of nine
constitutional isomers, with a selectivity of 18~mol\% referring to all
C$_{{10}}$ olefin fractions}~\cite{8}. Compound \textbf{b}
was also observed by GC as one of six co-oligomerization
products~\cite{9}. Compound \textbf{c} has no CAS number yet and seems
unknown. {The catalytic cycle for the synthesis of}
{\textbf{a}}{--}{\textbf{c}} and some main byproducts is depicted in
Figure~\ref{fig4}.
{The relevant catalytic cycles start from a cationic titanium species
bearing an ethyl ligand} {\textbf{I}}{. Coordination of an
${\upalpha}$-olefin or an ethylene molecule to the vacant coordination
site is followed by insertion into the Ti\textendash{}C bond}
{\textbf{II}}. Subsequent ethylene insertion {\textbf{III}} and
$\upbeta$-H elimination/transfer to a second ethylene molecule
{\textbf{IV}} {lead to the release of product} {\textbf{A}} {and
regeneration of the active species. In the absence of the
${\upalpha}$-olefin insertion, 1-butene} {\textbf{B}} {is predominantly
formed, which competes with the ${\upalpha}$-olefins added and leads to
concurrent 4-ethyl-1-hexene formation. The ${\upalpha}$-value (chain
propagation probability) increases with increasing temperature~\cite{2}
(and pressure) and leads to more byproduct formation at higher
temperature at both ends, linear byproducts and branched byproducts.}
{We always use the bisbenzyl titanium precatalysts since we have not
yet found ways of making any other suitable precatalyst despite the
activation with d-MAO. d-MAO gives slightly \mbox{better} co-trimer
selectivity than borate activators~\cite{3} and the bisbenzyl
precatalysts work fine with both activators. Regarding the catalyst
choice, we are very limited. We need a very low ${\upalpha}$-value to
avoid formation of longer linear ${\upalpha}$-olefins and their
branched derivatives. On the other hand, we need a sterically demanding
catalyst to avoid ${\upbeta}$-H elimination/transfer after the co-dimer
is formed. This is a very narrow gap, reducing the potential catalysts
to two examples yet: the precatalysts used in the original
contribution~\cite{2} and} {\textbf{1}} {used here. Both catalysts do
not isomerize ${\upalpha}$-olefins under the conditions \mbox{explored}
yet~\cite{2,3}. Precatalyst} {\textbf{1}} {reduces co-dimer formation to
about one third in comparison to precatalysts used in the original
contribution}.\looseness=1

\subsubsection{Synthesis of 4-ethyl-1-octene \textbf{a}}\label{sec3.1.1}

Compound \textbf{a} was synthesized on a multigram scale according to
the reaction scheme shown in \mbox{Figure~\ref{fig5}}. 1-Hexene was used as
the starting material. The corresponding reaction parameters are
summarized in Table~\ref{tabI}. The amount and mass of \textbf{a}
formed after consumption of a specific ethylene volume are given in
Table~\ref{tabII}. A plot of the amount of \textbf{a} synthesized
depending on the consumption of ethylene is depicted in
Figure~\ref{fig6}. The synthesis of the branched olefins desired on a
multigram scale requires a long-term stable catalyst system.
Figure~\ref{fig7} shows the ethylene consumption during
co-oligomerization of 1-hexene and ethylene plotted against the
reaction time required for the corresponding consumption.

\begin{figure}
\includegraphics{fig05}
\caption{\label{fig5}Synthesis of 4-ethyl-1-octene \textbf{a} from
ethylene and 1-hexene.}
\end{figure}


\begin{figure}
\includegraphics{fig06}
\caption{\label{fig6}Amount of 4-ethyl-1-octene \textbf{a} synthesized
as a function of ethylene consumption.}
\end{figure}

\begin{figure*}
\includegraphics{fig07}
\caption{\label{fig7}Ethylene consumption over time during the
synthesis of 4-ethyl-1-octene \textbf{a} from ethylene and 1-hexene.}
\end{figure*}


\begin{table*}
\caption{\label{tabI} Parameters for 4-ethyl-1-octene \textbf{a}
synthesis{*}}
\begin{tabular}{cccccccc}
\thead
$n_{{1}}$ (${\upmu}$mol) & $m_{\mathrm{d}\hyphen\mathrm{MAO}}$   (mg)  & 
$n_{\mathrm{TIBA}}$ (${\upmu }$mol)  & ${V}_{1\hyphen\mathrm{hexene}}$ (mL) &
${p}_{\mathrm{ethylene}}$ {(}bar{)} & ${T}$ {(}~\textdegree C{)} & 
${V}_{\mathrm{ethylene}}$ {(}L$_{\mathrm{n}}${)} & ${t}$ {(}h{)} \\
\endthead
1.0 & 2.0 & 400 & 300 & 2.5 & 30 & 200 & 7.5
\botline
\end{tabular}
\tabnote{*{${n}$}(d-MAO)/{${n}$}(Ti) ${=}$ 34.5.}
\end{table*}


\begin{table*}
\caption{\label{tabII} Amount and mass of 4-ethyl-1-octene \textbf{a}
synthesized by consumption of the indicated ethylene volumes}
\begin{tabular}{ccccccccc}
\tbody
${V}_{\mathrm{ethylene}}$ (L$_{\mathrm{n}}$) & 5 & 10 & 20 & 30 & 50 & 100 & 150 & 200 \\
$n_{\mathbf{a}}$ {(}mmol{)} & 11.3 & 19.2 & 33.0 & 60.3 & 100.8 & 198.9 & 280.5 & 381.4 \\
${m}_{\mathbf{a}}$ {(}g{)} & 1.6 & 2.7 & 4.6 & 8.5 & 14.1 & 27.9 & 39.4 & 53.5 
\botline
\end{tabular}
\end{table*}

The amount of \textbf{a} increases approximately linearly with the
progressing ethylene consumption, as shown in Figure~\ref{fig6}. Since
1-hexene, which was initially loaded, is also formed during the
catalytic process, its consumption appears to have had no observable
influence on the formation of \textbf{a}. After the consumption of 200
L$_{\mathrm{n}}$ ethylene, 53.5~g (381.4~mmol) of \textbf{a} was synthesized, as
determined by GC. Based on ethylene consumption, catalyst activity is
9500~kg$_{\mathrm{ethylene}}{\cdot}$mol$^{-1}{\cdot}$h$^{-1}{\cdot}$bar$^{-1}$.
Regarding the branched products synthesized, the catalyst system based
on \textbf{1} exhibited a selectivity of 58.0 wt\% (55.2~mol\%) toward
\textbf{a}. {Additionally 136.9~g 1-butene, 41.0~g 1-hexene, and 24.5~g
4-ethyl-1-hexene were formed}. A mass balance of all
${\upalpha}$-olefins synthesized during this catalytic process is
provided in the Electronic Supporting Information. As illustrated in
\mbox{Figure~\ref{fig7},} precatalyst \textbf{1} enables a continuous and long-term
stable oligomerization of the substrates. Moreover, a short
interruption of the ethylene feed for sampling appears to have had no
significant impact on the further progression of the reaction. After
the consumption of 200~L$_{\mathrm{n}}$ of ethylene, approximately 460~mL of the
liquid product and reactant fraction was obtained. The resulting
organic phase was subjected to fractional distillation to isolate
\textbf{a}. The compound exhibited a boiling point of 110~\textdegree C
at a pressure of 200 mbar. After purification, 38.5~g (274.5~mmol) of
\textbf{a} was isolated with a purity of 97.2~mol\%. Figure~\ref{fig8}
displays the $^{1}$H NMR {spectrum} of compound \textbf{a}.
All resonances observed can be assigned to the protons of
4-ethyl-1-octene. The integrals were referenced to the two vinylic
protons of \textbf{a}. As expected, the integral corresponding to the
two methyl groups accounts for approximately six protons. Additionally,
trace amounts of cumene are visible in the spectrum. Cumene was used as
the solvent for the stock solutions employed during synthesis. Its
boiling point of 152.4~\textdegree C at ambient pressure is close to
that of \textbf{a}, preventing complete removal by distillation. A
$^{13}$C NMR spectrum and a mass spectrum were recorded
for further characterization. Both are shown in the Electronic
Supporting Information.

\begin{figure*}
\includegraphics{fig08}
\caption{\label{fig8}$^{1}$H NMR spectrum of
4-ethyl-1-octene \textbf{a} (300 MHz, 293 K, C$_6$D$_6$).}
\end{figure*}
           
\subsubsection{Synthesis of 4-ethyl-1-decene \textbf{b}}\label{sec3.1.2}
The methodology applied for the synthesis of 4-ethyl-1-decene
\textbf{b} (Figure~\ref{fig9}) is analogous to that used for the preparation of \textbf{a}.
The corresponding reaction parameters are summarized in
Table~\ref{tabIII}. The amount and mass of \textbf{b} obtained after a
defined ethylene consumption are listed in Table~\ref{tabIV}.
Figure~\ref{fig10} displays the plot of the synthesized amount of
\textbf{b} versus the corresponding ethylene consumption.

\begin{figure}
\includegraphics{fig09}
\caption{\label{fig9}Synthesis of 4-ethyl-1-decene \textbf{b} from
ethylene and 1-octene.}
\end{figure}


\begin{figure}
\includegraphics{fig10}
\caption{\label{fig10}Amount of 4-ethyl-1-decene \textbf{b} synthesized
as a function of ethylene consumption.}
\end{figure}

\begin{table*}
\caption{\label{tabIII} Parameters for 4-ethyl-1-decene \textbf{b}
synthesis{*}}
\begin{tabular}{cccccccc}
\thead
$n_{{1}}$ {(}${\upmu}$mol{)}  & ${m}_{\mathrm{d}\hyphen\mathrm{MAO}}$ {(}mg{)}  & 
$n_{\mathrm{TIBA}}$ {(}${\upmu }$mol{)} & ${V}_{1\hyphen\mathrm{octene}}$ {(}mL{)} 
& ${p}_{\mathrm{ethylene}}$ {(}bar{)}  & \textit{T} {(}\textdegree C{)} 
& ${V}_{\mathrm{ethylene}}$ {(}L$_{\mathrm{n}}${)} & \textit{t}  {(}h{)} \\
\endthead
1.0 & 2.0 & 400 & 300 & 2.5 & 30 & 200 & 7.2 
\botline
\end{tabular}
\tabnote{{*}{\textit{n}}{(d-MAO)/}{\textit{n}}(Ti) ${=}$ 34.5.}
\end{table*}


\begin{table*}
\caption{\label{tabIV} Amount and mass of 4-ethyl-1-decene \textbf{b}
synthesized by consumption of the indicated ethylene volumes}
\begin{tabular}{ccccccccc}
\tbody
${V}_{\mathrm{ethylene}}$ {(}L$_{\mathrm{n}}${)} & 5 & 10 & 20 & 30 & 50 & 100 & 150 & 200 \\ 
$n_{\mathbf{b}}$ {(}mmol{)} & 8.0 & 17.9 & 37.3 & 51.6 & 87.6 & 200.9 & 282.1 & 383.8 \\ 
${m}_{\mathbf{b}}$ {(}g{)} & 1.3 & 3.0 & 6.3 & 8.7 & 14.7 & 33.8 & 47.5 & 64.6 
\botline
\end{tabular}
\end{table*}

\begin{table*}
\caption{\label{tabV} Parameters for the synthesis of
4-ethyl-6-methyl-1-heptene \textbf{c}{*}}
\tabcolsep3pt
\begin{tabular}{cccccccc}
\thead
$n_{{1}}$ {(}${\upmu}$mol{)}  & ${m}_{\mathrm{d}\hyphen\mathrm{MAO}}$ {(}mg{)}   & 
$n_{\mathrm{TIBA}}$ {(}${\upmu }$mol{)}  & 
${V}_{4\hyphen\mathrm{methyl}\hyphen1\hyphen\mathrm{pentene}}$ {(}mL{)} 
& $p_{\mathrm{ethylene}}$ {(}bar{)} & \textit{T} {(}\textdegree C{)} 
& ${V}_{\mathrm{ethylene}}$ {(}L$_{\mathrm{n}}${)} & \textit{t} {(}h{)} \\
\endthead
3.0 & 6.0 & 400 & 300 & 2.5 & 30 & 200 & 8.0 
\botline
\end{tabular}
\tabnote{*{\textit{n}}{(d-MAO) /} {\textit{n}}{(Ti) ${=}$ 34.5.}}
\end{table*}


The parameters of the synthesis affording 4-ethyl-1-decene \textbf{b}
(Table~\ref{tabIII}) are nearly identical to those used for the
synthesis of \textbf{a} (Table~\ref{tabI}). As shown in
Table~\ref{tabIV} and Figure~\ref{fig10}, the amount of \textbf{b}
increases approximately linearly with increasing ethylene consumption.
After the consumption of 200~L$_{\mathrm{n}}$ of ethylene, 64.6~g (383.8 mmol) of
4-ethyl-1-decene \textbf{b} was obtained, as determined by GC analysis.
Based on ethylene consumption, catalyst activity is
9900~kg$_{\mathrm{ethylene}}{\cdot}${mol}$^{{-1}}{\cdot}${h}$^{{-1}}{\cdot}${bar}$^{-1}$. 
Regarding the branched products synthesized, \textbf{1} exhibits a
selectivity of 58.9 wt\% (53.0~mol\%) toward \textbf{b}. {Additionally
126.9~g 1-butene, 43.7~g 1-hexene, 13.4~g 1-octene, and 22.3~g
4-ethyl-1-hexene were formed}. Upon completion of the synthesis,
approximately 490~mL of a liquid product and reactant mixture were
recovered. The workup was carried out analogously to \textbf{a}. The
organic phase was subjected to fractional distillation to isolate
\textbf{b}. 4-Ethyl-1-decene \textbf{b} exhibits a boiling point of
115~\textdegree C at 15~mbar. After purification, 44.7~g (265.6 mmol)
of \textbf{b} was isolated with a purity of 94.1~mol\%. $^{1}$H and
$^{13}$C NMR spectra as well as a mass spectrometry spectrum were
recorded for the characterization of \textbf{b}. The referring
$^{1}$H~NMR \mbox{spectrum} is shown in Figure~\ref{fig11}. All signals
observed in the spectrum can be assigned to the resonances of
4-ethyl-1-decene. The integrals were referenced to the two vinylic
protons of \textbf{b}. The high-field region contains the signals of
the protons in the two terminal methyl groups of the branched
4-ethyl-1-decene, showing an integral value corresponding to
approximately six protons.

\begin{figure*}
{\vspace*{-4pt}}
\includegraphics{fig11}
{\vspace*{-4pt}}
\caption{\label{fig11}$^{1}$H NMR spectrum of
4-ethyl-1-decene \textbf{b} (300 MHz, 293 K, C$_6$D$_6$).}
{\vspace*{-3pt}}
\end{figure*}

\subsubsection{Synthesis of 4-ethyl-6-methyl-1-heptene \textbf{c}}\label{sec3.1.3} 

\begin{figure}
\includegraphics{fig12}
\caption{\label{fig12}Synthesis of 4-ethyl-6-methyl-1-heptene
\textbf{c} from ethylene and 4-methyl-1-pentene.}
{\vspace*{-4pt}}
\end{figure}


\begin{figure}
\includegraphics{fig13}
\caption{\label{fig13}Amount of 4-ethyl-6-methyl-1-heptene \textbf{c}
synthesized as a function of ethylene consumption.}
\end{figure}


\begin{table*}
\caption{\label{tabVI} Amount and mass of 4-ethyl-6-methyl-1-heptene
\textbf{c} synthesized by consumption of the indicated ethylene
volumes}
\begin{tabular}{ccccccccc}
\tbody
${V}_{\mathrm{ethylene}}$ {(}L$_{\mathrm{n}}${)} & 5 & 10 & 20 & 30 & 50 & 100 & 150 & 200 \\
$n_{\mathbf{c}} $ {(}mmol{)} & 0 & 0.1 & 4.3 & 16.6 & 28.6 & 58.1 & 96.8 & 122.6 \\
${m}_{\mathbf{c}}$ {(}g{)} & 0 & 0.01 & 0.6 & 2.3 & 4.0 & 8.1 & 13.6 & 17.2
\botline
\end{tabular}
\end{table*}

\begin{figure*}
\includegraphics{fig14}
{\vspace*{3pt}}
\caption{\label{fig14}$^{1}$H NMR spectrum of
4-ethyl-6-methyl-1-heptene \textbf{c} (300 MHz, 293 K, C$_6$D$_6$).}
{\vspace*{3pt}}
\end{figure*}

The synthesis of \textbf{c} was carried out according to the reaction
sequence depicted in Figure~\ref{fig12} and proceeded analogously to the
synthesis of \textbf{a} and \textbf{b}. The corresponding reaction
parameters are summarized in Table~\ref{tabV}. The amount and mass of
\textbf{c} formed after the consumption of a specific volume of
ethylene are given in Table~\ref{tabVI}. A plot of the amount of
\textbf{c} synthesized depending on the consumption of ethylene is
depicted in Figure~\ref{fig13}.
The amount of precatalyst \textbf{1} required for the synthesis of
\textbf{c} had to be increased to 3.0 ${\upmu }$mol and the mass of
d-MAO had to be increased to 6.0 mg to achieve the consumption of 200
L$_{\mathrm{n}}$ of ethylene {within the desired reaction time of eight hours}.
This corresponds to a threefold increase in the amounts of both
precatalyst and activator compared to the synthesis of \textbf{a} and
\textbf{b}. Based on ethylene consumption, catalyst activity is 
3000~kg$_{\mathrm{ethylene}}{\cdot}$mol$^{-1}{\cdot}${h}$^{{-1}}{\cdot}
${bar}$^{{-1}}$. As evident from Table~\ref{tabVI} and
Figure~\ref{fig13}, the reactivity ratio of \textbf{1} toward branched
${\upalpha}$-olefins is significantly lower compared to linear
${\upalpha}$-olefins. Under otherwise identical conditions, the
synthesis of 4-ethyl-6-methyl-1-heptene \textbf{c} from ethylene and
4-methyl-1-pentene only afforded 122.6~mmol of \textbf{c}. This
corresponds to approximately one-third of the amount of product
obtained in the synthesis of \textbf{a} from ethylene and 1-hexene
(Table~\ref{tabII}) or \textbf{b} from ethylene and 1-octene
(Table~\ref{tabIV}). During the synthesis of \textbf{c}, an exponential
increase in product formation was observed between 5 and 30 L$_{\mathrm{n}}$ of
ethylene consumption (Figure~\ref{fig13}), which transitions into a linear
increase beyond 30 L$_{\mathrm{n}}$. After the consumption of 200~L$_{\mathrm{n}}$ of ethylene,
17.2~g (122.6~mmol) of 4-ethyl-6-methyl-1-heptene \textbf{c} was
obtained, as determined by GC analysis. Regarding the branched products
synthesized, \textbf{1} exhibits a selectivity of 25.3~wt\% (24.1~mol\%) 
toward \textbf{c}. {Additionally 136.7~g 1-butene, 32.8~g
1-hexene, and 24.4~g 4-ethyl-1-hexene were formed}. Upon completion of
the co-oligomerization, approximately 430~mL of a liquid product and
reactant mixture was recovered. After workup, the organic phase was
subjected to fractionated distillation. 4-Ethyl-6-methyl-1-heptene
\textbf{c} exhibits a boiling point of 95~\textdegree C at 300~mbar.
After purification, 15.3~g (109.1~mmol) of \textbf{c} was isolated
with a purity of 95.8~mol\%. The $^{1}$H NMR spectrum of \textbf{c} is
shown in Figure~\ref{fig14}. All resonances observed in the spectrum can be
assigned to the protons of \textbf{c}. The integrals were referenced to
the two vinyl protons in the low-field region of the spectrum. The
proton signal of the three terminal methyl groups appears in the
high-field region, with an integral corresponding to approximately nine
\mbox{protons}. Additionally, two signals assigned to the \mbox{protons} at the two
saturated tertiary carbon centers of \textbf{c} were observed in the
high-field region of the spectrum.\looseness=1

\section{Conclusion}\label{secIV}

In conclusion, we report the multigram synthesis of 4-ethyl-1-octene,
4-ethyl-1-decene, 4-ethyl-6-methyl-1-heptene from ethylene and
commercially and abundantly available, inexpensive starting materials.
The catalyst employed is stable in long-time runs of more than seven
hours and the change in product distribution over time results mostly
from 1-butene byproduct formation and its elongation/branching reaction
forming 4-ethyl-1-hexene. The same phenomenon is relevant for the
synthesis of 4-ethyl-1-hexene~\cite{3} but in a
beneficial way since 1-butene concentration is kept high via byproduct
formation. We work with a well-defined precatalyst under relatively low
ethylene pressure and at low temperature. All three conditions are
helpful to avoid polymer byproduct formation. In addition, we form a
lot of branched products with a better solubility than purely linear
products. We do not see polymer formation/precipitation even in an
over-seven-hour run. Conventional oligomerization catalysts that
operate via a Cossee--Arlman mechanism type eliminate as
soon as the co-dimer is formed and cannot insert another ethylene
molecule according to the formal
ethylene--${\upalpha}$-olefin--ethylene insertion
sequence~\cite{3} needed to form 4-ethyl-1-alkenes.

\section*{Acknowledgements} 
We thank the Chair of Macromolecular Chemistry II of the University of
Bayreuth for MS measurements.

%\section*{Declaration of interests}
%The authors do not work for, advise, own
%shares in, or receive funds from any organization that could benefit
%from this article, and have declared no affiliation other than their
%research organizations.

\printCOI

\section*{Funding}
The research was supported by the 
Deutsche Forschungsgemeinschaft (DFG KE756 / 35-1).

\section*{Supplementary data}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.  

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crchim-434-suppl.pdf}}

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