Organochromium complexes as catalysts for the carboalumination of unactivated terminal olefins

Article abstract of DOI:10.1039/b902609f

The addition of aluminium alkyls to terminal olefins leads to branched organoaluminium compounds which can be converted into functionalised alkanes. This carboalumination reaction is efficiently catalysed by a donor functionalised Cp-chromium(III) complex

Full text of DOI:10.1039/b902609f

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www.rsc.org/dalton | Dalton Transactions
Organochromium complexes as catalysts for the carboalumination
of unactivated terminal olefins
Stefan Mark,a Nikola Gaidzik,b Sven Doye†b and Markus Enders*a
Received 9th February 2009, Accepted 8th April 2009
First published as an Advance Article on the web 7th May 2009
DOI: 10.1039/b902609f
The addition of aluminium alkyls to terminal olefins leads to branched organoaluminium compounds
which can be converted into functionalised alkanes. This carboalumination reaction is efficiently
catalysed by a donor functionalised Cp-chromium(III) complex. The active catalyst is obtained by
activation of the chromium(III) dichloride precursor with MAO or with a mixture of trialkylaluminium
and N,N-dimethylanilinium tetrakispentaflourphenylborate. Primary aminoalkenes deactivate the
catalyst whereas secondary and tertiary aminoalkenes can also be carboaluminated.
are used and stoichiometric amounts of aluminium alkyls are
Introduction
present. The investigations described here were performed with
compound 1 (Fig. 1) as catalyst precursor14 and 1-dodecene as
well as aminoalkenes as terminal olefins. Compared to other
derivatives with the same ligand backbone, 1 has an increased
air stability and solubility in organic solvents. In addition to that,
the activated catalyst is stable for several days at room temperature
in the absence of air.
The addition of metal organyls to alkenes or alkynes is an advan-
tageous method for the synthesis of many organic compounds.
Such carbometallations are known for a variety of metal organyls
and have been studied thoroughly.1 As early as 1928 Ziegler and
Ba¨hr reported the carbometalation of olefins using alkaline metal
alkyls.2 In 1952 the same group described the carboalumination
of terminal olefins with AlEt3 at elevated temperature.3 Since
then catalytically controlled carboalumination reactions using e.g.
titanium4 or zirconocene systems5 have been developed. Chiral
zirconocenes enable enantio- and diastereoselective reactions6,7
In 2005 Molander and Sommers employed donor functionalised
cyclopentadienyl Cr-complexes for the synthesis of allenes via
carboalumination of alkynes.8 We now report the carboalumina-
tion of unactivated, terminal olefins, mediated by chromium(III)
half-sandwich complexes, which consist of rigid 8-quinolyl-
cyclopentadienyl ligands that lead to a predefined geometry and
increased stability. These are related to chromium complexes
developed by Jolly and co-worker.9 and, upon activation with
established co-catalysts (e.g. MAO), they show excellent prop-
erties in the polymerisation and co-polymerisation of olefins.9,10
Although the catalytically active species has not yet been isolated,
all experimental and theoretical investigations are in accordance
with an unsaturated cationic chromium(III) alkyl complex.10e,11 In
the presence of additional donors, related complexes have been
structurally characterised by Theopold and others.12 However,
certain neutral chromium(III) complexes have also been reported
to be active in olefin polymerisation without addition of any co-
catalyst.13 In olefin polymerisation the chain transfer from the Cr-
catalyst to aluminium alkyls is an important chain termination
reaction and therefore influences the molecular weight of the
polyolefin. If this transfer becomes dominant, a carboalumination
reaction results. For our system this is the case if higher a-olefins
Fig. 1 Complex 1 used as catalyst precursor.
Results and discussion
For better comparison with published results6 1 mol% of 1, and 1-
dodecene as substrate, was used in preparative scale. More detailed
kinetic analysis then showed that for the carboalumination of
1-dodecene the catalytic system is able to achieve complete
conversion after a few hours only (Table 1, entry 1). Thus, kinetic
studies via NMR spectroscopy were accomplished using 0.1 mol%
of 1.
Carboalumination of 1-dodecene
The catalyst precursor 1 was activated with different co-catalysts
and various organoaluminium reagents ([Al]-R) were added in
stoichiometric amounts (relative to the substrate).
Initially methylalumoxane (MAO) was used as both co-catalyst
and [Al]-R reagent. The reaction progress was monitored by the
decreasing olefinic 1H-NMR signals. 1 combined with MAO leads
to a nearly complete conversion after 30 h (Table 1, entry 2).
A reaction mixture containing AlMe3 and MAO in a 4 : 1 Al-
ratio yielded over 80% of the methylated product after 30 h
(Table 1, entry 4). To further clarify the role of AlMe3 as methyl-
transmitter and MAO as the generator of the active species, we
aAnorganisch-Chemisches Institut, Ruprecht-Karls-Universita¨t, Heidelberg,
Germany. E-mail: markus.enders@uni-hd.de; Fax: +49-6221-541616247;
Tel: +49-6221-546247
bOrganisch-Chemisches Institut, Ruprecht-Karls-Universita¨t, Heidelberg,
Germany
† Present address: Institut fu¨r Reine und Angewandte Chemie, Carl von
Ossietzky Universita¨t, Oldenburg Germany
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Table 1 Results of the carboalumination of 1-dodecene and aminoalkenes mediated by 1
Entry
Substrate
Methylation-reagent/co-catalyst
t/h
Conversion (%)
Cr : Al
mol% Cr
Al : Substrate
1
2
3
4
5
6
7
8
9
Dodecene
Dodecene
Dodecene
Dodecene
Dodecene
Dodecene
Dodecene
CH2=CHCH2C(CH3)2CH2NH2
CH2=CH(CH2)3N(H)CH2Ph
CH2=CH(CH2)3N(CH3)CH2Ph
MAO/MAOa
4
30
30
30
30
30
30
24
24
24
98c
99c
86c
81c
68c
2c
1 : 200
1
2 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
3 : 1
3 : 1
3 : 1
MAO/MAOa
1 : 1000
1 : 1000
1 : 1000
1 : 1000
1 : 1000
1 : 1000
1 : 300
0.1
0.1
0.1
0.1
0.1
0.1
1
AlEt3/HNMe2C6H5]+ [B(C6F5)4]-b
AlMe3/MAOa (4 : 1)
AlMe3/[HNMe2C6H5]+ [B(C6F5)4]-b
AlMe3/—
AlEt3/—
1c
MAO/MAOa
0d
MAO/MAOa
9d,e
22d,e
1 : 300
1 : 300
1
1
10
MAO/MAOa
a MAO as 10 wt% solution in toluene. b Anilinium borate : Cr = 1 : 1. c NMR analysis. d GC-MS analysis. e Isolated yield.
used AlMe3 together with [HNMe2C6H5]+[B(C6F5)4]- (“anilinium
borate”) in catalytic amounts. Such anilinium salts are known
to generate cationic alkyl complexes from metal dialkyls under
elimination of alkane resulting in particularly active catalysts for
olefin insertion and polymerisation.15–17 With 1 and “anilinium
borate” in combination with AlMe3 70% conversion after 30 h
was obtained (Table 1, entry 5). This result is in accordance with
Scheme 1 Carboalumination of terminal olefins using different co-cata-
a cationic complex as active species. Consequently, when using
AlMe3 alone (without MAO), hardly any reactivity was observed
(Table 1, entry 6). Although AlMe3 alkylates the catalyst precursor,
it is not able to abstract an alkyl group from the chromium dialkyl
species, which is a precondition for the formation of the cationic
active catalyst.
To proof that the carboalumination reaction is indeed catalyt-
ically driven, several tests with only MAO as aluminium alkyl
source, but without any chromium complex, have been performed.
No reaction progress could be observed after one day.
Fig. 2 shows the reaction progress of the carboalumination
monitored by NMR spectroscopy. The activity of the catalytic
system depends on the activator and on the alkylaluminium
reagent. AlEt3 shows a higher reactivity compared to AlMe3.
Although the reaction time for a complete conversion varies
between 3 h and more than 70 h, the catalyst remains active until
complete conversion is achieved.
lysts, terminal olefins and organoaluminium compounds.
compound where one or more alkyl groups originate from the
applied terminal olefin. This proposed structure is in accordance
with the hydrolysis products and with the observations of other
carboalumination reactions studied earlier with and without
catalysts.6,18
The most abundant side products after hydrolysis of the reaction
mixture were alkylated dimers of the terminal olefins used. This
is not surprising, since the catalyst is known to polymerise olefins.
The amount of alkylated dimers, analysed by gas chromatography,
is 30–40% when MAO was used, but only 10% for the catalyst
activated with AlR3/aniliniumborate. The regioselectivity of the
carboalumination mediated by 1 was investigated by treating
the reaction mixture (conditions Table 1, entry 4) with D2O
and the resulting product was analysed by 13C DEPT NMR
and 2H NMR. The 13C NMR-spectrum showed the signal of
the CDH2 group which is split into a triplet due to coupling
to deuterium. No deuterium coupled tertiary carbon signal was
observed, which would have been the result of a 2,1-insertion. The
2H NMR-spectrum showed 2,1-insertion product in an amount of
about 3%.
The capability of triethylaluminium (AlEt3) instead of AlMe3
or MAO as aluminium alkyl source was also examined. With-
out any co-catalyst 1/AlEt3—analogous to 1/AlMe3—is al-
most inactive in the carboalumination reaction. By adding
[HNMe2C6H5]+[B(C6F5)4]- in catalytic amounts activities result,
which are comparable to those achieved with MAO.
Fig. 2 Reaction progress of the carboalumination of 1-dodecene with
AlMe3, AlEt3 or MAO respectively, catalysed by 1 and various co-catalysts.
The curves correspond to entries 1–6 in Table 1.
Carboalumination of aminoalkenes
Aminoalkenes are substrates for the intramolecular hydroamina-
tion catalysed by titanocene- or zirconocene-type complexes.19 Un-
der the conditions used in this work no hydroamination reaction
could be observed. Thus, we investigated the carboalumination
The structure of the intermediate organoaluminium species
(Scheme 1) is supposed to be a dimeric tris-alkyl aluminium
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of aminoalkenes and the tolerance of activated chromium(III)-
species towards amine groups. A primary, secondary or tertiary
amine was used, together with MAO as methylation reagent and
co-catalyst, and 1 mol% of the catalyst precursor 1 (see Table 1).
Primary amines completely deactivate the catalytic system, while
the reaction with a secondary amine shows little conversion
(9% isolated product after 24 h). With tertiary amines moderate
conversion was observed (22% isolated product after 24 h).
Acknowledgements
The authors gratefully acknowledge financial support from the
Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 623).
Notes and references
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Conclusion
The potential of organochromium complexes like 1 as efficient
catalyst precursors for the carboalumination of unactivated olefins
with AlMe3 or AlEt3 respectively was demonstrated. For the for-
mation of the active catalyst, a suitable activator like MAO or the
aniliuniumborate [HNMe2C6H5]+[B(C6F5)4]- is indispensable. The
high catalytic activity outranges those of established zirconocene
systems, which makes this chromium system attractive for non-
enantioselective functionalisation of olefins.
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General procedure for carboalumination of olefins
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In a schlenk tube 10-4 equivalents (0.1 mol%) of 1 are dissolved
in dry toluene. 1 equivalent of aluminium alkyl and an activator
(10-4 equivalents if anilinium borate is used) is added followed
by addition of the olefin. After stirring at room temperature the
mixture is quenched with ice water/HCl, neutralised with NaOH
and extracted with diethyl ether. The solvent is removed under
vacuum and the isolated product is analysed by GC-MS and
NMR-spectroscopy.
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Procedures for entries in Table 1
Entry 1: 10.0 mg (23.30 mmol) of 1 was dissolved in 1 ml of
dry C6D6. 4.66 mmol of aluminium (MAO, 10 wt% in toluene,
supplier: Sigma-Aldrich) was added followed by addition of 0.39 g
of 1-dodecene (2.33 mmol). Then 0.5 ml of the reaction mixture
was transferred into a NMR-tube and the reaction progress was
followed by 1H-NMR spectroscopy.
Entries 2, 4, 6 and 7: 2.0 mg (4.66 mmol) of 1, 1000 equivalents
(4.66 mmol) of aluminium (AlEt3, AlMe3 and/or MAO) and 0,78 g
of 1-dodecene (4.66 mmol) were used.
Entries 3 and 5: The procedure follows the procedure for entries
2, 4, 6 and 7. After adding the aluminium compound, 3.73 mg
(4.66 mmol) [HNMe2C6H5]+[B(C6F5)4]- was added.
Entries 8–10: 10.00 mg (23.30 mmol) of 1 was dissolved in
2 ml of dry toluene. A total of 300 equivalents (6.99 mmol) of
aluminium (MAO) was added followed by addition of 0.23 mmol
of aminoalkene. The reaction mixture was stirred at room
temperature overnight and quenched with ice–water/HCl. The
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chromatography and analysed by GC-MS.
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This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 4875–4877 | 4877
©