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to [Ni-2] was optimal to achieve high catalyst activity. Chlori-
nated solvents were the most effective solvents at a w(solvent)/
w(1-butene)-ratio of 2 for producing a high yield of the
desired dimers. It was also necessary to use a mild reaction
ꢀ
temperature near 30 C to obtain high dimer yields of 67%. The
trimers were built with only 21%. Finally, we were able to
generate a catalyst complex that can dimerize 1-butene to the
highly desired linear octenes at selectivities of 70%.
Fig. 9 Results of the solvent screening (c([Ni-2]) ¼ 1 mol%, n(hfacac)/
ꢀ
n([Ni-2]) ¼ 1, w(solvent)/w(1-butene) ¼ 2, T ¼ 30 C, rt ¼ 24 h, Y4a–d
¼
ꢀ
yield of dimers and trimers determined by GC; * ¼ [Ni-1], T ¼ 70 C;
Experimental section
Typical oligomerization experiment
8
Y(C ), Y(C12)).
The oligomerization reactions were carried out using the
general Schlenk method. All solvents were dried over mol sieves
˚
(
4 A) and degased. In a representative experiment, 0.1064 g of
[
Ni-2] (0.0356 mmol, 1 mol%) were dissolved in 4 g
dichlormethane.
Then 0.0746 g hfacac (0.0356 mmol, 1 mol%) were added
dropwise to the solution and stirred for 1 h. The solution turned
from brown to red. Then the solution was transferred to an
autoclave and 2 g of 1-butene were added. The reaction was
analyzed with GC-FID to determine the dimer and trimer yields
Fig. 10 Variation of the reaction temperature (c([Ni-2]) ¼ 1 mol%,
n(hfacac)/n([Ni-2]) ¼ 1, rt ¼ 24 h, solvent ¼ dcm, w(solvent)/w(1- and GC-MS to characterize both compounds.
butene) ¼ 2, Y4a–d ¼ yield of dimers and trimers determined by GC;
To determine the yield of linear dimers a hydrogenation of
Y(C ), Y(C12)).
8
the reaction solution was carried out. This was necessary
because the isomers could not be separated on our GC-column
(
30 m HP-5.5% phenylmethylsiloxane column). The hydroge-
The highest yield of dimers (67% and 57%) was obtained
ꢀ
nation takes place at a reaction temperature of 80 C, 30 bar
hydrogen with 1 mol% of the Wilkinson-catalyst RhCl(PPh ) .
ꢀ
with dichlormethane (dcm) and dichlorethane (dce) at 30 C. It
can be assumed that chlorinated solvents are advantageous for
the oligomerization of alkenes with allylic nickel complexes,
whereas coordinating solvents like thf or 1,4-dioxan led to a
blockage of the catalyst. A remarkable difference in the catalyst
activity of the [Ni-1] and allylic nickel complexes appeared with
toluene as a solvent. A dimer yield of 40% was obtained with the
3
3
Product characterization. As the octene isomers could not be
separated by column chromatography, a 1H-NMR spectrum of
the isomeric mixture was prepared.
1
3
H-NMR [400 MHz, CDCl ] of the octene mixture: d [ppm]
5
1
.46–5.35 (m, 2H), 2.02–1.94 (m, 3H), 1.64–1.63 (m, 2H), 1.39–
2
.24 (m, 6H), 0.903–086 (t, 3H, J ¼ 7.2).
[
Ni-1] in toluene, whereas the allylic nickel complex had no
activity in toluene.
Mass spectroscopic characterization
cis- and trans-2-Octene. MS (GC): 112 (33) [M ], 97 (2), 84
Temperature screening
+
Temperature plays an important role in most chemical reactions (10), 83 (22), 82 (3), 71 (6), 70 (56), 69 (32), 68 (6), 67 (7), 65 (2), 57
because it provides the energy required to bridge the activation (20), 56 (59), 55 (100), 54 (10), 53 (10), 52 (4), 51 (3), 50 (2).
+
energy of the reaction. Furthermore, most homogenous transi-
cis- and trans-3-Octene. MS (GC): 112 (45) [M ], 97 (3), 84
tion metal complexes are not stable at high temperatures, thus (12), 83 (30), 82 (3), 81 (2), 79 (2), 77 (2), 71 (8), 70 (70), 69 (64), 68
the optimal temperature, at which the catalyst activity and (8), 67 (14), 65 (4), 57 (20), 56 (63), 55 (100), 54 (9), 53 (13), 52 (2),
stability are maximized, must be identied. For this purpose, the 51 (5), 50 (2).
+
oligomerization of 1-butene was conducted at 5 different
cis- and trans-4-Octene. MS (GC): 112 (34) [M ], 97 (2), 84 (9),
temperatures in dcm with 1 mol% of [Ni-2]. The results in Fig. 10 83 (28), 82 (2), 81 (2), 79 (2), 77 (2), 71 (5), 70 (50), 69 (35), 68 (5),
ꢀ
show that reaction temperatures over 30 C led to a decrease in 67 (10), 66 (2), 65 (3), 57 (13), 56 (47), 55 (100), 54 (9), 53 (9), 52
the dimer yield. In addition, a precipitation of nickel was (2), 51 (4), 50 (2).
ꢀ
+
observed at reaction temperatures over 70 C.
Methylheptenes. MS (GC): 112 (38) [M ], 111 (36), 97 (6), 83
(100), 81 (7), 79 (4), 78 (4), 77 (2), 76 (2), 70 (10), 68 (10), 67 (13),
6
5
5 (3), 64 (4), 63 (3), 57 (3), 56 (7), 55 (79), 54 (10), 53 (13), 52 (6),
1 (6), 50 (6).
Dimethylhexenes. MS (GC): 112 (23) [M ], 111 (20), 84 (7), 83
Conclusion
+
We demonstrated that it is possible to use hfacac as an activator
for allylic nickel complexes for the oligomerization of 1-butene. (58), 82 (61), 81 (4), 80 (5), 70 (10), 69 (11), 67 (9), 66 (6), 56 (8), 54
In addition, we observed that only the equimolar ratio of hfacac (100), 54 (10), 53 (11), 51 (5).
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RSC Adv., 2015, 5, 41372–41376 | 41375