3
516 Organometallics, Vol. 29, No. 16, 2010
Son and Waymouth
Figure 1. Metallacycle mechanism for Cr-catalyzed ethylene trimerization and tetramerization.
Coordination complexes of Cr are among the most active
and selective catalysts for trimerization and tetrameriza-
intermediacy and reactivity of metallacycles with main-
group alkyls.
2
tion. The activity and selectivity of Cr-catalyzed ethylene
Results and Discussion
oligomerization depends sensitively on the ligand environment
2
1-27
28
at Cr
cocatalyst,
ditions, the Cr catalysts afford polyethylene (PE),
as well as the reaction conditions (pressure,
2
9,30
25,31
Selective oligomerizations of ethylene were performed
with the biphosphonamine catalytic system Cr(PNP)Cl
and solvent
). Depending on the con-
2
4,32,33
3
i
PN( Pr)PPh
35
27
34,35
(PNP = Ph
ethylene in the presence of ZnR (R = Me, Et, Bu) as a
2
2
)
at 25 and 45 °C at 200 psig
ethylene oligomers, or 1-hexene/1-octene
as the major
product. The selectivities for 1-octene versus 1-hexene depend
2
38
transmetalation reagent. Reactions were quenched with
21,22,36
upon the steric bulk of the substituent on the ligand.
For
our studies, we targeted the Cr catalysts ligated by the (Ph P) -
D O, and the influence of transmetalation agents on the
2
product distribution and selectivity was investigated by
analysis of the product distributions for every C by gas
n
chromatography/mass spectrometry (GC/MS) using nonane
as an internal standard (Table 1).
In the absence of zinc alkyls (entries 1 and 5, Table 1),
2
2
i
NPr (=PNP) ligand, which produces up to 70% 1-octene along
16,35
with some 1-hexene and PE.
We envisioned that the intermediacy of metallacycles
could provide an opportunity to generate new classes of
functionalized ethylene oligomers by chain transfer to
3
7
oligomerization of ethylene with Cr(PNP)Cl
and 14 bar (200 psig) of ethylene afforded 1-hexene (1 wt %),
3
/MAO at 25 °C
a transmetalation reagent. Herein, we report the selec-
tive oligomerization of ethylene in the presence of ZnR2
1
-octene (42 wt %), C -C alkenes (35 wt %), and solid
10
22
(R = Me, Et, Bu) as a strategy to generate new classes
of functionalized ethylene oligomers and to test the
polyethylene (19 wt %). Cyclized C (methylcyclopentane and
6
methylenecyclopentane, 1 wt %), octane (3 wt %), and bran-
ched C12 and C14 oligomers (7 wt %) were also observed as side
products. At 45 °C, the product distributions were slightly
different, yielding 1-hexene (3 wt %), 1-octene (58 wt %),
C -C alkenes (11 wt %), and solid polyethylene (22 wt %).
(21) Blann, K.; Bollmann, A.; Dixon, J. T.; Hess, F. M.; Killian, E.;
Maumela, H.; Morgan, D. H.; Neveling, A.; Otto, S.; Overett, M. J.
Chem. Commun. 2005, 620–621.
(22) Overett, M. J.; Blann, K.; Bollmann, A.; Dixon, J. T.; Hess, F.;
Killian, E.; Maumela, H.; Morgan, D. H.; Neveling, A.; Otto, S. Chem.
Commun. 2005, 622–624.
10
22
These products are similar to those observed previously with
this catalyst system, although the distribution of products
is slightly different due to the different oligomerization
(
23) McGuinness, D. S.; Wasserscheid, P.; Morgan, D. H.; Dixon,
J. T. Organometallics 2005, 24, 552–556.
24) Jones, D. J.; Gibson, V. C.; Green, S. M.; Maddox, P. J.; White,
A. J. P.; Williams, D. J. J. Am. Chem. Soc. 2005, 127, 11037–11046.
25) Elowe, P. R.; McCann, C.; Pringle, P. G.; Spitzmesser, S. K.;
Bercaw, J. E. Organometallics 2006, 25, 5255–5260.
26) Xu, T.; Mu, Y.; Gao, W.; Ni, J.; Ye, L.; Tao, Y. J. Am. Chem.
Soc. 2007, 129, 2236–2237.
27) Junges, F.; Kuhn, M. C. A.; dos Santos, A. H. D.; Rabello,
1
6,35,39,40
(
conditions.
Influence of Transmetalation Agents: ZnMe . The addition
2
(
of 600 equiv of ZnMe2 to the ethylene oligomerization
system at 25 °C under otherwise identical conditions led to
an increase in activity, an increase in the C -C oligomers,
(
6
22
(
and a corresponding decrease in the amount of PE formed
C. R. K.; Thomas, C. M.; Carpentier, J.-F.; Casagrande, O. L. Organo-
metallics 2007, 26, 4010–4014.
(entry 2, Table 1). While the selectivity for 1-octene decreases
slightly (42 to 33 wt %) upon addition of ZnMe , the amount
(
28) Kuhlmann, S.; Dixon, J. T.; Haumann, M.; Morgan, D. H.;
Ofili, J.; Spuhl, O.; Taccardi, N.; Wasserscheid, P. Adv. Synth. Catal.
006, 348, 1200–1206.
29) McGuinness, D. S.; Rucklidge, A. J.; Tooze, R. P.; Slawin,
A. M. Z. Organometallics 2007, 26, 2561–2569.
30) Albahily, K.; Al-Baldawi, D.; Gambarotta, S.; Koc, E.;Duchateau,
R. Organometallics 2008, 27, 5943–5947.
31) Morgan, D. H.; Schwikkard, S. L.; Dixon, J. T.; Nair, J. J.;
Hunter, R. Adv. Synth. Catal. 2003, 345, 939–942.
2
of C -C oligomers increased significantly (35 to 60 wt %).
1
0
22
2
The decrease in the amount and the molecular weight of the
polyethylene generated (M = 80 500, entry 1 vs M = 320,
(
n
n
(
entry 2; see Figures S7 and S8) indicates that ZnMe medi-
2
14,38
ates chain transfer,
which increases the amount of
(
C -C oligomers at the expense of higher oligomers and
10
22
(32) Smith, K. M. Curr. Org. Chem. 2006, 10, 955–963.
polyethylene. The increase in activities is likely due to the
influence of the added alkylzinc on catalyst activation.
(
33) K o€ hn, R. D.; Haufe, M.; Mihan, S.; Lilge, D. Chem. Commun.
2
000, 1927–1928.
34) Zhang, J.; Braunstein, P.; Hor, T. S. A. Organometallics 2008, 27,
277–4279.
35) Bollmann, A.; Blann, K.; Dixon, J. T.; Hess, F. M.; Killian, E.;
(
4
(38) van Meurs, M.; Britovsek, G. J. P.; Gibson, V. C.; Cohen, S. A.
J. Am. Chem. Soc. 2005, 127, 9913–9923.
(
Maumela, H.; McGuinness, D. S.; Morgan, D. H.; Neveling, A.; Otto,
S.; Overett, M.; Slawin, A. M. Z.; Wasserscheid, P.; Kuhlmann, S. J. Am.
Chem. Soc. 2004, 126, 14712–14713.
(39) Two major isomers in C14 products are 7-methylene tridecane
and 7-methyl-1-tridecene. Formation of branched products was pre-
viously reported and attributable to co-oligomerization of ethylene with
1-octene.
(40) Even though no Zn is present, there are some deuterio alkanes in
the product mixture, which are likely generated by transmetalation by
trace trimethyl aluminum in MAO.
(
36) Klemps, C.; Payet, E.; Magna, L.; Saussine, L.; Le Goff, X. F.;
Le Floch, P. Chem. Eur. J. 2009, 15, 8259–8268.
37) Mark, S.; Gaidzik, N.; Doye, S.; Enders, M. Dalton Trans. 2009,
875–4877.
(
4