Stereoselective olefin polymerization catalysts generated by the transfer-epimetalation of olefins or acetylenes with dialkyltitanium(IV) complexes: Three-membered metallocycles as proposed chiral sites

Article abstract of DOI:10.1021/om0303092

Efficient transfer-epimetalations of simple olefins and acetylenes by R₂TiL₂ reagents (R = Buⁿ, Bu^t; L = X) are readily achieved in THF at -78°C to generate titanacyclopropa(e)ne intermediates, readily capable o

Full text of DOI:10.1021/om0303092

4172
Organometallics 2003, 22, 4172-4174
Ster eoselective Olefin P olym er iza tion Ca ta lysts
Gen er a ted by th e Tr a n sfer -Ep im eta la tion of Olefin s or
Acetylen es w ith Dia lk yltita n iu m (IV) Com p lexes:
Th r ee-Mem ber ed Meta llocycles a s P r op osed Ch ir a l Sites1
J ohn J . Eisch* and J ohn N. Gitua
Department of Chemistry, P.O. Box 6000, The State University of New York at Binghamton,
Binghamton, New York 13902-6000
Received April 28, 2003
Summary: Efficient transfer-epimetalations of simple
olefins and acetylenes by R2TiL2 reagents (R ) Bun, But;
L ) X) are readily achieved in THF at -78 °C to generate
titanacyclopropa(e)ne intermediates, readily capable of
inserting various unsaturated addends (olefin, acetylene,
nitrile). Analogous epimetalations conducted in hydro-
carbons lead to the stereoselective polymerization of
1-alkenes and the cyclotrimerizations of acetylenes. The
2-substituted-1-halotitanacyclopropyl cation is proposed
as the active site for stereoselective olefin polymerization.
We now wish to report that such transfer-epimetala-
tions by R2TiL2 reagents are readily achieved with
simple olefins and acetylenes as well. More significantly,
however, the resulting three-membered titanocycles
show a dramatic difference in reactivity as a function
of solvent. In donor solvents, for example, as in THF,
such adducts as 6 undergo a slow insertion of one or
two olefin or acetylene units, as in eq 2, leading to the
dimerization or trimerization of the monomer.6 In
hydrocarbon suspensions, on the other hand, R2TiL2
(especially L ) X) causes the immediate polymerization
of ethylene and of R-olefins as well as the cyclotrimer-
ization of monosubstituted acetylenes, even at temper-
atures as low as -78 °C. Moreover, the olefins, such as
propylene, 1-hexene, and styrene were found to have
undergone preponderantly isotactic polymerization in
such hydrocarbon media.6 Comparison of the 1H and 13C
NMR spectra of the resulting polymers with authentic
spectra of both the isotactic and atactic poly-
olefin has permitted the following estimates of the
overall empirical isotacticity of the polymer produced
The oxidative addition of subvalent transition metal
reagents (MtmLn, 1) to various CdC, CtC, CdO, or CdN
linkages (as in 2) with the formation of three-membered
metallocycles (3) has been termed epimetalation (eq
1).2The bonding character of the metallocyclic adduct
can range from a metallacycloprop(en)yl ring (3) having
two σ-like C-Mt bonds and a higher oxidation number
for the metal center, Mtm+2, to a π-complex (4) having
m
little change in the Mt oxidation number. Assigning
the relative importance of resonance structures 3 and
4 requires careful evaluation of the structural param-
eters of the individual adduct if isolated or of its
observed chemical reactions.2-4 Recently, we have dis-
covered that such epimetalations can be achieved more
rapidly, cleanly, and in higher yields by means of
dialkyltitanium(IV) complexes (5) by a process of coor-
dination-induced reductive elimination leading to tita-
nium(II) carbenoid transfer (eq 2).5 The adducts of such
transfer-epimetalations (6) reflect their titanacyclopro-
pa(e)ne-ring character by undergoing the insertion of
various addends A ) B (CO2, R-CtC-H, R-CtN,
R2CdO) and thereby expanding to five-membered ti-
tanocycles 7. In donor solvents such as THF such
insertions are generally limited to single units of A )
B.
by But TiCl2 (8): propylene (88%), 1-hexene (100%), and
2
styrene (77%).6
That the three-membered titanocycle (11 or 12) is
formed in a donor solvent like THF at 25 °C from the
transfer-epimetalating action of But TiCl2 (8) or Bun TiCl2
2
2
with ethylene (9) or with diphenylacetylene (10), re-
spectively, is demonstrated by the chemical-trapping
reactions depicted in Scheme 1. Titanacyclopropane 11
inserted benzonitrile to yield upon deuteriolysis â-
deuteriopropiophenone (14), and titanacyclopropene 12
underwent deuteriolysis with acetic acid-d to produce
(5) (a) Eisch, J . J .; Gitua, J . N. Organometallics 2003, 22, 24. (b)
The characterization of the complexes But2TiCl2 (8) and Bun2TiCl2
follows from their mode of formation from the interaction of 2 equiv of
RLi with 1 equiv of TiCl4 in THF at -78 °C. Warming up, removal of
volatiles, and extraction of the residue with toluene led to the
quantitative separation of the LiCl byproduct and the isolation of
analytically pure TiCl2‚2THF in 95% yield. The high yield of titanium
dichloride implies the presence of the precursors Bun2TiCl2 in ∼95%
yield. (Eisch, J . J .; Shi, X.; Lasota, J . Z. Naturforsch. 1995, 50b, 342).
(6) Experimental details concerning the transfer-epititanation of
ethylene, propylene, and diphenylacetylene by But2TiCl2 in THF, the
polymerization of ethylene and R-olefins by But2TiCl2 in hexane, and
the determination of the isotacticity of the resulting polyolefins by NMR
spectroscopy are given in the Supporting Information.
* Corresponding author. Fax: 607-777-4865. E-mail: jjeisch@
binghamton.edu.
(1) Part 26 of the series Organic Chemistry of Subvalent Transition
Metal Complexes; Part 25: Eisch, J . J .; Gitua, J . N. Organometallics
2003, 22, 24.
(2) Eisch, J . J . J . Organomet. Chem. 2001, 617-618, 148.
(3) Eisch, J . J .; Ma, X.; Han, K. I.; Gitua, J . N.; Kru¨ger, C. Eur. J .
Inorg. Chem. 2001, 77.
(4) Eisch, J . J .; Gitua, J . N.; Otieno, P. O.; Shi, X. J . Organomet.
Chem. 2001, 624, 229.
10.1021/om0303092 CCC: $25.00 © 2003 American Chemical Society
Publication on Web 09/13/2003

Communications
Organometallics, Vol. 22, No. 21, 2003 4173
Sch em e 1
Sch em e 2
Sch em e 3
cis-stilbene-d1,2 (15). That both the three-membered
titanacycles (11 and 12) underwent some insertion of a
second monomer unit to form 16 and 18, respectively,
was revealed by the chemical derivatization with ben-
zonitrile leading as well to ꢀ-deuteriovalerophenone (16
f 17) and the isolation of some hexaphenylbenzene
from the deuteriolytic workup (12 f 18 f 19).6 When,
Sch em e 4
on the other hand, the interaction of But TiCl2 (8) with
2
ethylene (9) or with diphenylacetylene (10) is conducted
starting at -78 °C in either hexane or toluene, the
ethylene (9) is immediately and exothermically poly-
merized to linear polyethylene (20) and the diphenyl-
acetylene (10) undergoes rapid cyclotrimerization to
hexaphenylbenzene (19) (Scheme 2).6,7 The absence of
polymerization of ethylene (9) and the slow cyclotri-
merization of diphenylacetylene (10) by But TiCl2 in
2
tetrahydrofuran solution may readily be attributed to
the THF solvation of the three-membered titanocycle
active site (e.g., 218a from 11), which would reduce the
electrophilicity of the titanium(IV) initiator and thus the
ease of its attack upon the π-electron system of 9 or
generated in hexane or toluene (Scheme 3), most likely
is due to the coordination complex 24, which is formed
from 22 and the hydrocarbon-soluble lithium salt 23.9
Such coordination would reduce the electrophilic char-
acter of the titanium(IV) center in 22.
10.8b,c Similarly, the failure of But Ti(OPri)2 (22) to
2
initiate the polymerization of ethylene or the cyclotri-
merization of diphenylacetylene at -78 °C, even when
In a parallel fashion, passing propylene gas (25) into
a THF solution of But TiCl2 or Bun2TiCl2 starting at -78
2
(7) Previous work has reported the polymerization of ethylene by
passing the gas into mixtures of n-butyllithium and TiCl4 combined
in ratios of 0.75:1.0 to 6.0:1.0 in hydrocarbon media under a nitrogen
atmosphere at room temperature: Friedlander, H. N.; Oita, K. Ind.
Eng. Chem. 1957, 49, 1885. Frankel, M.; Rabani, J .; Zilkha, A. J .
Polym. Sci. 1958, 28, 387. Again, by use of n-butyllithium and TiCl4
propylene has been polymerized under similar conditions and the poly-
(propylene) found to be about 60% isotactic: Zilkha, A.; Calderon, N.;
Ottolenghi, A.; Frankel, M. J . Polym. Sci. 1959, 40, 149. With reference
to our present work, however, we wish to point out that Bun2TiCl2 and
But2TiCl2 intermediates, generated from 2 equiv of RLi and 1 equiv of
TiCl4, are kinetically stable to decomposition into TiCl2 at -78 °C. But
at 25 °C such intermediates in THF decompose rapidly to TiCl2, and
their solutions in hydrocarbons decompose slowly as well. The hydro-
carbon suspensions of the resulting TiCl2 and LiCl at 25 °C can
polymerize either ethylene or propylene, albeit at a slower rate (5 g
PE/g Ti‚atm‚h, refs 2, 4, and Eisch, J . J .; Pombrik, S. I.; Shi, X.; Wu,
S. C. Macromol. Symp. 1995, 89, 221). Noteworthy also is that TiCl2
is very reactive toward the “inert” nitrogen atmosphere via a redox
reaction, such that ammonia is evolved upon hydrolysis (Eisch, J . J .;
Chan, T., unpublished studies). Thus although previous workers may
have generated Bun2TiCl2 in their experiments, it is not clear that this
alkyl survived under their conditions or was the specific catalyst for
the observed polymerizations. It equally follows that under our
conditions (under argon and at -78 °C) our observed polymerizations
cannot be attributed to TiCl2 catalysts.
°C and bringing the solution to 25 °C causes the
formation of 26 by transfer-epimetalation. The presence
of 26 was established by trapping with benzonitrile (13)
and observing the generation of principally butyro-
phenone (27) (Scheme 4). Thus benzonitrile inserted
selectively into the sterically more accessible C-Ti bond,
a , of 26; only a minor amount of isobutyrophenone (28),
the insertion product of 13 into bond b, was detectable.10
The selectivity exhibited in the benzonitrile insertion
into titanacyclopropane 26 forms the basis for an
(8) (a) The representation of 21 as a bis-tetrahydrofuran solvate in
THF solution is consistent with the known complexes: TiCl4‚2THF;
(C6H5)4Ti‚2Et2O, and MeTiCl3‚2THF (Wailes, P. C.; Coutts, R. S. P.;
Weigold, H. Organometallic Chemistry of Titanium, Zirconium and
Hafnium; Academic Press: New York, 1974; Chapter 2. (b) Stoichio-
metric quantities of Lewis bases with respect to Ti and Al reagents in
Ziegler-Natta catalysts have long been known to inhibit olefin
polymerization (ref 11a, pp 112-115). (c) Eisch, J . J .; Boleslawski, M.
P. J . Organomet. Chem. 1987, 334, C1.
(9) Eisch, J . J .; Gitua, J . N. Eur. J . Inorg. Chem. 2002, 3091.

4174 Organometallics, Vol. 22, No. 21, 2003
Communications
Sch em e 5
the transfer-epimetalation of olefins or acetylenes with
the formation of titanacyclopropane or titanacyclopro-
pene intermediates. Such intermediates from simple
olefins should no doubt be formed either in solution, as
we have shown, or in a heterogeneous phase, as we
propose. These three-membered titanocycles in donor
solvents are able to insert one or more unsaturated
monomers with the generation of five- or seven-
membered titanocycles. These findings support the
proposal that such titanocycles are also formed from
R2TiCl2 and the olefins or acetylenes in hydrocarbon
media.14 In such noncoordinating media, however, such
titanocycles are able to promote rapid and stereoselec-
tive olefin polymerization and acetylene cyclotrimeriza-
tion most likely via insertion of monomers into titana-
cycloalk(en)yl cations. The postulated 2-substituted-1-
titanacyclopropyl cation intermediate, such as 29 in
Scheme 5, thus appears to be an excellent model for
rationalizing the observed stereoselectivity in the for-
mation of isotactic poly(R-olefins) from terminal alkenes.
appealing model explaining the isotactic polymerization
of propylene and other R-olefins by these R2TiCl2
catalysts in hydrocarbon medium.11 Up to the present,
the most widely accepted model for the stereoselective
polymerization of R-olefins in heterogeneous phase has
been that proposed by Cossee and Arlman and modified
by Allegra.12 According to their scheme, an alkylated
Ti center, Ti-R, located on the lateral face of the TiCl3
crystal lattice, interacts with an R-olefin coordinated on
an adjacent octahedral site. On steric grounds the
addition of the Ti-C bond to the coordinated olefin is
greatly favored in one conformation, and thereby the
ensuing polymer chain growth occurs preferentially with
an isotactic configuration.
The present metallocyclopropane model hypothesizes
that a titanacyclopropane, such as 26, is formed in
solution or in heterogeneous phase by the transfer-
epimetalation of the R-olefin by a dialkyltitanium(IV)
dichloride, as depicted in Scheme 5. Because of the high
rate of polymerization, even at -78 °C, 26 most likely
is converted to the titanacyclopropyl cation 29 by
coordinative abstraction of the chloride anion into the
LiCl lattice.13 For steric reasons, the next propylene unit
would insert into bond a of 29, approaching the ring
from the side opposite (underneath) that of the first
projecting methyl group and with its own methyl group
distal to the Ti-Cl cationic center and oriented exocyclic
to the ring (30) (Scheme 5). The resulting five-membered
titanacyclic cation 31 would have established a pattern
for the head-to-tail, isotactic union of propylene units,
which union could be propagated by further, similar
insertions of propylene units at the sterically more
accessible C-Ti bond, namely, the underside of bond c
in 31.
Ack n ow led gm en t. Our research on reactions of
group 4 metal alkyls over the past decade has been
supported at various times by Akzo Corporate Research
America, The Boulder Scientific Company, the U.S.
National Science Foundation, and Solvay, S.A., Brus-
sels, Belgium. The present investigation has been
funded by the Alexander von Humboldt Stiftung, Bonn,
Germany, which has provided the principal investigator
with a Senior Scientist Award. Technical advice and
assistance have been generously given by Drs. Fredrick
A. Owuor and Peter O. Otieno.
Note Ad d ed In P r oof. In most recent studies we
have extended the present proposal of titanacyclopropyl
chiral cations as olefin polymerization sites to the
conventional Ziegler catalyst system, TiCl4 with Et3Al.
Thus admixing 1 equiv of TiCl4 with 2 equiv of Et3Al in
hexane at -78 °C gave an initially homogeneous solution
of the products, Et2TiCl2 and 2 equiv of Et2AlCl. In the
first orienting reaction the addition of diphenylacetylene
to such solution, warming to 25 °C, and then adding
D2O led to almost a quantitative isolation of 1,2-
dideuterio-cis-stilbene. This finding is consistent with
the tranfer-epititanation of the acetylene by Et2TiCl2
(cf. Scheme 1, 8 + 10 f 12). In the second reaction the
addition of ethylene to such a solution led to the
exothermic formation of linear, high-density polyethyl-
ene. This observation fits the outcome depicted in
Scheme 2, 8 f 11 f 20. In the final reaction the
addition of propylene to such a solution caused the rapid
precipitation of polypropylene that was up to 80%
isotactic, a stereoselective polymerization readily ac-
commodated by the model outlined in Scheme 5.
In summary, we have demonstrated that dialkyltita-
nium(IV) dichlorides in donor solvents are able to effect
(10) The observed ratio of 27 to 28 in the reaction of 26 with
benzonotrile is 4.0:1.0.
(11) (a) The vade mecum in the field of Ziegler-Natta olefin
polymerization up to 1979: Boor, J ., J r. Ziegler-Natta Catalysts and
Polymerization; Academic Press: New York, 1979. (b) More recent and
authoritative surveys include: Brintzinger, H. H.; Fischer, D.; Mu¨l-
haupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl.
1995, 34, 1143. Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem.
Rev. 2000, 100, 1253.
(12) (a) Arlman, E. J .; Cossee, P. J . Catal. 1964, 3, 99. (b) Arlman,
E. J . J . Catal. 1964, 3, 89. (c) Arlman, E. J . J . Catal. 1966, 5, 178. (d)
Arlman, E. J . Recl. Trav. Chim. Pays-Bas 1968, 87, 1217. (e) Allegra,
G. Makromol. Chem. 1971, 145, 235.
(13) The Lewis acidity of unsolvated ion-pairs of lithium chloride
for chloride anions is evident in the experimental detection of diamond-
shaped, bridged lithium chloride dimers in the vapor phase at over
800 °C by means of electron diffraction ( Cl-Li-Cl ) 108° ( 4°):
Bauer, S. H.; Tadishi, I.; Porter, R. F. J . Chem. Phys. 1958, 33, 685.
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM0303092
(14) Pertinent to note, moreover, is that Bun2TiCl2 or Bun2Ti(OPri)2
can also effect the stoichiometric transfer-epimetalation in hexane or
toluene solution (ref 5) of 1,2-substituted ethylenes, such as cis-stilbene
and acenaphthylene, without ensuing polymerization. The substituents
at the olefinic carbons appear to block further insertions into the C-Ti
bonds of the titanacyclopropane intermediate. But such successful,
stoichiometric transfer-epimetalations in hydrocarbons argue for the
similar epimetalation of ethylene and R-olefins as the crucial first step
in their polymerization.