Intramolecular cyclization of benzyl-substituted cyclopentadienyl titanum dichlorides promoted by boron tribromide

Article abstract of DOI:10.1016/S0022-328X(01)01397-3

Intramolecular cyclization of α,α-alkyl-ortho-methoxybenzylcyclopentadienyl titanium complexes to form titanoxacycle complexes promoted by BBr₃ has been studied. A probable two-step mechanism involving halogen exchange and intramolecular elimination was proposed. Four related benzyl-substituted cyclopentadienyl titanium complexes have been studied by X-ray crystallography.

Full text of DOI:10.1016/S0022-328X(01)01397-3

Journal of Organometallic Chemistry 645 (2002) 59–64
www.elsevier.com/locate/jorganchem
Intramolecular cyclization of benzyl-substituted cyclopentadienyl
titanium dichlorides promoted by boron tribromide
Yanlong Qian a,*, Jiling Huang a, Kai Ding a, Yue Zhang a, Qinglan Huang a,
Xiao Ping Chen a, Albert S.C. Chan b,*, Wing Tak Wong c
a Laboratory of Organometallic Chemistry, East China Uni6ersity of Science and Technology, 130 Meilong Road, P.O. Box 421,
Shanghai 200237, China
b Department of Applied Biology and Chemical Technology, Hong Kong Polytechnic Uni6ersity, Hung Hom, Kowloon, Hong Kong, China
c Department of Chemistry, Uni6ersity of Hong Kong, Pokfulam Road, Hong Kong, China
Received 12 June 2001; received in revised form 9 October 2001; accepted 19 October 2001
Abstract
Intramolecular cyclization of a,a-alkyl-ortho-methoxybenzylcyclopentadienyl titanium complexes to form titanoxacycle com-
plexes promoted by BBr3 has been studied. A probable two-step mechanism involving halogen exchange and intramolecular
elimination was proposed. Four related benzyl-substituted cyclopentadienyl titanium complexes have been studied by X-ray
crystallography. © 2002 Published by Elsevier Science B.V.
Keywords: Titanoxacycle; Halogen exchange; Intramolecular cyclization; Boron tribromide
1. Introduction
The cyclization reaction is promoted by halides, such
as LiBr. If one of the substituents on the benzylic
carbon atom of the complexes is a methyl group or
1,5-pentylidene group, the cyclization will not easily
take place, usually only mixtures were obtained. The
large substituents on the benzylic carbon atom cause
the benzene ring to shift towards the titanium atom,
Because of the importance of the cyclopentadiene
ligand in organometallic chemistry, much attention has
been paid to the synthesis of new types of substituted
cyclopentadienes and their transition metal complexes.
Recent interest in our laboratory is focused on the
chemistry of new types of substituted titanocene and
zirconocene complexes [1–5], which may be used as
effective catalysts for hydrogenation [6–8], isomeriza-
tion [9–12] and polymerization [13–21]. One of the
main themes of this study is an understanding of in-
tramolecular coordination and elimination of a,a-di-
alkyl-ortho-methoxybenzylcyclopentadienyl
titanium
complexes to form titanoxacycle complexes [22,23],
which seem to have potential use as catalysts in olefin
polymerization. Our previous investigations have
demonstrated that in the preparation of ortho-MeO-
containing benzyl-substituted titanium complexes, when
the benzylic carbon atom is substituted with an ethyl or
larger group, titanoxacycle complexes are always
formed, as shown in Scheme 1 [22,23].
* Corresponding authors. Tel./fax: +86-21-64702573.
E-mail address: qianling@online.sh.cn (Y. Qian).
Scheme 1.
0022-328X/02/$ - see front matter © 2002 Published by Elsevier Science B.V.
PII: S0022-328X(01)01397-3

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Y. Qian et al. / Journal of Organometallic Chemistry 645 (2002) 59–64
mixture of chloride and bromide. If the reaction time is
prolonged for more than 100 h on heating, it is possible
to isolate a pure bromide product (see pathway B). In
this case, we believe that dibromides 3 and 4 should
also formed as intermediates in the reaction mixtures,
but never were isolated as pure products. After exten-
sive screening of metal halides as promoters, BBr3 was
found to be a more effective and controllable promoter
for the cyclization. In the presence of BBr3, the reaction
may be stopped after the halogen-exchange step. There-
fore, two dibromides 3 and 4, were isolated as pure
products which may be converted into titanoxacycle
complexes easily on heating, or in the presence of BBr3
for a longer reaction time (see pathway C). Of course,
the dichlorides may be converted into titanoxacycle
complexes directly by BBr3 and a mixture of titanoxa-
cycle chloride and bromide was also converted conve-
niently into pure bromide.
In the present work, a series of benzyl-substituted
cyclopentadienyl titanium compounds have been syn-
thesized and well characterized. Among them, the
molecular structures of complexes 1, 4, 5, and 6, have
been studied by X-ray diffraction. Their crystal data,
selected bond lengths and angles are listed in Tables 1
and 2. The structures are shown in Figs. 1–4.
The attempts to prepare titanocene dibromides by
the reaction of titanocene dichlorides with KBr or LiBr
were unsuccessful, and the products isolated proved to
be mixtures of unconverted chloro complex, corre-
sponding bromo compound and mixed halo com-
pounds [24]. Instead, BBr3 is a more effective halogen
exchanging reagent for preparative reactions of metal-
locenes dibromides from corresponding dichlorides in
high yields [25]. Therefore, the mechanism of the cy-
clization may be proposed as follows (Scheme 3):
Scheme 2.
thus facilitating a favorable spatial orientation for the
cyclization. In order to extend this work, we have
performed the corresponding cyclizations using a,a-
dimethyl or ꢀ(CH2)5-ortho-methoxybenzylcyclopentadi-
enyl titanium complexes, in an attempt to obtain a
series of novel titanoxacycle complexes. Thus, a series
of benzyl-substituted cyclopentadienyl titanium com-
plexes has been synthesized. Such pure complexes have
never been isolated by using our previous method, by
heating or by the LiBr-promoted reaction [23], and four
of them have been studied by X-ray crystallography.
2. Results and discussion
After further study on such cyclization methodology
the reaction may be outlined as shown in Scheme 2.
In the case of R1=R2=Et or Prn, the cyclization
takes place readily [23] on heating without halide as
promoter (see pathway A). In the presence of halide,
such as LiBr, the cyclization may be completed, but it is
sometimes accompanied by halogen exchange to give a
Scheme 3.
The first step is halogen exchange from the dichloride
to the dibromide. The second step is intramolecular
coordination between Ti···O and Br···CH3 through a
four-membered ring transition state; then the titanoxa-
cycle bromide complex is formed by elimination of

Y. Qian et al. / Journal of Organometallic Chemistry 645 (2002) 59–64
61
CH3Br. Besides, BBr3 is a good Lewis acid which easily
producing a Br− anion and thus promotes demethyla-
tion of aryl methyl ether [26,27]. In our case, it seems to
be that the BBr3 as a Lewis acid for demethylation is
also involved, and thus in the fact formation of ti-
tanocyclic compounds with BBr3 is a complicated reac-
tion. A further study of the mechanism will be done in
the future.
CH2Cl2 was added dropwise to a solution of 1 (400 mg,
1 mmol) in 10 ml of CH2Cl2 with stirring at 0 °C. It
was stirred for 2 h at room temperature (r.t.). After the
solvent was removed, a dark black solid was obtained
as crude product. It was passed though a microcrystal
column with petroleum ether–CHCl3 as eluant. The
solvent was removed, and the remaining solid was
recrystallized from CHCl3–petroleum in the refrigera-
tor to afford 100 mg of dull red pure sample in 20%
1
yield, m.p. 174–176 °C, H-NMR (l ppm): 6.95–6.94
3. Experimental
(m, 4H), 6.66 (s, 5H), 6.59–6.85 (m, 4H), 3.67 (s, 3H),
1.78 (s, 6H). MS (m/e): 405 (M−Br, 2), 390 (M−
CH3Br, 32), 325 (M−CH3Br−Cp, 56), 311 (M−
CH3Br−Br, 42), 94 (CH3Br, 100). IR (KBr, cm−1):
3136w, 3114m, 3082m, 2998w, 2935w, 2874w, 2831m,
1593w, 1580w, 1490m, 1479m, 1444m, 1434m, 1291m,
1243s, 1181w, 1148w, 1091m, 1031m, 835s, 824s, 744s.
Anal. Calc. for C20H22Br2OTi: C, 49.41; H, 4.57.
Found: C, 48.74; H, 4.49%.
All reactions were carried out under an inert atmo-
sphere using standard Schlenck techniques. Solvents
were refluxed over Na wire and benzophenone and
distilled under Ar prior to use. Melting points were
uncorrected. 1H-NMR spectra were recorded on a
GERMINI-300 spectrometer using CDCl3 as solvent
and Me4Si as an internal standard. IR spectra were
measured on a NICOLET MAGNA-IR550. MS were
obtained on a HP5989A Mass Spectrometer. The com-
pounds 1, 2, 7, 8, 9 and 10 were synthesized according
to our previous method [23].
3.2. Synthesis of 4
To a solution of 2 (440 mg, 1 mmol) in 7 ml of
CH2Cl2, BBr3 (0.1 ml, 1 mmol) in 6 ml of CH2Cl2 was
added dropwise with stirring at 0 °C. It was stirred for
2 h. The solvent was removed. It was passed through a
microcrystal column with petroleum ether–CHCl3 as
eluant. After the solvent was removed, the remaining
solid was recrystallized from chloroform and petroleum
ether in refrigerator. Black crystals (70 mg) were ob-
X-ray data were collected on a Rigaku AFC7R dif-
fractometer with graphite monochromated Mo–Ka ra-
diation and a 12 kW rotating anode generator.
3.1. Synthesis of 3
A solution of BBr3 (2.4 ml, 0.22 mol) in 5 ml of
Table 1
Crystallographic data
Compound
1
4
5
6
Empirical formula
Formula weight
Color and habit
Crystal system
Space group
C20H22TiOCl2
397.20
Orange, block
Triclinic
C23H26TiOBr2
526.17
Red, block
Monoclinic
P21/a (c14)
13.881(2)
C
391.16
Orange, block
Orthorhombic
P212121 (c19)
11.028(1)
19H19TiOBr
C22H23TiOBr
431.23
Red, block
Triclinic
(
(
P1 (c2)
P1 (c2)
,
a (A)
6.684(1)
10.870(1)
13.450(1)
101.74(1)
101.27(1)
100.15(1)
914.3(2)
2
10.126(2)
12.340(2)
8.269(2)
100.58(2)
95.44(2)
112.58(1)
922.2(3)
2
,
b (A)
11.652(2)
14.632(2)
14.992(2)
9.976(1)
,
c (A)
h (°)
i (°)
k (°)
117.447(9)
3
,
V (A )
Z
2100.1(6)
4
1.664
42.35
1056.00
ꢀ–2q
1649.2(3)
4
1.575
29.48
792.00
ꢀ–2q
Dcalc (g cm−3
)
1.443
7.64
412.00
ꢀ–2q
1.553
26.44
440.00
ꢀ–2q
v(Mo–Ka) (cm−1
F(000)
)
Scan type
Max. 2q (°)
Crystal size (mm)
47.9
48.0
45.0
48.0
0.19×0.20×0.31
4–16.0
3154
0.23×0.28×0.29
6–16.0
3639
0.21×0.21×0.32
6–16.0
1275
0.14×0.22×0.34
6–16.0
3091
Scan rate (° min−1
)
Reflections observed
R
0.030
0.050
0.054
0.033
Rw
0.030
0.047
0.047
0.032
Goodness-of-fit
1.99
3.20
2.90
1.48

62
Y. Qian et al. / Journal of Organometallic Chemistry 645 (2002) 59–64
Table 2
,
Selected bond lengths (A) and angles (°) of 1, 4, 5, 6
1
4
5
6
Bond lengths
Ti(1)ꢀCl(1)
Ti(1)ꢀCl(2)
Ti(1)ꢀC(1)
Ti(1)ꢀC(2)
Ti(1)ꢀC(3)
Ti(1)ꢀC(4)
Ti(1)ꢀC(5)
Ti(1)ꢀC(6)
Ti(1)ꢀC(7)
Ti(1)ꢀC(8)
Ti(1)ꢀC(9)
Ti(1)ꢀC(10)
2.3739(9)
2.3490(9)
2.357(3)
2.398(3)
2.389(3)
2.389(3)
2.351(3)
2.482(3)
2.389(3)
2.317(3)
2.363(3)
2.417(3)
Ti(1)ꢀBr(1)
Ti(1)ꢀBr(2)
Ti(1)ꢀC(1)
Ti(1)ꢀC(2)
Ti(1)ꢀC(3)
Ti(1)ꢀC(4)
Ti(1)ꢀC(5)
Ti(1)ꢀC(6)
Ti(1)ꢀC(7)
Ti(1)ꢀC(8)
Ti(1)ꢀC(9)
Ti(1)ꢀC(10)
2.539(2)
2.496(2)
2.37(1)
2.34(1)
2.37(1)
2.38(1)
2.38(1)
2.49(1)
2.38(1)
2.30(1)
2.35(1)
2.41(1)
Ti(1)ꢀBr(1)
Ti(1)ꢀO(1)
Ti(1)ꢀC(1)
Ti(1)ꢀC(2)
Ti(1)ꢀC(3)
Ti(1)ꢀC(4)
Ti(1)ꢀC(5)
Ti(1)ꢀC(15)
Ti(1)ꢀC(16)
Ti(1)ꢀC(17)
Ti(1)ꢀC(18)
Ti(1)ꢀC(19)
2.535(4)
1.86(1)
2.32(2)
2.30(2)
2.35(2)
2.36(2)
2.36(2)
2.44(2)
2.41(2)
2.35(3)
2.34(3)
2.39(2)
Ti(1)ꢀBr(1)
Ti(1)ꢀO(1)
Ti(1)ꢀC(1)
Ti(1)ꢀC(2)
Ti(1)ꢀC(3)
Ti(1)ꢀC(4)
Ti(1)ꢀC(5)
Ti(1)ꢀC(18)
Ti(1)ꢀC(19)
Ti(1)ꢀC(20)
Ti(1)ꢀC(21)
Ti(1)ꢀC(22)
2.565(1)
1.878(3)
2.369(5)
2.373(5)
2.405(5)
2.372(5)
2.332(5)
2.377(5)
2.402(5)
2.382(5)
2.378(5)
2.387(5)
Bond angles
Cl(1)ꢀTi(1)ꢀCl(2)
Cl(1)ꢀTi(1)ꢀC(1)
Cl(1)ꢀTi(1)ꢀC(2)
Cl(1)ꢀTi(1)ꢀC(3)
Cl(1)ꢀTi(1)ꢀC(4)
Cl(1)ꢀTi(1)ꢀC(5)
Cl(2)ꢀTi(1)ꢀC(6)
Cl(2)ꢀTi(1)ꢀC(7)
Cl(2)ꢀTi(1)ꢀC(8)
Cl(2)ꢀTi(1)ꢀC(9)
Cl(2)ꢀTi(1)ꢀC(10)
92.80(3)
104.89(9)
77.53(8)
84.78(9)
117.86(9)
133.85(9)
85.07(7)
116.82(8)
137.35(9)
109.81(9)
80.89(8)
Br(1)ꢀTi(1)ꢀBr(2)
Br(1)ꢀTi(1)ꢀC(1)
Br(1)ꢀTi(1)ꢀC(2)
Br(1)ꢀTi(1)ꢀC(3)
Br(1)ꢀTi(1)ꢀC(4)
Br(1)ꢀTi(1)ꢀC(5)
Br(2)ꢀTi(1)ꢀC(6)
Br(2)ꢀTi(1)ꢀC(7)
Br(2)ꢀTi(1)ꢀC(8)
Br(2)ꢀTi(1)ꢀC(9)
Br(2)ꢀTi(1)ꢀC(10)
91.63(8) Br(1)ꢀTi(1)ꢀO(1)
93.8(4)
116.1(5)
83.3(5)
Br(1)ꢀTi(1)ꢀO(1)
Br(1)ꢀTi(1)ꢀC(1)
Br(1)ꢀTi(1)ꢀC(2)
Br(1)ꢀTi(1)ꢀC(3)
Br(1)ꢀTi(1)ꢀC(4)
Br(1)ꢀTi(1)ꢀC(5)
Br(1)ꢀTi(1)ꢀC(18)
Br(1)ꢀTi(1)ꢀC(19)
Br(1)ꢀTi(1)ꢀC(20)
Br(1)ꢀTi(1)ꢀC(21)
Br(1)ꢀTi(1)ꢀC(22)
O(1)ꢀTi(1)ꢀC(1)
O(1)ꢀTi(1)ꢀC(2)
O(1)ꢀTi(1)ꢀC(3)
O(1)ꢀTi(1)ꢀC(4)
O(1)ꢀTi(1)ꢀC(5)
96.3(1)
121.2(2)
87.3(1)
77.4(1)
103.0(1)
134.3(1)
135.3(1)
112.7(2)
81.4(1)
80.8(2)
111.3(2)
77.8(1)
85.0(2)
57.1(2)
57.9(2)
34.7(2)
106.2(4)
134.2(3)
117.1(3)
84.5(4)
Br(1)ꢀTi(1)ꢀC(1)
Br(1)ꢀTi(1)ꢀC(2)
Br(1)ꢀTi(1)ꢀC(3)
Br(1)ꢀTi(1)ꢀC(4)
Br(1)ꢀTi(1)ꢀC(5)
Br(1)ꢀTi(1)ꢀC(15)
Br(1)ꢀTi(1)ꢀC(16)
Br(1)ꢀTi(1)ꢀC(17)
Br(1)ꢀTi(1)ꢀC(18)
Br(1)ꢀTi(1)ꢀC(19)
O(1)ꢀTi(1)ꢀC(1)
O(1)ꢀTi(1)ꢀC(2)
O(1)ꢀTi(1)ꢀC(3)
O(1)ꢀTi(1)ꢀC(4)
O(1)ꢀTi(1)ꢀC(5)
81.3(5)
109.9(6)
136.3(5)
91.0(10)
123.3(10)
132.0(9)
102.0(1)
79.5(7)
78.0(5)
88.7(6)
124.2(7)
135.0(6)
103.5(6)
78.1(4)
86.6(3)
118.5(3)
137.3(3)
109.3(3)
81.4(3)
tained in 15% yield, m.p. 190–196 °C. 1H-NMR (l
ppm): 7.50–6.90 (m, 4H), 6.70–6.20 (m, 9H), 3.80 (s,
3H), 1.98–1.00 (m, 10H). MS (m/e): 430 (M−CH3Br,
6), 365 (M−CH3Br−Cp, 10), 351 (M−CH3Br−Br,
6). IR (KBr, cm−1): 1595w, 1480m, 1460m, 1235s,
1175s, 1100s, 1020s, 820s, 750s.
3.3. Synthesis of 5
3.3.1. Method 1
BBr3 (0.2 ml, 2 mmol) was added to a solution of 1
(400 mg, 1 mmol) in 15 ml of CH2Cl2 with stirring
under Ar. The reaction mixture changed in color from
red to black immediately. It was stirred for 2 h at r.t.
After the solvent was removed, it was passed through a
microcrystal column with CHCl3 as eluant. It was
stirred overnight at 50 °C. Under reduced pressure the
solvent was removed. The remaining solid was recrys-
tallized from C6H6 and petroleum ether. An analytically
pure sample was obtained as red crystals in 52% yield,
m.p. 192–194 °C.
1H-NMR (l ppm): 7.35–6.88 (m, 4H), 6.52 (s, 5H),
6.47–5.54 (m, 4H), 1.59 (s, 3H),1.30 (s, 3H). MS (m/e):
Fig. 1. Molecular structure of 1.

Y. Qian et al. / Journal of Organometallic Chemistry 645 (2002) 59–64
63
eluant. The eluant was concentrated, petroleum ether
was added, and orange red crystals were obtained in
1
45% yield. H-NMR spectrum was identical with the
authentic sample.
3.3.3. Method 3
A solution of 3 (485 mg, 1 mmol) in 5 ml of CHCl3
with stirring for 20 h at 60 °C. The solvent was re-
moved and it was recrystallized from C6H5CH3 and
n-C6H14. Compound 5 was obtained in 50% yield.
3.4. Synthesis of 6
By using compound 2 as starting material, compound
6 was synthesized in the same way as in Section 3.3.1.
Red crystals were obtained, in 30% yield, m.p. 195–
1
196 °C. H-NMR (l ppm): 7.48 (d-m, 1H), 7.04 (t-m,
1H), 6.87 (t-m, 1H), 6.51, 6.47 (s, 5H), 6.49 (m, 1H),
6.38 (m, 1H), 6.29 (m, 1H), 6.13 (m, 1H), 5.43–5.34 (m,
1H), 1.30–1.80 (m, 10H). MS (m/e): 430 (M, 41), 365
(M−Cp, 100), 351 (M−Br, 42). IR (KBr, cm−1):
3200–3100w, 2970m, 2950m, 2880m, 1600m, 1580m,
1480s, 1440s, 1300w, 1230s, 1160m, 1120m, 1040m,
880s, 820s, 770s. Anal. Calc. for C22H23BrOTi: C,
61.27; H, 5.39. Found: C, 61.47; H, 5.39%.
Fig. 2. Molecular structure of 4.
4. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC nos. 135658, 135659, 135660 and
135661 for compounds 1, 4, 5 amd 6, respectively.
Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (Fax: +44-1223-336033;
e-mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.ac.uk).
Fig. 3. Molecular structure of 5.
390 (M, 36), 325 (M−Cp, 55), 311 (M−Br, 100), 310
(M−Cp−CH3, 30), 296 (M−Br−CH3, 24), 65 (Cp,
12). IR (KBr, cm−1): 3110m, 2960m, 2870m, 1593m,
1571w, 1475s, 1446s, 1362m, 1255s, 1020m, 840s, 820s,
750s. Anal. Calc. for C19H19BrOTi: C, 58.33; H, 4.91.
Found: C, 58.39; H, 4.91%.
3.3.2. Method 2
A solution of LiBr (227 mg, 2.2 mmol) and 1 (215
mg, 0.5 mmol) in 30 ml of THF was stirred for 100 h at
40 °C. Then after the solvent was removed it was
passed through a microcrystal column with CHCl3 as
Fig. 4. Molecular structure of 6.

64
Y. Qian et al. / Journal of Organometallic Chemistry 645 (2002) 59–64
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