Ancillary ligand control of the regiochemistry of coupling of 3,3-dimethyl-1-butyne at titanium metal centers

Article abstract of DOI:10.1021/om0209677

The ambient-temperature sodium amalgam (2 Na per Ti) reduction of hydrocarbon solutions of [Cp(ArO)TiCl₂] (ArO = 2,3,5,6-tetraphenylphenoxide) (1) in the presence of 3,3-dimethyl-1-butyne yields the 2,5-di(tert-butyl)titanacyclopentadiene compound [Cp(ArO)Ti(C₄H₂Bu₂^t-2,5)] (2). An X-ray diffraction study of 2 confirms the regiochemistry and shows carbon - carbon distances of 1.343(3) and 1.492(3) A for the double and single bonds, respectively. In contrast the analogous reaction of either [Cp₂TiCl₂] or [(ArO)₂TiCl₂] in the presence of 3,3-dimethyl-1-butyne yields the corresponding 2,4-di(tert-butyl)titanacyclopentadiene compounds. When 2 is heated at 100 °C for a few days in C₆D₆ solution, isomerization to the more stable 2,4-di(tertbutyl) regioisomer 3 is observed by NMR. An attempt is made to rationalize the regiochemistry of the kinetically formed titanacyclopentadiene in terms of steric factors within the intermediate bis(alkyne) complex.

Full text of DOI:10.1021/om0209677

1546
Organometallics 2003, 22, 1546-1549
Notes
An cilla r y Liga n d Con tr ol of th e Regioch em istr y of
Cou p lin g of 3,3-Dim eth yl-1-bu tyn e a t Tita n iu m Meta l
Cen ter s
J ongtaik Lee, Phillip E. Fanwick, and Ian P. Rothwell*
Department of Chemistry, Purdue University, 560 Oval Drive,
West Lafayette, Indiana 47907-2038
Received November 25, 2002
Summary: The ambient-temperature sodium amalgam
(2 Na per Ti) reduction of hydrocarbon solutions of [Cp-
(ArO)TiCl2] (ArO ) 2,3,5,6-tetraphenylphenoxide) (1) in
the presence of 3,3-dimethyl-1-butyne yields the 2,5-di-
(tert-butyl)titanacyclopentadiene compound [Cp(ArO)Ti-
dramatic difference in regiochemistry of the coupling
of tert-butyl acetylene by the mixed Cp/OAr system
compared to the titanocene and bis(aryloxide) systems.
Resu lts a n d Discu ssion
(C4H2But -2,5)] (2). An X-ray diffraction study of 2
2
The 2,3,5,6-tetraphenylphenoxide compound [CpTi-
(OC6HPh4-2,3,5,6)Cl2] (1) can be readily obtained in high
yield by the addition of 1 equiv of the parent phenol to
a mixture of [CpTiCl3] and pyridine in benzene.4c The
one-electron reduction of related aryloxide compounds
has been shown to lead to dinuclear species [Cp(ArO)-
Ti(µ-Cl)2Ti(OAr)Cp]. When a benzene solution of 1 is
reduced by sodium amalgam (2 Na per Ti) in the
presence of excess 3,3-dimethyl-1-butyne, formation of
the titanacyclopentadiene compound [Cp(ArO)Ti(C4H2-
confirms the regiochemistry and shows carbon-carbon
distances of 1.343(3) and 1.492(3) Å for the double and
single bonds, respectively. In contrast the analogous
reaction of either [Cp2TiCl2] or [(ArO)2TiCl2] in the
presence of 3,3-dimethyl-1-butyne yields the correspond-
ing 2,4-di(tert-butyl)titanacyclopentadiene compounds.
When 2 is heated at 100 °C for a few days in C6D6
solution, isomerization to the more stable 2,4-di(tert-
butyl) regioisomer 3 is observed by NMR. An attempt is
made to rationalize the regiochemistry of the kinetically
formed titanacyclopentadiene in terms of steric factors
within the intermediate bis(alkyne) complex.
But -2,5)] (2) (ArO ) OC6HPh4-2,3,5,6) (Scheme 1) takes
2
place. Analysis by 1H NMR of the crude reaction
mixture from the reduction shows no evidence for the
presence of the corresponding 2,4-regioisomer (see
below). An X-ray diffraction study of 2 confirmed the
formulation (Figure 1, Table 1).
In tr od u ction
Titanocene dichloride, [Cp2TiCl2], first synthesized by
Wilkinson and co-workers,1 has proven to be a valuable
stoichiometric reagent and catalyst precursor in orga-
nometallic chemistry. Past research in our group has
focused on the use of the bis(aryloxide) compounds
[(ArO)2TiCl2], in which the [(ArO)2Ti] unit has an
isolobal relationship to the [Cp2Ti] fragment,2 for car-
rying out both novel and complementary organic trans-
formations.3 More recently we have begun to explore the
chemistry and synthetic utility of “hybrid” [Cp(ArO)-
TiCl2] systems containing a variety of chiral and achiral
aryloxide ligands.4 In this note we wish to report on the
1
In the H NMR spectrum of 2, a sharp Cp resonance
is observed at δ 6.56 ppm along with a single But
resonance at δ 1.01 ppm. When a C6D6 solution of 2 is
heated at 100 °C in a sealed tube, these resonances
slowly drop in intensity with the corresponding buildup
of peaks due to a new organometallic product 3. Com-
pound 3 is characterized by a Cp resonance at δ 5.78
ppm and two new, equal intensity But resonances at δ
0.93 ppm and 0.82 ppm. The spectroscopic data for 3
are entirely consistent with the 2,4-regioisomer [Cp-
(ArO)Ti(C4H2But -2,4)] (Scheme 1). There is precedence
2
for exactly this type of behavior in the work of Wigley
et al.5 The kinetically formed tantalacyclopentadiene
[(ArO)2(Cl)Ta(C4H2But -2,5)] (ArO ) OC6H3Pri -2,6) was
* Corresponding author. E-mail: rothwell@purdue.edu.
(1) (a) Wilkinson, G.; Paulson, P. L.; Birmingham, J . L.; Cotton, F.
A. J . Am. Chem. Soc. 1953, 75, 1011. (b) Wilkinson, G.; Birmingham,
J . L. J . Am. Chem. Soc. 1954, 76, 4281.
(2) Bradley, D. C.; Mehrotra, R. C.; Rothwell, I. P.; Singh, A. Alkoxo
and Aryloxo Derivatives of Metals; Academic Press: San Diego, 2002.
(3) (a) J ohnson, E. S.; Balaich, G. J .; Rothwell, I. P. J . Am. Chem.
Soc. 1997, 119, 7685. (b) Thorn, M. G.; Gill, J . E.; Waratuke, S. A.;
J ohnson, E. S.; Fanwick P. E.; Rothwell, I. P. J . Am. Chem. Soc. 1997,
119, 8630. (c) J ohnson, E. S.; Balaich, G. J .; Fanwick, P. E.; Rothwell,
I. P. J . Am. Chem. Soc. 1997, 119, 11086.
(4) (a) Vilardo, J . S.; Thorn, M. G.; Fanwick, P. E.; Rothwell, I. P.
J . Chem. Soc., Chem. Commun. 1998, 2425. (b) Thorn, M. G.; Vilardo,
J . S.; Fanwick, P. E.; Rothwell, I. P. J . Chem. Soc., Chem. Commun.
1998, 2427. (c) Thorn, M. G.; Vilardo, J . S.; Lee, J .; Hanna, B.; Fanwick,
P. E.; Rothwell, I. P. Organometallics 2000, 19, 5636.
2
2
shown to cleanly isomerize to the thermodynamically
more stable [(ArO)2(Cl)Ta(C4H2But -2,4)] upon heating.
2
However, the kinetic formation of 2 (Scheme 1) contrasts
dramatically with the previously reported formation of
the 2,4-regioisomer [(ArO)2Ti(C4H2But -2,4)] (4) (ArO )
2
OC6H3Ph2-2,6) by reduction of the 2,6-diphenylphenox-
ide [Ti(OC6H3Ph2-2,6)2Cl2] in the presence of 3,3-dim-
(5) Smith, D. P.; Strickler, J . R.; Gray, S. D.; Bruck, M. A.; Holmes,
R. S.; Wigley, D. E. Organometallics 1992, 11, 1275.
10.1021/om0209677 CCC: $25.00 © 2003 American Chemical Society
Publication on Web 03/04/2003

Notes
Organometallics, Vol. 22, No. 7, 2003 1547
Sch em e 1
ethyl-1-butyne (Scheme 1).3a We have also shown that
use of titanocene dichloride, [Cp2TiCl2], as substrate also
tungstacyclopentatriene ring has also been investi-
gated.7 In these systems the ground state structure of
the precursors does not involve coplanar η2-acyl, η2-
iminoacyl, or alkyne moieties. Instead, solid and solution
ground state structures with parallel units aligned
almost along the O-M-O axis are found ([A] and [B],
Scheme 2). In the heteroatom systems in which the
units are oriented in a head-to-tail fashion, coupling
then occurs by a conrotatory motion, leading to carbon-
carbon bond formation. It is interesting that in the
actinide-mediated coupling of two acyl units, a ground
state coplanar orientation of the two acyl units is
followed by a conrotatory acyl motion allowing formation
of the final ene-diolate.
leads to the 2,4-regioisomer [Cp2Ti(C4H2But -2,4)] (5)
2
(Scheme 1).
We attempt to rationalize the changes in regiochem-
istry observed for the kinetically formed titanacyclo-
pentadienes in terms of steric effects within the inter-
mediate bis(alkyne) complexes. The related coupling of
two η2-acyl and/or η2-iminoacyl groups at group 4 bis-
(aryloxide) metal centers has been the focus of theoreti-
cal investigations.6 The coupling of two alkyne units at
a tungsten bis(aryloxide) metal center to generate a
An important question therefore that arises is what
is the ground state structure for intermediate bis-
(alkyne) complexes [(X)(Y)Ti(RCCR′)2] (X, Y ) ArO or
Cp), and how does this vary with the nature of the
ancillary ligands? Recently our group has isolated and
structurally characterized “hybrid” bis(iminoacyl) com-
pounds [(ArO)(Cp)Zr(η2-R′NCR)2].8 The solid state struc-
ture shows a head-to-tail arrangement of iminoacyl
units very similar to that found in the bis(aryloxide)
derivatives ([C], Scheme 2). No corresponding bis-
(iminoacyl) derivatives of the group 4 metallocenes have
been isolated, with ene-diolate formation occurring via
a different mechanism.9 However, a bis(alkyne) adduct
of zirconocene has been formed and structurally char-
acterized.10 In this important case the two alkyne units
lie in a coplanar orientation similar to [D] in Scheme 2.
For the symmetrical bis(aryloxide) system the most
sterically favored situation is one in which C2 symmetry
F igu r e 1. ORTEP (50% thermal ellipsoids) view of [CpTi-
(OC6HPh4-2,3,5,6)(C4H2But -2,5)] (2).
2
Ta ble 1. Selected Bon d Dista n ces (Å) a n d An gles
(6) (a) Tatsumi, K.; Nakamura, A.; Hofmann, R.; Moloy, K. G.;
Marks, T. J . J . Am. Chem. Soc. 1986, 108, 4467. (b) Fagan, P. J .;
Manriquez, J . M.; Vollmer, S. H.; Day, C. S.; Day, V. W.; Marks, T. J .
J . Am. Chem. Soc. 1981, 103, 2206. (c) Hofmann, P.; Stauffert, P.;
Frede, M.; Tatsumi, K. Chem. Ber. 1989, 122, 1559. (d) Hardesty, J .
H.; Albright, T. A.; Kahlal, S. Organometallics 2000, 19, 4159.
(7) (a) Kriley, C. E.; Kerschner, J . L.; Fanwick, P. E.; Rothwell, I.
P. Organometallics 1993, 12, 2051. (b) Kerschner, J . L.; Fanwick, P.
E.; Rothwell, I. P. J . Am. Chem. Soc. 1988, 110, 8235.
(d eg) for [Cp Ti(ÃC6HP h 4-2,3,5,6)(C4H2Bu t2-2,5)] (2)
Ti-O
C(12)-C(13)
C(14)-C(15)
1.831(1)
1.343(3)
1.343(3)
Ti-C(12)
C(13)-C(14)
Ti-C(15)
2.070(2)
1.492(3)
2.067(2)
O-Ti-C(15)
102.94(7) O-Ti-C(12)
95.0(2)
103.91(8)
C(12)-C(13)-C(14) 126.6(2)
Ti-C(12)-C(13)
C(13)-C(14)-C(15) 126.4(2)
Cp-Ti-O
Cp-Ti-C(15)
C(14)-C(15)-Ti
95.2(2)
111.41(9)
96.60(9)
(8) Thorn, M. G.; Lee, J .; Fanwick, P. E.; Rothwell, I. P. J . Chem.
Soc., Dalton Trans. 2002, 3398.
(9) Roddick, D. M.; Bercaw, J . E. Chem. Ber. 1989, 122, 1579.
127.69(9) Cp-Ti-C(12)
109.71(9) C(12)-Ti-C(15)

1548 Organometallics, Vol. 22, No. 7, 2003
Notes
Sch em e 2
is present with bulky But groups away from each other
([E], Scheme 2). This then leads to the 2,4-regioisomer
upon coupling (the 3,4-isomer being highly unlikely on
steric grounds). In the case of the hybrid system, the
2,3,5,6-tetraphenylphenoxide ligand is much bulkier
than the Cp unit. Hence a ground state geometry for
the bis(alkyne) adduct in which both But groups are
distal to the aryloxide is expected to be favored ([F ],
Scheme 2). This would lead to the observed, 2,5-
disubstituted kinetic isomer, which thermally rear-
ranges to the more stable 2,4-regioisomer (Figure 2) in
[G] in Scheme 2. We presume that the metallocene
product arises via in-plane coupling of a bis(alkyne)
adduct in which the But substituents are arranged as
shown in [H] in Scheme 2.
molecule per Ti. Yield ) 0.29 g (26%). Anal. Calcd for C53H52
-
1
OTi; 2‚C6H6: C, 84.55; H, 6.96. Found: C, 84.15; H, 6.86. H
NMR (C6D6, 30 °C): δ 7.38-6.92 (aromatics); 6.56 (s, C4-
But H2); 5.64 (s, C5H5); 1.01 (s, CMe3). 13C NMR (C6D6, 30 °C):
2
δ 221.9 (TiCBut); 110.7 (C5H5); 40.8 (CMe3); 31.1 (CMe3).
A sample of 2 was dissolved in C6D6 in a J . Young valve
NMR tube. Over the course of a few days the sample slowly
converted to the 2,4-isomer 3 at 100 °C as monitored by NMR.
During the course of the isomerization, other products were
also formed in solution and a pure sample of 3 was not isolated.
1H NMR (C6D6, 30 °C): δ 7.38-6.92 (aromatics); 6.61 (d, C4-
But H2, 4J ) 4.5 Hz); 5.78 (s, C5H5); 0.93 (s), 0.82 (s, CMe3).
2
13C NMR (C6D6, 30 °C): δ 194.9 (TiCBut); 181.3 (TiCH); 138.7
(TiCCH); 131.7 (TiCCBut); 142.4 (Ti-O-C); 112.3 (C5H5); 40.9,
36.4 (CMe3), 31.4, 28.3 (CMe3). [Cp 2Ti(C4H2Bu t -2,4)] (5). A
2
mixture of [Cp2TiCl2] (1.0 g, 4.02 mmol) and 3,3-dimethyl-1-
butyne (1.5 mL, 12.3 mmol) in benzene (25 mL) was stirred
above a sodium (0.19 g, 8.26 mmol) amalgam. Over 15 h the
color of the mixture changed from red to green to brown. The
solution was then separated from Hg pool and metal salts by
Exp er im en ta l Section
Gen er a l Deta ils. All operations were carried out under a
dry nitrogen atmosphere using standard Schlenk techniques.
The hydrocarbon solvents were distilled from sodium/ben-
zophenone and stored over sodium ribbons under nitrogen
until use. The compound [CpTi(OC6HPh4-2,3,5,6)Cl2] (1) was
prepared as previously reported.4c The 1H and 13C NMR spectra
were recorded on a Varian Associates Gemini-200, Inova-300,
or General Electric QE-300 spectrometer and referenced to
protio impurities of commercial benzene-d6 as internal stan-
dard. Elemental analyses and molecular structures were
obtained through Purdue in-house facilities.
decanting and filtering through
evacuated to dryness, leaving the product as a brown solid.
a Celite plug and then
Yield ) 0.59 g (43%). Microanalytical data consistently gave
1
low C%. H NMR (C6D6, 30 °C), see Supporting Information:
4
δ 5.99 (d), 5.17 (d, TiC4But H2, J ) 16.9 Hz); 5.90 (s, C5H5);
2
[Cp(Ar O)Ti(C4H2Bu t2 -2,5)] (2) an d [Cp(Ar O)Ti(C H2Bu t
2
4
-2,4)] (3) (Ar O ) OC6HP h 4-2,3,5,6). A mixture of [CpTi(OC6-
HPh4-2,3,5,6)Cl2] (1) (1.0 g, 1.72 mmol) and 3,3-dimethyl-1-
butyne (1.0 mL, 8.17 mmol) in benzene (25 mL) was stirred
above a sodium (0.08 g, 3.48 mmol) amalgam. Over 15 h the
color of the mixture changed from orange to greenish brown.
The solution was then separated from Hg pool and metal salts
by decanting and filtering through a Celite plug and then
evacuated to dryness, leaving the crude product as a brown
solid. Recrystallization from benzene/hexane yielded the pure
compound 2 as orange chunks containing one benzene solvate
(10) (a) Warner, B. P.; Davis, W. M.; Buchwald, S. L. J . Am. Chem.
Soc. 1994, 116, 5471. (b) Buchwald, S. L.; Watson, B. T.; Wannamaker,
M. W.; Dewan, J . C. J . Am. Chem. Soc. 1989, 111, 4486. (c) Buchwald,
S. L.; Wannamaker, M.W.; Watson, B. T. J . Am. Chem. Soc. 1989, 111,
776.
F igu r e 2. Energy correlation for the conversion of 2,5-
disubstituted kinetic isomer 2 to the thermodynamically
more stable 2,4-disubstituted isomer 3.

Notes
Ta ble 2. Cr ysta l Da ta a n d Da ta Collection s
Organometallics, Vol. 22, No. 7, 2003 1549
DIRDIF92.13 The remaining atoms were located in succeeding
difference Fourier syntheses. Hydrogen atoms were included
in the refinement but restrained to ride on the atom to which
they are bonded. The structure was refined in full-matrix least-
P a r a m eter s for
[Cp Ti(ÃC6HP h 4-2,3,5,6)(C4H2Bu t2-2,5)] (2)
2‚C6H6
2
2
squares where the function minimized was ∑w(|Fo| - |Fc| )2
formula
fw
TiO1C53H52
752.90
and the weight w is defined as w ) 1/[σ2(Fo2) + (0.0585P)2 +
2
1.4064P] where P ) (Fo + 2Fc2)/3. Scattering factors were
space group
a, Å
b, Å
P21/n (No. 14)
19.3113(10)
10.8549(4)
21.2798(8)
90
108.766(2)
90
4223.6(6)
4
taken from International Tables for Crystallography.14 Refine-
ment was performed on an AlphaServer 2100 using SHELX-
97.15 Crystallographic drawings were done using ORTEP
programs.16
c, Å
R, deg
â, deg
γ, deg
V, Å
Ack n ow led gm en t. We thank the National Science
Foundation (Grant CHE 0078405) for financial support
of this research.
Z
F
calc, g cm-3
1.184
150.
temperature, K
radiation (wavelength)
R
Mo KR (0.71073 Å)
0.058
Su p p or tin g In for m a tion Ava ila ble: 1H NMR (C6D6, 30
°C) spectrum of 5. Tables of thermal parameters, bond
distances and angles, intensity data, torsion angles, and
multiplicities for 2. This material is available free of charge
via the Internet at http://pubs.acs.org.
Rw
0.129
1.24 (s), 1.00 (s, CMe3). 13C NMR (C6D6, 30 °C): δ 182.2
(TiCBut); 143.3 (TiCH); 113.4 (C5H5); 106.2 (TiCCH); 100.2
(TiCCBut); 34.8, 31.7 (CMe3); 29.9, 28.6(CMe3).
X-r a y Da ta Collection a n d Red u ction . A suitable crystal
was mounted on a glass fiber in a random orientation under
a cold stream of dry nitrogen. Preliminary examination and
final data collection were performed with Mo KR radiation (λ
) 0.71073 Å) on a Nonius Kappa CCD. Lorentz and polariza-
tion corrections were applied to the data.11 An empirical
absorption correction using SCALEPACK was applied.12 In-
tensities of equivalent reflections were averaged. The structure
was solved using the structure solution program PATTY in
OM0209677
(13) Beurskens, P. T.; Admirall, G.; Beurskens, G.; Bosman, W. P.;
Garcia-Granda, S.; Gould, R. O.; Smits, J . M. M.; Smykalla, C. The
DIRDIF92 Program System, Technical Report; Crystallography Labo-
ratory, University of Nijmegen: The Netherlands, 1992.
(14) International Tables for Crystallography, Vol. C; Kluwer Aca-
demic Publishers: Dordrecht, The Netherlands, 1992; Tables 4.2.6.8
and 6.1.1.4.
(15) Sheldrick, G. M. SHELXS97. A Program for Crystal Structure
Refinement; University of Gottingen: Germany, 1997.
(16) J ohnson, C. K. ORTEPII, Report ORNL-5138; Oak Ridge
National Laboratory: Oak Ridge, TN, 1976.
(11) McArdle, P. C. J . Appl. Crystallogr. 1996, 239, 306.
(12) Otwinowski, Z.; Minor, W. Methods Enzymol. 1996, 276.