Organic Functional Group Transformations in the Periphery of a Group-4 Metallocene Complex: 2H-Pyrrole Formation at a Pendant Boron Lewis Acid

Article abstract of DOI:10.1002/ejic.200300066

The allyl-substituted ansa-zirconocene dichloride [Me₂Si(C ₅H₄)(C₅H₃CH₂CH=CH ₂)ZrCl₂] (1) cleanly adds the borane 9-BBN to the allyl C=C double bond to yield the corresponding hydroboration product 2. The borane HB(C₆F₅)₂ (3) is also added regioselectively to 1 to yield [Me₂Si(C₅H₄){C₅H ₃(CH₂)₃B(C₆F₅) ₂}ZrCl₂] (4). The strongly electrophilic boron Lewis acid in 4 adds 2-methyl-4,5-dihydrooxazole to yield 6. Treatment of 4 with pyrrole results in the formation of the respective borane adduct 7 of the pyrrole tautomer 2H-pyrrole. The 2H-pyrrole ligand in 7 is deprotonated at C-2 upon treatment with pyridine, followed by proton transfer and replacement of pyrrole with formation of the corresponding pyridine adduct 12. The complexes 1, 4 and 7 were characterized by X-ray diffraction. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.

Full text of DOI:10.1002/ejic.200300066

FULL PAPER
Organic Functional Group Transformations in the Periphery of a Group-4
Metallocene Complex: 2H-Pyrrole Formation at a Pendant Boron Lewis Acid
Michael Hill,[a] Gerald Kehr,[a] Roland Fröhlich,[a][‡] and Gerhard Erker*[a]
Keywords: Boron / Hydroboration / Lewis acids / Tautomerism / Zirconium
The
allyl-substituted
ansa-zirconocene
dichloride
the formation of the respective borane adduct 7 of the pyrrole
tautomer 2H-pyrrole. The 2H-pyrrole ligand in 7 is depro-
[Me2Si(C5H4)(C5H3CH2CH=CH2)ZrCl2] (1) cleanly adds the
borane 9-BBN to the allyl C=C double bond to yield the cor- tonated at C-2 upon treatment with pyridine, followed by
responding hydroboration product 2. The borane HB(C6F5)2 proton transfer and replacement of pyrrole with formation of
(3) is also added regioselectively to to yield the corresponding pyridine adduct 12. The complexes 1, 4
1
[Me2Si(C5H4){C5H3(CH2)3B(C6F5)2}ZrCl2] (4). The strongly
electrophilic boron Lewis acid in 4 adds 2-methyl-4,5-dihy-
drooxazole to yield 6. Treatment of 4 with pyrrole results in
and 7 were characterized by X-ray diffraction.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2003)
Introduction
Group-4 metal centre. A first example is described in this
article. We hope that our findings may contribute to the
eventual development of novel bifunctional transition me-
tal/main group element systems well suited for selective or-
ganic reactions, that may otherwise be hard to achieve,
within such specific frameworks.
There are numerous examples showing that typical or-
ganic functional group chemistry can be carried out at the
Cp ligands of many transition metals from the middle and
the right-hand side of the Periodic Table.[1] This is much
more difficult to achieve in the case of the Group-4 metal-
locenes (and the corresponding complexes of the oxophilic
f-block elements). Functional group chemistry here is usu-
ally carried out at the stage of the free ligand. Usually the
functionalized Cp ligands are then subsequently attached
to the respective metal centre.[2,3] Introduction of functional
groups at the Cp rings of Cp2ZrCl2, for example, or conver-
sion of functional groups at the zirconocene Cp rings is
much more difficult, and consequently only a limited num-
ber of examples of such chemistry has so far been described
in the literature. Noteworthy examples include Mannich-
type enamine coupling reactions[4] and photochemical
[2ϩ2] cycloadditions,[5] both producing novel variants of
ansa-metallocenes. The olefin metathesis reaction has been
used for intra- and intermolecular carbon coupling reac-
tions at bent metallocenes.[6] Moreover, various types of
borylation[7] and hydroboration reactions[8] have been
shown to be compatible with the general chemical features
of many Group-4 metallocene complexes and related sys-
tems. We have now used such a reaction to attach a highly
electrophilic borane at a Cp side chain of an ansa-metallo-
cene and to carry out Lewis acid promoted reactions at the
boron centre in the direct vicinity of the mildly electrophilic
Results and Discussion
We had previously shown that [(allyl-C5H4)Cp] zir-
conium dichloride complexes can be cleanly hydroborated
at the pendant allyl group.[8] Piers recently showed that
[9]
similar HB(C6F5)2 additions at such systems were feas-
ible.[10] Such hydroboration reactions at open, unbridged
Group-4 metallocenes (and at some related ‘‘constrained
geometry’’ Zr complexes[10]) provided the synthetic basis for
this work. We have now extended these reactions to the al-
lyl-substituted rigid ansa-metallocene dichloride complex 1.
In contrast to the previously employed unbridged bent met-
allocenes,[8,10] the system 1 is no longer conformationally
flexible but has its pendant functional group oriented
towards the (chemically active) front side of the bent metal-
locene wedge.
Complex 1 was prepared as recently described by us[11]
by adaptation of established literature procedures.[12] The
X-ray crystal structure analysis of 1 was carried out and
described previously.[11]
Treatment of 1 with 9-BBN was carried out in toluene at
room temperature. The reaction was rather slow and re-
quired overnight stirring to go to completion. The hydrobo-
rated product 2 was isolated as a solid in almost quantitat-
ive yield. Complex 2 contains a chiral ansa-metallocene
[a]
Organisch-Chemisches Institut der Universität Münster,
Corrensstr. 40, 48149 Münster, Germany
[‡]
X-ray crystal structure analyses
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
1
backbone, and therefore shows a total of seven H NMR
Eur. J. Inorg. Chem. 2003, 3583Ϫ3589
DOI: 10.1002/ejic.200300066
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3583

M. Hill, G. Kehr, R. Fröhlich, G. Erker
FULL PAPER
1
methine signals of the two substituted Cp-type ligands [δ ϭ chiral metallocene core shows a total of seven H NMR
6.61, 5.91, 5.54 (C5H3), 7.02, 6.92, 5.98, 5.82 (C5H4) ppm] methane signals, at δ ϭ 6.56, 5.87, 5.47 (C5H3) and 7.00,
1
and a 1:1 intensity pair of SiϪCH3 signals (δ ϭ 0.72, 6.92, 5.96, 5.80 (C5H4) ppm. The H NMR resonances of
1
0.69 ppm). It also shows three distinct H NMR multiplets the trimethylene side chain of 4 occur at δ ϭ 2.72, 1.96 and
for the connecting trimethylene bridging unit (δ ϭ 2.69, 1.80 ppm, with corresponding 13C NMR features at δ ϭ
1.75, 1.40 ppm) and the signals of the BBN residue.
32.3, 30.9 and 25.5 ppm. As would be expected, the pair
Single crystals suitable for the X-ray crystal structure de- of C6F5 groups at the boron atom behaves in symmetry-
termination of 2 were obtained from a dichloromethane equivalent fashion in solution, producing a single set of o-,
solution at Ϫ30 °C. In the solid state, complex 2 has a rigid p- and m-19F resonances at δ ϭ Ϫ130.2, Ϫ148.6 and
ansa-metallocene framework. Because of the geometric con- Ϫ160.8 ppm, respectively. The strongly electrophilic boron
straints in such a system the ZrϪC(Cp) bond lengths vary centre does not interact with the ZrCl2 moiety in 4. The
from 2.443(3) to 2.596(3) A inside the C5H3 unit and from 11B NMR chemical shift of δ ϭ 75.6 ppm (ν1/2 ഠ 190 Hz)
˚
˚
2.471(3) to 2.535(3) A inside the Zr(C5H4) moiety. The indicates the presence of a coordinatively unsaturated
smaller values are found at the narrow backside of the bent tricoordinate boron centre [similarly to B(C6F5)3:[13]
metallocene wedge oriented toward the SiMe2 unit. The δ(11B) ϭ 60.0 (ν1/2 ഠ 400 Hz); in contrast the dimeric
C1ϪSi and C8ϪSi bond lengths are 1.866(3) and 1.862(3) [HB(C6F5)2]2 reagent features δ(11B{1H}) ϭ 19.5 (ν1/2
ഠ
2
˚
A, respectively. These (sp )CϪSi bonds are slightly longer 320 Hz), as would be expected for a dimeric doubly H-
3
˚
than the adjacent (sp )CϪSi linkages [C6ϪSi 1.846(4) A, bridged structure with two tetracoordinate boron atoms
˚
C7ϪSi 1.849(4) A], which is an additional indication that present].
the ansa-metallocene framework is strained. The
Complex 4 reacts rapidly with a variety of two-electron
C1ϪSiϪC8 angle is found at 93.6(1)° and the donor ligands.[14] A typical product is the 2-methyl-4,5-di-
Cp(centroid)ϪZrϪCp(centroid) angle in 2 is 126.0°. The hydrooxazole adduct 6, the molecular structure of which
Cl1ϪZrϪCl2 angle is in the normal range at 99.60(4)°.
The attached Ϫ(CH2)3ϪBBN side chain is pointing to- treatment of 4 with the heterocyclic donor ligand in toluene
is depicted in Figure 2. Complex 6 was formed readily by
ward the open front side of the bent metallocene wedge solution at room temperature (see the Exp. Sect. for details).
˚
[C11ϪC13 1.499(5) A] and is arranged at a maximum ex- The attached trimethylene chain again adopts a fully ex-
tension such as to bring the bulky ϪBBN group away from tended ‘‘zigzag’’ conformation featuring a series of antiperi-
the ZrCl2 unit (see Figure 1). This extended conformational planar
conformational
arrays
[dihedral
angles
arrangement is characterized by the corresponding dihedral C11ϪC13ϪC14ϪC15 Ϫ175.4(2)°, C13ϪC14ϪC15ϪB
angles C12ϪC11ϪC13ϪC14 Ϫ85.0(4)°, C11ϪC13Ϫ Ϫ176.6(2)°]. This arrangement resembles the conformation
C14ϪC15 Ϫ179.2(3)°, and C13ϪC14ϪC15ϪB Ϫ172.3(4)°. of the side chain in 4 (see above and Figure 1), except that
its torsional orientation relative to the adjacent Cp ring is
quite different. In 4 (see above), the substituent chain is
rotated away from the Cp plane, whereas in 6 it is rotated
almost by 90° so that the w-shaped carbon skeleton of the
side chain is now lying almost coplanar with its adjacent
C8ϪC11
η5-C5H4
ring
[dihedral
angle
C10ϪC11ϪC13ϪC14 Ϫ25.8(3)°]. This brings the boron
centre in the adduct 6 much closer to the zirconium atom
˚
˚
(B···Zr separation in 6: 6.395A, in 2: 7.621A) and it brings
the attached heterocycle almost in a direct opposition to
the ZrCl2 unit at the front side of the bent metallocene unit.
The corresponding dihedral angles are Ϫ77.5(3)°
(C14ϪC15ϪBϪN1) and Ϫ2.8(4)° (C15ϪBϪN1ϪC18).
The bonding features of the dihydrooxazole heterocycle
have not changed much upon complexation to the boron
Figure 1. A view of the molecular geometry of complex 2; selected
˚
bond lengths [A] and angles (°): ZrϪCl1 2.431(1), ZrϪCl2 2.431(1),
SiϪC1 1.866(3), SiϪC8 1.862(3), SiϪC6 1.846(4), SiϪC7 1.849(4),
C11ϪC13 1.499(5), C13ϪC14 1.526(5), C14ϪC15 1.517(5),
C15ϪB 1.555(6), BϪC16 1.569(6), BϪC20 1.566(6); Cl1ϪZrϪCl2
99.60(4), C1ϪSiϪC8 93.6(1), C1ϪSiϪC6 112.6(2), C1ϪSiϪC7
110.8(2), C8ϪSiϪC6 112.7(2), C8ϪSiϪC7 110.4(2), C6ϪSiϪC7
114.8(2), C11ϪC13ϪC14 111.5(3), C13ϪC14ϪC15 113.6(3),
C14ϪC15ϪB 118.3(3), C15ϪBϪC16 126.3(4), C15ϪBϪC20
122.5(4), C16ϪBϪC20 111.1(4)
˚
˚
atom [N1ϪC16 1.277(3) A, C16ϪO1 1.323(3) A, N1ϪC18
[15]
˚
˚
1.507(4) A]. The N1ϪB bond length is 1.626(4) A, which
is similar to the adjacent BϪC bond lengths [BϪC(aryl)
˚
˚
1.651(4) and 1.664(4) A; BϪC15 1.636(4) A]. The
NϪBϪC15 bond angle was found to be 110.1(2)°. The re-
From its monomer/dimer equilibrium[9] the borane maining bond angles at the boron atom were found to lie
HB(C6F5)2 (3) reacts much more rapidly with 1. The hydro- in a range between 105.0(2)° (N1ϪBϪC31) and 114.0(2)°
boration reaction was complete after ca. 30 min at room (C21ϪBϪC31). With regard to their conformational orien-
temperature in toluene solution, and the product 4 was iso- tation, the BϪN vector is oriented almost coplanar with
lated in a close to quantitative yield. It shows NMR spec- the C21ϪC26 aryl plane [dihedral angle C22ϪC21ϪBϪN1
troscopic features very similar to those of 2, which let us 14.3(3)°] whereas it adopts a gauche orientation with the
assume that both complexes have similar structures. The C31ϪC36 aryl ring [C32ϪC31ϪBϪN1 Ϫ69.5(3)°].
3584
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2003, 3583Ϫ3589

2H-Pyrrole Formation at a Pendant Boron Lewis Acid
FULL PAPER
ture for 18 h. During this time a close to quantitative con-
version of the hetarene took place, with formation of the
2H-pyrrole adduct 7. The coordination of the ‘‘isopyrrole
tautomer’’ to the boron atom was apparent from the
characteristic NMR spectra. The 2H-pyrrole moiety gave
rise to a set of three methine 1H NMR signals, one at rather
low field (δ ϭ 7.65 ppm, ϪNϭCHϪ), the other two at δ ϭ
6.42 ppm (10-H, see Scheme 2) and 5.61 (11-H) [corre-
sponding 13C NMR signals at δ ϭ 169.3 (ϪNϭCHϪ),
127.1 (C-11), 154.2 ppm (C-10)]. The remaining NϪCH2
group gives rise to a characteristic AB pattern at δ ϭ 3.72
and 3.68 ppm (2JH,H ϭ 27 Hz, 13C signal at δ ϭ 65.0 ppm).
Figure 2. Molecular structure of the adduct 6; selected bond
1
˚
The diastereotopic splitting of the 9-H/HЈ H NMR reson-
lengths [A] and angles [°]: ZrϪCl1 2.440(1), ZrϪCl2 2.445(1),
SiϪC1 1.868(2), SiϪC8 1.877(2), SiϪC6 1.847(3), SiϪC7 1.846(3),
C11ϪC13 1.501(3), C13ϪC14 1.527(3), C14ϪC15 1.528(3),
C15ϪB 1.636(4), BϪN1 1.626(4), BϪC21 1.651(4), BϪC31
1.664(4), N1ϪC16 1.277(3), N1ϪC18 1.507(4), C16ϪO1 1.323(3),
C16ϪC19 1.467(4), O1ϪC17 1.451(4), C17ϪC18 1.491(4);
Cl1ϪZrϪCl2 95.56(3), C1ϪSiϪC8 93.8(1), C1ϪSiϪC6 110.9(1),
C1ϪSiϪC7 112.6(1), C8ϪSiϪC6 112.7(1), C8ϪSiϪC7 111.7(1),
C6ϪSiϪC7 113.5(1), C11ϪC13ϪC14 113.3(2), C13ϪC14ϪC15
114.1(2),
C15ϪBϪC21 106.4(2), C15ϪBϪC31 110.1(2), N1ϪBϪC21
111.2(2), N1ϪBϪC31 105.0(2), C21ϪBϪC31 114.0(2),
BϪN1ϪC16 128.9(2), BϪN1ϪC18 123.4(2), C16ϪN1ϪC18
107.7(2), N1ϪC16ϪO1 116.5(3), N1ϪC16ϪC19 127.6(3),
C19ϪC16ϪO1 115.9(2), C16ϪO1ϪC17 107.6(2), O1ϪC17ϪC18
105.3(2), C17ϪC18ϪN1 102.9(2)
ance of the 2H-pyrrole moiety is due to the chirality of the
metal complex backbone. This also becomes evident in 7 in
the partial splitting of the 7-H/HЈ (δ ϭ 1.52/1.36) and 8-H/
HЈ (δ ϭ 1.36/1.21) signals of the bridging trimethylene unit
1
(6-H/HЈ H NMR resonance at δ ϭ 2.83 ppm). As would
be expected, a 1:1 intensity pair of SiMe2 1H NMR methyl
resonances is observed for 7 (δ ϭ 0.14 and 0.08 ppm). The
diastereotopic splitting is even extended into the B(C6F5)2
unit. The two pentafluorophenyl groups at the tetravalent
boron centre [δ(11B) ϭ 4.7, ν1/2 ഠ 414 Hz)] have become
diastereotopic. Consequently, we observed equal intensity
pairs of 19F NMR signals of the ortho- (δ ϭ Ϫ158.1 and
Ϫ158.2 ppm) and para-C6F5 signals (δ ϭ Ϫ132.6 and
Ϫ132.9 ppm). The meta-C6F5 resonances of the two dia-
C14ϪC15ϪB
115.8(2),
C15ϪBϪN1
110.1(2),
We had previously shown that unstable tautomers of a
variety of aromatic and heteroaromatic compounds can be
formed and stabilized by coordination to the strong or-
ganometallic Lewis acid B(C6F5)3. The first examples in-
cluded the α-naphthol/benzocyclohexadienone tautomerism
(see Scheme 1).[16] We have also formed the 2H-pyrrole tau-
tomer from its parent hetarene in the B(C6F5)3 coordi-
nation sphere by a three-step procedure involving depro-
tonation at the nitrogen atom, attachment at the boron
atom, and then protonation at the carbon atom to yield
the 2H-pyrrole/B(C6F5)3 adduct 10.[17] Resconi has recently
found a direct route yielding examples of the compound
type 10 by treatment of pyrrole derivatives with
B(C6F5)3.[18]
stereotopic C6F5 ligands are isochronous (δ
Ϫ163.3 ppm).
ϭ
Scheme 1
Scheme 2
We were able to carry out such a pyrrole/2H-pyrrole taut-
omerism reaction at the ansa-zirconoceneϪtrimeth-
yleneϪB(C6F5)2 complex 7. A mixture of 4 and pyrrole was
Single crystals of 7 were obtained from a concentrated
stirred in toluene/diethyl ether solution at room tempera- toluene solution at room temperature. The X-ray crystal
Eur. J. Inorg. Chem. 2003, 3583Ϫ3589
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M. Hill, G. Kehr, R. Fröhlich, G. Erker
FULL PAPER
structure analysis confirms the formation of the 2H-pyrrole acid activator component for generating homogeneous
unit. The C4H5N moiety shows a short NϭC bond inside ZieglerϪNatta catalyst systems.[17] The 2H-pyrrole methyl-
the heterocyclic ring system [see Figure 3, N1ϪC16 1.294(4) ene hydrogen atoms are also acidic. Treatment with pyridine
˚
˚
A], whereas the adjacent N1ϪC19 linkage [1.462(4) A] de- as a base removed one of these hydrogen atoms. We assume
notes the presence of a single bond. The remaining frame- that salt 11 may be formed as an intermediate, and may
work of the five-membered nitrogen heterocycle exhibits a then have reacted further in an acid/base reaction to yield
˚
typical bond alternation [C16ϪC17 1.443(5) A, C17ϪC18 the stable adduct 12 and free pyrrole (see Scheme 3).
1.326(5) A, C18ϪC19 1.474(4) A]. The N1ϪB bond length
amounts to 1.612(4) A, and the N1ϪBϪC15 bond angle is
˚
˚
˚
found at 107.9(2)°. The remaining bond angles at the boron
atom are within a range of 103.3(2) to 116.1(2)°.
Scheme 3
We next intend to try to make use of the specific coordin-
ative features of such main group element/transition metal
combinations and to investigate whether the zirconocene
units in adducts such as 6 or 7 can be activated to undergo,
for example, CϪH or CϪC activation reactions with sub-
strates attached at their neighbouring electrophilic borane
centre.
Figure 3. A projection of the molecular geometry of the 2H-pyrrole
˚
addition product 7; selected bond lengths [A] and angles [°]:
ZrϪCl1 2.448(1), ZrϪCl2 2.414(1), SiϪC1 1.877(3), SiϪC8
1.875(3), SiϪC6 1.845(4), SiϪC7 1.857(4), C11ϪC13 1.506(4),
C13ϪC14 1.541(4), C14ϪC15 1.531(4), C15ϪB 1.631(4), BϪN1
1.612(4), BϪC20 1.653(5), BϪC26 1.651(4), N1ϪC16 1.294(4),
N1ϪC19 1.462(4), C16ϪC17 1.443(5), C17ϪC18 1.326(5),
C18ϪC19 1.474(4); Cl1ϪZrϪCl2 98.76(4), C1ϪSiϪC8 94.8(1),
C1ϪSiϪC6 112.5(2), C1ϪSiϪC7 110.7(2), C8ϪSiϪC6 110.6(2),
C8ϪSiϪC7 112.1(2), C6ϪSiϪC7 114.5(2), C11ϪC13ϪC14
113.8(3), C13ϪC14ϪC15 113.1(3), C14ϪC15ϪB 115.9(2),
C15ϪBϪN1 107.9(2), C15ϪBϪC20 116.1(2), C15ϪBϪC26
Experimental Section
General: All reactions were carried out under dry argon in Schlenk-
type glassware or in a glove-box. Solvents, including deuterated
solvents used for NMR spectroscopy, were dried and distilled prior
to use. For additional general conditions, including a list of instru-
ments used for a physical characterization of the compounds see
ref.[6] Most NMR assignments were secured by carrying out a vari-
ety of 2D NMR experiments.[19]
108.4(2),
N1ϪBϪC20
103.3(2),
N1ϪBϪC26
108.8(2),
C20ϪBϪC26 112.0(2), BϪN1ϪC16 126.9(3), BϪN1ϪC19
125.3(2), C16ϪN1ϪC19 107.8(3), N1ϪC16ϪC17 111.8(3),
C16ϪC17ϪC18
C18ϪC19ϪN1 104.3(3)
107.3(3),
C17ϪC18ϪC19
108.7(3),
X-ray Crystal Structure Analyses: Data sets were collected with a
The 2H-pyrrole unit is oriented in plane with the Nonius KappaCCD diffractometer, equipped with
a Nonius
FR591 rotating anode generator. Programs used: data collection
COLLECT (Nonius B.V., 1998), data reduction Denzo-SMN,[20]
absorption correction SORTAV,[21] structure solution SHELXS-
B1ϪC15 vector [dihedral angle C16ϪN1ϪBϪC15 2.5(4)°].
The connecting trimethylene chain between the ansa-metal-
locene framework and the borate section of 7 adopts a con-
formation different from those of the two complexes 2 and
6 (see above). As can be seen from the projection of 7, de-
picted in Figure 3, the Ϫ(CH2)3Ϫ chain in 7 contains an
additional gauche conformation [C11ϪC13ϪC14ϪC15
Ϫ63.0(4)°]. The C13ϪC14ϪC15ϪB1 arrangement is anti-
periplanar [θ ϭ Ϫ165.9(3)°] and the C10ϪC11ϪC13ϪC14
dihedral angle [84.1(4)°] removes the borate end away from
the centre of the metallocene unit. Consequently, the Zr···B
97,[22]
structure
refinement
SHELXL-97,[23]
graphics
SCHAKAL.[24] CCDC-202081 to -202083 contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html
[or from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; Fax: (internat.) ϩ 44-1223/336-
033; E-mail: deposit@ccdc.cam.ac.uk].
Preparation of the ansa-Zirconocene Dichloride Complex 1: Chilled
toluene (250 mL, Ϫ78 °C) was added to a solid mixture of the
˚
˚
separation in 7 (6.837A) is smaller than in 2 (7.621A) but
reagent
[Me2Si(3-C5H3CH2CHϭCH2)(C5H4)Li2]
(3.47 g,
˚
larger than in 6 (6.395A).
14.5 mmol) and ZrCl4 (3.37 g, 14.5 mmol). The suspension im-
mediately became yellow. It was allowed to warm to room tempera-
ture with stirring. After stirring overnight, the yellow suspension
The methylene hydrogen atoms of the previously pre-
pared and studied 2H-pyrrole/B(C6F5)3 adduct 10 are
rather acidic. The system 10 has been used as a Brønsted was filtered through Celite. Removal of the volatiles in vacuo pro-
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Eur. J. Inorg. Chem. 2003, 3583Ϫ3589

2H-Pyrrole Formation at a Pendant Boron Lewis Acid
vided the product (3.23 g, 57%) as a yellow solid. M.p. 104 °C. H
FULL PAPER
C6F5), Ϫ148.6 (m, p-C6F5), Ϫ160.8 (m, m-C6F5) ppm.
1
NMR (CDCl3, 600 MHz): δ ϭ 7.03 (m, 1 H, 4Ј-H), 6.93 (m, 1 H, C27H19BCl2F10SiZr (734.5): calcd. C 44.15, H 2.61; found C 43.23,
3Ј-H), 6.60 (m, 1 H, 3-H), 5.98 (m, 1 H, 2Ј-H), 5.92 (m, 3 H, 2-H,
7-H), 5.82 (m, 1 H, 5Ј-H), 5.54 (m, 1 H, 5-H), 5.06, 5.04 (each m,
each 1 H, 8-H, 8-HЈ), 3.47, 3.38 (each m, each 1 H, 6-H, 6-HЈ),
0.72, 0.69 [each s, each 3 H, Si(CH3)2] ppm. 13C{1H} NMR
(CDCl3, 150 MHz): δ ϭ 141.0 (C-4), 135.9 (C-7), 128.0 (C-3), 127.8
(C-3Ј), 127.7 (C-4Ј), 116.7 (C-8), 115.3 (C-2), 114.1 (C-2Ј), 113.6
(C-5Ј), 113.5 (C-5), 107.9 (C-1), 107.8 (C-1Ј), 34.3 (C-6), Ϫ5.1,
Ϫ5.3 [Si(CH3)2] ppm. C15H18Cl2SiZr (388.5): calcd. C 46.37, H
4.67; found C 46.26, H 4.70.
H 2.56.
Treatment of 4 with 2-Methyl-4,5-dihydrooxazole; Formation of the
Dihydrooxazole Adduct 6: A solution of 2-methyl-4,5-dihydroox-
azole (9 mg, 104.8 mmol) in toluene (20 mL) was added at room
temperature to a light yellow solution of 4 (77 mg, 104.8 mmol) in
toluene (20 mL) and the mixture was stirred for 0.5 h. Removal of
the volatiles in vacuo provided the product (85 mg, 99%) as a light
yellow solid. Crystals suitable for an X-ray crystal structure analy-
sis were obtained from a concentrated solution in benzene at room
temperature. 1H NMR (C7D8, 600 MHz): δ ϭ 6.68 (m, 1 H, 4Ј-H),
6.67 (m, 1 H, 3Ј-H), 6.50 (m, 1 H, 3-H), 5.38 (m, 1 H, 2Ј-H), 5.37
(m, 1 H, 2-H), 5.33 (m, 1 H, 5Ј-H), 5.15 (m, 1 H, 5-H), 3.45/3.39
Treatment of Complex 1 with 9-BBN; Preparation of Complex 2: 9-
BBN (95 mg, 78.0 mmol) and 1 (302 mg, 78.0 mmol) were com-
bined in a flask, and toluene (20 mL) was added at room tempera-
ture. The reaction mixture was stirred for 12 h. The solvent was (each m, each 1 H, dihydrooxazole-CH2), 3.38/3.26 (each m, each
removed in vacuo, which provided the product (381 mg, 97%) as a 1 H, dihydrooxazole-CH2), 2.80/2.75 (each m, each 1 H, 6-H, 6-
light yellow solid. Crystals suitable for an X-ray crystal structure HЈ), 1.43/1.29 (each m, each 1 H, 7-H, 7-HЈ), 1.37/1.12 (each m,
analysis were obtained from a concentrated solution in dichloro- each 1 H, 8-H, 8-HЈ), 1.36 (s, 3 H, dihydrooxazole-CH3), 0.13/0.06
methane at Ϫ30 °C. M.p. 155 °C. 1H NMR (CDCl3, 600 MHz): [each s, each H, Si(CH3)2] ppm. 13C{1H} NMR (C7D8,
3
δ ϭ 7.02 (m, 1 H, 4Ј-H), 6.92 (m, 1 H, 3Ј-H), 6.61 (m, 1 H, 3-H), 150 MHz): δ ϭ 174.4 (dihydrooxazole-C), 142.5 (C-4), 129.2 (C-
5.98 (m, 1 H, 2Ј-H), 5.91 (m, 1 H, 2-H), 5.82 (m, 1 H, 5Ј-H), 5.54 3Ј), 128.4 (C-3), 125.8 (C-4Ј), 114.7 (C-5Ј), 114.6 (C-5), 113.6 (C-
(m, 1 H, 5-H), 2.69 (m, 2 H, 6-H), 1.81, 1.64 (each m, each 4 H,
2), 112.4 (C-2Ј), 107.8 (C-1Ј), 107.2 (C-1), 67.9/51.3 (dihydroox-
10-H), 1.81, 1.18 (each m, each 2 H, 11-H), 1.75 (m, 2 H, 7-H), azole-CH2), 33.1 (C-6), 26.9 (C-7), 22.9 (C-8), 13.1 (dihydroox-
3
1.64 (m, 2 H, 9-H), 1.40 (t, JH,H ϭ 7.8 Hz, 2 H, 8-H), 0.72, 0.69
[each s, each 3 H, Si(CH3)2] ppm. 13C{1H} NMR (CDCl3, (1JC,F ϭ 240.6 Hz), 137.6 (1JC,F ϭ 253.9 Hz), 120.6 (o-, p-, m-, ipso-
150 MHz): δ ϭ 143.9 (C-4), 127.9 (C-3), 127.7 (C-3Ј), 127.5 (C-4Ј), C6F5) ppm. 11B{1H} NMR (C7D8, 64 MHz): δ ϭ Ϫ5.0 (ν1/2
115.2 (C-2), 113.9 (C-2Ј), 113.7 (C-5), 113.6 (C-5Ј), 107.6 (C-1),
290 Hz) ppm. 19F NMR (C7D8, 564 MHz): δ ϭ Ϫ133.2/Ϫ133.7
azole-CH3), Ϫ5.7, Ϫ6.4 [Si(CH3)2]; 148.5 (1JC,F ϭ 239.7 Hz), 139.7
ϭ
107.5 (C-1Ј), 33.1 (C-6), 33.1 (C-10), 31.0 (br., C-9), 27.9 (br., C- (each m, each 2 F, o-C6F5), Ϫ158.1/Ϫ158.4 (each t, each 1 F,
8), 25.4 (C-7), 23.2 (C-11), Ϫ5.1, Ϫ5.2 [Si(CH3)2] ppm. 11B{1H} 3JFF ϭ 20.6/20.8 Hz, p-C6F5), Ϫ163.4/Ϫ163.5 (each m, each 2 F,
NMR (CDCl3, 64 MHz):
δ ϭ 86.8 (ν1/2 ϭ 320 Hz) ppm. m-C6F5) ppm. C31H26BCl2F10NOSiZr (819.6): calcd. C 45.43, H
C23H33BCl2SiZr (510.5): calcd. C 54.11, H 6.52; found C 53.09,
H 6.54.
3.20, N 1.71; found C 45.15, H 3.05, N 1.65.
X-ray Crystal Structure Analysis of 6: Empirical formula
C31H26BCl2F10NOSiZr·1/2C6H6, M ϭ 858.60, colourless crystal
X-ray Crystal Structure Analysis of 2: Empirical formula
C23H33BCl2SiZr, M ϭ 510.51, colourless crystal 0.40 ϫ 0.25 ϫ 0.30 ϫ 0.15 ϫ 0.10 mm, a ϭ 8.017(1), b ϭ 11.120(1), c ϭ 20.434(1)
3
˚
˚
˚
0.05 mm, a ϭ 8.175(1), b ϭ 26.251(1), c ϭ 11.251(1) A, β ϭ A, α ϭ 87.13(1), β ϭ 84.78(1), γ ϭ 81.23(1)°, V ϭ 1791.7(3) A ,
98.13(1)°, V ϭ 2390.2(4) A , ρcalcd. ϭ 1.419 g cmϪ3, µ ϭ 7.41 cmϪ1
empirical absorption correction (0.756 Յ T Յ 0.964), Z ϭ 4, mono-
,
ρ
calcd. ϭ 1.592 g cmϪ3, µ ϭ 5.71 cmϪ1, empirical absorption correc-
tion (0.847 Յ T Յ 0.945), Z ϭ 2, triclinic, space group P1 (no. 2),
3
˚
¯
˚
˚
clinic, space group P21/n (no. 14), λ ϭ 0.71073 A, T ϭ 198 K, ω λ ϭ 0.71073 A, T ϭ 198 K, ω and ϕ scans, 10918 reflections col-
and ϕ scans, 9625 reflections collected (Ϯh, Ϯk, Ϯl), [(sinθ)/λ] ϭ lected (Ϯh, Ϯk, Ϯl), [(sinθ)/λ] ϭ 0.65 AϪ1, 7958 independent
˚
0.65 AϪ1, 5428 independent (Rint ϭ 0.028) and 4617 observed re-
(Rint ϭ 0.017) and 6149 observed reflections [I Ն 2 σ(I)], 463 re-
˚
flections [I Ն 2 σ(I)], 255 refined parameters, R ϭ 0.045, wR2
ϭ
fined parameters, R ϭ 0.037, wR2 ϭ 0.083, max. residual electron
Ϫ3
density 0.38 (Ϫ0.40) e·AϪ3, hydrogen atoms calculated and refined
˚
˚
0.105, max. residual electron density 1.27 (Ϫ0.68) e·A close to
the ZrCl2 core, hydrogen atoms calculated and refined as riding
atoms.
as riding atoms.
Treatment of 4 with Pyrrole; Formation of the 2H-Pyrrole Adduct
7: A solution of pyrrole (37 mg, 0.54 mmol) in toluene (10 mL) was
added at room temperature to a light yellow solution of 4 (406 mg,
Treatment of Complex 1 with HB(C6F5)2 (3); Preparation of Com-
plex 4: Bis(pentafluorophenyl)borane (254 mg, 73.0 mmol) and 1
(285 mg, 73.0 mmol) were combined in a flask, and toluene 0.51 mmol) in toluene/diethyl ether (10:1, 20 mL), and the mixture
(50 mL) was transferred into the flask at 0 °C. The reaction mixture
was stirred while being allowed to warm to room temperature, and
the volatiles were removed in vacuo, resulting in the product
was stirred for 18 h. Removal of the volatiles in vacuo provided the
product (407 mg, 99%) as a light yellow solid. Crystals suitable for
an X-ray crystal structure analysis were obtained from a concen-
(524 mg, 97%) as a light yellow solid. M.p. 142 °C. 1H NMR trated solution in toluene at room temperature. 1H NMR (C7D8,
(CDCl3, 600 MHz): δ ϭ 7.00 (m, 1 H, 4Ј-H), 6.92 (m, 1 H, 3Ј-H),
6.56 (m, 1 H, 3-H), 5.96 (m, 1 H, 2Ј-H), 5.87 (m, 1 H, 2-H), 5.80 (m, 1 H, 3Ј-H), 6.48 (m, 1 H, 3-H), 6.42 (dq, J10-H,11-H ϭ 5.5,
600 MHz): δ ϭ 7.65 (br. s, 1 H, 12-H), 6.72 (m, 1 H, 4Ј-H), 6.66
3
3
(m, 1 H, 5Ј-H), 5.47 (m, 1 H, 5-H), 2.72 (m, 2 H, 6-H), 1.96 (m, 2
4J10-H,12-H ϭ 0.9 Hz, 1 H, 10-H), 5.61 (dq, J10-H,11-H ϭ 5.5,
H, 7-H), 1.80 (m, 2 H, 8-H), 0.72, 0.66 [each s, each 3 H, Si(CH3)2] 3J11-H,12-H ϭ 1.1, 4J9-H,11-H ϭ 1.3 Hz, 1 H, 11-H), 5.43 (m, 1 H, 2Ј-
1
ppm. 13C{1H} NMR (CDCl3, 150 MHz): δ ϭ 146.8 (dm, JC,F
ϭ
H), 5.38 (m, 1 H, 2-H), 5.32 (m, 1 H, 5Ј-H), 5.17 (m, 1 H, 5-H),
1
1
2
246.1 Hz), 143.4 (dm, JC,F ϭ 249.7 Hz), 137.4 (dm, JC,F
ϭ
3.72/3.68 (AB, JH,H ϭ 26.7 Hz, each 1 H, 9-H, 9-HЈ), 2.83 (m, 2
258.0 Hz) (o-, p-, m-C6F5), 142.1 (C-4), 128.2 (C-3Ј), 127.8 (C-3), H, 6-H, 6-HЈ), 1.52/1.36 (each m, each 1 H, 7-H, 7-HЈ), 1.36/1.21
126.9 (C-4Ј), 114.9 (C-2), 114.3 (C-5Ј), 113.8 (C-2Ј), 113.6 (C-5),
(each m, each 1 H, 8-H, 8-HЈ), 0.14/0.08 [each s, each 3 H,
107.7 (C-1), 107.6 (C-1Ј), 32.3 (C-6), 30.9 (C-7), 25.5 (C-8), Ϫ5.6, Si(CH3)2] ppm. 13C{1H} NMR (C7D8, 150 MHz): δ ϭ 169.3 (C-
Ϫ5.7 [Si(CH3)2] ppm. 11B{1H} NMR (CDCl3, 64 MHz): δ ϭ 75.6 12), 154.2 (C-10), 143.1 (C-4), 128.7 (C-3Ј), 128.6 (C-3), 127.1 (C-
(ν1/2 ϭ 190 Hz). 19F NMR (CDCl3, 564 MHz): δ ϭ Ϫ130.2 (m, o-
11), 125.6 (C-4Ј), 114.2 (C-2), 114.0 (C-5), 114.0 (C-5Ј), 113.1 (C-
Eur. J. Inorg. Chem. 2003, 3583Ϫ3589
www.eurjic.org
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3587

M. Hill, G. Kehr, R. Fröhlich, G. Erker
FULL PAPER
[2]
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2Ј), 107.8 (C-1Ј), 107.4 (C-1), 65.0 (C-9), 33.4 (C-6), 27.0 (C-7),
24.2 (C-8), Ϫ6.8, Ϫ5.9 [Si(CH3)2]; 148.4 (1JC,F ϭ 240.0 Hz), 139.8
(1JC,F ϭ 248.6 Hz), 137.6 (1JC,F ϭ 250.3 Hz), 121.4 (o-, p-, m-, ipso-
C6F5) ppm. 11B{1H} NMR (C7D8, 64 MHz): δ ϭ 4.7 (ν1/2
ϭ
415 Hz) ppm. 19F NMR (C7D8, 564 MHz): δ ϭ Ϫ132.6/Ϫ132.9
(each m, each 2 F, o-C6F5), Ϫ158.1/Ϫ158.2 (each t, each 1 F,
3JFF ϭ 21.0/20.6 Hz, p-C6F5), Ϫ163.3 (m, 4 F, m-C6F5) ppm.
C31H24BCl2F10NSiZr (801.5): calcd. C 46.45, H 3.02, N 1.75; found
C 46.84, H 3.30, N 1.40.
X-ray Crystal Structure Analysis of 7: Empirical formula
C31H24BCl2F10NSiZr·C7H8, M ϭ 893.67, light yellow crystal 0.30
ϫ 0.20 ϫ 0.20 mm, a ϭ 10.371(1), b ϭ 14.683(1), c ϭ 15.239(1) A,
˚
3
[3]
˚
α ϭ 115.75(1), β ϭ 96.40(1), γ ϭ 107.10(1)°, V ϭ 1919.0(3) A ,
ρ
calcd. ϭ 1.547 g cmϪ3, µ ϭ 5.35 cmϪ1, empirical absorption correc-
¯
tion (0.856 Յ T Յ 0.901), Z ϭ 2, triclinic, space group P1 (no. 2),
˚
λ ϭ 0.71073 A, T ϭ 198 K, ω and ϕ scans, 21470 reflections col-
lected (Ϯh, Ϯk, Ϯl), [(sinθ)/λ] ϭ 0.66 AϪ1, 9013 independent
˚
(Rint ϭ 0.039) and 7127 observed reflections [I Ն 2 σ(I)], 490 re-
fined parameters, R ϭ 0.048, wR2 ϭ 0.109, max. residual electron
[4]
Ϫ3
˚
density 1.38 (Ϫ0.70) e·A close to the solvate molecule, hydrogen
atoms calculated and refined as riding atoms.
Treatment of 7 with Pyridine; Formation of the Pyridine Adduct 12:
A solution of pyridine (7.6 mg, 96 mmol) in toluene (20 mL) was
added at room temperature to a light yellow solution of 7 (77 mg,
96 mmol) in toluene (20 mL), and the mixture was stirred for 2 h.
Removal of the volatiles in vacuo provided the product (77 mg,
99%) as a light yellow solid. 1H NMR (C7D8, 600 MHz): δ ϭ 8.13
[5]
[6]
[7]
3
(d, JH,H ϭ 5.8 Hz, 2 H, 9-H), 6.73 (m, 1 H, 11-H), 6.72 (m, 1 H,
D. Hüerländer, N. Kleigrewe, G. Kehr, G. Erker, R. Fröhlich,
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2000, 12, 1049Ϫ1050.
4Ј-H), 6.66 (m, 1 H, 3Ј-H), 6.45 (m, 2 H, 10-H), 6.44 (m, 1 H, 3-
H), 5.42 (m, 1 H, 2Ј-H), 5.38 (m, 1 H, 2-H), 5.32 (m, 1 H, 5Ј-H),
5.16 (m, 1 H, 5-H), 2.84 (t, JH,H ϭ 5.8 Hz, 2 H, 6-H, 6-HЈ), 1.62/
3
1.42 (each m, each 1 H, 8-H, 8-HЈ), 1.50/1.26 (each m, each 1 H,
7-H, 7-HЈ), 0.12/0.08 [each s, each 3 H, Si(CH3)2] ppm. 13C{1H}
NMR (C7D8, 150 MHz): δ ϭ 145.4 (C-9), 143.0 (C-4), 140.5 (C-
11), 128.2 (C-3Ј), 128.1 (C-3), 126.5 (C-4Ј), 125.3 (C-10), 114.3 (C-
2), 113.8 (C-5), 113.7 (C-5Ј), 113.0 (C-2Ј), 107.7 (C-1Ј), 107.3 (C-
1), 33.7 (C-6), 27.1 (C-7), 24.2 (C-8), Ϫ5.8, Ϫ6.3 [Si(CH3)2]; 148.6
[8]
[9]
G. Erker, R. Aul, Chem. Ber. 1991, 124, 1301Ϫ1310.
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(1JC,F ϭ 241.4 Hz), 140.0 (1JC,F ϭ 249.6 Hz), 137.7 (1JC,F
ϭ
249.7 Hz), 120.9 (o-, p-, m-, ipso-C6F5) ppm. 11B{1H} NMR (C7D8,
64 MHz): δ ϭ Ϫ0.6 (ν1/2 ϭ 335 Hz) ppm. 19F NMR (C7D8,
564 MHz): δ ϭ Ϫ131.6 (m, 4 F, o-C6F5), Ϫ157.5/Ϫ157.7 (each t,
3
each 1 F, JFF ϭ 20.5/20.6 Hz, p-C6F5), Ϫ163.1/Ϫ163.3 (each m,
each 2 F, m-C6F5) ppm. C32H24NBCl2F10SiZr (813.6): calcd. C
47.24, H 2.97, N 1.72; found C 46.88, H 2.88, N 1.62.
Supporting Information: See footnote on the first page of this arti-
cle. Details of the NMR spectroscopic characterization of the com-
pounds 1, 2, 4, 6, 7 and 12.
[10]
[11]
[12]
[13]
Acknowledgments
Financial support from the Fonds der Chemischen Industrie and
the Deutsche Forschungsgemeinschaft (SPP 1118) is gratefully
acknowledged.
[1]
[14]
M. Rosenblum, R. B. Woodward, J. Am. Chem. Soc. 1958, 80,
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3588
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2003, 3583Ϫ3589

2H-Pyrrole Formation at a Pendant Boron Lewis Acid
FULL PAPER
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