Generation of weakly bound Al-N Lewis pairs by hydroalumination of ynamines and the activation of small molecules: Phenylethyne and dicyclohexylcarbodiimide

Article abstract of DOI:10.1021/om3001179

Treatment of aminoalkynes (ynamines), Me₃Si-C≡C-NR ₂ [NR₂ = N(CH₂CH₂)₂NMe, N(CHMe-CH₂)₂CH₂], with dialkylaluminum hydrides, H-AlR′₂ (R′ = CH₂tBu, iBu, tBu), afforded by hydroalumination the corresponding alkenyl derivatives R′₂Al-(Me₃Si)CC(H)-NR₂ (3 to 6) in which the opposite Lewis functionalities adopted a cis-arrangement with the aluminum and nitrogen atoms on the same side of the resulting CC double bonds. Intramolecular Al-N interactions gave four-membered AlC₂N heterocycles, but ring strain may cause relatively long Al-N distances of 2.07 to 2.15 A. These compounds represent a new type of Lewis acid-base pairs, and we observed C-H bond activation by opening of the Al-N bond upon treatment with H-C≡C-C₆H₅. Compound 7 was isolated, which had the proton attached to nitrogen and the anionic ethynyl group bonded to aluminum. The formation of 7 is reversible in solution with an increasing dissociation into the starting compounds at elevated temperatures. Two different structural motifs were obtained upon reaction of the Lewis acid-base pairs with dicyclohexylcarbodiimide, which inserted selectively into the bond between the aluminum and the vinylic carbon atom. In the first compound (8) an amidinato ligand was formed, which coordinated the metal atom by both nitrogen atoms to give an AlCN₂ heterocycle. A six-membered heterocycle (9) resulted from the insertion of only one CN double bond of the carbodiimide into the Al-C(vinyl) bond. 9 had an unchanged interaction of the metal atom with the α-nitrogen atom of the former ynamine. The latter compound may be considered an intermediate, and rearrangement to yield an amidinato compound analogous to 8 was indeed observed with extended reaction time.

Full text of DOI:10.1021/om3001179

Article
pubs.acs.org/Organometallics
Generation of Weakly Bound Al−N Lewis Pairs by Hydroalumination
of Ynamines and the Activation of Small Molecules: Phenylethyne
and Dicyclohexylcarbodiimide
Thorsten Holtrichter-Roßmann,‡ Christian Rosener,† Johannes Hellmann,‡ Werner Uhl,*,†
̈
̈
Ernst-Ulrich Wurthwein,*,‡ Roland Frohlich,‡ and Birgit Wibbeling‡
̈
̈
†Institut fur Anorganische und Analytische Chemie der Universitat Munster, Corrensstraße 30, D-48149 Munster, Germany
‡Organisch-Chemisches Institut der Universitat Munster, Corrensstraße 40, D-48149 Munster, Germany
̈
̈
̈
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: Treatment of aminoalkynes (ynamines), Me3Si-
CC-NR2 [NR2 = N(CH2CH2)2NMe, N(CHMe-
CH2)2CH2], with dialkylaluminum hydrides, H-AlR′2 (R′ =
CH2tBu, iBu, tBu), afforded by hydroalumination the
corresponding alkenyl derivatives R′2Al-(Me3Si)CC(H)-
NR2 (3 to 6) in which the opposite Lewis functionalities
adopted a cis-arrangement with the aluminum and nitrogen
atoms on the same side of the resulting CC double bonds. Intramolecular Al−N interactions gave four-membered AlC2N
heterocycles, but ring strain may cause relatively long Al−N distances of 2.07 to 2.15 Å. These compounds represent a new type
of Lewis acid−base pairs, and we observed C−H bond activation by opening of the Al−N bond upon treatment with H-CC-
C6H5. Compound 7 was isolated, which had the proton attached to nitrogen and the anionic ethynyl group bonded to aluminum.
The formation of 7 is reversible in solution with an increasing dissociation into the starting compounds at elevated temperatures.
Two different structural motifs were obtained upon reaction of the Lewis acid−base pairs with dicyclohexylcarbodiimide, which
inserted selectively into the bond between the aluminum and the vinylic carbon atom. In the first compound (8) an amidinato
ligand was formed, which coordinated the metal atom by both nitrogen atoms to give an AlCN2 heterocycle. A six-membered
heterocycle (9) resulted from the insertion of only one CN double bond of the carbodiimide into the Al−C(vinyl) bond. 9
had an unchanged interaction of the metal atom with the α-nitrogen atom of the former ynamine. The latter compound may be
considered an intermediate, and rearrangement to yield an amidinato compound analogous to 8 was indeed observed with
extended reaction time.
pairs based on aluminum and nitrogen atoms and their
application in some preliminary activation reactions.
INTRODUCTION
■
The activation of small molecules by frustrated Lewis pairs is a
topic of considerable current interest.1 A key discovery in this
area was the heterolytic cleavage of the H−H bond of
dihydrogen molecules by a boron−phosphorus compound in
which the quench of the conflicting Lewis-acidic and -basic
properties was prevented by steric shielding and a rigid
perfluorinated phenylene backbone.2 This finding initiated
enormous research activities with the activation or fixation of
further molecules such as carbon dioxide, alkynes, and ketones.3
Most experiments have been conducted with compounds that
had sterically shielded phosphorus atoms and highly Lewis-
acidic bis(pentafluorophenyl)boryl groups. In some cases the
phosphanyl groups were replaced by other basic moieties such
as carbenes,4 thioethers,5 etc.6 Very few frustrated Lewis pairs
were reported that had aluminum atoms as the Lewis-acidic
centers.5,7−9 They have the advantage that their Lewis acidity is
relatively high and does not require activation by electron-
withdrawing groups. They were able to activate carbon dioxide,
C−H groups of terminal alkynes, or CC triple bonds. We
now report on the generation of a new class of active Lewis
RESULTS AND DISCUSSION
■
Synthesis of Aluminum−Nitrogen Compounds by
Hydroalumination of Ynamines. Hydroalumination10 of
the CC triple bonds of alkynylphosphines proved to be a
very efficient method for the generation of frustrated Lewis
pairs based on phosphorus and aluminum atoms.8,9 The
addition of the Al−H bonds to the alkynyl groups is highly
regio- and stereoselective and determined by charge distribu-
tion in the starting materials and by steric interactions in the
products. The alkynyl carbon atoms bonded to phosphorus
bear a relatively high negative partial charge and are preferably
attacked by the electrophilic aluminum atoms, which results in
geminal positions of the donor and acceptor functions. A cis-
arrangement of Al and H atoms in the resulting alkenyl groups
characterizes the kinetically favored products. Rearrangement
to give the thermodynamically favored trans-products with Al
Received: February 14, 2012
Published: March 28, 2012
© 2012 American Chemical Society
3272
dx.doi.org/10.1021/om3001179 | Organometallics 2012, 31, 3272−3283

Organometallics
Article
and H on different sides of the CC double bonds seems to
require intermolecular activation11 and can be prevented by
bulky substituents. The corresponding alkynylnitrogen starting
compounds (ynamines) have an amino group attached to the
CC triple bond. They can be prepared by a facile route
according to a literature procedure12 by the reaction of the
corresponding lithium amide with trichloroethene, subsequent
treatment of the crude product with n-butyllithium and
addition of chlorotrimethylsilane. Two new trimethylsilylethy-
nylamines (1 and 2, eq 1) have been prepared in the course of
Figure 1. Molecular structure of 3. The thermal ellipsoids are drawn at
the 40% probability level. Hydrogen atoms are omitted. Important
bond lengths (Å) and angles (deg): Al(1)−C(7) 1.982(4), C(6)−
C(7) 1.320(6), C(6)−N(1) 1.472(5), Al(1)−N(1) 2.144(3), C(6)−
C(7)−Al(1) 91.6(3), C(6)−C(7)−Si(1) 121.9(3), Al(1)−C(7)−
Si(1) 146.6(2), C(7)−C(6)−N(1) 116.8(4), C(6)−N(1)−Al(1)
81.4(2), C(7)−Al(1)−N(1) 70.3(2).
our investigations and applied for secondary hydroalumination
reactions. These ynamines reveal a relatively low thermal
stability and must be stored under argon at low temperatures
(1, 2 °C; 2, −15 °C). However, they can easily be handled at
room temperature for a short time.
Treatment of the ynamines 1 and 2 with dialkylaluminum
hydrides, R2AlH (R = CMe3, CH2CMe3, CH2CHMe2),
afforded the corresponding alkenyl compounds 3 to 6 by
selective hydroalumination reactions in 65−91% yields (eq 2).
Figure 2. Molecular structure of 4. The thermal ellipsoids are drawn at
the 40% probability level. Hydrogen atoms are omitted. Important
bond lengths (Å) and angles (deg): Al(1)−C(11) 1.992(4), C(11)−
C(12) 1.320(5), C(12)−N(1) 1.481(5), Al(1)−N(1) 2.103(3),
C(12)−C(11)−Al(1) 90.7(3), C(12)−C(11)−Si(1) 119.5(3),
Al(1)−C(11)−Si(1) 149.8(2), C(11)−C(12)−N(1) 116.1(3),
C(12)−N(1)−Al(1) 82.2(2), C(11)−Al(1)−N(1) 70.9(1).
intramolecular bonding interaction between the Lewis-acidic
and -basic centers. The resulting Al−N distances (2.073(2) (6),
2.103(3) (4), 2.144(3) (3), and 2.145(1) Å (5)) are in the
upper range observed for adducts between neutral aluminum
and nitrogen compounds.13 They may reflect a relatively weak
Al−N interaction between the tetracoordinated central atoms.
The strain of the AlNC2 heterocycles, which include a CC
double bond, becomes evident from an almost ideal rectangular
arrangement of the CC-Al groups with an average angle of
91.2°. These angles are much smaller than usually observed for
products of hydroalumination or hydrogallation reactions with
silylalkynes (110−120°) where no Lewis base is present in β-
position of the resulting alkenyl groups.11a,b,14,15 In compounds
3 to 6 this situation leads to relatively narrow contacts of the
metal atoms to the β-carbon atoms of the alkenyl moieties
(240.8 pm on average). In contrast, the second endocyclic
angle at the CC double bonds (CC−N) deviates less
Crystal structure determinations (Figures 1 and 4) showed that
the aluminum atoms are bonded to those carbon atoms of the
alkenyl moieties that are attached to the trimethylsilyl groups.
This position may be preferred for essentially two reasons: (i)
The corresponding carbon atom of the ynamine bears a
relatively high negative partial charge, which favors the attack of
the electrophilic aluminum atom. (ii) The negative partial
charge induced in the products by the bonding to the
electropositive aluminum atoms could be delocalized by
hyperconjugation into the trimethylsilyl groups. Aluminum
and hydrogen atoms are in a trans-arrangement on opposite
sides of the CC double bonds. This configuration allows an
approach of the aluminum and nitrogen atoms and a significant
3273
dx.doi.org/10.1021/om3001179 | Organometallics 2012, 31, 3272−3283

Organometallics
Article
for the starting ynamines (see below for quantum-chemical
calculations). Intramolecular repulsive interactions between the
relatively bulky neopentyl or tert-butyl groups bonded to
aluminum and the methyl groups of the piperidine substituent
in 4 and 5 may help to stabilize the a,a-isomer in these cases.
The NMR data correspond to these molecular structures.
The carbon atoms of the CC double bonds have the
expected chemical shifts in narrow ranges at δ = 162 (Al-C
C) and 148 (N-CC) in the 13C NMR spectra.11 The
resonances of the vinylic hydrogen atoms were observed in the
1H NMR spectra over a relatively broad range between δ = 6.00
for the sterically less shielded compound 3 and δ = 7.12 for the
3
di(tert-butyl) derivative 5. The JSi−H coupling constants across
the CC double bonds are usually very indicative of the
configuration of the alkenyl groups, with values >20 Hz for the
trans- and <12 Hz for the cis-arrangement of hydrogen and
silicon atoms.11,15,16 These coupling constants were difficult to
determine for compounds 3 to 6, and only for 5 could a
relatively small value of 6.4 Hz be detected with reasonable
accuracy. The CH2 hydrogen atoms of the neopentyl and
isobutyl groups in 3, 4, and 6 are diastereotopic and gave two
Figure 3. Molecular structure of 5. The thermal ellipsoids are drawn at
the 40% probability level. Hydrogen atoms are omitted. Important
bond lengths (Å) and angles (deg): Al(1)−C(11) 1.982(2), C(11)−
C(12) 1.323(2), C(12)−N(1) 1.480(2), Al(1)−N(1) 2.145(1),
C(12)−C(11)−Al(1) 92.0(1), C(12)−C(11)−Si(1) 121.5(1),
Al(1)−C(11)−Si(1) 146.5(1), C(11)−C(12)−N(1) 116.1(2),
C(12)−N(1)−Al(1) 81.56(9), C(11)−Al(1)−N(1) 70.33(6).
1
signals in the H NMR spectra. Two resonances were also
observed for the methyl carbon and hydrogen atoms of the
isobutyl groups. These observations clearly indicate that the
molecular structures detected in the solid state are retained in
solution. A boron−phosphorus compound comparable to 3 to
6 that has the donor and acceptor atoms bridged by an ethylene
group has recently been published, Mes2P-CH2CH2-B-
(C6F5)2.17 NMR spectroscopic characterization and quantum
chemical calculations revealed an internal adduct formation via
intramolecular B−P interactions, which is supported by π-
stacking between two aromatic rings. This compound was
applied for the activation of small molecules such as
dihydrogen. In order to check the propensity of our
aluminum−nitrogen compounds to react in a similar way by
opening of the Al−N bond and diminution of the ring strain,
we treated them in a few preliminary experiments with
phenylethyne and a carbodiimide.
Reaction of Compound 5 with the Terminal Alkyne H-
CC-C6H5. Terminal alkynes showed two different kinds of
activation upon treatment with frustrated Lewis pairs on the
base of boron or aluminum and phosphorus atoms:3,8,9 (i) The
C−H bonds were cleaved with the protons attached to the
basic phosphorus atoms and the anionic ethynyl groups
coordinated to the boron or aluminum atoms. (ii) The CC
triple bonds were activated, and in some cases five-membered
heterocycles with an endocyclic CC double bond resulted by
cycloaddition. Charge distribution determines the structures of
the products also in these cases, and the carbon atoms attached
to hydrogen are selectively bonded to boron or aluminum. We
treated compound 5 with equimolar quantities of phenylethyne
in cyclopentane and isolated reproducibly colorless crystals of 7
in quantitative yield (eq 3). Crystal structure determination
(Figure 5) verified C−H bond cleavage. The proton of the
former ethyne is bonded to the nitrogen atom, and the ethynyl
group is attached with its electron-rich terminal carbon atom to
the Lewis-acidic aluminum atom. Bond lengths and angles
changed considerably upon coordination of the alkyne. The
Al−C distance to the carbon atom of the CC double bond is
lengthened to 2.057(2) Å (1.982(2) Å in 5), which is in
accordance with the increased coordination number of the
metal atom. Interestingly also the Si−C bond length increased
significantly from 1.855(2) to 1.895(1) Å in 7, which may be
Figure 4. Molecular structure of 6. The thermal ellipsoids are drawn at
the 40% probability level. Hydrogen atoms are omitted. Important
bond lengths (Å) and angles (deg): Al−C(6) 1.993(2), C(6)−C(7)
1.322(3), C(7)−N(1) 1.483(2), Al−N(1) 2.073(2), C(7)−C(6)−Al
90.3(1), C(7)−C(6)−Si 123.5(2), Al−C(6)-Si 145.9(1), C(6)−
C(7)−N(1) 115.2(2), C(7)−N(1)−Al 83.1(1), C(6)−Al−N(1)
71.27(7).
drastically from the ideal value (116.1° on average). The
remaining angles of the heterocycles (C−Al−N, 70.7°, and C−
N−Al, 82.1°) are very acute, which may be favored by the
relatively high ionic character, in particular of the Al−N bond.
Al−C (vinyl) (1.987 Å), C−N (1.479 Å), and CC bond
lengths (1.321 Å) of the central heterocycles are unexceptional
and do not require a detailed discussion. The compounds 3, 4,
and 5, which were obtained by hydroalumination of the
ynamine 1, showed a different conformation of their 2,6-
dimethylpiperidyl substituents. The bisaxial form (a,a) was
observed for the neopentyl and tert-butyl derivatives 4 and 5,
while the sterically less shielded n-butyl compound 3 had the
bisequatorial arrangement (e,e) of both methyl groups. The e,e-
form was calculated to be thermodynamically slightly favored
3274
dx.doi.org/10.1021/om3001179 | Organometallics 2012, 31, 3272−3283

Organometallics
Article
small changes of chemical shifts. The ethenyl carbon atoms
were observed at δ = 162.8 and 138.0 in the 13C NMR
spectrum, and the 29Si NMR resonance appeared at δ = −3.6 (δ
= −10.4 for 5). The most significant difference from the
uncoordinated compound 5 occurred for the vinylic hydrogen
atom, which had a chemical shift of δ = 7.12 for 5 compared to
δ = 5.43 for 7. The resonance of 7 was split to a doublet by
coupling with the proton bonded to the nitrogen atom (3JH−H
=
7.3 Hz). Further important resonances were observed in the 1H
NMR spectrum at a very low field (δ = 10.43, broad) for the
N−H group and in the 13C NMR spectrum at δ = 118.1 and
116.1 for the carbon atoms of the ethynyl group. Dissolution of
single crystals of 7 in benzene afforded a mixture of
compounds. Besides the resonances of 7 the characteristic
resonances of the starting compounds 5 and phenylethyne were
detected. The concentration of the latter substances increased
with temperature in accordance with an increasing dissociation
upon warming. In addition a strong broadening of the
resonances was observed at elevated temperatures, indicating
a fast exchange process. The ratio between the free compound
5 and its phenylethyne adduct 7 is about 1:6 at 10 °C, 1:2.5 at
room temperature, and 1.4:1 at 35 °C. Further reactions of the
aluminum−nitrogen compounds 3 to 6 with phenylethyne led
to some changes in the chemical shifts of the starting
compounds; however, we were not able to isolate another
adduct in a pure and crystalline form.
Reactions of Compounds 3 and 6 with Di(cyclohexyl)-
carbodiimide. Treatment of the aluminum−nitrogen com-
pound 3 with di(cyclohexyl)carbodiimide in n-hexane gave the
complete consumption of the starting materials after stirring at
room temperature for 2 h (eq 4). Compound 8 was formed
almost quantitatively and isolated in 74% yield after
crystallization from the concentrated reaction mixture. Crystal
Figure 5. Molecular structure of 7. The thermal ellipsoids are drawn at
the 40% probability level. Hydrogen atoms with the exception of N−H
are omitted. Important bond lengths (Å) and angles (deg): Al(1)−
C(11) 2.057(2), C(11)−C(12) 1.323(2), Al(1)−C(011) 2.008(2),
C(011)−C(012) 1.209(2), Al(1)−C(11)−C(12) 127.3(1), Si(1)−
C(11)−C(12) 108.7(1), Al(1)−C(11)−Si(1) 123.97(8), C(11)−
C(12)−N(1) 126.1(1), C(11)−Al(1)−C(011) 104.97(6), Al(1)−
C(011)−C(012) 174.0(1), C(011)−C(012)−C(013) 177.3(2).
influenced by steric crowding in this part of the molecule. The
C−N bond is only slightly lengthened from 1.480(2) to
1.499(2) Å, and the CC double bond is unaffected (1.323(2)
Å). The angle Al−CC is very acute in 5 (92.0(1)°), which
verifies a considerable ring strain. In 7 this angle increased to
127.3(1)°. Steric repulsion may also cause the relatively large
CC−N angle of 126.1(1)° (116.1(3)° in 5). The angle
Al(1)−C(11)−Si(1) of 123.97(8)° is larger than the ideal
value, while the angle CC−Si (108.7(1)°) is smaller than
expected. Both observations reflect the particular steric
interactions in the molecule. The shortest Al−C bond length
(2.008(2) Å) was observed to the sp-hybridized carbon atom of
the alkynyl group (C(011)), which is in the normal range of
distances between four-coordinate aluminum and sp-hybridized
carbon atoms.18 An interesting structural detail is the short
contact between the α-carbon atom of the alkyne and the
proton attached to nitrogen (2.19 Å; N(1)···C(011) 3.000 Å),
which indicates intramolecular N−H···C hydrogen bonding.19
It may reflect an incomplete and reversible cleavage of the C−
H bond and may help to understand the behavior of compound
7 in solution.
NMR spectroscopic characterization gave resonances similar
to those of the aluminum−nitrogen compound 5 with only
3275
dx.doi.org/10.1021/om3001179 | Organometallics 2012, 31, 3272−3283

Organometallics
Article
structure determination (Figure 6) showed that a carbodiimide
molecule was inserted selectively into the Al−C bond to the α-
carbon atom of the vinyl group. Both nitrogen atoms of the
expected ranges. In contrast, the resonance of the second
carbon atom of the CC double bond has an unusual
chemical shift of δ = 90.4, which compared to the starting
compound 3 is considerably shifted to higher field by more
than 70 ppm. Only one resonance was observed at room
temperature for the methyl groups of the piperidine group.
This may indicate an accidental coincidence of the resonances
or a fast rotation about the C−N bond despite the π-
delocalization in this part of the molecule as derived from the
crystal structure (see above). Two different sets of resonances
were observed for the isobutyl groups attached to aluminum,
which indicates a hindered rotation about the C−C single bond
of the central N2C−CC moiety. An isobutyl group is on the
same side as the trimethylsilyl substituent, while the other one
is parallel to the piperidine group. Steric interactions may
prevent a fast rotation and an equilibration of the resonances.
1
Temperature-dependent H NMR experiments between −50
and +80 °C did not result in any significant alteration of the
shape of the spectra. The resonances became expectedly a little
broader at the lowest temperature.
The corresponding reaction of dicyclohexylcarbodiimide
with compound 6, which has a piperazine instead of a
piperidine group, was slower and gave another result in its
first step. Stirring of the reaction mixture for 30 min at room
temperature, concentration, and cooling yielded colorless
crystals of compound 9 (eq 4), which showed completely
different NMR spectra compared with 8. Two different sets of
resonances of cyclohexyl groups showed that 9 contains two
chemically different N-alkyl groups. The resonance of the inner
carbon atom of the NC−N group (δ = 156.4) shows a
considerable high-field shift compared to 6. The signal of the
vinylic carbon atom that is attached to the silicon atom appears
in a normal range (δ = 138.0; δ = 90.4 in 8). The isobutyl
groups show two sets of resonances. Crystal structure
determination revealed indeed a molecular structure that is
completely different compared with that of 8 (Figure 7). A
carbodiimide molecule has been inserted into the Al−C bond
to the vinylic carbon atom, as before. However, the aluminum
atom is coordinated to only one of its nitrogen atoms (N(01)),
and the second one (N(02)) occupies a terminal position. The
localized bonding situation results in different C−N bond
lengths (C(01)−N(01) 1.372(2) and C(01)−N(02) 1.287(2)
Å). The coordination sphere of the aluminum atom is
completed by the central nitrogen atom of the piperazine
group with an Al−N distance of 2.047(1) Å, in accordance with
a dative bonding interaction (see above). The remaining
distances are unexceptional (C(11)C(12) 1.329(2), C(01)−
C(11) 1.520(2), Al(1)−N(01) 1.871 Å) and do not require a
detailed discussion.
Compound 9 represents the first step of the insertion of
carbodiimides in the Al−C bond of the aluminum nitrogen
compounds 3 to 6. Only a single CN double bond inserts
into the Al−C bond to the vinylic carbon atom, and the
interaction of the aluminum atom to the α-nitrogen atom of the
piperazine ring is still intact. Rearrangement by opening of this
Al−N bond and the chelating coordination of the metal atom
by both nitrogen atoms of an amidinate ligand results in the
formation of a compound analogous to 8. Such a secondary
reaction was observed when the reaction mixture of 6 and
dicyclohexylcarbodiimide was stirred for 2 d instead for only 30
min at room temperature. Single crystals of the product (10)
could not be isolated, but the NMR spectroscopic character-
ization gave clear evidence that a compound analogous to 8 had
Figure 6. Molecular structure of 8. The thermal ellipsoids are drawn at
the 40% probability level. Hydrogen atoms are omitted. Important
bond lengths (Å) and angles (deg): Al(1)−N(01) 1.931(1), Al(1)−
N(02) 1.933(1), N(01)−C(01) 1.341(2), N(02)−C(01) 1.346(2),
C(01)−C(11) 1.484(2), C(11)−C(12) 1.359(2), N(01)−Al(1)−
N(02) 68.83(5), N(01)−C(01)−N(02) 108.8(1), C(01)−C(11)−
C(12) 127.6(1), Si(1)−C(11)−C(12) 114.4(1), Si(1)−C(11)−C(01)
117.6(1), C(11)−C(12)−N(1) 133.8(1).
resulting amidinate coordinate to the metal atom in a chelating
manner to form a four-membered AlN2C heterocycle with a
delocalized π-electronic system across the N−C−N group (C−
N 1.344 Å; Al−N 1.932 Å (average)). C−C bond formation
between an enamine and a carbodiimide yielded a unique
(Al)N2C−CC(H)-NC2 moiety with the second NC2 group
being part of the terminal dimethylpiperidine substituent. To
the best of our knowledge, such reactions have not been
reported previously for other enamines. π-Conjugation between
the (Al)N2C heterocycle and the CC double bond can be
excluded by the almost perpendicular arrangement of both
groups with torsion angles N(01)−C(01)−C(11)−C(12) and
N(02)−C(01)−C(11)−C(12) of −115.6(2)° and 72.7(2)°
and the C−C bond length (C(01)−C(11) 1.484(2) Å), which
is in the normal range of single bonds between sp2-hybridized
carbon atoms. π-Conjugation between the CC double bond
and the nitrogen atom of the piperidine group (N(1)) is
verified by an almost ideal planar environment of N(1) with the
sum of the angles of 356.6°. Further evidence is provided by the
short C(12)−N(1) bond (1.367(2) Å) and the slight
lengthening of the CC bond compared to the starting
compound 3 (1.359(2) versus 1.320(6) Å). The torsion angles
C(11)−C(12)−N(1)−C(13) and C(11)−C(12)−N(1)−
C(17) (16.4(3)° and 175.5(2)°) verify the coplanar arrange-
ment the these atoms. Insertion reactions of carbodiimides into
the Al−C bonds of trialkylaluminum compounds and further
amidinate complexes of aluminum have been reported
previously.20
The NMR spectra are complicated. They reflect the
particular constitution of compound 8 as derived from crystal
structure determination. In the 13C NMR spectrum the central
carbon atom of the chelating NCN ligand (δ = 177.4) and the
vinylic carbon atom attached to the nitrogen atom of the
terminal piperidine group (δ = 146.1) have resonances in the
3276
dx.doi.org/10.1021/om3001179 | Organometallics 2012, 31, 3272−3283

Organometallics
Article
TZVP level including zero-point corrections (at the B97-D/
def2-TZVP level).
The mechanism of the hydroalumination reaction of the
ynamine 1 with di(tert-butyl)aluminum hydride to give the
Lewis acid−base pair 5 was studied in detail by quantum-
chemical calculations (B2PLYP-D/def2-QZVP//M06-2x/def2-
TZVP and B2PLYP-D/def2-QZVP//B97-D/def2-TZVP in-
cluding zero-point correction) (Scheme 1). 1 can adopt two
possible conformations with respect to the positions of the two
Scheme 1. Hydroalumination of 1-a,a and 1-e,e with
HAltBu2 and Formation of 5-a,a and 5-e,e (B2PLYP-D/def2-
QZVP//B97-D/def2-TZVP+ZPE) [kcal/mol]
Figure 7. Molecular structure of 9. The thermal ellipsoids are drawn at
the 40% probability level. Hydrogen atoms and methyl groups of the
isobutyl substituents are omitted. Important bond lengths (Å) and
angles (deg): Al(1)−N(01) 1.871(1), Al(1)−N(1) 2.047(1), N(01)−
C(01) 1.372(2), C(01)−N(02) 1.287(2), C(01)−C(11) 1.520(2),
C(11)−C(12) 1.329(2), C(12)−N(1) 1.466(2), N(01)−Al(1)−N(1)
94.17(6), Al(1)−N(01)−C(01) 122.0(1), N(01)−C(01)−C(11)
112.5(1), C(01)−C(11)−C(12) 121.5(1), C(11)−C(12)−N(1)
124.2(1), C(12)−N(1)−Al(1) 99.77(9).
been obtained. Only one set of resonances was observed for the
cyclohexyl groups attached to nitrogen. Further, the chemical
shifts of the central carbon atom of the NCN chelating ligand
(δ = 177.3) and in particular of the carbon atom of the CC
double bond attached to silicon (δ = 92.1) are almost identical
to those obtained for 8. Resonances of two chemically different
isobutyl groups were detected, which is also in accordance with
the results obtained for 8.
Quantum-Chemical Calculations. High-level quantum-
chemical calculations on the structures and reactivity of these
novel compounds were performed by applying the GAUSSIAN
09 package of programs21 (geometry optimizations) and the
program TURBOMOLE22 (B2PLYP-D single-point calcula-
tions23). Structure optimizations for three of the Lewis acid−
base pairs (3, 5, and 6) were carried out with three different
current DFT methods (B97-D24/def2-TZVP, M06-2x25/def2-
TZVP, and B3LYP/6-311+G(d,p)) using basis sets26 as
implemented in the GAUSSIAN 09 package. The verification
of the experimental Al−N distances was particularly important
because they may have considerable influence on the specific
reactivity of these Lewis pairs. The relatively best fit with the X-
ray data was obtained with the B97-D functional (Table 1),
which indicates the importance of dispersion interactions for
species with relatively weak interactions. Thus, in the following
we mainly discuss geometries obtained at the B97-D/def2-
TZVP level of theory with energies from single-point
calculations at the B2PLYP-D/def2-QZVP//B97-D/def2-
methyl groups of the piperidine subunit. As anticipated, the e,e-
isomer is lower in energy by about 3.5 kcal/mol compared with
the a,a-isomer. Treatment of 1 with dialkylaluminum hydrides
yields the cis-addition products in the first step10,11 (Z-
configuration at the CC bond), which reflects the expected
configuration from the generally accepted concerted reaction
pathway of hydroalumination reactions.27 These compounds
exhibit π-delocalization of the lone pair at the nitrogen atom,
which results in an almost ideally planar surrounding of
nitrogen (sum of the angles: 356.6° (a,a) and 359.4° (e,e)),
relatively short C−N bonds (1.377 and 1.374 Å, respectively),
and slightly lengthened CC bonds (1.371 and 1.370 Å,
respectively). The delocalized π-system causes a coplanar
arrangement of the alkenyl group and the average plane of the
piperidine heterocycle. The addition of the Al−H bond is
calculated to be quite exothermic, by 31−35 kcal/mol, with a
slight preference for the a,a-product by ca. 3 kcal/mol, which
may be caused by steric repulsion between the trimethylsilyl
substituent and an in-plane piperidine methyl group in the e,e-
derivative. cis/trans-Isomerization across the CC bond yields
the E-isomers (formal trans-addition of Al−H bonds) with Al
and N atoms in neighboring positions. This isomerization
process was often observed for hydroalumination reactions, and
a possible pathway has been discussed in a previous
Table 1. Al−N Distances [Å] Obtained by Three
Computational Levels and Compared to the X-ray Data
B97D/def2-
TZVP
M06-2x/def2-
TZVP
B3LYP/6-
311+G(d,p)
X-ray
3
5
6
2.158
2.158
2.116
2.127
2.113
2.071
2.240
2.263
2.148
2.143
2.145
2.073
3277
dx.doi.org/10.1021/om3001179 | Organometallics 2012, 31, 3272−3283

Organometallics
Article
publication.11 The E-isomers (5-a,a and 5-e,e) are calculated to
be lower in energy by 5−10 kcal/mol. Structural data calculated
for the energetically preferred a,a-isomer (5-a,a) are similar to
the experimental values obtained by X-ray diffraction of 5. The
cis-arrangement of Al and N atoms allows a bonding interaction
between the Lewis-acidic and -basic centers, which may
contribute significantly to the comparably high stability of the
molecules in their E-forms. The Al−N distances were calculated
to be 2.159 (5-a,a; X-ray: 2.145(1)) and 2.249 Å (5-e,e), which
indicates a weaker Al−N bonding interaction in the less
thermodynamically stable e,e-isomer. The sum of the angles at
the nitrogen atoms (340.7° and 332.3°) verifies their distorted
tetrahedral coordination spheres. CC (1.338 Å in both cases)
and C−N bond lengths (1.468 (5-a,a) and 1.475 Å (5-a,a))
correspond to standard values and are in accordance with the
absence of any π-delocalization in the CC−N group (see the
intermediate Z-isomers for comparison).
changes from a,a in the starting compound 5 to e,e in 7, in
accordance with the experimental observations. Rearrangement
to the generally preferred e,e-form results already in the first
step of this reaction with the formation of the transient van der
Waals complex.
Two slightly different reaction pathways were found for the
reactions of the Lewis acid−base pairs 3 and 6 with
dicyclohexylcarbodiimide (B2PLYP-D/def2-QZVP//B97-D/
def2-TZVP incl. ZPE) (cf. eq 4). The piperidinyl compound
3 gives a primary complex in which the aluminum atom is
coordinated to one of the carbodiimide nitrogen atoms (Erel
=
−10.6 kcal/mol) (Scheme 3). Interestingly, the piperidyl group
(symbolized by NR2 in the Scheme) is in a cis-position with
Scheme 3. Reaction of 3-e,e with Carbodiimide and
Synthesis of the Amidinate Complex 8-a,a (B2PLYP-D/def2-
QZVP//M06-2x/def2-TZVP) [kcal/mol]
Phenylethyne reacted with the Lewis acid−base pair (E)-5 by
opening of the Al−N bond and C−H bond activation as
described above (7, eq 3, Figure 5). Quantum-chemical
calculations for the gas phase showed that in a first step a
weakly bound primary van der Waals complex (Erel = 0.1 kcal/
mol at B2PLYP-D/def2-QZVP//M06-2x/def2-TZVP without
ZPE, no minimum at B97-D/def2-TZVP) is formed with a
short contact of the acetylenic hydrogen atom to the amine
nitrogen atom (1.839 Å, 1.809 Å at M06-2x) and a long α-
alkynyl carbon−aluminum distance (2.450, 2.436 Å at M06-2x)
(Scheme 2). Via a negligible barrier of only 0.3 kcal/mol (SCS-
MP2/B3LYP; 0.1 kcal/mol at B2PLYPD/M06-2x) the final
Scheme 2. Reaction of 5-a,a with Phenylethyne to Yield 6-e,e
(B2PLYP-D/def2-QZVP//M06-2x/def2-TZVP) [kcal/mol]
respect to the trimethylsilyl group as a result of a simultaneous
cis/trans-isomerization about the CC bond. Carbon−carbon
bond formation between the carbodiimide carbon atom and the
vinylic carbon atom bearing the trimethylsilyl group starts via a
cyclic transition state (−3.1 kcal/mol) and affords another
intermediate (−21.9 kcal/mol) with a three-coordinate
aluminum atom and a central chain of thee carbon atoms. A
second cis/trans-isomerization about the CC bond of the
original Lewis pair and coordination of the piperidine nitrogen
atom to aluminum result in the formation of a six-membered
ring (−40.4 kcal/mol) with the bisaxial arrangement of the
product 7 is formed, which is exothermic compared with the
starting compounds by −15.9 kcal/mol (−14.2, B2PLYPD/
M06-2x). Cleavage of the acetylenic C−H bond results in the
formation of a protonated amino group (N−H: 1.049; 1.041 Å
M06) and a strong carbon−aluminum bond (2.022, 2.022 Å
M06-2x; X-ray: 2.008(2) Å). The intramolecular hydrogen
bond between the α-carbon atom of the ethynyl group and the
N−H proton was confirmed by a relatively short C···H distance
of 1.929 Å (N···C 2.973 Å). The conformation of the
dimethylpiperidine heterocycle in the minimum structures
3278
dx.doi.org/10.1021/om3001179 | Organometallics 2012, 31, 3272−3283

Organometallics
Article
literature procedures. Trichloroethene, chlorotrimethylsilane, 2,6-
dimethylpiperidine, 1-methylpiperazine, and solutions of HAliBu2 or
n-butyllithium in n-hexane were applied as purchased. The chemical
shifts of the NMR spectra were assigned on the basis of homo- and
heteronuclear 2D NMR experiments (COSY, HSQC, HMBC,
ROESY). NMR spectra were recorded at ambient probe temperature
using the following Bruker instruments: Avance I (1H, 400.13; 13C,
100.62; 29Si, 79.49 MHz) or Avance III (1H, 400.03; 13C, 100.59; 29Si,
79.47 MHz) and referenced internally to residual solvent resonances
(chemical shift data in δ). 13C NMR spectra were all proton-
decoupled. With the exception of compounds 1 and 2 IR spectra were
recorded as a paraffin mull between KBr or CsI plates on a Shimadzu
Prestige 21 spectrometer. Electron impact mass spectra were obtained
on a Varian mass spectrometer. Elemental analysis was determined by
piperidyl methyl groups (the e,e-form is energetically slightly
less favored by −36.1 kcal/mol). Cleavage of the Al−N donor−
acceptor bond to the piperidinyl nitrogen atom and rotation
about the C−N bond afford the amidinate complex 8 as the
final product (−58.7 kcal/mol), which has been obtained
experimentally and characterized by crystal structure determi-
nation (eq 4; Figure 6). The Lewis pair 6, which has a
piperazine instead of a piperidine substituent, gives a slightly
different reaction pathway upon treatment with carbodiimide.
cis/trans-Isomerization does not occur parallel to the initial
adduct formation (−14.8 kcal/mol) (Scheme 4). It is only
observed for the second step, which comprises C−C bond
the microanalytic laboratory of the Westfalische Wilhelms Universitat
̈
̈
Scheme 4. Reaction of 6 with Dicyclohexylcarbodiimide and
Formation of 10 via 9 (B2PLYP-D/def2-QZVP//B97D/
def2-TZVP+ZPE) [kcal/mol]
Munster.
̈
Synthesis of the Ynamines 1 and 2; General Procedure. A
solution of one equivalent of the corresponding secondary amine (2,6-
dimethylpiperidine, 1-methylpiperazine) in 30 mL of THF was treated
with 1.2 equivalents of n-butyllithium in n-hexane at −78 °C. The
solution was allowed to warm to room temperature and stirred for 1 h.
It was slowly added to a solution of one equivalent of trichloroethene
in 50 mL of THF at −78 °C. After stirring at room temperature for 1 h
and cooling to −78 °C two equivalents of n-butyllithium were added.
The solution was warmed to room temperature and stirred for a
further 2 h. After cooling the mixture to −78 °C one equivalent of
chlorotrimethylsilane in 10 mL of THF was added. The mixture was
stirred at room temperature for 16 h and concentrated to ca. 20 mL
under vacuum at room temperature. All ynamines were subsequently
purified by vacuum distillation. They are air and moisture sensitive and
must be stored under argon at low temperatures (1, 2 °C; 2, −15 °C).
Characterization of 2,6-Dimethyl-1-(trimethylsilylethynyl)-
piperidine (1). Yield: 32%; colorless liquid. Bp: 48 °C (4.8 × 10−2
bar). Anal. Calcd for C12H23NSi (209.2): C, 68.9; H, 11.1; N, 6.7.
Found: C, 68.7; H, 11.0; N, 6.6. 1H NMR (C6D6, 300 MHz): δ 2.54 (2
H, m, NCH), 1.44 (1 H, m, CH2), 1.25 (6 H, pseudo-d, N-CH-CH3),
1.20 (5 H, m, CH2), 0.25 (9 H, m, SiMe3). 13C NMR (C6D6, 75
MHz): δ 105.7 (N-CC), 67.5 (N-CC), 56.7 (NCH), 32.2 (N-
CH-CH2), 24.8 (N-CH-CH2-CH2), 20.8 (N-CH-CH3), 1.3 (SiMe3).
29Si NMR (C6D6, 60 MHz): δ −20.5. IR (neat, cm−1): 2965 m, 2932 s,
2860 vw ν(CH); 2143 vs ν(CC); 1447 m, 1375 m, 1356 m, 1314
m, 1246 s δ(CH3); 1163 w, 1117 w, 1094 m, 1074 w, 1055 w ν(CC),
ν(CN); 854 vs, 837 s ρ(CH3Si); 695 m, 635 vs ν(SiC). MS (EI, 20 eV,
25 °C): m/z (%): 209 (57) [M]+, 194 (100) [M − CH3]+.
Characterization of 1-Methyl-4-(trimethylsilylethynyl)-
piperazine (2). Yield: 72%; colorless liquid. Bp: 45 °C (8 × 10−3
bar). Anal. Calcd for C10H20N2Si (196.1): C, 61.2; H, 10.3; N, 14.3.
Found: C, 61.3; H, 10.2; N, 14.2. 1H NMR (C6D6, 300 MHz): δ 2.89
(4 H, m, N-CH2-CH2-N-CH3), 2.00 (4 H, m, N-CH2-CH2-N-CH3),
1.92 (3 H, m, NCH3), 0.23 (9 H, m, SiMe3). 13C NMR (C6D6, 75
MHz): δ 109.9 (N-CC), 62.2 (N-CC), 54.1 (N-CH2-CH2-N-
CH3), 51.9 (N-CH2-CH2-N-CH3), 46.3 (N-CH3), 1.2 (SiMe3). 29Si
NMR (C6D6, 60 MHz): δ −20.1 ppm. IR (neat, cm−1): 2957 m, 2941
m, 2847 m, 2795 ν(CH); 2153 vs, 2139 sh ν(CC); 1452 m, 1371 m,
1287 m, 1248 m δ(CH3); 1209 w, 1144 m, 1007 w ν(CC), ν(CN);
854 s, 839 s, 758 w ρ(CH3Si); 690 w, 657 w, 633 vs ν(SiC). MS (EI,
20 eV, 25 °C): m/z (%) 196 (100) [M]+, 181 (47) [M − CH3]+, 168
(31) [M − C2H4]+, 123 (24) [M − SiMe3]+.
S y n t h e s i s o f C o m p o u n d 3 . 2 , 6 - D i m e t h y l - 1 -
(trimethylsilylethynyl)piperidine (1) (0.50 mL, 0.430 g, 2.06 mmol)
was dissolved in 15 mL of n-hexane and treated with neat HAliBu2
(0.37 mL, 0.292 g, 2.06 mmol) at room temperature. Stirring was
continued for 2 h. Colorless crystals of 3 were obtained upon
concentration of the reaction mixture and cooling to −28 °C. Yield:
0.593 g (82%). Mp (argon, sealed capillary): 47 °C. Anal. Calcd for
C20H42AlNSi (351.3): C, 68.4; H, 12.1; N, 4.0. Found: C, 67.8; H,
11.8; N, 4.1. 1H NMR (C6D6, 400 MHz): δ 6.00 (1 H, s, br, N-CH
C), 2.13 (2 H, m, br, N-CH-CH3), 2.10 (2 H, septet, 3JH−H = 6.6 Hz,
Al-CH2-CH), 1.37 and 1.06 (1 H, m, N-CH-CH2-CH2), 1.34 (4 H, m,
N-CH-CH2-CH2), 1.21 and 1.19 (each 6 H, s, br, CH(CH3)2), 1.09 (6
formation and cleavage of the Al−vinyl bond (−20.5 kcal/mol).
The cyclic transition state is similar to that of the piperidine
case (−0.5 kcal/mol). Rotation about the C−N bond, cis/trans-
isomerization, and coordination of the inner piperazine
nitrogen atom to aluminum gives a six-membered heterocycle
(−43.4 kcal/mol), which has been isolated and characterized by
X-ray diffraction (9, Figure 7). Cleavage of the Al−N donor−
acceptor bond and rotation about the C−N bond gives an
amidinate complex (10), which, however, in this case is only
slightly favored compared with 9 (−43.4 versus −45.6 kcal/
mol). This low energetic driving force may cause the relatively
long reaction times for the rearrangement of 9 to 10 and may
facilitate the isolation and characterization of the intermediate
compound 9.
EXPERIMENTAL SECTION
■
All procedures were carried out under purified argon using standard
Schlenk techniques. n-Hexane and cyclopentane were dried over
LiAlH4; THF was dried over potassium. The starting compounds
28
29
HAl(CH2CMe3)2 and HAl(CMe3)2 were obtained according to
3279
dx.doi.org/10.1021/om3001179 | Organometallics 2012, 31, 3272−3283

Organometallics
Article
3
H, d, JH−H = 6.5 Hz, N-CH-CH3), 0.38 and 0.29 (2 H, m, Al-CH2-
Calcd for C18H39AlN21Si (338.3): C, 63.9; H, 11.6; N, 8.3. Found: C,
62.7; H, 11.4; N, 8.1. H NMR (C6D6, 400 MHz): δ 6.14 (1 H, s, N-
CHC), 2.68 and 2.26 (each 2 H, m, CCH-N-CH2), 2.21 and 1.88
(each 2 H, m, N-CH2-CH2-N-CH3), 2.13 (2 H, septet, 3JH−H = 6.7 Hz,
Al-CH2-CH), 1.87 (3 H, s, N-CH3), 1.24 (12 H, d, 3JH−H = 6.5 Hz, Al-
CH), 0.23 (s, 9 H, SiMe3). 13C NMR (C6D6, 100 MHz): δ 162.3 (br,
Al-CC), 147.6 (br, Al-CC), 62.2 (br, N-CH-CH3), 32.9 (br, N-
CH-CH2−CH2), 29.1 and 28.8 (Al-CH2-CHMe2), 27.1 (br, Al-CH2-
CH), 26.9 (br, Al-CH2), 24.5 (br, N-CH-CH2-CH2), 19.8 (N-CH-
CH3), 0.1 (SiMe3). 29Si NMR (C6D6, 79 MHz): δ −10.8. IR (CsI
plates, paraffin, cm−1): 1591 m, 1553 vs, 1545 sh ν(CC), 1462 vs,
1375 s (paraffin); 1362 s, 1310 s, 1244 vs δ(CH3); 1204 w, 1177 s,
1123 s, 1045 vs, 980 w ν(CC), ν(CN); 943 m, 914 w, 891 s, 845 s, 814
m, 789 w, 750 m ρ(CH3Si); 723 m (paraffin); 687 m, 671 m νas(SiC);
615 s νs(SiC); 598 s, 530 vs, 501 vs, 449 vs ν(AlC), ν(AlN). MS (EI,
20 eV, 30 °C): m/z (%) 296 (5), 295 (19), 294 (83) [M − iBu]+, 239
(2), 238 (14) [M − iBu − butene]+, 197 (14), 196 (100) [iBuAl-
N{CH(Me)-CH2}2CH2]+, 98 (24) [H-CC-SiMe3]+.
3
2
CH2-CHMe2), 0.39 and 0.26 (each 2 H, dd, JH−H = 7.6 Hz, JH−H
=
13.9 Hz, Al-CH2), 0.25 (9 H, s, SiMe3). 13C NMR (C6D6, 100 MHz):
δ 161.5 (br, Al-CC), 148.3 (Al-CC), 53.2 (N-CH2-CH2-N-CH3),
52.1 (N-CH2-CH2-N-CH3), 45.2 (N-CH3), 28.53 and 28.46 (Al-CH2-
CHMe2), 26.7 (Al-CH2-CH), 23.1 (br, Al-CH2), −0.5 (SiMe3). 29Si
NMR (C6D6, 79 MHz): δ −10.6. IR (CsI plates, paraffin, cm−1): 1599
vs, 1549 vs ν(CC); 1462 vs (paraffin); 1400 m δ(CH3); 1366 vs
(paraffin); 1315 vs, 1290 vs, 1246 vs δ(CH3); 1202 m, 1155 vs, 1107 s,
1057 vs, 1009 s, 978 s, 962 s ν(CC), ν(CN); 910 w, 841 vs, 754 s
ρ(CH3Si); 727 s (paraffin); 632 vs ν(SiC); 575 w, 523 s, 500 w, 478 w,
447 s ν(AlC), ν(AlN). MS (EI, 20 eV, 25 °C): m/z (%) 338 (0.1)
[M]+, 283 (7), 282 (30), 281 (65) [M − iBu]+, 199 (15), 198 (100)
[Me3Si-C(H)C(H)-N(CH2CH2)2NMe]+ (hydrolysis?), 184 (13),
183 (100) [iBuAl-N(CH2CH2)2NMe]+, 98 (7) [H-CC-SiMe3]+.
Synthesis of the Phenylethyne Adduct 7. Compound 5 (0.950
g, 2.70 mmol) was dissolved in 30 mL of cyclopentane and treated
with phenylethyne (0.297 mL, 0.276 g, 2.70 mmol) at room
temperature. Stirring was continued for 2 h. Colorless crystals were
obtained upon concentration of the reaction mixture and cooling to 2
°C. Yield: 1.216 g (99%); a temperature-dependent equilibrium
between 7 and its constituents was observed in solution. Mp (argon,
sealed capillary): 104 °C. Anal. Calcd for C28H48AlNSi (453.3): C,
S y n t h e s i s o f C o m p o u n d 4 . 2 , 6 - D i m e t h y l - 1 -
(trimethylsilylethynyl)piperidine (1) (0.270 mL, 0.233 g, 1.11
mmol) was dissolved in 5 mL of n-hexane and treated with
HAl(CH2tBu)2 (0.189 g, 1.11 mmol) in 10 mL of n-hexane at room
temperature. Stirring was continued for 1 h. Colorless crystals of 4
were obtained upon concentration of the reaction mixture and cooling
to −28 °C. Yield: 0.272 g (65%). Mp (argon, sealed capillary): 55 °C.
Anal. Calcd for C22H46AlNSi (379.3): C, 69.7; H, 12.2; N, 3.7. Found:
1
C, 69.3; H, 12.0; N, 3.7. H NMR (C6D6, 400 MHz): δ 6.84 (1 H, s,
br, N-CHC), 3.20 (2 H, m, br, N-CH-CH3), 1.54 and 1.12 (2 H, m,
N-CH-CH2-CH2), 1.31 (18 H, s, CMe3), 0.94 (8 H, m, br, N-CH-CH3
2
and N-CH-CH2-CH2), 0.68 and 0.49 (4 H, d, JH−H = 13.8 Hz, Al-
CH2), 0.31 (9 H, s, SiMe3). 13C NMR (C6D6, 100 MHz): δ 164.3 (br,
Al-CC), 147.9 (Al-CC), 54.5 (br, N-CH-CH3), 35.5 (Al-CH2-
CMe3), 32.4 (br, Al-CH2), 32.2 (Al-CH2-C), 30.6 (N-CH-CH2-CH2),
16.0 (N-CH-CH3 and N-CH-CH2-CH2), 0.6 (SiMe3). 29Si NMR
(C6D6, 79 MHz): δ −9.8. IR (CsI plates, paraffin, cm−1): 1585 m,
1537 s ν(CC); 1460 vs, 1377 s (paraffin); 1320 vw, 1310 vw, 1242 s
δ(CH3); 1227 s, 1181 vw, 1123 s, 1098 m, 1022 vs, 976 m ν(CC),
ν(CN); 885 w, 837 s, 752 m ρ(CH3Si); 721 (paraffin); 685 m
νas(SiC); 617 m νs(SiC); 579 vw, 554 vw, 521 w, 494 m, 463 vs, 417 m
ν(AlC), ν(AlN). MS (EI, 20 eV, 25 °C): m/z (%) 380 (0.2), 379 (1)
[M]+; 310 (6), 309 (24), 308 (100) [M − CH2tBu]+, 212 (2), 211
(18), 210 (45) [M − tBu − N{CH(Me)-CH2}2CH2]+.
1
74.2; H, 10.7; N, 3.1. Found: C, 73.4; H, 10.5; N, 3.0. H NMR
(C6D5-CD3, 400 MHz, 280 K): δ 10.43 (1 H, br, s, NH), 7.33 (2 H,
pseudo-d, o-H of phenyl), 6.99 (2 H, m, m-H of phenyl), 6.96 (1 H, m,
p-H of phenyl), 5.43 (1 H, d, 3JH−H = 7.3 Hz, N-CHC), 1.64 (2 H,
m, N-CH-CH3), 1.47 (18 H, s, Al-CMe3), 1.23 (6 H, br, N-CH-CH3),
1.21 and 0.99 (2 H, m, N-CH-CH2-CH2), 0.94 and 0.63 (1 H, m, N-
CH-CH2−CH2), 0.39 (9 H, s, SiMe3). 13C NMR (C6D5-CD3, 100
MHz, 280 K): δ 162.8 (br, Al-CC), 138.0 (Al-CC), 131.8 (o-C of
phenyl), 128.6 (m-C of phenyl), 127.5 (p-C of phenyl), 126.1 (ipso-C
of phenyl), 118.1 (br, Al-CC), 116.1 (Al-CC), 60.5 (N-CH-CH3),
33.6 (Al-CMe3), 31.0 (N-CH-CH2-CH2), 22.3 (N-CH-CH2-CH2),
20.3 (N-CH-CH3), 16.1 (br, Al-CMe3), 1.8 (SiMe3). 29Si NMR (C6D5-
CD3, 79 MHz, 280 K): δ −3.6. IR (CsI plates, paraffin, cm−1): 3319 w,
3179 w ν(NH); 2922 vs, 2851 vs (paraffin); 2361 m, 2342 m ν(C
C); 1884 w, 1593 vs, 1560 vs ν(CC), phenyl; 1462 vs (paraffin);
1400 m δ(CH3); 1377 s (paraffin); 1352 m, 1306 m, 1244 s δ(CH3);
1207 s, 1169 s, 1115 s, 1069 m, 1051 m, 1024 m, 1003 m, 988 w, 962
m ν(CC), ν(CN); 932 m, 918 w, 897 m, 878 m, 847 m, 829 m, 812 m,
785 w, 744 m 754 s ρ(CH3Si); 723 s (paraffin); 694 s νas(SiC); 646 w,
625 w νs(SiC); 561 vs, 538 vs, 486 vs, 449 vs, 438 s, 410 m ν(AlC),
ν(AlN). MS (EI, 20 eV, 60 °C): m/z (%) 296 (3), 295 (16), 294 (66)
[M − PhCCH − tBu]+, 197 (13), 196 (100) [iBuAl-N{CH(Me)-
CH2}2CH2]+, 102 (37) [Ph-CC-H].
S y n t h e s i s o f C o m p o u n d 5 . 2 , 6 - D i m e t h y l - 1 -
(trimethylsilylethynyl)piperidine (1) (2.181 mL, 1.876 g, 8.97
mmol) was dissolved in 20 mL of n-hexane and treated dropwise
(30 min) with a solution of HAltBu2 (1.274 g, 8.97 mmol) in 70 mL of
n-hexane at room temperature. Stirring was continued for 2 h.
Colorless crystals were obtained upon concentration of the reaction
mixture and cooling to −15 °C. Yield: 2.653 g (84%). Mp (argon,
sealed capillary): 121 °C. Anal. Calcd for C20H42AlNSi (351.3): C,
68.4; H, 12.1; N, 4.0. Found: C, 68.2; H, 11.8; N, 4.0. 1H NMR (C6D6,
400 MHz): δ 7.12 (1 H, s, 3JH−Si = 6.4 Hz, N-CHC), 3.36 (2 H, m,
N-CH-CH3), 1.62 and 1.06 (2 H, m, N-CH-CH2-CH2), 1.35 and 1.05
(each 1 H, m, N-CH-CH2-CH2), 1.27 (18 H, s, CMe3), 0.94 (6 H, d,
3JHH = 7,2 Hz, N-CH-CH3), 0.27 (9 H, s, SiMe3). 13C NMR (C6D6,
100 MHz): δ 160.8 (br, Al-CC), 148.1 (Al-CC), 54.1 (N-CH-
CH3), 32.3 (CMe3), 29.3 (N-CH-CH2-CH2), 18.0 (N-CH-CH3), 17.6
(br, CMe3), 12.7 (N-CH-CH2-CH2), 0.10 (SiMe3). 29Si NMR (C6D6,
79 MHz): δ −10.4. IR (CsI plates, paraffin, cm−1): 1572 m, 1541 s
ν(CC); 1464 vs, 1371 vs (paraffin); 1319 m, 1283 w, 1242 δ(CH3);
1174 m, 1128 s, 1091 m, 1061 m, 1020 s, 1001 s ν(CC), ν(CN); 939
sh, 887 m, 837 vs, 806 s, 748 s ρ(CH3Si); 719 vs (paraffin); 685 m
νas(SiC); 619 m νs(SiC); 575 s, 559 s, 498 s, 449 s, 420 s ν(AlC),
ν(AlN). MS (EI, 20 eV, 25 °C): m/z (%) 296 (4), 295 (14), 294 (69)
[M − tBu]+, 197 (11), 196 (100) [iBuAl-N{CH(Me)-CH2}2CH2]+,
98 (2) [H-CC-SiMe3]+.
Synthesis of the Carbodiimide Insertion Product 8. The
hydroalumination product 3 was generated in situ by the treatment of
a solution of 2,6-dimethyl-1-(trimethylsilylethynyl)piperidine (1)
(0.50 mL, 0.430 g, 2.06 mmol) in 15 mL of n-hexane with neat
HAliBu2 (0.365 mL, 0.292 g, 2.06 mmol) at room temperature.
Stirring was continued for 2 h, and solid dicyclohexylcarbodiimide
(0.424 g, 2.06 mmol) was added. The mixture was concentrated after 2
h at room temperature. Colorless crystals were obtained upon cooling
of the solution to −45 °C. Yield: 0.850 g (74%). Mp (argon, sealed
capillary): 94 °C. Anal. Calcd for C33H64AlN3Si (557.5): C, 71.1; H,
1
11.6; N, 7.5. Found: C, 70.6; H, 11.4; N, 7.6. H NMR (C6D6, 400
MHz): δ 5.99 (1 H, s, N-CHC), 3.57 (2 H, m, N-CH of piperidine),
3.17 (2 H, m, N-CH of cyclohexyl), 2.31 and 2.22 (each 1 H, septet,
3JH−H = 6.7 Hz, Al-CH2-CH), 1.95 and 1.35 (each 4 H, m, N-CH-CH2
of cyclohexyl), 1.72 and 1.26 (each 4 H, m, N-CH-CH2-CH2 of
cyclohexyl), 1.53 and 1.11 (each 2 H, m, N-CH-CH2-CH2−CH2 of
cyclohexyl), 1.46 and 1.13 (each 2 H, m, N-CH-CH2-CH2 of
piperidine), 1.45 and 1.25 (each 2 H, m, N-CH-CH2 of piperidine),
1.33 and 1.30 (each 6 H, d, 3JH−H = 6.4 Hz, CH2-CHMe2), 1.03 (6 H,
d, 3JHH = 7,1 Hz, N-CH-CH3), 0.51 and 0.40 (each 2 H, d, 3JH−H = 7.1
Synthesis of Compound 6. 1-Methyl-4-(trimethylsilylethynyl)-
piperazine (2) (0.68 mL, 0.592 g, 3.02 mmol) was dissolved in 10 mL
of n-hexane and treated dropwise with a solution of HAliBu2 (1 M,
0.365 mL, 0.429 g, 3.02 mmol) in 5 mL of n-hexane at room
temperature. Stirring was continued for 3 h. A colorless, waxy solid
with some crystalline material embedded was obtained upon
concentration of the reaction mixture and cooling to −28 °C. Yield:
0.927 g (91%); highly oxygen- and water-sensitive compound. Anal.
3280
dx.doi.org/10.1021/om3001179 | Organometallics 2012, 31, 3272−3283

Organometallics
Article
Hz, Al-CH2), 0.28 (9 H, s, SiMe3). 13C NMR (C6D6, 100 MHz): δ
177.4 (N-C-N), 146.1 (N-CC), 90.4 (N-CC), 54.4 (N-CH of
cyclohexyl), 51.4 (N-CH-CH3), 37.0 and 36.8 (N-CH-CH2 of
cyclohexyl), 30.9 (N-CH-CH2 of piperidine), 29.3 (Al-CH2-CHMe2),
27.23 and 27.17 (Al-CH2-CH), 26.3 and 26.0 (N-CH-CH2-CH2 of
cyclohexyl), 26.1 (N-CH-CH2-CH2-CH2 of cyclohexyl), 24.1 and 23.9
(Al-CH2), 21.4 (N-CH-CH3), 14.1 (N-CH-CH2-CH2), 1.1 (SiMe3).
29Si NMR (C6D6, 79 MHz): δ −4.2. IR (CsI plates, paraffin, cm−1):
1651 w, 1584 vs, br ν(CC), ν(CN2); 1460 vs, 1375 s (paraffin);
1366 sh, 1342 w, 1312 w, 1290 w, 1248 m δ(CH3); 1211 m, 1171 w,
1152 m, 1117 m, 1090 w, 1053 s, 1018 m, 982 m ν(CC), ν(CN); 945
w, 891 m, 839 s, 754 m ρ(CH3Si); 719 w (paraffin); 694 m, 669 m
νas(SiC); 644 m, 619 m νs(SiC); 596 w, 563 w, 519 m, 451 m, 432 w
ν(AlC), ν(AlN). MS (EI, 20 eV, 80 °C): m/z (%) 502 (38), 500 (100)
[M − iBu]+, 444 (1), 445 (4) [M − iBu − butene]+.
C31H61AlN4Si (544.4): C, 68.4; H, 11.3; N, 10.3. Found: C, 68.2; H,
11.0; N, 10.2. 1H NMR (C6D6, 400 MHz): δ 5.88 (1 H, s N-CHC),
3.19 (2 H, m, N-CH of cyclohexyl), 3.05 (2 H, m, Al-N-CH2 of
piperazine), 2.29 and 2.22 (each 1 H, septet, 3JH−H = 6.7 Hz, Al-CH2-
CH), 2.11 (4 H, m, Al-N-CH2-CH2 of piperazine), 1.99 (3 H, s, N-
CH3), 1.94 and 1.38/1.91 and 1.40 (each 2 H, m, N-CH-CH2 of
cyclohexyl), 1.67 and 1.22 (each 4 H, m, N-CH-CH2-CH2 of
cyclohexyl), 1.50 and 1.07 (each 2 H, m, N-CH-CH2-CH2-CH2 of
3
cyclohexyl), 1.32 and 1.29 (each 6 H, d, JH−H = 6.7 Hz, Al-CH2-
CHMe2), 0.50 and 0.46 (each 2 H, d, 3JH−H = 7.1 Hz, Al-CH2), 0.25 (9
H, s, SiMe3). 13C NMR (C6D6, 100 MHz): δ 177.3 (N-C-N), 145.2
(N-CC), 92.1 (N-CC), 55.3 (Al-N-CH2-CH2 of piperazine), 54.2
(N-C of cyclohexyl), 49.3 (Al-N-CH2 of piperazine), 46.1 (N-CH3),
36.9 and 36.3 (N-CH-CH2 of cyclohexyl), 29.21 and 29.19 (Al-CH2-
CHMe2), 27.18 and 27.15 (Al-CH2-CH), 26.11 and 26.06 (N-CH-
CH2-CH2), 26.0 (N-CH-CH2-CH2-CH2), 23.9 and 23.2 (Al-CH2), 0.7
(SiMe3). 29Si NMR (C6D6, 79 MHz): δ −4.8. IR (CsI plates, paraffin,
cm−1): 1597 vs, br, 1557 sh ν(CC), ν(CN2); 1460 vs, 1371 vs
(paraffin); 1292 s, 1250 s δ(CH3); 1225 w, 1200 s, 1165 s, 1140 s,
1070 m, 1030 s, 1009 vs ν(CC), ν(CN); 939 m, 891 m, 837 vs, 754 m
ρ(CH3Si); 669 s νas(SiC); 637 s νs(SiC); 523 m, 496 w, 451 m
ν(AlC), ν(AlN). MS (EI, 20 eV, 60 °C): m/z (%) 490 (3), 488 (36)
[M − butene]+, 472 (1) [M − iBu − CH3]+, 332 (16), 331 (61) [M −
SiMe3 − cHex − iBu and M − 2iBu − N(CH2CH2)2NMe]+, 306 (34),
305 (100) [M − AliBu2 − cHex − Me]+.
Synthesis of the Carbodiimide Insertion Product 9. The
hydroalumination product 6 was generated in situ. 1-Methyl-4-
(trimethylsilyl)ethynylpiperazine (0.25 mL, 0.221 g, 1.13 mmol) was
dissolved in 10 mL of n-hexane and treated dropwise with neat
HAliBu2 (0.20 mL, 0.160 g, 1.13 mmol) at room temperature. Stirring
was continued for 1 h, and solid dicyclohexylcarbodiimide (0.232 g,
1.13 mmol) was added. Stirring was continued for 30 min, and
compound 9 was obtained upon concentration of the reaction mixture
and cooling to −15 °C. Yield: 0.375 (61%). Mp (argon, sealed
capillary): 98 °C. Anal. Calcd for C31H61AlN4Si (544.4): C, 68.4; H,
11.3; N, 10.3. Found: C, 67.6; H, 11.0; N, 10.1. 1H NMR (C6D5-CD3,
400 MHz): δ 5.64 (1 H, s, N-CHC), 3.75 (1 H, m, CN-CH of
cyclohexyl), 3.49 and 2.91/2.98 and 2.43 (each 1 H, Al-N-CH2 of
piperazine), 3.34 (1 H, m, C-N-CH of cyclohexyl), 2.43 and 1.96/2.39
and 2.07 (each 1 H, m, CN-CH-CH2 of cyclohexyl), 2.15 and 2.07
(each 1 H, m, Al-CH2-CH), 2.00 and 1.88/2.00 and 1.81 (each 1 H, m,
Al-N-CH2-CH2 of piperazine), 1.93 and 1.39 (each 1 H, m, CN-
CH-CH2-CH2-CH2 of cyclohexyl), 1.88 and 1.37/1.87 and 1.69 (1 H,
m, CN-CH-CH2-CH2 of cyclohexyl), 1.87 and 1.69/1.58 and 1.37
(each 1 H, m, C-N-CH-CH2-CH2), 1.81 (3 H, s, N-CH3), 1.81 and
1.37 (each 1 H, m, C-N-CH-CH2-CH2-CH2 of cyclohexyl), 1.70 and
1.36 (each 2 H, m, C-N-CH-CH2 of cyclohexyl), 1.21 and 1.20 (each 6
H, d, 3JH−H = 6.2 Hz, Al-CH2-CHMe2), 0.45 and 0.14/0.12 and −0.01
(each 1 H, m, Al-CH2), 0.29 (9 H, s, SiMe3); unambiguous assignment
was difficult owing to the overlap of resonances and the complicated
splitting patterns. 13C NMR (C6D5-CD3, 100 MHz): δ 156.4 (N-C-N),
138.6 (N-C(H)-C(Si)), 138.0 (N-C(H)C(Si)), 56.9 (C-N-CH of
cyclohexyl), 56.4 (CN-CH of cyclohexyl), 51.54 and 51.46 (Al-N-
CH2 of piperazine), 51.1 and 49.8 (Al-N-CH2-CH2), 45.7 (N-CH3),
36.9 and 36.5 C-N-CH-CH2 of cyclohexyl), 33.8 and 31.8 (CN-CH-
CH2 of cyclohexyl), 29.0 and 28.7 (Al-CH2-CHMe3), 27.7 and 27.1
(CN-CH-CH2-CH2 of cyclohexyl), 27.1 and 26.7 (C-N-CH-CH2-
CH2 of cyclohexyl), 27.0 and 26.8 (Al-CH2-CH), 25.3 (C-N-CH-CH2-
CH2-CH2 of cyclohexyl), 24.9 (CN-CH-CH2-CH2-CH2 of cyclo-
hexyl), 23.0 and 22.1 (Al-CH2), 0.5 (SiMe3). 29Si NMR (C6D5-CD3, 79
MHz): δ −4.1. IR (CsI plates, paraffin, cm−1): 1595 vs, br, 1551 s
ν(CC); 1464 vs, 1375 vs (paraffin); 1315 m, 1292 s, 1248 vs
δ(CH3); 1225 m, 1200 m, 1153 s, 1117 s, 1093 m, 1061 s, 1034 s,
1011 vs, 986 s, 970 m ν(CC), ν(CN); 951 m, 907 m, 889 s, 839 vs,
797 m, 775 w, 756 m ρ(CH3Si); 718 m (paraffin); 694 m, 665 m
νas(SiC); 633 s νs(SiC); 588 w, 557 w, 523 s, 496 m, 478 w, 451 m,
437 m ν(AlC), ν(AlN). MS (EI, 20 eV, 40 °C): m/z (%) 489 (4), 488
(12), 487 (40) [M − butyl]+, 334 (83), 335 (21), 336 (4) [M+ −
cyclohexene − butyl − butene − CH3], 333 (3), 332 (13), 331 (55)
[M − SiMe3 − cHex − butyl and M − 2butyl − N(CH2CH2)2NMe]+,
307 (9), 306 (32), 305 (100) [M − AliBu2 − cHex − Me]+.
Crystal Structure Determinations. Single crystals were obtained
by cooling of the reaction mixtures as described above. The
crystallographic data were collected with a Bruker APEX diffrac-
tometer. The structures were solved by direct methods and refined
with the program SHELXL-9730 by a full-matrix least-squares method
based on F2. Hydrogen atoms were positioned geometrically and
allowed to ride on their respective parent atoms. Only the position of
the N−H hydrogen atom of compound 7 was taken from a difference
Fourier map and refined with isotropic U. The molecules of
compounds 3 and 4 are localized on a crystallographic mirror plane.
An isobutyl group of 6 was disordered; the atoms were refined on split
positions with occupation factors of 0.86:0.14. An isobutyl group of 8
was disordered (C121); its atoms were refined on split positions
(0.73:0.27). Further details of the crystal structure determinations are
available from the Cambridge Crystallographic Data Center on
quoting the depository numbers CCDC-865846 (3), -865847 (4),
-865848 (5), -865849 (6), -865850 (7), -865851 (8), and -865852 (9).
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files giving the crystal data for compounds 3 to 9. Results
of quantum chemical calculations including optimized
Cartesian coordinates (B97-D/def2-TZVP, M06-2x/def2-
TZVP, and B3LYP/6-311+G(d,p) and single-point energies
at the level B2PLYP-D/def2-QZVP//B97-D/def2-TZVP incl.
ZPE). This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Fax: +49-251-8336660. E-mail: uhlw@uni-muenster.de;
wurthwe@uni-muenster.de.
Rearrangement of 9; Synthesis of 10. The intermediates 6 and
9 were generated in situ. 1-Methyl-4-(trimethylsilyl)ethynylpiperazine
(1.0 mL, 0.87 g, 4.44 mmol) was dissolved in 35 mL of n-hexane and
treated dropwise with neat HAliBu2 (0.787 mL, 0.630 g, 4.44 mmol) at
room temperature. Stirring was continued for 2 h, and solid
dicyclohexylcarbodiimide (0.914 g, 4.44 mmol) was added. Stirring
was continued for 2 d. Compound 10 was obtained upon
concentration of the reaction mixture and cooling to −28 °C. Yield:
2.162 g (89%). Mp (argon, sealed capillary): 98 °C. Anal. Calcd for
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the Deutsche Forschungsgemeinschaft (SFB
858) and the Fonds der Chemischen Industrie (Frankfurt) for
generous financial support.
3281
dx.doi.org/10.1021/om3001179 | Organometallics 2012, 31, 3272−3283

Organometallics
Article
(9) Appelt, C.; Westenberg, H.; Bertini, F.; Ehlers, A. W.; Slootweg, J.
C.; Lammertsma, K.; Uhl, W. Angew. Chem. 2011, 123, 4011−4014;
Angew. Chem., Int. Ed. 2011, 50, 3925−3928.
REFERENCES
■
(1) (a) Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. Angew. Chem.
2010, 122, 1444−1447; Angew. Chem., Int. Ed. 2010, 49, 1402−1405.
(b) Ullrich, M.; Lough, A. J.; Stephan, D. W. Organometallics 2010, 29,
(10) Uhl, W. Coord. Chem. Rev. 2008, 252, 1540−1563.
(11) (a) Uhl, W.; Bock, H. R.; Claesener, M.; Layh, M.; Tiesmeyer,
́
3647−3654. (c) Rokob, T. A.; Hamza, A.; Papai, I. J. Am. Chem. Soc.
I.; Wurthwein, E.-U. Chem.Eur. J. 2008, 14, 11557−11564. (b) Uhl,
̈
2009, 131, 10701−10710. (d) Jiang, C.; Blacque, O.; Berke, H.
Organometallics 2010, 29, 125−133. (e) McCahill, J. S. J.; Welch, G.
C.; Stephan, D. W. Angew. Chem. 2007, 119, 5056−5059; Angew.
W.; Claesener, M.; Haddadpour, S.; Jasper, B.; Tiesmeyer, I.; Zemke,
S. Z. Anorg. Allg. Chem. 2008, 634, 2889−2896. (c) Uhl, W.;
Claesener, M.; Hepp, A.; Jasper, B.; Vinogradov, A.; van Wullen, L.;
̈
Chem., Int. Ed. 2007, 46, 4968−4971. (f) Momming, C. M.; Otten, E.;
̈
Koster, T. K.-J. Dalton Trans. 2009, 10550−10562.
̈
Kehr, G.; Frohlich, R.; Grimme, S.; Stephan, D. W.; Erker, G. Angew.
̈
(12) Himbert, G.; Naßhahn, H.; Gerulat, O. Synthesis 1999, 293−
294.
Chem. 2009, 121, 6770−6773; Angew. Chem., Int. Ed. 2009, 48, 6643−
6646. (g) Berkefeld, A.; Piers, W. E.; Parvez, M. J. Am. Chem. Soc.
2010, 132, 10660−10661. (h) Ashley, A. E.; Thompson, A. L.; O’Hare,
D. Angew. Chem. 2009, 121, 10023−10027; Angew. Chem., Int. Ed.
2009, 48, 9839−9843. (i) Otten, E.; Neu, R. C.; Stephan, D. W. J. Am.
Chem. Soc. 2009, 131, 9918−9919. (j) Neu, R. C.; Otten, E.; Stephan,
D. W. Angew. Chem. 2009, 121, 9889−9892; Angew. Chem., Int. Ed.
2009, 48, 9709−9712. (k) Bertini, F.; Lyaskovskyy, V.; Timmer, B. J.
J.; de Kanter, F. J. J.; Lutz, M.; Ehlers, A. W.; Slootweg, J. C.;
Lammertsma, K. J. Am. Chem. Soc. 2012, 134, 201−204. (l) Marwitz,
A. J. V.; Dutton, J. L.; Mercier, L. G.; Piers, W. E. J. Am. Chem. Soc.
2011, 133, 10026−10029.
(13) (a) Hill, J. B.; Eng, S. J.; Pennington, W. T.; Robinson, G. H. J.
Organomet. Chem. 1993, 445, 11−11. (b) Gardiner, M. G.; Raston, C.
L.; Cloke, F. G. N.; Hitchcock, P. B. Organometallics 1995, 14, 1339−
1353. (c) Mitzel, N. W.; Lustig, C. Z. Naturforsch. 2004, 59b, 1532−
1539. (d) Venugopal, A.; Kamps, I.; Bojer, D.; Berger, R. J. F.; Mix, A.;
Willner, A.; Neumann, B.; Stammler, H.-G.; Mitzel, N. W. Dalton
Trans. 2009, 5755−5765. (e) Trepanier, S. J.; Wang, S. Organometallics
1994, 13, 2213−2217. (f) Schumann, H.; Wassermann, B. C.; Schutte,
S.; Heymer, B.; Nickel, S.; Seuß, T. D.; Wernik, S.; Demtschuk, J.;
Girgsdies, F.; Weimann, R. Z. Anorg. Allg. Chem. 2000, 626, 2081−
2095.
(14) (a) Uhl, W.; Kovert, D.; Layh, M.; Hepp, A. Chem.Eur. J.
(2) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W.
Science 2006, 314, 1124−1126.
2011, 17, 13553−13561. (b) Uhl, W.; Claesener, M.; Kovert, D.;
Hepp, A.; Wurthwein, E.-U.; Ghavtadze, N. Organometallics 2011, 30,
̈
(3) (a) Stephan, D. W.; Erker, G. Angew. Chem. 2010, 122, 50−81;
Angew. Chem. Int. Ed 2010, 49, 46−76. (b) Stephan, D. W. Dalton
Trans. 2009, 3129−3136. (c) Stephan, D. W. Org. Biomol. Chem. 2008,
6, 1535−1539. (d) Erker, G. C. R. Chim. 2011, 14, 831−841.
(4) (a) Holschumacher, D.; Bannenberg, T.; Hrib, C. G.; Jones, P.
G.; Tamm, M. Angew. Chem. 2008, 120, 7538−7542; Angew. Chem.,
Int. Ed. 2008, 47, 7428−7432. (b) Chase, P. A.; Stephan, D. W. Angew.
Chem. 2008, 120, 7543−7547; Angew. Chem., Int. Ed. 2008, 47, 7433−
7437. (c) Kronig, S.; Theuergarten, E.; Holschumacher, D.;
Bannenberg, T.; Daniliuc, C. G.; Jones, P. G.; Tamm, M. Inorg.
3075−3082. (c) Uhl, W.; Rohling, M.; Wurthwein, E.-U.; Ghavtadze,
̈
N.; Bergander, K. Organometallics 2010, 29, 5236−5240. (d) Uhl, W.;
Matar, M. J. Organomet. Chem. 2002, 664, 110−115.
(15) Uhl, W.; Bock, H. R.; Breher, F.; Claesener, M.; Haddadpour, S.;
Jasper, B.; Hepp, A. Organometallics 2007, 26, 2363−2369.
(16) (a) Uhl, W.; Claesener, M. Inorg. Chem. 2008, 47, 729−735.
(b) Uhl, W.; Claesener, M. Inorg. Chem. 2008, 47, 4463−4470.
(c) Uhl, W.; Claesener, M.; Hepp, A. Organometallics 2008, 27, 2118−
2122. (d) Uhl, W.; Heller, D.; Rohling, M.; Kosters, J. Inorg. Chim.
Acta 2011, 374, 359−365.
(17) (a) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.; Frohlich, R.;
̈
Chem. 2011, 50, 7344−7359. (d) Theuergarten, E.; Schluns, D.;
̈
̈
Grunenberg, J.; Daniliuc, C. G.; Jones, P. G.; Tamm, M. Chem.
Commun. 2010, 8561−8563. (e) Holschumacher, D.; Bannenberg, T.;
Ibrom, K.; Daniliuc, C. G.; Jones, P. G.; Tamm, M. Dalton Trans.
2010, 39, 10590−10592. (f) Alcarazo, M.; Gomez, C.; Holle, S.;
Goddard, R. Angew. Chem. 2010, 122, 5924−5927; Angew. Chem., Int.
Ed. 2010, 49, 5788−5791.
Grimme, S.; Stephan, D. W. Chem. Commun. 2007, 5072−5074. For
a similar B−N compound see: (b) Schwendemann, S.; Frohlich, R.;
̈
Kehr, G.; Erker, G. Chem. Sci. 2011, 2, 1842−1849.
(18) (a) Uhl, W.; Er, E.; Matar, M. Main Group Chem. 2006, 5, 31−
40. (b) Uhl, W.; Er, E.; Hubner, O.; Himmel, H.-J. Z. Anorg. Allg.
̈
Chem. 2008, 634, 2133−2139. (c) Cui, C.; Schmidt, J. A. R.; Arnold, J.
J. Chem. Soc., Dalton Trans. 2002, 2992−2994. (d) Schiefer, M.;
Reddy, N. D.; Ahn, H.-J.; Stasch, A.; Roesky, H. W.; Schlicker, A. C.;
Schmidt, H.-G.; Noltemeyer, M.; Vidovic, D. Inorg. Chem. 2003, 42,
4970−4976. (e) Schiefer, M.; Hatop, H.; Roesky, H. W.; Schmidt, H.-
G.; Noltemeyer, M. Organometallics 2002, 21, 1300−1303. (f) Zhu,
H.; Chai, J.; Fan, H.; Roesky, H. W.; He, C.; Jancik, V.; Schmidt, H.-
G.; Noltemeyer, M.; Merrill, W. A.; Power, P. P. Angew. Chem. 2005,
117, 5220−5223; Angew. Chem., Int. Ed. 2005, 44, 5090−5093.
(g) Zhu, H.; Oswald, R. B.; Fan, H.; Roesky, H. W.; Ma, Q.; Yang, Z.;
Schmidt, H.-G.; Noltemeyer, M.; Starke, K.; Hosmane, N. S. J. Am.
Chem. Soc. 2006, 128, 5100−5108.
(19) (a) Kiely, A. F.; Nelson, C. M.; Pastor, A.; Henling, L. M.; Day,
M. W.; Bercaw, J. E. Organometallics 1998, 17, 1324−1332.
(b) Dewhurst, R. D.; Hill, A. F.; Smith, M. K. Angew. Chem. 2004,
116, 482−484; Angew. Chem., Int. Ed. 2004, 43, 476−478.
(c) Viswamitra, M. A; Radharkrishnan, R.; Bandekar, J.; Desiraju, G.
R. J. Am. Chem. Soc. 1993, 115, 4868−4869. (d) Steiner, T. Angew.
Chem. 2002, 114, 50−80; Angew. Chem., Int. Ed. 2002, 41, 48−76.
(e) Desiraju, G. R. Acc. Chem. Res. 1996, 29, 411−449. (f) Braga, D.;
Grepioni, F.; Desiraju, G. Chem. Rev. 1998, 98, 1375−1405.
(5) Dureen, M. A.; Brown, C. C.; Stephan, D. W. Organometallics
2010, 29, 6594−6607.
(6) (a) Sumerin, V.; Schulz, F.; Nieger, M.; Leskela, M.; Repo, T.;
̈
Rieger, B. Angew. Chem. 2008, 120, 6090−6092; Angew. Chem., Int. Ed.
2008, 47, 6001−6003. (b) Sumerin, V.; Schulz, F.; Atsumi, M.; Wang,
C.; Nieger, M.; Leskela, M.; Repo, T.; Pyykko, P.; Rieger, B. J. Am.
̈
̈
Chem. Soc. 2008, 130, 14117−14119. (c) Geier, S. J.; Stephan, D. W. J.
Am. Chem. Soc. 2009, 131, 3476−3477. (d) Geier, S. J.; Gille, A. L.;
Gilbert, T. M.; Stephan, D. W. Inorg. Chem. 2009, 48, 10466−10474.
(e) Chase, P. A.; Welch, G. C.; Jurca, T.; Stephan, D. W. Angew. Chem.
2007, 119, 8196−8199; Angew. Chem., Int. Ed. 2007, 46, 8050−8053.
(f) Geier, S. J.; Gille, A. L.; Gilbert, T. M.; Stephan, D. W. Inorg. Chem.
2009, 48, 10466−10474. (g) Geier, S. J.; Chase, P. A.; Stephan, D. W.
Chem. Commun. 2010, 46, 4884−4886. (h) Geier, S. J.; Dureen, M. A.;
Ouyang, E. Y.; Stephan, D. W. Chem.Eur. J. 2010, 16, 988−993.
(7) (a) Dureen, M. A.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131,
́
8396−8397. (b) Menard, G.; Stephan, D. W. J. Am. Chem. Soc. 2010,
132, 1796−1797. (c) Zhang, Y.; Miyake, G. M.; Chen, E. Y.-X. Angew.
Chem. 2010, 122, 10356−10360; Angew. Chem., Int. Ed. 2010, 49,
10158−10162. (d) Menard, G.; Stephan, D. W. Angew. Chem. 2011,
123, 8546−8549; Angew. Chem Int. Ed. 2011, 50, 8396−8399.
(e) Boudreau, J.; Courtemanche, M.-A.; Fontaine, F.-G. Chem.
Commun. 2011, 47, 11131−11133.
(20) (a) Rowley, C. N.; DiLabio, G. A.; Barry, S. T. Inorg. Chem.
2005, 44, 1983−1991. (b) Lechler, R.; Hausen, H.-D.; Weidlein, J. J.
Organomet. Chem. 1989, 359, 1−12. (c) Schmidt, J. A. R; Arnold, J.
Organometallics 2002, 21, 2306−2313. (d) Chang, C.-C.; Hsiung, C.-
S.; Su, H.-.L.; Srinivas, B.; Chiang, M. Y.; Lee, G.-H.; Wang, Y.
Organometallics 1998, 17, 1595−1601. (e) Coles, M. P.; Hitchcock, P.
(8) Westenberg, H.; Slootweg, J. C.; Hepp, A.; Kosters, J.; Roters, S.;
Ehlers, A. W.; Lammertsma, K.; Uhl, W. Organometallics 2010, 29,
1323−1330.
̈
3282
dx.doi.org/10.1021/om3001179 | Organometallics 2012, 31, 3272−3283

Organometallics
Article
B. Eur. J. Inorg. Chem. 2004, 2662−2672. (f) Coles, M. P.; Swenson, D.
C.; Jordan, R. F.; Young, V. G. Jr. Organometallics 1997, 16, 5183−
5194. (g) Kottmair-Maieron, D.; Lechler, R.; Weidlein, J. Z. Anorg.
Allg. Chem. 1991, 593, 111−123. (h) Ergezinger, C.; Weller, F.;
Dehnicke, K. Z. Naturforsch. 1988, 43b, 1621−1627. (i) Duchateau, R.;
Meetsma, A.; Teuben, J. H. J. Chem. Soc., Dalton Trans. 1996, 223−
224. (j) Coles, M. P.; Swenson, D. C.; Jordan, R. F.; Young, V. G. Jr.
Organometallics 1998, 17, 4042−4048. (k) Kenney, A. P.; Yap, G. P. A;
Richeson, D. S.; Barry, S. T. Inorg. Chem. 2005, 44, 2926−2933.
(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A.,
Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.;
Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.;
Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi,
J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J.
B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador,
̈
P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision A.1; Gaussian, Inc.: Wallingford, CT, 2009.
(22) TURBOMOLE V6.3, a development of University of Karlsruhe
and Forschungszentrum Karlsruhe GmbH, TURBOMOLE GmbH,
2010; available from http://www.turbomole.com.
(23) (a) Schwabe, T.; Grimme, S. Phys. Chem. Chem. Phys. 2007, 9,
3397−3406. (b) Grimme, S. J. Chem. Phys. 2006, 124, 034108.
(24) Grimme, S. J. Comput. Chem. 2006, 27, 1787−1799.
(25) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215−241.
(26) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7,
3297−3305; as taken from the EMSL basis set exchange (https://bse.
pnl.gov/bse/portal).
(27) (a) Chey, J.; Choe, H.-S.; Chook, Y.-M.; Jensen, E.; Seida, P. R.;
Francl, M. M. Organometallics 1990, 9, 2430−2436. (b) Bundens, J.
W.; Francl, M. M. Organometallics 1993, 12, 1608−1615. (c) Gropen,
O.; Haaland, A. Acta Chem. Scand., Ser. A 1982, 36, 435−438.
(d) Bundens, J. W.; Yudenfreund, J.; Francl, M. M. Organometallics
1999, 18, 3913−3920.
(28) Beachley, O. T. Jr.; Victoriano, L. Organometallics 1988, 7, 63−
67.
(29) Uhl, W.; Cuypers, L.; Graupner, R.; Molter, J.; Vester, A.;
Neumuller, B. Z. Anorg. Allg. Chem. 2001, 627, 607−614.
̈
(30) SHELXTL-Plus, REL. 4.1; Siemens Analytical X-Ray Instru-
ments Inc.: Madison, WI, 1990. Sheldrick, G. M. SHELXL-97, Program
for the Refinement of Structures; Universitat Gottingen: Germany, 1997.
̈
̈
3283
dx.doi.org/10.1021/om3001179 | Organometallics 2012, 31, 3272−3283