(N-Arylaminomethyl)pyridine-N-oxides: Synthesis and characterization of potential ligand systems and the formation of their N,O-chelate aluminum complexes

Article abstract of DOI:10.1016/j.jorganchem.2008.05.032

Pyridine-N-oxide-2-carbaldehyde (4a) was converted to the corresponding imine (5a) by treatment with 2,6-diisopropylaniline. Subsequent reduction with a sodium borohydride gave the corresponding (N-arylaminomethyl)pyridine-N-oxide derivative (6a). A series of analogous compounds was prepared starting from the respective (aldimino)quinoline-N-oxide (4b) or (ketimino)pyridine-N-oxide (10) systems. Deprotonation of the (aminomethyl)pyridine-N-oxides resulted in a series of unexpected reactions, such as coupling, internal redox reactions or fragmentation. Eventually, the N,O-chelate aluminum complexes (22, 23) derived from the (aminoethyl)pyridine-N-oxide ligand systems could be obtained by treatment of the respective iminopyridine-N-oxides with trimethylaluminum. Many products were characterized by X-ray diffraction.

Full text of DOI:10.1016/j.jorganchem.2008.05.032

Journal of Organometallic Chemistry 693 (2008) 3063–3073
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Journal of Organometallic Chemistry
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(N-Arylaminomethyl)pyridine-N-oxides: Synthesis and characterization of
potential ligand systems and the formation of their N,O-chelate aluminum
complexes
1
1
*
Katrin Nienkemper, Gerald Kehr, Seda Kehr , Roland Fröhlich , Gerhard Erker
Organisch-Chemisches Institut der Universität Münster, Corrensstrasse 40, 48149 Münster, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Pyridine-N-oxide-2-carbaldehyde (4a) was converted to the corresponding imine (5a) by treatment with
2,6-diisopropylaniline. Subsequent reduction with a sodium borohydride gave the corresponding (N-aryl-
aminomethyl)pyridine-N-oxide derivative (6a). A series of analogous compounds was prepared starting
from the respective (aldimino)quinoline-N-oxide (4b) or (ketimino)pyridine-N-oxide (10) systems.
Deprotonation of the (aminomethyl)pyridine-N-oxides resulted in a series of unexpected reactions, such
as coupling, internal redox reactions or fragmentation. Eventually, the N,O-chelate aluminum complexes
(22, 23) derived from the (aminoethyl)pyridine-N-oxide ligand systems could be obtained by treatment
of the respective iminopyridine-N-oxides with trimethylaluminum. Many products were characterized
by X-ray diffraction.
Received 15 January 2008
Received in revised form 9 May 2008
Accepted 26 May 2008
Available online 30 July 2008
Keywords:
Aluminum
Chelate ligands
Pyridine-N-oxides
Imines
Ó 2008 Elsevier B.V. All rights reserved.
Arylamines
1. Introduction
2. Results and discussion
Mono-anionic N,O-chelate ligands have found extensive use in
homogeneous catalysis. Important examples include the various
salicylaldiminato systems (A), derivatives of which have found
many practical catalytic applications ranging from polymerization
reactions [1,2] to asymmetric organic synthesis [3]. In a way the
(amidomethyl)pyridine-N-oxides (B) represent complementary
systems to the ubiquitous salicylaldiminates, where formally the
negatively charged center was moved from the oxy-arene site to
a side-chain nitrogen (see Scheme 1).
We had previously described the synthesis of pyridine-N-oxide
aldimines [4], the neutral analogues of the negatively charged sal-
icylaldiminates (A), and a series of their coordination compounds.
We have now investigated a variety of synthetic routes to systems
aimed at generating and using the ligands (B) in some straightfor-
ward fashion. This turned out to be difficult but was eventually
successful by making some respective N,O-chelate aluminum com-
plexes of examples of B readily available. Some of these develop-
ments will be described in this account.
2.1. The parent (aminomethyl)pyridine-N-oxide systems
The synthesis of the parent compound (6a, see Scheme 2)
started from commercially available 2-(hydroxymethyl)pyridine
(2a). It was treated with 30% aqueous H2O2 in glacial acetic acid
to yield the substituted pyridine-N-oxide (3a, 34% after recrystalli-
zation from methanol) [5]. Subsequent oxidation with SeO2 gave
the corresponding pyridine-N-oxide carbaldehyde (4a) [6] in close
to 90% yield. Acid-catalyzed condensation of 4a with 2,6-diisopro-
pylaniline then gave the respective aldimine derivative 5a which
was eventually reduced to the 2-(N-arylaminomethyl)pyridine-N-
oxide derivative 6a (54% isolated) by treatment with the NaBH4/
CH3COOH reagent.
Pyridine-N-oxide-2-carbaldehyde (4a) was also condensed with
pentafluoroaniline to yield the pyridine-N-oxide carbaldimine
derivative 5c. Its (in situ) reduction with sodium cyanoboro-
hydride
eventually
furnished
the
N–C6F5-substituted
(aminomethyl)pyridine-N-oxide derivative 6c (39% isolated, see
Scheme 2).
Both the imine (5a) and its reduction product (6a) were charac-
terized by X-ray diffraction. Single crystals of 5a were obtained
from methanol. The X-ray crystal structure analysis of 5a (see
Fig. 1, left (values taken from molecule A)) shows a nearly to copla-
nar arrangement of the imine –C@N– unit (C7–N8 1.261(2) Å) [7]
with the pyridine-N-oxide nucleus (dihedral angle N1–C2–C7–N8
* Corresponding author. Fax: +49 251 83 36503.
E-mail address: erker@uni-muenster.de (G. Erker).
X-ray crystal structure analyses.
1
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.05.032

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K. Nienkemper et al. / Journal of Organometallic Chemistry 693 (2008) 3063–3073
168.5(1)°). The bond lengths around the pyridine-N-oxide nitrogen
atom amount to N1–O 1.298(2) Å, N1–C2 1.366(2) Å and N1–C6
1.361(2) Å. The imine functional group is E-configurated (C2–C7–
N8–C9 174.0(1)°) and the plane of the 2,6-diisopropylphenyl sub-
stituent is close to normal to the C@N plane (C7–N8–C9–C14
ꢀ99.2(2)°).
C7–N8 unit in 6a are found in an anti-periplanar orientation with
dihedral angle of C2–C7–N8–C9 163.6(3)° (N1–C2–C7–N8
179.8(3)°). The plane of the bulky 2,6-diisopropylphenyl substitu-
ent at N8 is oriented almost normal to the central pyridine plane
(dihedral angle C7–N8–C9–C14 ꢀ74.1(5)°).
a
The related compound 6c was also characterized by X-ray dif-
fraction (single crystals from methanol/triethylamine by evapora-
tion). It features similar structural parameters of the pyridine-N-
oxide core (N1–O1 1.318(2) Å, C2–C7 1.503(2) Å, N1–C2–C7–N8
ꢀ178.6(1)°, see Fig. 2) but exhibits a markedly different conforma-
tional arrangement of the attached N-pentafluorophenylamino-
methyl substituent. While the C6F5 group is rotated almost
perpendicular from the pyridine plane (C2–C7–N8–C9 86.4(2)°),
its plane is found almost coplanar with the adjacent C7–N8 vector
(C7–N8–C9–C10 ꢀ8.8(2)°).
Compound 6a features a pyridine-N-oxide N1–O1 bond length
of 1.317(4) Å. The adjacent C2–C7 bond length is found at
1.484(5) Å and the C7–N8 bond (1.469(4) Å) is now in the typical
C(sp3)–N single bond range [7]. The substituents at the central
R'
Ar
Ar
R
N
O
R
N
The conformation of 5a in the crystal contains an element of ax-
ial prochirality at the N8–C9 vector. However, in solution diaste-
reotopic splitting of the methyl NMR resonances of the attached
pair of isopropyl groups was not observed [1H/13C: d 1.16 (d,
12H)/23.4 CH(CH3)2]. Similarly, compound 6a features one set of
1H/13C NMR isopropyl methyl resonances (d 1.19/24.3).
N O
B
A
Scheme 1.
SeO2
H2O2
CH3CO2H
OH
OH
O
N
O
N
N
O
4a
H
2a
3a
H2N-Ar, HCOOH, mol. sieves (4 Å)
Ar = 2,6-diisopropylphenyl
Ar = C6F5
F5
N
N
N
O
N
O
H
H
5a
5c
6c
NaBH(OAc)3
NaBH3CN
H
H
N
F5
N
N
O
N
O
6a
Scheme 2.
Fig. 1. Views of the molecular structures of the compounds 5a (left) and 6a (right).

K. Nienkemper et al. / Journal of Organometallic Chemistry 693 (2008) 3063–3073
3065
Fig. 2. Molecular geometry of 6c.
The synthesis of the related quinoline-N-oxide derivatives
started from 2-methyl-quinoline-N-oxide (1b), which was ob-
tained by oxidation (m-CPBA in chloroform) of chinaldine accord-
ing to the literature procedure [8]. It was converted to 2-
hydroxymethylquinoline (2b) by a Boekelheide reaction [9] by
treatment with trifluoroacetic anhydride (for
a mechanistic
scheme see the Supplementary material). Then, first the quinoline
nitrogen was oxidized with meta-chloroperbenzoic acid to get 3b
and subsequent oxidation with SeO2 gave the corresponding alde-
hyde (4b) [10], which was then converted to the imine (5b) by
treatment with 2,6-diisopropylaniline (see Scheme 3). The imine
5b was isolated in ca. 80% after crystallization from methanol.
Reduction of 5b was carried out with NaBH4/CH3COOH to eventu-
ally yield 6b (86% isolated).
Fig. 3. Projections of the molecular structures of compounds 5b (top) and 6b
(bottom).
Compound 6b shows a N1–O1 bond length of 1.303(1) Å and an
adjacent C(sp2)–C(sp3) linkage (C2–C7 1.492(2) Å) to the –CH2–
NH–Ar substituent. The C7–N8 single bond (1.462(2) Å) is oriented
almost coplanar with the adjacent quinoline-N-oxide ring (N1–C2–
C7–N8 ꢀ177.9(1)°) but the adjacent N–C(ar) vector is markedly ro-
tated out of this plane (C2–C7–N8–C9 141.2(1)°). Again the bulky
2,6-diisopropylphenyl substituent is oriented near to normal to
the adjacent C7–N8 vector (dihedral angle C7–N8–C9–C14
ꢀ72.8(1)°).
Both compounds 5b and 6b were characterized by X-ray crystal
structure analyses (see Fig. 3). Single crystals were obtained by
crystallization from methanol (5b) or by a slow evaporation of sol-
vent from a methanol/triethylamine solution (6b).
Compound 5b features a N1–O1 bond length of 1.287(2) Å. The
C@N unit (C7–N8 1.266(2) Å) is E-configurated (dihedral angles
N1–C2–C7–N8 178.4(2)°, C2–C7–N8–C9 176.5(2)°) and the aryl
substituent at N8 is oriented close to orthogonal (C7–N8–C9–C14
94.0(2)°) (values are taken from molecule A).
TFAA
mCPBA
OH
O
N
O
N
1b
2b
SeO2
H2N-Ar
mol. sieves
(4 Å)
OH
N
O
N
O
H
3b
4b
NaBH(OAc)3
H
N
N
N
O
N
H
O
5b
6b
Ar = 2,6-diisopropylphenyl
Scheme 3.

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K. Nienkemper et al. / Journal of Organometallic Chemistry 693 (2008) 3063–3073
In solution compound 5b features a single set of isopropyl
[12] (for a mechanistic scheme see the Supplementary material).
Subsequent nucleophilic methylation, followed by hydrolysis, N-
oxidation and condensation gave the iminopyridin-N-oxide sys-
tems (11) (see Scheme 4). Their structures in the crystal had previ-
ously been determined and described [4].
methyl NMR resonances [1H/13C: d 1.19 (d, 3J = 6.9 Hz)/23.5] as
does 6b [1H/13C: d 1.17 (d, 3J = 6.9 Hz)/24.3].
2.2. Systems derived from 2-acetylpyridine-N-oxide
Imines can often be converted to amines by the addition of, e.g.
alkyllithium nucleophiles [13], followed by hydrolysis. We have,
therefore, treated the ketimino compound 11a with methyl lith-
ium. Workup gave a new compound (12) in ca. 50% yield that
clearly was not the corresponding amine but rather a coupled
product between two pyridine cores of the starting material (see
Scheme 5). This became evident from the observation of two imino
13C NMR signals at d 167.4 and d 165.6. The 1H NMR spectrum (see
Fig. 4) showed a total of five separated signals originating from the
six pyridine protons [d 9.19, 8.45 (2H), 8.01, 7.63, 7.49 (in THF-d8)]
in addition to a pair of isopropyl CH septets (d 3.03, d 2.80) and four
corresponding isopropyl methyl doublets (d 1.26, 1.17, 1.16, 1.14).
The structural assignment of 12 was confirmed by an X-ray
crystal structure analysis of the compound (see Fig. 5). It shows
the presence of a pyridine ring ortho-coupled to a pyridine-N-oxide
unit [dihedral angle between the units: N1–C6–C22–N23
143.6(4)°]. The C28–N30 imine moiety is close to coplanar to its
adjacent pyridine ring (N23–C24–C28–N30 ꢀ175.0(4)°) with its at-
tached 2,6-diisopropylphenyl substituent being oriented almost
normal to that plane (C28–N30–C31–C36 79.2(5)°). The more
bulky pyridine-N-oxide forces its attached imine away from
coplanarity (N1–C2–C7–N9 127.0(4)°). Both imino-C@N double
bonds in 12 are E-configurated. Due to the relatively high R-factor
The compounds 11a/b were prepared as was described by us
previously [4,11]. The pyridine-N-oxides (7) were converted to
the 2-cyanopyridines (8) by a modified Reissert–Henze reaction
Me3SiCN
1) MeLi
2) H2O
Me2NCOCl
O
R
N
O
R
N
C
R
N
N
CH3
8
7
9
H2N-Ar
H2O2
HCOOH
O
N
R
N
O
R
N
CH3
O H3C
10
11
Ar = 2,6-diisopropylphenyl
R = H (a), -CHMe2 (b)
Scheme 4.
4'
5'
3'
3
O
N
CH3
N
CH3Li
Ar
Ar
N
N
N
N
Et2O
Ar
Ar
N
Ar
-78 °C
O
CH3
CH3
5
4
12
11a
CH3Li/-CH4
-LiOH
H
O
N
CH3
11a
N
Li
N
Ar
N
O
N
O
CH3
13
Ar = 2,6-diisopropylphenyl
CH3
Li
14
Scheme 5.
Fig. 4. 1H NMR spectrum of 12 (500 MHz, 298 K, THF-d8).

K. Nienkemper et al. / Journal of Organometallic Chemistry 693 (2008) 3063–3073
3067
Fig. 5. Molecular structure of 12.
(9.8%) no further details of the crystal structure analysis of 12 shall
be discussed.
A potential mechanistic pathway of the formation of 12 is de-
picted in Scheme 5. It is well established that pyridine-N-oxides
are readily deprotonated in the 2-position by treatment with suit-
able bases [14]. This o-lithiation can in some cases [15,16] initiate
non-transition metal induced pathways [17] to the formation of
bipyridyl-N-oxide derivatives. So it may be that o-lithiation of
11a leads to the formation of 13, which could subsequently under-
go nucleophilic attack at the 2-position of a molecule of the start-
ing material (11a) to generate 14. Elimination of lithium hydroxide
from this reactive intermediate would then provide a suitable
route to the observed product 12 (see Scheme 5) [15].
Since methyl lithium proved to be too basic for the attempted
nucleophilic imine to amine conversion we searched for a suited
less basic carbon nucleophile for addition to the imine carbon atom
of the (ketimino)pyridine-N-oxide reagents 11. We found it in
trimethylaluminum. Treatment of 11a with excess trimethylalumi-
num followed by aqueous work up furnished the corresponding N-
aryl-1-methyl-aminoethyl-substituted pyridine-N-oxide system
(15a) in >50% yield. The analogous reaction of 11b with (AlMe3)2
gave the corresponding product (15b) in >60% yield (see Scheme
6). Compound 15b features a single 1H NMR isopropyl methyl dou-
blet of the 2,6-diisopropylphenyl substitutent at
d 0.95 (d,
3J = 6.9 Hz, 12 Hz) plus a corresponding signal of the isopropyl
group at the pyridine-N-oxide nucleus of half the intensity at d
1.33 (d, 3J = 6.9 Hz, 6H). The newly introduced –C(CH3)2–N 1H
NMR methyl resonance occurs at d 1.54 (s, 6H) (13C: d 26.8).
Both the compounds 15a and 15b were characterized by X-ray
diffraction (see Fig. 6). Compound 15a (values are taken from mol-
ecule A) features a N1–O1 bond length of 1.309(2) Å of the pyri-
dine-N-oxide unit. The adjacent C2–C7 bond length is found at
Fig. 6. Views of the molecular structures of the compounds 15a (top) and 15b
(bottom).
1. (AlMe3)2
2. H2O
CH3
CH3
N
1.533(2) Å and the C7–N10 linkage (1.489(2) Å) is in the typical
R
N
O
R
N
O
Ar
C(sp3)–N
r-bond range. It may be that the N–H group forms a
HN
CH3
weak hydrogen bond to the pyridine-N-oxide oxygen atom [O1–
H(N10) 2.10(2) Å]. However, the conformation of the respective
part of the molecule 15a is markedly non-planar [dihedral angle
N1–C2–C7–N10 55.1(2)°]. The bulky 2,6-diisopropylphenyl group
attains a conformational orientation that minimizes steric hin-
drance (C2–C7–N10–C11 89.7(2)°, C7–N10–C11–C16 93.4(2)°).
Compound 15b exhibits an analogous structure in the crystal
(see Fig. 6).
The parent (aldimino)pyridine-N-oxide compound 5a reacts
analogously with (AlMe3)2. After aqueous workup, we have iso-
lated the (aminoethyl)pyridine-N-oxide derivative 16 in 75% yield.
Compound 16 contains a chiral center [1H NMR: d 4.00 (1H), d 1.69
(d, 3J = 6.9 Hz, 3H), –CH(CH3)–N]. Therefore, a set of diastereotopic
Ar
11
15
R = H (a), -CHMe2 (b)
1. (AlMe3)2
2. H2O
H
N
H
N
N
Ar
N
O
Ar
O H3C
H
5a
Ar = 2,6-diisopropylphenyl
16
Scheme 6.

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K. Nienkemper et al. / Journal of Organometallic Chemistry 693 (2008) 3063–3073
1H/13C NMR isopropyl methyl signals was observed for this com-
pound [1H: d 3.47 (sept. 1H), 1.18/1.03 (each d, 3J = 6.8 Hz, each
6H)/13C: d 27.8 (CHMe2), 24.6/24.4 (CH(CH3)2)].
and 19b were positively identified by comparison with the authen-
tic separately prepared heterocyclic aldimines [18,19]. A potential
mechanistic scheme for this intramolecular redox reaction is de-
picted in Scheme 8. However, this needs to be supported by further
specific investigations (see Scheme 7).
We have also reacted the (1-methyl-aminoethyl)pyridine-N-
oxide derivative 15a with methyl lithium. In this case, the intramo-
lecular redox reaction is prevented to take place by the presence of
Compound 16 was also characterized by X-ray diffraction. It
features a remarkably different conformation from the related
15a/15b pair of compounds (see Fig. 7). The bond lengths of the
characteristic functional groups of compound 16 are in the typical
range (N1–O1: 1.312(2) Å, C2–C7 1.512(2) Å, C7–N9 1.464(2) Å),
but the C7–N9 vector is rotated away from the pyridine-N-oxide
moiety (dihedral angles N1–C2–C7–N9 152.3(1)°, C2–C7–N9–C10
ꢀ72.6(2)°). Again the bulky 2,6-diisopropylphenyl substituent at-
tains a position that probably minimizes unfavorable steric inter-
action (C7–N9–C10–C11 ꢀ61.0(2)°).
the pair of geminal methyl groups at the a-position of the substi-
tuent. However, in this case the apparently generated amide anion
system proved to be unstable with regard to a fragmentation reac-
tion to yield the acetone imine product (21) (see Scheme 9).
In view of these synthetic complications we decided to build up
the (amidomethyl)pyridine-N-oxide N,O-chelate ligands directly in
the coordination sphere of a metal. Since the reactions of the
respective imine systems with trimethylaluminum had success-
fully been used for the alkylated (aminomethyl)pyridine-N-oxide
synthesis (see above) [20] we decided to use this reaction for the
preparation and isolation of the respective (N,O-chelate ligand)alu-
minum complexes (see Scheme 10).
2.3. Metallation reactions
For use of the (aminomethyl)pyridine-N-oxide derivatives as li-
gands in transition metal chemistry it would be desirable to gener-
ate the respective anions by deprotonation at the amine nitrogen
of the side chain. Therefore, we treated compound 6a with n-butyl
lithium in THF. Unexpectedly, this led to the sole formation of the
pyridine aldimine product (19a). Treatment of 6a with several
other bases (LDA in THF; KH in THF and even Zr(NMe2)4 in toluene)
gave the same result. The reaction of the (aminomethyl)quinoline-
N-oxide derivative 6b with LDA (in THF) took the analogous course
to yield the quinoline aldimine product 19b. Both the products 19a
Treatment of compound 11a with trimethylaluminum gave the
(N,O-chelate)Al complex 23 as a bright yellow solid. It features a 1H
NMR singlet for the geminal C(CH3)2 group inside the chelate ring
base
H
H
H
H
H
N
O
N
O
N
O
HN
N
HN
Ar
Ar
Ar
6
17
18
H
H
-OH
N
N
O
N
N
Ar
H
Ar
18'
19
Scheme 8.
H3C
CH3
CH3Li
CH3
CH3
CH3
N
N
O
CH3
N
O
HN
N
Ar
Ar
15a
20
21
Fig. 7. A projection of the molecular structure of compound 16.
Scheme 9.
n
BuLi
½ (AlMe3)2
toluene
H
N
H
N
N
N
THF
N
O
Ar
N
Ar
N
O
CH3
N
O
Ar
Ar
H
H
Ar
6a
6b
19a
Al
Me2
5a
22
½ (AlMe3)2
toluene
CH3
H
N
LDA
THF
N
N
N
O
N
O
CH3
N
O
Ar
N
dipp
CH3
11a
Ar = 2,6-diisopropylphenyl
N
H
Ar
Al
19b
Me2
23
Ar = 2,6-diisopropylphenyl
Scheme 7.
Scheme 10.

K. Nienkemper et al. / Journal of Organometallic Chemistry 693 (2008) 3063–3073
3069
deprotonation of these systems resulted in reactive amide anions
that were unstable with regard to either an intramolecular redox
reaction or the formation of a coupling product or fragmentation.
Under the reaction conditions applied by us in this study this has
prohibited the usual use and application of such ligands by means
of employment of their amide anions (i.e. their alkali metal com-
pounds). A potential synthetic solution of this serious problem
was eventually provided by the clean reaction of the pyridine-N-
oxide aldimines or ketimines with trimethylaluminum; from these
reaction mixtures the corresponding N,O-chelate (amidoethyl)pyr-
idine-N-oxide Al complexes could be obtained. It will be seen if
such Al systems might prove useful, be it as reagents for transmet-
allation to transition metals or themselves as catalysts, e.g. in poly-
merization reactions [21,22].
4. Experimental
All reactions involving air or moisture sensitive compounds
were carried out under inert atmosphere using Schlenk-type glass-
ware or in a glovebox. Solvents were dried and distilled prior to
use. The following instruments were used for physical character-
ization of the compounds: Melting points, DSC 2010 TA-instru-
ments; elemental analyses, Foss-Heraeus CHNO-Rapid; MS,
Micromass Quattro LC-Z electrospray mass spectrometer; NMR,
Bruker AC 200 P (1H: 200 MHz), Bruker AV 400 (1H: 400 MHz), Var-
ian INOVA NMR spectrometer (1H: 500 MHz, 13C: 126 MHz), or
Varian UNITY plus NMR spectrometer (1H: 600 MHz, 13C:
151 MHz, 19F 564 MHz). X-ray crystal structure determinations:
Data sets were collected with Nonius KappaCCD diffractometers,
in case of Mo-radiation a rotating anode generator was used. Pro-
grams used: data collection COLLECT (Nonius B.V., 1998), data reduc-
tion Denzo-SMN (Z. Otwinowski, W. Minor, Methods in
Enzymology, 1997 (276) 307), absorption correction SORTAV
(R.H. Blessing, Acta Crystallogr., Sect. A, 1995 (51) 33; R.H. Blessing,
J. Appl. Crystallogr. 1997 (30) 421) and Denzo (Z. Otwinowski, D.
Borek, W. Majewski, W. Minor, Acta Crystallogr., Sect. A, 2003
(59) 228), structure solution SHELXS-97 (G.M. Sheldrick, Acta Crys-
tallogr., Sect. A, 1990 (46) 467), structure refinement SHELXL-97
(G.M. Sheldrick, Universität Göttingen, 1997), graphics SCHAKAL
(E. Keller, Universität Freiburg, 1997).
Fig. 8. Molecular structure of the N,O-chelate Al complex 23.
at d 1.45 (13C: d 28.0) and a Al(CH3)2 resonance at d ꢀ0.23 (1H) (13C:
d ꢀ6.9). The N-2,6-diisopropylphenyl unit exhibits axial prochiral-
ity and, thus, features one isopropyl 1H NMR CH septet (d 3.37, 2 H
intensity) and two CH(CH3)2 doublets (d 1.27, d 1.10) each of 6H
intensity [13C: d 27.8 (CHMe2), d 26.2, 25.1 (CH(CH3)2)].
Complex 23 was characterized by X-ray diffraction (single crys-
tals from pentane at ꢀ20 °C). The structural analysis confirms that
a methyl anion equivalent was added to the former imine carbon
center of the starting material (11a). The aluminum atom is coor-
dinated to both the nitrogen atom of the side chain and the oxygen
atom of the pyridine-N-oxide subunit (bond lengths Al–N10
1.844(2) Å, Al–O1: 1.859(2) Å). This coordination has resulted in
a marked lengthening of the pyridine-N-oxide N1–O1 bond (23:
1.355(3) Å, cf. 11a: 1.301(1) Å [4]). The Al atom in 23 features a dis-
torted pseudotetrahedral coordination sphere [bond angles N10–
Al–O1 94.2(1)°, N10–Al–C24 114.6(1)°, O1–Al–C24 109.7(1)°,
N10–Al–C23 120.6(1)°, O1–Al–C23 102.9(1)°, C24–Al–C23
112.0(2)°]. The metallacyclic six-membered chelate ring adopts a
shallow envelope-like conformation (C2–C7–N10–Al ꢀ59.5(3)°,
N1–C2–C7–N10 52.4(3)°, O1–N1–C2–C7 2.7(4)°). The bulky 2,6-
diisopropylphenyl substituent at N10 is markedly rotated out of
the mean central plane of 23 (C7–N10–C11–C12 ꢀ89.2(3)°) (see
Fig. 8).
The (ketimino)pyridine-N-oxides 11a and 11b [4] were synthe-
sized according to procedures reported in the literature.
4.1. 2-{[(E)-2,6-Diisopropylphenylimino]methyl}pyridine-N-oxide
(5a)
Treatment of the aldimino-substituted pyridine-N-oxide system
(5a) with trimethylaluminum yielded the corresponding six-mem-
bered N,O-chelate Al complex 22 (see Scheme 10). In this case, the
ring contains a newly formed chirality center. Consequently we
have observed a pair of 1H NMR isopropyl CH septets (at d 3.86,
3.09) and four –CH(CH3)2 doublets (d 1.18, 1.17, 1.10, 1.04) of the
orthogonally oriented 2,6-diisopropylphenyl substituent. Complex
22 features a pair of Al(CH3) 1H NMR resonances at d ꢀ0.54 and
ꢀ0.77 (13C: ꢀ7.2, ꢀ9.4; both are very broad).
2,6-Diisopropylaniline (3.01 g, 17.0 mmol) and
a catalytic
amount of formic acid were added to a solution of 4a (2.00 g,
16.2 mmol) in dichloromethane (50 mL). After refluxing the reac-
tion mitxture in the presence of moleculare sieve (4 Å) for 4 h, it
was filtered and then the solvent was removed in vacuo to yield
a yellow solid. Crystallisation from methanol gave the pure prod-
uct (2.80 g, 61%) as a shiny yellow crystalline solid. Fractional crys-
tallization from methanol gave crystals suitable for X-ray
diffraction. M.p. 133 °C (DSC). MS-ESI (m/z, ES+, in methanol):
283.2 [M+H]+, 305.2 [M+Na]+, 587.3 [2M+Na]+. Anal. Calc. for
C18H22N2O: C, 76.56; H, 7.85; N, 9.92. Found: C, 76.47; H, 7.94;
3. Conclusions
N, 9.81%. 1H NMR (600 MHz, CDCl3, 298 K): d = 8.89 (s, 1H, N@CH),
8.23 (m, 1H, 6-HPy), 8.18 (m, 1H, 3-HPy), 7.35 (m, 1H, 5-HPy), 7.33
(m, 1H, 4-HPy), 7.15 (m, 2H, 3-HAr), 7.11 (m, 1H, 4-HAr), 2.91 (sept,
3J = 6.9 Hz, 2H, CH(CH3)2), 1.16 (d, 3J = 6.9 Hz, 12H, CH(CH3)2).
13C{1H} NMR (151 MHz, CDCl3, 298 K): d = 155.1 (1JCH = 176 Hz,
N@CH), 148.3 (C1Ar), 145.5 (C2Py), 140.0 (C6Py), 137.2 (C2Ar),
127.3 (C5Py), 125.4 (C4Py), 124.9 (C4Ar), 124.5 (C3Py), 123.1 (C3Ar),
28.0 (CH(CH3)2), 23.4 (CH(CH3)2).
This study has shown that a variety of (aminomethyl)pyridine-
N-oxide systems is readily available via their respective aldimines
or ketimines. Reduction can be effected by treatment with sodium
borohydride reagents. Alternatively, treatment with trimethylalu-
minum leads to a clean transfer of a methyl anion equivalent. Sub-
sequent
hydrolysis
then
liberates
the
corresponding
(aminoethyl)pyridine-N-oxide derivatives. Unexpectedly, the

3070
K. Nienkemper et al. / Journal of Organometallic Chemistry 693 (2008) 3063–3073
X-ray crystal structure analysis for 5a: formula C18H22N2O,
1.19 (d, 3J = 6.9 Hz, CH(CH3)2). 13C{1H} NMR (151 MHz, CDCl3,
298 K): d = 149.7 (C2Py), 143.0 (C2Ar), 142.1 (C1Ar), 139.6 (C6Py),
126.0 (C4Py), 125.6 (C3Py), 124.6 (C5Py), 124.2 (C4Ar), 123.6 (C3Ar),
51.5 (NCH2), 27.5 (CH(CH3)2), 24.3 (CH(CH3)2).
M = 282.38, yellow crystal 0.40 ꢁ 0.25 ꢁ 0.15 mm, a = 10.9335(3),
b = 22.6543(7), c = 13.4702(5) Å, b = 100.451(1)°, V = 3281.09(18) Å3,
q
calc = 1.143 g cmꢀ3 = 0.557 mmꢀ1, empirical absorption correc-
, l
tion (0.808 6 T 6 0.921), Z = 8, monoclinic, space group P21/n,
(No. 14), k = 1.54178 Å, T = 223 K, and / scans, 27251 reflections
collected ( h, k, l), [(sinh)/k] = 0.60 Åꢀ1
5859 independent
(I)], 387 refined
X-ray crystal structure analysis for 6a: formula C18H24N2O,
x
M = 284.39,
a = 35.478(2), b = 5.819(1), c = 15.606(1) Å, V = 3221.8(6) Å3,
calc = 1.173 g cmꢀ3 = 0.567 mmꢀ1, empirical absorption correc-
tion (0.848 6 T 6 0.983), Z = 8, orthorhombic, space group Pbcn,
(No. 60), k = 1.54178 Å, T = 223 K, and / scans, 13626 reflections
collected ( h, k, l), [(sinh)/k] = 0.59 Åꢀ1
2243 independent
(I)], 198 refined
light
yellow
crystal
0.30 ꢁ 0.06 ꢁ 0.03 mm,
,
(Rint = 0.042) and 5194 observed reflections [I P 2
r
q
, l
parameters, R = 0.050, wR2 = 0.139, max. residual electron density
0.54 (ꢀ0.37) e Åꢀ3, hydrogen atoms calculated and refined as
riding atoms.
x
,
(Rint = 0.068) and 1182 observed reflections [I P 2
r
4.2. 2-{[(E)-2,6-Diisopropylphenylimino]methyl}quinoline-N-oxide
(5b)
parameters, R = 0.061, wR2 = 0.213, max. residual electron density
0.28 (ꢀ0.28) e Åꢀ3, hydrogen atom at N8 from difference fourier
map, other calculated and refined as riding atoms.
2,6-Diisopropylaniline (1.09 mL, 5.80 mmol) was added to a
solution of 4b (1.00 g, 5.80 mmol) in dichloromethane (30 mL)
with moleculare sieve (4 Å). After refluxing the reaction mitxture
for 4 h, it was filtered and then the solvent was removed in vacuo
to yield a yellow solid. Crystallisation from methanol gave the pure
product (1.58 g, 81%) as a shiny yellow solid. Fractional crystalliza-
tion from methanol gave crystals suitable for X-ray diffraction.
M.p. 133 °C (DSC). MS-ESI (m/z, ES+, in methanol): 333.2 [M+H]+,
355.2 [M+Na]+, 687.4 [2M+Na]+. Anal. Calc. for C22H24N2O: C,
79.48; H, 7.28; N, 8.43. Found: C, 78.57; H, 7.14; N, 8.33%. 1H
NMR (600 MHz, CDCl3, 298 K): d = 9.14 (s, 1H, N@CH), 8.78 (d,
3J = 8.8 Hz, 1H, 8-HCh), 8.22 (d, 3J = 8.8 Hz, 1H, 3-HCh), 7.88 (d,
3J = 8.1 Hz, 1H, 5-HCh), 7.77 (ddd, 3J = 8.8 Hz, 3J = 7.0 Hz,
4J = 1.3 Hz, 1H, 7-HCh), 7.76 (d, 3J = 8.8 Hz, 1H, 4-HCh), 7.68 (ddd,
3J = 8.1 Hz, 3J = 7.0 Hz, 4J = 1.2 Hz, 1H, 6-HCh), 7.18 (m, 2H, 3-HAr),
7.14 (m, 1H, 4-HAr), 2.98 (sept, 3J = 6.9 Hz, CH(CH3)2), 1.19 (d,
3J = 6.9 Hz, CH(CH3)2).
4.4. 2-[(2,6-Diisopropylphenylamino)methyl]quinoline-N-oxide (6b)
(according to the procedure described for the synthesis of 6a)
Sodium borohydride (0.26 g, 6.87 mmol) in THF (15 mL) and
glacial acetic acid (5 mL) reacts with a solution of 5b (0.76 g,
2.29 mmol) in THF (5 mL). The described workup using 50 mL
dichloromethane to dissolve the residue was carried out. Finally
the product was obtained after column chromatography (SiO2;
pentane/methanol/dichloromethane/triethylamine 30:1:1:1) as a
white solid (0.66 g, 86%). Crystallization from a methanol/triethyl-
amine mixture gave crystals suitable for X-ray diffraction. M.p.
124 °C (DSC). MS-ESI (m/z, ES+, in methanol): 335.2 [M+H]+,
357.2 [M+Na]+. Anal. Calc. for C22H26N2O: C, 79.01; H, 7.84; N,
8.38. Found: C, 78.40; H, 7.83; N, 8.29%. 1H NMR (600 MHz, CDCl3,
298 K): d = 8.79 (d, 3J = 8.8 Hz, 1H, 8-HCh), 7.84 (d, 3J = 8.2 Hz, 1H, 5-
HCh), 7.77 (ps t, 1H, 7-HCh), 7.69 (d, 3J = 8.5 Hz, 1H, 4-HCh), 7.61 (ps
t, 1H, 6-HCh), 7.41 (d, 3J = 8.5 Hz, 3-HCh), 7.08 (m, 3H, 3-/4-HAr), 4.41
(s, 2H, NCH2), 3.37 (sept, 3J = 6.9 Hz, 2H, CH(CH3)2), 1.17 (d,
3J = 6.9 Hz, 12H, CH(CH3)2). 13C{1H} NMR (151 MHz, CDCl3,
298 K): d = 146.5 (C2Ch), 143.0 (C2Ar), 142.2 (C1Ar), 141.6 (C8aCh),
130.5 (C7Ch), 129.6 (C4aCh), 128.2 (C6Ch), 128.1 (C5Ch), 125.6
(C4Ch), 124.2 (C4Ar), 123.6 (C3Ar), 121.5 (C3Ch), 119.4 (C8Ch), 52.1
(NCH2), 27.5 (CH(CH3)2), 24.3 (CH(CH3)2).
X-ray crystal structure analysis for 5b: formula C22H24N2O,
M = 332.43, yellow crystal 0.25 ꢁ 0.15 ꢁ 0.15 mm, a = 8.578(1),
b = 10.941(1), c = 20.396(8) Å,
a = 79.58(1)°, b = 83.87(1)°, c =
80.30(1)°, V = 1850.0(3) Å3,
q , l ,
calc = 1.194 g cmꢀ3 = 0.572 mmꢀ1
empirical absorption correction (0.870 6 T 6 0.919), Z = 4, triclinic,
ꢀ
space group P1, (No. 2), k = 1.54178 Å, T = 223 K,
x and / scans,
20302 reflections collected ( h, k, l), [(sinh)/k] = 0.60 Åꢀ1, 6416
independent (Rint = 0.037) and 5520 observed reflections
X-ray crystal structure analysis for 6b: formula C22H26N2O,
[I P 2
r
(I)], 489 refined parameters, R = 0.050, wR2 = 0.139, max.
M = 334.45,
a = 24.044(1), b = 16.651(1), c = 9.290(1) Å, b = 95.09(1)°, V =
3696.7(5) Å3, calc = 1.202 g cmꢀ3 = 0.572 mmꢀ1
empirical
absorption correction (0.825 6 T 6 0.967), Z = 8, monoclinic, space
light
yellow
crystal
0.35 ꢁ 0.20 ꢁ 0.06 mm,
residual electron density 0.70 (ꢀ0.17) e Åꢀ3, hydrogen atoms cal-
culated and refined as riding atoms.
q
,
l
,
4.3. 2-[(2,6-Diisopropylphenylamino)methyl]pyridine-N-oxide (6a)
group C2/c, (No. 15), k = 1.54178 Å, T = 223 K, x and / scans,
23809 reflections collected ( h, k, l), [(sinh)/k] = 0.60 Åꢀ1, 3303
independent (Rint = 0.042) and 3042 observed reflections
Sodium borohydride (1.24 g, 32.7 mmol) was added carefully to
a solution of THF (49 mL) and glacial acetic acid (19 mL) at 0 °C in a
way that the temperature was always below 20 °C. After the
hydrogen evolution has stopped, the rection mixture was cooled
to ꢀ78 °C and a solution of 5a (3.09 g, 10.9 mmol) in THF (10 mL)
was added. Then the reaction mixture was stirred over night at
room temperature. The solvent was removed in vacuo and the res-
idue was dissolved in dichloromethane (100 mL). Aqueous Na2CO3
solution was added, the organic phase was separated, dried over
MgSO4, and the solvent was removed in vacuo. The product was
obtained after column chromatography (SiO2; pentane/methanol/
[I P 2r
(I)], 233 refined parameters, R = 0.040, wR2 = 0.106, max.
residual electron density 0.14 (ꢀ0.15) e Åꢀ3, hydrogen atom at
N8 from difference fourier map, other calculated and refined as rid-
ing atoms.
4.5. 2-(Pentafluorophenylaminomethyl)pyridine-N-oxide (6c)
Pentafluoroaniline (811 mg, 4.43 mmol) was added to a solu-
tion of 4a (519 mg, 5.69 mmol) and acetic acid (0.50 mL) in meth-
anol (10 mL). The reaction mixture was refluxed for 24 h in the
presence of molecular sieve (3 Å). Subsequently, sodium cyano-
borohydride (796 mg, 12.7 mmol) was added carefully and the
reaction mixture was refluxed for further two days. Then the reac-
tion was quenched with aqueous NaHCO3 solution. After the sepa-
ration of the organic phase the aqueous phase was extracted with
dichloromethane three times, the combined organic phases were
dried over MgSO4 and the solvent was removed in vacuo. Finally
the product was obtained after column chromatography (SiO2;
pentane/chloroform/methanol/triethylamine 80:1:1:1) as a white
dichloromethane/triethyl-amine 15:1:1:1) as
a
white solid
(1.66 g, 54%). Crystallization from a chloroform/methanol/triethyl-
amine mixture gave crystals suitable for X-ray diffraction. M.p.
92 °C (DSC). MS-ESI (m/z, ES+, in methanol): 285.1 [M+H]+, 307.1
[M+Na]+, 591.4 [2M+Na]+. Anal. Calc. for C18H24N2O: C, 76.02; H,
8.51; N, 9.85. Found: C, 75.98; H, 8.43; N, 9.77%. 1H NMR
(600 MHz, CDCl3, 298 K): d = 8.27 (m, 1H, 6-HPy), 7.32 (m, 1H, 3-
HPy), 7.21 (m, 2H, 4-/5-HPy), 7.06 (m, 3H, 3-/4-HAr), 4.47 (br s,
1H, NH), 4.20 (s, 2H, NCH2), 3.32 (sept, 3J = 6.9 Hz, 2H, CH(CH3)2),

K. Nienkemper et al. / Journal of Organometallic Chemistry 693 (2008) 3063–3073
3071
solid (0.65 g, 39%). Crystallization from a methanol/triethylamine
mixture gave crystals suitable for X-ray diffraction. MS-ESI (m/z,
ES+, in methanol): 291.0 [M+H]+, 313.0 [M+Na]+. Anal. Calc. for
tion (0.781 6 T 6 0.985), Z = 4, monoclinic, space group P21/n,
(No. 14), k = 1.54178 Å, T = 223 K, and / scans, 20993 reflections
collected ( h, k, l), [(sinh)/k] = 0.60 Åꢀ1
5794 independent
(I)], 398 refined
x
,
C12H7N2OF5: C, 49.67; H, 2.43; N, 9.65. Found: C, 49.56; H, 2.32;
(Rint = 0.054) and 4600 observed reflections [I P 2r
N, 9.57%. 1H NMR (600 MHz, C6D6, 298 K): d = 7.74 (ddd,
3J = 6.4 Hz, 4J = 1.2 Hz, 5J = 0.5 Hz, 1H, 6-HPy), 6.41 (dd, 3J = 7.7 Hz,
4J = 1.9 Hz, 1H, 3-HPy), 6.15 (td, 3J = 7.7 Hz, 4J = 1.2 Hz, 1H, 4-HPy),
6.06 (ddd, 3J = 7.7 Hz, 3J = 6.4 Hz, 4J = 1.9 Hz, 1H, 5-HPy), 5.60 (br s,
1H, NH), 4.21 (d, 3J = 7.3 Hz, 2H, CH2NH). 13C{1H} NMR (151 MHz,
C6D6, 298 K): d = 147.9 (C2Py), 139.4 (C6Py), 125.1 (C3Py), 124.7
(C5Py), 123.5 (C4Py), 46.4 (CH2NH), n.o. (C6F5). 19F NMR (564 MHz,
C6D6, 298 K): d = ꢀ159.3 (m, 2F, o-PhF), ꢀ164.9 (m, 2F, m-PhF),
ꢀ171.6 (m, 1F, p-PhF).
parameters, R = 0.098, wR2 = 0.260, max. residual electron density
0.37 (ꢀ0.24) e Åꢀ3, hydrogen atoms calculated and refined as rid-
ing atoms.
4.7. 2-[1-(2,6-Diisopropylphenylamino)methylethyl]pyridine-N-oxide
(15a)
Trimethylaluminum (1.97 mL, 1.44 g, 20.0 mmol) was added
carefully to a solution of 11a (3.00 g, 10.1 mmol) in toluene
(100 mL) at 0 °C and the reaction mixture was stirred over night
at room temperature. After adding aqueous NaOH solution, the or-
ganic phase was separated and the aqueous phase was extracted
with chloroform. The combined organic phases were dried over
MgSO4 and the solvent was removed in vacuo. Finally the product
was obtained after column chromatography (SiO2; pentane/trieth-
ylamine/chloroform/methanol 15:2:1:1) as a white solid (1.79 g,
57%). Crystallization from a chloroform/methanol/triethylamine
mixture gave crystals suitable for X-ray diffraction. M.p. 88 °C
(DSC). MS-ESI (m/z, ES+, in methanol): 313.2 [M+H]+, 335.2
[M+Na]+, 647.4 [2M+Na]+. Anal. Calc. for C20H28N2O: C, 76.88; H,
9.03; N, 8.97. Found: C, 76.89; H, 9.01; N, 8.88%. 1H NMR
(500 MHz, CDCl3, 298 K): d = 8.29 (dd, 3J = 6.4 Hz, 4J = 1.0 Hz, 1H,
6-HPy), 7.33 (br d, 1H, 3-HPy), 7.25 (br t, 1H, 4-HPy), 7.19 (br t,
1H, 5-HPy), 7.06 (m, 1H, 4-HAr), 7.02 (m, 2H, 3-HAr), 5.77 (br s,
1H, NH), 3.28 (sept, 3J = 6.8 Hz, 2H, CH(CH3)2), 1.54 (s, 6H,
NC(CH3)2), 1.02 (d, 3J = 6.8 Hz, 12H, CH(CH3)2). 13C{1H} NMR
(126 MHz, CDCl3, 298 K): d = 156.3 (C2Py), 148.4 (C2Ar), 141.4
(C6Py), 138.9 (C1Ar), 126.3 (C4Py), 125.1 (C4Ar), 124.0 (C5Py), 123.1
(C3Py), 122.8 (C3Ar), 58.0 (NC(CH3)2), 28.1 (CH(CH3)2), 26.1
(NC(CH3)2), 24.2 (CH(CH3)2).
X-ray crystal structure analysis for 6c: formula C12H7F5N2O ꢂ 1/2
H2O, M = 299.20, colorless crystal 0.40 ꢁ 0.25 ꢁ 0.15 mm,
a = 25.262(1),
b = 5.629(1),
q
c = 17.097(1) Å,
b = 103.22(1)°,
V = 2366.8(5) Å3,
calc = 1.679 g cmꢀ3 = 0.165 mmꢀ1, empirical
, l
absorption correction (0.937 6 T 6 0.976), Z = 8, monoclinic, space
group C2/c, (No. 15), k = 0.71073 Å, T = 198 K,
x and / scans,
7512 reflections collected ( h, k, l), [(sinh)/k] = 0.67 Åꢀ1, 2834
independent (Rint = 0.042) and 1978 observed reflections
[I P 2r
(I)], 195 refined parameters, R = 0.041, wR2 = 0.116, max.
residual electron density 0.20 (ꢀ0.25) e Åꢀ3, hydrogen atom at
N8 and water from difference fourier map, other calculated and re-
fined as riding atoms.
4.6. Reaction of 11a with methyllithium: formation of 12
Methyllithium (1.6 M in diethylether, 4.6 mL, 7.30 mmol) was
added to a solution of 11a (2.06 g, 6.95 mmol) in diethylether
(100 mL) at ꢀ78 °C. Subsequently, the reaction mixture was stirred
for 2 h at ꢀ78 °C then for 12 h at room temperature. After addition
of aqueous ammonium chloride solution, the phases were sepa-
rated and the aqueous phase was extracted with dichloromethane
(30 mL). The combined organic phases were dried over MgSO4, fil-
tered, and the solvent was removed in vacuo. The obtained yellow
oil was purified by column chromatography (SiO2; cyclohexane/
ethyl acetate/triethylamine 30:1:1) to yield the product as a yel-
low, crystalline solid (1.05 g, 53%). Crystallization from a pen-
tane/chloroform/methanol/triethylamine mixture gave crystals
suitable for X-ray diffraction. M.p. 189 °C (DSC). MS-ESI (m/z, ES+,
in methanol): 575.4 [M+H]+, 597.4 [M+Na]+. Anal. Calc. for
C38H46N4O: C, 79.40; H, 8.07; N, 9.75. Found: C, 78.73; H, 7.93;
N, 9.69%. 1H NMR (500 MHz, THF-d8, 298 K): d = 9.19 (dd,
3J = 7.9 Hz, 4J = 1.0 Hz, 1H, 30-HPy), 8.454 (dd, 3J = 7.9 Hz,
4J = 1.0 Hz, 1H, 50-HPy), 8.450 (dd, 3J = 7.8 Hz, 4J = 2.1 Hz, 1H, 3-
HPy), 8.01 (t, 3J = 7.9 Hz, 1H, 40-HPy), 7.63 (dd, 3J = 7.8 Hz,
4J = 2.1 Hz, 1H, 5-HPy), 7.49 (t, 3J = 7.8 Hz, 1H, 4-HPy), 7.16 (m, 2H,
3-HAr), 7.157 (m, 2H, 30-HAr), 7.05 (ps t, 1H, 4-HAr), 7.045 (ps t,
1H, 40-HAr), 3.03 (sept, 3J = 6.9 Hz, 2H, CH(CH3)2), 2.80 (sept,
3J = 6.9 Hz, 2H, CH(CH3)20 ), 2.29 (s, 3H, @CCH30 ), 2.18 (s, 3H, @CCH3),
1.26 (d, 3J = 6.9 Hz, 6H, CH(CH3ACH3B)), 1.171 (d, 3J = 6.9 Hz, 6H,
X-ray crystal structure analysis for 15a: formula C20H28N2O,
M = 312.44,
colorless
crystal
c = 36.415(1) Å,
0.35 ꢁ 0.30 ꢁ 0.15 mm,
a = 17.666(1),
b = 8.845(1),
b = 100.21(1)°,
V = 5599.9(7) Å3,
q
calc = 1.112 g cmꢀ3
,
l
= 0.529 mmꢀ1, empirical
absorption correction (0.837 6 T 6 0.925), Z = 12, monoclinic,
space group P21/c, (No. 14), k = 1.54178 Å, T = 293 K,
x and / scans,
50552 reflections collected ( h, k, l), [(sinh)/k] = 0.60 Åꢀ1, 9993
independent (Rint = 0.046) and 8034 observed reflections
[I P 2r
(I)], 652 refined parameters, R = 0.050, wR2 = 0.129, max.
residual electron density 0.18 (ꢀ0.19) e Åꢀ3, hydrogen atom at
N10 from difference fourier map, other calculated and refined as
riding atoms.
4.8. 2-[1-(2,6-Diisopropylphenylamino)methylethyl]-6-
isopropylpyridine-N-oxide (15b) (according to the procedure
described for the synthesis of 15a)
Trimethylaluminum (113 lL, 85.0 mg, 1.18 mmol) was reacted
CH(CH3ACH3 )0), 1.165 (d, 3J = 6.9 Hz, 6H, CH(CH3ACH3B)), 1.14 (d,
with a solution of 11b (0.20 g, 0.59 mmol) in toluene (15 mL).
The described workup was carried out, to yield the product after
column chromatography (SiO2; pentane/triethylamine/chloro-
form/methanol 60:1:1:1) as a white solid (0.13 g, 63%). Crystalliza-
tion from a n-heptane gave crystals suitable for X-ray diffraction.
MS-ESI (m/z, ES+, in methanol): 355.3 [M+H]+, 377.3 [M+Na]+,
731.5 [2M+Na]+. Anal. Calc. for C23H34N2O: C, 77.92; H, 9.67; N,
7.90. Found: C, 77.67; H, 9.66; N, 7.65%. 1H NMR (600 MHz, CDCl3,
298 K): d = 7.18 (m, 1H, 5-HPy), 7.16 (m, 1H, 4-HPy), 7.11 (dd,
3J = 7.3 Hz, 4J = 2.4 Hz, 1H, 3-HPy), 7.04 (m, 1H, 4-HAr), 6.97 (m,
2H, 3-HAr), 5.99 (br s, 1H, NH), 3.87 (sept, 3J = 6.9 Hz, 1H,
CH(CH3)2Py), 3.14 (br sept, 3J = 6.9 Hz, 2H, CH(CH3)2Ar), 1.54
(s, 6H, NC(CH3)2), 1.33 (d, 3J = 6.9 Hz, 6H, CH(CH3)2Py), 0.95 (d,
3J = 6.9 Hz, 12H, CH(CH3)2Ar). 13C{1H} NMR (151 MHz, CDCl3,
B
3J = 6.9 Hz, 6H, CH(CH3ACH3 )0). 13C{1H} NMR (126 MHz, THF-d8,
B
298 K): d = 167.4 (@CCH30 ), 165.6 (@CCH3), 156.5 (C60Py), 151.7
(C6Py), 149.5 (C20Py), 148.0 (C2Py), 147.4 (C10Ar), 146.0 (C1Ar),
137.3 (C40Py), 136.5 (C2Ar), 136.2 (C20Ar), 128.5 (C3Py), 127.4
(C30Py), 126.0 (C5Py), 125.0 (C4Py), 124.6, 124.3 (C4Ar C40Ar),
,
0
0
Py
123.59, 123.56 (C3Ar, C3 Ar), 122.1 (C5 ), 29.1 (CH(CH3)02), 28.7
(CH(CH3)2), 23.7 (CH(CH3ACH3B)), 23.5 (CH(CH3ACH3 )0), 23.0
B
(CH(CH3ACH3B)), 22.9 (CH(CH3ACH3 )0), 19.6 (@CCH3), 17.2
B
(@CCH03).
X-ray crystal structure analysis for 12: formula C38H46N4O,
M = 574.79, yellow crystal 0.50 ꢁ 0.30 ꢁ 0.03 mm, a = 11.2767(4),
b = 10.4211(3), c = 29.1668(9) Å, b = 96.249(2)°, V = 3407.19(19) Å3,
q
calc = 1.121 g cmꢀ3 = 0.522 mmꢀ1, empirical absorption correc-
, l

3072
K. Nienkemper et al. / Journal of Organometallic Chemistry 693 (2008) 3063–3073
298 K): d = 159.1 (C6Py), 156.1 (C2Py), 148.8 (C2Ar), 139.6 (C1Ar),
125.3 (C4Py), 125.0 (C4Ar), 122.7 (C3Ar), 120.5 (C5Py), 120.2 (C3Py),
57.9 (NC(CH3)2), 28.0 (CH(CH3)2Ar), 27.9 (CH(CH3)2Py), 26.8
(NC(CH3)2), 24.1 (br, CH(CH3)2Ar), 20.3 (CH(CH3)2Py).
CH(CH3ACH3B)), 1.10 (d, 3J = 6.8 Hz, 3H, CH(CH3ACH3 )0), 1.04 (d,
B
3J = 6.8 Hz, 3H, CH(CH3ACH3 )0), ꢀ0.54 (br. s, 3H, Al(CH3)2), ꢀ0.77
B
(br s, 3H, Al(CH3)2). 13C{1H} NMR (151 MHz, C7D8, 333 K):
d = 158.9 (C2Py), 149.7 (C2Ar), 148.8 (C20Ar), 145.9 (C1Ar), 141.7
(C6Py), 138.3 (C4Py), 125.1 (C4Ar), 124.6 (C3Py), 124.3 (C30Ar),
123.9 (C3Ar), 123.7 (C5Py), 60.9 (NCH(CH3)), 28.1 (CH(CH3)02), 27.4
X-ray crystal structure analysis for 15b: formula C23H34N2O,
M = 354.52,
a = 11.626(1), b = 18.960(1), c = 19.166(1) Å, V = 4224.7(5) Å3,
calc = 1.115 g cmꢀ3 = 0.068 mmꢀ1, empirical absorption correc-
tion (0.967 6 T 6 0.998), Z = 8, orthorhombic, space group Pbca,
(No. 61), k = 0.71073 Å, T = 198 K, and / scans, 36160 reflections
collected ( h, k, l), [(sinh)/k] = 0.62 Åꢀ1
4287 independent
(I)], 262 refined
colorless
crystal
0.50 ꢁ 0.50 ꢁ 0.03 mm,
B
(CH(CH3)2), 26.0 (CH(CH3ACH3B)), 25.7 (CH(CH3ACH3 )0), 25.57
B
q
,
l
(CH(CH3ACH3 )0), 25.55 (CH(CH3ACH3B)), 20.1 (NCH(CH3)), ꢀ7.2,
ꢀ9.4 (br, Al(CH3)2).
x
,
4.11. jN,O-{2-[1-(2,6-Diisopropylphenylamido)methylethyl]pyridine-
(Rint = 0.072) and 2874 observed reflections [I P 2
r
N-oxide}dimethylaluminum (23)
parameters, R = 0.058, wR2 = 0.141, max. residual electron density
0.31 (ꢀ0.30) e Åꢀ3, hydrogen atom at N10 from difference fourier
map, other calculated and refined as riding atoms.
Trimethylaluminum (1.97 mL, 1.44 g, 20.0 mmol) was added to
a solution of 11a (3.00 g, 10.1 mmol) in toluene (50 mL) at 0 °C and
stirred over night. Then the volatiles were removed in vacuo and
the residue was washed with pentane, to get the product as a light
yellow solid. Crystallization from a pentane solution at ꢀ20 °C gave
crystals suitable for X-ray diffraction. M.p. 160 °C (DSC, Decomp.).
1H NMR (600 MHz, C6D6, 298 K): d = 7.43 (d, 3J = 6.2 Hz, 1H, 6-HPy),
7.16 (m, 1H, 4-HAr), 7.13 (m, 2H, 3-HAr), 6.53 (dd, 3J = 7.9 Hz,
4J = 1.7 Hz, 1H, 3-HPy), 6.43 (td, 3J = 7.9 Hz, 4J = 1.3 Hz, 1H, 4-HPy),
5.95 (ddd, 3J = 7.9 Hz, 3J = 6.2 Hz, 4J = 1.7 Hz, 1H, 5-HPy), 3.37 (sept,
3J = 6.8 Hz, 2H, CH(CH3)2), 1.45 (s, 6H, NC(CH3)2), 1.27 (d,
3J = 6.8 Hz, 6H, CH(CH3ACH3B)), 1.10 (d, 3J = 6.8 Hz, 6H,
CH(CH3ACH3B)), ꢀ0.23 (s, 6H, Al(CH3)2). 13C{1H} NMR (151 MHz,
C6D6, 298 K): d = 161.6 (C2Py), 150.0 (C2Ar), 145.1 (C1Ar), 140.4
(C6Py), 133.9 (C4Py), 124.7 (C4Ar), 123.5 (C3Ar), 122.9 (C5Py), 121.8
(C3Py), 58.6 (NC(CH3)2), 28.0 (NC(CH3)2), 27.8 (CH(CH3)2), 26.2
(CH(CH3ACH3B)), 25.1 (CH(CH3ACH3B)), ꢀ6.9 (br, Al(CH3)2).
4.9. 2-[1-(2,6-Diisopropylphenylamino)ethyl]pyridine-N-oxide (16)
(according to the procedure described for the synthesis of 15a)
Trimethylaluminum (14.2 mL, 28.4 mmol) was reacted with a
solution of 5a (4.00 g, 14.2 mmol) in toluene (100 mL). The de-
scribed workup was carried out, to yield the product after column
chromatography (SiO2; pentane/chloroform/methanol 10:3:1) as a
white solid (3.18 g, 75%). Crystallization from a chloroform/metha-
nol/triethylamine mixture gave crystals suitable for X-ray diffrac-
tion. M.p. 93 °C (DSC). MS-ESI (m/z, ES+, in methanol): 299.2
[M+H]+, 321.2 [M+Na]+. Anal. Calc. for C19H26N2O: C, 76.47; H,
8.78; N, 9.39. Found: C, 76.33; H, 8.80; N, 9.40%. 1H NMR
(600 MHz, C7D8, 298 K): d = 7.71 (dd, 3J = 6.1 Hz, 4J = 1.4 Hz, 1H,
6-HPy), 6.97 (m, 2H, 3-HAr), 6.94 (m, 1H, 4-HAr), 6.32 (dd,
3J = 7.4 Hz, 4J = 2.2 Hz, 1H, 3-HPy), 6.14 (td, 3J = 7.4 Hz, 4J = 1.4 Hz,
1H, 4-HPy), 6.11 (ddd, 3J = 7.4 Hz, 3J = 6.1 Hz, 4J = 2.2 Hz, 1H, 5-
HPy), 5.64 (br s, 1H, NH), 4.00 (m, 1H, NCH(CH3)), 3.47 (sept,
3J = 6.8 Hz, 2H, CH(CH3)2), 1.69 (d, 3J = 6.9 Hz, 3H, NCH(CH3)), 1.18
(d, 3J = 6.8 Hz, 6H, CH(CH3ACH3B)), 1.03 (d, 3J = 6.8 Hz, 6H,
CH(CH3ACH3B)). 13C{1H} NMR (151 MHz, C7D8, 298 K): d = 152.3
(C2Py), 143.0 (C1Ar), 142.9 (C2Ar), 140.5 (C6Py), 125.1 (C3Py),
123.93 (C4Ar), 123.86 (C3Ar), 123.79, 123.77 (C4Py, C5Py), 61.1
(NCH(CH3)), 27.8 (CH(CH3)2), 24.6 (CH(CH3ACH3B)), 24.4
(CH(CH3ACH3B)), 17.7 (NCH(CH3)).
X-ray crystal structure analysis for 23: formula C22H33AlN2O,
M = 368.48, yellow crystal 0.30 ꢁ 0.25 ꢁ 0.10 mm, a = 14.9530(6),
b = 10.2917(3), c = 28.8857(12) Å, b = 94.139(1)°, V = 4433.7(3) Å3,
q
calc = 1.104 g cmꢀ3 = 0.104 mmꢀ1, empirical absorption correc-
, l
tion (0.970 6 T 6 0.990), Z = 8, monoclinic, space group C2/c, (No.
15), k = 0.71073 Å, T = 198 K, and / scans, 13025 reflections col-
lected ( h, k, l), [(sinh)/k] 3911 independent
0.59 Åꢀ1
(I)], 243 refined
x
=
,
(Rint = 0.094) and 1932 observed reflections [I P 2
r
parameters, R = 0.060, wR2 = 0.155, max. residual electron density
0.21 (ꢀ0.25) e Åꢀ3, hydrogen atoms calculated and refined as rid-
ing atoms.
X-ray crystal structure analysis for 16: formula C19H26N2O,
M = 298.42, colorless crystal 0.15 ꢁ 0.10 ꢁ 0.10 mm, a = 13.240(1),
b = 10.673(1), c = 13.257(1) Å, b = 112.16(1)°, V = 1735.0(2) Å3,
Acknowledgements
q
calc = 1.142 g cmꢀ3 = 0.071 mmꢀ1, empirical absorption correc-
, l
tion (0.990 6 T 6 0.993), Z = 4, monoclinic, space group P21/c, (No.
14), k = 0.71073 Å, T = 198 K, and / scans, 14030 reflections col-
lected ( h, k, 4121 independent
l), [(sinh)/k] = 0.66 Åꢀ1
(I)], 208 refined
Financial support from the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie is gratefully acknowl-
edged. We thank the BASF for a generous gift of solvents.
x
,
(Rint = 0.062) and 2511 observed reflections [I P 2
r
parameters, R = 0.051, wR2 = 0.131, max. residual electron density
0.14 (ꢀ0.21) e Åꢀ3, hydrogen atom at N9 from difference fourier
map, other calculated and refined as riding atoms.
Appendix A. Supplementary material
CCDC 658474, 658475, 658476, 658477, 658478, 658479,
658480, 658481, 658482 and 658483 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2008.05.032.
4.10. jN,O-{2-[1-(2,6-Diisopropylphenylamido)ethyl]pyridine-N-
oxide}dimethylaluminum (22)
Trimethylaluminum (0.49 mL, 0.37 g, 5.10 mmol) was added to
a solution of 5a (0.72 g, 2.55 mmol) in toluene (25 mL) at 0 °C and
stirred over night. Then the volatiles were removed in vacuo and
the residue was washed with less pentane, to get the product as
a yellow solid (0.61 g, 67%). 1H NMR (600 MHz, C7D8, 333 K):
d = 7.80 (br d, 3J = 6.6 Hz, 1H, 6-HPy), 7.02 (m, 3H, 3-/4-HAr), 6.62
(br t, 3J = 7.8 Hz, 1H, 4-HPy), 6.54 (br d, 3J = 7.8 Hz, 1H, 3-HPy),
6.13 (br dd, 3J = 7.8 Hz, 3J = 6.6 Hz, 1H, 5-HPy), 4.18 (q, 3J = 6.9 Hz,
1H, NCH(CH3)), 3.86 (sept, 3J = 6.8 Hz, 1H, CH(CH3)2), 3.09 (sept,
3J = 6.8 Hz, 1H, CH(CH3)20 ), 1.28 (d, 3J = 6.9 Hz, 3H, NCH(CH3)),
1.18 (d, 3J = 6.8 Hz, 3H, CH(CH3ACH3B)), 1.17 (d, 3J = 6.8 Hz, 3H,
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