Facile generation of aluminum 1,2-dihydropyridyl and hydroxyl derivatives from an aluminum cyclopropene analogue

Article abstract of DOI:10.1021/om060722k

Reaction of LAl(η²-Me₃SiC₂SiMe ₃) (1; L = HC[(CMe)-(NDipp)]₂ Dipp = 2,6-iPr ₂C₆H₃) with pyridine afforded the (1,2-dihydropyridyl)aluminum species LAl[C₂(SiMe₃) ₂CHN-(C₄H₄)] (2), having an AlC₃N five-member-ed ring, as a major product via insertion into one of the Al-C bonds, while hydrolysis of 1 at low temperature resulted in ring cleavage to yield the monomeric aluminum alkenyl hydroxide LAl(OH)-[C(SiMe ₃)CH(SiMe₃)] (3). The molecular structures of 2 and 3 were determined by X-ray single-crystal analysis.

Full text of DOI:10.1021/om060722k

Organometallics 2006, 25, 5665-5667
5665
Notes
Facile Generation of Aluminum 1,2-Dihydropyridyl and Hydroxyl
Derivatives from an Aluminum Cyclopropene Analogue
Xiaofei Li, Lili Duan, Haibin Song, Chengbao Ni, and Chunming Cui*
State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai UniVersity,
Tianjin 300071, People’s Republic of China
ReceiVed August 9, 2006
Summary: Reaction of LAl(η2-Me3SiC2SiMe3) (1; L ) HC[(CMe)-
(NDipp)]2, Dipp ) 2,6-iPr2C6H3) with pyridine afforded the
(1,2-dihydropyridyl)aluminum species LAl[C2(SiMe3)2CHN-
(C4H4)] (2), haVing an AlC3N fiVe-membered ring, as a major
product Via insertion into one of the Al-C bonds, while
hydrolysis of 1 at low temperature resulted in ring cleaVage to
yield the monomeric aluminum alkenyl hydroxide LAl(OH)-
[C(SiMe3)CH(SiMe3)] (3). The molecular structures of 2 and 3
were determined by X-ray single-crystal analysis.
followed by subsequent transformations. The high reactivity of
1 combined with its tendency to form stable five-membered
aluminum heterocycles by insertion into CdX (X ) O, N) bonds
suggested that 1 could be a promising candidate to generate
1,2-dihydropridyl subunits, provided that 1 could insert into the
CdN bond of aromatic pyridines. Herein we report the facile
generation of dihydropyridylaluminum heterocycles by the
reaction of 1 with pyridine, along with the hydrolytic reaction
of 1 to yield an aluminum alkenyl hydroxide.
Introduction
Results and Discussion
Simple 1,2-dihydropyridines are electron-rich diene compo-
nents, which can undergo Diels-Alder reactions and reactions
characteristic of enamines.1 The most convenient preparation
of such species is the direct addition of alkyllithium and
Grignard reagents to pyridine or N-substituted electrophilic
pyridinium salts.1 Lithium aluminum hydride (LAH) in pyridine
also is known to reduce ketones selectively in the presence of
ester and carboxyl groups. It has been proposed, on the basis
of NMR investigations, that several isomers of lithium tetrakis-
(dihydropyridyl)aluminate containing 1,2- and 1,4-dihydro-
pyridyl ligands which formed initially are active species for the
reduction reactions.2 Recently the thermodynamically stable
(1,4-dihydropyridyl)aluminates and the corresponding neutral
species have been isolated and structurally characterized by
X-ray single-crystal analysis.3 However, it is well-known that
aluminum alkyls do not undergo insertion reactions with simple
pyridine derivatives under normal conditions but form adducts
such as Py‚AlR3.4 Recent studies of the reactivity of the AlC2
strained-ring compound LAl[η2-(Me3SiC2SiMe3)] (1; L )
HC[(CMe)(NDipp)]2, Dipp ) 2,6-iPr2C6H3)5a have shown that
it displayed diverse reactivity toward a number of functional
groups, such as ketones, nitriles, isonitriles, organic azides, and
small molecules such as O2, CO, and CO2.5 These reactions
either afforded insertion products or involved an initial insertion
Reaction of 1 with 1 equiv of pyridine from -78 °C to room
temperature followed by removal of volatiles under vacuum
1
afforded an orange solid. The H NMR spectrum of the crude
product showed that it contains two species, the ratio of which
was estimated to be ca. 10:1 according to the integration of the
SiMe3 resonances (Scheme 1). Orange crystals of 2 were
obtained by crystallization from diethyl ether. They were
characterized by 1H and 13C NMR and IR spectroscopy and by
1
X-ray single-crystal analysis. The H and 13C NMR spectra of
2 exhibit characteristic resonances for a 1,2-dihydropyridyl unit.
The IR spectrum displays relatively strong absorption bands
centered at 1637 and 1602 cm-1, which were assigned to the
stretching frequencies of the diene group. The molecular array
has been further verified by an X-ray single-crystal structural
analysis. Unfortunately, we were unable to isolate the minor
product in pure form, because its solubility was quite similar
to that of the major product. It is postulated that the minor
product could be the 1,4-addition product, since the proton NMR
spectrum of the crude product mixture also showed weak
separated resonances at δ 4.31, 5.09, 5.75, and 6.52 ppm in the
dihydropyridine unit region1a and two singlets at δ 0.29 and
0.48 ppm for the two SiMe3 groups, while the other resonances
for the compound overlapped with those of the major product.
The high selectivity to undergo a pyridine 1,2-insertion reaction
is attributed to the AlC2 three-membered-ring structure of 1, as
this type of insertion leads to the formation of the more stable
AlC3N five-membered ring in compound 2. Proton NMR studies
on 2 in C6D6 indicated that it did not decompose or isomerize
over 4 h at 75 °C. This behavior is in contrast to the
* To whom correspondence should be addressed. E-mail: cmcui@
nankai.edu.cn.
(1) For reviews, see: (a) Eisner, U.; Kuthan, J. Chem. ReV. 1972, 72, 1.
(b) Stout, D. M.; Meyers, A. I. Chem. ReV. 1982, 82, 223. (c) Kuthan, J.;
Kurfu¨rst, A. Ind. Eng. Chem. Prod. Res. DeV. 1982, 21, 191. (d)
(2) (a) Lansbury, P. T.; Rogozinski, J. R.; Coblentz, F. L. J. Org. Chem.
1961, 26, 2277. (b) Lansbury, P. T.; Peterson, J. O. J. Am. Chem. Soc.
1963, 85, 2236. (c) Tanner, D. D.; Yang, C.-M. J. Org. Chem. 1993, 58,
1840.
(3) Hensen, K.; Lemke, A,; Stumpf, T.; Bolte, M.; Fleischer, H.; Pulham,
C. R.; Gould, R. O.; Harris, S. Inorg. Chem. 1999, 38, 4700.
(4) Lichtenberger, D. L.; Hogan, R. H.; Healy, M. D.; Barron, A. R.
Organometallics 1991, 10, 609.
(5) (a) Cui, C.; Ko¨pke, S.; Herbst-Irmer, R.; Roesky, H. W.; Noltemeyer,
M.; Schmidt, H.-G.; Wrackmeyer, B. J. Am. Chem. Soc. 2001, 123, 9091.
(b) Zhu, H.; Chai, J.; Ma, Q.; Jancilk, V.; Roesky, H. W.; Fan, H.; Herbst-
Irmer, R. J. Am. Chem. Soc. 2004, 126, 10194. (c) Li, X.; Song, H.; Duan,
L.; Cui, C.; Roesky, H. W. Inorg. Chem. 2006, 45, 1912. (d) Li, X.; Ni, C.;
Song, H.; Cui, C. Chem. Commun. 2006, 1763.
10.1021/om060722k CCC: $33.50 © 2006 American Chemical Society
Publication on Web 10/13/2006

5666 Organometallics, Vol. 25, No. 23, 2006
Notes
Scheme 1. Synthesis of 2 and 3
hydroalumination of pyridine, in which dynamic isomerization
from 1,2- to 1,4-pyridinyl has been reported to occur.4
Single crystals of 2 suitable for an X-ray diffraction analysis
were obtained from diethyl ether at -20 °C. The structure of 2
is shown in Figure 1, together with selected bond parameters.
The structure reveals the presence of the AlC3N five-membered
ring, formed via pyridine addition to one of the Al-C bonds.
The C30-C31 (1.348(9) Å), C31-C32 (1.429(10) Å), and
C32-C33 (1.313(9) Å) bond lengths indicate the C30-C31-
C32-C33 diene moiety. The N3-C30 (1.376(7) Å) bond length
is shorter than that of N3-C34 (1.470(8) Å) because of the
partial overlap of the lone-pair electron on the nitrogen atom
with the diene π electrons. The dihydropyridyl ring is not planar,
and the internal angles range from 109.5(5) to 123.9(6)°. These
structural features correlate well with those of 1,2-dihydro-
pyridine derivatives.6 C34-C35 (1.527(8) Å) and C35-C36
(1.365(8) Å) are typical of C-C single and double bonds,
respectively.
crystallization from diethyl ether (Scheme 1). In contrast,
controlled hydrolysis of LAlR2 and LAlH2 leads to ligand
cleavage. Several ligand L supported aluminum hydroxides are
known, but they are generated from the corresponding aluminum
halides in the presence of NHC carbenes or in ammonia and
from hydrolysis of the aluminum amide and oxide.7 None were
prepared by the direct hydrolytic cleavage of an Al-C bond.
The selective cleavage of the Al-C bond in 1 may result from
1
ring strain reactivity. The H NMR spectrum of 3 displays a
broad singlet at δ 0.42 ppm, attributed to the resonance of the
hydroxyl proton. The IR spectrum shows the characteristic OH
absorption centered at 3731 cm-1. The structure was finally
confirmed by an X-ray single-crystal analysis, which is shown
in Figure 2. The Al1-O1 (1.7036(15) Å) bond length is very
close to those found in LAl(OH)2 (1.7107(16) and 1.6947(15)
Å).7a The Al-C bond length (1.9845(19) Å) is in the reported
range for a Al-C single bond.5a The C30-C31 bond length
(1.353(3) Å) is consistent with a C-C double bond.5a
Reaction of 1 with water in diethyl ether proceeded instantly
at -78 °C to give pale yellow crystals of 3 in high yield after
Compound 3 is thermally stable and could be stored under
an inert atmosphere for a long time. The OH group does not
seem to dissociate in the presence of pyridine or to interact
1
with a strong Lewis acid, B(C6F5)3, according to H NMR
studies.
In summary, reaction of 1 with pyridine resulted in the
insertion of pyridine into one of the Al-C bonds with high
Figure 1. Ortep drawing of 2 (30% probability). One of the two
independent molecules in the unit cell and hydrogen atoms have
been omitted for clarity. Selected bond lengths (Å) and angles
(deg): Al1-N1 ) 1.916(5), Al1-N2 ) 1.913(5), Al1-N3 )
1.838(4), Al1-C36 ) 1.974(6), N3-C30 ) 1.376(7), N3-C34 )
1.470(8), C30-C31 ) 1.348(9), C31-C32 ) 1.429(10), C32-
C33 ) 1.313(9), C33-C34 ) 1.536(8), C34-C35 ) 1.527(8),
C35-C36 ) 1.365(8); N1-Al1-N2 ) 96.5(2), N3-Al1-C36 )
92.0(2), Al1-N3-C30 ) 132.1(4), Al1-N3-C34 ) 110.7(3),
N3-C30-C31 ) 123.9(6), N3-C34-C33 ) 109.5(5), C30-N3-
C34 ) 115.7(5), C30-C31-C32 ) 116.8(6), C31-C32-C33 )
121.9(6), C32-C33-C34 ) 118.4(6).
Figure 2. Ortep drawing of 3 (30% probability). Hydrogen atoms
have been omitted for clarity. Selected bond lengths (Å) and angles
(deg): Al1-N1 ) 1.9075(16), Al1-N2 ) 1.9249(17), Al1-O1
) 1.7036(15), Al1-C30 ) 1.9845(19), C30-C31 ) 1.353(3); N1-
Al1-N2 ) 96.27(7), N1-Al1-O1 ) 108.17(7), N1-Al1-C30
) 114.48(8), N2-Al1-O1 ) 113.03(7), N2-Al1-C30 ) 110.51(8),
O1-Al1-C30 ) 113.25(8).

Notes
Organometallics, Vol. 25, No. 23, 2006 5667
LAl(OH)[C(SiMe3)CH(SiMe3)] (3). To a solution of 1 (0.31 g,
0.5 mmol) in diethyl ether (20 mL) at -78 °C was added a solution
of water in diethyl ether (1 mL, 0.5 M, 0.5 mmol). The black-red
solution immediately turned to pale yellow. The mixture was
warmed to room temperature and stirred for 30 min. The resulting
yellow solution was concentrated (to ca. 5 mL) under vacuum and
stored at -30 °C overnight to give colorless crystals of 3 (0.25 g,
region selectivity. The formed (1,2-dihydropyridyl)aluminum
compound is thermally stable, and its isomerization to the 1,4-
dihydropyridyl subunit was not observed. Hydrolysis of 1
resulted in selective cleavage of one of the Al-C bonds and
yielded an aluminum alkenyl hydroxide. These results indicate
that the ring strain results not only in the high reactivity of 1
but also in the high selectivity in the two reactions. Because of
the dihydropyridyl unit in 2 and OH group in 3, both compounds
could be used for further transformations.
1
79%). Mp: 172 °C. H NMR (CDCl3, 400 MHz): δ -0.37 (s,
9H, SiMe3), -0.20 (s, 9H, SiMe3), 0.42 (s, 1H, OH), 1.04 (d, 6H,
J ) 6.40 Hz, CHMe2), 1.08 (m, 12H, CHMe2), 1.19 (d, 6H, J )
6.80 Hz, CHMe2), 1.78 (s, 6H, â-CMe), 3.04 (sept, 2H, J ) 6.80
Hz, CHMe2), 3.38 (sept, 2H, J ) 6.80 Hz, CHMe2), 5.24 (s, 1H,
γ-CH), 7.0-7.24 (m, 6H, Ar H), 7.27 (s br, 1H, CHSiMe3). 13C
NMR (CDCl3): δ 1.22, 2.10 (SiMe3), 24.14 (â-Me), 24.87, 25.05
(CHMe2), 28.23 (CHMe2), 98.66 (γ-C), 109.8 (CdCHSiMe3),
124.3, 124.6, 126.9, 141.4, 143.6, 144.6 (Ar C), 152.0 (Al-C),
169.6, 170.5 (NC). IR (cm-1): 3731 (OH), 3057, 2964, 2869, 2760,
2610, 2475, 2360, 1932, 1867, 1803, 1702, 1662, 1585, 1548, 1524,
1463, 1439, 1392, 1317, 1252, 1174, 1055, 1018, 937, 860, 833,
800, 761, 735, 685, 668, 596, 534. Anal. Calcd for C37H61AlN2-
OSi2 (633.05): C, 70.20; H, 9.71; N, 4.43. Found: C, 70.05; H,
10.19; N, 4.32.
Experimental Section
All experiments were carried out under an argon atmosphere
using Schlenk line and glovebox techniques. The solvents were
dried over activated molecular sieves and then refluxed over sodium
1
and distilled prior to use. H and 13C NMR spectra were recorded
on a Bruker AMD-400 NMR spectrometer. IR spectra were
recorded on a Bio-Rad FTS 6000 spectrometer. Compound 1 was
prepared according to literature methods.5a
LAl[C2(SiMe3)2CHN(C4H4)] (2). To a solution of 1 (1.28 g,
2.0 mmol) in n-hexane at -78 °C was added neat pyridine (0.16
g, 2 mmol). The mixture was warmed slowly. A gradual color
change from black-red to orange was observed over 30 min. The
mixture was stirred for an additional 2 h at room temperature. The
solution was concentrated (to ca. 10 mL) and stored at -20 °C for
1 week to give orange crystals of 2 (0.59 g, 41%). Mp: 137-139
°C. 1H NMR (C6D6): δ -0.24, 0.38 (s, 9H + 9H, SiMe3), 1.02 (d,
3H, J ) 6.40 Hz, CHMe2), 1.11 (d, 3H, J ) 6.80 Hz, CHMe2),
1.12 (d, 6H, J ) 6.80 Hz, CHMe2), 1.19 (d, 6H, J ) 6.00 Hz,
CHMe2), 1.29 (d, 3H, J ) 6.40 Hz, CHMe2), 1.31 (d, 3H, J ) 6.80
Hz, CHMe2), 1.44 (s, 3H, CMe), 1.57 (s, 3H, CMe), 3.04, 3.20,
3.37, 3.83 (sept, 1H + 1H + 1H + 1H, J ) 6.80 Hz, CHMe2),
4.87 (s br, 1H, NCH), 4.99 (s, 1H, γ-CH), 5.19 (d, 1H, CH), 5.45
(t, 1H, CH), 6.24 (t, 1H, CH), 6.95-7.11 (m, 7H, Ar H + CH).
13C NMR (C6D6): δ 2.01, 2.44 (SiMe3), 23.54, 24.54, 24.76, 24.88,
25.12, 25.34, 25.63, 26.03 (Me and CHMe2), 28.05, 28.54, 28.64,
28.94 (CHMe2), 68.40 (NCHCSi), 100.48, 100.63 (CH and γ-CH),
100.82 (CH), 124.38, 124.53, 125.00, 125.24, 127.21, 127.54,
140.36, 141.30, 142.03, 142.45, 143.90, 145.25, 145.60 (Ar C and
CH), 170.87, 172.09 (NCMe), 179.72 (NCCH). UV-vis (hex-
anes): λmax/nm (ꢀ/mol-1 L cm-1) 351.4 (20 700). IR: ν/cm-1 3404,
3058, 2960, 2926, 2868, 1638, 1601, 1548, 1463, 1440, 1382, 1315,
1258, 1094, 1018, 935, 875, 835, 801, 758, 633, 486, 453. Anal.
Calcd for C42H64AlN3Si2 (693.45): C, 72.67; H, 9.29; N, 6.05.
Found: C, 72.60; H, 9.60; N, 5.45.
X-ray Structural Determination. Data were collected at 294(2)
K on a Bruker Smart-Apex II diffractometer using graphite-
monochromated Mo KR (λ ) 0.710 73 Å) radiation. All structures
were solved by direct methods (SHELXS-97)8 and refined by full-
matrix least squares on F2. All non-hydrogen atoms were refined
anisotropically, and hydrogen atoms were refined by a riding model
(SHELXL-97).9 Crystallographic data for 2: C42H64AlN3Si2, Mr
) 694.12, orange plate, 0.30 × 0.26 × 20 mm, monoclinic, space
group Pn, a ) 12.67554(17) Å, b ) 21.369(3) Å, c ) 16.374(2)
Å, â ) 99.367(2)°, V ) 4369.2(10) Å,3 Z ) 4, Dc ) 1.055 g cm-3
,
F(000) ) 1512, 24 627 reflections measured (12 113 unique). R1
) 0.0642 (I > 2σ(I)), wR2 ) 0.1929 (all data), GOF ) 1.007 for
896 parameters and 26 restraints. Crystallographic data for 3:
C37H62AlN2OSi2, Mr ) 634.05, colorless plate, 0.52 × 0.46 × 0.26
mm, monoclinic, space group P21/n, a ) 11.326(4) Å, b )
19.690(7) Å, c ) 18.246(6) Å, â ) 102.217(4)°, V ) 3977(2) Å,3
Z ) 4, Dc ) 1.059 g cm-3, F(000) ) 1388, 16 271 reflections
measured (11 213 unique). R1 ) 0.0454 (I > 2σ(I)), wR2 ) 0.1355
(all data), GOF ) 1.031 for 404 parameters and 3 restraints.
Acknowledgment. This work was supported by the National
Natural Science Foundation of China (Grant No. 20421202) and
the NCET.
Supporting Information Available: CIF files for the structures
of 2 and 3. This material is available free of charge via the Internet
at http://pubs.acs.org.
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OM060722K
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