Aluminium-containing ring systems and N-heterocycle formation via nitrile insertions into Al-N bonds

Article abstract of DOI:10.1039/b007810g

Reactions of Me₃Al with 1,2-diaminobenzene [1,2-(H₂N)₂C₆H₄] or anthranilic acid, [1,2-(H₂N)(HO₂-C)C₆H₄], followed by treatment with acetonitrile, afford tetranuclear and hexanuclear aluminium-containing ring systems; a single crystal X-ray structure on the hexametallic product reveals the construction of quinazoline ligands arising via insertion of acetonitrile into Al-N bonds.

Full text of DOI:10.1039/b007810g

Aluminium-containing ring systems and N-heterocycle formation via nitrile
insertions into Al–N bonds
Vernon C. Gibson,* Carl Redshaw,† Andrew J. P. White and David J. Williams
Department of Chemistry, Imperial College, South Kensington, London, UK SW7 2AY. E-mail: v.gibson@ic.ac.uk
Received (in Cambridge, UK) 7th September 2000, Accepted 6th November 2000
First published as an Advance Article on the web 15th December 2000
Reactions of Me3Al with 1,2-diaminobenzene [1,2-
(H2N)2C6H4] or anthranilic acid, [1,2-(H2N)(HO2- C)C6H4],
followed by treatment with acetonitrile, afford tetranuclear
and hexanuclear aluminium-containing ring systems; a
single crystal X-ray structure on the hexametallic product
reveals the construction of quinazoline ligands arising via
insertion of acetonitrile into Al–N bonds.
a related manner to that postulated for the reaction of hydrazines
with Me3Al/MeCN.5
Similar treatment of anthranilic acid, [1,2-(NH2)-
(HO2C)C6H4] (twice sublimed) with 2 equivalents of a toluene
solution of Me3Al affords, after work-up in acetonitrile, large
yellow prisms for which the 1H NMR spectrum possesses nine
distinct singlets in the Al–methyl region. The X-ray analysis§ of
the product revealed the chiral trimeric hexanuclear complex
[(Me2AlL)(MeAl)(m3-O)(m-O)]}3 3 (L = quinazoline) shown
in Fig. 1. The central Al3O3 ring has a twisted boat
conformation, whereas the three attached Al2O2CN rings each
have a folded envelope geometry. All six aluminium atoms
exhibit marked departures from ideal tetrahedral geometry with
angles in the range 100.9(2)–124.4(3)°. Although not possess-
ing strict C3 symmetry [the methyls on Al(2) and Al(4) are ‘up’,
whilst that on Al(6) is ‘down’], the pattern of bonding
throughout the structure is essentially three-fold symmetric. It is
interesting to note that whilst all of the Al–O bond lengths
within the central Al3O3 ring and also those to O(2), O(4) and
O(6) are all essentially the same [1.770(4)–1.794(4) Å], those to
Al(1), Al(3) and Al(5) are all significantly longer
[1.811(4)–1.827(4) Å]. As 3 is not the product of a chiral
synthesis, the presence in the crystals of molecules of only one
chirality is a consequence of spontaneous resolution upon
crystallisation.
It is well over one hundred years since the trimerisation of
nitriles to triazines by organometallic reagents was first noted,
by Hofmann1 using sodium, and by Frankland2 in his studies on
Et2Zn. During the 1960s, Wade3 and Lappert4 did much to
advance the understanding of nitrile binding and insertion
reactions at main group centres; however, the potential of such
reactions for synthesising useful heterocycles and interesting
metal-containing ring systems has remained largely undevel-
oped.
Recently, we described the synthesis of large aluminium-
containing ring systems via treatment of Me3Al with hydra-
zines;5 these included a highly unusual octa-aluminium struc-
tural analogue of a tetrapyrrole. The key macrocycle-forming
step revolves around the insertion of acetonitrile into the
aluminium–nitrogen bonds of intermediate amide species. With
a view to probing the generality of this approach for the
synthesis of aluminium-containing macrocycles, and also to
evaluate the potential of this methodology for constructing
nitrogen heterocycles, we have extended the study to other
classes of amine substrate. Here, we describe the reactivity of
Me3Al towards 1,2-diaminobenzene [1,2-(H2N)2C6H4] and
anthranilic acid [1,2-(H2N)(HO2C)C6H4]. The former gives rise
to a tetranuclear complex, the latter to an unprecedented
hexanuclear species incorporating N-heterocyclic ligands.
Slow addition of a solution of Me3Al (2 equivalents) in
toluene to 1,2-(H2N)2C6H4, followed by a 12 h reflux, afforded
a pale brown solution. After removal of the solvent under
reduced pressure, the residue was dissolved in acetonitrile and
heated to reflux for 2–3 min. Slow cooling of this solution,
followed by standing at room temperature for 2 days, gave
colourless needles of 1 in 40% yield (Scheme 1). The 1H NMR
spectrum‡ of 1 consists of four equal intensity singlets in the
Al–methyl region (d 20.37 to 20.75) together with a similar
intensity singlet at d 1.29 attributable to carbon-bonded methyl
groups. X-Ray analysis§ reveals the product to be the Ci
symmetric twelve-membered macrocyclic complex 1 compris-
ing four aluminium atoms (two bridging and two chelating), six
nitrogen atoms and two carbon atoms, The structure of this
product is closely related to that obtained from the reaction of
MePhNNH2 with Me3Al,5 and is not described in further detail
here.
Whereas the formation of 1 can be rationalised in terms of
acetonitrile insertion into the aluminium–nitrogen bonds of 2,
the carboxylic acid group of anthranilic acid contributes oxygen
atoms to the Al3O3 core around which the quinazoline ligands
are clustered. The reaction reproducibly affords 3 in ca. 45%
yield; its hydrolysis then readily releases the 2-methyl-4(3H)-
quinazolinone heterocycle 4 (identified by comparison of NMR
data with those of an authentic sample) in excellent yield. We
In the absence of acetonitrile, the 2+1 reaction of Me3Al and
1,2-(H2N)2C6H4 (in toluene) has been shown to afford the
asymmetric complex [(Me2Al)2AlMe(C6H4(NH)2)2]·AlMe3 2.6
This, or a closely related derivative, is the likely precursor to 1.
Formation of the 12-membered ring product is brought about by
insertion of two acetonitrile molecules into the Al–N bonds, in
† Present address: School of Chemical Sciences, University of East Anglia,
Scheme 1 Compounds 1–4 (inserted molecules of acetonitrile are shown in
Norwich, UK NR4 7TJ.
bold).
DOI: 10.1039/b007810g
Chem. Commun., 2001, 79–80
This journal is © The Royal Society of Chemistry 2001
79

(s, 3H, Me-quin), 2.88 (s, 3H, Me-quin), 2.89 (s, 3H, Me-quin), 7.61 (m, 3H,
quinH), 7.82 (m, 3H, quinH), 7.92 (m, 3H, quinH), 8.35 (m, 3H, quinH). 13
C
NMR (100.6 MHz, CDCl3, 298 K), d 212.24, 211.56 (2 3 m, 3 3 AlMe),
27.22, 26.79, 26.45, 25.43, 25.10, 24.57 (6 3 s, 3 3 AlMe2), 25.53,
25.82, 25.93 (3 3 s, Me-quin), 117.15 (m, 3 3 aryl C), 125.28–127.61
(overlapping m, 9 3 aryl C), 136.33 (m, 3 3 aryl C), 150.98 (m, 3 3 aryl
C), 158.45 (m, 3 3 aryl C), 169.31 (m, 3 3 aryl C). EI-MS: m/z: 807 (M+
2 Me). IR: n(m3-O)Al3 800 cm21
.
§ Crystal data: For 1: C24H42N6Al4, M = 522.6, orthorhombic, space group
Pbca (no. 61), a = 8.717(1), b = 16.868(1), c = 20.416(2) Å, V =
3002.0(4) Å3, Z = 4 (the complex has crystallographic Ci symmetry), Dc =
1.156 g cm23, m(Cu-Ka) = 16.1 cm21, F(000) = 1120, T = 183 K; clear
prisms, 0.27 3 0.23 3 0.13 mm, Siemens P4/RA diffractometer, w-scans,
2182 independent reflections. The structure was solved by direct methods
and the non-hydrogen atoms were refined anisotropically using full-matrix
least squares based on F2 to give R1 = 0.069, wR2 = 0.173 for 1479
independent observed reflections [|Fo| > 4s(|Fo|), 2q < 120°] and 162
parameters.
For 3: C36H48N6O6Al6, M = 822.7, orthorhombic, space group P212121
(no. 19), a = 12.152(1), b = 16.054(1), c = 22.310(1) Å, V = 4352.3(6)
Å3, Z = 4, Dc = 1.256 g cm23, m(Cu-Ka) = 17.9 cm21, F(000) = 1728,
T = 173 K; yellow rhombs, 0.50 3 0.23 3 0.23 mm, Siemens P4/RA
diffractometer, w-scans, 4017 independent reflections. The structure was
solved by direct methods and the non-hydrogen atoms were refined
Fig. 1 The molecular structure of 3. Selected bond lengths (Å); Al(1)–O(1)
1.811(4), Al(1)–N(2) 2.021(5), Al(2)–O(1) 1.785(4), Al(2)–O(3) 1.788(4),
Al(2)–O(2) 1.779(4), Al(3)–O(3) 1.827(4), Al(3)–N(22) 2.034(5), Al(4)–
O(3) 1.770(4), Al(4)–O(4) 1.783(5), Al(4)–O(5) 1.785(4), Al(5)–O(5)
1.825(4), Al(5)–N(42) 2.015, Al(6)–O(1) 1.784(4), Al(6)–O(5) 1.794(4),
Al(6)–O(6) 1.785(5).
anisotropically using full-matrix least squares based on F2 to give R1
=
0.055, wR2 = 0.133 for 3483 independent observed reflections [|Fo| >
4s(|Fo|), 2q < 128°] and 488 parameters. The absolute structure of 3 was
determined by use of the Flack parameter which refined to a value of
20.07(7).
CCDC 182/1855. See http://www.rsc.org/suppdata/cc/b0/b007810g/ for
crystallographic files in .cif format.
note that the formation of N-heterocycles by this methodology
is related to the intramolecular Ritter reaction7–9 in which
nitrilium salts, generated in the presence of Friedel–Crafts
reagents, react with a second nitrile molecule to give quinazo-
line ring systems.9
1 A. W. Hofmann, Chem. Ber., 1868, 1, 194.
Future studies will focus on further exploiting nitrile
insertions into aluminium (and gallium) bonds to access unusual
inorganic ring systems and nitrogen heterocycle products.
The EPSRC and the Leverhulme Trust (for a Research
Fellowship to C. R.) are thanked for financial support. Professor
Charles Rees is thanked for helpful discussions.
2 E. Frankland and J. C. Evans, J. Chem. Soc., 1880, 37, 563.
3 H. J. Emeléus and K. Wade, J. Chem. Soc., 1960, 2614; J. E. Lloyd and
K. Wade, J. Chem. Soc., 1964, 1649; J. E. Lloyd and K. Wade, J. Chem.
Soc., 1965, 2662; J. R. Jennings, J. E. Lloyd and K. Wade, J. Chem. Soc.,
1965, 5083; I. Pattinson and K. Wade, J. Chem. Soc., 1968, 57; J. R.
Jennings and K. Wade, J. Chem. Soc. A, 1968, 1946.
4 W. Gerrard, M. F. Lappert and J. W. Wallis, J. Chem. Soc., 1960, 2178;
M. F. Lappert and B. Prokai, Adv. Organomet. Chem., 1967, 5, 225 and
references therein.
Notes and References
‡ Satisfactory microanalyses have been obtained.
5 V. C. Gibson, C. Redshaw, A. J. P. White and D. J. Williams, Angew.
Chem., Int. Ed., 1999, 961.
For 1: 1H NMR (400 MHz, C6D6, 298 K), d 20.75, 20.48, 20.42, 20.37
(4 3 s, 8 3 3H, AlMe), 1.29 (s, 2 3 3H, CMe), 6.36–7.27 (3 3 m, 8H, aryl
H), NH not seen. 13C NMR (100.6 MHz, C6D6, 298 K), d 212.24, 212.81
(br, AlMe), 28.91 (br, AlMe), 25.99 (br, AlMe). IR: n(N–H) 3247
6 R. L. Wells, H. Rahbarnoohi, P. B. Glaser, L. M. Liable-Sands and A. L.
Rheingold, Organometallics, 1996, 15, 3204.
7 J. J. Ritter and P. P. Minieri, J. Am. Chem. Soc., 1948, 70, 4045; J. J. Ritter
and J. Kalish, J. Am. Chem. Soc., 1948, 70, 4048.
8 L. I. Krimen and D. J. Cota, Org. React. (N.Y.), 1969, 17, 213.
9 R. Bishop, in Comprehensive Organic Synthesis, ed. B. M. Trost and I.
Fleming, Pergamon, Oxford, UK, 1991, vol. 6 and references therein.
cm21
.
For 3: 1H NMR (400 MHz, CDCl3, 298 K), d 20.87, 20.82, 20.59,
20.47, 20.46, 20.45, 20.24, 20.23, 20.22 (9 3 s, each 3H, AlMe), 2.79
80
Chem. Commun., 2001, 79–80