Dialkyl aluminium amides: New reagents for the conversion of C=O into C=NR functionalities

Article abstract of DOI:10.1039/b203693b

A new methodology for the preparation of α-diimines and β-aminoenones has been devised and represents an alternative route to these and related nitrogenous ligands bearing highly electronegative substituents.

Full text of DOI:10.1039/b203693b

Dialkyl aluminium amides: new reagents for the conversion of CNO into
CNNR functionalities†
John C. Gordon,*a Piyush Shukla,b Alan H. Cowley,b Jamie N. Jones,b D. Webster Keoghc and Brian
L. Scotta
a Los Alamos National Laboratory, Chemistry Division, MS J514, Los Alamos, New Mexico 87545, USA.
E-mail: jgordon@lanl.gov
b Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712, USA
c
Los Alamos National Laboratory, Chemistry Division, MS G739, Los Alamos, New Mexico 87545, USA.
E-mail: jgordon@lanl.gov
Received (in Columbia, MO, USA) 16th April 2002, Accepted 1st October 2002
First published as an Advance Article on the web 18th October 2002
A new methodology for the preparation of a-diimines and b-
aminoenones has been devised and represents an alternative
route to these and related nitrogenous ligands bearing highly
electronegative substituents.
a-Diimines, b-diimines and b-ketoimines are assuming increas-
ingly important roles as ligands in catalytic processes1 and also
for the stabilization of unusual bonding situations.2 The
standard method for the synthesis of such ligands involves the
reaction of an a- or b-diketone with the appropriate primary
amine in the presence of an acid catalyst.3 While many of these
reactions proceed in high yields, this methodology is less
satisfactory for acid-sensitive carbonyls and weakly nucleo-
philic primary amines.4 In order to develop catalytic systems
with enhanced activity at metal centers, it has become desirable
Fig. 1 Thermal ellipsoid plot (30% probability level) for 2. Selected bond
distances (Å) and angles (°): Al(1)–N(1) 1.971(2), Al(1)–N(1A) 1.989(2),
Al(1)–C(1) 1.937(3), Al(1)–C(2) 1.939(3), N(1)–Al(1)–N(1A) 87.64(9),
to attach highly electronegative substituents to the nitrogen
atoms of these classes of ligands.5 Since primary amines
containing electron withdrawing groups are anticipated to be
poor nucleophiles, we envisioned that a new synthetic method-
ology for converting CNO into CNNR functionalities was called
for (where R may be for example, a (per)fluorinated aromatic
group).
The fact that Al–O and O–H bonds are appreciably stronger
than Al–N and N–H bonds suggested that primary aminoalanes
would be suitable reagents for effecting the desired transforma-
tions (eqn. 1).
Al(1)–N(1)–Al(1A) 92.22(9), C(1)–Al(1)–C(2) 123.12(16).
corresponding yields for the primary amine route are 70, 0.7 and
0.2%, respectively. By way of comparison, Tilset et al.5a
reported that a 26% yield of ArNNCMeCMeNNAr (Ar =
3,5-(CF3)2C6H3) can be obtained from the reaction of 3,5-bis-
(trifluoromethyl)aniline with 2,3-butanedione. A further ad-
vantage of the aminoalane route is that the procedures are much
cleaner for the majority of the reactions investigated, thus
greatly facilitating product separation and purification. The
crystalline diimines 4 and 6 possesses a trans molecular
structure exemplified by that of 6 which is shown in Fig. 2.
The aminoalanes 1, 2 and 3 also react with 2,4-pentanedione
to afford the b-aminoenones ONCMeCHNC(Me)N(H)Ar, 7 (Ar
= p-fluorophenyl), 8 (Ar = 3,5-difluorophenyl) and 9 (Ar =
C6F5) in yields of 68, 57 and 52%, respectively. The yields of 7,
8 and 9 obtained via the conventional approach are somewhat
inferior (54, 14 and 11%, respectively). Compounds 7–9 adopt
the b-aminoenone structure in the solid state rather than the b-
ketoimine tautomeric alternative and the CNO and C–N(H)Ar
functionalities are arranged in a syn fashion (Fig. 3). Such a
(1)
Further support for this concept stems from the observation6
that lithium aluminium amides will convert aldehydes and
cyclic ketones into the corresponding imines. It is not known,
however, whether this route is effective for aldehydes and
ketones with highly electronegative substituents.
The following aminoalanes have been prepared in moderate
to good yields by treatment of the appropriate primary amine
with AlMe3 in toluene solution: [Me2Al-m-N(H)Ar]2 (Ar = p-
fluorophenyl (1), 3,5-difluorophenyl (2), and pentafluorophenyl
(3)).‡ The dimeric nature of 2 and 3 in the solid state was
confirmed by X-ray crystallography.§ Compound 2 adopts a cis
geometry with respect to the nitrogen substituents on the Al2N2
ring (Fig. 1) while in the case of 3, the stereochemistry is trans.
Structurally characterized examples of aluminoalane complexes
have been previously reported in the literature.7
In two of the three a-diketone reactions studied, the use of the
new aminoalane reagents results in a higher yield of the a-
diimine product than that afforded by use of the primary amine
methodology. Specifically, the reactions of 2,3-butanedione
with 1, 2 and 3 afford the a-diimines, ArNNCMeCMeNNAr, 4
(Ar = p-fluorophenyl), 5 (Ar = 3,5-difluorophenyl), and 6 (Ar
= C6F5) in yields of 59, 22 and 15%, respectively, while the
Fig. 2 Thermal ellipsoid plot (30% probability level) for 6. Selected bond
distances (Å) and angles (°): N(1)–C(10) 1.293(4), C(10)–C(10A) 1.494(5),
C(10)–C(11) 1.487(5), N(1)–C(1) 1.408(4), C(10A)–C(10)–C(11)
118.8(3), C(10A)–C(10)–N(1) 114.8(3), C(11)–C(10)–N(1) 126.4(3).
† Electronic supplementary information (ESI) available: HRMS, 1H and 19
NMR data for 1–9. See http://www.rsc.org/suppdata/cc/b2/b203693b/
F
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This journal is © The Royal Society of Chemistry 2002

Fig. 3 Thermal ellipsoid plot (30% probability level) for 9. Selected bond distances (Å) and angles (°): N(1)–C(10) 1.411(4), N(1)–C(2) 1.349(4), C(2)–C(3)
1.374(4), C(3)–C(4) 1.427(5), C(4)–C(5) 1.506(5), C(4)–O 1.240(4), C(10)–N(1)–C(2) 125.2(3), N(1)–C(2)–C(1) 117.7(3), C(1)–C(2)–C(3) 121.6(3), N(1)–
C(2)–C(3) 120.7(3), C(3)–C(4)–O 121.9(3), C(3)–C(4)–C(5) 119.2(3), C(5)–C(4)–O 118.9(3).
2,4-pentanedione. (The use of either 1+1 or 2+1 mole ratios of aminoalane-
+diketone produced the same result.) In the case of 9, the stirred reaction
mixture was refluxed for 45 h, while for 7 and 8 the reaction mixture was
stirred for 90 h at ambient temperature. Crystals of 8 and 9 were grown in
the same manner as 4 and 5, while crystals of 7 were grown as described for
6.
structure assignment is consistent with the pattern of bond
distances in the NC3O skeleton (Fig. 3 caption) and the
detection of N–H resonances at d (ppm, C6D6) 12.8 (7), 12.7 (8)
and 12.5 (9) in the 1H NMR spectra. The coordination chemistry
of these new and related nitrogenous ligands bearing highly
electronegative substituents is under active investigation.
In summary, the aminoalane method represents a useful
alternative methodology for the conversion of CNO into CNNR
functionalities. Complementarity between the amine and ami-
noalane routes is nicely illustrated by the fact that the reaction
of 2,4-pentanedione with (C6F5)2NH and p-toluene sulfonic
acid in refluxing toluene results in the corresponding b-
diketimine8 while the reaction of 2,4-pentanedione with 3
affords b-aminoalane 9.
§ Crystal data for 2: C16H20Al2F4N2, monoclinic, space group C2/c, a =
14.241(5), b
= 7.324(2), c = 18.512(7) Å, b = 106.376(6)°, V =
1852.7(11) Å3, Z = 4, Dc = 1.328 g cm23, m(Mo-Ka) = 0.194 mm21
.
¯
Crystal data for 6: C16H6F10N2, triclinic, space group P1, a = 6.3833(13),
b = 7.7232(15), c = 8.2726(17) Å, a = 91.79(3), b = 107.64(3), g =
103.10(3)°, Z = 1, Dc = 1.837 g cm23, m(Mo-Ka) = 0.197 mm21. Crystal
data for 9: C11H7F5NO, monoclinic, space group P21/c, a = 10.830(5), b =
8.734(5), c = 11.608(5) Å, b = 90.433(3)°, V = 1098.0(9) Å3, Z = 4, Dc
= 1.604 g cm23, m(Mo-Ka) = 0.161 mm21. All three structures were
solved by direct methods and refined to R1 values of 0.0914, 0.0596, and
We are grateful to the U.S. Department of EnergyAs Defense
Programs Education Office, the Laboratory Directed Research
and Development Program at Los Alamos National Laboratory,
D.O.EAs Office of Basic Energy Sciences, and the Robert A.
Welch Foundation for support of this work. Los Alamos
National Laboratory is operated by the University of California
under contract W-7405-ENG-36.
0.0723 for 2,
6 and 9, respectively. CCDC reference numbers
184628–184630. See http://www.rsc.org/suppdata/cc/b2/b203693b/ for
crystallographic data in CIF or other electronic format.
1 See, for example S. D. Ittel, L. K. Johnson and M. Brookhart, Chem. Rev.,
2000, 100, 1169 and references therein.
2 See, for example N. J. Hardman, B. E. Eichler and P. P. Power, Chem.
Commun., 2000, 1491; C. Cui, H. W. Roesky, H.-G. Schmidt, M.
Noltemeyer, H. Hao and F. Cimpoesu, Angew. Chem., Int. Ed., 2000, 39,
4274.
Notes and references
3 H. tom Dieck, M. Svoboda and T. Grieser, Z. Naturforsch., B, 1981, 36,
823.
‡
Synthetic procedures: note that standard Schlenk-line and glovebox
techniques were used when appropriate.
4 For a review of imine-forming methodologies, see B. E. Love, T. S.
Boston, B. T. Nguyen and J. R. Rorer, Org. Prep. Proced. Int., 1999, 31,
399.
5 (a) L. Johansson, O. B. Ryan and M. Tilset, J. Am. Chem. Soc., 1999, 121,
1974; (b) H. Heiberg, L. Johansson, O. Gropen, O. B. Ryan, O. Swang
and M. Tilset, J. Am. Chem. Soc., 2000, 122, 10831; (c) L. Johansson and
M. Tilset, J. Am. Chem. Soc., 2001, 123, 739.
(a) Aminoalane complexes 1–3. A toluene solution of the appropriate
primary amine was added dropwise to an equimolar quantity of AlMe3 in
toluene solution. Following the cessation of gas evolution, the reaction
mixture was stirred for an additional 1.0 h at ambient temperature. Colorless
crystals of 1, 2 and 3 were obtained upon storage of resulting solutions
overnight at 230 °C in yields of 45, 28, and 27%, respectively.
(b) a-Diimines 4–6 and b-aminoenones 7–9. The a-diimines 4–6 were
prepared by addition of a toluene solution of the aminoalane complex 1, 2
or 3 to an equimolar quantity of 2,3-butanedione in toluene solution. In each
case, the resulting solution was treated with methanol and DI water,
followed by extraction with CHCl3. After drying over MgSO4, the organic
layer was filtered through a frit fitted with a pad of alumina. Colorless
crystals of 4 and 5 were obtained by storage of the saturated 2+1 pentane–
Et2O solutions overnight at 230 °C. The b-aminoenones 7–9 were prepared
by addition of a toluene solution of 1, 2 or 3 to a toluene solution of
6 A. Solladié-Cavallo, M. Bencheqroun and F. Bonne, Synth. Commun.,
1993, 23, 1683.
7 For previous examples of structurally characterized aminoalanes, see e.g.
(a) M. G. Davidson, D. Elilio, S. L. Less, A. Martin, P. R. Raithby, R.
Snaith and D. S. Wright, Organometallics, 1993, 12, 1; (b) J. J. Byers, B.
Lee and G. H. Robinson, Polyhedron, 1992, 11, 967; (c) K. M. Waggoner
and P. P. Power, J. Am. Chem. Soc., 1991, 113, 3385.
8 A. Parda, M. Stender, R. J. Wright, M. M. Olmstead, P. Klavins and P. P.
Power, Inorg. Chem., 2002, 41, 3909.
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