Synthesis and structure of neutral and cationic aluminum complexes incorporating bis(oxazolinato) ligands

Article abstract of DOI:10.1021/om0400245

Treatment of the bis(oxazolines) 1,1-bis[4,4-dimethyl-1,3-oxazolin-2-yl] ethane (1a, {BOX-Me₂}H) and 1,1-bis[(4S)-4-isopropyl-1,3-oxazolin-2- yl] ethane (1b, {BOX-(S)-ⁱPr}H) with ⁿBuLi, followed by the addition of 1 equiv of ClAlMe₂, affords the corresponding dimethyl Al complexes {BOX-Me₂}AlMe₂ (2a) and {BOX-(S)-ⁱPr}AlMe₂ (2b) in reasonable yields. The dichloro Al complex {BOX-Me₂}AlCl₂ (3a) was synthesized in a similar way using 1 equiv of AlCl₃. Compounds 2a,b and 3a are monomeric four-coordinate mono[bis(oxazolinato)-aluminum] complexes, on the basis of X-ray analyses for 2b and 3a and NMR data for 2a,b and 3a. The dimethylaluminum complexes {BOX-Me₂}AlMe₂ (2a) and {BOX-(S)-ⁱPr}AlMe₂ (2b) react in C₆D ₅Br with B(C₆F₅)₃ to yield the quantitative formation of cations {BOX-Me₂}AlMe⁺ (4a ⁺) and {BOX-(S)-ⁱPr}AlMe⁺ (4b⁺), respectively, as fully dissociated MeB(C₆F₅) ₃-salts. Cations 4a,b⁺, which are most likely either base-free three-coordinate cationic species or four-coordinate cationic Al-C₆D₅Br adducts, are unstable at room temperature in C₆D₅-Br and decompose to unidentified species. When these ionization reactions are performed in the presence of a Lewis base L (L = THF, NMe₂Ph), corresponding four-coordinate Al-L Lewis base adducts are cleanly generated, {BOX-Me₂}Al(Me)(L)⁺ (5a⁺, L = THF; 6a⁺, L = NMe₂Ph) and {BOX-(S)-ⁱPr}Al(Me) (L)⁺ (5b⁺, L = THF; 6b⁺, L = NMe ₂Ph), as determined by solution studies and, in the case of 6a ⁺, by X-ray analysis. In contrast, the reaction of {BOX-Me ₂}AlMe₂ (2a) with [Ph₃C] [B(C₆F ₅)₄] yields the bis(imine) Al cation 7a⁺, as determined by NMR and X-ray analysis. The formation of 7a⁺ most likely proceeds via a hydride abstraction by Ph₃C⁺ at the Me group located at the back of the bis(oxazolinato) ligand in 2a.

Full text of DOI:10.1021/om0400245

Organometallics 2004, 23, 3053-3061
3053
Syn th esis a n d Str u ctu r e of Neu tr a l a n d Ca tion ic
Alu m in u m Com p lexes In cor p or a tin g Bis(oxa zolin a to)
Liga n d s
Samuel Dagorne,*,† Ste´phane Bellemin-Laponnaz,*,‡ and Richard Welter§
Laboratoire de Chimie Organome´tallique, UMR CNRS 7576, Ecole Nationale Supe´rieure
de Chimie de Paris, 11, rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France, and
Laboratoire de Chimie Organome´tallique et de Catalyse, UMR 7513, and
Laboratoire DECMET, Institut Le Bel, Universite´ Louis Pasteur, 4,
rue Blaise Pascal, 67000 Strasbourg, France
Received February 27, 2004
Treatment of the bis(oxazolines) 1,1-bis[4,4-dimethyl-1,3-oxazolin-2-yl]ethane (1a , {BOX-
n
Me2}H) and 1,1-bis[(4S)-4-isopropyl-1,3-oxazolin-2-yl]ethane (1b, {BOX-(S)-iPr}H) with
-
BuLi, followed by the addition of 1 equiv of ClAlMe2, affords the corresponding dimethyl Al
complexes {BOX-Me2}AlMe2 (2a ) and {BOX-(S)-iPr}AlMe2 (2b) in reasonable yields. The
dichloro Al complex {BOX-Me2}AlCl2 (3a ) was synthesized in a similar way using 1 equiv of
AlCl3. Compounds 2a ,b and 3a are monomeric four-coordinate mono[bis(oxazolinato)-
aluminum] complexes, on the basis of X-ray analyses for 2b and 3a and NMR data for 2a ,b
and 3a . The dimethylaluminum complexes {BOX-Me2}AlMe2 (2a ) and {BOX-(S)-iPr}AlMe2
(2b) react in C6D5Br with B(C6F5)3 to yield the quantitative formation of cations {BOX-
Me2}AlMe+ (4a+) and {BOX-(S)-iPr}AlMe+ (4b+), respectively, as fully dissociated MeB(C6F5)3
-
salts. Cations 4a ,b+, which are most likely either base-free three-coordinate cationic species
or four-coordinate cationic Al-C6D5Br adducts, are unstable at room temperature in C6D5-
Br and decompose to unidentified species. When these ionization reactions are performed
in the presence of a Lewis base L (L ) THF, NMe2Ph), corresponding four-coordinate Al-L
Lewis base adducts are cleanly generated, {BOX-Me2}Al(Me)(L)+ (5a +, L ) THF; 6a +, L )
NMe2Ph) and {BOX-(S)-iPr}Al(Me)(L)+ (5b+, L ) THF; 6b+, L ) NMe2Ph), as determined
by solution studies and, in the case of 6a +, by X-ray analysis. In contrast, the reaction of
{BOX-Me2}AlMe2 (2a ) with [Ph3C][B(C6F5)4] yields the bis(imine) Al cation 7a +, as
determined by NMR and X-ray analysis. The formation of 7a + most likely proceeds via a
hydride abstraction by Ph3C+ at the Me group located at the back of the bis(oxazolinato)
ligand in 2a .
In tr od u ction
In this regard, low-coordinate Al cations are particularly
attractive, since they combined a cationic charge and a
low coordination number and are thus expected to be
highly electrophilic species. Recent studies in this area
showed that chelate cationic Al alkyls such as three-
and four-coordinate Al alkyl cations {L-X}AlR+ and
{L-X}Al(R)(L)+ (L-X- is a monoanionic bidentate
ligand; L is a labile ligand), readily obtained by reaction
of {L-X}AlR2 with [Ph3C][B(C6F5)4] or B(C6F5)3, are
powerful Lewis acids.5 As such, these cationic species
may mediate unusual transformations in Lewis acid
based catalysis.
To extend the scope of applications of low-coordinate
Al cations, we are interested in the synthesis of chiral
Al alkyl complexes {L-X*}Al(R)(L)+, where L-X*- is
a monoanionic chiral bidentate chelating ligand and L
is labile, because such species may be useful for ap-
Cationic aluminum complexes are of interest because
the enhanced Lewis acidity of the Al center versus that
of their neutral analogues is attractive for potential
applications in catalysis.1 Some of these cationic com-
plexes have already found applications in ethylene,2
alkene oxide,3 and D,L-lactide4 polymerization catalysis.
* To whom correspondence should be addressed. Fax: +33 1 43 26
00
61.
E-mail:
dagorne@ext.jussieu.fr
(S.D.);
bellemin@
chimie.u-strasbg.fr (S.B.-L.).
† Ecole Nationale Supe´rieure de Chimie de Paris.
‡ Laboratoire de Chimie Organome´tallique et de Catalyse, Univer-
site´ Louis Pasteur.
§ Laboratoire DECMET, Universite´ Louis Pasteur.
(1) Atwood, D. A. Coord. Chem. Rev. 1998, 176, 407.
(2) For leading references, see: (a) Bochmann, M.; Dawson, D. M.
Angew. Chem., Int. Ed. Engl. 1996, 35, 2226. (b) Coles, M. P.; J ordan,
R. F. J . Am. Chem. Soc. 1997, 119, 8125. (c) Bruce, M.; Gibson, V. C.;
Redshaw, C.; Solan, G. A.; White, A. J . P.; Williams, D. J . Chem.
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J . Am. Chem. Soc. 2002, 124, 7662.
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Soc. 1995, 117, 6779. (b) J egier, J . A.; Atwood, D. A. Inorg. Chem. 1997,
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J . Am. Chem. Soc. 1999, 121, 6747.
(5) (a) Korolev, A. V.; Ihara, E.; Guzei, I. A.; Young, V. G., J r.;
J ordan, R. F. J . Am. Chem. Soc. 2001, 123, 8291. (b) Dagorne, S.; Guzei,
I. A.; Coles, M. P.; J ordan, R. F. J . Am. Chem. Soc. 2000, 122, 274. (c)
Radzewich, C. E.; Guzei, I. A.; J ordan, R. F. J . Am. Chem. Soc. 1999,
121, 8673. (d) Radzewich, C. E.; Coles, M. P.; J ordan, R. F. J . Am.
Chem. Soc. 1998, 120, 8673. (e) Ihara, E.; Young, V. G., J r.; J ordan,
R. F. J . Am. Chem. Soc. 1998, 120, 8277.
(4) Emig, N.; Nguyen, H.; Krautscheid, H.; Re´au, R.; Cazaux, J .-B.;
Bertrand, G. Organometallics 1998, 17, 3599.
10.1021/om0400245 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 05/05/2004

3054 Organometallics, Vol. 23, No. 12, 2004
Dagorne et al.
Ch a r t 1
Sch em e 1
plications in asymmetric catalysis. For this purpose, our
interest was driven toward the use of C2-symmetric
chiral bis(oxazolinato) ligands (BOX-) of type B (Chart
1) for coordination to aluminum. Whereas neutral bis-
(oxazolines) have been extensively used in numerous
catalytic asymmetric reactions,6 anionic versions of such
ligands remain less studied. Efficient catalysts that bear
the anionic ligand family were reported with titanium,7
rhodium,8 copper,6a zinc,9 magnesium,10and more re-
cently lanthanides.11 Moreover, the catalysts are usually
prepared in situ and not isolated. Only few examples
have been completely characterized.8,11-15
As for the aluminum metal center, although there
have been no reports of group 13 metal complexes
containing the bis(oxazolinato) ligand, this chelate
ligand, i.e., a monoanionic bidentate dinitrogen donor
with a six-electron π-delocalized system, should yield a
rather stable and rigid six-membered metallacycle ring
when coordinated to an Al center.
Here we report our initial efforts in the synthesis and
structural characterization of neutral and cationic,
achiral and chiral mono[bis(oxazolinato)aluminum] com-
plexes. The coordination chemistry of the achiral ligand
A (Chart 1) with Al was first investigated, because the
dimethyl substitution of a carbon R to the nitrogen of
the oxazolinyl ring in A should provide a significant
steric shielding of the metal center and thus yield stable
and monomeric Al complexes. We then extended our
studies, when appropriate, to the chiral version B (Chart
1).
Resu lts a n d Discu ssion
The mono[bis(oxazolinato)aluminum] dimethyl com-
plexes 2a ,b and the dichloro Al derivative 3a were
synthesized via a salt metathesis pathway by reaction
of the desired bis(oxazolinato) lithium salt, generated
in situ, and the corresponding Al chloride reagent
(AlClMe2 or AlCl3) (Scheme 1). For example, the reaction
of the bis(oxazoline) ligand {BOX-Me2}H (1a ) with
nBuLi at -78 °C in pentane followed by the addition,
at -40 °C, of AlClMe2 or AlCl3 affords the corresponding
Al complex {BOX-Me2}AlMe2 (2a ) or {BOX-Me2}AlCl2
(3a ), respectively, in reasonable yields (2a , 57% yield;
3a , 62% yield). The chiral C2-symmetric dimethyl Al
complex {BOX-(S)-iPr}AlMe2 (2b) was obtained in mod-
erate yield (48%) by
a similar procedure using
{BOX-(S)-iPr}H and AlClMe2. Alternatively, it is note-
worthy that the NMR-scale reaction of {BOX-Me2}H
with AlMe3 in C6D6 at room temperature yields, along
with methane generation, the quantitative formation of
2a , thus showing that the alkane elimination method
is also a viable pathway to access bis(oxazolinato)-
aluminum dialkyl derivatives. Compounds 2a ,b and 3a
were isolated as colorless crystalline solids that are
quite soluble in hydrocarbon solvents and stable for
months under N2 either in hydrocarbon solution or in
the solid state.
(6) (a) Lowenthal, R. E.; Abiko, A.; Masamune, S. Tetrahedron Lett.
1990, 31, 6005. (b) Evans, D. A.; Woerpel, K. A.; Hinman, M. M.; Faul,
M. M. J . Am. Chem. Soc. 1991, 113, 726. (c) Corey, E. J .; Imai, N.;
Zhang, H. Y. J . Am. Chem. Soc. 1991, 113, 728. (d) Mu¨ller, D.;
Umbricht, G.; Weber, B.; Pfaltz, A. Helv. Chim. Acta 1991, 74, 232.
(e) For a review of applications of C2-symmetric bis(oxazolines) in
asymmetric catalysis, see: Ghosh, A. K.; Mathivanan, P.; Cappiello,
J . Tetrahedron: Asymmetry 1998, 9, 1. (f) For a recent review of
nitrogen-containing ligands in asymmetric catalysis, see: Fache, F.;
Schulz, E.; Lorraine Tommasino, M.; Lemaire, M. Chem. Rev. 2000,
100, 2159.
(7) (a) Bandini, A.; Cozzi, P. G.; Negro, L.; Umani-Ronchi A. Chem.
Commun. 1999, 39. (b) Bandini, M.; Bernardi, F.; Bottoni, A.; Cozzi,
P. G.; Miscione, G. P.; Umani-Ronchi A. Eur. J . Org. Chem. 2003, 2972.
(8) Brown, J . M.; Guiry, P. J .; Price, D. W.; Hursthouse, M. B.;
Karalulov, S. Tetrahedron: Asymmetry 1994, 5, 561.
(9) Nakamura, M.; Hirai, A.; Nakamura, E. J . Am. Chem. Soc. 1996,
118, 8489.
(10) (a) Corey, E. J .; Wang, Z. Tetrahedron Lett. 1993, 34, 4001. (b)
Schulze, V.; Hoffmann, R. W. Chem. Eur. J . 1999, 5, 337.
(11) Hong, S.; Tian, S.; Metz, M. V.; Marks, T. J . J . Am. Chem. Soc.
2003, 125, 14768.
(12) Hall, J .; Lehn, J . M.; De Cian, A.; Fisher, J . Helv. Chim. Acta
1991, 74, 1.
(13) Go¨rlitzer, H. W.; Spiegler, M.; Anwander, R. J . Chem. Soc.,
Dalton Trans. 1999, 4287.
(14) Anionic bis(oxazolinato) ligands have recently been used to
stabilize rhodium(II) complexes; see: Willems, S. T. H.; Russcher, J .
C.; Budzelaar, P. H. M.; de Bruin, B.; de Gelder, R.; Smits, J . M. M.;
Gal, A. W. Chem. Commun. 2002, 148.
The molecular structures of the chiral Al complex 2b
and the dichloro Al compound 3a were determined by
X-ray crystallographic analysis, establishing their mon-
omeric nature as well as the bidentate chelation of one
bis(oxazolinato) ligand (Figures 1 and 2 and Tables 1
and 2).
The Al metal center in both complexes adopts a
distorted-tetrahedral structure with N-Al-N bite angles
(2b, 93.13(7)°; 3a , 97.35(8)°) similar to those in the
related â-diketiminato Al complexes (TTP)AlX2 (Chart
2; C, 94.72(14)°; D, 99.41(12)°), which contain a similar
six-membered C3N2Al metallacycle.16 The Al-N bond
distances in 3a (1.857(2) and 1.843(2) Å) are comparable
to those in D (1.850(2) Å) but are slightly shorter than
those in 2b (1.906(2) and 1.899(2) Å), due to the more
electrophilic Al center in 3a . The Al-C bond distances
(15) Bandini, M.; Cozzi, P. G.; Monari, M.; Perciaccante, R.; Selva,
S.; Umani-Ronchi A. Chem. Commun. 2001, 1318.
(16) Qian, B.; Ward, D. L.; Smith, M. R. III. Organometallics 1998,
17, 3070.

Bis(oxazolinato)aluminum Complexes
Organometallics, Vol. 23, No. 12, 2004 3055
Ta ble 1. Cr ysta llogr a p h ic Da ta for 2b, 3a , [6a ][MeB(C6F 5)3], a n d [7a ][B(C6F 5)4]
2b
3a
[6a ][MeB(C6F5)3]
[7a ][B(C6F5)4]
formula
fw
C
16H29AlN2O2
C
12H19AlCl2N2O2
C
40H36AlBF15N3O2
C
37H18AlBF21N2O2
308.39
321.17
913.51
959.38
cryst size (mm)
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
0.09 × 0.08 × 0.07
orthorhombic
P212121
12.462(5)
16.896(5)
17.501(5)
90.00
90.00
90.00
3685(2)
8
1.112
0.13 × 0.10 × 0.08
orthorhombic
Pna21
8.528(1)
10.190(1)
18.087(1)
90.00
90.00
90.00
1571.8(3)
4
1.357
0.13 × 0.10 × 0.08
triclinic
P1h
0.09 × 0.08 × 0.07
monoclinic
P21/c
12.842(1)
19.392(2)
15.786(1)
90.00
98.21(5)
90.00
3890.9(6)
4
13.349(5)
16.348(5)
18.623(5)
89.25(5)
89.68(5)
89.30(5)
4063(2)
4
Z
Dcalcd (g cm-3
)
1.493
1.638
no. of indep rflns
no. of params
R1a
10 777
379
0.0488
4554
168
0.0521
23 476
1117
0.1043
9290
589
0.0555
wR2 (all data)
goodness of fit
0.1306
0.971
0.1370
1.090
0.2449
1.109
0.1761
1.025
a
R1 (I > 2σ(I)).
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 2b a n d 3a
2b
3a
Al(1)-N(1)
Al(1)-N(2)
Al(1)-C(1)
Al(1)-C(2)
N(1)-C(3)
N(2)-C(5)
C(5)-C(4)
C(4)-C(3)
1.906(2)
1.899(2)
1.967(2)
1.968(3)
1.331(3)
1.336(3)
1.384(3)
1.394(3)
Al-N(1)
1.857(2)
1.843(2)
2.123(1)
2.136(2)
1.328(3)
1.341(3)
1.393(3)
1.393(3)
Al-N(2)
Al-Cl(1)
Al-Cl(2)
N(1)-C(5)
N(2)-C(8)
C(5)-C(6)
(C(6)-C(8)(
F igu r e 1. Molecular structure of complex 2b. The H atoms
are omitted for clarity. Selected torsion angles (deg): N(2)-
Al(1)-N(1)-C(3) ) 6.09(17), Al(1)-N(2)-C(5)-C(4) )
-1.5(3), N(2)-C(5)-C(4)-C(3) ) 4.1(3).
N(2)-Al(1)-N(1)
N(2)-Al(1)-C(1)
N(1)-Al(1)-C(1)
N(2)-Al(1)-C(2)
N(1)-Al(1)-C(2)
C(1)-Al(1)-C(2)
93.13(7)(
110.30(9)(
115.9(1)(
115.2(1)
109.3(1)
111.8(1)(
N(2)-Al-N(1)
N(2)-Al-Cl(1)
N(1)-Al-Cl(1)
N(2)-Al-Cl(2)
N(1)-Al-Cl(2)
Cl(2)-Al-Cl(1)
97.35(8)
114.27(8)
113.71(7)
111.20(8)
112.06(7)
108.06(4)
Ch a r t 2
(1.39(1) Å average), which are close to the C-C bond
distances in aromatic systems (1.395 Å), and the C-N
bond distances (1.33(1) Å average), which are interme-
diate between CdN double-bond distances in imines
(1.28 Å) and C(sp2)-N single-bond distances (1.47 Å).
As a result of the nearly planar Al metallacycle com-
bined bonding π-delocalization of the ligand backbone,
complexes 2b and 3a approach overall C2 and C2v
symmetry, respectively.
F igu r e 2. Molecular structure of complex 3a . The H atoms
are omitted for clarity. Selected torsion angles (deg): Al-
N(1)-C(5)-C(6) ) 4.9(3), N(1)-Al-N(2)-C(8) ) 6.5(2),
C(5)-C(6)-C(8)-N(2) ) -2.5(4).
in 2b and the Al-Cl bond distances in 3a are also
similar to those in C and D, respectively.
The 1H and 13C NMR data for 2b and 3a are
consistent with their solid-state structure being retained
in solution, and these data are similar to those for 2a .
In both 2b and 3a complexes, the six-membered ring
{BOX}Al moiety forms a nearly planar metallacycle
(|C-C-Al-N| < 7° in 2b and (|C-C-Al-N| < 6° in 3a ),
with the carbon and nitrogen atoms exhibiting a trigo-
nal-planar coordination (sum of angles ca. 360°). The
bonding in the Me2CNdC(O)C(CH3)C(O)dNC(Me2) back-
bone of the bis(oxazolinato) ligand in both 2b and 3a is
delocalized, as shown by the C-C bond distances
1
For example, the H NMR spectra of 2a and 3a in C6D6
each exhibit one O-CH2 resonance (4H) and one CMe2
resonance (12H), which is consistent with an effective
C2v symmetry for these two complexes. As for 2b, the
presence of one N-CH resonance (2H), one AlMe2
resonance (6H), and two CH3-iPr resonances are in

3056 Organometallics, Vol. 23, No. 12, 2004
Dagorne et al.
Sch em e 2
Sch em e 3
agreement with a C2-symmetric complex under the
studied conditions.
Rea ction of th e Mon o[bis(oxa zolin a to)a lu m i-
n u m ] Dim eth yl Com p lexes (2a ,b) w ith B(C6F 5)3.
The conversion of neutral Al complexes 2a ,b to cationic
Al alkyl species was investigated using the well-known
strong Lewis acid B(C6F5)3 for Me- abstraction at the
Al metal center.
The instability of cations 4a ,b+ prompted us to study
the ionization chemistry of 2a ,b in the presence of Lewis
bases such as NMe2Ph and THF, to stabilize the formed
cationic Al alkyls. The NMR-scale reaction of the
dimethylaluminum complexes 2a ,b with 1 equiv of
B(C6F5)3 (CD2Cl2, room temperature, 15 min) in the
presence of 1 equiv of L (L ) THF, NMe2Ph) leads to
the quantitative formation of the corresponding four-
coordinate cationic Al-L adducts {BOX-Me2}Al(Me)(L)+
(5a +, L ) THF; 6a +, L ) NMe2Ph) and {BOX-(S)-iPr}-
Al(Me)(L)+ (5b+, L ) THF; 6b+, L ) NMe2Ph), respec-
tively, as MeB(C6F5)3- salts (Scheme 3). The generation
of [5a ,b][MeB(C6F5)3] and [6a ,b][MeB(C6F5)3] on a
preparative scale (CH2Cl2, room temperature, 30 min)
allowed, in each case, their isolation in a pure form as
colorless solids in good yields (see Experimental Sec-
tion). These four salt compounds are stable for several
days in CD2Cl2 solution and for months in the solid state
under an inert atmosphere. To the best of our knowl-
edge, complexes 5b+ and 6b+ are the first examples of
chiral cationic Al alkyls.
The reaction of the {BOX}AlMe2 complexes 2a ,b with
1 equiv of B(C6F5)3 (C6D5Br, room temperature, 15 min)
yields the quantitative formation of cations {BOX-Me2}-
AlMe+ (4a +) and {BOX-(S)-iPr}AlMe+ (4b+), respec-
tively, as MeB(C6F5)3- salts (Scheme 2). Cations 4a ,b+
are highly unstable species at room temperature in
C6D5Br and rapidly decompose to unidentified species,
thus preventing the isolation of these salts in a pure
form. In the present case, although the decomposition
pathway is unknown, it does not appear to involve the
MeB(C6F5)3 anion, since the 1H, 13C, and 19F NMR
-
resonances of this anion remain unaffected all along the
decomposition process. Degradation reactions between
cationic Al alkyls incorporating various bidentate L-X-
ligands and the MeB(C6F5)3- anion have been previously
-
reported and usually proceed cleanly via a C6F5
transfer from the anion to the cationic center.5b,16,17
Compounds [5a,b][MeB(C6F5)3] and [6a,b][MeB(C6F5)3]
are fully dissociated in CD2Cl2 with no cation-anion
interactions at room temperature, as observed by 1H and
19F NMR spectroscopy. In particular, the 1H NMR
spectrum in CD2Cl2 of each salt exhibits a MeB reso-
nance (δ 0.48) characteristic of a free MeB(C6F5)3- anion
The H, 13C, and 19F NMR data for the [4a ,b][MeB-
1
(C6F5)3] salts in C6D5Br at -20 °C show that the
-
MeB(C6F5)3 anion is free in solution.18 As for cations
4a ,b+, the 1H and 13C NMR data are consistent with
C2v-symmetric and C2-symmetric structures for 4a + and
4b+, respectively. For example, key 1H NMR resonances
for 4a + are (i) the AlMe+ resonance (δ -0.21) shifted
downfield as compared to that of the neutral precursor
(δ -0.51), a result of the cationic charge on Al, and (ii)
1
in solution. The H NMR data at room temperature for
the cationic THF and NMe2Ph adducts 5a ,b+ and 6a ,b+
all contain AlMe resonances that are significantly
shifted downfield as compared to those of the neutral
precursors 2a ,b, as expected from the presence of the
cationic charge on the Al center. Similarly, the coordi-
nation of the Lewis base L (L ) THF, NMe2Ph) to the
Al cationic center is also evidenced by significant
1
the presence of only three H NMR singlet resonances
for the ligand backbone (CMe2, O-CH2, and CMe). The
latter observation agrees with a C2v-symmetric structure
for 4a +. Overall, on the basis of NMR data and under
the studied conditions, cations 4a ,b+ in C6D5Br solution
are most likely either base-free three-coordinate cationic
species similar to cationic three-coordinate â-diketimi-
nato Al complexes previously reported,5c or four-
coordinate cationic Al solvent adducts (i.e. C6D5Br
adducts) with a fast C6D5Br coordination/decoordination
process on the NMR time scale under the studied
conditions. Solid-state structures of four-coordinate
cationic Al-ClPh adducts have been recently reported.19
1
downfield shifts of the H and 13C NMR resonances of
the coordinated L versus those of free L.20 Overall, the
NMR data for the THF-adduct Al cations 5a + and 5b+
at room temperature are consistent with effective C2v-
and C2-symmetric structures, respectively, which can
be ascribed to a fast face exchange of THF on the NMR
time scale under the studied conditions. In contrast,
under the same conditions, the Al-NMe2Ph cationic
adducts 6a + and 6b+ exhibit lower overall symmetries:
i.e., Cs symmetry for 6a + and C1 symmetry for chiral
(17) Dagorne, S.; Lavanant, L.; Welter, R.; Chassenieux, C.; Haquette,
P.; J aouen, G. Organometallics 2003, 22, 3732.
(18) The MeB 1H NMR resonance (δ 1.43) in [4a ,b][MeB(C6F5)3] is
nearly identical with that of [NBu3(CH2Ph)][MeB(C6F5)3] (δ 1.39) in
C6D5Br at -20 °C.
(20) For information, NMR data for free THF in CD2Cl2 are as
follows. 1H NMR: δ 3.67, 1.81. 13C{1H} NMR: δ 67.4, 25.2. NMR Data
for free NMe2Ph in CD2Cl2 are as follows. 1H NMR: δ 7.23 (Ph, 2H),
6.75 (Ph, 2H), 6.70 (Ph, 1H), 2.94 (Me, 6H). 13C{1H} NMR: δ 150.1
(Cipso Ph), 128.6 (Ph), 116.0 (Ph), 112.2 (Ph), 40.0 (Me).
(19) Korolev, A.; Delpech, F.; Dagorne, S.; Guzei, I.; J ordan, R. F.
Organometallics, 2001, 20, 3367.

Bis(oxazolinato)aluminum Complexes
Organometallics, Vol. 23, No. 12, 2004 3057
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 6a + a n d 7a +
6a +
7a +
Al(1)-N(1)
Al(1)-N(2)
Al(1)-C(1)(
Al(1)-N(3)
N(1)-C(6)
N(2)-C(9)
C(6)-C(7)
C(7)-C(9)
1.852(5)(
1.857(4)(
1.942(6)(
2.018(4)(
1.325(8)(
1.319(7)(
1.413(8)(
1.395(7)
Al(1)-N(1)
Al(1)-N(2)
N(1)-C(5)
N(2)-C(8)
C(6)-C(8)
C(5)-C(6)
C(6)-C(7)
1.936(3)
1.933(3)
1.283(4)
1.281(4)
1.465(5)
1.476(5)
1.320(5)
N(1)-Al(1)-N(2)
N(1)-Al(1)-C(1)
N(2)-Al(1)-C(1)
N(1)-Al(1)-N(3)
N(2)-Al(1)-N(3)
C(1)-Al(1)-N(3)
95.8(2)
N(1)-Al(1)-N(2)
N(2)-Al(1)-C(13)
N(1)-Al(1)-C(13)
N(2)-Al(1)-C(14)
N(1)-Al(1)-C(14)
C(13)-Al(1)-C(14)
92.8(1)
119.3(2)
117.0(2)
105.6(2)
106.1(2)
111.3(2)
109.5(1)
111.2(1)
113.5(1)
111.9(1)
115.7(1)
F igu r e 3. Molecular structure of cation 6a +. The H atoms
are omitted for clarity. Selected torsion angles (deg): N(2)-
6b+. These NMR observations agree with an effective
and nonlabile coordination of NMe2Ph in both 6a + and
6b+, which may reflect the more Lewis basic character
of NMe2Ph vs THF. In fact, cations 6a + and 6b+ are
rather robust adducts, since NMe2Ph face exchange does
not proceed at room temperature even in the presence
C(9)-C(7)-C(6)
) 15.3(1), N(1)-C(6)-C(7)-C(9) )
12.7(2), N(1)-Al(1)-N(2)-C(9) ) -24.5(4), N(2)-Al(1)-
N(1)-C(6) ) 26.3(4).
dimethyl complex 2b (1.906(2) and 1.899(2) Å), which
may reflect the increased ionic character of the Al-N
bond distances resulting from the cationic charge at the
Al center. The Al-N(3) bond distance (2.018(4) Å) is in
the normal range for Al-N dative bonds (1.957(3)-
2.238(4) Å)23 and nearly identical with that in the
dinuclear Al cation (tBuMe2SiO)2Al2Me3(NMe2Ph)+, which
also contains an aniline-stabilized four-coordinate cat-
ionic Al center.
Unlike 2b and 3a , the Al chelate six-membered-ring
metallacycle (C3N2Al) is significantly distorted from
planarity, with the Al center being displaced 0.46 Å out
of the N(2)-C(9)-N(1)-C(6) average plane toward the
coordinated NMe2Ph. The bis(oxazolinato) ligand back-
bone (C3N2) is itself slightly distorted from planarity,
as shown by the N(2)-C(9)-C(7)-C(6) and N(1)-C(6)-
C(7)-C(9) torsion angles (15.3 and 12.7°, respectively).
In fact, as shown in Figure 3, the bidentate chelating
ligand appears to be pushed away from the Al-NMe2-
Ph group, which may be to avoid significant steric
interactions between the aniline and the bidentate
ligand.24
1
of free NMe2Ph, as observed by H NMR spectroscopy.
Preliminary studies on the use of cations 5a ,b+ as
Lewis acid catalysts in a hetero-Diels-Alder (DA)
reaction between a diene and a glyoxylate derivative
showed the quantitative conversion of the reactants to
the hetero-DA product and the ene product in a 1/1
ratio.21 In the case of chiral Al cation 5b+, chiral GC
analysis revealed that the ene product is racemic,
whereas the hetero-DA product was obtained with 5%
ee. Although disappointing in term of enantioselectivity,
the excellent activity of 5a ,b+ first shows that such
cationic Al alkyls appear suitable for hetero-Diels-Alder
catalysis.
Solid -Sta te Str u ctu r e of [6a ][MeB(C6F 5)3]. The
molecular structure of the salt compound [6a ][MeB-
(C6F5)3] was confirmed by X-ray crystallographic analy-
sis (Tables 1 and 3). [6a ][MeB(C6F5)3] crystallizes as 6a +
-
and MeB(C6F5)3 anions with no cation-anion close
contacts. The 6a + cation (Figure 3) is an aniline-
stabilized four-coordinate cationic Al species. The ge-
ometry at the Al center is best described as distorted
tetrahedral with a N-Al-N bite angle (95.8(2)°) similar
to that in 2b and 3a . In contrast to the neutral
derivatives {BOX}AlX2 (2b, X ) Me; 3a , X ) Cl), in
which the two X ligands are symmetrically displaced
from the N-Al-N plane, the Al-Me and Al-NMe2Ph
groups in 6a + are displaced from the N-Al-N plane
by 45.3(5) and 66.0(5)°, respectively. This difference in
the coordination geometry of 6a + vs that of 2b and 3a
may be related to the different donor abilities of Me-
vs NMe2Ph. A similar structural trend has previously
been observed with related In complexes when compar-
ing {LX}InX2 to {LX}In(Me)(NMe2Ph)+.22 The Al-N(1)
and Al-N(2) bond distances (1.852(5) and 1.857(4) Å,
respectively) are a bit shorter than those in the Al
Hydr ide Abstr action Reaction by P h 3C+ on {BOX-
Me2}AlMe2 (2a ). The NMR-scale reaction of 2a with 1
equiv of [Ph3C][B(C6F5)4] (CD2Cl2, room temperature,
15 min) results in the quantitative formation of the
+
diimine cationic Al complex {H2CdC(OX-Me2)2}AlMe2
(7a +), as a B(C6F5)4 salt, and Ph3CH in a 1/1 ratio
-
(Scheme 4). The preparative-scale generation of [7a ]-
[B(C6F5)4] allowed its isolation in a pure form as a bright
yellow solid. The formation of cation 7a + proceeds via
an hydride abstraction by Ph3C+ at the Me group
(23) (a) Hogerheide, M. P.; Wesseling, M.; J astrzebski, J . T. B. H.;
Boersma, J .; Kooijman, H.; Spek, A. L.; van Koten, G. Organometallics
1995, 14, 4483. (b) Hill, J . B.; Eng, S. J .; Pennington, W. T.; Robinson,
G. H. J . Organomet. Chem. 1993, 445, 11. (c) Kumar, R.; Sierra, M.
L.; Oliver, J . P. Organometallics 1994, 13, 4285.
(21) For a preliminary study, 2,3-dimethyl-1,3-butadiene was used
as a diene and ethyl glyoxylate as a dienophile. The catalysis was
performed using 10% mol of 5a ,b+ at 0 °C in toluene. For more details
concerning the hetero-Diels-Alder reaction between the aforemen-
tioned diene and ethyl glyoxylate, see: J ohannsen, M.; J orgensen, K.
A. J . Org. Chem. 1995, 60, 5757.
(24) A close analysis of the molecular structure of 6a + reveals that
the shortest C-H distances between the phenyl carbons in NMe2Ph
and the hydrogens bonded to C(3) (Figure 3) are 2.82 Å, which is
slightly less than the sum of the van der Waals radii (2.97 Å). It is
likely that the observed distortion of the BOX ligand is to minimize
the steric interactions between the aniline and the aforementioned
ligand that would otherwise result. For van der Waals radii, see:
Bondi, A. J . Phys. Chem. 1964, 68, 441.
(22) Delpech, F.; Guzei, I. A.; J ordan, R. F. Organometallics 2002,
21, 1167.

3058 Organometallics, Vol. 23, No. 12, 2004
Dagorne et al.
the electrophilic Al center in 7a +, but are longer than
those in 2b (1.906(2) and 1.899(2) Å), due to the poorer
donor ability of the bis(imine) ligand vs that of BOX-.
Apart from ligand backbone changes, the structural
features for 7a + can be related to those of 2b and 3a .
The 1H and 13C NMR data for cation 7a + (room
temperature, CD2Cl2) are consistent with a C2v-sym-
metric structure and with the solid-state structure being
retained in solution. For example, the 1H NMR spec-
trum of 7a + in CD2Cl2 only contains four singlet
resonances (δ 7.38, 4.59, 1.59, -0.52) assigned to the
CH2 (olefinic), O-CH2, CMe2, and AlMe2 groups, re-
spectively, which is consistent with a C2v-symmetric
structure for 7a +. In addition, the combination of 13C
NMR and DEPT NMR data for 7a + allowed the assign-
ment of two characteristic 13C NMR resonances to the
CdCH2 olefinic moiety (see Experimental Section), thus
confirming the presence of such a group in 7a + in
solution.
F igu r e 4. Molecular structure of cation 7a +. The H atoms
are omitted for clarity, except those on C(4), C(9), and C(7).
Sch em e 4
Su m m a r y a n d Con clu sion s
Our initial studies show that the bidentate bis-
(oxazolinato) ligand is suitable for coordination to
aluminum, allowing the synthesis of neutral Al dimethyl
and dichloro complexes 2a ,b and 3a via salt metathesis
routes. The alkane elimination method appears to be
also effective for the preparation of 2a . The solution and
solid-state data for the chiral compound 2b are consis-
tent with a C2-symmetric structure. Complexes 2a ,b
react in C6D5Br with B(C6F5)3 to afford the highly
unstable cations 4a ,b+, which are either three-coordi-
nate base-free Al cations or four-coordinate Al-C6D5-
Br cationic adducts, on the basis of NMR data. When
these ionization reactions are performed in the presence
of Lewis bases such THF and NMe2Ph, the correspond-
ing four-coordinate Al-Lewis base adducts are cleanly
generated, 5a ,b+ and 6a ,b+, as determined by solution
studies and, in the case of 6a +, by X-ray crystallographic
analysis. These cationic adducts are quite stable in
solution and in the solid state, illustrating the suitability
of the bis(oxazolinato) ligand for the generation of
cationic chiral Al alkyls such as 5b+ and 6b+. Initial
studies showed that cations 5a ,b+ effectively catalyze
a hetero-Diels-Alder reaction between a simple diene
and a glyoxylate derivative. Unlike B(C6F5)3, [Ph3C]-
[B(C6F5)4] does not abstract a Me- at the Al center when
reacted with 2a but, instead, abstracts a H- at the back
of the bis(oxazolinato) ligand to afford the unexpected
bis(imine) Al cation 7a +, as determined by X-ray crys-
tallographic analysis.
located at the back of the BOX ligand in complex 2a .
Typically, hydride abstraction reactions by Ph3C+ at
hydride or alkyl metal complexes occur either at the
metal center for metal hydrides to yield Ph3CH and a
M-H+ cation25 or in its vicinity for some alkylmetal
complexes: at the Câ of a metal-bonded alkyl group, for
instance.26 In particular, [Ph3C][B(C6F5)4] has been
shown to â abstract a H- at neutral Al complexes {LX}-
Al(iBu)2 to lead to the formation of Ph3CH, isobutene,
and a Al-iBu+ cation.5a However, to our knowledge,
hydride abstraction reactions of the type observed here:
i.e., at the bidentate LX- ligand chelating the metal
center and rather far away from the metal center, have
not been previously reported. The observed reactivity
further illustrates the strong Lewis acid character of
Ph3C+ and shows an unexpected Lewis base character
for the BOX ligand incorporated in 2a .
The molecular structure of 7a + was determined by
X-ray crystallographic analysis of the salt [7a ][B(C6F5)4],
-
which crystallizes as 7a + and B(C6F5)4 ions with no
cation-anion interactions. The molecular structure of
7a + is illustrated in Figure 4. As expected, removal of
a hydride from the formally anionic BOX- ligand results
in a disruption of the π-delocalization C3N2 backbone
and the formation of a formally neutral bidentate ligand,
which is now best described as a classical neutral bis-
(oxazoline). The C(5)-N(1) and C(8)-N(2) bond dis-
tances of the ligand backbone (1.283(4) and 1.281(4) Å)
are characteristic of C(sp2)dN double bonds (1.30 Å),
whereas the C(6)-C(7) bond distance (1.320(5) Å) is
comparable with that of a CdC double bond (1.337 Å).
The Al-Nimine bond distances (Al(1)-N(1), 1.936(3) Å;
Al(1)-N(2), 1.933(3) Å) are shorter than those in neutral
Al imine complexes (1.97 Å average),27 as expected from
Exp er im en ta l Section
Gen er a l P r oced u r es. All experiments were carried out
under N2 using standard Schlenk techniques or in a MBraun
Unilab glovebox. Toluene, pentane, and THF were distilled
from Na/benzophenone and stored over activated molecular
sieves (4 Å) in a glovebox prior to use. CH2Cl2 and CD2Cl2 were
(27) For representative examples of neutral chelate Al complexes
of the type {LX}AlMe2 containing an Al-Nimine bond, see: (a) Kanters,
J . A.; Van Mier, G. P. M.; Nijs, R. L. L. M.; van der Steen, F.; van
Koten, G. Acta Crystallogr., Sect. C 1988, 44, 1391. (b) Cameron, P.
A.; Gibson, V. C.; Redshaw, C.; Segal, J . A.; Solan, G. A.; White, A. J .
P.; Williams, D. J . Dalton 2001, 1472.
(25) (a) Beck, W.; Su¨nkel, K. Chem. Rev. 1988, 88, 1405. (b) Cheng,
T.-Y.; Bullock, R. M. Organometallics 2002, 21, 2325.
(26) For two recent examples, see: (a) Reference 5a. (b) Mehrkho-
davanti, P.; Schrock, R. R. J . Am. Chem. Soc. 2001, 123, 10746.

Bis(oxazolinato)aluminum Complexes
Organometallics, Vol. 23, No. 12, 2004 3059
distilled from CaH2 and stored over activated molecular sieves
(4 Å) in a glovebox prior to use. C6D6 was degassed under an
N2 flow and stored over activated molecular sieves (4 Å) in a
glovebox prior to use. (S)-Valinol was synthesized by reduction
of the commercially available (S)-valine according a a litera-
ture procedure.28 B(C6F5)3 was purchased from Strem Chemi-
cals and was extracted with dry pentane prior to use. [Ph3C]-
[B(C6F5)4] was purchased from Asahi Glass Europe and used
as received. CD2Cl2, C6D6, and C6D5Br were purchased from
Eurisotope. All other chemicals were purchased from Aldrich
and were used as received. All NMR spectra were recorded at
room temperature (unless otherwise indicated) on a Bruker
AC 200 or 400 MHz spectrometer, except those for 1a ,b, which
were recorded on a Bruker Avance 300 MHz spectrometer. 1H
and 13C chemical shifts are reported versus SiMe4 and were
determined by reference to the residual 1H and 13C solvent
peaks. 11B and 19F chemical shifts are reported respectively
versus BF3‚Et2O in CD2Cl2 and versus neat CFCl3. Elemental
analyses were all performed by Mikroanalytisches Labor
Pascher, Remagen-Bandorf, Germany, except those for 1a ,b
performed by the microanalysis laboratory of the Universite´
Louis Pasteur, Strasbourg, France. EI mass spectra of 1a ,b
were recorded by the mass spectroscopy laboratory of the
Universite´ Louis Pasteur, Strasbourg, France. The cationic Al
complexes 5a ,b+ and 6a ,b+ were all obtained as fully dissoci-
ated MeB(C6F5)3- salts in CD2Cl2 solutions. The corresponding
1,1-Bis[4,4-dim eth yl-1,3-oxazolin -2-yl]eth an e (1a, {BOX-
Me2}H). The bis(oxazoline) 1a was prepared from diethyl
methylmalonate and 2-amino-2-methylpropanol in 42% overall
yield, following the same procedure as that for 1b. 1H NMR
(300 MHz, CDCl3): δ 3.94 (s, 4H, CH2), 3.45 (q, J ) 7.3 Hz,
1H, CHCH3), 1.47 (d, J ) 7.3 Hz, CHCH3), 1.27 (s, 12H,
C(CH3)2). 13C NMR (75.5 MHz, CDCl3): δ 164.2 (CdN), 79.4
(CH2), 67.0 (C(CH3)), 34.0 (CHCH3), 28.1 (C(CH3)2), 15.2 (CH3-
(apical)). MS (EI; m/z (%)): 224.4 [M]+ (50), 209.3 [M - CH3]+
(100).
n
{BOX-Me2}AlMe2 (2a ). In a glovebox, BuLi (2.38 mL of a
1.6 M hexanes solution, 3.80 mmol) was added dropwise via a
pipet to a pentane solution (15 mL) of {BOX-Me2}H (852 mg,
3.81 mmol) which had been precooled to -40 °C in a freezer.
Upon addition of nBuLi, immediate precipitation of a colorless
solid occurs, along with gas bubbles formation (nBuH). After
the addition, the reaction mixture was warmed to room
temperature and stirred overnight. The resulting thick color-
less suspension was then filtered through a glass frit and the
obtained colorless solid washed with cold pentane and dried
under vacuum for 1 h. After this time, the solid residue was
dissolved in toluene (10 mL) and the resulting pale yellow
solution stored at -40 °C for 30 min. The solution was then
taken out of the freezer and ClAlMe2 (3.80 mL of a 1 M
hexanes solution, 3.80 mmol), also precooled to -40 °C, was
quickly added to the toluene solution. Immediate precipitation
occurs most likely due to LiCl formation. The reaction mixture
was then warmed to room temperature and vigorously stirred
overnight at this temperature to yield a suspension of a
colorless solid in a pale yellow solution. The mixture was
evaporated to dryness under vacuum to yield an oily and sticky
orange residue which was extracted with a 10/1 pentane/Et2O
mixture (11 mL). The mixture was then filtered through a frit
under vacuum. The obtained yellow filtrate was concentrated
to ∼4 mL and stored overnight at -40 °C, causing the
precipitation of a colorless crystalline solid. Filtration through
a glass frit and subsequent drying of the obtained solid in
vacuo afforded pure 2a (607 mg, 57% yield) as colorless
crystals. 1H NMR (400 MHz, C6D6): δ -0.26 (s, 6H, AlMe2),
1.09 (s, 12H, CMe2), 2.18 (s, 3H, MeCCN), 3.38 (s, 4H, OCH2).
13C{1H} NMR (100 MHz, C6D6): δ -5.4 (AlMe2), 9.7 (MeCCN),
27.4 (CMe2), 63.2 (CMe2), 64.0 (MeCCN), 79.1 (OCH2), 171.5
-
NMR data for the MeB(C6F5)3 anion are listed below for all
compounds.
Da ta for MeB(C6F 5)3-. 1H NMR (400 MHz, CD2Cl2): δ 0.48
(BMe). 11B{1H} NMR (128 MHz, CD2Cl2): δ -11.9 (br s, BMe).
1
13C{1H} NMR (100 MHz, CD2Cl2): δ 148.6 (d, J CF ) 233 Hz,
o-C6F5), 137.9 (d, 1J CF ) 238, p-C6F5), 136.7 (d, 1J CF ) 233 Hz,
m-C6F5), 10.3 (MeB). 19F NMR (376 MHz, CD2Cl2): δ -133.5
3
3
(d, J FF ) 19 Hz, 2F, o-C6F5), -165.7 (t, J FF ) 20 Hz, 1F,
3
p-C6F5), -168.2 (m, J FF ) 19 Hz, 2F, p-C6F5).
1,1-Bis[(4S)-4-isop r op yl-1,3-oxa zolin -2-yl]et h a n e (1b ,
{BOX-(S)-iP r }H). Diethyl methylmalonate (2.06 g, 11.8 mmol)
and (S)-valinol (2.44 g, 23.6 mmol) were added in a Schlenk
flask. NaH (10 mg, 0.25 mmol; 60% dispersion in mineral oil)
was then added under nitrogen to the flask, which was sealed
and placed at 130-140 °C (sand bath). After 3-4 h, the ethanol
was removed under vacuum to leave a white solid pure enough
to be used for the next step without further purification (3.35
g, 98% yield). 1H NMR (300 MHz, CDCl3): δ 7.26-7.13 (s, 2H,
NH), 3.85-3.38 (m, 6H, -C(C3H7)HCH2-), 3.32 (q, J ) 7.3
Hz, 1H, CHCH3), 1.80 (m, 2H, -CH(CH3)2), 1.49 (d, J ) 7.3
Hz, 3H, CHCH3), 0.96-0.87 (m, 12H, CH(CH3)2).
To an ice-cooled solution of the dihydroxy diamide (3.35 g,
11.6 mmol) and triethylamine (5.9 g, 58 mmol) in CH2Cl2 (75
mL) was added MsCl (3.3 g, 29 mmol). The mixture was
warmed to room temperature, stirred for 1 h, and washed with
a solution of NH4Cl. The organic phase was dried with Na2-
SO4 and concentrated in vacuo to give a yellow oil, which was
used in the next step without purification. The bis-mesylated
compound was treated with NaOH (2.0 g, 50 mmol) in 80 mL
of a 1:1 MeOH/H2O mixture. The solution was refluxed for 2
h and then concentrated to remove methanol and extracted
with CH2Cl2 (3 × 50 mL). The organic phase was dried with
Na2SO4 and concentrated in vacuo. Distillation under vacuum
afforded 1.2 g (41% from diethyl methylmalonate) of 1b as a
colorless oil. 1H NMR (300 MHz, CDCl3): δ 4.23 (m, 4H, CH2),
3.98 (m, 2H, CH(C3H7)), 3.53 (q, 1H, CHCH3(apical)), 1.77 (m,
2H, CH(CH3)2), 1.47 (d, J ) 7.1 Hz, 3H, CH3(apical)), 0.95-
0.84 (m, 12H, CH(CH3)2)). 13C NMR (75.5 MHz, CDCl3): δ
165.5 (CdN), 71.8 (CHC3H7), 70.2 (CH2), 34.0 (CHCH3), 32.4
(CH(CH3)2), 18.7 (CH(CH3)2), 17.7 (CH(CH3)2), 15.3 (CH3-
(apical)). MS (EI; m/z (%)): 252.3 [M]+ (2), 209.3 [M - (CH-
(CH3)2)]+ (94), 123.3 (100). Anal. Calcd for C14H24N2O2: C, 66.6;
H, 9.6; N, 11.1, Found: C, 66.8; H, 9.5; N, 10.8.
1
(NCO). H NMR (300 MHz, C6D5Br, -20 °C): δ -0.51 (s, 6H,
AlMe2), 1.18 (s, 12H, CMe2), 1.94 (s, 3H, MeCCN), 3.64 (s, 4H,
OCH2). Anal. Calcd for C14H25AlN2O2: C, 59.98; H, 8.99.
Found: C, 59.71; H, 8.85.
NMR-Sca le Gen er a tion of {BOX-Me2}AlMe2 (2a ) via
Meth a n e Elim in a tion . In a drybox, the neutral ligand {BOX-
Me2}H (41.0 mg, 0.183 mmol) was charged in a J . Young NMR
tube and dissolved in 0.5 mL of CD2Cl2, resulting in a colorless
solution. The NMR tube was then stored in a freezer at -35
°C for 30 min, after which AlMe3 (17.5 uL, 0.183 mmol) was
quickly added via a syringe. The tube was then vigorously
shaken and warmed to room temperature. After 30 min at
1
room temperature, a H NMR spectrum of the reaction mixture
was recorded, showing the quantitative formation of {BOX-
Me2}AlMe2, along with methane formation (δ 0.17).
{BOX-(S)-iP r }AlMe2 (2b). Compound 2b was synthesized
by following the same procedure as that for 2a , using equi-
n
molar amounts of {BOX-(S)-iPr}H (138.0 mg, 0.534 mmol), -
BuLi (0.34 mL of a 1.6 M hexanes solution, 0.534 mmol), and
ClAlMe2 (0.53 mL of a 1 M hexanes solution, 0.534 mmol).
After evaporation of the reaction mixture, the solid residue
was extracted twice with pentane (2 × 10 mL). The pentane
filtrate was then concentrated to ∼2 mL and stored in a freezer
at -40 °C for 2 days to yield pure 2b as colorless crystals (77
mg, 48% yield). 1H NMR (400 MHz, C6D6): δ -0.29 (s, 6H,
3
3
AlMe2), 0.48 (d, J ) 7.0 Hz, 6H, Me iPr), 0.74 (d, J ) 6.6 Hz,
i
3
3
6H, Me Pr), 2.05 (d of septet, J doublet ) 3.5 Hz, J septet ) 7.0
i
Hz, 2H, CH Pr), 2.13 (s, 3H, MeCCN), 3.60 (t, J ) 8.6 Hz, 2H,
(28) Abiko, A.; Masamune, S. Tetrahedron Lett. 1992, 33, 5517.
OCH2), 3.69 (d of d, 2J ) 8.6 Hz, 3J ) 5.9 Hz, 2H, OCH2), 3.82

3060 Organometallics, Vol. 23, No. 12, 2004
Dagorne et al.
(m, 2H, CHN). 13C{1H} NMR (100 MHz, C6D6): δ -8.5 (AlMe2),
H(â) THF), 4.15 (m, 4H, H(R) THF), 4.19 (s, 4H, OCH2 BOX).
13C{1H} NMR (100 MHz, CD2Cl2): δ -12.8 (AlMe), 8.1
(MeCCN), 25.0 (C(â) THF), 27.5 (CMe2), 63.2 (CMe2), 68.1
(MeCCN), 73.5 (C(R) THF), 79.7 (OCH2 BOX), 172.4 (NCO).
Anal. Calcd for C36H33AlBF15N2O3: C, 50.02; H, 3.85. Found:
C, 50.47; H, 3.96.
i
9.9 (MeCCN), 14.2 (CH3 iPr), 18.9 (CH3 iPr), 31.0 (CH Pr), 62.9
(MeCCN), 65.8 (OCH2), 67.3 (NCH), 172.3 (NCO). Anal. Calcd
for C16H29AlN2O2: C, 62.31; H, 9.48; N, 9.08. Found: C, 62.53;
H, 9.25; N, 9.21.
{BOX-Me2}AlCl2 (3a ). The same procedure as that for 2a ,b
was used, with equimolar amounts of {BOX-Me2}H (136.0 mg,
0.605 mmol), nBuLi (0.38 mL of a 1.6 M hexanes solution, 0.605
mmol), and AlCl3 (colorless solid, 80.7 mg, 0.607 mmol). After
filtration of the reaction mixture through a glass frit to remove
LiCl and subsequent drying under vacuum, crude 3a was
obtained as a colorless powder and was recrystallized from a
5/1 Et2O/toluene mixture (5 mL) to afford pure 3a as a colorless
crystalline solid (121 mg, 62% yield). 1H NMR (400 MHz,
C6D6): δ 1.23 (s, 12H, CMe2), 1.98 (s, 3H, MeCCN), 3.27 (s,
4H, OCH2). 13C{1H} NMR (100 MHz, C6D6): 9.2 (MeCCN), 27.1
(CMe2), 63.9 (CMe2), 67.1 (MeCCN), 79.7 (OCH2), 172.1 (NCO).
Anal. Calcd for C12H19AlCl2N2O2: C, 44.87; H, 5.96; N, 8.72.
Found: C, 44.85; H, 6.17; N, 8.59.
Da ta for 5b+. 1H NMR (400 MHz, CD2Cl2): δ -0.36 (s, 3H,
3
3
AlMe), 0.84 (d, J ) 6.8 Hz, 6H, Me iPr), 0.96 (d, J ) 6.9 Hz,
6H, Me iPr), 1.75 (s, 3H, MeCCN), 1.84 (d of septet, J doublet
)
3
3
i
3.3 Hz, J septet ) 6.9 Hz, 2H, CH Pr), 2.13 (m, 4H, H(â) THF),
2
4.05-4.17 (m, 6H, CHN and H(R) THF), 4.36 (d of d, J ) 9.2
3
Hz, J ) 5.8 Hz, 2H, OCH2), 4.41 (t, J ) 8.6 Hz, 2H, OCH2).
13C{1H} NMR (100 MHz, CD2Cl2): δ -14.8 (AlMe2), 8.1
(MeCCN), 13.3 (CH3 iPr), 18.2 (CH3 iPr), 24.7 (C(â) THF), 31.6
i
(CH Pr), 64.5 (NCH), 67.1 (MeCCN), 68.2 (OCH2), 74.1 (C(R)
THF), 172.9 (NCO). Anal. Calcd for C38H37AlBF15N2O3: C,
51.14; H, 4.18. Found: C, 50.85; H, 4.12.
[{BOX-Me2}Al(Me)(NMe2P h )][MeB(C6F 5)3] ([6a ][MeB-
(C6F5)3)]) an d [{BOX-(S)-iP r }Al(Me)(NMe2P h )][MeB(C6F5)3]
([6b][MeB(C6F 5)3]). The salt compounds [6a ,b][MeB(C6F5)3]
were obtained as analytically pure colorless solids ([6a ][MeB-
(C6F5)3], 73% yield; [6b][MeB(C6F5)3], 55% yield), following the
same procedure as that for the synthesis of [5a ,b][MeB(C6F5)3]-
using equimolar amounts of 2a ,b (2a , 60.0 mg, 0.214 mmol;
2b, 51.0 mg, 0.195 mmol), NMe2Ph, and B(C6F5)3.
NMR -Sca le Gen er a t ion of [{BOX-Me2}AlMe][MeB-
(C6F5)3] ([4a][MeB(C6F5)3]) an d [{BOX-(S)-iP r }AlMe][MeB-
(C6F 5)3] ([4b][MeB(C6F 5)3]). In a drybox, equimolar amounts
of the bis(oxazolinato) aluminum dimethyl complex 2a ,b (2a ,
9.1 mg, 0.0324 mmol; 2b, 10 mg, 0.0324 mmol) and B(C6F5)3
(16.6 mg, 0.0324 mmol) were weighed into a small sample vial
and were quickly dissolved in C6D5Br (0.5 mL). The resulting
colorless solution was transferred to a J . Young NMR tube,
and a 1H NMR spectrum was immediately recorded at -20
°C, showing the quantitative formation of [4a ][MeB(C6F5)3] and
Da ta for 6a +. 1H NMR (300 MHz, CD2Cl2): δ -0.13 (s, 3H,
AlMe), 1.09 (s, 6H, CH2CMe), 1.22 (s, 6H, CH2CMe), 1.71 (s,
2
3H, MeCCN), 3.05 (s, 6H, NMe2Ph), 3.85 (d, J ) 8.7 Hz, 2H,
2
3
OCH2 BOX), 4.01 (d, J ) 8.7 Hz, 2H, OCH2 BOX), 7.27 (d, J
-
3
3
[4b][MeB(C6F5)3], respectively, as fully dissociated MeB(C6F5)3
) 7.4 Hz, 2H, Ph), 7.42 (t, J ) 7.2 Hz, 1H, Ph), 7.53 (t, J )
6.9 Hz, 2H, Ph). 13C{1H} NMR (100 MHz, CD2Cl2): δ -13.2
(AlMe), 8.1 (MeCCN), 22.0 (CH2CMe2), 27.9 (CH2CMe2), 46.2
(NMe2Ph), 63.5 (CMe2), 70.9 (MeCCN), 78.8 (OCH2 BOX),
120.6 (CH Ph), 128.2 (CH Ph), 131.4 (CH Ph), 120.6 (Ph), 144.1
(Cipso Ph), 173.2 (NCO). Anal. Calcd for C40H36AlBF15N3O2: C,
52.59; H, 3.97. Found: C, 52.86; H, 3.61.
salt species in solution under the studied conditions. The poor
stability of these salt compounds precluded their isolation in
pure form.
Da ta for 4a +. 1H NMR (400 MHz, C6D5Br, -20 °C): δ -0.21
(s, 3H, AlMe), 0.96 (s, 12H, CMe2), 1.75 (s, 3H, MeCCN), 3.61
(s, 4H, OCH2). 13C{1H} NMR (100 MHz, C6D5Br, -20 °C): δ
-6.3 (AlMe), 11.1 (MeCCN), 29.5 (CMe2), 65.7 (CMe2), 72.1
(MeCCN), 81.1 (OCH2), 174.6 (NCO).
Da ta for 6b+. 1H NMR (400 MHz, CD2Cl2): δ -0.17 (s, 3H,
3
3
AlMe), 0.55 (d, J ) 7.0 Hz, 3H, Me iPr), 0.56 (d, J ) 7.0 Hz,
Da ta for 4b+. 1H NMR (400 MHz, C6D5Br, -20 °C): δ -0.21
3H, Me iPr), 1.01 (d, 3J ) 6.7 Hz, 3H, Me iPr), 1.03 (d, 3J ) 6.7
3
i
3
i
3
3
(s, 3H, AlMe), 0.45 (d, J ) 6.5 Hz, 6H, Me Pr), 0.57 (d, J )
Hz, 3H, Me Pr), 1.45 (d of septet, J doublet ) 3.5 Hz, J septet
)
6.7 Hz, 6H, Me iPr), 1.61 (m, 2H, CH Pr), 1.78 (s, 3H, MeCCN),
i
i
6.6 Hz, 1H, CH Pr), 1.70 (s, 3H, MeCCN), 1.84 (d of septet,
3.81-3.90 (m, 6H, CHN and OCH2). 13C{1H} NMR (100 MHz,
3J doublet ) 3.3 Hz, 3J septet ) 6.9 Hz, 2H, CH Pr), 2.02 (d of triplet,
i
i
3J doublet ) 8.6 Hz, 3J triplet ) 3.5 Hz, 1H, NCH), 2.98 (s, 3H, NMe2-
Ph), 3.09 (s, 3H, NMe2Ph), 3.86 (t, 3J ) 9.0 Hz, 1H, OCH2),
C6D5Br, -20 °C): δ -7.3 (AlMe), 11.6 (MeCCN), 16.0 (Me -
Pr), 20.8 (Me iPr), 34.2 (CH Pr), 65.7 (CMe2), 66.7 (CHN), 71.1
i
2
3
(OCH2), 71.2 (MeCCN), 175.3 (NCO).
4.09 (d. of d., J ) 9.0 Hz, J ) 3.5 Hz, 1H, OCH2), 4.16 (ddd,
3J ) 9.8 Hz, J ) 7.0 Hz, J ) 2.7 Hz, 1H, NCH), 4.36 (d of d,
3
3
-
Da ta for MeB(C6F 5)3
.
1H NMR (400 MHz, C6D5Br, -20
2J ) 9.0 Hz, J ) 7.0 Hz, 1H, OCH2), 4.42 (t, J ) 9.4 Hz, 1H,
3
°C): δ 1.43 (MeB). 13C{1H} NMR (100 MHz, C6D5Br, -20 °C):
δ 16.4 (MeB), 139.2 (d, 1J CF ) 241 Hz, MeB(C6F5)3-), 140.7 (d,
3
3
OCH2), 7.39 (d, J ) 7.8 Hz, 1H, Ph), 7.50 (t, J ) 7.0 Hz, 1H,
Ph), 7.59 (t, 3J ) 7.6 Hz, 1H, Ph). 13C{1H} NMR (100 MHz,
CD2Cl2): δ -13.6 (AlMe), 8.7 (MeCCN), 13.7 (CH3 iPr), 15.0
1J CF ) 247 Hz, MeB(C6F5)3-), 150.7 (d, J CF ) 246 Hz,
1
MeB(C6F5)3-). 19F NMR (376 MHz, C6D5Br, -20 °C): δ -164.7
(CH3 iPr), 18.0 (CH3 iPr), 20.3 (CH3 iPr), 31.8 (CH Pr), 32.1
i
3
3
(t, J FF ) 19.2 Hz, 2F, C6F5), -160.4 (t, J FF ) 20.3 Hz, 1F,
C6F5), -133.4 (d, J FF ) 19.2 Hz, 2F, C6F5).
(CH iPr), 44.3 (NMe2Ph), 48.2 (NMe2Ph), 64.8 (NCH), 66.0
(NCH), 68.3 (OCH2), 68.4 (MeCCN), 68.9 (OCH2), 120.7 (CH
Ph), 129.1 (CH Ph), 131.0 (CH Ph), 144.1 (Cipso Ph), 173.2
(NCO), 174.6 (NCO). Anal. Calcd for C42H40AlBF15N3O2: C,
53.58; H, 4.28. Found: C, 54.74; H, 4.37.
[{H 2CdC(OX-Me2)2}AlMe2][B(C6F 5)4] ([7a ][B(C6F 5)4]).
NMR Sca le. Equimolar amounts of compound 2a (8.0 mg,
0.028 mmol) and [Ph3C][B(C6F5)4] were added to a small
sample vial and dissolved in CD2Cl2 (0.5 mL). The resulting
bright orange solution was then transferred to a J . Young
NMR tube, and a 1H NMR spectrum was immediately re-
corded, showing the quantitative formation of [7a ][B(C6F5)4]
and Ph3CH in a 1/1 ratio.
P r ep a r a tive Sca le. In a glovebox, compound 2a (12.0 mg,
0.043 mmol) and [Ph3C][B(C6F5)4] (39.5 mg, 0.043 mmol) were
charged in a small Schlenk flask and dissolved in CH2Cl2 (1
mL). The resulting bright orange solution was stirred for 30
min at room temperature, after which the volatiles were
removed under vacuum to yield an orange foam. This foamy
residue was washed twice with toluene (2 × 2 mL) to remove
3
[{BOX-Me2}Al(Me)(THF)][MeB(C6F5)3] ([5a][MeB(C6F5)3)
a n d [{BOX-(S)-iP r }Al(Me)(THF )][MeB(C6F 5)3] ([5b][MeB-
(C6F 5)3). In a drybox, equimolar amounts of the appropriate
bis(oxazolinato)aluminum dimethyl complex (2a , 50.0 mg,
0.178 mmol; 2b, 55.5 mg, 0.180 mmol) and THF (14.5 and 14.6
µL, respectively) were dissolved in 0.75 mL of CH2Cl2, resulting
in a colorless solution. One equivalent of B(C6F5)3 (91.3 and
92.1 mg, respectively) was added all at once. The colorless
solution was charged in a small Schlenk flask and stirred at
room temperature for 30 min, after which it was evaporated
under vacuum to yield a colorless foam. In both cases,
trituration of the foamy residue with cold pentane (precooled
at -40 °C) caused the precipitation of a colorless solid which,
after filtration through a glass frit and drying under vacuum,
afforded the salts [5a ][MeB(C6F5)3] and [5b][MeB(C6F5)3] in a
pure form ([5a ][MeB(C6F5)3], 101 mg, 66% yield; [5b][MeB-
(C6F5)3], 125 mg, 78% yield), respectively.
Da ta for 5a +. 1H NMR (400 MHz, CD2Cl2): δ -0.23 (s, 3H,
AlMe), 1.41 (s, 12H, CMe2), 1.76 (s, 3H, MeCCN), 2.17 (m, 4H,

Bis(oxazolinato)aluminum Complexes
Organometallics, Vol. 23, No. 12, 2004 3061
Ph3CH and any excess [Ph3C][B(C6F5)4], as follows: addition
of toluene provoked the formation of a reddish orange oil at
the bottom of the flask. The supernatant toluene solution was
discarded and the oily residue dried under vacuum to afford a
sticky red oil. Subsequent trituration of the residue with cold
pentane (precooled at -35 °C) caused the precipitation of a
yellow solid. The mixture was then filtered under reduced
pressure through a glass frit and the solid residue dried under
vacuum to afford pure [7a ][B(C6F5)4] as a bright yellow solid
(32 mg, 78% yield). Anal. Calcd for C38H24AlBF20N2O2: C,
For complex 3a , two positions must be considered for the
C10 atom (C10 and C10A). The same disorder was applied on
the bonded C11 and C12 methyl groups. In the case of [7a ][MeB-
(C6F5)3)], rigorous interpretation of the electronic density
around the aluminum atom was required to consider a partial
substitution of the methyl group’s carbon atoms by fluorine
atoms. Best refinements are obtained for equal occupancies
(50% carbon, 50% fluorine) on each position. Crystallographic
data (excluding structure factors) have been deposited in the
Cambridge Crystallographic Data Centre as Supplementary
Publication Nos. CCDC 231414-231417. Copies of the data
can be obtained free of charge on application to the CCDC, 12
Union Road, Cambridge CB2 1EZ, U.K. (fax (+44)1223-336-
033; e-mail deposit@ccdc.cam.ac.uk).
1
47.62; H, 2.52. Found: C, 47.10; H, 2.25. H NMR (400 MHz,
CD2Cl2): δ -0.52 (s, 6H, AlMe2), 1.59 (s, 12H, CMe2), 4.59 (s,
4H, OCH2), 7.38 (s, 2H, H2CdC). 13C{1H} NMR (100 MHz, CD2-
Cl2): δ -8.4 (AlMe2), 26.8 (CMe2), 69.4 (CMe2), 81.4 (OCH2),
118.0 (H2CdC), 136.7 (d, 1J CF ) 243 Hz, B(C6F5)4-), 138.6 (dt,
1J CF ) 243 Hz, 2J CF ) 14 Hz, B(C6F5)4-), 147.6 (H2CdC), 148.5
1
(d, J CF ) 239 Hz, B(C6F5)4-), 164.3 (NdCO).
Ack n ow led gm en t. We thank the Centre National
de la Recherche Scientifique (CNRS) for financial sup-
port and M.-N. Rager (ENSCP, Paris, France) for low-
temperature NMR assistance. S.D. and S.B.-L. are
grateful to Prof. G. J aouen (ENSCP, UMR CNRS 7576)
and Prof. L. H. Gade (University of Strasbourg, UMR
CNRS 7513), respectively, for allowing this independent
work to be carried out in their laboratories.
X-r a y Str u ctu r e An a lysis of Com p lexes 2b, 3a , [6a ]-
[MeB(C6F 5)3)], a n d [7a ][B(C6F 5)4]. Selected crystals were
mounted on a Nonius Kappa-CCD area detector diffractometer
(Mo KR, λ ) 0.710 73 Å). The complete conditions of data
collection (Denzo software) and structure refinements are
given in Table 1. The cell parameters were determined from
reflections taken from one set of 10 frames (1.0° steps in ψ
angle), each at 20 s exposure. The structures were solved using
2
direct methods (SIR97) and refined against F using the
SHELXL97 software. In the case of 2b and [6a ][MeB(C6F5)3)],
the absorption was corrected empirically (with Sortav). All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
were generated according to stereochemistry and refined using
a riding model in SHELXL97.
Su p p or tin g In for m a tion Ava ila ble: CIF files for 2b, 3a ,
[6a ][MeB(C6F5)3], and [7a ][B(C6F5)4]. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM0400245