Synthesis and structure of a four-coordinate aluminum Alkyl cation/HB(C6F5)3 salt: Implication in a B(C6F5)3-catalyzed hydroalumination reaction of benzophenone or benzaldehyde

Article abstract of DOI:10.1021/om0400806

The reaction of the bulky aminophenol 2-(CH₂NMe ₂)-4-Me-6-CPh₃-C₆H₃OH (1) with Al(ⁱBu)₃ yields via isobutane elimination the formation of the diisobutyl Al complex {6-(CH₂-NMe₂)-2-CPh ₃-4-Me-C₆H₂O}Al(ⁱBu)₂ (2) in high yield. Reaction of 2 with B(C₆F₅)₃ in the presence of NMe₂Ph affords, along with isobutene, the salt compound [{6-(CH₂NMe₂)-2-CPh₃-4-Me-C ₆H₂O}Al(ⁱBu)(NMe₂Ph)][HB(C ₆F₅)₃] (3), consisting of a four-coordinate Lewis acidic Al cation associated with the borohydride HB(C₆F ₅)₃^-, whose solid-state structure was determined by X-ray crystallography. When complex 2 is reacted with B(C ₆F₅)₃ (1 and 0.05 equiv) in the presence of benzophenone and benzaldehyde, the quantitative conversion to the hydroalumination products 4 and 5, respectively, is observed. This B(C ₆F₅)₃-catalyzed hydroalumination reaction, via a B(C₆F₅)₃-mediated hydride abstraction/transfer reaction, represents a new type of reactivity for Al alkyl cations and illustrates the higher reactivity of Al cation/HB(C ₆F₅)₃^- salts versus the classical Al cation/MeB(C₆F₅)₃^- and B(C ₆F₅)₄^- salts.

Full text of DOI:10.1021/om0400806

4706
Organometallics 2004, 23, 4706-4710
Syn th esis a n d Str u ctu r e of a F ou r -Coor d in a te Alu m in u m
Alk yl Ca tion /HB(C6F 5)3 Sa lt: Im p lica tion in a
B(C6F 5)3-Ca ta lyzed Hyd r oa lu m in a tion Rea ction of
Ben zop h en on e or Ben za ld eh yd e
Samuel Dagorne,*,† Izabela J anowska,† Richard Welter,‡
J anusz Zakrzewski,§ and Ge´rard J aouen†
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,
Laboratoire DECMET, Institut Le Bel, Universite´ Louis Pasteur, 4, rue Blaise Pascal,
67000 Strasbourg, France, and Department of Organic Chemistry, University of Lodz,
Narutowicza 68, 90-136 Lodz, Poland
Received J une 2, 2004
The reaction of the bulky aminophenol 2-(CH2NMe2)-4-Me-6-CPh3-C6H3OH (1) with Al(i-
Bu)3 yields via isobutane elimination the formation of the diisobutyl Al complex {6-(CH2-
NMe2)-2-CPh3-4-Me-C6H2O}Al(iBu)2 (2) in high yield. Reaction of 2 with B(C6F5)3 in the
presence of NMe2Ph affords, along with isobutene, the salt compound [{6-(CH2NMe2)-2-
CPh3-4-Me-C6H2O}Al(iBu)(NMe2Ph)][HB(C6F5)3] (3), consisting of a four-coordinate Lewis
-
acidic Al cation associated with the borohydride HB(C6F5)3 , whose solid-state structure was
determined by X-ray crystallography. When complex 2 is reacted with B(C6F5)3 (1 and 0.05
equiv) in the presence of benzophenone and benzaldehyde, the quantitative conversion to
the hydroalumination products 4 and 5, respectively, is observed. This B(C6F5)3-catalyzed
hydroalumination reaction, via a B(C6F5)3-mediated hydride abstraction/transfer reaction,
represents a new type of reactivity for Al alkyl cations and illustrates the higher reactivity
of Al cation/HB(C6F5)3- salts versus the classical Al cation/MeB(C6F5)3- and B(C6F5)4- salts.
In tr od u ction
Alternatively, although synthetically challenging, the
use of more reactive and more Lewis basic counterions
for association with a Lewis acidic Al cation appears
interesting, since the resulting salt compound would
include both a Lewis acidic and basic entity. This
complementarity may be useful for the functionalization
of some unsaturated substrates: i.e., (i) activation of the
substrate by coordination to the Al cation followed by
(ii) reaction of the Lewis basic anion with the coordi-
nated substrate.
Low-coordinate cationic Al species have recently
received increased attention because the potent Lewis
acid character of the obtained Al cations, a result of the
cationic charge at the Al center, is of potential interest
for Lewis acid catalysis.1 In this regard, cationic alkyl
Al species of the type {LX}AlMe+ are particularly
interesting targets, since they can be readily accessible
from {LX}AlMe2 via a Me- abstraction by strong Lewis
acids such as B(C6F5)3 at the Al center.2 Several studies
have shown that the stability of these cationic species
is strongly dependent on the inertness of the counterion
and on the presence of a Lewis base L in the medium
to stabilize the Al cation by forming {LX}Al(Me)(L)+
adducts.3 Thus, the MeB(C6F5)3- and B(C6F5)4- anions,
which are among the least reactive anions known so far,
are typically associated with low-coordinate Al cations,
yielding, in most cases, stable {LX}Al(Me)(L)+ cations.2,3
We reported earlier that the reactions of monoamino-
phenolate-N,O dialkyl Al complexes {2-(CH2NMe2)-6-
t
R-C6H3O}AlMe2 (R ) Bu, Ph) with B(C6F5)3 lead to
-
stable dinuclear cationic Al species as MeB(C6F5)3
salts, whose reactivity was then studied.4 To prevent
such aggregation and thus to favor the obtainment of
mononuclear cationic species, we became interested in
the synthesis of Al alkyl cations incorporating a more
bulky bidentate aminophenolate. The CPh3 ortho-
substituted aminophenolate ligand A (Chart 1) seemed
a good candidate for this purpose, because the signifi-
cant steric crowding around the Al center should
promote the formation and the stability of mononuclear
Al species. As part of our studies on the reactivity of
{2-(CH2NMe2)-4-Me-6-CPh3-C6H3O}AlR2 with B(C6F5)3,
we here report that the ionization reaction of {2-(CH2-
NMe2)-4-Me-6-CPh3-C6H3O}AliBu2 with B(C6F5)3 in the
* To whom correspondence should be addressed. Fax: +33 1 43 26
00 61. E-mail: dagorne@ext.jussieu.fr.
† Ecole Nationale Supe´rieure de Chimie de Paris.
‡ Universite´ Louis Pasteur.
§ University of Lodz.
(1) Atwood, D. A. Coord. Chem. Rev. 1998, 176, 407.
(2) For leading references, see: (a) Coles, M. P.; J ordan, R. F. J .
Am. Chem. Soc. 1997, 119, 8125. (b) Bruce, M.; Gibson, V. C.; Redshaw,
C.; Solan, G. A.; White, A. J . P.; Williams, D. J . Chem. Commun. 1998,
2523. (c) Radzewich, C. E.; Guzei, I. A.; J ordan, R. F. J . Am. Chem.
Soc. 1999, 121, 8673.
(3) (a) Dagorne, S.; Guzei, I. A.; Coles, M. P.; J ordan, R. F. J . Am.
Chem. Soc. 2000, 122, 274. (b) Korolev, A. V.; Ihara, E.; Guzei, I. A.;
Young, V. G., J r.; J ordan, R. F. J . Am. Chem. Soc. 2001, 123, 8291.
(4) Dagorne, S.; Lavanant, L.; Welter, R.; Chassenieux, C.; Haquette,
P.; J aouen, G. Organometallics 2003, 22, 3732.
10.1021/om0400806 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 08/27/2004

A Four-Coordinate Aluminum Alkyl Cation
Organometallics, Vol. 23, No. 20, 2004 4707
Al-iBu moiety of complex 2, as evidenced by the
presence of isobutene and of the HB(C6F5)3- anion.7 As
anticipated, compound 3 is less stable than related
[{LX}Al(Me)(NMe2Ph)][MeB(C6F5)3] salt compounds and
slowly decomposes over 2-3 days in CD2Cl2 at room
temperature to yield unknown products. The prepara-
tive-scale generation of salt compound 3 allowed its
obtainment as a colorless solid, although not analytically
pure (see Experimental Section).
Ch a r t 1
Sch em e 1
Attempts to perform similar ionization reactions using
less bulky aminophenolate Al-diisobutyl complexes {2-
t
(CH2NMe2)-6-R-C6H3O}AliBu2 (R ) Bu, Ph)8 only al-
lowed the fleeting observation of the desired species [{2-
(CH2NMe2)-6-R-C6H3O}Al(iBu)(NMe2Ph)][HB(C6F5)3],
followed by fast decomposition to unknown species.
These observations strongly suggest that a significant
steric crowding at the Al center is a key requirement to
obtain stable [{LX}Al(iBu)(L)][HB(C6F5)3]. In addition,
the nature of the L Lewis base coordinated to the
cationic Al center also appears to be critical for the
stability of these salt compounds. Thus, when the Al
diisobutyl complex 2 is reacted with B(C6F5)3 in the
presence of THF, an intractable mixture of products is
obtained, which contrasts with the general straightfor-
ward generation and good stability of [{LX}Al(Me)-
(THF)][MeB(C6F5)3] salts. Overall, changing from MeB-
Sch em e 2
presence of an appropriate Lewis base L affords an
unprecedented salt compound, associating a Lewis
acidic Al-L cation with the HB(C6F5)3- anion, a hydride
source. The potential reactivity of this salt is illustrated
here by its implication in the B(C6F5)3-catalyzed hy-
(C6F5)3- to the more Lewis basic HB(C6F5)3 anion, a
-
formal hydride source, dramatically decreases the sta-
bility of the formed salts.
droalumination of benzophenone or benzaldehyde, which
involves the reduction of a carbonyl group by HB(C6F5)3
-
.
Solu tion a n d Solid -Sta te Str u ctu r es of th e Sa lt
1
Com p ou n d 3. The H, 11B, and 19F NMR data for 3 in
Resu lts a n d Discu ssion
C6D6 at room temperature are consistent with the
-
presence of the HB(C6F5)3 anion and with this anion
The reaction of the aminophenol 2-(CH2NMe2)-4-Me-
6-CPh3-C6H3OH (1; Scheme 1), synthesized according
to known literature procedures,5 with 1 equiv of Al(iBu)3
at -40 °C in pentane affords, along with isobutane
evolution, the corresponding Al diisobutyl complex {6-
(CH2NMe2)-2-CPh3-4-Me-C6H2O}Al(iBu)2 (2; Scheme 1)
in high yield as an analytically pure colorless solid. The
NMR data for complex 2 at room temperature in C6D6
are consistent with the chelation of one aminophenolate
ligand to the Al center. Under the studied conditions,
the NMR data for 2 show an effective Cs symmetry,
which may be ascribed to a fast conformation change of
the six-membered-ring Al metallacycle on the NMR time
scale, as previously observed for related monoamino-
phenolate group 13 complexes.6 For example, the 1H
NMR spectrum of the diisobutyl Al complex 2 exhibits
two CH3 iBu resonances (δ 0.69, 0.64) and two CH2 iBu
resonances (δ -0.39, -0.45) but contains only one CH
iBu signal (δ 1.48), as expected for the proposed sym-
metry.
being free in solution under these conditions, as its NMR
data closely matched those for the ammonium salt
[Bu4N][HB(C6F5)3].9 For example, the 1H NMR spec-
trum of 3 contains a broad doublet (δ 4.15) assigned to
the HB hydride, whereas the 11B NMR spectrum
exhibits a doublet (δ -25.7), which is in agreement with
a BH-type four-coordinate borate.9 The NMR data for
the Al-NMe2Ph cation at room temperature clearly
evidence the coordination of the aniline to the cationic
Al center and show an effective overall C1 symmetry
for the Al cation, as a result of the coordination of the
1
aniline to the Al center. In particular, the H and 13C
NMR spectra of 4 both display two resonances for the
Me groups of the coordinated NMe2Ph as well as two
signals for the NMe2 group of the aminophenolate
ligand.
The solid-state structure of complex 3 was determined
by X-ray crystallographic analysis. The molecular struc-
ture of 3 and selected bond distances and angles are
shown respectively in Figure 1 and Table 2, and
crystallographic data are given in Table 1. The salt
compound 3 crystallizes as {6-(CH2NMe2)-2-CPh3-4-Me-
C6H2O}Al(iBu)(NMe2Ph)+ and HB(C6F5)3- ions with no
close cation-anion interactions. Each asymmetric unit
contains two independent molecules of 3, and both of
The NMR-scale reaction of the diisobutyl Al complex
2 with 1 equiv of B(C6F5)3 (CD2Cl2, 15 min, room
temperature) in the presence of 1 equiv of NMe2Ph
yields the quantitative formation of the four-coordinate
-
cationic Al-NMe2Ph adduct as a HB(C6F5)3 salt, [{6-
(CH2NMe2)-2-CPh3-4-Me-C6H2O}Al(iBu)(NMe2Ph)][HB-
(C6F5)3] (3, Scheme 2), along with isobutene formation,
as observed by 1H NMR. This reaction most likely
proceeds via a hydride abstraction reaction at Câ of one
(7) [Ph3C][B(C6F5)4] was also reported to â-abstract an hydride at
neutral {LX}Al(iBu)2Al complexes to yield Ph3CH, isobutene, and an
Al-iBu+ cation. For more details, see: Ihara, E.; Young, V. G., J r.;
J ordan, R. F. J . Am. Chem. Soc. 1998, 120, 8277.
(8) Dagorne, S.; J aouen, G. Unpublished results.
(5) Tramontini, M. Synthesis 1973, 103.
(6) Hagen, H.; Reinoso, S.; Albrecht, M.; Boersma, J .; Spek, A. L.;
van Koten, G. J . Organomet. Chem. 2000, 608, 27. See also ref 4.
(9) Blackwell, J . M.; Morrison, D. J .; Piers, W. E. Tetrahedron 2002,
58, 8247.

4708 Organometallics, Vol. 23, No. 20, 2004
Dagorne et al.
Sch em e 3
ture, in which the boron adopts a nearly ideal tetrahe-
dral geometry. The Al cation is an aniline-stabilized
four-coordinate Al species, in which the Al center adopts
a slightly distorted tetrahedral structure with a N-Al-O
chelate bite angle (98.0(2)° average). The six-membered-
ring chelate Al metallacycle is significantly twisted, with
the NMe2 group above the nearly planar OAlC3 back-
bone, as shown by the torsion angle values of |Al(1)-
O(1)-C(22)-C(16)| and |Al(1)-O(1)-C(16)-C(15)| (3.3
and 1.8°, respectively) versus those of |O(1)-Al(1)-
N(2)-C(15)| and |O(1)-C(22)-C(15)-N(2)| (53.5 and
35.8°, respectively). The Al-O and Al-N bond distances
of the chelated aminophenolate (1.735(6) and 1.99(1) Å,
respectively) are slightly shorter than those in the
related neutral Al dimethyl complex {6-(CH2NMe2)-2-
tBu-C6H2O}AlMe2 (1.758(1) and 2.036(5) Å, respec-
tively),4 as expected from the more electrophilic cationic
Al center in 2 vs that in a neutral derivative. The Al-
Naniline bond distance (1.99(1) Å) is normal for a Al-N
dative bond and is comparable with that in the di-
nuclear Al cation (tBuMe2SiO)2Al2Me3(NMe2Ph)+, which
also contains an aniline-stabilized four-coordinate Al
cationic center. Overall, the solid-state structure of 3
clearly shows the significant steric bulk around the Al
center, which, in part, justifies the relatively good
stability of this salt.
F igu r e 1. Molecular structure of the salt compound 3. The
H atoms (except BH) and the C6F5 groups of the HB(C6F5)3
-
anion are omitted for clarity. Some atoms of the Al cation
are not labeled for the same reason. Selected torsion angles
(deg): |Al(1)-O(1)-C(22)-C(16)| ) 3.3(1), |Al(1)-O(1)-
C(16)-C(15)| ) 1.8(1), |O(1)-Al(1)-N(2)-C(15)| ) 53.5(2),
|O(1)-C(22)-C(15)-N(2)| ) 35.8(1).
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
Deta ils for 3
formula
formula wt
cryst syst
space group
a (Å)
C41H48AlN2OHBC18F15
1124.79
triclinic
P1h (No. 2)
11.4160(10)
21.473(3)
24.265(3)
68.61(5)
77.52(5)
81.59(5)
5392.9(11)
4
1.385
b (Å)
c (Å)
B(C6F 5)3-Ca ta lyzed Hyd r oa lu m in a tion of P h 2CO
a n d P h CHO in th e P r esen ce of Com p lex 2. To probe
R (deg)
-
the reactivity of Al cation/HB(C6F5)3 salt compounds
â (deg)
as a Lewis acid and base system, carbonyl substrates
seemed appropriate, since they may act as Lewis bases
γ (deg)
V (Å3)
Z
toward the formed Al cation while being susceptible to
calcd density (g cm-3
)
-
hydride attack from HB(C6F5)3
.
µ(Mo KR) (mm-1
F(000)
)
0.133
2312
The diisobutyl Al complex 2 cleanly reacts with 1
equiv of B(C6F5)3 in the presence of 1 equiv of benzophe-
none and benzaldehyde (CD2Cl2, 15 min, room temper-
ature) to quantitatively afford the corresponding neutral
monoalkoxy monoisobutyl complexes {6-(CH2NMe2)-2-
CPh3-4-Me-C6H2O}Al(O-C(H)(R)Ph)(iBu) (4, R ) H; 5,
R ) Ph; Scheme 3), along with the evolution of isobutene,
cryst color
colorless
cryst size (mm)
temp (K)
0.13 × 0.10 × 0.08
173
min, max θ (deg)
data set (h; k; l)
total and unique
no. of data; R(int)
1.59, 27.85
-14 to +14; -25 to +28; 0-31
25 464, 25 463; 0.026
1
no. of obsd data (I > 2σ(I))
18 101
as observed by H NMR on a NMR-scale reaction. The
N
rfln, Nparam
25 463, 1431
0.0904, 0.2485, 0.895
0.001, 0.00
19F NMR data of the reaction mixture only contain
signals for 1 equiv of B(C6F5)3. When this reaction is
performed using 0.05 equiv of B(C6F5)3, complete con-
version to complexes 4 and 5 (CD2Cl2, room tempera-
ture, complete conversion to 4 after 3 h and to 5 after 5
R1, wR2, GOFa
max, av shift/error
min, max desd. dens (e Å-3
)
-0.469, 1.123
w ) 1/[σ2(Fo2) + (0.1749P)2 + 2.2774P]; P ) (Fo + 2Fc2)/3.
a
2
1
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for th e Ca tion in 3
h) is also observed by H NMR, along with isobutene
formation, without consumption of any B(C6F5)3, as
observed by 19F NMR.10 Compounds 4 and 5 were
generated on a preparative scale in CH2Cl2 and isolated
in a pure form after evaporation of the volatiles and a
subsequent pentane wash (see Experimental Section).
Al(1)-O(1)
Al(1)-N(1)
1.735(3)
1.991(4)
Al(1)-N(2)
Al(1)-C(1)
1.985(4)
1.946(4)
O(1)-Al(1)-N(2)
O(1)-Al(1)-C(1)
97.8 (1)
122.2(2)
O(1)-Al(1)-N(1)
N(2)-Al(1)-N(1)
104.9(1)
109.7(1)
(10) The NMR-scale generation of complexes 4 and 5 was carried
out using hexamethylbenzene (0.3 equiv vs 2) and Li[B(C6F5)4] (0.5
equiv vs 2) as internal standards for 1H and 19F NMR analysis,
respectively.
them exhibit structural features nearly identical with
one another. The HB(C6F5)3- anion, shown in Figure 1
without the C6F5 rings for clarity, has a normal struc-

A Four-Coordinate Aluminum Alkyl Cation
Organometallics, Vol. 23, No. 20, 2004 4709
Sch em e 4
Further studies will focus on the use of different
unsaturated organic substrates as well as on the use of
different Lewis basic anions that could be associated
with a Al cation.
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 an MBraun
Unilab glovebox. Toluene, pentane, and diethyl ether were
distilled from Na/benzophenone and stored over activated
molecular sieves (4 Å) in a glovebox prior to use. CH2Cl2 and
CD2Cl2 were distilled from CaH2 and stored over activated
molecular sieves (4 Å) in a glovebox prior to use. C6D6 was
degassed under a N2 flow and stored over activated molecular
sieves (4 Å) in a glovebox prior to use. The phenol derivative
2-CPh3-4-Me-C6H3OH was prepared by following a literature
procedure.12 B(C6F5)3 was purchased from Strem and extracted
with dry pentane prior to use. All other chemicals were
purchased from Aldrich and were used as received, except
NMe2Ph, which was stored over activated molecular sieves (4
Å) prior to use. NMR spectra were recorded on Bruker AC 200
and 400 MHz NMR spectrometers, in Teflon-valved J . Young
NMR tubes at ambient temperature, unless otherwise indi-
cated. 1H and 13C chemical shifts are reported vs SiMe4 and
were determined by reference to the residual 1H and 13C
solvent peaks. 11B and 19F chemical shifts are reported versus
BF3‚Et2O in CD2Cl2 and neat CFCl3, respectively. Elemental
analyses were performed by Mikroanalytisches Labor Pascher
(Remagen-Bandorf, Germany), except that for compound 1,
which was carried out by the microanalysis laboratory of the
Universite´ Pierre et Marie Curie (Paris, France).
The NMR data for both complexes agree with the
proposed structures and with overall C1-symmetry
structures, as expected. For example, the 1H NMR
spectrum of 4 exhibits an AB pattern (δ 4.37) consistent
with the benzyl protons of the Al-OCH2Ph moiety,
whereas that of 5 contains a singlet resonance (δ 5.36)
characteristic of the Al-OCHPh2 benzylic proton. Thus,
these results are consistent with B(C6F5)3 catalytically
converting the diisobutyl complex 2 to the monoalkoxy
complexes 4 and 5 in the presence of the appropriate
carbonyl derivative. In contrast, attempts to perform
these hydroalumination reactions using enolizable ke-
tones such as acetophenone yielded intractable mixtures
of products, in which neither a hydroalumination prod-
uct nor an enolized product was present.
6-(CH2NMe2)-2-CP h 3-4-Me-C6H2OH (1). The disubstituted
phenol 2-CPh3-4-Me-C6H2OH (1.00 g, 2.85 mmol), dimethyl-
amine (2.15 mL of a 40% aqueous solution, 17,1 mmol), and
formaldehyde (1.25 mL of a 37% aqueous solution, 17.1 mmol)
were added to a 50 mL round-bottom flask and dissolved in
15 mL of ethanol. The reaction mixture was refluxed for 40 h,
and upon cooling to room temperature, 6-(CH2NMe2)-2-CPh3-
4-Me-C6H3OH precipitated out of solution as a colorless solid.
The mixture was filtered and the colorless solid washed with
cold ethanol and pentane to afford, after drying under vacuum,
pure 6-(CH2NMe2)-2-CPh3-4-Me-C6H2OH (900 mg, 78% yield).
Anal. Calcd for C29H29NO: C, 85.47; H, 7.17; N, 3.44. Found:
C, 85.47; H, 7.28; N, 3.31. 1H NMR (400 MHz, CDCl3): δ 7.35-
7.13 (m, 15H, CPh3), 6.89 (d, 4J (HH) ) 2.0 Hz, 1H, Ph-O),
6.78 (d, 4J (HH) ) 2.0 Hz, 1H, Ph-O), 3.54 (s, 2H, PhCH2),
2.21 (s, 3H, MePh), 2.13 (s, 6H, NMe2). 13C NMR (100 MHz,
CDCl3): d 154.1 (O-Ph), 146.0 (Ph), 133.8 (Ph), 131.0 (Ph),
130.5 (Ph), 128.1 (Ph), 126.7 (Ph), 126.4 (Ph), 125.8 (Ph), 122.3
(Ph), 63.1 (CPh3), 62.6 (PhCH2), 43.8 (NMe2), 20.8 (MePh).
{6-(CH2NMe2)-2-CP h 3-4-Me-C6H2O}Al(iBu )2 (2). In a glove-
box, a pentane suspension (6 mL) of the aminophenol 1 (800.0
mg, 1.96 mmol) precooled to -40 °C was slowly added via a
pipet to a vial containing a pentane solution (5 mL) of Al(iBu)3
(389.3 mg, 1.96 mmol). After the addition, the reaction mixture
(colorless suspension) was warmed to room temperature with
the vial loosely capped to allow isobutane escape and was
stirred for 18 h. The resulting suspension was filtered through
a glass frit to afford, after drying in vacuo, the diisobutyl Al
complex 2 as a colorless solid (912 mg, 85% yield). Anal. Calcd
for C37H46AlNO: C, 81.13; H, 8.46. Found: C, 80.84; H, 8.32.
1H NMR (400 MHz, CD2Cl2): δ 7.19-7.10 (m, 15H, CPh3), 6.98
(br. s, 1H, O-Ph), 6.73 (br. s, 1H, O-Ph), 3.67 (s, 2H, Ph-
CH2), 2.27 (s, 6H, NMe2), 2.15 (s, 3H, Ph-Me), 1.48 (septet,
A possible mechanism for the hydroalumination reac-
tion described here is presented in Scheme 4. The initial
reaction of complex 2 with B(C6F5)3, in the presence of
the carbonyl derivative acting as a Lewis base L,
probably leads to the formation of a transient species
that can be related to 3: i.e., the four-coordinate cationic
Al-L adduct {6-(CH2NMe2)-2-CPh3-4-Me-C6H2O}Al(iBu)-
(L)+ as a HB(C6F5)3- salt, along with release of isobutene.
The HB(C6F5)3- borohydride may then transfer back an
hydride to the quite electrophilic CdO carbon, thus
producing complex 4 or 5 and regenerating B(C6F5)3.
The mechanism proposed here, i.e., a B(C6F5)3-mediated
hydride abstraction/transfer reaction, is reminiscent of
that found in the B(C6F5)3-catalyzed hydrosilation of
carbonyl functions, studied by Piers and al.11
Con clu sion s
The synthesis and structure of the salt compound [{6-
(CH2NMe2)-2-CPh3-4-Me-C6H2O}Al(iBu)(NMe2Ph)][HB-
(C6F5)3] show that low-coordinate cationic Al species of
the type {LX}Al(R)(L)+ may be stable in the presence
of a borohydride anion such as HB(C6F5)3-, provided
there is a significant steric protection of the Al metal
center and a Lewis base L not susceptible to hydride
-
attack is used. The increased reactivity of the HB(C6F5)3
vs the MeB(C6F5)3- or B(C6F5)4- salts is illustrated here
by a B(C6F5)3-catalyzed hydroalumination of Ph2CO and
PhCHO, in which a four-coordinate Al cation/HB-
-
(C6F5)3 salt is most probably involved. This new type
3
i
3
2H, J (HH) ) 6.7 Hz, CH Bu), 0.69 (d, 6H, J (HH) ) 6.4 Hz,
of reactivity for Al alkyl cations may be of interest for
other catalytic applications.
3
CH3 iBu), 0.64 (d, 6H, J (HH) ) 6.4 Hz, CH3 iBu), -0.39 (dd,
2
3
2H, J (HH) ) 14.1 Hz, J (HH) ) 7.3 Hz, CH2 iBu), -0.45 (dd,
2H, 2J (HH) ) 14.1 Hz, 3J (HH) ) 7.3 Hz, CH2 iBu). 13C{1H}
(11) (a) Parks, D. J .; Piers, W. E. J . Am. Chem. Soc. 1996, 118, 9440.
(b) Parks, D. J .; Blackwell, J . M.; Piers, W. E. J . Org. Chem 2000, 65,
3090.
(12) Schorigin, P. Chem. Ber. 1927, 60, 8.

4710 Organometallics, Vol. 23, No. 20, 2004
Dagorne et al.
NMR (100 MHz, CD2Cl2): δ 155.1 (O-Ph), 146.2 (Ph), 135.7
(Ph), 131.8 (Ph), 130.7 (Ph), 128.8 (Ph), 126.5 (Ph), 124.6 (Ph),
123.9 (Ph), 120.4 (Ph), 63.0 (CPh3), 62.5 (PhCH2), 44.3 (NMe2),
7.02 (br s, 1H, O-Ph), 6.72 (br. s, 1H, O-Ph), 4.37 (AB system,
2J (HH) ) 14.9 Hz, 2H, O-CH2Ph), 4.00 (d, 2J (HH) ) 13.8 Hz,
2
1H, Ph-CH2), 3.18 (d, J (HH) ) 13.9 Hz, 1H, Ph-CH2), 2.46
i
27.6(0) (CH3 iBu), 27.5(8) (CH3 iBu), 25.3 (CH Bu), 20.2 (br,
(s, 3H, Me-Ph or NMe), 2.16 (s, 3H, Me-Ph or NMe), 2.15 (s,
3
CH2 iBu), 20.1 (MePh).
3H, Me-Ph or NMe), 1.47 (septet, 2H, J (HH) ) 6.2 Hz, CH
iBu), 0.73 (d, 6H, 3J (HH) ) 6.5 Hz, CH3 iBu), 0.66 (d, 6H,
[{6-(CH2NMe2)-2-CP h 3-4-Me-C6H2O}Al(iBu )(NMe2P h )]-
[HB(C6F 5)3] (3). In a small Schlenk tube, the diisobutyl Al
complex 2 (120.0 mg, 0.219 mmol) and NMe2Ph (26.5 mg, 0.219
mmol) were dissolved in 1 mL of CH2Cl2 to yield a colorless
solution. B(C6F5)3 (112.1 mg, 0.219 mmol) was then added at
room temperature all at once, provoking gas formation
(isobutene) for a few seconds. The resulting pale yellow
solution was then stirred at room temperature for 1 h and was
evaporated. Trituration of the colorless foamy residue with cold
pentane caused the precipitation of a colorless solid which,
after filtration and further drying under vacuum, was revealed
to be the salt compound 3 (151 mg, 84% yield).13 1H NMR (200
MHz, CD2Cl2): δ 7.35-7.19 (m, 15H, CPh3), 7.01 (br s, 1H,
O-Ph), 6.66 (br. s, 1H, O-Ph), 4.15 (br. d, 1H, HB), 3.33 (d,
2
3J (HH) ) 6.5 Hz, CH3 iBu), -0.25 (dd, 2H, J (HH) ) 13.8 Hz,
3J (HH) ) 7.0 Hz, CH2 iBu), -0.42 (dd, 2H, J (HH) ) 13.9 Hz,
2
3J (HH) ) 7.1 Hz, CH2 iBu). 13C{1H} NMR (100 MHz, CD2Cl2):
δ 154.6 (O-Cipso Ph), 148.2 (Cipso Ph2CHO), 146.1 (Cipso CPh3),
144.9 (Cipso PhCH2-O), 135.8 (Cquat PhO), 131.6 (CH Ph), 130.7
(CH CPh3), 128.7 (CH Ph), 128.4 (CH Ph), 127.4 (CH Ph), 126.5
(CH CPh3), 125.5 (CH Ph), 125.4 (CH Ph), 124.6 (Cquat PhO),
120.5 (Cquat PhO), 64.0 (PhCH2N or PhCH2O), 63.0 (CPh3), 61.8
(PhCH2N or PhCH2O), 45.0 (NMe), 42.3 (NMe), 42.4 (NMe),
i
27.3 (CH3 iBu), 27.1 (CH3 iBu), 24.9 (CH Bu), 20.1 (MePh),
15.3 (br, CH2 iBu).
{6-(CH2NMe2)-2-CP h 3-4-Me-C6H2O}Al(O-CHP h 2)(iBu )
(5). The Al complex 5 was synthesized by following the same
experimental procedure as that for complex 4, using equimolar
amounts of compound 2 (80.0 mg, 0.146 mmol) and benzophe-
none (26.6 mg, 0.146 mmol) and 0.05 equiv of B(C6F5)3 (3.7
mg, 0.007 mmol), and was isolated in pure form as a colorless
solid (53 mg, 54% yield). Anal. Calcd for C46H48AlNO2: C,
2
2J (HH) ) 14.9 Hz, 1H, Ph-CH2), 2.78 (d, J (HH) ) 14.5 Hz,
1H, Ph-CH2), 2.77 (s, 3H, Me-Ph or NMe), 2.47 (s, 3H, Me-
Ph or NMe), 2.39 (s, 3H, Me-Ph or NMe), 2.24 (s, 3H, Me-Ph
or NMe), 2.14 (Me-Ph or NMe), 1.47 (septet, 2H, 3J (HH) )
i
3
6.5 Hz, CH Bu), 0.68 (d, 6H, J (HH) ) 5.8 Hz, CH3 iBu), 0.57
1
3
3
(d, 6H, J (HH) ) 6.5 Hz, CH3 iBu), 0.06 (d, 2H, J (HH) ) 7.3
81.99; H, 7.18. Found: C, 81.23; H, 7.13. H NMR (300 MHz,
Hz, CH2 iBu). 13C{1H} NMR (100 MHz, CD2Cl2):14 δ 151.3 (O-
CD2Cl2): δ 7.33-6.82 (m, 26H, aromatics), 6.63 (br s, 1H,
O-Ph), 5.36 (s, 1H, O-CHPh2), 3.56 (d, 2J (HH) ) 13.8 Hz,
1H, O-CH2Ph), 2.93 (d, 2J (HH) ) 13.8 Hz, 1H, Ph-CH2), 2.28
(s, 3H, Me-Ph or NMe), 2.16 (s, 3H, Me-Ph or NMe), 2.07 (s,
2
Cipso Ph), 147.9 (d, J (CF) ) 235 Hz, C6F5), 145.8 (Ph), 143.7
(Cipso PhNMe2), 136.5 (Ph), 136.3 (d, J (CF) ) 235 Hz, C6F5),
2
133.6 (Ph), 130.6 (Ph), 130.3 (Ph), 129.3 (Ph), 128.8 (Ph), 127.3
(Ph), 126.6 (Ph), 125.8 (Ph), 120.0 (Ph), 118.8 (Ph), 62.8 (CPh3
and PhCH2), 46.5 (NMe), 46.3 (NMe), 45.2 (NMe), 45.1 (NMe),
3
3H, Me-Ph or NMe), 1.38 (septet, 2H, J (HH) ) 7.2 Hz, CH
iBu), 0.67 (d, 6H, 3J (HH) ) 6.4 Hz, CH3 iBu), 0.54 (d, 6H,
3J (HH) ) 6.6 Hz, CH3 iBu), -0.36 (dd, 1H, J (HH) ) 14.0 Hz,
2
i
27.1 (CH3 iBu), 26.0 (CH3 iBu), 23.4 (CH Bu), 20.0 (MePh),
3J (HH) ) 6.8 Hz, CH2 iBu), -0.52 (dd, 1H, J (HH) ) 14.0 Hz,
2
16.1 (br, CH2 iBu). 11B NMR (128.4 MHz, C6D6): δ -25.7 (d,
2J (BH) ) 54 Hz). 19F NMR (376.5 MHz, C6D6): δ 133.2 (m,
2F, C6F5), -164.2 (t, 1F, C6F5), -167.3 (m, 2F, C6F5).
3J (HH) ) 7.9 Hz, CH2 iBu). 13C{1H} NMR (100 MHz, CD2Cl2):
δ 154.5 (O-Cipso Ph), 148.2 (Cipso Ph2CHO), 148.0 (Cipso Ph2-
CHO), 146.1 (Cipso CPh3), 135.7 (Cquat PhO), 131.5 (CH Ph),
130.7 (CH Ph), 128.6 (CH Ph), 127.6 (CH Ph), 127.3 (CH Ph),
126.6 (CH Ph), 126.2 (CH Ph), 125.8 (CH-Ph), 125.7 (CH Ph),
125.6 (CH Ph), 124.8 (CH Ph), 124.6 (Cquat PhO), 120.7 (Cquat
PhO), 75.2 (CHPh2), 63.0 (CPh3), 61.4 (PhCH2), 46.5 (NMe),
44.8 (NMe), 42.4 (NMe), 27.5 (CH3 iBu), 27.0 (CH3 iBu), 24.9
{6-(CH2NMe2)-2-CP h 3-4-Me-C6H2O}Al(O-CH2P h )(iBu )
(4). In a glovebox, a stoichiometric amount of the diisobutyl
Al complex 2 (33.5 mg, 0.061 mmol) and benzaldehyde (5.5
µL, 0.061 mmol) were added to CH2Cl2 (3 mL) in a sample
vial to yield a colorless solution. With vigorous stirring, 0.05
equiv of B(C6F5)3 (1.6 mg, 0.003 mmol) was then added to the
solution. The mixture was stirred for 5 h at room temperature,
after which it was evaporated to dryness under vacuum to
yield a colorless solid. This solid was treated with pentane (5
mL) to wash off B(C6F5)3 to afford, after drying, complex 4 in
i
(CH Bu), 20.1 (MePh), 15.8 (br, CH2 iBu).
Ack n ow led gm en t. We thank the Centre National
de la Recherche Scientifique (CNRS) for financial sup-
port. I.J . thanks the European Community for a Marie
Curie Fellowship (HMPT-CT-2000-00186).
a pure form (24.4 mg, 67% yield). Anal. Calcd for C40H44
AlNO2: C, 80.37; H, 7.42. Found: C, 80.51; H, 7.21. H NMR
-
1
(400 MHz, CD2Cl2): δ 7.28-7.11 (m, 20H, CPh3 and OCH2Ph),
Su p p or tin g In for m a tion Ava ila ble: A CIF file giving
crystallographic data for 3. This material is available free of
charge via the Internet at http://pubs.acs.org.
(13) We were unable to obtain a satisfactory elemental analysis for
this salt compound.
(14) One 13C NMR C6F5 resonance is not listed. It is most likely
obscured by the other resonances in the aromatic region.
OM0400806