Synthesis, Characterization, and Catalytic Activity of a Series of Aluminium-Amidate Complexes

Article abstract of DOI:10.1071/CH14514

The aluminium complexes {[κ²-N,O-(t-BuNCOPh)]AlMe₂}₂ (2), [κ²-N,O-(t-BuNCOPh)]₂AlMe (3), and [κ²-N,O-(t-BuNCOPh)]₃Al (4) were prepared through the protonolysis reaction between trimethylaluminium and one, two, or three equivalents, respectively, of N-tert-butylbenzamide. Complex 2 was also prepared via a salt metathesis reaction between K(t-BuNCOPh) and dimethylaluminium chloride. Complexes 2-4 were characterized using ¹H and ¹³C NMR spectroscopy. Single-crystal X-ray diffraction analysis of the complexes corroborated ligand:metal stoichiometries and revealed that all the amidate ligands coordinate to the aluminium ion in a κ² fashion. The Al-amidate complexes 2-4 were viable catalyst precursors for the Meerwein-Ponndorf-Verley-Oppenauer reduction-oxidation manifold, successfully interconverting several classes of carbonyl and alcohol substrates.

Full text of DOI:10.1071/CH14514

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2015, 68, 357–365
Full Paper
http://dx.doi.org/10.1071/CH14514
Synthesis, Characterization, and Catalytic Activity
of a Series of Aluminium–Amidate Complexes*
A
A
B
Kevin P. Yeagle, Darryl Hester, Nicholas A. Piro,
B
B
A C
,
William G. Dougherty, W. Scott Kassel, and Christopher R. Graves
A
Department of Chemistry & Biochemistry, Albright College, 13th & Bern St, Reading,
PA 19612, USA.
B
Department of Chemistry, Villanova University, 800 Lancaster Ave, Villanova,
PA 19085, USA.
C
Corresponding author. Email: cgraves@alb.edu
The aluminium complexes {[k2-N,O-(t-BuNCOPh)]AlMe2}2 (2), [k2-N,O-(t-BuNCOPh)]2AlMe (3), and [k2-N,O-(t-
BuNCOPh)]3Al (4) were prepared through the protonolysis reaction between trimethylaluminium and one, two, or three
equivalents, respectively, of N-tert-butylbenzamide. Complex 2 was also prepared via a salt metathesis reaction between
K(t-BuNCOPh) and dimethylaluminium chloride. Complexes 2–4 were characterized using 1H and 13C NMR spectro-
scopy. Single-crystal X-ray diffraction analysis of the complexes corroborated ligand : metal stoichiometries and revealed
that all the amidate ligands coordinate to the aluminium ion in a k2 fashion. The Al–amidate complexes 2–4 were viable
catalyst precursors for the Meerwein–Ponndorf–Verley–Oppenauer reduction–oxidation manifold, successfully inter-
converting several classes of carbonyl and alcohol substrates.
Manuscript received: 23 August 2014.
Manuscript accepted: 29 September 2014.
Published online: 20 January 2015.
complexes was originally reported by Lappert and Horder[29]
and Wade et al.,[30] and was expanded on by the Lin group to
include various substituents in the amidate ligand.[31,32] The
MacBeth group has more recently reported the preparation of a
tris(amidate)aluminium complex implementing a tripodal
ligand.[33] Despite these chemistries, the application of alumin-
ium amidate complexes as catalysts has not been investigated.
The Stahl group has reported the Al2(NMe2)6-catalyzed transa-
midation of carboxamides,[34] and has shown that aluminium
amidate functional groups are important moieties under the
reaction conditions,[35,36] suggesting that Al–amidate com-
plexes may be viable catalytic species. Herein, we report the
synthesis of a series of Al–amidate complexes that vary in
ligand : metal stoichiometry. We also demonstrate the viability
of the complexes to serve as catalytic precursors for the
Meerwein–Ponndorf–Verley (MPV) reduction of carbonyls
and the related Oppenauer oxidation of alcohols.
Introduction
The development of new catalytic systems for small molecule
transformations that employ non-precious metal complexes is
an important challenge.[1] Aluminium is one of the most abun-
dant elements in the earth’s crust[2] and is an attractive choice for
such catalyst development because it is readily available,
inexpensive, and non-toxic.[3,4] The application of aluminium
complexes as catalysts for high impact reactions is therefore
very desirable. Indeed, aluminium complexes represent a pri-
vileged class of compounds in Lewis-acid catalysis[5] and
development of Al-based catalysts systems for processes such as
the ring-opening polymerization of cyclic esters,[6] the hydro-
functionalization reaction,[7–10] epoxide coupling with carbon
dioxide,[6b,11–14] and transfer hydrogenation[15–17] continues to
be an active area of research.
We have been investigating the coordination chemistry of
amidate ligands to aluminium(III) ions with the goal of develop-
ing a new class of Al-based catalysts. Amidates represent a
family of highly variable N,O-chelating, monoanionic ligands.
The amide ligand precursors are easily prepared via the nucleo-
philic acyl substitution of an acid chloride with a primary
amine[18] in a variety of substitution patterns such that the steric
and electronic properties of the amidate ligand can be readily
modulated. Amidate ligand complexes have been shown to
support a variety of transition[19–23] and rare-earth metals[24–28]
that has resulted in a family of useful reagents for metal-
mediated transformations. The synthesis of Al–amidate
Results and Discussion
Synthesis and Characterization of Aluminium–Amidate
Complexes 2–4
The 1 : 1 reaction between trimethylaluminium and various
simple amides has been previously reported by the Lin
group[31,32] who showed that the bonding mode of the ligand
and solid-state structure of the Al–amidate complexes were
dependent on the substitution pattern of the amide precursor.
*Dedicated to the memory of Professor Richard F. Langler, an inspirational teacher and mentor.
Journal compilation Ó CSIRO 2015
www.publish.csiro.au/journals/ajc

358
K. P. Yeagle et al.
Ph
Ph
N
t-Bu
t-Bu
H3C
H3C
N
Al
O
CH3
CH3
O
H3C
H3C
CH3
CH3
1 equiv. 1-H
3 equiv. 1-H
t-Bu
N
O
Al
0.5 equiv.
Al
Al
Ph
CH3
Al
t-Bu
N
hexane, 68ЊC
ϪCH4
hexane, 100ЊC
Ϫ3CH4
O
N
t-Bu
O
CH3
2 equiv. 1-H
hexane
Ph
Ϫ2CH4
Ph
[κ2-N,O-(t-BuNCOPh)]3Al
54 %
{[κ2-N,O-(t-BuNCOPh)]AlMe2}2
74 %
68ЊC
2
4
O
CH3
t-Bu
N
O
N
t-Bu
Al
Ph
N
Ph
Ph
H
O
1-H
t-Bu
[κ2-N,O-(t-BuNCOPh)]2AlMe
79 %
3
Scheme 1. Protonolysis routes to Al–amidate complexes with varying ligand/metal ratios.
O
t-Bu
{[κ2-N,O-(t-BuNCOPh)]AlMe2}2
50 %
ϩ
AlCl(CH3)2
Ph
N
Toluene, room temperature
2
K
ϪKCl
1-K
Scheme 2. Salt metathesis route to Al–amidate complex 2.
We were interested in examining how changing the number of
equivalents of amide ligand precursor in the protonation
chemistry with trimethylaluminium manifests in different alu-
minium coordination compounds. We focussed on the N-tert-
butylbenzamide ligand (t-BuNHCOPh, 1-H). The protonation
reaction between 1-H and Al2(CH3)6 in a 1 : 0.5, 2 : 0.5, and
3 : 0.5 ratio in refluxing hexane was utilized to prepare the alu-
minium complexes {[k2-N,O-(t-BuNCOPh)]AlMe2}2 (2), [k2-
N,O-(t-BuNCOPh)]2AlMe (3), and [k2-N,O-(t-BuNCOPh)]3Al
(4), respectively (Scheme 1).
A 1 : 1 reaction between amide 1-H and trimethylaluminium
gives 2 as a dimer in the solid state in good isolated yield. The
preparation and crystallization of compound 2 has previously
been reported by Lin et al.[31] Increasing the number of equiva-
lents of amide to two results in protonation of two Al–CH3
moieties to give compound 3 under analogous reaction condi-
tions. Compound 3 was isolated in 79 % yield after crystalliza-
tion from toluene at ꢀ258C. Addition of a third equivalent of
amide proved more difficult, with the third protonation reaction
occurring only at an increased temperature (1008C). Compound
4 was isolated in 54 % yield after crystallization from ether at
ꢀ258C. All three complexes 2–4 were prepared on a gram scale.
The 1H NMR spectra of the crude reaction products for all three
compounds indicate that the protonation reactions are occurring
essentially quantitatively with no apparent side products, and
the isolated yields are a reflection of the solubility of the
complexes under the crystallization conditions. Complexes 2–4
are stable in the solid state when stored under nitrogen at ꢀ258C
with no apparent decomposition even upon storage for longer
than one year.
toluene at room temperature gave 2 in 50 % yield after workup
and crystallization from toluene at ꢀ258C.[37]
Complexes 2–4 are readily characterized by NMR spectro-
scopy analyses (see Supplementary Material for copies of the
spectra). The 1H NMR spectra of all the complexes lack the N–H
resonance of the 1-H ligand precursor at ,6 ppm, indicating
reaction between the amide hydrogen and the basic Al–CH3
moieties. The integration ratio between the tert-butyl groups in
the ligand backbone and the unreacted Al–CH3 groups helps
support the formulation of the proposed compounds. In com-
pound 2, the tert-butyl-to-Al–CH3 ratio is 3 : 2, supporting the
addition of one ligand per aluminium ion. In complex 3, a ratio
6 : 1 was observed, indicating that two ligands have reacted.
Finally, complex 4 lacks a resonance below 0 ppm, supporting
that all Al–CH3 groups underwent protonolysis. The 13C NMR
spectra of 2–4 also support these assignments: resonances for the
Al–CH3 group are observed at ꢀ8.5 and ꢀ11.7 ppm for com-
plexes 2 and 3, respectively, whereas the 13C NMR spectrum of
compound 4 lacks a signal in this range.
The structures of complexes 3 and 4 were corroborated by
X-ray crystallography and crystallographic data are provided
in Table 1. The structure of complex 2 has previously been
reported.[31] Single crystals of 3 were grown from a concentra-
ted hexane solution at room temperature. The molecule crystal-
lizes with two independent half-molecules in the asymmetric
unit, where each lies on a crystallographic 2-fold rotation axis,
thus generating the full molecules. A representation of one
independent molecule is shown in Fig. 1. The two molecules
are very similar and the bond distances and angles are effec-
tively identical, so the discussion will only focus on the data for
the molecule consisting of Al(1). The geometry of the alumin-
ium ion in 3 is best described as square pyramidal based on a t5
of 0.16,[38] with the two nitrogen atoms and two oxygen atoms
We were also able to utilize a salt metathesis route to form
complex 2 (Scheme 2). Reaction of the potassium salt of the
amidate ligand (1-K) with dimethylaluminium chloride in

Aluminium Amidates
359
Table 1. Crystallographic data for complexes 3 and 4
3
4(Et2O)0.5
N(1)
Formula
Formula weight [g molꢀ1
˚
a [A]
˚
b [A]
˚
c [A]
C23H31AlN2O2
394.48
18.6325(17)
6.3247(6)
19.8089(19)
90
C35H47AlN3O3.50
592.74
30.729(3)
10.8742(11)
21.609(2)
90
]
O(1)
O(3)
O(2)
AI(1)
a [8]
b [8]
g [8]
N(2)
108.812(2)
90
111.098(4)
90
N(3)
3
˚
V [A ]
2209.7(4)
4
6736.5(11)
8
Z
Space group
P2/n
C2/c
T [K]
˚
Dcalc [Mg mꢀ3
m [mmꢀ1
100(2)
0.71073
1.186
120(2)
0.71073
1.169
l [A]
]
Fig. 2. Solid-state structure of [k2-N,O-(t-BuNCOPh)]3Al (4) with ellip-
soids at the 30 % probability level. H atoms have been omitted for clarity.
Respective selected bond distances and angles: Al(1)–N(1), 2.0248(13);
Al(1)–O(1), 1.9037(11); Al(1)–N(2), 2.0131(13); Al(1)–O(2), 1.9056(12);
]
0.112
0.099
R1 (I . 2s(I))
wR2 (all data)
0.0416
0.1125
1.07
0.0503
0.1219
1.01
T
max/Tmin
˚
and Al(1)–N(3), 2.0130(14); and Al(1)–O(3), 1.8939(12) A; and N(1)–
GOF (F2)
1.021
1.009
Al(1)–O(1), 67.25(5); N(2)–Al(1)–O(2), 67.21(5); N(3)–Al(1)–O(3),
67.59(5); N(1)–Al(1)–N(2), 105.46(5); N(1)–Al(1)–N(3), 154.16(6);
N(2)–Al(1)–N(3), 97.96(5); O(1)–Al(1)–O(2), 94.06(5); O(1)–Al(1)–O(3),
98.08(5); O(2)–Al(1)–O(3), 165.72(5); N(1)–Al(1)–O(2), 94.94(5); N(1)–
Al(1)–O(3), 96.67(5); N(2)–Al(1)–O(1), 159.81(6); N(2)–Al(1)–O(3);
101.54(5); N(3)–Al(1)–O(1); 93.87(5); and N(3)–Al(1)–O(2); 104.22(5)8.
C(1)
AI(1)
N(1)
O(1)
O(1A)
˚
those observed for 2 (Al–N ¼ 2.157(2) A and Al–O ¼ 1.9096
[31]
˚
(17) A).
In all three complexes 2–4, the N–Al–O angle
N(1A)
formed between the aluminium ion and a given amidate ligand
is in the range of 63.14(7)–67.90(3)8 and is the smallest X–Al–Y
angle (where X and Y are N or O) in the complexes.
Fig. 1. Solid-state structure of [k2-N,O-(t-BuNCOPh)]2AlMe (3) with
ellipsoids at the 30 % probability level. H atoms have been omitted for
clarity. Respective selected bond distances and angles: Al(1)–C(1), 1.9689
Catalytic Activity of the Al–Amidate Complexes
˚
(17); Al(1)–N(1), 2.0032(9); and Al(1)–O(1), 1.8886(8) A; and N(1)–Al(1)–
We were interested in investigating the ability of the Al–amidate
complexes 2–4 to serve as catalysts. As a starting point, we
focussed on the MPV reduction of carbonyls,[15,44–46] a benign
and chemoselective reduction protocol for the conversion of
carbonyls into their alcohol equivalents utilizing alcohols, most
often 2-propanol, as the hydrogen source.[47] Given the impor-
tance of carbonyl reduction in organic chemistry, the develop-
ment of new MPV protocols applicable for a wide range of
carbonyls is attractive. The MPV reduction has been shown to
proceed through internal hydrogen transfer between an alu-
minium alkoxide and a coordinated carbonyl.[48,49] We envi-
sioned that the Al–CH3 groups in complexes 2 and 3 would be
readily protonated by 2-propanol to generate catalytically viable
Al–alkoxide species under MPV reaction conditions. Indeed, at
10 mol-% in total aluminium,[50] complexes 2 and 3 are both
pre-catalysts for the reduction of various carbonyl compounds at
508C using 8 equivalents of 2-propanol as the reductant[51]
(Table 2). The catalyst system was applicable to the reduction of
ketones to secondary alcohols. Acetophenone was cleanly
reduced to sec-phenethyl alcohol in 60 % and 71 % conversions
using 2 and 3, respectively (entries 1 and 2). The related 2-
napthanone was reduced to 1-(2-naphthyl)ethanol, although the
conversions were lower for both aluminium compounds (entries
4 and 5). The reduction of both alkenyl and alkynyl substrates
proved more difficult. 4-Phenyl-3-buten-2-one was reduced to
the alcohol 4-phenyl-3-buten-2-ol in 29 % (for 2, entry 6) and
41 % (for 3, entry 7) conversion and 4-phenyl-3-butyne-2-one
was only reduced in 8 % conversion using either pre-catalyst
O(1), 67.90(3); N(1)–Al(1)–C(1), 107.85(3); O(1)–Al(1)–C(1), 112.72(3);
N(1)–Al(1)–N(1A), 144.31(6); O(1)–Al(1)–O(1A), 134.56(6); and N(1)–
Al(1)–O(1A), 98.02(4)8.
coordinating in mutually trans relationships in the basal plane.
˚
At 1.9689(17) A, the Al–C distance in 3 is longer than those in 2
(1.955(3) and 1.947(3) A),[31] but both are in the range of other
˚
reported Al–CH3 distances. For example, the SalentBuAlCH3
complex prepared by Atwood et al. has an Al–C distance of
1.963(17) A,[39] whereas the {(BINOLate)AlCH3(THF)}2 com-
˚
plex (BINOLate ¼ 1,10-bi-2-naphtholate) reported by Nguyen
[40]
˚
et al. has an Al–C distance of 1.961(2) A.
The (formamidi-
nate)Al(CH3)2 complex prepared by Deacon, Junk, Anwander
[41]
˚
et al. has Al–C bond distances of 1.950(3) A,
whereas the
(b-ketiminate)Al(CH3)2 complexes reported by Peng et al. have
˚
Al–C distances of 1.964(4) and 1.984(3) A depending on the
specific ketiminate ligand employed.[42] Finally, the (NON)
AlCH3 pincer complex (NON2ꢀ ¼ {CyNC6H4}2O2ꢀ) prepared
[43]
˚
by Dagorne et al. has an Al–C distance of 1.947(3) A.
Single crystals of 4 were grown from diethyl ether at ꢀ258C
(Fig. 2). The aluminium ion is six-coordinated with a distorted
octahedral geometry. The three nitrogen atoms and three
oxygen atoms coordinate in mutually meridional arrangements.
˚
The average Al–N bond distance in 4 is 2.017 A and the average
˚
Al–O distance is 1.901 A, which are slightly longer than the
corresponding distances in complex 3 (Al(1)–N(1) ¼ 2.0032
˚
˚
(9) A and Al(1)–O(1) ¼ 1.8886(8) A) although shorter than

360
K. P. Yeagle et al.
Table 2. MPV reduction of carbonyls catalyzed by 2 and 3
8 equiv. 2-Propanol, 10 mol-% Al
Alcohol (1)
Carbonyl
Toluene, 50ЊC, 24 h
Entry
CarbonylA
CatalystB
Alcohol
Conversion [%]C
1
2
3
2
3
4
60
71
69
O
OH
CH3
OH
Ph
CH3
Ph
O
4
5
2
3
44
44
CH3
CH3
O
O
OH
6
7
2
3
29
41
CH3
Ph
Ph
CH3
Ph
Ph
OH
8
9
2
3
8
8
CH3
CH3
O
OH
10
11
2
3
99
97
O
OH
12
13
2
3
.99
99
Ph
H
Ph
O
OH
14
15
2
3
.99
93
H
AReactions were run on a 1-mmol scale in carbonyl.
BAmounts of catalyst used were 0.05 mmol of 2, 0.1 mmol of 3, or 0.1 mmol of 4.
CDetermined by GC by relative integrations of the starting material to the product.
(entries 8 and 9). The aliphatic ketone cyclohexanone was
readily reduced to cyclohexanol in excellent conversion (entries
10 and 11). Finally, both benzylic (entries 12 and 13) and ali-
phatic (entries 14 and 15) aldehydes could also be reduced under
our reaction conditions to give the corresponding primary
alcohols in excellent conversions. The aluminium tris(amidate)
complex 4 does not have an Al–CH3 functional group and hence
should lack the ability to form an Al–alkoxide with 2-propanol,
rendering the complex catalytically inactive. However, aceto-
phenone is readily reduced using 4 as the pre-catalyst (entry 3),
suggesting that the solid-state structure of 4 may not be reflec-
tive of the solution dynamics of the complex under the reaction
conditions. Stahl demonstrated that a tris(amidate) aluminium
species was the resting state of the catalyst in his transamidation
chemistry and that catalytically viable species were generated
through facile ligand substitution reactions under the reaction
conditions.[35,36] We suggest similar fluxionality here, where
one of the amidate ligands is protonated off of the aluminium
ion by an equivalent of 2-propoanol to generate a catalytically
viable Al–alkoxide under the reaction conditions.[52]
We also investigated the competency of the Al–amidate
complexes to serve as catalysts for the Oppenauer oxidation[53]
of alcohols to the corresponding carbonyl compounds.[16] The
Oppenauer oxidation is the reverse of the MPV reduction, again
proceeding through an Al–alkoxide where a sacrificial carbonyl
serves as the oxidant. The Al–amidate complexes are viable
catalysts for the Oppenauer oxidation, and proved to be much
more active under the oxidation conditions relative to the
reduction protocol developed above. At 5 mol-% total alumi-
nium[54] and using pivaldehyde as the oxidant, various alcohols
were oxidized in good-to-excellent conversions at room tem-
perature (Table 3). Using 1.5 equivalents of pivaldehyde, sec-
phenethyl alcohol was oxidized to acetophenone in good
conversions of 81 % and 91 % using 2 or 3 as the catalytic

Aluminium Amidates
361
Table 3. Oppenauer oxidation of alcohols catalyzed by 2–4
O
x equiv.
, 5 mol-% Al
H
t-Bu
Carbonyl
(2)
Alcohol
Toluene, 3 h
Entry
AlcoholA
CatalystB
x [equiv.]
Carbonyl
Conversion [%]C
1
2
3
4
5
2
2
3
3
4
1.5
3
81
98
91
97
99
OH
O
1.5
3
Ph
CH3
CH3
Ph
1.5
OH
O
6
7
8
2
3
4
3.0
9 (16D)
12 (20D)
9 (10D)
CH3
CH3
OH
OH
O
O
9
2
3
4
3.0
3.0
46 (99D)
96
10
Ph
Ph
CH3
11
95
CH3
Ph
12
13
14
15
2
2E
29 (45D)
64D
5 (7D)
30 (39D)
CH3
3
CH3
4
Ph
OH
O
16
17
18
19
2
3
3
4
1.5
1.5
3.0
1.5
48 (75D)
72
88
40 (68D)
OH
O
20
21
22
2
3
4
1.5
3.0
74
80
90
H
Ph
Ph
OH
O
23
24
25
26
2
2E
,1
20D
,1
,1
H
3
4
AReactions were run on a 1-mmol scale in alcohol.
BAmounts of catalyst used were 0.025 mmol of 2, 0.05 mmol of 3, or 0.05 mmol of 4.
CDetermined by GC by relative integrations of the starting material to the product.
DObtained after 24 h.
EAmount of catalyst 2 employed was 0.05 mmol.
precursor, respectively, in 3 h (entries 1 and 3). Increasing the
amount of oxidant to 3 equivalents increased the conversions to
98 % (for 2, entry 2) and 97 % (for 3, entry 4) under otherwise
identical conditions and time. As with the reduction protocol,
despite lacking an Al–CH3 functional group, complex 4 was a
viable option as a catalytic precursor for this oxidation protocol.
In fact, complex 4 gave the highest conversion of alcohol into
carbonyl in the sec-phenethyl oxidation (entry 5). The Al–
amidate complexes did not perform as well in the oxidation of
1-(2-naphthyl)ethanol, giving only low conversions into ketone
even with 3 equivalents of pivaldehyde and after longer reaction
times (entries 6–8). The alkenyl alcohol 4-phenyl-3-buten-2-ol
was readily oxidized using all three Al–amidate complexes as
catalysts, giving 4-phenyl-3-buten-2-one in good conversions.
Oxidations using 3 and 4 as catalyst precursors were complete in
3 h, giving the ketone product in .95 % conversion (entries 10
and 11). The corresponding oxidation with 2 was more sluggish
(entry 9), proceeding in only 46 % conversion in 3 h, although
increasing the reaction time to 24 h did provide the ketone
product in comparable conversions to those obtained using the
other pre-catalysts. The alkynyl alcohol 4-phenyl-3-butyne-2-ol
was oxidized to the ketone 4-phenyl-3-butyne-2-one, although
the conversions were low. For Al–amidate complexes 2 and 4,
conversions of 29 % and 30 % (entries 12 and 15), respectively,

362
K. P. Yeagle et al.
were obtained which increased slightly with longer reaction
times. Increasing the amount of complex 2 to 10 mol-% total
aluminium did provide reasonable conversion (64 %) into the
ketone product after 24 h (entry 13). The oxidation using 3 as
the catalyst precursor was unsuccessful, giving product in less
than 10 % conversion even with longer reaction times (entry 14).
Cyclohexanol was readily oxidized to cyclohexanone; using
either 2 or 4 as the catalyst precursor gave the ketone product in
,50 % conversion after 3 h using 1.5 equivalents of pivalde-
hyde, although higher conversions were obtained after 24 h
(entries 16 and 19). Using Al–amidate complex 3 as the pre-
catalyst gave cyclohexanone in 72 % conversion under identical
reaction conditions (entry 17), and increasing the amount of
pivaldehyde to 3 equivalents increased the conversion to 88 %
after 3 h (entry 18). Our catalyst system was also applicable to
the oxidation of benzylic primary alcohols. Benzaldehyde was
formed in 74–90 % conversion from the oxidation of benzyl
alcohol using 1.5 equivalents of pivaldehyde and 2–4 as the
catalyst precursor (entries 20–22). The Al–amidate complexes
were unsuccessful catalyst precursors for the oxidation of
primary aliphatic alcohols (entries 23–26), giving only trace
amounts of product for all three Al–amidate complexes. Increas-
ing the amount of catalyst precursor to 10 mol-% total alumi-
nium did increase the conversion into the product to 20 %.
Both the reduction and oxidation procedures could be scaled
and reaction products isolated; acetophenone could be reduced
on a 5-mmol scale with 2-propanol using 2 as the catalyst to give
sec-phenethyl alcohol in 52 % yield after purification (Eqn 3).
Similarly, oxidation of 5 mmol of sec-phenethyl alcohol to
acetophenone could be carried out with 2 as the catalyst in
80 % isolated yield (Eqn 4).
of the Al–amidate complexes in other types of catalytic
processes.
Experimental
Physical Measurements
1H and 13C NMR spectra were recorded at ambient temperature
in deuterated solvents using a Varian 400 MHz spectrometer
(399.78 MHz for 1H, 100.52 MHz for 13C). Chemical shifts
were referenced to residual solvent; s ¼ singlet, d ¼ doublet,
t ¼ triplet, q ¼ quartet, m ¼ multiplet, b ¼ broad. Melting points
were determined with a DigiMelt SRS capillary melting point
apparatus using capillary tubes plugged with silicon grease
under nitrogen; values are uncorrected. Elemental analyses were
performed either at the University of California, Berkeley
Microanalytical Facility, on a Perkin–Elmer Series II 2400
CHNS analyzer (for 3 and 4) or at Complete Analysis Labora-
tories on a CHN analyzer by Thermo Electron (for 1-K). Gas
chromatography (GC) analyses were carried out on a computer-
interfaced Varian 3800 instrument equipped with an flame
ionization detector (FID). A 30-m Zebron capillary column with
a 0.25-mm inner diameter and a 0.25-mm film thickness was
used. The flow rate was 1.2 mL minꢀ1
.
X-Ray Structure Determination
X-Ray diffraction data were collected on a Bru¨ker-AXS Kappa
˚
APEX II CCD diffractometer with 0.71073 A MoKa radiation.
Cell parameters were retrieved using APEX II software[55] and
further refined on all observed reflections during integration
using SAINT1.[56] Each dataset was treated with SADABS[57]
absorption corrections based on redundant multi-scan data. The
structures were solved by direct methods and refined by least-
squares method on F2 using the SHELXTL program package.[58]
All non-hydrogen atoms were refined with anisotropic dis-
placement parameters and all hydrogen atoms were treated as
idealized contributions. Details regarding specific solution
refinement for each compound are provided in the following
paragraphs.
OH
O
5 mmol
OH
OH
8 equiv.
, 5 mol-% 2
(3)
(4)
Toluene, 50ЊC, 24 h
Ph
CH3
Ph
CH3
52 %
X-Ray structural analysis for 3: A single colourless plate
(0.41 ꢁ 0.13 ꢁ 0.07 mm3) was mounted using NVH immersion
oil onto a nylon fibre and cooled to the data collection tempera-
ture of 100(2) K. The systematic absences in the data were
consistent with the centrosymmetric, monoclinic space group
P2/n. The asymmetric unit contains two independent half
molecules of 12AlMe for Z0 ¼ 1 and Z ¼ 4.
OH
O
3 equiv.
, 2.5 mol-% 2
H
tBu
Toluene, 3 h
Ph
CH3
Ph
CH3
80 %
5 mmol
X-Ray structural analysis for 4: A single colourless block
(0.20 ꢁ 0.14 ꢁ 0.06 mm3) was mounted using NVH immersion
oil onto a nylon fibre and cooled to the data collection tempera-
ture of 120(2) K. The systematic absences in the data were
consistent with the centrosymmetric, monoclinic space group
C2/c. The asymmetric unit contains one molecule of 13Al and a
half molecule of diethyl ether solvent located on the inversion
centre.
Conclusions
We have demonstrated that aluminium–amidate complexes
with varying ligand-to-metal ratios can be prepared systemati-
cally through the protonation reaction between trimethylalu-
minium and amide ligand precursor. We have also demonstrated
that salt metathesis is a viable route to install amidate ligands at
the aluminium ion. The structural and spectroscopic charac-
terization of the complexes support the ligand-to-metal stoi-
chiometry observed. Our investigations into the catalytic
chemistry of Al–amidate complexes have shown that 2–4 are
all viable catalyst precursors for the Meerwein–Ponndorf–
Verley–Oppenauer manifold for a variety of carbonyl and
alcohol substrates. We are currently investigating how changes
in the amidate ligand backbone affect the Al–amidate coordi-
nation complexes and how these changes manifest in differ-
ences in catalytic activity. We are also expanding the use
Preparation of Compounds
All reactions and manipulations were performed under an inert
atmosphere (N2) using standard Schlenk techniques or in a
Vacuum Atmospheres, Inc. Nexus II drybox equipped with a
molecular sieves 13X/Q5 Cu-0226S catalyst purifier system.
Glassware was dried overnight at 1508C before use. CDCl3 was
purchased from Sigma Aldrich and was degassed and stored
˚
over 4 A molecular sieves before use. [D8]THF was purchased
from Sigma Aldrich and dried over sodium metal before use.

Aluminium Amidates
363
Toluene, hexanes, and diethyl ether were purchased from Fisher
Scientific. These solvents were sparged for 20 min with dry
argon and dried using a commercial two-column solvent puri-
fication system comprising columns packed with Q5 reactant
and neutral alumina respectively (for hexanes) or two columns
of neutral alumina (for toluene and Et2O). Celite was purchased
from Sigma Aldrich and was dried under reduced pressure at
2508C for 48 h before use. N-tert-Butylbenzamide (1-H) was
prepared through the nucleophilic acyl substitution reaction of
benzyl chloride with tert-butyl amine according to literature
methods.[18] Complex 2 was prepared via a protonolysis route
between 1-H and trimethylaluminium as reported by Lin
et al.[31] on a 5.6-mmol total aluminium scale in 74 % yield.
2-Propanol was dried and distilled over calcium oxide before
use. Pivaldehyde was dried and distilled over CaH2 before use.
4-Phenyl-3-buten-2-ol was prepared by sodium borohydride
reduction of 4-phenyl-3-buten-2-one. Silica gel (230–400 mesh)
was purchased from Fisher Scientific. All other reagents were
purchased from commercial sources and used as received.
and the reaction vessel was sealed. The reaction was heated at
688C for 24 h after which volatiles were removed under vacuum.
Crude materials were dissolved in toluene and filtered over a
Celite-padded frit, and the filtrate was concentrated to ,20 mL.
Colourless crystals were obtained after 24 h at ꢀ258C, and were
collected by filtration, washed with cold toluene (3 ꢁ 5 mL), and
dried under vacuum (1.76 g, 79 %), mp 141–1438C. dH (CDCl3)
7.42 (10H, m, Ph), 1.40 (18H, s, C(CH3)3), ꢀ0.55 (3H, s, Al-
CH3). dC (CDCl3) 177.0 (C¼O), 135.4, 129.4, 128.1, 126.8 (Ph),
51.8 (N-C(CH3)3), 31.3 (N-C(CH3)3), ꢀ11.7 (Al-CH3). Anal.
Calc. for C23H31AlN2O2: C 70.03, H 7.92, N 7.10. Found: C
69.89, H 7.94, N 7.50 %.
[k2-N,O-(t-BuNCOPh)]3Al (4)
A Schlenk flask equipped with a magnetic stir bar was
charged with N-tert-butylbenzamide (2.99 g, 16.9 mmol) and
hexane (50 mL). A solution of trimethylaluminium in heptane
(5.6 mL, 5.6 mmol) was added to the stirring solution and the
reaction vessel was sealed. The reaction was heated at 1008C for
24 h after which volatiles were removed under vacuum. Crude
materials were dissolved in ether and filtered over a Celite-
padded frit, and the filtrate was concentrated to ,20 mL.
Colourless crystals were obtained after 24 h at ꢀ258C, which
were collected by filtration, washed with cold ether (3 ꢁ 5 mL),
and dried under vacuum (1.69 g, 54 %), mp 130–1348C. dH
(CDCl3) 7.41 (15H, m, Ph), 1.26 (27H, s, C(CH3)3). dC (CDCl3)
160.3 (C¼O), 136.1, 128.9, 128.0, 126.7 (Ph), 52.0 (N-C
(CH3)3), 31.9 (N-C(CH3)3). Anal. Calc. for C33H42AlN3O3: C
71.33, H 7.62, N 7.56. Found: C 71.29, H 7.46, N 7.62 %.
K(t-BuNCOPh) (1-K)
An Erlenmeyer flask equipped with a magnetic stir bar was
charged with N-tert-butylbenzamide (2.50 g, 14.1 mmol). The
reagent was dissolved in diethyl ether (50 mL). To the rapidly
stirring solution was added KN(SiMe3)2 (2.80 g, 14.1 mmol) in
small portions over 0.5 h. The resulting mixture was stirred for
12 h at room temperature after which a white solid was collected
by filtration over a medium-porosity frit. The solid was washed
with diethyl ether (25 mL) followed by hexanes (25 mL) and
then dried under vacuum. 1-K was collected as a white powder
(2.67 g, 88 %). dH ([D8]THF) 8.11(2H, m), 7.12 (3H, m), 1.33
(9H, s, C(CH3)3). dC ([D8]THF) 164.5 (C¼O), 128.5, 127.1,
126.8, 51.6 (N-C(CH3)3), 31.6 (N-C(CH3)3). Anal. Calc. for
C11H14KNO: C 61.36, H 6.55, N 6.50. Found: C 57.97, H 7.34,
N 6.48 %.
General Procedure for the Al–Amidate-Catalyzed Reduction
of Ketone Substrates by 2-Propanol
An 8-mL vial equipped with a magnetic stirring bar was charged
with pre-catalyst (0.05 mmol for 2; 0.01 mmol for 3 or 4) and
toluene (4 mL). 2-Propanol (610 mL, 8.0 mmol) was added and
the vial was capped with a Teflon-lined silicone septum, and the
reaction was stirred for 0.5 h. Carbonyl substrate (1.0 mmol) was
added neat and the reaction was heated to 508C. After 24 h, an
aliquot (100 mL) of the reaction was collected with a gas-tight
syringe, loaded onto a plug of alumina, rinsed with methanol
(15 mL), and analyzed directly by GC. The GC retention times
of the products were confirmed with those of commercially
available samples.
{[k2-N,O-(t-BuNCOPh)]AlMe2} (2)
K(t-BuNCOPh) (1.20 g, 5.6 mmol) was added to an Erlen-
meyer flask equipped with a magnetic stir bar. The reagent was
suspended in toluene (50 mL) after which dimethylaluminium
chloride was added as a 1.0 M solution in hexanes (5.6 mL,
5.6 mmol) using a syringe. The reaction vessel was sealed and
the reaction mixture was stirred at room temperature for 12 h
after which it was filtered over a Celite-padded frit and washed
with toluene (10 mL). Solvents were removed and crude product
was dissolved in boiling hexanes (50 mL) and filtered over a
Celite-padded frit. Solvents were removed from the filtrate and
the crude product was dissolved in toluene and filtered over a
Celite-padded frit, and the filtrate was concentrated to ,20 mL.
Colourless crystals were obtained after 24 h at ꢀ258C, and were
collected by filtration, washed with cold toluene (3 ꢁ 5 mL), and
dried under vacuum (0.65 g, 50 %). The 1H and 13C NMR data of
this material match those reported by Lin et al. as follows.[31]
General Procedure for the Al–Amidate-Catalyzed Oxidation
of Alcohol Substrates by Pivaldehyde
An 8-mL vial equipped with a magnetic stirring bar was charged
with pre-catalyst (0.025 mmol for 2; 0.05 mmol for 3 or 4) and
toluene (4 mL). Alcohol (1.0 mmol) was added neat and the vial
was capped with a Teflon-lined silicone septum, and the reaction
was stirred for 0.5 h. Pivaldehyde (163 mL, 1.5 mmol or 325 mL,
3.0 mmol) was added and the reaction was stirred at room
temperature. After 3 h, an aliquot (100 mL) of the reaction was
collected with a gas-tight syringe, loaded onto a plug of alumina,
rinsed with methanol (15 mL), and analyzed directly by GC. The
GC retention times of the products were confirmed with those
of commercially available samples.
d
H (CDCl3) 7.41 (10H, m, Ph), 1.09 (18H, s, C(CH3)3), ꢀ0.87
(12H, s, Al-CH3). dC (CDCl3) 167.8 (C¼O), 133.8, 129.8, 128.1,
127.2, 52.9 ((C(CH3)3), 31.1 (C(CH3)3), ꢀ8.5 (Al-CH3).
[k2-N,O-(t-BuNCOPh)]2AlMe (3)
Synthesis of sec-Phenethyl Alcohol
A Schlenk flask equipped with a magnetic stir bar was
charged with N-tert-butylbenzamide (1.99 g, 11.2 mmol) and
hexane (50 mL). A 1.0 M solution of trimethylaluminium in
heptane (5.6 mL, 5.6 mmol) was added to the stirring solution
To a flask equipped with a magnetic stir bar was added 2 (0.12 g,
0.25 mmol) and toluene (25 mL). 2-Propanol (3.0 mL,
40.0 mmol) was added to the solution and the reaction was

364
K. P. Yeagle et al.
stirred for 0.5 h after which acetophenone (0.60 g, 5.0 mmol)
was added. The reaction vessel was sealed and the reaction
heated to 508C with stirring. After 24 h, the reaction was cooled
to room temperature and quenched with 3 M HCl (25 mL). The
resultant mixture was extracted with diethylether (3 ꢁ 50 mL)
and the combined organics were washed with saturated sodium
bicarbonate (3 ꢁ 25 mL) and brine (25 mL), and then dried over
anhydrous Na2SO4. The drying agent was removed by filtration
and solvents were removed from the filtrate under reduced
pressure to give the crude products. The product was purified by
column chromatography on silica gel with hexane/ethyl acetate
(8 : 2) eluent.[59] sec-Phenethyl alcohol was collected as a col-
[2] F. A. Cotton, G. Wilkinson, C. A. Murillo, M. Bochmann, in Advanced
Inorganic Chemistry 6th ed. 1999, p. 1355 (John Wiley & Sons, Inc.:
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[3] D. Rabinovich, Nat. Chem. 2013, 5, 76. doi:10.1038/NCHEM.1535
[4] Aluminum, December 2013 (U. S. Geological Survey: Reston, VA)
[5] T. Taguchi, H. Al Yanai III, in Acid Catalysis in Modern Organic
Synthesis (Eds H. Yamamoto, K. Ishihara) 2008, Vol. 1, pp. 241–345
(Wiley-VCH: Weinheim).
[6] For recent selected examples, see: (a) D. Li, Y. Peng, C. Geng, K. Liu,
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Part A: Polym. Chem. 2013, 51, 1137. doi:10.1002/POLA.26476
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Marell, C. J. Cramer, W. B. Tolman, Inorg. Chem. 2013, 52, 13692.
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1
ourless oil (0.32 g, 52 %) and characterized by H NMR spec-
troscopy, with the data matching that of commercial samples. dH
(CDCl3) 7.34 (5H, m, Ph), 4.90 (1H, q, J 7, HC-OH), 1.78 (1H, b,
OH), 1.50 (3H, d, J 7, CH3).
Synthesis of Acetophenone
To a 125-mL flask equipped with a magnetic stir bar was added
2 (0.058 g, 0.125 mmol). Toluene (25 mL) was added to the
flask followed by sec-phenethyl alcohol (0.61 g, 5.0 mmol). The
resultant solution was stirred for 0.5 h after which pivaldehyde
(1.63 mL, 15.0 mmol) was added. The reaction was stirred for
3 h at room temperature after which the reaction was quenched
with 3 M HCl (25 mL). The resultant mixture was extracted with
diethylether (3 ꢁ 50 mL) and the combined organics were
washed with saturated sodium bicarbonate (3 ꢁ 25 mL) and
brine (25 mL), and then dried over anhydrous Na2SO4. The
drying agent was removed by filtration and solvents were
removed from the filtrate under reduced pressure to give the
crude products. The product was purified by column chroma-
tography on silica gel with hexane/ethyl acetate (8 : 2) eluent.[59]
Acetophenone was collected as a colourless oil (0.48 g, 80 %)
and characterized by 1H NMR spectroscopy, with the data
matching that of commercial samples. dH (CDCl3) 7.96 (2H,
d, J 8, Ph), 7.56 (1H, t, J 8, Ph), 7.46 (2H, t, J 8, Ph), 2.60 (3H,
s, CH3).
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Supplementary Material
1H and 13C NMR spectra of compounds 2, 3, and 4, GC methods
and traces for the carbonyl and alcohol substrates, and 1H NMR
spectra of sec-phenethyl alcohol and acetophenone from the
large-scale reactions are available from the Journal’s website.
CCDC 1020534 and CCDC 1020533 contain the supplementary
crystallographic data for complexes 3 and 4. The data can be
obtained from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: þ44 1223 336 033;
email: deposit@ccdc.cam.ac.uk or data can be obtained from
http://www.ccdc.cam.ac.uk/data_request/cif.
[15] For examples of catalytic MPV reductions protocols employing
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(d) J. Yin, M. A. Huffman, K. M. Conrad, J. D Armstrong, J. Org.
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Acknowledgements
We thank Albright College for financial support of this work. We thank the
George I. Alden Trust for funding towards the purchase of the NMR spec-
trometer employed in this work. KPY and DH thank the Albright College
Undergraduate Research Council for Summer Research Fellowships.
[16] For examples of catalytic Oppenauer oxidation protocols, see:
(a) T. Ooi, H. Otsuka, T. Miura, H. Ichikawa, K. Maruoka, Org.
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