Organoaluminum cations for carbonyl activation

Article abstract of DOI:10.1039/c9cc08272g

In search of stable, yet reactive aluminum Lewis acids, we have isolated an organoaluminum cation, [(Me₂NC₆H₄)₂Al(C₄H₈O)₂]⁺, coordinated with two labile tetrahydrofuran ligands. Its catalytic performance in aldehyde dimerization reveals turn-over frequencies reaching up to 6000 h^-1, exceeding that of the reported main group catalysts. The cation is further demonstrated to catalyze hydroelementation of ketones. Mechanistic investigations reveal that aldehyde dimerization and ketone hydrosilylation occur through carbonyl activation.

Full text of DOI:10.1039/c9cc08272g

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Organoaluminum cations for carbonyl activation†
a
a
b
a
Ramkumar Kannan,
Raju Chambenahalli,b
Sandeep Kumar, Athul Krishna,
Cite this: Chem. Commun., 2019,
5, 14629
a
a
Alex P. Andrews, Eluvathingal D. Jemmis
and Ajay Venugopal
*
5
Received 22nd October 2019,
Accepted 12th November 2019
DOI: 10.1039/c9cc08272g
rsc.li/chemcomm
+
In search of stable, yet reactive aluminum Lewis acids, we have isolated Et Al initiates uncontrolled side-reactions including alkylation of
2
+
14
+
2 6 4 2 4 8 2
2
an organoaluminum cation, [(Me NC H ) Al(C H O) ] , coordinated the solvent. To subjugate the reactivity of R Al and to explore
with two labile tetrahydrofuran ligands. Its catalytic performance in controlled catalytic activity, we propose to introduce additional
aldehyde dimerization reveals turn-over frequencies reaching up to neutral donor functionality (L) on the anionic alkyl/aryl ligands (X)
À1
+
6
000 h , exceeding that of the reported main group catalysts. The and explore the reactivity of aluminum cations of the type (LX)
2
Al .
cation is further demonstrated to catalyze hydroelementation of There have been efforts to isolate such cations, for example,
+
15
ketones. Mechanistic investigations reveal that aldehyde dimerization [NC
6 5 3 2 2
H C(SiMe ) ] Al . However, sterics on the ligand have
and ketone hydrosilylation occur through carbonyl activation.
prevented access to the electrophilic aluminum center. Herein,
we report the use of 2-[(dimethylamino)]-phenyl (Me NC H )
2
6
4
Friedel–Crafts alkylation and acylation employing AlCl is a ligand, offering less steric hindrance, to prepare aluminum
3
1
+
landmark discovery in Lewis acid catalysis. It has impelled cations of the type (LX) Al . We have probed the Lewis acidic
chemists to explore aluminum compounds in a wide range of properties of (LX)
2
+
2
Al and its catalytic relevance in the activation
catalytic processes in the chemical industry. Production of long of aldehydes and ketones.
chain primary alkenes and alcohols are the most notable
examples of aluminum compounds as Lewis acid catalysts.
The reaction between one equivalent of AlCl
3
and two
AlCl
2
16
equivalents of [Me
2
NC
6
H
4
Li] resulted in (Me
2
NC
6
H
4
)
2
New directions towards achieving molecular aluminum super- (1) in 70% yield (Scheme 1). In the solid state, 1 is a five
acids for small molecule activation have triggered the synthesis coordinate complex with a distorted trigonal bi-pyramidal
3
4
5
6
of Al{OC(CF ) } , Al(OC F ) , Al{N(C F ) } and Al(C F ) .
geometry (Fig. 1). Two carbon atoms of the aryl ligands and
3
3 3
6
5 3
6
5 2 3
6 5 3
The twenty-first century has witnessed the development of one chlorine atom bound to the aluminum atom form the
aluminum compounds for catalytic reduction of unsaturated trigonal equatorial plane. Intramolecular coordination of the
7
bonds through the hydroboration pathway. Hydroboration of two nitrogen atoms of the aryl ligands with the aluminum
8
9
10
11
12
alkenes, alkynes, aldehydes, ketones, imines and nitriles
center in 1 results in two four-member chelate rings. The two
has been performed employing molecular aluminum catalysts. nitrogen atoms bind axially to the aluminum center with
In search of highly reactive low coordinate aluminum com- distances of 2.156(4) and 2.184(4) Å (Fig. 1 and Table 1).
+
pounds, R Al cations (R = Me, Et and Aryl) bearing weakly coordi-
The cationic complex, [(Me NC H ) Al(C H O) ][B(C H Cl ) ]
2 6 4 2 4 8 2 6 3
2
2 4
17
13
nating anions have been isolated. Exceptional Lewis acidity in (2) was generated from 1 using [(CH CN) Ag][B(C H Cl ) ] in
these cations initiates C–H activation and olefin polymerization.
Efforts to employ Et
deoxygenation of CO
3
2
6
3
2 4
13c
1
THF (Scheme 1). The H NMR spectrum of 2 recorded in CD Cl
2 2
+
2
Al in catalytic CO hydrosilylation have led to confirmed the presence of two coordinated THF molecules and
4
resulting in CH . Extreme reactive nature of
2
2
a
School of Chemistry, Indian Institute of Science Education and Research
Thiruvananthapuram, Vithura, Thiruvananthapuram 695551, India.
E-mail: venugopal@iisertvm.ac.in
b
Department of Inorganic and Physical Chemistry, Indian Institute of Science,
Bangalore 560012, India
†
Electronic supplementary information (ESI) available: Synthesis and character-
isation of compounds 1–4, catalytic experiments, computational details and CIF
for 1–4. CCDC 1959215–1959218. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c9cc08272g
Scheme 1 Synthesis of organoaluminum cations.
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Scheme 2 Aldehyde dimerization catalyzed by compound 2. Yields were
determined by NMR spectroscopy.
Inferring two Lewis acidic sites on the aluminum cation, we
began probing the reactivity of 2 with carbonyl compounds.
When a catalytic amount of 2 was introduced into a neat sample of
1
9
Fig. 1 Solid state structures of the cations in 1–4. Hydrogen atoms are C H CHO, the classical Tischenko reaction took place. It resulted
6
11
omitted for clarity. Thermal ellipsoids drawn at 35% probability level.
in the quantitative dimerization of aldehyde to the corresponding
ester at ambient temperature with a turn-over number and frequency
of 200 and 6000 h respectively (Scheme 2). While aluminum
isopropoxide has been traditionally used as a catalyst in Tischenko
reaction, several electrophilic main group compounds have also
À1
Table 1 Selected bond lengths (Å) in 1–4
20
21
Compound
1
2
3
4
been probed. Our investigations point out that 2 exhibits the highest
Al–N1
Al–N2
Al–O1
Al–O2
Al–Cl
2.156(4)
2.184(4)
—
2.408(3)
2.487(3)
1.939(2)
1.922(2)
—
2.769(8)
3.351(8)
1.809(7)
1.794(7)
—
2.866(6)
2
1
3.321(7) turn-over number and frequency among the main group and
1.819(4)
1.785(5)
—
22
f-element catalysts for aldehyde dimerization. The turn-over
—
number and frequency shown by 2 resemble the efficiency seen
2.1723(1)
23
in the Shvo catalyst but are lower than those for the recently
24
reported bench-mark precious-metal osmium hydride catalysts.
the solid state structure reveals Al–O distances of 1.939(2) and Following this successful dimerization of C 11CHO, we probed
.922(2) Å (Fig. 1 and Table 1). The coordination of THF the activity of 2 with other aromatic and aliphatic aldehydes
molecules to the aluminum center in 2 elongates the AlÁ Á ÁN (Scheme 2). As expected, aromatic aldehydes showed less reactivity
distances substantially to 2.408(3) and 2.487(2) Å as compared to 1 compared to C 11CHO. 1-Butanone showed lesser reactivity
6
H
1
6
H
(Table 1). The geometry of the aluminum cation in 2 can be best because it exhibited keto–enol tautomerism in solution.
described as a bi-capped tetrahedron (Fig. 1). Each of the two
To understand the mechanism of aldehyde dimerization
carbon and oxygen atoms occupy the corners of the tetrahedron catalyzed by 2, we performed further experimental and theoretical
and the nitrogen atoms are capping the tetrahedron. investigations. Kinetic studies revealed that benzyl benzoate for-
THF molecules bound to the aluminum center in 2 were mation follows second order kinetics with respect to PhCHO (see
successfully substituted by the more nucleophilic OPEt and the ESI,† Fig. S16). When PhCHO dimerization was performed in
ligands to obtain 3 and 4 (Scheme 1). The P NMR the presence of 0.25 equivalents of 2 in CD Cl , benzyl benzoate
exhibited a single sharp was detected in the H and C NMR spectra. The absence of the
signal at d 75.5 ppm for the two coordinated OPEt3 ligands ketone Me NC H C(O)C H in the spectra confirms the ancillary
3
3
1
OP(NMe
spectrum of 3 recorded in CD
2
)
3
2
2
1
13
2
Cl
2
2
6
4
6 5
(
see the ESI,† Fig. S9). This observation is indicative of high nature of the 2-[(dimethylamino)]-phenyl (Me NC H ) ligand bound
2 6 4
Lewis acidity at the aluminum center in 3. In line with our to the aluminum center and rules out the reactivity of the aryl
expectations, the oxygen atoms in the stronger bases, OPEt ligand with benzaldehyde in the initial step of catalysis (see the
and OP(NMe , bind to the aluminum centers in 3 and 4 with ESI,† Fig. S17 and S18). DFT calculations using the M06-2X
3
2 3
)
Al–O distances shorter than the ones in 2 (Fig. 1, Table 1). The functional and cc-pVTZ basis set were performed to establish the
shorter Al–O distances in 3 and 4 elongate the AlÁ Á ÁN inter- plausible catalytic cycle. In the initial step, two of the THF
action to the limits of the sum of van der Waals radii of molecules are substituted by PhCHO molecules to form INT-1 with
1
8
À1
aluminum and nitrogen (SrvdW(Al & N) = 3.39 Å, Table 1). the reaction accompanied by an energy change of +2.7 kcal mol
Thus, 3 and 4 can be primarily described as tetrahedral com- (Scheme 3). The coordination of PhCHO molecules to the aluminum
plexes (Fig. 1). The trans-influence exerted by the incoming center leads to an elongation as well as polarization of the CQO
2
5
nucleophiles displaces the NMe
opportunity to employ (Me NC
promoted catalytic reactions.
2
groups, thus offering an bond. In the second step, an incoming PhCHO molecule interacts
+
2
6
H
4
)
2
Al species in Lewis acid with INT-1 to form TS involving a four member ring with an
À1
activation barrier of DEact = 25.6 kcal mol . In the third step, TS
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Scheme 3 Proposed catalytic cycle for aldehyde dimerization catalyzed
by compound 2.
re-organizes into INT-2 and forms the ester bound to the aluminum
Scheme 5 Ketone hydrosilylation and deoxygenation catalyzed by com-
a
b
center. This step is highly exothermic with the energy change of pound 2. Reaction performed in C
6
D
6
.
Reaction performed in CD
2 2
Cl .
À1
Yields were determined by NMR spectroscopy.
À59.0 kcal mol . It is evident from the mechanism that
+
[
(Me NC H ) Al(aldehyde) ] is the active species catalyzing aldehyde
2
6
4 2
2
dimerization differing from the well known metal alkoxide mediated
Meerwein–Ponndorf–Verley-type reduction of PhCHO.
Prompted by the success with the aldehyde dimerization cata-
lyzed by a Lewis acidic aluminum center, we set out to investigate
ketone reduction using pinacol borane (Bpin–H) and triethylsilane
observed that the use of PhSiH
Ph CO and di-p-tolylmethanone at room temperature.
To understand the mechanism of ketone hydrosilylation,
3
resulted in the de-oxygenation of
19–22
2
a few test reactions were performed. There was no interaction
1
observed in the H NMR spectrum when 2 was treated with
3 3
(Et Si–H). Bpin–H and Et SiH reduced di-p-tolylmethanone to
excess of Et
hand, treatment of 2 with excess of Ph
3
SiH in C
6
D
6
(see the ESI,† Fig. S50). On the other
CO (sixteen equivalents)
the corresponding alkoxy derivative in the presence of catalytic
amounts of 2 (Scheme 4). Notably, hydrosilylation proceeded
more efficiently than hydroboration. We also observed that
hydrostannylation could be performed at ambient temperature
but required longer completion time.
2
led to the substitution of the two THF ligands bound to the
aluminum center in the former by the latter (see ESI,† Fig. S49).
Further, gradual increase of Ph
2
CO concentration with respect
CO
to Et SiH resulted in the decrease of reaction time for Ph
3
2
Catalytic hydrosilylation is a topical area in Lewis acid
hydrosilylation (see the ESI,† Table S3). However, increasing
26
catalysis. Examples of catalytic ketone hydrosilylation are mainly
Et SiH concentration with respect to Ph CO did not lead to any
3
2
27
28
restricted to boron and phosphorus compounds in main group
chemistry. Reports on aluminum catalyzed hydrosilylation of
appreciable change in the reaction time. These evidences are
indicative of a CQO activation pathway for ketone hydrosilylation
reminiscent of the proposed carbonyl activation mechanisms for
2
9
ketones are scarce. Based on the preliminary results on di-p-
tolylmethanone hydrosilylation, we proceeded to study other ketones
30
aldehydes using main group Lewis acids.
(
Scheme 5). In general, these reactions required a minimum tem-
perature of 60 1C and proceeded faster at increased temperatures.
Benzophenone (Ph CO) hydrosilylation with Et SiH was slower
In conclusion, we have demonstrated that introduction of
additional N-donor functionality to an aryl ligand (Ar) can
2
3
control the reactivity at the electrophilic aluminum center in
compared to di-p-tolylmethanone (Scheme 5). Aliphatic substituents
on the carbonyl moiety decreased the reaction duration. We also
+
Ar Al . Aldehyde dimerization and hydroelementation of
2
ketones provide an insight into the potential applicability of 2
in Lewis acid catalysis. To the best of our knowledge, 2
represents a rare example of a main group Lewis acid catalyzing
3
1
ketone hydrosilylation via CQO activation. We are currently
exploring the reactivity in organic transformations involving
carbonyl activation.
We thank MHRD-STARS (STARS/Rec-final/Apr19/01, P. ID 250)
for providing generous funding, IISER Thiruvananthapuram for
top-class research facilities, and fellowships to R. K. and R. C. and,
SERC-IISc for computational facilities at SERC-IISc. EDJ thanks
SERB-DST (Year of Science Chair Professorship) for research
support.
Scheme 4 Hydroelementation of di-p-tolylmethanone catalyzed by
compound 2. Yields were determined by NMR spectroscopy.
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Conflicts of interest
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P. E. Whitley, Organometallics, 2013, 32, 1674.
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14632 | Chem. Commun., 2019, 55, 14629--14632
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