Controlled hydrosilylation of carbonyls and imines catalyzed by a cationic aluminum alkyl complex

Article abstract of DOI:10.1021/om2008277

The synthesis, characterization, and unprecedented catalytic activity of cationic aluminum alkyl complexes toward hydrosilylation are described. X-ray crystallographic analysis of Tp*AlMe₂ (1) and [Tp*AlMe][I₃] (3) revealed the prefe

Full text of DOI:10.1021/om2008277

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pubs.acs.org/Organometallics
Controlled Hydrosilylation of Carbonyls and Imines Catalyzed
by a Cationic Aluminum Alkyl Complex
J€urgen Koller and Robert G. Bergman*
Department of Chemistry, University of California, Berkeley, California 94720, United States
S Supporting Information
b
ABSTRACT: The synthesis, characterization, and unprece-
dented catalytic activity of cationic aluminum alkyl complexes
toward hydrosilylation are described. X-ray crystallographic
analysis of Tp*AlMe2 (1) and [Tp*AlMe][I3] (3) revealed
the preference of Al for a tetrahedral coordination environment
and the versatility of the Tp* ligand in stabilizing Al in bi- and
tridentate coordination modes. [Tp*AlMe][MeB(C6F5)3] (2) is highly active toward the hydrosilylation of a wide variety of
carbonyls and imines, thus providing an inexpensive and versatile alternative to late transition metal catalysts.
Scheme 1. Synthesis of Compounds 1ꢀ3
he direct formation of silyl-protected alcohols via reduction
of carbonyls, also known as hydrosilylation, is an exception-
T
ally useful reaction and has therefore become an important
synthetic tool in organic chemistry.1 Few chemical transforma-
tions have received a comparable amount of attention, and thus
a plethora of catalysts with high activities have been reported.
Examples of active catalysts span the entire transition metal
series, some of which have been highly refined to achieve extra-
ordinary turnover numbers and unmatched potential for asym-
metric synthesis.2
Although noble metals have excelled in this chemistry, recent
interest has shifted toward catalyst systems based on inexpensive
and readily available metals such as iron.3 Recent reports of a
simple yet highly effective Fe amide catalyst show the potential
of such approaches while eliminating the need for extensive
ligand synthesis.4 Examples of active catalysts based on main
group elements have lagged behind their transition metal coun-
terparts, although some systems based on remarkably simple
Lewis acids such as B(C6F5)3 have recently been documented.5
The homoleptic and typically monodentate ligation of such
species, however, does not allow for significant reactivity control
and thus limits their synthetic utility. Despite the Lewis acidic
character of Al, few Al-based catalysts have been shown to
participate in productive hydrosilylation chemistry, with most
examples limited to alkynes and strained cyclic substrates.6
A significant challenge using such Lewis acid catalysts is the
typically difficult reaction control, often resulting in complex
product mixtures. Partially masking the reactivity by using bulky
ligands can alleviate some of the mentioned issues, although
typically at the expense of catalytic activity.
exceptionally qualified for this task, as they can support multiple
coordination modes and potentially stabilize a highly reactive
metal center. Al complexes featuring Tp* ligands have been known
since the late 1980s, and their synthesis was achieved using a
slightly modified literature procedure.7 Accordingly, simple salt
metathesis between commercially available KTp* and AlMe2Cl
resulted in the formation of 1 in quantitative yield on a multigram
scale via the elimination of KCl (Scheme 1).
1H NMR spectroscopic analysis of 1 (CDCl3 at 25 °C)
revealed fluxional coordination of the Tp* ligand to the metal
center, as evidenced by average ligand resonances at 5.81, 2.29,
and 2.28 ppm, corresponding to the aromatic pyrazolyl protons
and the methyl substituents, respectively. As expected, both
Al-bound methyl moieties are magnetically equivalent, appear-
ing as a single resonance at ꢀ0.57 ppm. Resonances observed by
13C NMR spectroscopy confirm the structural assignment.
Expectedly, the fluxional η3-coordination observed in solution
is not sustained in the solid state, as evidenced by the X-ray
crystallographic analysis of 1 (Figure 1a). The Tp* ligand is found
to adopt a bidentate coordination mode, likely a result of the
preference of Al for tetrahedral coordination in combination
with crystal packing forces. This effect is well documented in
We thus sought a catalyst system that would stabilize a highly
Lewis acidic cationic Al complex by exploiting the distinct
preference of Al for tetrahedral coordination geometry. Such
systems, designed to display a unique balance of reactivity and
stability, are expected to provide highly active catalysts with
unprecedented reaction control. Tridentate scorpionate ligands
such as hydro-tris(1,3-dimethylpyrazol-1-yl)borate (Tp*) seemed
Special Issue: F. Gordon A. Stone Commemorative Issue
Received: September 2, 2011
Published: September 14, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/om2008277 Organometallics 2012, 31, 2530–2533
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Figure 1. Thermal ellipsoid diagrams of the molecular structures of (a) 1 and (b) 3. Thermal ellipsoids are drawn at 50% probability. H atoms (except
H1), counterions, and cocrystallized solvents are omitted for clarity. Selected bond lengths (Å) and angles (deg): 1: Al1ꢀN1 1.9169(17), Al1ꢀC11
1.956(3), Al1ꢀC12 1.989(3), C11ꢀAl1ꢀC12 115.44(14); 3: Al1ꢀC16 1.912(3), Al1ꢀN1 1.896(3), Al1ꢀN3 1.901(3), Al1ꢀN5 1.898(3),
N1ꢀAl1ꢀC16 120.65(13).
the literature for Al alkyl complexes bearing similar supporting
ligands.8
Table 1. Silane Screening and Reaction Optimization
On the basis of the similarities to known systems, we
anticipated the facile abstraction of a methyl ligand by Lewis
acidic reagents such as BArF , resulting in a cationic Al complex
3
with the Tp* ligand adopting a nonfluxional η3-coordina-
tion mode. In fact, treating 1 with one equivalent of B(C6F5)3
resulted in the almost instantaneous formation of 2, which was
isolated in good yield. 1H NMR analysis of 2 clearly shows two
distinct methyl resonances at 0.26 and 0.41 ppm. The former
corresponds to the Al-Me moiety and indicates a significantly
more electron-deficient metal center compared to that of 1
(Δδ = 0.83 ppm). The latter broad resonance is diagnostic
for a B-bound methyl group that is well-separated from the Al
center.9 The average signals for the Tp* ligand are consistent with
symmetric η3-coordination to Al.
cat
conc
entry (mol %) (mol/L)
silane
temp (°C) time (h) conv (%)
1
2
3
4
5
6
7
8
9
3.8
4.2
4.0
4.3
3.0
1.7
2.5
1.0
1.0
0.1
0.1
0.1
0.1
0.1
0.1
1.0
1.0
1.0
HSiPh3
75
75
17
19
14
18
4
0
73
HSiEt3
H2SiPh2
H2SiEt2
H2SiMePh
H3SiPh
75
>98
>98
>98
>98
>98
>98
>98
75
75
75
1.5
HSiEt3
100
100
100
1
Due to crystallization difficulties often observed for complexes
involving B-based anions, we sought a similar complex bearing an
alternative counterion. Treating 1 with two equivalents of I2
resulted in the formation of complex 3 accompanied by elimina-
tion of MeI (Scheme 1). X-ray quality crystals of 3 can be grown
within minutes from concentrated DCM solutions at ꢀ35 °C,
H2SiMePh
H3SiPh
1
0.5
reductant since hydrosilylation reactions involving tertiary si-
lanes are of greater industrial importance and typically yield
single products. A wide variety of substrates were successfully
converted to their respective silyl ethers (4aꢀp), as illustrated in
Table 2. Unsurprisingly, conversion of aldehydes was efficient
at catalyst loadings of 1.0 mol % (entries 1ꢀ3), while undesired
Lewis acid-catalyzed side reactions such as the Tishchenko
dimerization were kept below the detection limit (entries
1ꢀ3).11 Consistent with recent studies by Mayr and co-workers,
1,2-addition of HSiEt3 to cinnamaldehyde was observed as a
result of the highly electrophilic character of the carbonyl moiety,
whereas 1,4-addition was prevalent for α,β-unsaturated ketones
(vide infra).12 Aromatic ketones were identified as equally
suitable substrates for hydrosilylation, demonstrated by the clean
conversion of acetophenone and even the sterically demanding
4,40-dichlorobenzophenone (entries 4, 5), although higher cat-
alyst loadings (2.5 mol %) and extended reaction times were
necessary to achieve quantitative conversion. Reduction of a wide
variety of aliphatic ketones bearing various functional groups to
the respective silyl ethers was successful under standard reaction
conditions (entries 6ꢀ10).
ꢀ
likely a result of the improved crystal packing capability of the I3
anion. As expected, η3-coordination of the Tp* ligand is observed
in the solid state, resulting in a tetrahedral coordination environ-
ment around Al (Figure 1b) with almost identical AlꢀN bonds
(∼1.90 Å). The AlꢀC16 bond (1.912(3) Å) is noticeably shorter
compared to the AlꢀC bonds in compound 1 (Δd > 0.05 Å),
reflecting the electron-deficient nature of the metal center. Both
1H and 13C NMR data indicate that these structural features are
maintained in solution (CDCl3 at 25 °C).
Initial NMR-scale experiments (CDCl3) revealed the activity
of 2 toward hydrosilylation chemistry at temperatures above
75 °C, as evidenced by the clean conversion of acetophenone
and H2SiPh2 to the corresponding silyl ether.10 Encouraged by
these results, wescreened various silanes for theirefficiencytoward
this transformation (Table 1). H3SiPh exhibited the highest activity,
whereas secondary and tertiary silanes were significantly less reactive
(entries 1ꢀ6). Furthermore, the absolute substrate/silane concen-
tration had a major impact on the rate of the reaction, with reactions
proceeding significantly faster at high concentrations (entries 7ꢀ9).
Although H3SiPh showed the highest activity, preparative
scale (1 mmol) reactions were performed using HSiEt3 as the
The catalyst proved effective toward the hydrosilylation of
more challenging substrates such as an electron-rich cyclic ester
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Table 2. Hydrosilylation of Aldehydes, Ketones, and Imines
a Isolated yields. b +20% of unidentified product. c Reaction run at 75 °C. d NMR yields, determined by integration of product signals versus internal
standard. e Product isolated as mixture of Z:E (4:1) isomers. f Reaction run at 135 °C.
(δ-valerolactone), which was converted to the corresponding
acetal (entry 11) even though Al complexes are known for the
ring-opening polymerization of such species.14 As noted pre-
viously, complex 2 was highly active toward the direct conversion
of α,β-unsaturated ketones to the corresponding silyl enol ethers
(entries 12ꢀ14), a rare transformation typically observed only
with late transition metal catalysts.13 Conversion of N-benzyli-
deneaniline and N-benzylidene-4-methylbenzene-sulfonamide
to the respective amines required higher temperatures and longer
reaction times (entries 15, 16), although these examples illustrate
the versatility of 2 as a hydrosilylation catalyst.
Although mechanistic details of this transformation remain
largely unexplored, clean formation of compound 4h in the
absence of ring-opening byproduct is inconsistent with a radical
pathway.15 Moreover, coordination of acetophenone to complex
2 was not observed during variable-temperature 1H NMR studies
(see Supporting Information). On the contrary, loss of the J
coupling between SiꢀH and the methylene protons of the ethyl
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groups suggests an interaction of HSiEt3 with 2 (at elevated
temperatures) analogous to other Lewis acid-catalyzed systems.5c
These findings are corroborated by deuterium labeling studies,
which show the statistical H/D redistribution between DSiEt3 and
H2SiPh2 in the presence of catalytic amounts of 2 (eq 1). Un-
doubtedly, additional studies are required for the conclusive me-
chanistic elucidation of these Al-catalyzed hydrosilylation reactions.
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DSiEt3 þ H2SiPh2 2 ð1 mol %Þ HSiEt þ H D2ꢀxSiPh2 ð1Þ
F
R
3
x
100 °C
CDCl
3
In conclusion, cationic Al alkyl complexes supported by
scorpionate ligands have been prepared and structurally char-
acterized. Crystallographic analysis revealed the versatility of the
Tp* ligand as well as a change in coordination geometry upon
formation of the cationic complexes. Compound 2 is active
toward the hydrosilylation of a variety of substrates, including
ketones, aldehydes, imines, and even electronically unfavorable
lactones, highlighting the curiously gentle yet highly active character
of the cationic Al complex. Such catalysts may thus provide a cheap
and effective alternative for expensive late transition metal species in
addition to a remarkably broad substrate scope.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, X-ray
b
crystallographic data (CIF), and additional characterization data.
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Corresponding Author
*Tel: +1-510-642-2156. Fax: +1-510-642-7714. E-mail: rbergman@
cchem.berkeley.edu.
’ DEDICATION
This paper is dedicated to the memory of Prof. Gordon Stone,
with gratitude for the immense impact that his scientific con-
tributions have made to the field of organometallic chemistry.
’ ACKNOWLEDGMENT
This material is based upon work supported as part of the
Center for Catalytic Hydrocarbon Functionalization, an Energy
Frontier Research Center funded by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences, under
Award Number DE-SC0001298.
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