An Aluminum Hydride That Functions like a Transition-Metal Catalyst

Article abstract of DOI:10.1002/anie.201503304

The reaction of [LAlH₂] (L=HC(CMeNAr)₂, Ar=2,6-iPr₂C₆H₃) with MeOTf (Tf=SO₂CF₃) resulted in the formation of [LAlH(OTf)] (1) in high yield. The triflate substituent in 1 increases the positive charge at the aluminum center, which implies that 1 has a strong Lewis acidic character. The excellent catalytic activity of 1 for the hydroboration of organic compounds with carbonyl groups was investigated. Furthermore, it was shown that 1 effectively initiates the addition reaction of trimethylsilyl cyanide (TMSCN) to both aldehydes and ketones. Quantum mechanical calculations were carried out to explore the reaction mechanism.

Full text of DOI:10.1002/anie.201503304

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201503304
German Edition: DOI: 10.1002/ange.201503304
Homogeneous Catalysis
Hot Paper
An Aluminum Hydride That Functions like a Transition-Metal
Catalyst**
Zhi Yang,* Mingdong Zhong, Xiaoli Ma, Susmita De, Chakkittakandiyil Anusha,
Pattiyil Parameswaran,* and Herbert W. Roesky*
Dedicated to Professor Jean-Marie Lehn on the occasion of his 75th birthday
Abstract: The reaction of [LAlH2] (L = HC(CMeNAr)2, Ar =
2,6-iPr2C6H3) with MeOTf (Tf = SO2CF3) resulted in the
formation of [LAlH(OTf)] (1) in high yield. The triflate
substituent in 1 increases the positive charge at the aluminum
center, which implies that 1 has a strong Lewis acidic character.
The excellent catalytic activity of 1 for the hydroboration of
organic compounds with carbonyl groups was investigated.
Furthermore, it was shown that 1 effectively initiates the
addition reaction of trimethylsilyl cyanide (TMSCN) to both
aldehydes and ketones. Quantum mechanical calculations were
carried out to explore the reaction mechanism.
bimetallic system, where alkaline bases are crucial for the
activation of inert C H bonds,[7] the THF coordinate anion is
À
stabilized in this complex only by the Al center. This
observation prompted us to investigate aluminum(III) hy-
dride as a main-group catalyst based on the principles of
transition-metal catalysts.[8] Previous examples of single-site
main-group metal catalysts require a strong Lewis acid
cocatalyst.[9] One consideration to increase the reactivity of
aluminum hydride species is the opening of the coordination
site for substrate binding and activation. This feature is made
possible by conferring a higher positive charge to the Al
center, while concurrently retaining its requisite hydridic
À
G
roup 13 hydrides have been studied for hydrogen stor-
character of the Al H bond.
The electrophilic MeOTf (Tf = SO2CF3) was treated with
[LAlH2] resulting in 1 in high yield (Scheme 1). Compound
age,[1] and used for various organic transformations.[2] The
chemistry of the aluminum hydrides is well-developed for the
reduction of various polar functional and unsaturated sub-
strates.[3] Considering the distinct reactivity of [LAlH2] (L =
HC(CMeNAr)2, Ar= 2,6-iPr2C6H3), it can be used for the
activation of nonpolar sulfur–sulfur bonds.[4] Moreover, the
b-diketiminato substituent has found widespread application
as a supporting ligand for metal-mediated catalysis.[5] Con-
sequently, functionalized [LAlH2] can be a viable alternative
to transition-metal compounds for catalysis through rational
ligand design. Recently, Driess and co-workers reported on
Scheme 1. Synthesis of aluminum monohydride complex 1.
Ar=2,6-iPr2C6H3, Tf=SO2CF3.
À
À
the activation of C H and C O bonds mediated by the
hydrido-Al(I)!Fe complex.[6] In contrast to the reported
1 was characterized by multinuclear NMR spectroscopy,
single-crystal X-ray studies, and elemental analysis. The
1H NMR spectrum of 1 shows splitting of the isopropyl
group signals. The infrared spectrum of 1 shows only one
absorption band (1797 cmÀ1) in contrast to that of [LAlH2]
(1832 and 1795 cmÀ1),[10] thus confirming the existence of only
[*] Prof. Z. Yang, Dr. M. Zhong, Dr. X. Ma
School of Chemical Engineering and Environment
Beijing Institute of Technology
100081 Beijing (P.R. China)
E-mail: zhiyang@bit.edu.cn
À
one Al H bond. The calculated gas-phase reaction energy for
the formation of 1 from [LAlH2] (À67.9 kcalmolÀ1), indicates
the thermodynamic feasibility of the reaction (Scheme S1).[11]
Single crystals of X-ray quality were obtained from a toluene
solution of 1 at low temperature. The molecular structure as
well as the selected bond lengths and angles are shown in the
caption of Figure S1 (see the Supporting Information). The
Prof. H. W. Roesky
Institut für Anorganische Chemie, Universität Gçttingen
Tammannstrasse 4, 37077 Gçttingen (Germany)
E-mail: hroesky@gwdg.de
Dr. S. De, M. Sc. C. Anusha, Dr. P. Parameswaran
Department of Chemistry,
National Institute of Technology Calicut
NIT Campus P.O., Calicut, Kerala, 673 601 (India)
E-mail: param@nitc.ac.in
À
Al N bond length (average 1.8674 Š) of 1 is shorter than that
of [LAlH2] (average 1.8989 Š).[10] This observation suggests
that the OTf substituent in 1 increases the positive charge at
the aluminum center, which implies a strong Lewis acidic
character of 1.[12] This behavior is consistent with the NBO
charge on Al, which is 1.41 e in [LAlH2] and 1.81 e in
compound 1 (Table S3).
[**] We are grateful to the Beijing Natural Science Foundation (2132044)
and Beijing Higher Education Young Elite Teacher Project
(YETP1191). Support of the Deutsche Forschungsgemeinschaft is
gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201503304.
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Table 1: Hydroboration of aldehydes and ketones catalyzed by 1.[a]
Jones and co-workers recently reported on the catalytic
hydroboration of ketones and aldehydes using Group 14
metal(II) hydrides with two-coordinate metal atoms.[13] In
order to compare the results of the well-defined p-block
metals in this catalysis, we used 1 as an active catalyst for
hydroboration of organic carbonyl groups. As shown in
Scheme 2, the reaction of 1 with one equivalent of benzalde-
Entry
Substrate
Cat.
[mol%]
t
[h]
Yield
[%][b]
2a
2b
0
1
1
1
trace
99%
2c
2d
1
1
1
1
99%
99%
2e
2 f
1
1
1.5
1
99%
99%
2g
2h
1
1
1.5
1.5
99%
71%
Scheme 2. Reaction mechanism for the hydroboration of benzaldehyde
calculated at the M06/def2-TZVPP/BP86/def2-SVP level of theory.[11b]
2i
2j
1
2
6
6
99%
51%
hyde (PhCHO) is rapid at ambient temperature and affords
the hydrometalation product [LAl(OCH2Ph)(OTf)] (Int-3),
1
in essentially quantitative yield (as determined by H NMR
2k
2l
2
1
6
1
58%
99%
spectroscopic analyses of the reaction mixture). Finally, Int-3
was reacted with a stoichiometric amount of pinacolborane
(HBpin) at room temperature in C6D6. The formation of the
desired molecule PhCH2OBpin (Pdt-1) was monitored by
1H NMR spectroscopy, and shows that s-bond exchange
[a] All reactions carried out in [D6]benzene at room temperature using
1 equiv of HBpin. [b] Conversion was determined by NMR spectroscopy
on the basis of the consumption of the aldehyde/ketone, and the identity
of the product was confirmed by RCH2OBpin or RR1CHOBpin reso-
nances.
À
À
occurs between the Al O bond of the catalyst with the B H
bond of pinacolborane. To the best of our knowledge, this is
the first example of an organoaluminum hydride acting as
a catalyst for the hydroboration of organic carbonyl groups. In
contrast, the uncatalyzed hydroboration of benzaldehyde
showed little reactivity (Table 1, entry 2a), while the addition
of only 1 mol% of 1 led to full conversion to [PhCH2OBpin]
within 1 h at room temperature (Table 1, entry 2b).
bipyramidal geometry at the Al center, which has PhCHO
and one of the N atoms of the b-diketiminato ligand at the
À
We have carried out quantum mechanical calculations at
the M06/def2-TZVPP//BP86/def2-SVP level of theory to
explore the reaction mechanism (Scheme 2 and Figure S2).[11]
The first step of the reaction can be considered as the
formation of a weakly bound complex (Int-1), where the
incoming substrate PhCHO approaches the tetrahedral Al
axial positions. The elongated Al H bond (1.608 Š) in Int-2
À
compared to the Al H bond (1.585 Š) in catalyst 1 indicates
a weak bonding interaction of the nucleophilic hydrogen
atom (NBO charge = À0.48 e) with the carbonyl carbon atom
(NBO charge = 0.50 e). The increase in the coordination
number at the Al center from four in 1 to five in Int-2 also
À
À
center from the side opposite to one of the Al N bonds (Al–
contributes to the elongation of the Al H bond. However, the
OOCHPh distance is 3.581 Š). The reaction energy (DE) and
Gibbs free energy (DG) for this step are À4.2 and 6.8 kcal
molÀ1, respectively.[11b] The nucleophilic attack of the carbon-
yl oxygen atom of PhCHO to the electrophilic tetrahedral Al
center (NBO charge on Al = 1.81 e) results in the complex
Int-2 (Al–OPhCHO = 2.090 Š), which has a distorted trigonal-
large distance between the hydrogen atom at the aluminum
and the carbonyl carbon atom is significant (2.839 Š). The DE
(À4.6 kcalmolÀ1) and DG (À1.8 kcalmolÀ1) values for the
formation of Int-2 from Int-1 are slightly negative and the
corresponding barriers (DE° and DG°) are quite low. The
next step is the formation of the tetrahedral Al benzyloxide
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complex (Int-3) by a hydride shift to the carbonyl carbon
atom through a four-membered transition state (TS-Int-2_Int-
3; Figure S2). The DE (À31.7 kcalmolÀ1) and DG (À32.7 kcal
molÀ1) values for this step are large and negative and the
corresponding kinetic barrier is also very high (DE° = 22.1
and DG° = 23.8 kcalmolÀ1). Hence, this step can be consid-
ered as the overall rate-determining step of the hydroboration
reaction (Figure S2). Here, the Al center in catalyst 1 acts as
a hydride donor to the electrophilic carbonyl carbon atom. In
temperature (ca. 238C) and catalyst loading of 3 mol% was
used. The addition of p-tolylaldehyde (1.0 mmol) and PhSiH3
(1.6 mmol) to 1 in C6D6 results in 60% yield of 3e (see the
Supporting Information) within 6 h. We found that butyr-
aldehyde reacts with (EtO)3SiH under similar conditions to
form the desired product 3j in a comparable yield of 60%.
Further products of the hydrosilylation reaction with alde-
hydes are given in the Supporting Information. However, the
results of hydrosilylation are so far not optimized.
À
the next step, the addition of the B H bond of pinacolborane
The addition reaction of trimethylsilyl cyanide (TMSCN)
to either an aldehyde or a ketone to form cyanohydrin
trimethylsilyl ethers (O-trimethylsilyl cyanohydrins) can be
catalyzed by a variety of Lewis acids and bases.[15,16] We found
that 1 is an effective catalyst for the addition reaction of
TMSCN to either aldehydes or ketones. The reaction of
TMSCN with benzaldehyde gives an almost quantitative yield
of product within 1 h with 0.5 mol% loading of the catalyst at
ambient temperature in C6D6 (Table 2, entry 4a). When the
loading of the catalyst is reduced to 0.1 mol% the reaction is
completed within 5 h (Table 2, entry 4b). This reaction does
not proceed without the catalyst.
À
across the Al OOCH Ph bond of Int-3 results in intermediate
2
Int-4 having a four-membered Al-O-B-H ring. The reaction
proceeds through a four-membered transition state (TS-Int-
3_Int-4), which is similar to that of s-bond metathesis
reactions. The B center in this intermediate is tetrahedral
and shows significant B H bond activation (1.308 Š) as
compared to the B H bond (1.209 Š) in pinacolborane. The
Al H bond length in Int-4 is also significantly longer
(1.898 Š) than those in 1 and Int-2. The reaction energy and
Gibbs free energy for this step are positive. Hence, Int-4
undergoes rapid ring opening by the breakage of Al O and
B H bonds to result in the hydroboration product and
regeneration of the catalyst 1. The DG value for this step is
À15.4 kcalmolÀ1 and the kinetic barrier (TS-Int-4_Pdt-1) is
significantly low (DG° = 4.4 kcalmolÀ1). In this step, the
catalyst 1 is regenerated by the addition of hydride from
pinacolborane and the subsequent release of Pdt-1.
We have also calculated the energetics for the pathways
involving ionic intermediates (Schemes S3 and S4 in the
Supporting Information). Since the dissociation of OTfÀ from
the catalyst is a highly endothermic process (DE = 116.5 kcal
molÀ1), we have not considered reaction pathways involving
ionic intermediates for further discussion.
À
À
À
À
À
Quantum mechanical calculations indicate that the initial
nucleophilic attack by PhCHO to give Int-2 (DE = À8.8 and
DG = 5.0 kcalmolÀ1; Scheme 2) and by TMSCN to give Int-B
(DE = À6.1 and DG = 6.2 kcalmolÀ1, Scheme S2) are compa-
Table 2: Addition reaction of TMSCN to aldehydes and ketones
catalyzed by 1.[a]
Encouraged by this result, the scope of the catalytic
reaction with a variety of aldehydes was briefly examined.
Aliphatic aldehydes (Table 1, entries 2c,2d) were investi-
gated for hydroboration activity with HBpin at room temper-
ature using catalyst loadings of 1 mol%, which afforded the
borate esters in essentially quantitative yield. Excellent yields
were also obtained when aldehydes with electron-withdraw-
ing or electron-donating aromatic groups were investigated
(Table 1, entries 2e,2f). The analogous reaction with sub-
strates including heterocycles was also found to be effective
for the hydroboration reaction (Table 1, entries 2g,2h). This
compound underwent hydroboration with a catalyst loading
of 0.5 mol% in 6 h (Table 1, entry 2i). It is obvious from the
results presented (Table 1, entries 2j,2k) that the hydro-
boration of ketones is significantly less efficient than those of
aldehydes. The ketone substituent (R1) reduces the Lewis
basicity of the O center of [LAlOC(R)R1]. In order to
investigate if the rate was affected by bulky aldehydes, the
reaction of trimethylacetaldehyde with HBpin was also
investigated (Table 1, entry 2l).
Entry
Substrate
Product
Cat.
[mol%]
t
[h]
Yield
[%][b]
4a
4b
4c
4d
4e
0.5
0.1
0.5
0.5
0.5
1
5
1
1
1
99%
99%
99%
99%
99%
4 f
0.5
0.5
1
1
99%
99%
4g
4h
4i
2
2
1
1
99%
99%
Recently, Takagi and Sakaki reported on a computational
investigation and predicted that hydrosilylation of the acti-
vated ketone is viable when taking advantage of kinetic and
thermodynamic stabilization of germylene hydride with
three-coordinate germanium.[14] Thus we investigated 1 as
the catalyst for the hydrosilylation of aldehydes. Mild
[a] Aldehyde or ketone, 2 mmol; TMSCN, 3 mmol; at ambient temper-
ature. [b] Yield was obtained according to 1H and 13H NMR spectro-
scopic analysis.
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rable. However, the formation of Int-5 by the reaction of
TMSCN with Int-2 is less favorable (Scheme S3) as compared
to the formation of Int-C by the reaction of PhCHO with Int-
complex undergoes a s bond metathesis reaction with the
À
B H bond of pinacolborane. As a result, catalyst 1 is finally
regenerated by hydride addition. In contrast to the hydro-
À
B (Scheme S2). Moreover, the activation of the Si C bond
boration reaction, catalyst 1 plays a significant role in
À
À
and the addition of the bulky Si(CH3)3 group to the Al O
activating the Si C bond in TMSCN and subsequently
bond of the pentacoordinate Al (Int-6, Scheme S3) are
practically unfeasible from the geometrical point of view.[11b]
Hence, we have considered the reaction pathway for the
facilitates the cycloaddition reaction with PhCHO.
initial attack of TMSCN. The nucleophilic group (TMSCN) Experimental Section
À
Synthesis of 1: A solution of [LAlH2] (0.446 g, 1.0 mmol) in toluene
attacks from the side opposite to that of the Al O bond to
(15 mL) was added dropwise to MeOTf (0.164 g, 1 mmol) in toluene
(15 mL) at 08C with elimination of CH4. After the addition was
complete, the reaction mixture was allowed to warm to room
temperature and stirring was continued overnight. The solvent was
removed in vacuo, and the crude product was crystallized from
result in a weakly bound intermediate (Int-A) that rearranges
to Int-B (Figure S4). Int-B possesses a trigonal bipyramidal
geometry at the Al center, where OTf and TMSCN occupy
À
the axial positions. The longer Si C (1.904 Š) bond as
compared to TMSCN (1.886 Š) indicates that the Si C
bond is activated in Int-B. The reaction is exothermic by
3.0 kcalmolÀ1 for this step and the DG value is close to zero
(0.04 kcalmolÀ1) at 298.15 K. The addition of PhCHO to this
intermediate results in another weakly bound complex Int-C.
À
toluene to afford colorless crystals of 1 (0.512 g, 86%); mp 240–
À1
2428C; IR (KBr): n ¼1797 cm (s, Al H); 1H NMR (400 MHz,
À
~
CDCl3, 258C, TMS): d = 7.35–7.25 (m, 6H, Ar-H), 5.35 (s, 1H, g-H),
3.14 (sept, 3JH-H = 6.8 Hz, 2H, CHMe2), 3.01 (sept, 3JH–H = 6.8 Hz, 2H,
3
CHMe2), 1.96 (s, 3H, Me), 1.94 (s, 3H, Me), 1.29 (d, JH–H = 6.8 Hz,
6H, CHMe2), 1.19 (d, 3JH–H = 6.8 Hz, 6H, CHMe2); elemental analysis
calculated for C30H42AlF3N2O3S: C 60.48, H 7.03, N 4.69. Found: C
60.80, H 7.16, N 4.51.
À
The cycloaddition of the Si C s bond of the TMSCN
fragment to the O C bond of PhCHO fragment in Int-C
=
proceeds through a concerted cyclic four-membered transi-
tion state and results in the formation of cyanohydrin
aluminum complex Int-D (Figure S4). The DE and DG
values for this step are negative (À13.8 and À12.0 kcalmolÀ1,
respectively) and the corresponding DE° and DG° values are
29.4 and 32.0 kcalmolÀ1, respectively. This step can be
considered as the rate-determining step for the catalytic
formation of cyanohydrin. The DG° value for the release of
the product is only 2.1 kcalmolÀ1, and the DG value of the
reaction is À3.6 kcalmolÀ1. In contrast to the hydroboration
Keywords: aluminum · homogeneous catalysis · hydrides ·
hydroboration · hydrosilylation
How to cite: Angew. Chem. Int. Ed. 2015, 54, 10225–10229
Angew. Chem. 2015, 127, 10363–10367
´
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À
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PhCHO.
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reaction of TMSCN with a number of aldehydes and ketones.
The results shown in Table 2 indicate that the reaction
involving a variety of aryl aldehydes worked well with 1 as
a catalyst. Thus, reaction of p-tolylaldehyde or phenylacrolein
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the desired product in 99% yield (Table 2, entry 4e). We also
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À
PhCHO. The Al O bond of the resulting Al alkoxide
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[10] C. Cui, H. W. Roesky, H. Hao, H.-G. Schmidt, M. Noltemeyer,
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[11] a) See the Supporting Information for the details of computa-
tional methodology and related references; b) we could not
locate a transition state for rearrangement of Int5 to Int6 or
directly to the product and the catalyst 1 (Scheme S3).
Received: April 12, 2015
Revised: May 28, 2015
Published online: June 18, 2015
Angew. Chem. Int. Ed. 2015, 54, 10225 –10229
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 10229