An Aluminum Dihydride Working as a Catalyst in Hydroboration and Dehydrocoupling

Article abstract of DOI:10.1021/jacs.6b00032

The well-defined aluminum dihydride LAlH₂ (L = HC(CMeNAr)₂, Ar = 2,6-Et₂C₆H₃) (1) operates in catalysis like a transition metal complex. The catalytic activity of 1 for hydroboration of terminal alkynes was investigated. Furthermore, catalyst 1 effectively initiated the dehydrocoupling of boranes with amines, thiols, and phenols, respectively, to form compounds with B-E bonds (E = N, S, O) under elimination of H₂. Quantum mechanical calculations indicate that hydroboration and dehydrocoupling reactions occur via three consecutive cycloaddition reactions involving the activation of the X-H (X = Al, B, C, and O) σ-bonds.

Full text of DOI:10.1021/jacs.6b00032

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Communication
An Aluminum Dihydride Working as a Catalyst
in Hydroboration and Dehydrocoupling
Zhi Yang, Mingdong Zhong, Xiaoli Ma, Karikkeeriyil Nijesh,
Susmita De, Pattiyil Parameswaran, and Herbert W. Roesky
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b00032 • Publication Date (Web): 10 Feb 2016
Downloaded from http://pubs.acs.org on February 10, 2016
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An Aluminum Dihydride Working as a Catalyst in Hydroboration
and Dehydrocoupling
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Zhi Yang,*,† Mingdong Zhong,† Xiaoli Ma,† Karikkeeriyil Nijesh,# Susmita De# and Pattiyil
Parameswaran,*,# and Herbert W. Roesky*,‡
†School of Chemical Engineering and Environment, Beijing Institute of Technology, 100081 Beijing, China
‡Institut für Anorganische Chemie der GeorgꢀAugustꢀUniversität Göttingen, Tammannstrasse 4, Dꢀ37077 Göttingen,
Germany
#Department of Chemistry, National Institute of Technology Calicut, NIT Campus P.O., Kozhikhode 673 601 Kerala, India
Supporting Information Placeholder
lineꢀearth metals, respectively, while the related catalytic reacꢀ
tions with group 13 elements have not been mentioned.
ABSTRACT: The wellꢀdefined aluminum dihydride LAlH2
(L=HC(CMeNAr)2, Ar=2,6ꢀEt2C6H3) (1) operates in catalysis like
a transition metal complex. The catalytic activity of 1 for
hydroboration of terminal alkynes was investigated. Furthermore,
catalyst 1 effectively initiated the dehydrocoupling of boranes
with amines, thiols, and phenols, respectively to form compounds
with B−E bonds (E=N, S, O) under elimination of H2. Quantum
mechanical calculations indicate that hydroboration and
dehydrocoupling reactions occur via three consecutive
cycloaddition reactions involving the activation of the XꢀH (X =
Al, B, C, and O) σꢀbonds.
Recently, Stephan and coꢀworkers reported on the Lewis acidic
phosphonium salt [(C6F5)3PF][B(C6F5)4] which functioned as a
main group catalyst effecting the dehydrocoupling of silanes with
amines, thiols, phenols, and carboxylic acids, respectively, to
form compounds containing the Si−E bond (E=N, S, O) by elimiꢀ
nation of H2.10 Our group revealed the hydridic reactivity of
LAlH(OTf) (L=HC(CMeNAr)2, Ar=2,6ꢀiPr2C6H3) with carbonyl
compounds featuring the electrophilic Al center, which interacted
with a polar σꢀbond of the substrates.11 In 2012, we reported on
three structures containing the Al−E−C moiety using just the reꢀ
verse polarity of the hydrides in LAlH2 and the proton in C−EH
compounds ( E= N, S, O).12 Previously the reactions of alumiꢀ
num(III) hydrides with alkynes and their application for bond
activation have been proposed.13 Inspired by these results, we
used the LAlH2 compound as the catalyst for the formation of
compounds with B−E (E= C, N, S, O) bonds. Herein, we report
on the main group metal hydride catalyzed addition of HBPin to
compounds with the carbonꢀcarbon triple bond and the dehydroꢀ
coupling of boranes with amines, thiols, and phenols, respectively,
with LAlH2 (1) (L=HC(CMeNAr)2, Ar=2,6ꢀEt2C6H3), as a
precatalyst (Scheme S1).
In quest of a new generation of commercial useful and enviꢀ
ronmentally benign catalysts for important chemical transforꢀ
mations main group compounds are promoted as viable alternaꢀ
tives to transition metal catalysts.1 Organoboron compounds are
versatile precursors in organic synthesis due to their application as
nucleophilic reagents in C−C bond forming reactions.2 Conseꢀ
quently, great efforts have been accomplished on the development
of tools for the preparation of organoborane derivatives.3 A numꢀ
ber of research groups have investigated catalytic hydroboration
reactions with transition metal complexes, in order to reduce the
cost of the hydroboration catalysts, to complement the selectivity
of the late transition metals systems, and to discover new catalytic
pathways.4 Recently, Chirik et al. reported on alkyl compounds
with lowꢀcoordinate cobalt and iron dinitrogen complexes, respecꢀ
tively as catalyst for alkene and alkyne hydroboration.5 Nikonov
and coꢀworkers described the catalytic addition of HBCat with
nitriles to form the bis(borylated) amines RCH2N(BCat)2.6 These
methods directly add the borane only to unsaturated precursors for
the reason, that an initial oxidative addition of the B−H bond to
the metal center is followed by the πꢀcoordination of the unsatuꢀ
rated C≡C and C≡N bond, respectively to the metal center. In
2015 Hill et al. reported on dehydrocoupling reactions between
deactivated boranes (HBPin or 9ꢀborabicyclo[3.3.1]nonane) with
a number of amines and anilines to form aminoboranes by an
alkalineꢀearth metal catalyst.7 Sadow et al. published magnesiumꢀ
catalyzed ester hydroboration reactions, which rapidly and effiꢀ
ciently provide alkoxy borane products via ester cleavage.8 Nolan
and coꢀworkers used a ruthenium based catalyst, that demonstratꢀ
ed a great ability to catalyze the coupling of thiols with boranes.9
However, all of the reported methods focus on transition and alkaꢀ
At the outset of this investigation, the stoichiometric reaction of
1 with HBPin and the terminal alkyne (PhCCH), respectively, was
carried out as a feasibility study for the proposed catalytic protoꢀ
col.14 Encouraged by this result, the initial catalytic experiments
were conducted with LAlH2 (1) showing the utility of the hydrobꢀ
oration with a variety of alkynes. Stirring an anhydrous CDCl3
solution containing a 1.2:1 mixture of phenylacetylene and HBPin
in the presence of 3 mol% of 1 at 30 °C for 12 h produced a 73%
conversion to the (E)ꢀvinylboronate ester arising from antiꢀ
Markovnikov addition of the borane (Scheme 1, product 1). Good
yield and (E) selectivity were observed for the propargyl ethers,
propargyl arylꢀ, and alkylꢀsubstituted alkynes, respectively
(Scheme 1, products 2ꢀ9). More electronꢀrich alkynes with secꢀ
ondary alkyl substituents underwent slower hydroboration and
required neat conditions and a reaction time of 32 h. Aryl substiꢀ
tuted alkynes with electronꢀdonating (pꢀOMe) and electronꢀ
withdrawing (mꢀF) groups maintained high (E) selectivity and
produced good yields (Scheme 1, products 2 and 3). Under the
same conditions aliphatic alkynes showed higher yields than those
of aromatic alkynes in the catalytic system, while they failed to
react with internal alkynes to produce single hydroboration prodꢀ
ucts (Scheme 1, products 10 and 11).
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sistent with the longer reaction time of the hydroboration reacꢀ
Scheme 1. Hydroboration of Alkynes Catalyzed by 1
1
2
3
4
tions (Scheme 1). The overall reaction proceeds through three
successive metathesis reactions and the energetics indicate that
the formation of the aluminum acetylide (Int1) is the overall rate
determiningꢀstep (Figure S3).
In 2014 Jones et al. reported that Ge(II) and Sn(II) hydride
complexes can be efficient catalysts for the hydroboration of carꢀ
bonyl compounds.17 The carbonyl compound was used as subꢀ
strate, and the first step involves the addition of the metal hydride
to the C=O bond. Jones proposed that the metal alkoxide acts as
the intermediate then undergoes a σꢀbond metathesis with HBPin
to generate a borate ester by returning the catalyst. We found that
aluminum hydride LAlH2 (1) can react with phenol to form the
metal alkoxide under elimination of H2.12 1 was also used to cataꢀ
lyze the dehydrocoupling of boranes with phenols. The reactions
were carried out in CDCl3 with 5 mol% loading of the catalyst at
ambient temperature. Scheme 3 (products 1ꢀ10) summarizes the
investigation of 1 to catalyze the dehydrocoupling of phenols with
HBPin and 9ꢀBBN, respectively. Moreover, we also observed that
both 9ꢀBBN and less Lewis acidic HBPin exhibit high reactivity
patterns. In general, these reactions were complete in 1 h with a
noticeable bubbling of hydrogen.
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Hydroboration of terminal alkynes with LAlH2. Reaction
conditions: 3 mol % of LAlH2, 30 °C, in CDCl3, 12 h. The
E selectivity was determined by NMR spectroscopy. Reꢀ
ported numbers are the NMR yields. Reaction time 32h
with neat substrate.
a
Scheme 3. Dehydrocoupling of Boranes with Amines, Pheꢀ
nols, and Thiols Catalyzed by 1
Scheme 2. Reaction Mechanism for the Hydroboration of
Phenylacetylene
O
H
B
O
O
O
E
B
R
2 equiv
or
E
+
H
R
5 mol%
1
2 equiv
or
r.t., CDCl3
H
B
B
E= N, O, S
H
E
B
R
R= alkyl or aryl group
1 equiv
O
O
R
O
O
R
R
R
R
F
O
Cl
MeO
Me
a3, HBPin 99%
a5, HBPin 99%
a9, HBPin 99%
a10, 9-BBN 99%
a7, HBPin 99%
a8, 9-BBN 99%
a1, HBPin 99%
a2, 9-BBN 99%
a4, 9-BBN 99% a6, 9-BBN 99%
A mechanism for the aluminum catalyzed hydroboration of
terminal alkynes at the M06/Def2ꢀTZVPP//BP86/Def2ꢀSVP level
of theory15 is presented in Scheme 2 (Figure S3). The molecular
electrostatic potential (Figure S2c) and NBO charge analysis (Taꢀ
ble S2) on 1 show that the Al atom (qAl = 1.40 e) is electrophilic
and hydrogen atoms (qH = ꢀ0.41 e) connected to the Al center are
nucleophilic (Table S2). Note that, hydrogen atom connected to
the sp hybridized carbon in phenylacetylene is acidic (qH = 0.23 e)
and the hydrogen atom connected to boron in HBPin is basic (qH
= ꢀ0.11 e). Accordingly, the first step is the σꢀbond metathesis
reaction via concerted [2σ + 2σ] cycloaddition of HꢀCCCPh of
phenylacetylene across the AlꢀH bond leading to the aluminum
acetylide (Int1) and H2. Even though, this step demands a high
kinetic energy barrier (ꢁE‡ = 33.5 and ꢁG‡ = 43.7 kcal/mol), it is
exothermic by 13.4 and exergonic by 11.1 kcal/mol. Int1 acts as
the active species for the catalyst cycle.16 The second step (ꢁE = ꢀ
26.2 and ꢁG = ꢀ11.0 kcal/mol) is [2σ + 2π] cycloaddition reaction
where the B−H bond of pinacolborane (HBPin) adds to the C≡C
bond of Int1. The energy barrier for the formation of the resulting
intermediate (Z)ꢀalkene (Int2) is very high (ꢁE‡ = 26.2 and ꢁG‡ =
40.4 kcal/mol). The AlꢀC bond in Int2 is highly polarized (qAl =
1.65 e and qC = ꢀ0.89 e) as compared to the AlꢀH bond in 1,
which in turn favors the σꢀbond metathesis [2σ + 2σ] reaction by
the addition of the AlꢀC bond with the HꢀCCCPh bond of the seꢀ
cond molecule of phenylacetylene resulting in the product (E)ꢀ
alkene (Pd1) and the active catalyst Int1. Similar to the previous
two cycloaddition steps, the final step is also thermodynamically
favorable but involves a high kinetic energy barrier. This is conꢀ
S
R
S
S
S
R
R
R
R
S
F
Me
Cl
b11, HBPin 50%
c12, 9-BBN 93%
b15, HBPin trace
b16, 9-BBN 90%
b13, HBPin trace
b14, 9-BBN 87%
b17, HBPin 61%
b18, 9-BBN 90%
b19, HBPin 63%
b20, 9-BBN 95%
H
N
R
R
R
N
O
O
N
or
B
R =
B
H
b21, HBPin 32%
b22, 9-BBN 90%
b23, HBPin 99%
b24, 9-BBN 99%
b25, HBPin 99%
b26, 9-BBN 99%
Reaction condition: 5 mol% of LAlH2, at rt, in CDCl3. Reꢀ
a
ported numbers are the yields by NMR. Reaction time 1h
b
with neat substrate. Reaction time 24 h. cReaction time 18
h.
This facile dehydrocoupling route to organoboranes can be
used to access other useful compounds. For example, through the
hydridic (B−H) and the protic (E−H, E=N, O, S) properties of
hydrogen atoms the elimination of dihydrogen was observed. In
this case the Al center is attacked by the Lewis base (N, O, S) to
generate the corresponding transient fourꢀmembered species. Iniꢀ
tially, 5 mol% of 1, C6H5CH2SH, and HBPin were allowed to
react for 24 h to result in a 50% yield at rt (Scheme 3, product 11).
Interestingly, when HBPin was used instead of the enhanced Lewꢀ
is acidic 9ꢀBBN, the reaction was complete within 18 h to result
in 93% conversion (Scheme 3, product 12). In contrast, HBPin
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reactions with aryl substituted thiophenols bearing electronꢀ
acetylene to the AlꢀH bond of 1 in the first step of hydroboration
1
2
3
4
5
6
7
8
withdrawing (mꢀF and mꢀCl) groups are not yielding the desired
products even by increasing the reaction time (Scheme 3, products
13 and 15). However, the more Lewis acidic 9ꢀBBN provided
higher yield and required neat conditions and longer reaction time
(24 h) (Scheme 3, products 14 and 16). In case of the reaction of
(pꢀMe) thiophenol with HBPin and 9ꢀBBN, respectively, dehyꢀ
drocoupling of the SH group with borane resulted in 61% and
90% yield, respectively (Scheme 3, products 17 and 18). As
shown in Scheme 3, under the same condition butanethiol gave
nearly the same yield compared to that of thiophenol (Scheme 3,
products 19 and 20).
Aminoboranes are used in a variety of interesting transforꢀ
mations, although the activation of the N−H bond often relies on
the oxidative addition of the N−H to a transitionꢀmetal center.18
Thus, the metal catalytic dehydrocoupling of amines and boranes
is quite limited.4 Berben and coworkers reported on the heterolytꢀ
ic activation of amines by Al pincer complexes via a metal−ligand
cooperative pathway.19 Herein, we used the aluminum hydride
effect as the catalyst for the synthesis of aminoboranes from
readily available amine and borane precursors. The reaction of nꢀ
butylamine with HBPin gives a high yield of product within 24 h
with 5 mol% loading of the catalyst at ambient temperature in
CDCl3. Encouraged by this result, we studied the scope of the
aluminumꢀcatalyzed dehydrocoupling reactivity. As shown in
Scheme 3, the weak Lewis acid HBPin was observed to couple
readily with primary amine (aniline and butyl amine) and secondꢀ
ary amine, respectively (Scheme 3, products 21, 23, 25). However,
failed to perform in the reaction of two molar equiv of borane
with one equiv of amine and aniline, respectively, providing conꢀ
version into the bisborylated amine, albeit with gentle heating and
slightly extended reaction time. The same result can also be obꢀ
served for the more Lewis acidic 9ꢀBBN. While aromatic and
aliphatic amines coupled readily with one molar equiv of 9ꢀBBN
resulting in higher yields than those of HBPin under the same
conditions (Scheme 3, products 22, 24, 26).
reaction (Scheme 2). In this reaction, the intermediate Inta can be
considered as the active species for the catalytic cycle.20 The next
step is the [2σ + 2σ] cycloaddition of the HꢀB bond of pinacolꢀ
borane to one of the AlꢀO bond in Intb resulting in an intermediꢀ
ate, Intc having a fourꢀmembered AlꢀOꢀBꢀH ring. This step is
slightly endothermic (ꢁE = 7.8 kcal/mol) and endergonic (ꢁG =
23.2 kcal/mol). The final step is the concomitant transfer of hyꢀ
dride of the HBPin unit to the Alꢀcenter as well as the cleavage of
the Al–O bond leading to the dehydrocoupling product Pd2 and
regeneration of the active catalyst Inta. This step is exergonic
(ꢁG = ꢀ13.9 kcal/mol) and involves a low kinetic energy barrier
(ꢁE‡ = 6.7 and ꢁG‡ = 5.7 kcal/mol). The overall energetics indiꢀ
cate that the formation of the active catalyst, Inta, from comꢀ
pound 1 is the rate determiningꢀstep. However, the slight endoꢀ
thermicity/endergonicity observed in the formation of the dehyꢀ
drocoupling product is duly compensated by the subsequent forꢀ
mation of the stable intermediate, Intb. This can be correlated
with the less reaction time of the dehydrocoupling reactions as
compared to the hydroboration reactions.
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In conclusion, an aluminum hydride catalyzed method for hyꢀ
droboration of terminal alkynes and dehydrocoupling of boranes
with amines, phenols, and thiols, respectively was developed. The
quantum mechanical calculations indicate that the dehydrocouꢀ
pling reaction is found to be kinetically more favorable than the
hydroboration reaction.
ASSOCIATED CONTENT
Supporting Information
Complete experimental and computational details, representative
NMR spectra, crystallographic data for 1 in a cif format and the
Cartesian coordinates of all calculated molecules. The Supporting
Information is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
Scheme 4. Reaction Mechanism for the Dehydrocoupling
of pꢀFluoroꢀphenol with HBPin
*Eꢀmail: zhiyang@bit.edu.cn.,*Eꢀmail: hroesky@gwdg.de.
*Eꢀmail: param@nitc.ac.in
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The support of the Beijing Natural Science Foundation (2132044),
Deutsche Forschungsgemeinschaft (RO224/64ꢀ1), and Departꢀ
ment of Science and Technology, India are highly acknowledged.
REFERENCES
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The reaction mechanism of the dehydrocoupling of pꢀfluoroꢀ
phenol with pinacolborane (HBPin) at the M06/Def2ꢀ
TZVPP//BP86/Def2ꢀSVP level of theory (Scheme 4) was also
explored.15 The first and second step of the reaction are the two
consecutive [2σ + 2σ] σꢀbond metathesis involving addition of
each of the AlꢀH bonds of 1 across the O–H bond of two moleꢀ
cules of pꢀfluoroꢀphenol through fourꢀmembered transition state
(Scheme 4, Figure S5). The formation of the resultant intermediꢀ
ates Inta and Intb occurs with the liberation of H2. The addition
of more polar OꢀH bonds is thermodynamically and kinetically
favorable as compared to the addition of the CꢀH bond of phenylꢀ
(5) (a) Obligacion, J. V.; Neely, J. M.; Yazdani, A. N.; Pappas, I.; Chirik,
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S. P.; Chirik, P. J. J. Am. Chem. Soc. 2014, 136, 4133. (c) Obligacion, J.
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(14) We assume that 1 is the active species in the stoichiometric and cataꢀ
lytic reactions reported herein (see Schemes S2ꢀS5).
1
2
3
4
5
6
7
8
(15) See supporting information for details of computational methodology.
(16) We have explored the alternate reaction mechanisms. (a) The energetꢀ
ics of the reaction mechanism in which two AlꢀH bonds of the catlayst 1
are involved are found to be less favorable (Scheme S22 and Figure S4).
(b) Scheme 24 depicts the activation of phenyl acetylene by metalꢀligand
cooperation, where both the metal and one nitrogen atom of the betaꢀ
diketiminate ligand are involved for the activation process. The initial
steps of this pathway are eneretically less favorable as compared to those
in Scheme 2 and also leads to the breakage of the AlꢀN bond of the cataꢀ
lyst (Scheme S24).
(10) Pérez, M.; Caputo, C. B.; Dobrovetsky, R.; Stephan, D. W. Proc.
Natl. Acad. Sci. U. S. A. 2014, 111, 10917.
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Roesky, H. W. Angew. Chem., Int. Ed. 2015, 54, 10225.
(12) Yang, Z.; Hao, P.; Liu, Z.; Ma, X.; Roesky, H. W.; Sun, K.; Li, J.
Organometallics 2012, 31, 6500.
(17) Hadlington, T. J.; Hermann, M.; Frenking, G.; Jones, C. J. Am. Chem.
Soc. 2014, 136, 3028.
(18) (a) Fafard, C. M.; Adhikari, D.; Foxman, B. M.; Mindiola, D. J.;
Ozerov, O. V. J. Am. Chem. Soc. 2007, 129, 10318. (b) Zhao, J.; Goldman,
A. S.; Hartwig, J. F. Science 2005, 307, 1080. (c) Kanzelberger, M.;
Zhang, X.; Emge, T. J.; Goldman, A. S.; Zhao, J.; Incarvito, C.; Hartwig, J.
F. J. Am. Chem. Soc. 2003, 125, 13644. (d) Sykes, A. C.; White, P.;
Brookhart, M. Organometallics 2006, 25, 1664.
(19) (a) Myers, T. W.; Berben, L. A. J. Am. Chem. Soc. 2013, 135, 9988.
(20) The alternate reaction mechanism (Scheme S23 and Figure S6) inꢀ
volving an attack of the HꢀB bond of HBPin to the AlꢀO bond of Inta
gives Intb', formation of which is thermodynamically less favorable as
compared to the formation of Intb. The highly exothermic step Inꢀ
ta→Intb governs the catalytic cycle and hence formation of the product.
(13) (a) Zheng, W.; MöschꢀZanetti, N. C.; Roesky, H. W.; Hewitt, M.;
Cimpoesu, F.; Schneider, T. R.; Stasch, A.; Prust, J. Angew. Chem., Int.
Ed. 2000, 39, 3099. (b) Cui, C.; Roesky, H. W.; Hao, H.; Schmidt, H. ꢀG.;
Noltemeyer, M. Angew. Chem., Int. Ed. 2000, 39, 1815. (c) Jancik, V.;
Peng, Y.; Roesky, H. W.; Li, J.; Neculai, D.; Neculai, A. M.; HerbstꢀIrmer,
R. J. Am. Chem. Soc. 2002, 125, 1452. (d) GonzálezꢀGallardo, S.; Jancik,
V.; CeaꢀOlivares, R.; Toscano, R. A.; MoyaꢀCabrera, M. Angew. Chem.,
Int. Ed. 2007, 46, 2895. (e) Franz, D.; Inoue, S. Chem. −Eur. J. 2014, 20,
10645. (f) Franz, D.; Irran, E.; Inoue, S. Dalton Trans. 2014, 43, 4451. (g)
Tan, G.; Szilvási, T.; Inoue, S.; Blom, B.; Driess, M. J. Am. Chem. Soc.
2014, 136, 9732.
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