Lanthanide aryloxides catalyzed hydroboration of aldehydes and ketones

Article abstract of DOI:10.1016/j.catcom.2018.04.014

The lanthanide aryloxides Ln(OAr)₃(THF)₂ (Ar = Ar¹ = 2,6-^tBu₂-4-MeC₆H₂, Ln = Yb (1), Y (2); Ar = Ar² = 2,6-ⁱPr₂C₆H₃, Ln = Y (3); Ar = Ar³ = 2,6-Me₂C₆H₃, Ln = Y (4); Ar = Ar¹, Ln = Sm (5), Nd (6)) could be served as highly efficient catalysts for the hydroboration of aldehydes and ketones with good functional group tolerance and excellent chemoselectivity. Computational studies were carried out to probe a feasible mechanism of the Ln-aryloxides catalyzed hydroboration of aldehydes/ketones.

Full text of DOI:10.1016/j.catcom.2018.04.014

CatalysisCommunications112(2018)26–30
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Lanthanide aryloxides catalyzed hydroboration of aldehydes and ketones
Zhangye Zhu, Ping Dai, Zhenjie Wu, Mingqiang Xue⁎, Yingming Yao, Qi Shen, Xiaoguang Bao⁎
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Dushu Lake Campus, Soochow University,
Suzhou 215123, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Lanthanide aryloxide
Hydroboration
Carbonyl compound
Chemoselectivity
DFT calculation
The lanthanide aryloxides Ln(OAr)3(THF)2 (Ar = Ar1 = 2,6-tBu2-4-MeC6H2, Ln = Yb (1),
Y
(2);
Ar = Ar2 = 2,6-iPr2C6H3, Ln = Y (3); Ar = Ar3 = 2,6-Me2C6H3, Ln = Y (4); Ar = Ar1, Ln = Sm (5), Nd (6))
could be served as highly efficient catalysts for the hydroboration of aldehydes and ketones with good functional
group tolerance and excellent chemoselectivity. Computational studies were carried out to probe a feasible
mechanism of the Ln-aryloxides catalyzed hydroboration of aldehydes/ketones.
1. Introduction
2. Results and discussion
The catalytic hydroboration of unsaturated C-X bond (X = C, N or O
atoms) is a straightforward and atom-economical method for the con-
struction of a variety of boronic esters. Boronic esters are ubiquitous in
organic syntheses and capable of being transformed into various im-
portant functional groups [1–3]. Among these transformations, the se-
lective hydroboration/reduction of carbonyl compounds represents a
powerful tool for the synthesis of alcohols which are present in nu-
merous targets of synthetic interest [4–6].
To date, various organometallic complexes including main group
[7–17] and transition [18–23] metal complexes have been extensively
studied and described to be efficient for carrying out the hydroboration
of carbonyl compounds. However, the use of lanthanide (Ln) complexes
for this transformation is limited in spite of their excellent catalytic
performance [24–26]. In particular, the Ln-O complexes, as one of the
most important class of Ln complexes, have still remain unexplored in
driving the hydroboration reaction. This is probably due to the strong
affinity of the Ln-O bond of complexes, thus suppressing their activity
[27]. The unique advantages of Ln-O complexes such as relative sta-
bility in air, easy accessibility and their catalytic activities in organic
and polymer science have spurred our interest in exploring their utility
in hydroboration reactions.
2.1. Catalytic hydroborations
The catalysts Ln(OAr)3(THF)2 (Ar = Ar1 = 2,6-tBu2-4-MeC6H2,
Ln = Yb (1),
Y
(2); Ar = Ar2 = 2,6-iPr2C6H3, Ln = Y (3);
Ar = Ar3 = 2,6-Me2C6H3, Ln = Y (4); Ar = Ar1, Ln = Sm (5), Nd (6))
were synthesized according to previously described methods [28–30]. A
preliminary study was carried out using 1 of 0.01 mol% together with a
1:1 M ratio of acetophenone and pinacolborane (HBpin) under ambient
temperature (25 °C). Pleasingly, the hydroboration can occurr within
15 min as evidenced by NMR analysis (Table 1, entries 1–4), while there
is no reaction in the absence of a catalyst which is consistent with
previous studies [24,31]. Furthermore, the screening of solvents in-
dicates THF to be superior to toluene and hexane. It should be noted
that the reaction can also take place under neat condition and give the
product with similar yield as in THF (Table 1, entries 3 and 4). In ad-
dition, the steric effect of the OAr ligand on the catalytic activity was
evaluated (Table 1, entries 5–7). The catalytic performance could be
significantly improved by using bulkier ligands, which is consistent
with results from analogous Ln-OAr catalysts in driving the reaction of
amines with carbodiimides [32]. The screening of the central Ln metals
demonstrates that the order of catalytic activity follows Y and Yb <
Sm < Nd (Table 1, entries 4–5 and 8–9). The same trend has also been
found in the hydroborations with Ln-Cp complexes [25]. Full conver-
sion could be achieved by increasing the catalyst loading to 0.05 mol%
and adding a slight excess of HBpin (1.1 eq) (entry 13).
Herein, we report a highly efficient hydroboration of aldehydes and
ketones catalyzed by homoleptic Ln-OAr complexes, which exhibit ex-
cellent chemoselectivity and broad functional group compatibility.
With the optimized reaction conditions in hand, the scope of hy-
droboration with respect to a wide range of carbonyl compounds was
explored. In the case of aldehydes, most of the aromatic substrates
⁎
Corresponding authors.
E-mail addresses: xuemingqiang@suda.edu.cn (M. Xue), xgbao@suda.edu.cn (X. Bao).
https://doi.org/10.1016/j.catcom.2018.04.014
Received 27 January 2018; Received in revised form 16 April 2018; Accepted 17 April 2018
Availableonline19April2018
1566-7367/©2018ElsevierB.V.Allrightsreserved.

Z. Zhu et al.
CatalysisCommunications112(2018)26–30
Table 1
Table 2
Optimization of the reaction conditions.a
Hydroboration of aldehydes catalyzed by 6.a
Con
vb(%
)
Entry
1
2
3
4
5
6
7
8
Cat. (mol%)
1 (0.01%)
1 (0.01%)
1 (0.01%)
1 (0.01%)
2 (0.01%)
3 (0.01%)
4 (0.01%)
5 (0.01%)
6 (0.01%)
6 (0.05%)
6 (0.01%)
6 (0.03%)
6 (0.05%)
Sol
Hex
Tol
Substrate ratiob
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1.1
1:1.1
1:1.1
Convc (%)
34
54
78
79
77
64
58
88
91
97
93
97
Cat.
(mol%)
Time
(min)
Yields
c (%)
Ent-
ry
Alde-
hydes
Alcohols
–
1
2
3
0.05
0.05
0.05
10
10
10
>99
>99
>99
95
96
93
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
9
10
11
12
13
15
(10)
>99
(95)
4
0.05
93
> 99
5
6
0.05
0.05
20
15
99
91
89
a
Acetophenone, HBpin, and Ln(OAr)3(THF)2 at ambient temperature
>99
(25 °C).
b
c
Acetophenone, HBpin.
Conversion was determined by 1H NMR spectroscopy.
7
0.05
10
>99
87
required only 0.05 mol% of catalyst loading for complete conversion
into the corresponding boronate esters within a short period. For ex-
ample, benzaldehyde can be converted to the target product in just
10 min, which corresponds to a TOF > 11,800 h−1, demonstrating the
most active catalytic system among the lanthanide complexes reported
[24–26]. No steric inhibitory effect is observed with either ortho- or
para- methylbenzaldehyde (Table 2, entries 2 and 3). For a more
sterically hindered substrate such as 2,4,6-trimethylbenzaldehyde, a
slightly longer reaction time is required for full conversion (Table 2,
entry 4). The reaction shows good tolerance toward various functional
groups (Table 2, entries 5–10). For instance, the fluoro-containing
substrate can be completely converted to the desired product after
20 min (Table2, entry 5). The reaction of α, β-unsaturated cinna-
maldehyde affords the single carbonyl reduction product which is in
line with recently reported hydroboration protocols (Table 2, entry10)
[7,25,26,31]. Furthermore, no dearomatization of the hetero ring is
observed with either 2-pyridinecarboxaldehyde or thiophene-2-carbal-
dehyde, demonstrating the exclusive chemoselectivity towards C]O
bond (Table 2, entries 11 and 12). The hydroboration reaction is also
applicable to 2-naphthaldehyde and anthracene-9-carbaldehyde
(Table 2, entries 13 and 14). In addition, the aliphatic substrate hep-
tanal is successfully converted into the target product (Table 2, entry
15). In general, the lanthanide aryloxides catalyzed hydroboration of
aldehydes can produce the boronate esters with excellent yields. Sub-
sequently, the generated boronate esters can be efficiently hydrolyzed
into the corresponding alcohol products.
8
9
0.05
0.05
15
15
>99
>99
94
93
10
11
0.1
0.1
15
30
97
91
88
>99
12
13
14
0.1
0.05
0.1
15
15
30
>99
>99
>99
94
92
90
15
16
0.05
0.05
15
30
>99
>99
92
90
aAldehydes (1 mmol), pinacolborane (1.1 mmol) and 6 (0.05 mol%) at rt., THF
(0.2 mL) for solid substrate.
bConversion of aldehydes based on 1H NMR analysis.
cIsolated yield of 1° alcohol by column chromatography.
catalyst loading is necessary for the quantitative conversion of 2-acet-
ylthiophene (Table 3, entry 11). Finally, the aliphatic substrate 3-me-
thyl-2-butanone (Table 3, entry 12) exhibits comparable reactivity with
the aliphatic aldehyde (Table 2, entry 15). It is worth noting that
complex 6 shows unique compatibility toward OH and NH2 substituted
substrates. Both aldehyde and ketone can undergo full conversion as
well as satisfactory isolation yields of carbonyl groups with NH2 and OH
groups intact (Table 2, entry 16 and Table 3, entry 13), exhibiting the
unique chemoselectivity of complex 6, as those groups are readily re-
liable to BeN, BeO coupling [34]. Moreover, the very low catalyst
loading and short reaction period exhibit dominant advantages over the
previously reported cases [7,26].
To further evaluate the selectivity of hydroboration of aldehydes
versus ketones, a competitive intermolecular reaction involving a
mixture of benzaldehyde and acetophenone was carried out in the
presence of 6, which results in the preferential reduction of the alde-
hyde rather than ketone (Scheme 1). Meanwhile, separate experiments
involving 4-fluorobenzaldehyde, 4-methylbenzaldehyde and heptanal
firmly establish the selectivity of aldehydes over ketones. Notably, near
The lanthanide aryloxides catalyzed hydroboration protocol was
extended to ketones and a range of substrates were examined. All the
reactions can undergo smoothly and give the complete conversions of
various substrates to the corresponding products in high yields using 6
as a catalyst. In general, the reactivity of ketones is lower compared
with aldehydes. For instance, the complete conversion of acetophenone
requires 15 min, while only 10 min is required for benzaldehyde
(Table 2, entry 1 vs Table 3, entry 1). A similar trend is observed with p-
tolualdehyde/o-tolualdehyde
and
p-acetyltoluene/o-acetyltoluene
(Table 2, entries 2 and 3 vs Table 3, entries 2 and 3). This is supported
by DFT calculations which show that the energy barriers are sub-
stantially higher for ketones than for aldehydes [31]. This observation
is also in line with known catalysts [4,7,21,25,26,33], although in
contrast with La[N(SiMe3)2]3 [24]. In the cases of isobutyrophenone
and benzophenone, a higher catalyst loading (0.1 mol%) is required for
full conversion because of the steric hindrance in these substrates
(Table 3, entries 8 and 9). A slightly longer time as well as higher
27

Z. Zhu et al.
CatalysisCommunications112(2018)26–30
Table 3
Hydroboration of ketones catalyzed by 6.a
Scheme 2. Intramolecular chemoselective hydroboration of aldehydes cata-
lyzed by 6.
Cat.
(mol%)
Conv
b (%)
Yieldsc
(%)
Ent-
ry
Keton-
es
Time
(min)
Alcoho-
ls
toward aldehydes in comparison with ketones, which is consistent with
previous reports [7,25,26]. In addition, an intramolecular competition
experiment involving 4-acetylbenzaldehyde was conducted with the
aim of examining the selectivity toward an aldehyde or ketone con-
tained in the same molecule (Scheme 2). The outcome clearly indicates
the reductive hydroboration of aldehyde is almost completely dominant
over ketone, suggesting the performance of chemoselectivity is even
better than those for Ln-N and Ln-Cp complexes [24,25].
0.05
15
15
30
15
>99
>99
>99
>99
92
93
89
91
1
2
0.05
0.05
0.05
0.05
0.05
3
4
5
6
Reaction kinetic experiments were carried out by means of 1H NMR
monitoring using mesitylene as internal standard. The results showed
that the reaction rates were first-order for HBPin and zero-order for
both aldehyde and ketone, respectively. Full details have been ad-
dressed in the Supporting Information.
15
15
>99
>99
87
91
7
8
9
0.1
0.1
0.1
15
15
15
>99
>99
>99
94
89
91
2.2. DFT calculations
Computational studies were carried out to explore a mechanistic
understanding of the Ln-aryloxide catalyzed hydroboration of alde-
hydes/ketones with HBpin (the Y(OAr)3, Ar = Ar3, catalyst model was
employed in the computational study)1. First, the isodestmic Reaction
(1) is computed to be exothermic by 4.7 kcal/mol, indicating that al-
dehyde is more ready than HBpin to coordinate with the Y(OAr)3 cat-
alyst. The subsequent hydroboration was explored based on the initially
formed complex PhCHO…Y(OAr)3 (INT0). Next, two possible me-
chanistic pathways of hydroboration were considered. The first one is
the direct hydroboration via a metathesis reaction with HBpin. The
located transition state (TS1) for this path shows that the distances of
O1…B and C…H are shortened to 1.99 and 1.45 Å, respectively, while
the distances of B…H and C…O1 are lengthened to 1.28 and 1.30 Å,
respectively, leading to the final product in a concerted manner. The
predicted energy barrier for this step is 39.9 kcal/mol (Fig. 1, red line).
Alternatively, the electrophilic HBpin substrate could attack the OAr
ligand of the catalyst to form intermediate INT2. Subsequently, the
hydride of the pin(OAr)BH− group in INT2 could undergo a migration
step to the electrophilic C site of the carbonyl group of aldehyde to form
INT3. Next, the formed alkyloxide group in INT3 could undergo nu-
cleophilic attack to the electron deficient site of pinB-OAr to afford
INT4. Then, the cleavage of the BeO bond in INT4 can produce the
10
0.05
15
>99
93
11
12
0.1
20
15
>99
>99
95
86
0.05
13
0.5
60
>99
87
aKetones (1 mmol), pinacolborane (1.1 mmol) and 6 (0.05 mol%) at rt., THF
(0.2 mL) for solid substrates.
bConversion of ketones based on 1H NMR analysis.
cIsolated yield of 2° alcohol by column chromatography.
quantitative aldehyde hydroboration product is afforded in the last
example, displaying superior selectivity over Ln-N and Ln-Cp complexes
[24,25]. Generally, the catalyst 6 displays outstanding chemoselectivity
Scheme 1. Intermolecular chemoselective hydroboration of aldehydes catalyzed by 6.
1
The B3PW91 density functional method was used to carry out the computational
studies (see SI for computational details).
28

Z. Zhu et al.
CatalysisCommunications112(2018)26–30
Fig. 1. Energy profiles (in kcal/mol) for the Y(OAr)3 catalyzed hydroboration of benzaldehyde with pinacolborane. Bond lengths are shown in Å. (For interpretation
of the references to color in this figure, the reader is referred to the web version of this article.)
desired product and regenerate the catalyst. The computational results
show that the rate determining step for the stepwise pathway is the
hydride transfer step and the energy barrier is predicted to be
12.4 kcal/mol lower in energy than the former concerted path, in-
dicating that the stepwise mechanism is more favorable for the Y
(OAr)3-catalyzed hydroboration with aldehyde. The highly efficient Y
(OAr)3 catalyst in driving the hydroboration of aldehyde could be ra-
tionalized by the effective capture of the two substrates, the metal
center for PhCHO and the aryloxide ligand for HBpin, respectively.
Therefore, the hydroboration of aldehyde could take place in a stepwise
intramolecular style between the activated and HBpin (A possible re-
action pathway is proposed in Scheme 3).
HBpin‐‐‐Y(OAr)3 + PhCHO → PhCHO‐‐‐Y(OAr)3 + HBpin
(1)
In addition, the chemoselective hydroboration of aldehyde over
ketone was also computationally evaluated. Computational results
show that the energy barrier of the rate limiting step for ketone sub-
strate is ca. 5 kcal/mol higher in energy than that of the aldehyde,
suggesting that ketone is less reactive than aldehyde (SI, Fig. S1). Due
to the presence of methyl group, the C atom of the carbonyl group in
ketone is less electrophilic than that in aldehyde. Thus, the preferential
hydroboration of aldehyde over ketone is observed experimentally.
3. Conclusions
Scheme 3. Proposed catalytic cycle.
In this work, a highly efficient hydroboration of aldehydes and
29

Z. Zhu et al.
CatalysisCommunications112(2018)26–30
ketones catalyzed by lanthanide aryloxides was developed. This pro-
tocol demonstrates broad functional group compatibility and excellent
chemoselectivity. Computational studies suggest that the Ln-aryloxide
catalysts could effectively trap both substrates, the Ln metal for car-
bonyl compounds and the aryloxide ligand for HBpin, respectively.
Thus, the hydroboration of carbonyl group could occur in an in-
tramolecular stepwise style via the hydride transfer and the subsequent
alkyloxide migration steps to produce the final product.
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