Interconverting Lanthanum Hydride and Borohydride Catalysts for C=O Reduction and C?O Bond Cleavage

Article abstract of DOI:10.1002/anie.201813305

The high catalytic reactivity of homoleptic tris(alkyl) lanthanum La{C(SiHMe₂)₃}₃ is highlighted by C?O bond cleavage in the hydroboration of esters and epoxides at room temperature. The catalytic hydroboration tolerates functionality typically susceptible to insertion, reduction, or cleavage reactions. Turnover numbers (TON) up to 10 000 are observed for aliphatic esters. Lanthanum hydrides, generated by reactions with pinacolborane, are competent for reduction of ketones but are inert toward esters. Instead, catalytic reduction of esters requires activation of the lanthanum hydride by pinacolborane.

Full text of DOI:10.1002/anie.201813305

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Accepted Article
Title: Interconverting Lanthanum Hydride and Borohydride Catalysts for
C=O Reduction and C–O Bond Cleavage
Authors: Smita Patnaik and Aaron David Sadow
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201813305
Angew. Chem. 10.1002/ange.201813305
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http://dx.doi.org/10.1002/ange.201813305

10.1002/anie.201813305
Angewandte Chemie International Edition
COMMUNICATION
Interconverting Lanthanum Hydride and Borohydride Catalysts
for C=O Reduction and C–O Bond Cleavage
Smita Patnaik, and Aaron D. Sadow*[a]
Abstract: The high catalytic reactivity of homoleptic tris(alkyl)
[M]
O
O
H
lanthanum La{C(SiHMe2)3}3 is highlighted by C–O bond cleavage in
the hydroboration of esters and epoxides at room temperature. The
catalytic hydroboration tolerates functionality typically susceptible to
insertion, reduction, or cleavage reactions. Turnover numbers (TON)
up to 10 000 are observed for aliphatic esters. Lanthanum hydrides,
generated by reactions with pinacolborane, are competent for
reduction of ketones but are inert toward esters. Instead, catalytic
reduction of esters requires activation of the lanthanum hydride by
pinacolborane.
[M]
H
C
+
R
R
R
R
A.
increasing
hydridicity
more inert
alkoxide
challenging conversion
to metal hydride
E–OCR2H
E–OCR2H
E–H
metal hydride
reformed by
ether dissociation
O
[M]
E
H
H
H
avoids
metal
alkoxide
R
R
[M]
[M]
H
Strategies that rely on highly nucleophilic metal hydrides to
affect catalytic reduction and C–O bond cleavage of unsaturated
oxygenates are limited by the stability of the corresponding metal
alkoxide intermediates, which impedes regeneration of the metal
hydride (Scheme 1A).[1] More reactive catalysts are needed for
conversions of inert substrates, such as esters and ethers, and
new mechanisms are needed to ameliorate the thermodynamic
barriers associated with breaking strong M–O bonds. One
approach that could avoid M–O bond formation involves an
activating interaction of the hydride source with the electrophilic
catalytic center (Scheme 1B).
B.
E
E
+
H
O
H
increased
hydridicity
R
R
Scheme 1. Conceptual approach for kinetically-enhanced catalytic reduction of
oxygenates.
catalyzed by a large number of metal complexes,[7] fewer
catalysts, mainly magnesium[2f,
compounds,[9] mediate hydroboration of esters. Moreover,
hydroboration of epoxides is uncommon, with reports of only a
nickel catalyst (involving C–O oxidative addition)[10] and an iron
catalyst limited to aryl-substituted substrates.[11]
4,
8]
and molybdenum
Pinacolborane (HBpin) is promising in this regard because it
generates metal hydrides upon reaction with organometallic
compounds, and it also forms adducts with nucleophilic metal
hydrides.[1-2] For example, hydride species Cp*2LaH and
In the present contribution, we demonstrate that the easily
synthesized tris(alkyl)lanthanum compound La{C(SiHMe2)3}3
(1)[12] catalyzes the hydroboration of esters and epoxides. With 5
mol % 1 at room temperature, alkyl esters, alkyl benzoates, and
aryl benzoates are reduced to boryl esters. Hydrolysis gives the
corresponding alcohol in high isolated yield (Table 1). Halogen-,
nitro-, alkoxy-, and trifluoromethyl-substituted benzoate esters are
reduced without affecting the functional group. Olefins are
untouched, whereas La{N(SiMe3)2}3 is reported to catalyze
addition of alkenes and HBpin.[13] Electron-poor esters react more
rapidly than electron-rich ones.
{HC(CMeNDipp)2}MgH (Dipp
catalytic intermediates in ketone, aldehyde, imine, carbodiimide,
nitrile, and pyridine hydroborations.[2f,
= 2,6-C6H3iPr2) are proposed
3]
Alternatively, isolable
pinacolborohydrides ToMMg{HXBpin} (ToM = tris(4,4-dimethyl-2-
oxazolinyl)phenylborate; X = H, alkoxide) are proposed in
hydroboration of esters, and the postulated mechanism avoids
M–O intermediates.[4] In contrast, pinacolborohydride compounds
are also proposed as off-cycle states or as precursors to catalyst
deactivation.[2f, 2h, 3c]
Early trivalent lanthanide hydrides often provide high catalytic
activity.[5] For example, La{N(SiMe3)2}3 and LaCp3, which may
provide entry to lanthanum hydrides upon reaction with HBpin,
afford highly active catalysts for hydroboration of ketones and
aldehydes.[6] While hydroboration of aldehydes and ketones is
While the reduction of ethyl acetate is complete within 5 min.
using 5 mol % 1, catalyst loadings down to 0.01 mol % are also
effective. For example, hydroboration using
1 gives 3560
turnovers (TON) within 30 min providing an initial turnover
frequency (TOF) of 7120 h–1, (35% conversion within 1 h), and
quantitative conversion after 2 h (TON = 10,000); ([1]ini = 0.183 ×
10-4 M, [EtOAc]ini = 0.188 M, [HBpin]ini 0.662 M). Aryl esters react
more slowly. For example, hydroboration of benzyl benzoate
using 1 mol % 1 gives full conversion over 24 h, whereas methyl
p-iodobenzoate is reduced by 0.1 mol % 1 in the same time.
[a]
S. Patnaik, Prof. A. D. Sadow
Department of Chemistry and US Department of Energy Ames
Laboratory
Iowa State University
1605 Gilman Hall
E-mail: sadow@iastate.edu; https://sadow.chem.iastate.edu/
Supporting information for this article is given via a link at the end of
the document.
1
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10.1002/anie.201813305
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COMMUNICATION
Table 1. La{C(SiHMe2)3}3 (1) catalyzed hydroboration of esters.
Table 2. Catalytic hydroboration of epoxides.
Reaction[a]
Time (h)
kobs/[1]
Yield[f]
Reaction[a]
Yield[b]
O
0.5
100 (98)
1.352
1
HBpin
1
OBpin
O
OBpin
O
O
100 (92)
HBpin
0.954
OBpin
1
100 (92)
100 (98)
2
1
O
OBpin
98 (85)
98 (95)
100 (98)
O
HBpin
HBpin
O
OBpin
1
O
1.589
2.096
0.658
1
0.5
OBpin
HBpin
O
HBpin
Cl
Cl
Ph
OBpin
1
Ph
Ph
1
Ph
Ph
O
Ph
Ph
1
0.5
100 (98)
100
HBpin
O
OBpin
O
O
6[b]
HBpin
I
I
Ph Ph
24[c]
1
100
+
31:100[c]
98 (78)
HBpin
O
O
Ph OBpin
OBpin
1
1
92 (90)
OBpin
O
HBpin
O
O
OBpin
HBpin
O
1
100 (95)
1
[a] Reaction conditions: 5 mol % catalyst, C6D6, 1.3 equiv. of HBpin, 25 °C, 24 h.
[b] NMR yield was determined with respect to Si(SiMe3)4 as an internal standard,
and isolated yield is given in parenthesis. [c] 60 °C, 5 mol % 1 or 25 °C, 10 mol %
1.
OBpin
OBpin
O
HBpin
O2N
O2N
O
1
0.5
2.219
4.738
98 (95)
O
The catalysis is initiated by reaction of 1 and HBpin, which
instantaneously produces (Me2HSi)3CBpin (11B NMR: 33 ppm). In
contrast, mixtures of 1 and methyl anisate or 2,2-dimethyloxirane,
as representative substrates, contain only starting materials after
1 h at room temperature. Characterization of the lanthanum-
containing product(s) formed in reactions of 1 and HBpin requires
additional analysis. First, quantitative conversion of 1 and 3 equiv.
of HBpin gives 2.8 equiv. of (Me2HSi)3CBpin, and the precipitate
2 (Scheme 2) forms over 7 h. That solid is insoluble in
hydrocarbon solvents (e.g., benzene, pentane) and reacts with
methylene chloride or THF.
HBpin
F3C
F3C
O
1
100 (92)
100
OBpin
0.5
O
Ph
2
HBpin
24[b]
O
1
100 (85)
100
0.16
2[d]
OBpin
O
HBpin
O
1
OBpin
O
100 (96)
90 (72)
0.5
HBpin
Ph
Crude 2 contains small quantities of (Me2HSi)3CBpin (ca. 0.2
Ph
1
equiv. with respect to initial 1), and H NMR spectra of ketone-
O
1
quenched materials revealed unidentified aliphatic signals. The
latter problem is avoided by in situ precipitation of 2 from 1 and 3
1[e]
OBpin
O
Ph
HBpin
equiv. of HBpin. Insoluble 2 reacts quantitatively with 3 equiv. of
[a] Reaction conditions: 5 mol % catalyst in C6D6, 2.5 equiv. of HBpin, 25 °C.
Me2
Me2HSi
Si
H
MeOBpin is the by-product. 1 mol % catalyst. [c] 0.1 mol % catalyst. 0.01
mol % catalyst. [e] PhCH2OBpin is the byproduct. [f] NMR yield was determined
with respect to Si(SiMe3)4 as an internal standard, and isolated yield is given in
parenthesis. kobs/[1] is the ternary rate constant (M–2s–1) obtained from kinetic
experiments with 3.5 equiv. of HBpin at a known [1].
[b]
[d]
SiMe2
H
C
H
H
Me2Si
Me2Si
1 equiv. HBpin
La
3 equiv. HBpin
SiHMe2
C
C
H
SiMe2
SiMe2
Me2HSi
H
– 2.8 equiv.
(Me2HSi)3C–Bpin
– 1 equiv.
(Me2HSi)3C–Bpin
1
On the basis of the high catalytic reactivity of 1 towards esters,
we investigated ring-opening of epoxides to boryl esters, which
would also involve a C–O bond cleavage step. In fact, 1 is an
active catalyst for this process, giving high yields within 1 d at
room temperature (Table 2). The reaction is effective for 2,2-
disubstituted epoxides, 2,3-disubstituted epoxides, styrenic
derivatives, and cycloalkane oxides. Interestingly, trans-2,3-
stilbene oxide gives a single product at room temperature,
whereas the cis isomer gives a mixture of 1,2- and 2,2-diphenyl
ethanol after hydrolytic workup. The latter product results from a
1,2-aryl shift, which occurs both at room temperature and 60 °C.
6 HBpin
2·HBpin
[LaH3]n·(Me2HSi)3C–Bpin
soluble
{(Me2HSi)3C}3-xLaHx
soluble
HBpin
insoluble
3
2
O
O
O
O
Aryl
OMe
Aryl
OMe
Ph
Ph
{(Me2HSi)3C}3-xLa(OCHMePh)x
{La(OCHMePh)3}n
no reaction
no reaction
Scheme 2. Reactivity of 1 and HBpin to give lanthanum hydride species, which
are reactive toward ketones but not esters.
2
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10.1002/anie.201813305
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COMMUNICATION
1
acetophenone to afford soluble species. The H NMR spectrum
of this mixture revealed three doublets at 1.47, 1.42, 1.30 ppm
and a multiplet signal at 5.47 ppm, together assigned to
OCHMePh groups bonded to lanthanum. Signals associated with
PhMeHCOBpin could not be detected in the 11B NMR spectrum
of this mixture, and HBpin was not detected in the in situ
generated samples of 2, ruling out consumption of acetophenone
by hydroboration in these experiments. The product of 2 and
acetophenone was assigned as {La(OCHMePh)3}n on the basis of
hydrolysis which gives HOCHMePh. Compound 2 is tentatively
assigned as containing LaH3 on the basis of the stoichiometry of
its formation and reaction with ketones.[14] This approach was
previously used to assign reactive magnesium hydride.[3a] The
tris(alkoxide)lanthanum species, 1, and 2 are all competent
precatalysts for acetophenone and benzaldehyde hydroboration
in the presence of HBpin. Remarkably, in situ generated 2 and
esters, such as methyl anisate and methyl para-chlorobenzoate,
do not react under these conditions (Scheme 1, right) and 2 does
not dissolve upon addition of esters in the absence of HBpin.
Reaction of 1 and 6 equiv. of HBpin provides a mixture of
(Me2HSi)3CBpin and HBpin (1:0.9 ratio) as determined by 11B
NMR spectroscopy. Compound 2 does not precipitate under
these conditions. Evaporation of the volatile components,
including HBpin, provides a material that is insoluble in benzene
and appears equivalent to 2. Apparently, the presence of HBpin
solubilizes 2 in benzene (Scheme 1, center).
increases over a series of experiments in which [1] and [ester]ini
are held constant (Figure 1). Likewise, a plot of the initial rate
(d[product]/dt) vs [HBpin]ini shows saturation at high
concentrations of pinacolborane (Figure S43). Similar
experiments, in which [1]total and [HBpin]ini are held constant and
[ester]ini is varied, also show saturation behavior.
Figure 1. Plot of k’obs vs [HBpin], fit to the equation: k’obs = k2/{[HBpin] + B} where
the constant B = k-1/k1 +k2[ester]/k1.
In addition, reaction of 1 and 1 equiv. of HBpin produces 1
equiv. of (Me2HSi)3CBpin (based on 1H and 11B NMR
spectroscopy) and soluble La-containing species 3; however, the
1H NMR signals from -C(SiHMe2)3 groups on 1 and in this mixture
appear with equivalent chemical shifts. The product(s) 3 could be
a mixture of {(Me2HSi)3C}2LaH, {(Me2HSi)3C}LaH2, and 1 in
stoichiometry-required ratios, or a combination of 1/3 equiv. of 1
and 2/3 equiv. of 2. The possibility that 3 is actually a mixture of 1
and 2 is ruled out by its reaction with acetophenone, which gives
–OCHMePh signals in the 1H NMR spectrum distinct from those
obtained in the reaction of 2. It is further noteworthy that 3 and
methyl anisate also do not react under these conditions (Scheme
1, left). Thus, the lanthanum hydride species of 2 and 3 are
reactive toward the C=O bond of ketones but not competent for
reaction with esters. As is the case for many catalytic species,
isolation of 2 and 3 in pure form was not possible, and the related
YH3 species has been assigned on the basis of mass balance in
reactions trimethylyttrium and H2AlMes.[15]
Saturation in both [HBpin] and [ester] is consistent with a two-
step mechanism, in which one reactant reversibly binds to the
catalyst, followed by the adduct’s bimolecular combination with
the second reactant in the turnover-limiting step. This mechanism
is described by the rate law of eq. 2, where k’obs of eq. 1 equals
k1k2/{k1[HBpin]+k-1+k2[ester]}.
k1k2 1 ester HBpin
d ester
[ ][
][
]
[
]
=
–
(2)
dt
k HBpin +k–1+k2 ester
[
]
[
]
1
A value of k2 = 1.5 ± 1 M–1·s–1 is determined from a nonlinear
least-squares regression analysis of the data in Figure 1.
These kinetic data provide significant mechanistic insight. First-
order dependence on the [ester] indicates that its cleavage occurs
after the turnover-limiting step (Scheme 3A).[4] In contrast, the
ToMMg-catalyzed hydroboration of esters follows a rate law
exhibiting half-order ester dependence (Scheme 3B).[4]
Further mechanistic insight was sought from in situ-monitored
catalytic experiments. Second-order plots indicate that reaction
rate is directly proportional to the concentration of HBpin and
ester. The pseudo-second-order rate constants (kobs) plotted vs
HBpin
fast
O
cat
OBpin
OCH2R
~[ester]1
HBpin
+
2 RCH2OBpin
A.
[1] reveal
a
linear relationship consistent with first-order
R
OCH2R
R
slow
dependence on catalyst concentration, giving the ternary-order
rate constant k’obs = 1.95 ± 0.86 M–2s–1 for the experimental rate
law in eq. 1.
2 HBpin, cat
O
cat
O
2
~[ester]1/2
2 RCH2OBpin
B.
R
OCH2R
R
H
slow
fast
d ester
[
]
1
–
=k'obs 1 1 ester HBpin 1
(1)
[ ] [ ] [
]
Scheme 3. (A) First-order dependence on [ester] indicates C–O bond cleavage
occurs after the turnover-limiting step, whereas (B) half-order dependence on
ester indicates C–O bond cleavage happens prior to or during the turnover
limiting step.
dt
The rate law is further complicated by inverse dependence on
[HBpin]. The ternary rate constants k’obs decrease as [HBpin]ini
3
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10.1002/anie.201813305
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COMMUNICATION
Two mechanisms are consistent with the experimental rate law.
In the “ester-first” mechanism of Scheme 4A, [La]–H and ester
interact reversibly, likely via insertion/β-H elimination steps,
followed by irreversible reaction of the [La]–OCHR(OR)
intermediate and HBpin. Alternatively, an “HBpin-first”
mechanism involves [La]–H and HBpin reversibly binding to give
a lanthanum hydridoborate [La]{H2Bpin}, followed by irreversible
interaction with ester (Scheme 4B). In both cases, the
combination of ester and HBpin is slower than the second addition
of HBpin and C–O cleavage, as required by the rate law.
of [HBpin] and [ester]. Moreover, the data and the interpretation
of rate law of eq. 2 show that experimental rate laws change
drastically with variations in reactant concentration, with orders in
[HBpin] and [ester] varying from zero to one. In addition, the
apparent inhibition by HBpin is a natural consequence of
reversible formation of [La]{H2Bpin} adducts.
In conclusion, while lanthanum hydride is chemically competent
for hydroboration of ketones, likely via carbonyl insertion and
alkoxide transfer to boron, [La]{H2Bpin} is required for the more
challenging reductions of esters and epoxides.
A.
O
OCH2R
H
k1
+
[La]–H
[La]
O
CH2R
Acknowledgements
R
O
k–1
R
OCH2R
H
k2
This research was supported by the U.S. Department of Energy,
Office of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences, and Biosciences. The Ames Laboratory is operated
for the U.S. Department of Energy by Iowa State University under
Contract No. DE-AC02-07CH11358.
RH2CO
H
[La]
O
+ HBpin
O
Bpin
+ [La]–H
turnover-limiting
R
R
RH2CO
H
2
+
Bpin
HBpin
O
R
OBpin
fast
R
Keywords: hydroboration • C–O bond cleavage • lanthanum
B.
hydride • homogeneous catalysis • saturation kinetics
k1
k–1
+ HBpin
[La]H2Bpin
[La]–H
[1]
[2]
C. C. Chong, R. Kinjo, ACS Catal. 2015, 5, 3238-3259.
H
[La]
k2
a) M. S. Hill, D. J. Liptrot, C. Weetman, Chem. Soc. Rev. 2016, 45, 972-
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O
Bpin
H
O
C
+
[La]H2Bpin
CH2R
R
O
turnover-limiting
RH2CO
+
R
H
[La]
+ HBpin
[La]–H
Bpin
H
2
R
OBpin
O
C
fast
RH2CO
R
Scheme 4. Mechanisms for 1-catalyzed hydroboration of esters. Mechanisms
A and B are both consistent with kinetic experiments, but A is less likely based
on reactivity of in situ generate hydridolanthanum species.
Several points favor the mechanism of Scheme 4B (See SI).
Esters are not reactive toward hydridolanthanum compounds 2
and 3, whereas ketones react readily. The catalytic activity for
ester hydroboration is engendered in 2 and 3 by HBpin. Moreover,
2 is solubilized by HBpin, although a direct interaction between
these species is not detected. In contrast, esters do not solubilize
compound 2, suggesting that these compounds do not interact.
Although the slower nature of epoxide hydroboration has
limited extensive kinetic studies, and the mechanism of this
reaction is distinct from that of ester hydroboration, linear second-
order plots of ln{[HBpin]/[styrene oxide]} vs time suggest a similar
rate law. In this context, reversible cleavage of a C–O bond in the
epoxide by the hydridolanthanum catalyst, in analogy to the first
step of Scheme 4A (see Figure S45A), appears unlikely. Thus,
the mechanisms of Schemes 4B and S45B are favored by
analogous rate laws.
[3]
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The experimental rate law of eq. 1 is valid for the hydroboration
of all the aryl esters in this study, indicating one general
mechanism is operative. The kinetic studies, however, show that
comparisons of ternary experimental rate constants only partially
capture the relevant terms that describe relative catalytic
reactivity. Activity depends on rate constants and concentrations
4
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COMMUNICATION
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10.1002/anie.201813305
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Lanthanum hydride, generated by
reaction of lanthanum alkyl and
pinacolborane, is a catalyst for ester
and epoxide hydroboration. Yet,
lanthanum hydride reacts with
ketones by insertion, whereas
pinacolborane is required for ester
conversion.
Smita Patnaik, Aaron D. Sadow*
Page No. – Page No.
Interconverting Lanthanum Hydride
and Borohydride Catalysts for C=O
Reduction and C–O Bond Cleavage
6
This article is protected by copyright. All rights reserved.