Readily available lithium compounds as catalysts for the hydroboration of carbodiimides and esters

Article abstract of DOI:10.1016/j.jorganchem.2021.121924

Selective and efficient hydroboration of esters and carbodiimides to alcohols and amines by two well-defined and readily accessible lithium complexes, 2,6-di-tert-butyl phenolate lithium (1a) and 1,1’-dilithioferrocene (1b) are described. A range of aliphatic, aromatic, and cyclic esters with various functional groups were selectively converted into the corresponding boronate esters. Similarly, the single hydroboration of carbodiimides with aliphatic and aromatic substituents on the nitrogen atoms was studied. A possible mechanistic pathway of the hydroboration of carbodiimides with HBpin has been proposed using NMR studies and DFT calculations. These reactions are convenient alternatives to stoichiometric hydride reduction or hydrogenation. The employing of lithium complexes is also significant, because of the need to find cheap and green alternatives to noble metal complexes.

Full text of DOI:10.1016/j.jorganchem.2021.121924

Journal Pre-proof
Readily Available Lithium Compounds as Catalysts for the
Hydroboration of Carbodiimides and Esters
Milan Kumar Bisai , Kritika Gour , Tamal Das , Kumar Vanka ,
Sakya S. Sen
PII:
S0022-328X(21)00245-X
DOI:
Reference:
https://doi.org/10.1016/j.jorganchem.2021.121924
JOM 121924
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Journal of Organometallic Chemistry
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Accepted date:
27 March 2021
2 June 2021
4 June 2021
Please cite this article as: Milan Kumar Bisai , Kritika Gour , Tamal Das , Kumar Vanka ,
Sakya S. Sen , Readily Available Lithium Compounds as Catalysts for the Hydrobo-
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Readily Available Lithium Compounds as Catalysts for the
Hydroboration of Carbodiimides and Esters
Milan Kumar Bisai,ac Kritika Gour,ac Tamal Das,bc Kumar Vanka,*bc and Sakya S. Sen*ac
aInorganic Chemistry and Catalysis Division, CSIR-National Chemical Laboratory, Dr. Homi
Bhabha Road, Pashan, Pune 411008 (India); bPhysical and Material Chemistry Division, CSIR-
National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune 411008 (India);
cAcademy of Scientific and Innovative Research (AcSIR), Ghaziabad-201002 (India).
Correspondence should be addressed: ss.sen@ncl.res.in, k.vanka@ncl.res.in
Highlights
(a) The catalytic application of organolithium compounds is very limited due to their high reactivity and
aggregation phenomena. However, the paradigm started to shift recently, when the groups of Okuda (J.
Am. Chem. Soc. 2016, 138, 10790) and Mulvey (Chem. Eur. J., 2017, 23, 16853) reported the use of
organolithium compounds for the hydroboration of aldehydes and ketones. Nonetheless, the catalytic use
of organolithium compounds for reduction of more challenging and synthetically useful substrates such as
carbodimides, esters etc. is deemed desirable.
(b) Very recently, we have shown the use of organolithium compounds for the hydroboration of
aldehydes and ketones (Chem. Commun. 2018, 54, 6843) as well as alkenes and alkynes (Chem.
Commun. 2019, 55, 11711). Building upon our previous works, we demonstrate here catalytic
hydroboration of ester to alcohol and carbodiimide to imine.
(c) Prior to this work, there are few main-group catalysts reported for the hydroboration of carbodiimdies
(See Chart 1 in the manuscript), but no well-defined group 1 complex as a single site catalyst has been
used. The same is true for ester hydroboration. This is the first report of the catalytic hydroboration of
esters and carbodiimides using single site catalyst based on a s-block element.
(d) A thorough DFT studies were carried out to understand the mechanism of carbodiimide
hydroboration.
1

Abstract
Selective and efficient hydroboration of esters and carbodiimides to alcohols and amines by two
well-defined and readily accessible lithium complexes, 2,6-di-tert-butyl phenolate lithium (1a)
and 1,1‟-dilithioferrocene (1b) are described. A range of aliphatic, aromatic, and cyclic esters
with various functional groups were selectively converted into the corresponding boronate esters.
Similarly, the single hydroboration of carbodiimides with aliphatic and aromatic substituents on
the nitrogen atoms was studied. A possible mechanistic pathway of the hydroboration of
carbodiimides with HBpin is proposed using NMR studies and DFT calculations. These
reactions are convenient alternatives to stoichiometric hydride reduction or hydrogenation. The
employing of lithium complexes is also significant because of the need to find cheap and green
alternatives to the noble metal complexes.
Introduction
Hydroboration, the direct formation of X-B bond by formal addition of a borane to a C=X (X=C,
N, O) bond, is a powerful synthetic procedure for the reduction of carbonyls, amides, alkynes,
alkenes, nitriles etc. Although tremendous progress has been achieved in the field of transition
metal catalyzed hydroboration,1 the rising demand for sustainable chemical processes have led to
signuificant focus on the development of hydroboration catalysts involving main-group
elements. In this regard, there have been several important discoveries in the design of main-
group element based catalytic systems for hydroboration of unsaturated organic substrates.2
Among the main-group elements, lithium compounds have been neglected, but they could be the
alternatives to address today‟s urgent need to develop new catalysts and processes that are clean,
2

safe, and efficient. A further impetus for the lithium catalysts comes from their inexpensiveness,
ready accessibility, and non-involvement in Schlenk equilibrium unlike their adjacent group 2
elements.3 Moreover, catalysts based on main-group elements are usually synthesized from their
lithium compounds. Hence, the use of lithium compounds as catalysts benefits from the
avoidance of the additional salt metathesis step. It is said that Ziegler initially used nBuLi to
promote his olefin polymerization reaction, but later switched to trialkyl aluminum, presumably
due to price advantage.4 Although the lithium catalysts lost the first round, recent ground-
breaking works from the groups of Okuda,5,6,7 Mulvey,8,9 Wangelin,10 and others11,12,13,14,15 for
the hydroboration of aldehydes, ketones, nitriles, and other organic substrates using a range of
lithium compounds have brought them back into competition.
We have been actively developing operationally convenient methods for hydroboration
using simple and readily accessible main-group compounds.16,17,18 Over the past three years, our
laboratory has reported the use of lithium 2,6-di-tert-butyl phenolate (1a) and 1,1‟-
dilithioferrocene (1b) as catalysts for the reduction of aldehydes, ketones, alkenes, alkynes, all
three classes of amides.19,20,21 Herein, we report single-site hydroboration of carbodiimides and
esters using HBpin in the presence of 1a and 1b. Reduction of esters to the corresponding
alcohols is an important transformation in organic chemistry, while the reduction of
carbodiimides (R-N=C=N-R) is a practical method for the synthesis of amines without the
formation of any by-product.
Results and Discussion
Hydroboration of Carbodiimides. In comparison to aldehydes and ketones, the electrophilicity
of the carbodiimide group is considerably low, which makes the hydroboration of carbodiimides
a challenging task. Probably because of this reason, very few examples of the hydroboration of
3

carbodiimides using compounds with main-group elements have been reported. Hill and co-
workers described a β‐diketiminato magnesium alkyl complex, [CH{C(Me)NDipp}2}MgnBu]
(Dipp=2,6‐iPr2C6H3), as an effective pre‐catalyst for the catalytic hydroboration of carbodiimides
with pinacolborane (HBpin).22 Parkin reported hydroboration of two alkyl carbodiimides (R=iPr
and Cy) using a terminal magnesium hydride, [TismiPrBenz]MgH.23 Roesky and co-workers
reported a alkylaluminum complex, [CH{C(Me)NDipp}2}AlnBu2] as a pre-catalyst for
hydroboration of carbodiimides.24 Subsequently, the same group as well as Nembenna‟s team
studied a series of aluminum compounds as catalysts for the hydroboration of carbodiimides.25,26
Eisen and co-workers showed monohydroboration of carbodiimides using actinide-methyl
complexes of uranium and thorium.27 9-borabicyclo[3.3.1]nonane catalyzed hydroboration of
carbodiimides was reported by the groups of Ramos and Antinolo.28 Besides these developments,
simple main-group compounds such as SnCl2/SnBr2,29 KCH2Ph,30 NaH31 have been shown to
promote/catalyze the hydroboration of carbodiimides. Additionally, dihydroboration of
carbodiimides was reported with magnesium hydridotriphenylborate, [Mg(thf)6][HBPh3]2 from
the
groups
of
Okuda32
and
magnesium
tris(pentafluorophenyl)hydridoborate,
[CH{C(Me)NDipp}2}Mg][HB(C6F5)3] from the groups of Hill (Chart 1).33
4

Chart 1. Main group and actinide based catalysts for carbodiimide hydroboration.
We started our investigation with N,N'-diisopropyl carbodiimide (DIC) (0.25 mmol) in
the presence of equimolar amount of HBpin (0.28 mmol, 1.1 eq) at 80 °C in neat condition
without any solvent (Table 1), which afforded the desired singly reduced N-borylformamidine in
excellent yield within 12-15 h (Table 1, entry 6 and entry 13). It is important to point out that the
reaction between DIC and HBpin did not afford any product in the absence of catalysts under the
same reaction conditions. With the optimized conditions in hand, the scope of the reaction was
1
investigated and product N-borylformamidine [R1NCHN(BPin)R1] was identified through H
NMR spectrum with a singlet resonance at δ = 7.90 ppm. Also, other substrates such as N,N'-di-
tert-butylcarbodiimide,
N,N'-dicyclohexylcarbodiimide
(DCC),
and
bis(2,6-
diisopropylphenyl)carbodiimide provide moderate to good yields of the desired mono
hydroboration products under standard reaction conditions (Scheme 1, entries 2-4). It is worth
5

mentioning here that, even though DIC performed well at the room temperature (Entry 4 and 10),
other substrates underwent very poor conversion under the same reaction condition.
Table 1. Optimization table for hydroboration of N,N-diisopropyl carbodiimide by 1a and 1b.
1
Yields were determined by H NMR integration relative to mesitylene. Reaction Scale: 0.25
mmol (carbodiimide) and 0.28 mmol (HBpin) in neat condition. CDCl3 is used as the deuterated
solvent.
Entry
Catalyst
Catalyst (mol%)
Temp (°C)
Time (h)
NMR Yield
(%)
20
53
84
90
66
92
>99
54
87
96
92
95
99
86
62
1.
2.
3.0
3.0
5.0
5.0
3.0
4.0
4.0
3.0
5.0
5.0
2.0
3.0
4.0
4.0
-
rt
rt
1
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
1b
1b
-
15
13
15
15
15
18
1
3.
rt
4.
rt
5.
60
80
80
rt
6.
7.
8.
9.
rt
13
15
12
12
12
12
20
10.
11.
12.
13.
14.
15
rt
80
80
80
60
rt-120
The performance of 1a and 1b as catalysts is superior to the recently reported nBuLi catalyzed
carbodiimide hydroboration, where the quantitative conversion of aliphatic carbodiimides (DCC
6

and DIC) took 24 h at 80 °C and the hydroboration of Dipp-N=C=N-Dipp was reported to take
48 h.15 Between 1a and 1b, the latter is found to be more efficient as it requires less time than
the former to achieve the transformation. It is noteworthy that an attempted reaction of
iPrN=C=NiPr and two equivalents of HBpin afforded 52.0% mono hydroboration and 24.0%
double hydroboration product under the same reaction conditions.
Scheme 1. The scope of hydroboration with carbodiimide substrates. Reaction Scale: 0.25 mmol
(carbodiimide) and 0.28 mmol (HBpin). Reaction conditions: 4.0 mol% catalyst, 12-15 h
reaction at 80 °C in neat conditions. Superscripts “a” and “b” stand for the catalysts 1a and 1b
1
respectively. Yields were determined by H NMR integration relative to mesitylene. CDCl3 is
used as the deuterated solvent.
Entry
Substrate
Time
(h)
Yield Single
Hydroboration
(%)
Time
(h)
Yield Single
Hydroboration
(%)
At 80 °C
At room temperature
1.
2.
3.
15a/12b
15a/12b
92a/99b
15
90a/96b
3a/11b
30a/86b
15
15
64a/41b
15a/12b
15a/12b
98a/88b
90a/67b
4.
15
46a/0b
7

DFT studies for carbodiimide hydroboration. We have investigated the hydroboration
mechanism of carbodiimide only with 1a. The reaction of 1a with HBpin gives a peak at δ 4.7
ppm in the 11B NMR, which can be ascribed for a three-coordinated boron atom. However,
keeping the NMR tube for a longer time led to the generation of a singlet and a quintet resonance
at δ 21.62 and –39.83 ppm, reflecting the formation of a trialkoxyborane [2,6-tBu2-C6H3-OBpin]
-
and BH4 anion (See SI, Figure S44). Very recently, Thomas and coworkers divulged the
possible role of BH3 adducts as "true catalysts" in the hydroboration with HBpin of unsaturated
substrates using nucleophiles.34,35 Hence, we have performed the hydroboration of DIC in
presence of 10 mol% of TMEDA, however, the reaction afforded 86% yield implying the
negligible influence of BH3 as a hidden catalyst in this reaction (See SI, Figure S45). To obtain
additional mechanistic insight, full quantum chemical calculations were done with density
functional theory (DFT) at the dispersion corrected PBE/TZVP level of theory. In the first step of
the reaction in the presence of catalyst 1a, a weekly bound complex (Int-1) is formed between
catalyst 1a and pinacolborane as stated earlier for the hydroboration of aldehydes, ketones and
amides.19 The formation of Int-1´ was discarded based on the Gibbs‟ free energy value of the
reaction (15.2 kcal/mol), which is thermodynamically unfavorable. A very similar mechanism
was proposed by Pati and Khan in their recent germylene and stannylene catalyzed
hydroboration of aldehydes.36 Now, the carbodiimide approaches towards the B-H bond of the
Int-1. This is the prelude to the nucleophilic attack by the imide nitrogen of carbodiimide to the
boron center of the pinacolborane, with the hydride being transferred from the boron center to
the electrophilic sp-hybridized carbon of the imide. This occurs through a four-membered
transition state (TS-1) and leads to the formation of hydroboration product (Pdt) along with the
regeneration of the catalyst 1a (see Scheme 2 and Figure S46 in the SI). The ΔE (-12.1 kcal/mol)
8

and ΔG (-8.4 kcal/mol) values for this step are significantly negative and the activation energy
(ΔG#) barrier corresponding to the transition state is 25.2 kcal/mol. This is also the slowest step
of the overall hydroboration reaction. In this transition state there is a significant amount of B-H
bond activation (the B-H bond distance increases to 1.91 Å), which allows the hydride transfer
from the boron to the sp-hybridized imide carbon center along with the simultaneous C=N bond
cleavage and B-N bond formation.
Scheme 2. The catalytic cycle for the carbodiimide hydroboration by catalyst 1a. ΔG and ΔG‡
represent the Gibbs free energy of reaction and the barrier, respectively. All values are in
kcal/mol.
Hydroboration of esters. Similar to carbodiimides, ester hydroboration with main-group
compounds is seldom reported. Sadow and coworkers described ToMMgMe (ToM = tris(4,4-
dimethyl-2-oxazolinyl)phenylborate) catalyzed ester hydroboration.37 Nembenna and coworkers
showed magnesium amide Mg{N(SiMe3)2}2 and complexes derived from the latter, such as
9

(iPr2N)C(NAr2)MgN(SiMe3)2 (Ar = 2,6-Me2-C6H3 and 2,6-iPr2-C6H3)38 and the group of Ma has
recently demonstrated the use of a Mg(I) dimer as an efficient pre-catalyst for the hydroboration
of a wide range of esters (Chart 2).39 Apart from main-group elements, the groups of Sadow and
Marks independently used lanthanum catalysts for the hydroboration of esters.40,41 Given the
success of hydroboration of carbodiimides, we next turned our attention towards the
hydroboration of ester with HBpin. Accordingly, the reaction conditions were optimized to
realize the maximum yield.
Chart 2. Main group and lanthanide based (pre)catalysts for ester hydroboration.
As seen in Table 2, methyl benzoate was treated with HBpin in the presence of 6ꢀmol%
1a at room temperature to obtain the corresponding aryloxy boronate ester only in 35% yield
(entry 1). With the increase of temperature and time, the reaction yield started to gradually
increase (entry 2-5). A high yield of 95% was obtained when the reaction was conducted at 80
°C for 10 h (entry 5). The decrease of mol% of 1a or reaction time led to drop-off in yield (entry
6 and 7). When we have used 1b as the catalyst, we observed that it requires less catalyst mol%
and reaction time. The highest yields were obtained with 3-4 mol% of 1b with 10 h reaction time
(entry 10 and 11). When the reaction time was reduced to 8 h, there is a yield drop to 88% (entry
12). However, we chose the latter as the optimized condition in order to study the electronic
10

effect of the ester substituents. Unlike carbodiimide hydroboration, we did not perform the DFT
studies for the ester hydroboration.
Table 2. Optimization table for the hydroboration of methyl benzoate catalyzed by 1a and 1b.
Reaction conditions: 3.0-6.0 mol% catalyst, 8-10 h reaction at 45-80 °C temperature in neat
condition or in THF solvent (0.5 mL). Yields were determined by 1H NMR integration relative to
mesitylene. Reaction Scale: 0.25 mmol (ester) and 0.50 mmol (HBpin). CDCl3 is used as the
deuterated solvent.
Entry
Catalyst
Catalyst
(mol%)
6.0
Solvent
Temperature
Time
(h)
12
10
10
12
10
10
8
NMR yield
(%)
35
(°C)
25
60
60
80
80
80
80
25
25
45
45
45
45
45
1.
2.
Neat
Neat
THF
Neat
Neat
Neat
Neat
THF
THF
THF
THF
THF
THF
THF
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
1b
1b
6.0
44
3.
6.0
44.5
96.5
95
4.
6.0
5.
6.0
6.
5.0
81
7.
6.0
73
8.
6.0
12
12
10
10
8
98
9.
4.0
83
10.
11.
12.
13.
14.
4.0
>99
97
3.0
3.0
88
3.0
6
82
2.0
8
75
Scheme 3. The scope of hydroboration with ester substrates. Reaction Scale: 0.25 mmol (ester)
and 0.50 mmol (HBpin). Reaction conditions: 3.0-6.0 mol% catalyst, 8-10 h reaction at 45-80 °C
11

temperature in neat condition or in THF solvent. Superscripts “a” and “b” are for catalyst 1a and
1
1b respectively (See SI for further details). Yields were determined by H NMR integration
relative to mesitylene. CDCl3 is used as the deuterated solvent.
Both aliphatic and aromatic esters underwent hydroboration to form corresponding boronate
ester in good to excellent yield. Aromatic esters with short to long alkyl chains showed
negligible steric influence and provided the desired product in quantitative yields (3a-3e). Also,
aromatic esters with electron-withdrawing or donating substituents at the o/m/p positions react
12

with HBpin and show complete conversion (Scheme 3, 3f-3i) to the respective products.
Similarly, the aliphatic esters underwent smooth reduction when treated with HBpin (3m-3p).
The quantitative conversion was documented for the reaction of ε-caprolactone (3m) under
similar reaction conditions. We did not observe any evidence of polymerization. Chemoselective
reduction of esters is very important and quite challenging in the presence of other reducible
functional groups in organic synthesis. In this regard, we have demonstrated the hydroboration of
esters in the presence of other functional groups such as conjugated or non-conjugated C=C
double bonds (3j, 3k and 3n) and C≡C triple bonds (3l, 3n, and 6o). Esters with halide
substituents (3i and 3p) were also tolerated well. No side reaction with nitrile substituent (3g)
was observed, indicating the preference for the ester reduction.
Conclusion
In our previous papers, we have demonstrated the use of simple lithium compounds as catalysts
for hydroboration of aldehydes, ketones, alkenes, alkynes, primary-tertiary amides. Herein, the
scope and mechanism of lithium compound-catalyzed hydroboration of diverse series of esters
are studied. The catalyst shows complete selectivity for ester reduction over competing nitrile,
alkenes, and alkynes moieties, even at temperatures as high as 60 °C. Besides, the catalysts are
very efficient for the hydroboration of carbodiimides with HBpin. We believe that our
contributions in employing simple and readily accessible lithium compounds as catalysts and
will spur more interest in organolithium chemistry, a land that is undoubtedly worth exploring.42
Acknowledgment
This work was supported by Ramanujan Research Grant (SB/S2/RJN-073/2014) and CSIR-Young
Scientist Award Contingency Grant (YSA000726). MKB, KG, and TD thank the Council of
13

Scientific and Industrial Research (CSIR), India for research fellowships. The support and the
resources provided by „PARAM Brahma Facility‟ under the National Supercomputing Mission,
Government of India at the IISER Pune are gratefully acknowledged. We are thankful to the
reviewers for their critical feedbacks to improve the quality of the manuscript.
Dedication
This paper is dediated to Prof. Pradeep Mathur on the occasion of his 65th birthday
Supporting Information
General catalytic procedure for the hydroboration of carbodiimides and esters, analytical data of
the corresponding borylamines and boronate esters, their representative NMR spectra, and details
of theoretical calculations are provided in the supporting information.
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TOC
for
Readily Available Lithium Compounds as Catalysts for the
Hydroboration of Carbodiimides and Esters
We have demonstrated the use of simple and readily available lithium compounds, 2,6-di-tert-
butyl phenolate lithium (1a) and 1,1‟-dilithioferrocene (1b) as single site catalysts for
hydroboration of diverse esters and carbodiimides in high yields and excellent functional group
tolerance. DFT studies have been performed to understand the mechanism of carbodiimide
hydroboration.
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