Synthesis of DBpin using Earth-abundant metal catalysis

Article abstract of DOI:10.1016/j.tet.2020.131084

The synthesis of DBpin was achieved using (^EtBIP)CoCl₂ or (^tBuPNN)FeCl₂ as pre-catalysts activated with NaO^tBu. (^EtBIP)CoCl₂ was used as a pre-catalyst for the hydrogen isotope exchange of HBpin with D₂, and (^tBuPNN)FeCl₂ for deuterogenolysis of B₂pin₂. The one-pot, tandem hydrogenolysis-hydroboration/deuterogenolysis-deuteroboration reaction of terminal alkenes could be catalysed by (^tBuPNN)FeCl₂ to give alkyl boronic esters.

Full text of DOI:10.1016/j.tet.2020.131084

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Synthesis of DBpin using Earth-abundant metal catalysis
Andrew W.M. Cummins, Shuyang Li, Dominic R. Willcox, Tommi Muilu, Jamie H.
Docherty, Stephen P. Thomas
PII:
S0040-4020(20)30205-2
https://doi.org/10.1016/j.tet.2020.131084
TET 131084
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 24 January 2020
Accepted Date: 25 February 2020
Please cite this article as: Cummins AWM, Li S, Willcox DR, Muilu T, Docherty JH, Thomas
SP, Synthesis of DBpin using Earth-abundant metal catalysis, Tetrahedron (2020), doi: https://
doi.org/10.1016/j.tet.2020.131084.
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Synthesis of DBpin Using Earth-abundant
Metal Catalysis
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Andrew W. M. Cummins a, Shuyang Li a,
Dominic R. Willcox a, Tommi Muilu b, Jamie H.
Docherty a, *, Stephen P. Thomas a, *
a EaStCHEM School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road,
Edinburgh, EH9 3FJ, UK.
b EaStCHEM, School of Chemistry, University of St Andrews North Haugh, Fife, KY16 9ST, UK.

1
Tetrahedron
journal homepage: www.elsevier.com
Synthesis of DBpin Using Earth-abundant Metal Catalysis
Andrew W. M. Cumminsa, Shuyang Lia, Dominic R. Willcoxa, Tommi Muilub, Jamie H. Dochertya, *,
Stephen P. Thomasa, *
a EaStCHEM School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh, EH9 3FJ, UK.
b EaStCHEM, School of Chemistry, University of St Andrews North Haugh, Fife, KY16 9ST, UK
ARTICLE INFO
ABSTRACT
The synthesis of DBpin was achieved using (EtBIP)CoCl2 or (tBuPNN)FeCl2 as pre-
catalysts activated with NaOtBu. (EtBIP)CoCl2 was used as a pre-catalyst for the
hydrogen isotope exchange of HBpin with D2, and (tBuPNN)FeCl2 for deuterogenolysis
of B2pin2. The one-pot, tandem hydrogenolysis-hydroboration/deuterogenolysis-
deuteroboration reaction of terminal alkenes could be catalysed by (tBuPNN)FeCl2 to
give alkyl boronic esters.
Article history:
Received
Received in revised form
Accepted
Available online
Keywords:
Catalysis
Boron
Iron
Cobalt
Deuterium
1. Introduction
incorporation is common, and forcing conditions can be needed.
The first example of hydrogen isotope exchange of HBpin was
Deuterated compounds have found use across a wide range of
applications from mechanistic analysis to pharmacokinetic
studies. The strength of the C-D bond slows metabolism and can
therefore facilitate lower pharmaceutical doses and numerous
deuterated active pharmaceutical ingredients are currently
undergoing clinical trials, with Deutetrabenazine having been
fully approved by the FDA [1].
reported by Perutz and co-workers using a rhodium catalyst,
however the DBpin was not isolated from the reaction [19].
Further examples of hydrogen isotope exchange have been
shown with iridium and ruthenium complexes [13,20], but only a
single example has been reported using an Earth-abundant metal,
a
methyl-cobalt complex, which gave good deuterium
incorporation at room temperature [8].
Boronic esters are ubiquitous in modern organic chemistry
due to their functional group tolerance and broad applications [2–
4]. Boronic esters are commonly prepared by the metal-catalysed
hydroboration of alkenes and alkynes, with a wide range of
transition metal and main group catalysts having been developed
[5,6]. 4,4,5,5-Tetramethyl-1,3,2-dioxaborolane (HBpin) has
become a common reagent in organic synthesis, due to its
stability and that of alkyl and aryl pincacolboronic esters [7]. The
The deuterogenolysis of bis-(pincolato)borane (B2pin2)
provides a synthetically simple route to DBpin, with high
deuterium incorporation. The deuterogenolysis of B2pin2 was first
demonstrated by Hartwig and Hall using an iridium catalyst,
however due to low reactivity the reaction required multiple
charges of D2 over the period of 24 h [9]. Marder and
Braunschweig demonstrated the hydrogenolysis of B2pin2 using a
heterogenous catalyst [21]. Huang demonstrated the use of a
PNN-ligated cobalt catalyst for the deuterogenolysis of B2pin2,
with high reactivity and the generated DBpin could be used in
situ for the deuteroboration of alkenes and alkynes using the
same catalyst. NaHBEt3 was required to activate the cobalt pre-
catalyst [22].
deutero-isotopologue, DBpin offers
a
simple means of
deuterium-incorporation and has been commonly used for in situ
reaction monitoring of reactive species, and allows for a B-H
kinetic isotope effect to be determined [8–16].
The synthesis of HBpin was first reported by Knochel by
reaction of Me2S·BH3 with pinacol and distillation to give a high
yield of HBpin [17]. A similar synthetic route has been
commonly used to prepare DBpin, but requires the prior
synthesis of B2D6 gas, which is toxic, pyrophoric, and requires a
complex reaction setup [18]. DBpin has also been prepared by
hydrogen isotope exchange of HBpin, however low deuterium
We have previously demonstrated NaOtBu as an alternative to
organometallic activators in Earth-abundant metal catalysis, thus
we sought to extend this to the synthesis of DBpin [23]. Herein
we report the use of Earth-abundant metal catalysts for the
hydrogen isotope exchange of HBpin and deuterogenolysis of

2
Tetrahedron
B2pin2 using bench stable pre-catalysts and NaOtBu as the
We next turned our attention to the hydrogenolysis of B2pin2.
Various iron complexes were assessed, Milstein’s PNN and
activator to give DBpin with high deuterium incorporation and
simple purification.
Huang’s PONN complexes [24,25], (tBuPONNiPr)FeCl2 7 and
(
tBuPNN)FeCl2 8 were found to give the only active catalysts
using a combination of PhSiH3 and NaOtBu for pre-catalyst
activation (Scheme 3) [23]. We also found that Ni(COD)2 in
combination with PCy3 gave excellent conversion [22].
Scheme 3. Hydrogenolysis of B2pin2 with various catalysts. Values
a
listed are conversion by 11B NMR. Without PhSiH3 and NaOtBu,
Scheme 1. Synthesis of 2-deutero-4,4,5,5-tetramethyl-1,3,2-
dioxaboralane (DBpin) catalysed by transition metal complexes.
with THF as the solvent. b 5 mol% catalyst loading used.
Using (tBuPNN)FeCl2 8 we tested various organometallic
activators as a performance comparison (Table 1) [23]. All
organometallic activators were found to give significant amounts
of B2pin3 along with other degradation products, as observed by
11B NMR spectroscopy, and PhSiH3 and NaOtBu remained the
activator of choice.
2. Results and discussion
2.1. Hydrogen Isotope Exchange of HBpin
Chirik
reported
that
the
Co(I)-methyl
complex,
(
MesBIP)CoCH3, could catalyse hydrogen isotope exchange with
HBpin, therefore we attempted to access this reactivity by in situ
alkoxide activation from the parent Co(II) complex (EtBIP)CoCl2
[8]. We found that hydrogen isotope exchange occurred readily
using (EtBIP)CoCl2 1 as a catalyst and NaOtBu as an activator
when the reaction was carried out in neat HBpin under D2 (4 bar)
giving excellent deuterium incorporation (99%-d) and conversion
(>99%), but poor isolated yield (25%). Presumably when the
reaction was removed from the D2 atmosphere, the catalyst
promoted the degradation of DBpin into redistribution products,
such as B2pin3.
Table 1. Activator screen for the hydrogenolysis of B2pin2 by
(
tBuPNN)FeCl2. Conversion calculated from 11B NMR.
Entry
Activator
NaOtBu + PhSiH3
NaHBEt3
Conversion (%)
1
2
3
4
89
37
51
81
EtMgBr
PhLi
Scheme 2. Hydrogen isotope exchange of HBpin by (EtBIP)CoCl2
We then carried out a series of control reactions to ensure that
using NaOtBu as an activator.
the iron catalyst and activator were required. Interestingly, it was
found the exogenous PhSiH3 was not required to activate the pre-
catalyst, with B2pin2 able to perform the same function (Table 2,
2.2. Hydrogenolysis and deuterogenolysis of B2pin2

3
entry 2). The same results were obtained regardless of whether
the B2pin2 was purified by recrystallisation or not. We found that
the reaction was complete in 2 hours.
isotope effect using DBpin the yields were lower than the
analogous protio equivalent for all substrates. Deuterium
incorporation was observed at both the terminal and internal
position indicating the catalyst is operating by reversible
hydrometallation of the alkene. No deuterium incorporation was
observed at the internal alkene of a diene (10d), suggesting the
hydrometallation is chemoselective for terminal alkenes (Scheme
5, b).
Table 2. Control reactions. Conversion calculated from 11B NMR.
P(tBu)2
N
Cl
Fe
H
Cl
N
2
Bpin
Bpin
2, (1 mol%)
NaOtBu (2 mol%)
9a-e
R
10a-e
H2
2 HBpin
2
R
B2pin2
+
or
or
or
rt, 0.5-4 h
Et2O, r.t., 2-16 h
D
Entry
Variation from standard conditions
None
Conversion (%)
D2
2 DBpin
2
R
H
1
2
3
4
5
6
7
89
93
0
11a-e
(a) With HBpin:
No PhSiH3
H
H
No NaOtBu
Bpin
(EtO)3Si
Bpin
MeO
O
Bpin
Only (tBuPNN)FeCl2
Only NaOtBu
0
10a, (86%)
10b, (92%)
10c, (65%)
0
H
H
Bpin
Bpin
Only tBuPNN Ligand
Time = 2 h
0
89
10d, (94%)
10e, (96%)
Having optimised hydrogenolysis conditions, we then set out
to optimise the purification process. Et2O was found to be the
best reaction solvent, as it could easily be separated from the
HBpin by distillation to give good isolated yield (76%) on gram
scale. With optimised conditions for hydrogenolysis, we next
sought to apply this reaction to the preparation of DBpin.
Deuterogenolysis was found to be much slower than
hydrogenolysis, presumably due to a kinetic isotope effect and a
lower reaction pressure, therefore, the reaction time was extended
to 16 hours. This gave DBpin in high isolated yield (86%) on
gram-scale with >99%-d incorporation.
(b) With DBpin:
(66%)
(62%)
(64%)
D
D
D
MeO
Bpin
Bpin
(EtO)3Si
Bpin
O
D
D
D
(30%)
(38%)
(28%)
11a, (60%)
11b, (90%)
11c, (65%)
(66%)
(40%)
Bpin
D
D
Bpin
D
D
(32%)
(26%)
11d, (47%)
11e, (61%)
Scheme 5. (a) Scope of iron-catalysed one-pot hydrogenolysis-
hydroboration of terminal alkenes. (b) Scope of iron-catalysed one-
pot deuterogenolysis-deuteroboration of terminal alkenes. Deuterium
incorporation determined by integration of the residual signal in the
1H NMR spectrum.
3. Conclusions
We have successfully demonstrated the use of cobalt- and iron-
catalysis for the synthesis of DBpin with high deuterium
incorporation and isolated yield, using either hydrogen isotope
exchange or deuterogenolysis. The one-pot tandem
hydrogenolysis-hydroboration
deuteroboration were also developed and applied to terminal
alkenes.
and
deuterogenolysis-
Scheme 4. (a) Optimised hydrogenolysis of B2pin2. (b) Optimised
deuterogenolysis of B2pin2.
2.3. One-pot tandem hydrogenolysis-hydroboration and
deuterogenolysis-deuteroboration of B2pin2
4. Experimental section
4.1 General
As (tBuPNN)FeCl2 is also reported to be a highly efficient
hydroboration catalyst [26], we trialed a one-pot hydrogenolyis-
hydroboration using the HBpin that was formed in situ. This
hydroboration showed functional group tolerance to siloxanes
(10b), esters (10c), and internal alkenes (10d) (Scheme 5, a). We
extended this tandem reaction to the deuteroboration of terminal
alkenes (Scheme 5, b). Presumably due to a significant kinetic
Reaction setup: All reactions were performed in oven (185 ºC)
and/or flame dried glassware. All glassware was cleaned using
i
base (KOH, PrOH) and acid (HClaq) baths. All reported reaction
temperatures correspond to ambient room temperature, which
was approximately 20 ºC.

4
Tetrahedron
1
2
NMR spectroscopy: H, D, 13C and 11B spectra were recorded
CDCl3) 25.0, 83.3. Analytical data were in accordance with those
previously reported [22].
on Bruker Avance III 400 and 500 MHz; Bruker PRO 500 MHz;
Bruker Avance I 600 MHz spectrometers. Chemical shifts are
4.4. General procedure for one-pot hydroboration of
terminal alkenes.
reported in parts per million (ppm). H and 13C NMR spectra
1
were referenced to the residual solvent peak (CDCl3: 7.26 ppm).
Multiplicities are indicated by app. (apparent), br. (broad), s
(singlet), d (doublet), t (triplet), q (quartet), quin. (quintet), sext.
(sextet), sept. (septet), non. (nonet). Coupling constants, J, are
reported in Hertz and rounded to the nearest 0.1 Hz. Integration is
provided and assignments are indicated.
B2pin2 (0.500 mmol, 127 mg), (tBuPNN)FeCl2 (0.005 mmol,
2.2 mg), and NaOtBu (0.01 mmol, 0.8 mg) were dissolved in
Et2O (0.5 mL) in an autoclave. The vessel was purged with H2
then pressurised with H2 (10 bar). The reaction was stirred for 2
hours, then the pressure was released and the vessel transferred
into a glovebox. Alkene (1.00 mmol) was added, and the reaction
left to stir for 30 mins. The solvent was removed in vacuo and the
residue purified by flash column chromatography on silica (with
a mixture of 40/60 petroleum ether and diethyl ether). Volatiles
were removed in vacuo to give the alkyl boronic ester.
Chromatography: Column chromatography was carried out on a
Teledyne ISCO CombiFlash NextGen 300+ using RediSep Rf
Gold normal phase silica flash columns (12, 25, 40, or 80 g; 20-
40 microns). Substrates were purified using 40/60 petroleum
ether and EtOAc on a gradient of 100:0 to 0:100 with flow rates
of 10-110 mL min-1 depending on the size of column and ꢀRf.
4.5. Deuterogenolysis of B2pin2
B2pin2 (10.0 mmol, 2.54 g), (tBuPNN)FeCl2 (0.10 mmol, 44
mg) and NaOtBu (0.20 mmol, 19 mg) were dissolved in Et2O (10
mL) in an autoclave. The vessel was purged with D2 then
pressurised with D2 (10 bar). This was left to stir for 16 hours,
then the pressure was released. The reaction mixture was distilled
at atmospheric pressure to remove Et2O (60 °C). The DBpin was
distilled in vacuo (25 °C, 0.1 mbar), to give DBpin (2.22 g, 86%,
Mass Spectrometry: Mass spectrometry (MS) was performed by
the University of Edinburgh, School of Chemistry, Mass
Spectrometry Laboratory. High resolution mass spectra were
recorded on a VG autospec, or Thermo/Finnigan MAT 900, mass
spectrometer. Electron Impact (EI+) spectra were performed at 70
eV using methane as the carrier gas, with either a double
focusing sector field (DFSF) or time-of-flight (TOF) mass
analyzer. Chemical Ionization (CI+) spectra were performed with
methane reagent gas, with either a double focusing sector field
(DFSF) or time-of-flight (TOF) mass analyzer. Electrospray
Ionization (ESI+) spectra were performed using a time-of-flight
(TOF) mass analyzer. Data are reported in the form of m/z
(intensity relative to the base peak = 100).
1
>99%-d) as a colourless oil. H NMR (500.12 MHz, CDCl3):
1.27 (12H, s, CH3). 11B NMR (128.34 MHz, CDCl3): 28.2 (s br),
13C NMR (125.77 MHz, CDCl3) 25.0, 83.4. 2D NMR (76.75
MHz, toluene) 4.15 (br m). Analytical data were in accordance
with those previously reported [22].
4.6. General procedure for one-pot deuteroboration of
terminal alkenes.
Chemicals: All reagents and were purchased from Sigma
Aldrich, Alfa Aesar, Acros organics, Tokyo Chemical Industries
UK, Fluorochem and Apollo Scientific without further
purification. Sodium tert-butoxide (97%) was purchased from
Sigma Aldrich (UK). B2pin2 was purchased from Fluorochem
and recrystallised before use. All cobalt and iron species were
prepared according to reported procedures [23,24,26–28].
B2pin2 (0.500 mmol, 127 mg), (tBuPNN)FeCl2 (0.005 mmol, 2.2
mg), and NaOtBu (0.01 mmol, 0.8 mg) were dissolved in Et2O
(0.5 mL) in an autoclave. The vessel was purged with D2 then
pressurised with D2 (4 bar). This was left to stir for 16 hours, then
the pressure was released and the vessel transferred into a
glovebox. Alkene (1.00 mmol) was added, and the reaction left to
stir for 4 hours. The solvent was removed in vacuo and the
residue purified by flash column chromatography on silica (with
a mixture of 40/60 petroleum ether and diethyl ether). Volatiles
were removed in vacuo to give the deutero alkyl boronic ester.
4.2. Preparation of DBpin with (EtBIP)CoCl2
Under argon, to an autoclave was added (EtBIP)CoCl2 (0.14
mmol, 76 mg), and NaOtBu (0.28 mmol, 30 mg). HBpin (2.0 mL,
14 mmol) was added dropwise n.b. exothermic. The vessel was
sealed, flushed with D2, then pressurised with D2 (4 bar). The
reaction was left to stir at room temperature for 36 h, then the
flush/pressurise cycle was repeated, and the reaction was left to
stir for another 24 h. The DBpin was distilled in vacuo (25 °C,
0.1 mbar), to give DBpin (0.50 g, 25%, 99%-d) as a colourless
oil. The deuterium incorporation was determined by integration
Acknowledgements
S.P.T. thanks the Royal Society for a University Research
Fellowship. D.R.W. thanks the Royal Society for a PhD
studentship. J.H.D. and S.P.T. acknowledge GlaxoSmithKline,
EPSRC, and the University of Edinburgh (PIII002) for post-
doctoral funding.
1
of the residual HBpin signal. H NMR (500.12 MHz, CDCl3):
1.27 (12H, s, CH3). 11B NMR (128.34 MHz, CDCl3): 28.2 (s br),
13C NMR (125.77 MHz, CDCl3) 25.0, 83.4. 2D NMR (76.75
MHz, toluene) 4.15 (br m). Analytical data were in accordance
with those previously reported [22].
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: