One-Step Conversion of Potassium Organotrifluoroborates to Metal Organoborohydrides

Article abstract of DOI:10.1021/acs.orglett.8b01373

This letter describes the one-step conversion of heteroatom-substituted potassium organotrifluoroborates (KRBF₃) to metal monoorganoborohydrides (MRBH₃) using alkali metal aluminum hydrides. The method tolerates a variety of functional groups, expanding MRBH₃ diversity. Hydride removal with Me₃SiCl in the presence of dimethylaminopyridine (DMAP) affords the organoborane·DMAP (RBH₂·DMAP) adducts.

Full text of DOI:10.1021/acs.orglett.8b01373

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
One-Step Conversion of Potassium Organotrifluoroborates to Metal
Organoborohydrides
Christopher M. Bateman,† Hailey Beal,† Joshua E. Barker,† Brena L. Thompson,† Drake Donovan,†
Bradley J. Grant,† Jesse Shooter,† Jordan H. Arakawa,† Spencer Johnson,† Caleb J. Allen,†
James L. Yates,† Ryota Kato,§ Colby W. K. Tinsley,† Lev. N. Zakharov,‡ and Eric R. Abbey*,†
†Department of Chemistry, Biochemistry, & Physics, Eastern Washington University, Cheney, Washington 99004, United States
‡Department of Chemistry and Biochemistry, CAMCOR, University of Oregon, Eugene, Oregon 97403, United States
§Department of Chemistry for Materials, Mie University, Tsu, Mie 514-8507, Japan
S
* Supporting Information
ABSTRACT: This letter describes the one-step conversion of heteroatom-substituted potassium organotrifluoroborates
(KRBF3) to metal monoorganoborohydrides (MRBH3) using alkali metal aluminum hydrides. The method tolerates a variety of
functional groups, expanding MRBH3 diversity. Hydride removal with Me3SiCl in the presence of dimethylaminopyridine
(DMAP) affords the organoborane·DMAP (RBH2·DMAP) adducts.
etal organoborohydrides (MBRnH(4−n), abbreviated
MOBs) have been the subject of research for over 60
facilitate the creation of new three-coordinate organoboranes
(RBH2), a process recently used to prepare frustrated Lewis
pairs with −BH2 Lewis acidic sites.21
M
years, beginning with the pioneering work of Schlesinger.1 This
work dramatically expanded under Brown, who demonstrated
the tuning effects of organic substituents on the reactivity of
compounds containing B-H bonds as hydride donors and
reducing agents.2 Modification of the steric,3 stereochemical,4
hydrophobic,5 and electronic6 influences of organic substituents
permits creation of MOBs with reactivity ranging from milder
than NaBH4, with the creation of sodium cyanoborohydride7
and sodium triacetoxyborohydride,8 to much stronger than
LiAlH4, with the invention of “Super Hydride” LiBEt3H.9
Others, including Singaram,10 Paetzold,11 Soderquist,12 and
Noth,13 furthered this work. The recent explosion of frustrated
Lewis pair chemistry takes advantage of the hydride donation
ability of organoborohydrides14 with the first examples of metal-
free catalytic hydrogenation.
Despite the widespread application of MOBs in organoboron
research, most prior work has focused on metal triorganobor-
ohydride compounds (MR3BH), with limited examples of metal
diorgano-22 and monoorganoborohydrides (MR2BH2 and
MRBH3, respectively). The −BH3 unit is notable for its
isoelectronic and isoelectronic relationship with the methyl
group. With the exception of N-heterocyclic carbene−borane
adducts, MRBH3s synthesized and isolated to date are mostly
limited to alkyl, aryl, silyl,23 carbonyl,24 and fluoro25
substituents, with only a handful of examples of heteroatom
substitution on the organic substituent where the heteroatom is
not connected directly to boron.26 The relative absence of
diverse, highly functionalized R groups in MRBH3s in the
literature may be due to their limited synthetic access.
In addition to their widespread use as hydride donors, MOBs
have been employed as synthetic building blocks for the creation
of chiral tetracoordinate boron centers,15 CBN hydrogen
storage compounds,16 optoelectronic materials,17 boron-con-
taining polymers,18 and ligands for d- and f-block metals.19 Not
only can MOBs serve in these roles, they also act as four-
coordinate “protected boranes” (RnBH(3−n)) which can be
generated in situ upon addition of Bronsted or Lewis acids.20
Therefore, we envisioned that not only would a new method
enabling the creation of highly substituted MRBH3s expand the
chemistry of four-coordinate organoborohydrides, it would also
Three general methods of generating MRBH3s exist: (1)
addition of binary or complex metal hydrides to organo-
boranes27 or organohaloboranes28 (Scheme 1), (2) addition of
organolithium or organomagnesium compounds to borane−
Lewis base adducts (e.g., BH3·THF), and (3) addition of a
complex metal aluminohydride to a boronic ester or boronic
acid.29 Of these routes, method 3 appears to be the most
Received: April 30, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01373
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Synthetic Methods for the Preparation of MRBH3
Table 1. Optimization of Conditions for H/F Exchange on
1a
a
entry
hydride
LiAlH4
LiAlH4
LiAlH4
LiAlH4
LiAlH4
LiAlH4
Li(Ot-Bu)3AlH
NaAlH4
NaAlH4
KAlH4
LiH
NaH
CaH2
1. TMSCl
2. LiH
equiv
solvent
THF
time (h)
product
1
2
3
4
5
6
7
8
4
2
1.1
1.1
4
4
3.1
4
1.1
1.1
4
24
24
24
1
1b·THF
1b·THF
1b·THF
1b·THF
THF
THF
THF
Et2O
1,4-dioxane
THF
THF
THF
THF
THF
THF
THF
THF
24
24
24
24
18
18
24
24
24
24
−
incomplete
1b·THF
1c
1c
1d
−
−
−
−
convenient, general, and functional group tolerant, yet all three
routes rely on air- or water-sensitive sources of boron. Therefore,
we envisioned that KRBF3s could serve as ideal substrates for the
creation of a diverse family of heteroatom-substituted MRBH3s
and sought to discover the conditions with the greatest
functional group tolerance capable of this conversion.
9
10
11
12
13
14
4
4
4
4
Potassium organotrifluoroborates (KRBF3s) serve as attrac-
tive organoboron starting materials due to their ease of synthesis,
hydrolytic stability, and late-stage functionalization compati-
bility.30 The work of Molander and others has rapidly expanded
this chemistry, with application primarily as masked boronic
acids in Suzuki−Miyaura coupling. Others have demonstrated
the utility of KRBF3s as reagents for allyl and crotylborations and
for the generation of organohaloboranes (RBF2 and RBCl2),31
BN heterocycles,32 and chiral borane Lewis acid synthesis.33 To
the best of our knowledge, this is the first general method for the
conversion of KRBF3s to MRBH3s and is broadly tolerant of a
wide variety of heteroatom-containing functional groups.
We began screening hydride reagents in various solvents on
model compound 1a, discovering that LiAlH4 in THF was
capable of converting the B-F bond to the desired B-H bond34
(Table 1). Variation of stoichiometry revealed that only 1.1
equiv of LiAlH4 is needed for the quantitative conversion in 1
h.35 Due to the limited solubility of KRBF3s in other LiAlH4-
compatible solvents, THF was the only solvent that facilitated
this conversion. Unfortunately, we were only able to isolate the
THF adduct of the LiRBH3 compound 1b·THF, which was
insoluble in any solvent after concentration in vacuo. The milder
Li(t-BuO)3AlH accomplished the desired conversion, but also
yielded the same LiRBH3·THF adduct 1b·THF. After an
exhaustive screening of hydride sources and additives, we found
that 1.1 equiv of NaAlH4 in THF accomplished quantitative
conversion at 22 °C in 18 h, yielding compound 1c. While using
NaAlH4 overcomes the problems of the lithium salts, it is
problematic because the MRBH3 products contain a mixture of
sodium and potassium cations. Therefore, we attempted the
reaction with 1.1 equiv of KAlH4, which yielded compounds
1d−12d (except 9d) that contain only potassium cations. The
sodium and potassium MRBH3s do not suffer from the solubility
problems of the lithium salts, with moderate solubility in THF,
MeCN, and DMSO.
15
16
17
1. TMSCl
2. NaH
1. TMSCl
2. CaH2
4
4
4
4
THF
THF
24
24
24
−
−
−
b
Me2SiHCl
6
MeCN
b
a
Progress monitored by 11B NMR. Reaction run at 60 °C
Scheme 2. Sodium Organoborohydrides Synthesized Using
New Method
a
b
c
1.5 equiv NaAlH4 used. 1.0 equiv KAlH4 used. 1.5 equiv KAlH4
used. FW for compound was based on a 1:1 ratio of Na:K.
d
of MRBH3 compounds in the literature, where the metal is
lithium or a transition metal.
By screening a variety of KRBF3 substrates, we demonstrated
that this new method is compatible with benzyl, aryl, styryl,
amino, chloroaryl, bromoaryl, trifluoromethyl, thioether, acetal,
ether, and amide functional groups (Scheme 2). Purification of
the new MRBH3s is straightforward, with filtration through
Celite providing the desired products in moderate to good
yields.36 Furthermore, this method yields sodium and potassium
organoborohydrides, in contrast to almost every other example
Additionally, we discovered that the attenuated reducing
power of NaAlH4 and KAlH4 compared to LiAlH4 imparts
greater functional group tolerance to this method, permitting
synthesis of MRBH3s with greater structural diversity than
would have been possible with LiAlH4. For example, amides are
readily reduced by LiAlH4, but react only slowly with NaAlH4
and KAlH4. Using our conditions, we can synthesize amide-
substituted 12c and 12d. To the best of our knowledge, these are
B
DOI: 10.1021/acs.orglett.8b01373
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the first examples of amide-substituted borohydrides where the
carbonyl is not attached directly to boron. Curiously, NaAlH4
exhibited broader functional group tolerance than KAlH4, as we
were unable to synthesize dioxolane substituted 9d using KAlH4
and experienced greater difficulty obtaining pure product with
more sensitive functional groups such as styryl and amido
derivatives 3d and 12d, respectively. Furthermore, KAlH4 is not
commercially available, making NaAlH4 a more convenient
reagent if obtaining a mixed alakali metal salt is not problematic.
The barrier to F/H exchange appears to be enthalpic and
driven by favorable entropy. Conversion of BF3 and AlH3 to BH3
and AlF3 [eq 1] is endothermic, but exergonic in the gas phase,
ΔH°298 = +42.89 kJ mol−1 or +14.30 kJ mol−1 per bond; ΔG°298
= −11.31 kJ mol−1 or −3.77 kJ mol−1 per bond.37
the phenyl ring, with a C(7)−O(1)−C(4)−C(3) torsion angle
of −7.3(2)°, consistent with π-donation into the phenyl ring.
The geometries of the DMAP fragments of both 4e·DMAP
and 10e·DMAP are similar to other structures of DMAP−
borane adducts. The nitrogen of the dimethylamino fragments
appear to be sp2 hybridized with C(14)−N(3)−C(12) =
120.8(1)° and C(14)−N(2)−C(10) = 120.9(1)°, respectively,
and are close to coplanar with torsion angles C(15)−N(3)−
C(11)−C(11) = −3.3(2)° and C(14)−N(2)−C(10)−C(11) =
−1.7(2)°, respectively, consistent with π-donation into the
pyridine ring. The B(1)−N(1) distances of 1.594(2) Å and
1.597(2) Å for 4e·DMAP and 10e·DMAP, respectively, are
similar to the range observed for other DMAP−borane
adducts.40 Interestingly, in 10e·DMAP, the B-H exhibits an
intermolecular close contact of H(2)B−H(2) = 2.49(2) Å,
which may be evidence of an electrostatic interaction between
the hydridic B−H and an aromatic C−H, and is observed in
other borohydride crystal structures.41
In summary, we have discovered a convenient method for the
conversion of potassium organotrifluoroborates to alkali metal
monoorganoborohydrides that is tolerant of a wide range of
functional groups, which expands the chemical space of this class
of compounds. We then demonstrated the conversion of two
MRBH3s to heteroatom-substituted organoborane·DMAP
adducts. With synthetic access to diversely substituted
MRBH3s now available, our laboratory seeks to systematically
study the effects of various organic substituents on the reactivity
of B-H bonds and quantify the substituent effects on hydricity.
These new MRBH3s may find use as reducing agents, ligands,
and building blocks for the synthesis of organoboron-based
molecules and materials.
BF + AlH3 → BH3 + AlF
(1)
3
3
With the new organoborohydrides in hand, we sought to use
them as starting materials for the preparation of heteroatom-
substituted boranes. Reaction of 4b and 10b with 1.0 equiv
Me3SiCl THF in the presence of DMAP afforded the
organoborane·DMAP adducts 4e·DMAP and 10e·DMAP,
respectively (Scheme 3). X-ray quality crystals 4e·DMAP and
10e·DMAP38 were obtained from slow diffusion of pentane into
concentrated solutions in dichloromethane.
Scheme 3. Conversion of Sodium Organoborohydrides into
Organoborane·DMAP Adducts
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b01373.
Experimental procedures and characterization data
(PDF)
Accession Codes
CCDC 1833625 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
The unit cell of 4e·DMAP contains one symmetry
independent formula unit, in the monoclinic space group P21/
n, whereas the unit cell of 10e·DMAP contains two symmetry
independent formula units in the orthorhombic space group
Pbca. In both cases, boron appears to be sp3 hybridized, with
C(1)−B(1)−N(1) = 109.1(1)° for 4e·DMAP and C(1)−
B(1)−N(1) = 110.8(1)° for 10e·DMAP. The B(1)−C(1)
distances of 1.615(2) Å and 1.609(3) Å for 4e·DMAP and 10e·
DMAP, respectively, are consistent with a formal C−B single
bond.39
In 4e·DMAP the nitrogen of the dimethylaminophenyl
fragment appears to be mostly sp3 hybridized, with C(8)−
N(1)−C(7) = 111.4(1)° and a C(3)−C(4)−N(1)−C(7)
torsion angle of −46.2(2)°, which suggests little π-donation
into the phenyl ring. In contrast, The oxygen of the methoxy
group in 10e·DMAP appears to be mostly sp2 hybridized, with
C(7)−O(1)−C(4) = 117.2(1)°, and is close to coplanar with
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: eabbey@ewu.edu.
ORCID
Joshua E. Barker: 0000-0001-6139-8858
Eric R. Abbey: 0000-0001-6818-043X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Start Something Big Grant
and the Faculty Grants for Research and Creative Works. We
gratefully acknowledge the support of Travis Denton and Gang
Chen at Washington State University for assistance with HRMS
C
DOI: 10.1021/acs.orglett.8b01373
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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