A boron-oxygen transborylation strategy for a catalytic midland reduction

Article abstract of DOI:10.1021/acscatal.0c05168

The enantioselective hydroboration of ketones is a textbook reaction requiring stoichiometric amounts of an enantioenriched borane, with the Midland reduction being a seminal example. Here, a turnover strategy for asymmetric catalysis, boron.oxygen transb

Full text of DOI:10.1021/acscatal.0c05168

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Research Article
A Boron−Oxygen Transborylation Strategy for a Catalytic Midland
Reduction
Kieran Nicholson,∥ Joanne Dunne,∥ Peter DaBell, Alexander Beaton Garcia, Andrew D. Bage,
Jamie H. Docherty, Thomas A. Hunt, Thomas Langer, and Stephen P. Thomas*
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ABSTRACT: The enantioselective hydroboration of ketones is a textbook reaction requiring stoichiometric amounts of an
enantioenriched borane, with the Midland reduction being a seminal example. Here, a turnover strategy for asymmetric catalysis,
boron−oxygen transborylation, has been developed and used to transform the stoichiometric borane reagents of the Midland
reduction into catalysts. This turnover strategy was demonstrated by the enantioselective reduction of ketones, including derivatives
of biologically active molecules and those containing reducible groups. The enantioenriched borane catalyst was generated in situ
from commercially available reagents, 9-borabicyclo[3.3.1]nonane (H-B-9-BBN) and β-pinene, and B−O transborylation with
pinacolborane (HBpin) was used for catalytic turnover. Mechanistic studies indicated that B−O transborylation proceeded by B−O/
B−H boron exchange through a stereoretentive, concerted transition state, resembling σ-bond metathesis.
KEYWORDS: transborylation, hydroboration, enantioselective, boron, main group, asymmetric catalysis, ketone, reduction
atalysis underpins the sustainable future of chemical
synthesis yet remains dominated by second- and third-row
examples of transborylation in catalysis are limited to boron−
carbon bonds. Translation of this turnover pathway to boron−
oxygen bonds, B−O transborylation, would open up a new class
of reactivity for catalytic turnover.8
Asymmetric ketone hydroboration using stoichiometric
enantioenriched boranes has found widespread use in total
synthesis.9 The Midland reduction10 using Alpine-borane 2a
represents the most applied example (Scheme 1a). A major
drawback of this method is the concurrent destruction of the
stoichiometric enantioenriched reagent 2a upon hydrolysis of
the borinic ester 3 to give the enantioenriched alcohol 5.
Development of B−O transborylation would render this
C
transition-metal species.1 The entrenched mechanisms of
catalysisoxidative addition and reductive eliminationare
not easily translated beyond the d-block.2 Although great efforts
have been made to force redox activity on main-group species,
these have yet to be widely adopted.3 Many main-group catalysts
continue to rely on Lewis and Brønsted acid/base interactions to
facilitate substrate binding and catalyst turnover.4 New turnover
mechanisms are needed to further the development and use of
main-group catalysts.
Ligand redistribution is well established in the p-block and is
routinely used in the synthesis of organoboron and organo-
aluminum species.5 The ability to harness this redistribution
offers a redox-neutral approach for main-group catalyst
turnover. The hydroboration of alkenes and alkynes has been
catalyzed by organoborane species6 and is proposed to occur
through a redistribution event between two boron centers.7 This
boron−carbon transborylation, a subclass of σ-bond metathesis,
is analogous to transmetalation and has enabled the use of
primary and secondary borane species as catalysts. Current
Received: November 25, 2020
Revised: January 25, 2021
Published: February 1, 2021
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© 2021 American Chemical Society
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borylation (Scheme 1b). This requires chemoselective alkene
hydroboration in the presence of excess ketone. Five key
mechanistic challenges must be addressed for the successful
realization of B−O transborylation enabled asymmetric
catalysis:
Scheme 1. Transborylation for Catalytic Turnover in Borane
Reduction Reactions
a
(i) establishment of B−O/B−H transborylation.
(ii) conservation of enantiomeric excess during B−O/B−H
transborylation.
(iii) chemo- and stereoselective regeneration of the borane
catalyst.
(iv) suppression of unselective ketone reduction by achiral
boron reagents (H-B-9-BBN and HBpin).
(v) suppression of B−C/B−H transborylation to avoid
catalyst deactivation.
Herein B−O transborylation has been developed and used as
a strategy for catalytic turnover in asymmetric ketone reduction
(Scheme 1c). The previously stoichiometric Midland reduction
was rendered catalytic, demonstrating this mode of catalysis.
The validity of B−O transborylation was established using
single-turnover experiments with a range of enantiopure tertiary
boranes (Scheme 2). The stoichiometric reduction of 4-phenyl-
3-butyn-2-one 1a with enantiopure boranes, showed that Alpine
borane 2a, myrtanyl-B-9-BBN (myrtanyl borane)11 2b, and
Soderquist’s borane12 2c gave good enantioselectivity (Scheme
2a). Soderquist’s borane 2c was not investigated further due to
the lower ee achieved in comparison to other stoichiometric
enantioenriched reductants. The Midland reduction proceeds
by reaction of Alpine borane 2a with a propargylic ketone to give
the α-pinene α-4 and the enantioenriched borinic ester 3a,
which is hydrolyzed to alcohol 5a on workup. Here, B−O
transborylation of the borinic ester 3a and subsequent catalyst
1
regeneration were investigated by in situ H and 11B NMR
spectroscopy. The reaction of Alpine borane 2a (δ(11B) 87
ppm) with 4-phenyl-3-butyn-2-one 1a gave the borinic ester 3a
(δ(11B) 56 ppm) and free α-pinene α-4 (Scheme 2b). Addition
of HBpin 6, to induce B−O/B−H transborylation, gave the
alkoxyboronic ester 7a (δ(11B) 22 ppm) and H-B-9-BBN 2d
(δ(11B) 28 ppm, dimer). The presence of H-B-9-BBN 2d, rather
than catalyst 2a, suggested that α-pinene α-4 was too hindered
to undergo rapid hydroboration. Preventing catalyst regener-
ation allowed the unselective background reactions to dominate,
giving a product with reduced e.e. (Scheme 2a; 2d, 6). The
hydroboration of 1,1-disubstituted alkenes, such as in β-pinene
β-4, is fast13 and would enable catalyst regeneration (Scheme
2c). Reaction of β-pinene-derived myrtanyl borane 2b (δ(11B)
87 ppm) with 4-phenyl-3-butyn-2-one 1a gave the correspond-
ing borinic ester 3a (δ(11B) 56 ppm) in high enantioselectivity
(89% e.e.) and β-pinene β-4. Significantly, the addition of HBpin
6 showed formation of the alkoxyboronic ester 7a (δ(11B) 22
ppm), and re-formation of the borane catalyst 2b (δ(11B) 87
ppm). H-B-9-BBN 2d was not observed, indicating that B−O
transborylation was followed by rapid, chemoselective alkene
hydroboration of β-pinene β-4 to regenerate the catalyst 2b.
With the stoichiometric B−O transborylation having been
established using myrtanyl borane 2b, the use of substoichio-
metric loadings was explored (Scheme 2d). For this catalytic
protocol to be viable, the enantiomeric excess (e.e.) of the
substoichiometric (catalytic) reaction must match that achieved
using stoichiometric borane. This was quantified using
enantiofidelity (e.f.), defined as the degree of enantiomeric
excess retained in the substoichiometric reaction in comparison
to the stoichiometric reaction (Scheme 2d). The reaction
a
Legend: (a) Midland reduction using Alpine borane 2a; (b) missing
step in proposed catalytic Midland reduction; (c) B−O trans-
borylation as a turnover strategy for asymmetric catalysis.
reaction catalytic in borane 2a and provide an exemplar of this
turnover pathway in asymmetric catalysis (Scheme 1b).
The use of an isodesmic B−O/B−H transborylation for
catalytic turnover represents a previously unexploited mecha-
nism that enables catalyst regeneration. However, the activation
barrier for this exchange poses a challenge in application to the
Midland reduction due to the requirement of low temperature to
maintain enantioselectivity. A significant requirement of this
methodology is the regeneration of the catalyst after trans-
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Scheme 2. Assessment of Stoichiometric Borane Reagents for Asymmetric Ketone Reduction and Translation to a Catalytic
a
Method
a
Legend: (a) stoichiometric reduction of 4-phenyl-3-butyn-2-one 1a; (b) single-turnover experiments using boranes 2a and 2b (chemical shifts and
e.e. values refer to the reaction using myrtanyl borane 1b, corrected for use of 92% e.e. β-pinene); (c) hydroboration of β-pinene versus α-pinene
α-4 and comparison to background unselective reductions; (d) catalytic reactions using boranes 2a and 2b.
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f
Table 1. Substrate Scope for the Transborylation-Enabled Asymmetric Ketone Reduction
a
b
Reaction at 18 °C: 5a (92% yield, 35% e.e.), 5m (88% yield, 49% e.e.), 5s (92% yield, 34% e.e.) and 5v (83% yield, 26% e.e.). Reaction over 40 h.
c
d
e
An additional 1 mL of THF added (0.08 M, H-B-9-BBN 2d). HBpin 6 addition at 5.4 μL h−1. d.f. = 100 × (stoichiometric diastereomeric
f
excess)/(catalytic diastereomeric excess). Reaction conditions unless specified otherwise: (S)-β-pinene β-4 (0.2 equiv), H-B-9-BBN 2d (0.5 M in
THF, 0.2 equiv), substrate 1a−y, HBpin 6 (1.2 equiv), 16 h, 0 °C, then addition of H2O and SiO2. Isolated yields are reported. e.e. values for
catalytic reactions are shown in parentheses, with the ee values corrected for the use of 92% e.e. (S)-β-pinene.
development was focused on achieving high enantiofidelity and
not absolute enantioselectivity. To achieve high enantiofidelity,
the rate of catalyst regeneration must exceed the rate of
background reduction by the achiral boranes. The stoichio-
metric reaction of H-B-9-BBN 2d and HBpin 6 with 4-phenyl-3-
butyn-2-one 1a gave the racemic alcohol ( )-5a in 25% and 36%
yields, respectively, under conditions mimicking those of
catalysis (Scheme 2a).
After optimization of the catalytic reaction conditions
(Section S2 in the Supporting Information), the use of myrtanyl
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borane 2b (20 mol %) and HBpin 6 (1.2 equiv) at 0 °C enabled
the asymmetric reduction of 4-phenyl-3-butyn-2-one 1a in 76%
yield and 89% e.e. This matched the yield and enantioselectivity
obtained using stoichiometric myrtanyl borane 2b (90% yield,
89% e.e.), giving 99% e.f. and establishing B−O transborylation
as a mechanism of turnover for asymmetric main-group catalysis.
The catalytic asymmetric reduction was further applied to other
substrate classes; however, this proved unsuccessful in the cases
of acetophenone and 4-phenyl-3-buten-2-one (no reaction) and
an α-keto ester and an α-keto thioester (poor e.e.) (see Table S1
in the Supporting Information).
enantiofidelity in the reduction of ethyl- (1s) and trifluor-
omethyl-substituted (1v) ketones, with the enantiofidelity
increasing from 63% to 82% e.f. and from 19% to 83% e.f.,
respectively. The enantiofidelity could also be improved by
reducing the H-B-9-BBN 2d loading (10 mol %) while
maintaining the β-pinene β-4 loading (20 mol %); the
enantiofidelity of ethyl ketone 1s increased from 63% to 85%
e.f. Reducing the reaction temperature to −20 °C improved the
enantiofidelity (to 85% e.f.), albeit with reduced yield (22%).
Applying B−O transborylation to substrates derived from
biologically active compounds proved successful. Asymmetric
reduction of the galactopyranose-derived substrate 1x gave high
diastereofidelity (95% d.f.). Ketone 1y, derived from propofol
(Diprivan), was reduced with excellent enantiofidelity (99%
e.f.). Gram-scale reduction of ketone 1a under the standard
conditions gave excellent enantiofidelity and yield (80% yield,
98% e.f.).
Two mechanisms of boron−boron exchange have been
proposed: ligand redistribution and transborylation (Scheme
3a).7,8,14 For ligand redistribution, the B−O bond of the borinic
ester 3a is maintained and the supporting ligands are exchanged
with HBpin. Transborylation breaks the B−O bond on the
borinic ester 3a by σ-bond metathesis, with the boron atom of
alkoxyboronic ester 7a originating from HBpin. The reaction of
H10Bpin with borinic ester 3a gave only the 10B-labeled
alkoxyboronic ester 10B-7a, as determined by 10B and 11B
NMR spectroscopy (Scheme 3a). Therefore, exchange
proceeded by B−O transborylation, not ligand redistribution.
The thermodynamic properties of the B−O transborylation
were determined using an Eyring plot constructed over the
temperature range 301−315 K (Scheme 3b; see section S9 in the
Supporting Information).15 This supported a highly ordered
transition-state structure for B−O transborylation with a large
negative entropy value (ΔS⧧ = −21.5 eu)16 and a Gibbs free
energy (ΔG⧧298 = 22.7 kcal mol−1) similar to those of B−C(sp2)
(ΔG⧧ = 20.3 kcal mol−1)7b and B−C(sp3) (ΔG⧧ = 28 kcal
mol−1)7a transborylation reactions.
When all mechanistic investigations were taken into account,
a catalytic cycle for the B−O transborylation-driven asymmetric
ketone reduction was proposed (Scheme 3c). Enantioselective
hydroboration of the ketone 1 by the borane catalyst 2b through
a Meerwein−Ponndorf−Verley-type transition state gives the
enantioenriched borinic ester 3 and releases β-pinene β-4
(enantioselective hydroboration).11,17 B−O/B−H transborylation
of borinic ester 3 with HBpin 6 gives the alkoxyboronic ester
product 7 and releases H-B-9-BBN 2d (transborylation). The
borane catalyst 2b is regenerated by highly chemo-, regio-, and
diastereoselective hydroboration of β-pinene β-4 by H-B-9-BBN
2d (alkene hydroboration).
In summary, B−O transborylation has been established and
applied as a turnover mechanism for asymmetric main-group
catalysis. A catalytic Midland reduction has been enabled, using
B−O/B−H transborylation and myrtanyl borane 2b as the
asymmetric catalyst, across a range of functionalized substrates
with excellent enantiofidelity. B−O transborylation was found to
proceed by a σ-bond metathesis mechanism. Modification of the
catalytic protocol to reduce racemic background reductions by
achiral boron reagents (H-B-9-BBN 2d and HBpin 6) ensured
high enantiofidelity for challenging substrates. This application
of B−O/B−H transborylation demonstrates the potential of
transborylation to be used as a general platform for main-group
catalysis.
The substrate scope of the catalytic asymmetric hydro-
boration was explored using myrtanyl borane 2b as the catalyst,
generated in situ by reaction of H-B-9-BBN 2d (20 mol %) and
β-pinene β-4 (20 mol %) (Table 1). 4-Phenyl-3-butyn-2-one 1a
underwent hydroboration with excellent yield (90%) and
enantiofidelity (99% e.f.). Substitution on the aromatic ring
was tolerated, with excellent enantiofidelity observed for 4-tert-
butyl (1b, 89% e.f.), 4-methyl (1c, 94% e.f.), 3-methyl (1d, >99%
e.f.), and 2-methyl (1e, 97% e.f.) groups. Use of the 4-fluoro
derivative 1f gave good enantiofidelity (88% e.f.) whereas
decreased enantiofidelity was observed for the 3-chloro
analogue 1g (66% e.f.). Lewis basic ether substituents 1n
(84% e.f.) and 1m (92% e.f.) and the thioether 1o (90% e.f.) gave
high enantiofidelity, although the 4-methoxy-substituted 1r gave
lower enantiofidelity (50% e.f.). Reduced enantiofidelity was
observed with the dimethylamino-bearing ketone 1q (60% e.f.).
Excellent chemoselectivity was observed, with groups expected
to react with boranes being tolerated. Nitrile (1w, 91% e.f.), ester
(1j, 74% e.f.), and amide substituents (1i, >99% e.f.) all gave
excellent enantiofidelity. Propargylic ketones bearing electron-
withdrawing substituents, such as 1f (73% e.e.), 1g (46% e.e.),
and 1j (67% e.e.), were reduced in moderate to good e.e.,
presumably due to a greater rate of background, unselective
reduction by HBpin. Propargylic ketones bearing electron-
donating substituents 1a−e consistently gave improved
enantioselectivities (89−77% e.e.). However, in contrast to
ketones bearng electron-donating groups about the arene,
substrates bearing a mesomeric donor in the para position, 1q
(52% e.e.) and 1r (44% e.e.), gave moderate to poor
enantioselectivty. The greater Lewis basicity of these substrates
may increase the rate of unselective reduction, by greater
coordination to the achiral boranes. Although a higher rate of
reaction was achieved at 18 °C, the enantioselectivity was
decreased (5a (92% yield, 35% e.e.), 5m (88% yield, 49% e.e.), 5s
(92% yield, 34% e.e.), and 5v (83% yield, 26% e.e.), presumably
as a result of the low temperature required for enantioselectivity
in the Midland reduction.
Sterically encumbered ketones 1s (63% e.f.) and 1t (39% e.f.)
gave poor to moderate enantiofidelity. Presumably, slow
hydroboration by the enantioenriched borane allowed signifi-
cant background reduction by the less sterically demanding,
achiral boranes H-B-9-BBN 2d and HBpin 6. The trideuter-
iomethyl-substituted ketone 1h was tolerated, but electron-
withdrawing groups such as monofluoromethyl (1u, 44% e.f.)
and trifluoromethyl (1v, 19% e.f.) gave reduced enantiofidelity.
The trifluoromethyl ketone 1v was reduced to the racemic
alcohol ( )-5v by HBpin 6 in 86% yield under the reaction
conditions, indicating that unselective hydroboration by HBpin
6 outcompetes the enantioselective reaction.
Controlling the concentration of achiral boranes (H-B-9-BBN
2d and HBpin 6) could suppress the rate of unselective
hydroboration. The slow addition of HBpin improved the
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a
Scheme 3. Mechanistic Investigations
AUTHOR INFORMATION
Corresponding Author
■
Stephen P. Thomas − EaStCHEM School of Chemistry,
University of Edinburgh, Edinburgh EH9 3FJ, United
Kingdom; orcid.org/0000-0001-8614-2947;
Email: stephen.thomas@ed.ac.uk
Authors
Kieran Nicholson − EaStCHEM School of Chemistry,
University of Edinburgh, Edinburgh EH9 3FJ, United
Kingdom
Joanne Dunne − EaStCHEM School of Chemistry, University of
Edinburgh, Edinburgh EH9 3FJ, United Kingdom
Peter DaBell − EaStCHEM School of Chemistry, University of
Edinburgh, Edinburgh EH9 3FJ, United Kingdom
Alexander Beaton Garcia − EaStCHEM School of Chemistry,
University of Edinburgh, Edinburgh EH9 3FJ, United
Kingdom
Andrew D. Bage − EaStCHEM School of Chemistry, University
of Edinburgh, Edinburgh EH9 3FJ, United Kingdom;
orcid.org/0000-0002-5067-6899
Jamie H. Docherty − EaStCHEM School of Chemistry,
University of Edinburgh, Edinburgh EH9 3FJ, United
Kingdom
Thomas A. Hunt − Medicinal Chemistry, Early Oncology,
AstraZeneca, Cambridge CB4 0WG, United Kingdom
Thomas Langer − Pharmaceutical Technology & Development,
Chemical Development U.K., AstraZeneca, Macclesfield SK10
2NA, United Kingdom
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.0c05168
Author Contributions
∥K.N. and J.D. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.P.T. thanks the Royal Society for a University Research
Fellowship (URF/R/191015). S.P.T., A.D.B., and K.N. thank
AstraZeneca and the EPSRC for iCASE awards. J.D. and S.P.T.
thank the University of Edinburgh for Shaw Macfie Lang Ph.D.
scholarships. S.P.T. and P.D. thank the EPSRC and the
CRITICAT CDT for financial support (EP/L016419/1).
S.P.T. and J.H.D. thank the Royal Society for financial support.
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ASSOCIATED CONTENT
■
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* Supporting Information
this material is available free of charge via the Internet at . The
Supporting Information is available free of charge at https://
pubs.acs.org/doi/10.1021/acscatal.0c05168.
Additional discussion, experimental procedures, kinetic
data and analysis, computational details, characterization
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ACS Catal. 2021, 11, 2034−2040