Reduction of Weinreb amides to aldehydes under ambient conditions with magnesium borohydride reagents Dedicated to the memory of Professor Sheldon Shore

Article abstract of DOI:10.1016/j.tetlet.2014.12.066

Chloromagnesium dimethylaminoborohydride (ClMg⁺ [H₃BNMe₂]^-, MgAB) is an analogue of the versatile lithium dialkylaminoborohydrides (LAB reagents), prepared by the reaction of dimethylamine-borane with methylmagnesium chloride. MgAB is a partial reducing agent for Weinreb amides under ambient conditions and is complementary to the commonly utilized lithium aluminum hydride (LiAlH₄) and diisobutylaluminum hydride (DIBAL) reagents, while exhibiting enhanced chemoselectivity. To prevent over-reduction, the aldehyde products are readily isolated in good yields by forming the sodium bisulfite adducts. Aldehyde products can both be stored and later used as the bisulfite adducts, or can be regenerated from the bisulfite adducts by treatment with aqueous formaldehyde.

Full text of DOI:10.1016/j.tetlet.2014.12.066

Tetrahedron Letters xxx (2015) xxx–xxx
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Tetrahedron Letters
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Reduction of Weinreb amides to aldehydes under ambient
conditions with magnesium borohydride reagents
Christopher L. Bailey a, Jacob W. Clary a, Chittreeya Tansakul b, Lucas Klabunde a, Christopher L. Anderson a,
Alexander Y. Joh a, Alexander T. Lill a, Natalie Peer a, Rebecca Braslau a, Bakthan Singaram a,
⇑
a Department of Chemistry and Biochemistry, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA
b Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand
a r t i c l e i n f o
a b s t r a c t
Chloromagnesium dimethylaminoborohydride (ClMg+ [H3BNMe2]ꢀ, MgAB) is an analogue of the versatile
lithium dialkylaminoborohydrides (LAB reagents), prepared by the reaction of dimethylamine-borane
with methylmagnesium chloride. MgAB is a partial reducing agent for Weinreb amides under ambient
conditions and is complementary to the commonly utilized lithium aluminum hydride (LiAlH4) and dii-
sobutylaluminum hydride (DIBAL) reagents, while exhibiting enhanced chemoselectivity. To prevent
over-reduction, the aldehyde products are readily isolated in good yields by forming the sodium bisulfite
adducts. Aldehyde products can both be stored and later used as the bisulfite adducts, or can be regen-
erated from the bisulfite adducts by treatment with aqueous formaldehyde.
Article history:
Received 26 November 2014
Revised 10 December 2014
Accepted 12 December 2014
Available online xxxx
Dedicated to the memory of Professor
Sheldon Shore
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Synthetic methods
Aminoborohydride
Weinreb amide
Partial reduction
Aldehyde
Introduction
particular note are the N-methoxy-N-methylamides (Weinreb
amides).4 Weinreb amides are readily prepared from carboxylic
Reduction of amides with metal hydride reagents to the corre-
sponding alcohol and amine is not unusual.1 However, the reduc-
tion of amides to the corresponding aldehydes represents a
challenge and only a few methods are available to do this transfor-
mation. The partial reduction of amides to aldehydes is often sub-
strate specific, being highly dependent upon the nature of the
substituent on the amide nitrogen atom, with bulkier substituents
affording higher yields of aldehydes.2–4 In most cases, this transfor-
mation is carried out with aluminum hydrides such as LiAlH4,5
DIBAL,6 and their derivatives.7 The general reduction of amides to
aldehydes with commercially available metal hydride sources
results in poor yields of aldehydes.8 These reductions often require
cryogenic reaction conditions followed by exothermic workups,
resulting in over-reduction to amines or alcohols. Borohydrides
reduce tertiary amides, but are not as reactive as aluminum
hydrides.9
acids14–16 and their derivatives, such as acid chlorides,4 esters,17
lactones, and anhydrides.14 Since their discovery, these amides
have been utilized extensively as acylating agents in synthe-
sis.14,18–21 The presence of the methoxy group on the N-atom
allows the formation of stable chelates of the tetrahedral interme-
diate following one equivalent of organometallic nucleophile, pre-
venting further nucleophilic addition. Reaction of Weinreb amides
with Grignard and organolithium reagents is one of the best ways
to generate ketones. However, controlled reduction of Weinreb
amides to the corresponding aldehydes typically uses very strong
reducing agents such as DIBAL or LiAlH4 and cryogenic reaction
conditions.22 It should be noted that at ꢀ78 °C DIBAL is capable
of reducing amides, especially Weinreb amides, to the correspond-
ing aldehyde with very little over reduction arising from b-hydride
elimination. However, to our knowledge no one has reported the
reduction of Weinreb amides to the corresponding aldehydes using
DIBAL at 25 °C, as this gives a mixture of products, including alco-
hols, amines, and aminals.23
One synthetic strategy to prevent over-reduction employs spe-
cialized amide derivatives such as N-acylcarbazoles10 (acylimidaz-
oles11 and acylaziridines12), and morpholine amides.13 Of
Recent work toward the partial reduction of amides has
resulted in the reported use of transition metal hydrides, such as
titanium and zirconium hydrides.24,25 Buchwald reported that
the combination of titanium(IV) isopropoxide and diphenylsilane
⇑
Corresponding author. Tel.: +1 831 459 3154; fax: +1 831 459 2935.
E-mail address: singaram@ucsc.edu (B. Singaram).
http://dx.doi.org/10.1016/j.tetlet.2014.12.066
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
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C. L. Bailey et al. / Tetrahedron Letters xxx (2015) xxx–xxx
can reduce N,N-disubstituted amides to the corresponding alde-
hydes.26 However, this method is limited to
-enolizable sub-
the product chloromagnesium dimethylaminoborohydride has
JBH = 83 Hz.
a
strates via an enamine intermediate. Georg reported a promising
reduction of tertiary amides, including Weinreb amides, to the cor-
responding aldehydes using Cp2Zr(H)Cl, proceeding through an
imine intermediate.27 Ganem has also demonstrated a procedure
to reduce amides and lactams to the corresponding imines using
two equivalents of Cp2Zr(H)Cl, followed by anhydrous workup.25
However, the sensitivity of this zirconium reagent requires that it
be prepared fresh prior to use, limiting its synthetic application.28
It is highly desirable to develop a stable reducing agent based on
readily available metals, which is capable of controlled room tem-
perature reduction.
A recent observation prompted this study: during the synthesis
of vinyl ketone 2, the Weinreb amide precursor 1 was treated with
an aged commercial solution of vinylmagnesium bromide. Upon
work up, a mixture of the desired product 2 and aldehyde 3 was
isolated (Eq. 1).
When synthesizing MgAB, chloride-based methyl or ethyl Grig-
nard reagents give complete conversion. However bromide-based
Grignard reagents afford mixtures due to Schlenk disproportion-
ation.30 MgAB can be stored under inert atmosphere at room tem-
perature for at least three months without any disproportionation
as monitored by 11B NMR.
Partial reduction of Weinreb amides using MgAB
Previously, we have reported that various lithium aminoboro-
hydrides (Li+ [H3BNR2]ꢀ, LAB reagents) reduce both aliphatic and
aromatic amides to give either the corresponding alcohols or
amines, depending on both the steric environments of the tertiary
amide and the LAB reagent.31 Controlled reduction of Weinreb
amides using either LAB reagents or 9-borabicyclo3.3.1nonane
(9-BBN) was not demonstrated.32
ð1Þ
The partial reduction of N-methoxy-N-methylbenzamide to
benzaldehyde using 4 was investigated as a model substrate. One
equivalent of 4 reduced N-methoxy-N-methylbenzamide to benz-
aldehyde in 30 min at 25 °C, as evidenced by TLC analysis. How-
ever, after acidic quench and aqueous work up, 1H NMR analysis
of the crude mixture revealed the presence of benzyl alcohol in
addition to benzaldehyde. We speculated that an inexpensive sac-
rificial aldehyde, such as acetaldehyde, could be used as a hydride
scavenger during the quench. Indeed, dropwise transfer of the
reduction mixture to a pentane solution of acetaldehyde and acetic
acid prevented the over-reduction, but contamination was
observed. Attempted purification of the crude benzaldehyde by sil-
ica gel column chromatography resulted in the isolation of almost
pure benzyl alcohol. Dimethylaminoborane (Me2N-BH2), the by-
product from 4, exists as a stable dimer and is usually unreactive
to aldehydes.33 Evidently, activation of the aldehyde carbonyl by
silica gel results in the reduction of benzaldehyde, similar to that
observed for the silica gel-promoted reduction by N-heterocyclic
carbene boranes.34
Using freshly prepared vinylmagnesium bromide, only the
desired a,b-unsaturated ketone 2 was formed, indicating the pres-
ence of a hydride reducing agent contaminant in the commercial
sample of Grignard reagent. To verify this hypothesis, the older
commercial vinylmagnesium bromide was added to benzaldehyde.
1H NMR analysis of the product revealed a 2:1 mixture of the
expected allylic alcohol, 1-phenyl-2-propen-1-ol, and benzyl alco-
hol. Similar observations of reduction products from Grignard reac-
tions with Weinreb amides have been reported in the literature,17
thus warranting an investigation into the reduction of Weinreb
amides using magnesium-based hydrides. We have recently syn-
thesized chloromagnesium dimethylaminoborohydride (ClMg+ H3-
BNMeꢀ2 , MgAB 4), using the reaction of Grignard reagents with
dimethylamine-borane.29 Herein is a study of the ability of MgAB
to reduce Weinreb amides to the corresponding aldehydes.
Results and discussion
Even though MgAB is an excellent reducing agent to achieve the
controlled reduction of Weinreb amides to aldehydes, convenient
isolation of pure aldehyde products proved to be a challenge. Puri-
fication of aldehydes by addition of bisulfite is a well-established
procedure; ketones and aldehydes form insoluble solid bisulfite
adducts.35 Separating bisulfite adducts from the crude reaction
mixture, followed by the regeneration of the aldehyde, proved to
be convenient and practical. The aldehydes are readily regenerated
by treatment with either aqueous acid36 or base.37 Regeneration of
the aldehyde by treatment of the bisulfite adduct with aqueous
formaldehyde38 was straightforward, allowing for the isolation of
various aldehydes (Table 1).39
Synthesis of MgAB reagent from the reaction of Grignard
reagents and amine-borane
Chloromagnesium dimethylaminoborohydride (MgAB) was
quantitatively synthesized from methylmagnesium chloride and
dimethylamine-borane (Scheme 1).29b
In the 11B NMR spectrum in THF, the MgAB species appears as a
quartet at dB ꢀ16 ppm, while the starting amine-borane has a
chemical shift of dB ꢀ14 ppm. The starting material and the prod-
uct can be further distinguished by their coupling constants;
dimethylamine-borane exhibits a quartet with JBH = 98 Hz, while
Weinreb amide substrates of varying steric and electronic nat-
ure were reduced under these mild conditions. Aromatic substrates
bearing electron-donating (entries 2–4) or electron-poor
(entries 6–9) groups were amenable to reduction without
complication. Reduction is even observed for substrates with
increased steric demand (entries 2 and 7). Furthermore, the cin-
namic amide was reduced without any observed over-reduction
to cinnamyl alcohol as a side product (entry 5). The reduction of
Scheme 1. Synthesis of MgAB.
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C. L. Bailey et al. / Tetrahedron Letters xxx (2015) xxx–xxx
3
Table 1
7 in the presence of MgAB, which led to quantitative conversion to
4-cyanobenzaldehyde 8 in 74% yield (Eq. 2).
Reduction of Weinreb amidesa
Yieldb (%)
75
ð2Þ
Entry
1
Amide
Product
Gratifyingly, a similar result was obtained by the reaction of
methyl 4-(methoxy(methyl)carbamoyl)benzoate 9 with MgAB,
affording the corresponding aldehyde methyl 4-formylbenzoate
10 in 70% yield (Eq. 3).
2
3
4
5
74
81
82
70
ð3Þ
Lastly, reaction of MgAB with N-methoxy-N-methyl-4-nitrob-
enzamide 11 afforded 4-nitrobenzaldehyde 12 in 89% yield (Eq. 4).
ð4Þ
The clean reduction of the Weinreb amides in the presence of a
nitro group, a nitrile, an ester, and conjugated olefin functionalities
indicates a unique chemoselective profile of MgAB.
The bisulfite adducts of aldehydes formed in the workup proto-
col are stable crystalline solids, and are amenable to long term
storage or use in situ.40 Thus reduction of Weinreb amides to pro-
vide the robust bisulfite adducts directly was explored. Isolating
the bisulfite adducts in lieu of liberating the free aldehyde allowed
expansion of the study to include adducts of water-soluble alde-
hydes. For example, heteroaromatic amides were amenable to
the reduction procedure, however the corresponding aldehydes
have increased water solubility, hindering isolation (Table 2).
6
81
7
8
74
70
Table 2
Reduction of Weinreb amides to aldehydes isolated as bisulfite adductsa
9
75
10
80c
Entry
1
Amide
Product
Yieldb (%)
70
a
Reactions conducted on a 2 mmol scale with 1 equiv Weinreb amide and
1 equiv ClMg+ [H3BNMe2]ꢀ.
b
Isolated yield of aldehyde after liberation from bisulfite adduct.
60 min.
c
2
3
70
67
N-methoxy-N-methylcinnamide
previously
reported
with
Cp2Zr(H)Cl afforded significant amounts of cinnamyl alcohol.27
Aliphatic amides are also amenable to reduction with this method-
ology (entry 10).
Of particular interest is the chemoselective reduction of Weinreb
amides by MgAB. Reduction of the cinnamic amide demonstrates
1,2-reduction in the absence of 1,4-reduction (Table 1, entry 5).
The chemoselectivity was further explored by examining the room
temperature reaction of 4-cyano-N-methoxy-N-methylbenzamide
a
Reactions conducted on a 2 mmol scale with 1 equiv Weinreb amides and
1 equiv ClMg+ [H3BNMe2]ꢀ.
b
Isolated yield of solid bisulfite adduct.
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4
C. L. Bailey et al. / Tetrahedron Letters xxx (2015) xxx–xxx
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In summary, a mild, simple, and efficient method for the reduc-
tion of Weinreb amides to aldehydes under ambient conditions has
been developed using a new reducing agent, chloromagnesium
dimethylaminoborohydride (MgAB). This reagent can be prepared
by the reaction of methylmagnesium chloride with dimethyl-
amine-borane at 0 °C. MgAB exhibits a unique chemoselective pro-
file, capable of reducing Weinreb amides to aldehydes in the
presence of a nitro group, a nitrile, an ester, and a conjugated dou-
ble bond. The aldehyde product is effectively isolated as the corre-
sponding bisulfite adduct, which can be stored, or unveiled to
provide pure aldehydes. MgAB is milder, safer, and complementary
to reducing agents typically used to convert Weinreb amides to
aldehydes.
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Acknowledgments
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N.P. thanks the National Institutes of Health NIGMS Bridges to
the Future Program (GM 51765-10). C.T. thanks the Royal Thai
Government for a Higher Education Strategic Scholarship for Fron-
tier Research Network.
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39. Procedure for the reduction of Weinreb amides: To a stirred solution of N-
methoxy-N-methyl-4-nitrobenzamide (0.420 g, 2 mmol) in THF (1.7 mL) was
added MgAB (2 mL, 1 M, 2 mmol) dropwise via a syringe. After 30 min, the
reaction solution was added dropwise to a solution of acetaldehyde (2 mmol)
and acetic acid (2 mmol) in pentane (10 mL). After 15 min, saturated aqueous
NH4Cl (2 mL) was added. The organic layer was separated and the aqueous
phase was extracted with Et2O (2 ꢁ 10 mL). The combined organic layers was
washed with 1 M HCl (10 mL), dried with magnesium sulfate, and
concentrated under reduced pressure to yield crude aldehyde as orange oil.
To the crude aldehyde was added EtOH (3 mL) and EtOAc (5 mL) and the
solution was cooled with an ice bath. A saturated aqueous solution of NaHSO3
(1 mL) was added with stirring. After 4 h, the solid bisulfite adduct was isolated
by vacuum filtration, washed with Et2O (3 ꢁ 5 mL) and dried under vacuum to
yield a white solid. The bisulfite adduct was then added to a round-bottom
flask dissolved in H2O (5 mL) and a 37% formalin solution (2 mL) was added
followed by Et2O (20 mL). The biphasic solution was stirred for 1 h. The
aqueous layer was separated and extracted with a 1:1 mixture of THF/Et2O
(3 ꢁ 10 mL). The combined organic layers was dried over magnesium sulfate,
and concentrated under reduced pressure to give 12 (0.269 g, 75% yield).
40. Faul, M.; Larsen, R.; Levinson, A.; Tedrow, J.; Vounatsos, F. J. Org. Chem. 2013,
78, 1655–1659.
Supplementary data
Supplementary data (these data include NMR spectra for all
compounds described in this Letter) associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2014.12.066.
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Please cite this article in press as: Bailey, C. L.; et al. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2014.12.066