A Manganese Pre-Catalyst: Mild Reduction of Amides, Ketones, Aldehydes, and Esters

Article abstract of DOI:10.1002/anie.201709441

A new (N-phosphinoamidinate)manganese complex is shown to be a useful pre-catalyst for the hydrosilative reduction of carbonyl compounds, and in most cases at room temperature. The Mn-catalyzed reduction of tertiary amides to tertiary amines, with a useful scope, is demonstrated for the first time by use of this catalyst, and is competitive with the most effective transition-metal catalysts known for such transformations. Ketones, aldehydes, and esters were also successfully reduced under mild conditions by using this new Mn catalyst.

Full text of DOI:10.1002/anie.201709441

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Accepted Article
Title: A Manganese Pre-Catalyst for the Mild Reduction of Amides,
Ketones, Aldehydes, and Esters
Authors: Colin Kelly, Robert McDonald, Orson Sydora, Mark Stradiotto,
and Laura Turculet
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201709441
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A Manganese Pre-Catalyst for the Mild Reduction of Amides,
Ketones, Aldehydes, and Esters
Colin M. Kelly, Robert McDonald, Orson L. Sydora,* Mark Stradiotto,*,[a] and Laura Turculet*,[a]
[
a]
[b]
,[c]
Abstract: A new (N-phosphinoamidinate)manganese complex 1 is
shown to be a useful pre-catalyst for the hydrosilative reduction of
carbonyl compounds, in most cases at room temperature. The Mn-
catalyzed reduction of tertiary amides to tertiary amines with useful
scope is demonstrated for the first time by use of 1, and is competitive
with the most effective transition metal catalysts known for such
transformations. Ketones, aldehydes, and esters were also
successfully reduced under mild conditions by use of 1.
Scheme 1. Selective reduction of tertiary amides to tertiary amines by use
of 3d transition metal catalysts.
The reduction of carbonyl compounds is an important class of
transformations that is employed widely in synthetic chemistry.[1]
In moving beyond the use of harsh alkali metal hydrides and
related reductants, transition metal catalysis offers an alternative
for such transformations. While platinum-group metal catalysts
were examined early on, the high cost and low abundance of such
metals has created motivation for the development of competitive
late 3d transition metal catalysts.[2] As such, a diversity of effective
uncommon. Following a report by Mamillapalli and Sekar,[9]
whereby complete reduction of both keto and amide groups in -
keto amides was achieved by use of Ph
10 mol% TMEDA, 10 mol% KOtBu, 25 ˚C), Garg and co-
workers[10] disclosed the use of PhSiH
in amide reductions with
NiCl (dme) (10 mol% Ni, 115 ˚C, 24 h). This progress
notwithstanding, the reduction of amides remains a significant
2 2 2
SiH (5 mol% Ni(OAc) ,
3
2
base metal-catalyzed carbonyl reduction protocols have emerged, challenge. Moreover, 3d transition metal catalysts capable of
including atom-economical transformations using hydrogen as
the reductant. Complementary transformations such as
hydrosilylation[ are also of interest, in that they in some cases
are successful when direct hydrogenation fails, and can be carried
effecting the hydrosilative reduction of amides, ketones,
aldehydes, and esters are, to the best of our knowledge, unknown.
In moving beyond Fe, Co, and Ni, the application of Mn
catalysis in organic synthesis has experienced a renaissance,[
3]
11]
out conveniently without the need for pressurized reaction vessels. including recent examples of carbonyl reductions based on
transfer hydrogenation,[
12]
hydrogenation,[13] hydroboration,[14]
Whereas useful base metal catalysts for the hydrosilative
reduction of ketones, aldehydes, and/or esters have been
developed,[2, 3b] selective reductions of amides to amines have
proven to be significantly more challenging (Scheme 1).[4] The first
reports of such transformations were disclosed simultaneously by
and hydrosilylation.[ Conversely, prior to our work herein the
Mn-catalyzed reduction of amides to amines was limited to two
reports: one[16] involving the reduction of dimethylformamide or
diethylformamide employing a tertiary silane and [CpMn(CO)
mol%), either under thermal (120 ˚C, 24 h) or photochemical
activation (25 ˚C, 12-18 h); and another[17] using [Mn
(CO)10] for
the reduction of N-acetylpiperidine with Et SiH (2 mol% Mn, 5
mol% Et
NH co-catalyst, 100 ˚C, 16 h).[18] As part of our program
15]
3
] (5
the groups of Beller[ and Nagashima, whereby tertiary amides
5]
[6]
were
polymethylhydrosiloxane) in the presence of [Fe(CO)
Fe
reduced
by
use
of
PMHS
(PMHS
=
or
2
5
]
3
[
3
(CO)12] (6-30 mol% Fe), either under thermal (100 ˚C, 24 h)
2
[7]
or photochemical activation (25 ˚C, 9 h). The Co-catalyzed
targeting effective 3d transition metal catalysts supported by N-
phosphinoamidinate ligation,[19] we turned our attention to the
application of Mn pre-catalysts. We report herein that the new [(2-
hydrosilative reduction of amides is limited to the use of
[
˚
Co
2
(CO)
8
] (1 mol% Co) primarily in combination with PMHS (100
C, 3-16 h).[ Amide reductions employing Ni catalysis are also
8]
3 2
P,N)Mn(N(SiMe ) )] complex 1 can catalyze the hydrosilative
reduction of tertiary amides to tertiary amines in a manner that is
competitive with the most effective catalysts known for such
transformations. Pre-catalyst 1 is also shown to accommodate an
unusual breadth of carbonyl substrates (amides, ketones,
aldehydes, and esters), in most cases at room temperature.
The pre-catalyst 1 was selected for the Mn-catalyzed reduction
of amides and other carbonyl compounds, on the basis of the
outstanding performance of the analogous Fe complex (i.e., 1-Fe)
[
a]
C. M. Kelly, Prof. Dr. M. Stradiotto, Prof. Dr. L. Turculet
Department of Chemistry
Dalhousie University
Halifax, Nova Scotia B3H 4R2 (Canada)
E-mail: mark.stradiotto@dal.ca; laura.turculet@dal.ca
Dr. R. McDonald
X-ray Crystallography Laboratory, Department of Chemistry
University of Alberta
[
b]
c]
[19b]
Edmonton, Alberta T6G 2G2 (Canada)
Dr. O. L. Sydora
Research and Technology
in the reduction of ketones, aldehydes, and esters.
Treatment
[
H
of the pro-ligand (PN ) with [Mn(N(SiMe
3
)
2
)
2
] afforded 1, which
was isolated as an analytically pure yellow solid in 73% yield
(Figure 1).[20] The measured magnetic moment (Evan’s method,
Chevron Phillips Chemical Company LP
1862 Kingwood Drive
Kingwood, Texas 77339 (USA)
E-mail: sydorol@cpchem.com
300 K, µeff = 5.9 µ
/2) center, and X-ray data (Figure 1) confirm the three-coordinate,
distorted trigonal planar nature of 1 (anglesMn = 360˚).
B
) is consistent with a high-spin Mn(II) (SMn =
5
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.2017XXXXX.
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COMMUNICATION
Figure 2. Catalytic reduction of tertiary amides to tertiary amines
employing 1. Reaction conditions: Unless stated otherwise, amide (0.4
mmol), PhSiH (0.4 mmol), benzene (500 L), 1 (5 mol%), 75 ˚C, 1 h.
3
Target product identity confirmed on the basis of NMR analysis, and
conversion to 2 determined by use of calibrated GC methods. Isolated
yields in parentheses. [a] 1 (2 mol%), 25 ˚C, 18 h. [b] 3 h. [c] 2 mol% 1, 25
Figure 1. Synthesis and crystallographically determined structure of 1.
Selected interatomic distances (Å) and angles (˚) within the two
crystallographically independent molecules of 1: Mn-P 2.5319(4) and
2.5045(4), Mn-N(1) 2.0722(12) and 2.0946(11), Mn-N(3) 1.9742(12) and
1.9696(12), P-Mn-N(1) 78.93(3) and 78.25(3), P-Mn-N(3) 135.20(4) and
141.28(4), N(1)-Mn-N(3) 145.54(5) and 140.11(5).
˚C, 48 h.
Having prepared 1, we then sought to evaluate the ability of this
pre-catalyst to effect the reduction of tertiary amides under
conditions of modest heating and relatively short reaction times (5
It is worthy of mention that in the absence of photochemical
activation, the reduction of simple tertiary amides at room
temperature by use of a 3d transition metal catalyst is, to the best
of our knowledge, unknown.[21] In this context, we examined
3
mol% 1, 75 ˚C, 1 h). Our choice of PhSiH as the reductant was
made on the basis of our previous examination of acetophenone
reduction employing 1-Fe, whereby the use of alternative silanes
including PMHS afforded poor results. Under our chosen
conditions (Figure 2), pre-catalyst 1 proved effective in enabling
the high-yielding and selective reduction of various tertiary
amides to the corresponding tertiary amines. N-Benzyl
piperidines featuring substitution on both the piperidine and aryl
rings were successfully generated, including the hindered variant
subsequently the ability of 1 to effect such unprecedented
transformations (2 mol% 1, 25 ˚C, 18 h; Figure 2). We were
pleased to observe that for several of the tertiary amide substrates
under investigation herein, high conversion to the desired tertiary
amine was achieved under room temperature conditions,
including the formation of N-benzylpiperidine (2a), N-
benzylmorpholine (2f), tribenzylamine (2h), and N,N-
dimethylbenzylamine (2l).
Limitations to this amide reduction chemistry were encountered.
Despite the high isolated yield of 2g (89%) that was achieved by
use of 1 at elevated temperature, poor conversion of the
thiophene-derived amide substrate was observed at room
temperature. Moreover, efforts to effect the reduction (5 mol% 1,
2e; N-benzylmorpholine (2f) was obtained similarly in high
isolated yield (85%). The reduction of tertiary dibenzylamides to
tertiary dibenzylamines featuring (hetero)benzyl, substituted
benzyl, and aliphatic substituents (2g-2k) was also successful.
The reduction of tertiary amides leading to dialkyl or aryl/alkyl
benzylamine products (2l and 2m) also proved feasible.
75 ˚C, 1 h) of 1-benzyl-2-piperidone, or variants of 2i whereby the
Comparative experiments confirmed the superiority of
relative to both 1-Fe and [Mn(N(SiMe ]. In the case of amide
reductions leading to 2k (5 mol% pre-catalyst, 75 ˚C, 1 h) 70%
conversion was achieved by use of [Mn(N(SiMe ] (cf. >90%
1
methoxy group was replaced with either cyano or nitro groups,
resulted in complex product mixtures from which the desired
tertiary amine could not be isolated. As well, our efforts to extend
the use of 1 to the reduction of primary or secondary amides have
thus far proven unsuccessful.
The success of 1 in the room temperature reduction of tertiary
amides prompted us to explore further the ability of this pre-
catalyst to effect the reduction of alternative carbonyl substrates
under mild conditions (1 mol% 1, 25 ˚C, 4 h). Acetophenone and
derivatives featuring electron-donating, electron-withdrawing, and
extended aromatic substituents were each reduced successfully
3 2 2
) )
3 2 2
) )
with 1), whereas a complex mixture of products including
unreacted tertiary amide and 2k (~1:1 ratio) was formed when
using 1-Fe. Similarly, <50% conversion to 2l was achieved when
using either [Mn(N(SiMe
90% conversion to 2l was attained by use of 1 (5 mol% pre-
catalyst, 75 ˚C, 1 h). In keeping with our preliminary conjecture
vide supra), the use of PMHS (1 or 2 equiv, in a manner
3 2 2
) ) ] or 1-Fe under conditions where
>
(
[
5]
analogous to that described by Beller and co-workers ) with 1 in
several test reactions afforded low conversion of the tertiary
amide, under conditions where complete conversion to the target
(
3a-3d), as was benzophenone leading to 3e (Figure 3). Related
acetophenone variants featuring sterically demanding alkyl,
heteroaryl, or pendant alkene functionality, as well as 4-
heptanone, were also well accommodated, resulting in high
conversion to 3f-3i. Aldehyde reduction was also achieved under
mild conditions by use of 1, leading to benzyl alcohol variants
tertiary amine was achieved when using PhSiH
ability to conduct such reductions on a proportionally larger scale
3.55 mmol amide) was demonstrated in the successful synthesis
of 2k (0.930 g, 98% isolated yield; 5 mol% 1, 75 ˚C, 4 h).
3
(Figure 2). The
(
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featuring electron-donating, electron-withdrawing, ortho-, and
heteroaryl functionality (3j-3o). The observation of unreacted
PhSiH (each featuring three reducing equivalents) in these
3
reactions suggests that that sub-stoichiometric amounts of the
reductant may be employed.
To place this work in context, Trovitch’s Mn pre-catalysts
featuring phosphine-tethered bis(imino)pyridine ligation[15a, 15b, 15d]
can function effectively at <1 mol% loading, and represent the
most active catalysts known for the hydrosilylation of aldehydes
and ketones. While we did not explore lower loadings or shorter
reaction times exhaustively, it is evident that 1 is less active than
the Trovitch catalysts. However, 4-heptanone proved to be an
unusually reactive substrate in our study using 1, with vigorous
bubbling and a distinct exotherm noted upon mixing under our
standard conditions (Figure 3). Further examination revealed that
at the 0.25 mol% 1 loading level the reduction of 4-heptanone is
Figure 4. Catalytic reduction of esters employing 1. Reaction conditions:
Ester (0.8 mmol), PhSiH
3
(0.8 mmol), benzene (500 L), 1 (1 mol%), 25
˚C, 4 h. Conversion to 3 determined by use of calibrated GC methods.
Isolated yields in parentheses. [a]Conversion based on silyl ether
formation.
outlined in Figures 3 and 4, the formation of complex reaction
mixtures and/or low conversion to the target reduction product
occurred in the attempted reduction of 4-acetylpyridine,
benzylideneacetone,
ethyl
4-pyridylacetate,
methyl
4-
-1
complete within 2 minutes (TON = 400; TOFfinal = 12,000 h ); high
conversion is also achieved for this ketone at lower loadings (0.1
mol% 1, 25 ˚C, 4 h).
nitrobenzoate, or phthalide. In several cases vigorous bubbling
was observed, along with the formation of a precipitate. Studies
directed toward understanding the fate of
transformations are ongoing.
1
in these
In summary, the newly developed Mn pre-catalyst 1 is shown
to be broadly useful for the hydrosilative reduction of carbonyl
compounds, including at room temperature, with the established
scope (amides, ketones, aldehydes, and esters) exceeding that
demonstrated previously by any other 3d transition metal catalyst
system under such mild conditions. In particular, the normally
challenging reduction of tertiary amides to tertiary amines that is
achieved with useful scope employing
1 proceeds under
conditions that are competitive with the most effective 3d
transition metal catalysts known for such transformations, thereby
establishing a new class of synthetically useful Mn-catalyzed
transformations. Future research will focus on gaining an
understanding of the mechanism of these transformations, as well
as on developing catalyst variants that address the
aforementioned substrate scope limitations.
Figure 3. Catalytic reduction of ketones and aldehydes employing 1.
3
Reaction conditions: Ketone/aldehyde (0.8 mmol), PhSiH (0.8 mmol),
benzene (500 L), 1 (1 mol%), 25 ˚C, 4 h. Conversion to 3 determined by
use of calibrated GC methods. Isolated yields in parentheses.
Acknowledgements
We thank Chevron Phillips Chemical Company LP for supporting
this research and for permission to publish. NSERC of Canada
and Dalhousie University are acknowledged for their support of
this work.
In exploring further the scope of carbonyl reductions that can
be achieved by use of 1, we briefly examined the selective
reduction of methyl esters to alcohols under mild conditions (1
mol% 1, 25 ˚C, 4 h; Figure 4); the first examples of such room
temperature transformations were achieved by use of 1-Fe.[19b]
A
Keywords: amide • reduction • hydrosilylation • manganese
series of methyl benzoate derivatives featuring electron-donating
or electron-withdrawing groups were efficiently reduced under
these conditions, as was methyl heptanoate. Tracking the
reaction progress revealed that high (but incomplete) conversions
were achieved throughout after only 1-2 h. In this regard, pre-
catalyst 1 is apparently more effective for the hydrosilative
reduction of methyl esters, in comparison to Trovitch’s phosphine-
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reduction of ketones and esters by using 1. Under the conditions
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COMMUNICATION
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provided in the Supporting Information; CCDC 1569566 contain
the supplementary data for the crystallographic characterization of
[
[
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1.
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Entry for the Table of Contents
COMMUNICATION
Colin M. Kelly, Robert McDonald, Orson
L. Sydora,* Mark Stradiotto,* and Laura
Turculet*
Page No. – Page No.
A Manganese Pre-Catalyst for the
Mild Reduction of Amides, Ketones,
Aldehydes, and Esters
Hey Man(ganese): A newly prepared (N-phosphinoamidinate)manganese pre-
catalyst has been shown to be effective for the hydrosilative reduction of a diverse
scope of carbonyl compounds, in most cases at room temperature.
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