Hydrosilylative Reduction of Tertiary Amides to Amines Catalyzed by N-(Phosphinoaryl)anilido Complexes of Iron and Cobalt

Article abstract of DOI:10.1002/cctc.201900550

The synthesis and structural characterization of low-coordinate Fe(II) and Co(II) complexes supported by the monoanionic P,N-ligand N-(2-dicyclohexylphosphinophenyl)-2,6-diisopropylanilide are described. A three-coordinate (P,N)Fe-hexamethyldisilazide complex (2), and four-coordinate (P,N)Fe- (3-Fe) and (P,N)Co-alkyl (3-Co) complexes were evaluated as pre-catalysts for the hydrosilylative reduction of amides with PhSiH₃ (5 mol % pre-catalyst, 1 equiv. PhSiH₃, 80 °C, 1–24 h). The Fe complex 2 proved to be more broadly effective for the reduction of a variety of tertiary amide substrates, and was shown to mediate the reduction of N,N-dibenzylbenzamide at a loading of 1 mol %, to achieve near quantitative formation of tribenzylamine in 1 h (80 °C). Complex 2 also proved effective for the hydrosilylation of tertiary amides under ambient conditions (5 mol % Fe, 24 h), which is a unique example of room temperature amide hydrosilylation mediated by an Fe catalyst without the need for photochemical activation. Given the widespread use of amide reduction protocols in synthesis, the development of efficient Fe-based catalysts that operate under mild conditions is an important target.

Full text of DOI:10.1002/cctc.201900550

Accepted Article
Title: Hydrosilylative Reduction of Tertiary Amides to Amines Catalyzed
by N-(Phosphinoaryl)anilido Complexes of Iron and Cobalt
Authors: Dylan J. Hale, Luke J. Murphy, Robert McDonald, Michael J.
Ferguson, and Laura Turculet
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Hydrosilylative Reduction of Tertiary Amides to Amines
Catalyzed by N-(Phosphinoaryl)anilido Complexes of Iron and
Cobalt
Dylan J. Hale,[a] Luke J. Murphy,[a] Robert McDonald,[b] Michael J. Ferguson[b] and Laura Turculet*[a]
Abstract: The synthesis and structural characterization of low-
coordinate Fe(II) and Co(II) complexes supported by the
monoanionic P,N-ligand N-(2-dicyclohexylphosphinophenyl)-2,6-
comparatively low toxicity, and high abundance of Fe.[1e, 4] In the
context of amide hydrosilylation, early efforts towards this goal
were reported concurrently by Beller[5] and Nagashima,[6] who
demonstrated the utility of Fe carbonyl species in such catalysis
under both thermal (6 - 30 mol% Fe, 100 °C, 24 h) and
photochemical (10 mol% Fe, RT, 9 h) conditions. Since these
initial reports, the groups of Sortais and Darcel,[7] Buitrago and
Adolfsson,[8] and Driess[9] have each reported on the utility of Fe
complexes supported by N-heterocyclic carbene ligation for
amide hydrosilylation catalysis. Although the natural abundance
of Fe makes it the top choice amongst the transition metals from
diisopropylanilide are described.
A three-coordinate (P,N)Fe-
hexamethyldisilazide complex (2), and four-coordinate (P,N)Fe- (3-
Fe) and (P,N)Co-alkyl (3-Co) complexes were evaluated as pre-
catalysts for the hydrosilylative reduction of amides with PhSiH3 (5
mol% pre-catalyst, 1 equiv. PhSiH3, 80 °C, 1-24 h). The Fe complex
2 proved to be more broadly effective for the reduction of a variety of
tertiary amide substrates, and was shown to mediate the reduction
of N,N-dibenzylbenzamide at a loading of 1 mol%, to achieve near
quantitative formation of tribenzylamine in 1 h (80 °C). Complex 2
also proved effective for the hydrosilylation of tertiary amides under
ambient conditions (5 mol% Fe, 24 h), which is a unique example of
room temperature amide hydrosilylation mediated by an Fe catalyst
without the need for photochemical activation. Given the widespread
use of amide reduction protocols in synthesis, the development of
efficient Fe-based catalysts that operate under mild conditions is an
important target.
the perspective of sustainability,
a handful of noteworthy
examples of amide hydrosilylation mediated by other base
metals, such as Mn-,[10] Co-,[11] and Ni-,[12] as well as Cu-[13] and
[14]
Zn-based
catalysts, have also been disclosed, and efforts to
develop non-metal catalysts have also been undertaken.[15]
These notwithstanding, such reports are still very few in
comparison to related hydrosilylative reductions of carbonyl
species such as ketones and aldehydes.[4d-g, 16]
In a collaborative effort, our group has recently reported on a
versatile Mn catalyst featuring bidentate N-phosphinoamidinate
ligation that is broadly useful for the hydrosilylation of carbonyl
substrates under mild conditions, including rare examples of
room temperature hydrosilylative reduction of tertiary amides
Introduction
The reduction of amides to amines is a fundamental reaction of
broad utility in organic synthesis.[1] While reactive alkali metal
hydride or boron hydride reagents are commonly used as
stoichiometric reductants in this regard, significant effort has
been devoted to developing alternative routes that may prove
less harsh and more selective, including transition metal
catalyzed hydrosilylation.[1d] Notable examples of amide
hydrosilylation catalyzed by platinum-group metals have been
reported in recent years, including Ru-, Rh-, Ir-, and Pt-based
catalysts.[2] However, the high cost and low abundance of such
with PhSiH3 (5 mol% Mn, 75 °C, 1 h or 2 mol% Mn, 25 °C, 3 - 48
[17]
h).[10],
In a subsequent study, we evaluated the catalytic
performance of structurally related N-phosphinoamidinate Mn,
Fe, Co, and Ni pre-catalysts in amide hydrosilylation, and
developed a new three-coordinate Ni pre-catalyst that proved to
be particularly effective in such amide reduction chemistry.[12c]
We further demonstrated that the consideration of ancillary
ligation plays an important role in amide reduction catalysis, as
N-phosphinoamidinate ligated complexes showed far superior
reactivity in comparison with M(N(SiMe3)2)2 (M = Mn, Fe, and
Co) pre-catalysts.
precious metals has inspired
a broad effort to develop
increasingly sustainable catalysts, including those that utilize
earth-abundant 3d transition metals.[3] In this regard, Fe-based
catalysts are particularly attractive, given the low cost,
[a]
[b]
D. J. Hale, L. J. Murphy, Prof. Dr. L. Turculet
Department of Chemistry
Dalhousie University
6274 Coburg Rd., Halifax, Nova Scotia, B3H 4R2 (Canada)
E-mail: laura.turculet@dal.ca
Dr. M. J. Ferguson, Dr. R. McDonald
X-Ray Crystallography Laboratory, Department of Chemistry
University of Alberta
11227 Saskatchewan Dr., Edmonton, Alberta, T6G 2G2 (Canada)
Supporting information for this article is given via a link at the end of
the document.
Figure 1. Fe and Co pre-catalysts reported herein for the reduction of amides
to the corresponding amines with PhSiH3.
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In an effort to further elaborate ancillary ligation strategies for
reactive 3d-transition metal complexes that may be of catalytic
utility in such transformations, we have also pursued alternative
anionic P,N ligand motifs. Despite the rich chemistry of P,N
ligands used in conjunction with various transition metals,[18] the
specific class of monoanionic P,N ligands has received
considerably less attention, particularly with respect to the first
row transition metals. Phenylene-bridged anilido phosphine
equiv. of
1
(Scheme 2). The targeted three-coordinate
hexamethyldisilazide Fe(II) complex 2 was isolated in 79% yield
via this route. Solution magnetic moment measurements for 2
(Evans method,[21] benzene-d6, 300K) resulted in a calculated
μeff value of 4.9 μB, consistent with an S = 2 ground state, as was
also observed for N-phosphinoamidinate analogues of 2,[17f] and
as is typically the case for related three-coordinate β-
diketiminato Fe(II) complexes.[22] The solid state structure of 2
was determined by use of X-ray crystallographic techniques, and
is consistent with the proposed formulation of 2 as a
mononuclear hexamethyldisilazide complex featuring κ2-P,N
coordination of the diarylamido phosphine ligand (Figure 2b).
The complex is planar at the Fe center (Σ<
adopts a coordination geometry that is somewhat closer to T-
shaped (P-Fe-N1 84.90(6)°, N1-Fe-N2 153.09(9)°, P-Fe-N2
122.01(7)°) than previously observed for N-phosphinoamidinate
analogues (cf. P-Fe-Namidinate 81.10(6)°, Namidinate-Fe-NSiMe3
144.15(9)°, P-Fe-NSiMe3 132.86(6)° for an N-phosphinoamidinate
ligands are a structurally simple class of monoanionic P,N ligand
[20]
that have not been widely explored in this regard.[19],
Herein
we report a new anilido phosphine derivative (1) and evaluate
complexes of Fe and Co supported by this bidentate ligand as
pre-catalysts for the hydrosilylation of tertiary and secondary
amides with PhSiH3 (Figure 1). A three-coordinate Fe amido pre-
catalyst of this type (2) was found to be superior to related four-
coordinate Fe (3-Fe) and Co (3-Co) alkyl derivatives, enabling
= 360°) and
at Fe
facile reduction of
a variety of tertiary amide substrates,
including more challenging sterically demanding substrates such
as N,N-diisopropylbenzamide. The useful catalytic performance
of 2 under relatively mild conditions, including examples of room
temperature reactivity, is noteworthy given current interest in
iron catalysis.
analogue of
2
featuring P-tBu2 and Namidinate-2,6-iPr2C6H3
substitution).[17f] The Fe-P and Fe-N1 distances of 2.4425(7) and
1.9409(19) Å, respectively, are comparable to those reported for
related N-phosphinoamidinate Fe(II) species.[17f] Attempts to
access the Co analogue of 2 using a similar synthetic route to
that outlined in Scheme 2 were not successful.
Results and Discussion
Synthesis of a P,N Ligand Precursor. The Liang group has
previously reported the synthesis of bidentate, phenylene-
bridged anilido phosphine ligands and their coordination
chemistry with Ni and Zn.[19a-d] Examples of Cu and Ni
complexes supported by similar ligands have also been reported
by Peters[19e] and Mindiola,[19f] respectively. The closely related
pro-ligand 1 featuring cyclohexyl-phosphino substitution was
readily prepared on a multigram scale by the application of
sequential C-N and C-P cross-coupling, starting from o-
dibromobenzene (Scheme 1). Single crystal X-ray diffraction
analysis (Figure 2a) and NMR spectroscopic data for
confirmed the formulation of this precursor.
1
Scheme 2. Synthesis of Fe(II) and Co(II) complexes.
Synthesis of Fe(II) and Co(II) Alkyl Complexes. We further
sought to explore the feasibility of generating P,N-supported
Fe(II) alkyl complexes that may function as pre-catalysts for
carbonyl hydrosilylation applications. Towards this end, py4FeCl2
was treated with two equiv. of LiCH2SiMe3 and subsequently
with one equiv. of 1, which afforded the Fe alkyl complex 3-Fe
as a red, paramagnetic solid (Scheme 2).[23] Crystallographic
characterization of 3-Fe confirmed the formulation of this
complex as a four-coordinate alkyl complex featuring distorted
tetrahedral geometry at Fe, with κ2-P,N coordination of the
diarylamido phosphine ligand as well as an equiv. of pyridine
coordinated to the Fe center (Figure 2c). The Fe-C7 distance of
2.045(3) Å is comparable to values reported previously for low-
coordinate Fe(II)-CH2SiMe3 complexes.[24] Solution magnetic
Scheme1. Synthesis of the pro-ligand 1.
Synthesis of an Fe(II) Amido Complex. Our previous work
has identified Fe hexamethyldisilazide complexes as useful pre-
catalysts for carbonyl hydrosilylation applications,[17f] possibly via
reactive Fe-hydride intermediates generated under catalytic
conditions.[17a] In an effort to access such a hexamethyldisilazide
complex featuring anilido phosphine ancillary ligation, FeCl2 was
treated with two equiv. LiN(SiMe3)2 and subsequently with one
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moment measurements for 3-Fe (Evans method,[21] benzene-d6,
300K) resulted in a calculated μeff value of 5.2 μB, consistent with
an S = 2 ground state, as anticipated for a high-spin d6 metal
center in tetrahedral geometry.[25]
The analogous Co alkyl complex 3-Co was also prepared
using a similar synthetic route (Scheme 2). Thus treatment of
[26]
py4CoCl2 with two equiv. of LiCH2SiMe3
and subsequently
with one equiv. of 1 afforded dark red, paramagnetic 3-Co in
82% yield. Unlike for 3-Fe, solution magnetic moment
measurements for 3-Co (Evans method, benzene-d6, 300K)
resulted in a calculated μeff value of 2.3 μB, consistent with
square-planar, low-spin (S = ½) Co(II).[27] A high quality X-ray
crystal structure of 3-Co has remained elusive. However, the
connectivity and square-planar geometry in 3-Co were
unequivocally ascertained on the basis of a low resolution X-ray
crystal structure (Figure S2 in the Supporting Information),
whereby complete refinement was not feasible due to the lack of
data at high diffraction angles.
Complexes 3-Fe and 3-Co are among only a few reported
Fe and Co alkyl complexes supported by bidentate P,N ligation.
Some related examples of five- and six-coordinate CoII- and
CoIII-methyl complexes of 2-(diphenylphosphino)anilido ligands
have been reported previously by Klein and co-workers.[19g] As
well, examples of four-coordinate mono-alkyl complexes
supported by tridentate bis(phosphine) amido PNP ligands have
been reported for both Fe(II) and Co(II). Similar to the
observations reported herein, (PNP)MCH2SiMe3 complexes
(PNP = κ3-N(SiMe2CH2PPh2)2, M = Fe or Co) were found to
adopt tetrahedral high-spin configuration for M = Fe, and low-
spin square planar configuration for M = Co.[25b, 27a]
Fe and Co-catalyzed Reduction of Amides. Having
prepared mononuclear Fe and Co complexes supported by
bidentate anilido phosphine ligation, the utility of these
complexes as pre-catalysts for amide hydrosilylation was
evaluated. The choice of PhSiH3 as the reductant for this
application was based on our previous observations that PhSiH3
is effective for the reduction of ketones, aldehydes, esters, and
amides by related N-phosphinoamidinate 3d metal complexes.[10,
12c, 17f] We initially chose to evaluate the efficacy of 2, 3-Fe and 3-
Co as catalysts for the hydrosilylation of a series of tertiary
benzamide substrates using a pre-catalyst loading of 5 mol%
and one equiv. of PhSiH3 relative to amide (Table 1). Upon
heating at 80 °C for 1 h, near quantitative (>99%) conversion of
N,N-dibenzylbenzamide to tribenzylamine was observed for 2
and 3-Fe, but only 48% conversion to product was obtained for
3-Co (Table 1, Entry 1). Comparable reactivity was observed for
similar substrates under these conditions, with the Fe-based
pre-catalysts 2 and 3-Fe generally outperforming 3-Co. For the
Co-based pre-catalyst 3-Co, full conversion to product was
observed only for 1-benzoylpiperidine (Table 1, Entry 3), while
conversions for all other benzamides ranged from 55-83%. In
the case of 4-benzoylmorpholine (Table 1, Entry 6), longer
reaction times (18-23 h) were required to achieve >95%
conversion to the amine product for both the Fe-based pre-
catalysts. While 2 and 3-Fe generally performed in a similar
manner for this set of substrates, a significant divergence from
this was observed in the reduction of N,N-diisopropylbenzamide
(Table 1, Entry 5), which is a challenging substrate for all three
pre-catalysts. After 24 h at 80 °C only 2 afforded >90%
conversion to N,N-diisopropylbenzylamine (78% isolated yield),
while use of 3-Fe and 3-Co resulted in 59 and 57% conversion
Figure 2. Crystallographically determined structures of (a) 1, (b) 2, and (c) 3-
Fe; most hydrogen atoms in (b) and (c) have been omitted for clarity. Non-
hydrogen atoms are represented by Gaussian ellipsoids at the 30% probability
level. Selected interatomic distances (Å) and angles (°): for 1 P-C1 1.8406(11),
P-C11 1.8651(12), P-C21 1.8529(12), N-C2 1.3857(14), N-C31 1.4279(14),
C1-C2 1.4189(15); for 2 Fe-P 2.4425(7), Fe-N1 1.9409(19), Fe-N2 1.904(2),
P-C1 1.812(2), N1-C2 1.398(3), C1-C2 1.422(3), P-Fe-N1 84.90(6), P-Fe-N2
122.01(7), N1-Fe-N2 153.09(9); for 3-Fe Fe-P 2.4239(9), Fe-N1 2.007(2), Fe-
N2 2.160(2), Fe-C7 2.045(3), P-C1 1.821(3), N1-C2 1.376(3), C1-C2 1.430(4),
P-Fe-N1 82.86(7), P-Fe-N2 97.92(6), P-Fe-C7 117.56, N1-Fe-N2 105.50(9),
N1-Fe-C7 135.05(11), N2-Fe-C7 110.06(11).
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to product, respectively. In the case of N,N-dibenzylbenzamide
(Table 1, Entry 1), the loading of 2 could be dropped to 1 mol%
while still achieving near quantitative formation of tribenzylamine
in 1 h.
(Table 2, Entry 4), increasing reaction times from 1 to 24 h
facilitates >99% conversion to product for 2 (75% isolated yield),
but 3-Fe reaches only 65% conversion to the corresponding
amine under these conditions. The presence of an ortho-chloro
substituent also dramatically decreases catalytic performance
(Table 2, Entry 3), such that only 64% conversion to product was
achieved for 2 over 4 h, while 3-Fe and 3-Co achieve 13 and
27% conversion, respectively, under these conditions. Longer
reaction times did not lead to higher yield of the amine product
for this substrate. The Fe-based pre-catalysts can also tolerate
thiophene- and pivalamide-derived substrates reasonably well
(Table 2, Entries 6 and 7), with higher yields achieved in the
case of 2.
Table 1. Reduction of tertiary benzamides to tertiary benzylamines.
5 mol% 3-Co
5 mol% 2
5 mol% 3-Fe
Entry
Amide
Yield
(%)[a]
Time
(h)
Yield
(%)[a]
Time
(h)
Yield
(%)[a]
Time
(h)
>99[b]
98[c]
(74)
,
Table 2. Reduction of tertiary and secondary amides to amines.
1
2
3
1
1
1
>99
98
1
1
1
48
83
98
1
1
1
>99,
>99[c]
(57)[d]
5 mol% 3-Co
5 mol% 2
5 mol% 3-Fe
Entry
Amide
Yield
(%)[a]
Time
(h)
Yield
(%)[a]
Time
(h)
Yield
(%)[a]
Time
(h)
>99,
>99[c]
(74)
>99
>99,
>99[b]
(73)
1
2
3
4
1
4
95
97
13
65
1
4
59
98
27
39
1
4
>99,
66[c]
(72)
4
5
6
1
93
59
98
1
80
57
55
1
97,
83[b]
(>99)
94,
9[c]
(78)
24
23
24
18
24
18
64
4
4
4
97
(78)
24
24
24
>99
(75)
[a] Yield of product (average of two runs at 80 °C, unless otherwise stated)
determined by use of gas chromatography against an internal standard
(dodecane); in the case of incomplete reactions, the mass balance
corresponds primarily to unreacted starting material. Numbers in
parentheses are isolated yields following flash chromatography. [b] 1 mol %
2, 80 °C, 1 h. [c] 5 mol% 2, RT, 24 h. [d] Low isolated yield due in part to
volatility of the amine product.
5
93
4
96
4
45
4
(60)
Expanding the substrate scope to include substituted
benzamides (Table 2) further demonstrates that Fe-based pre-
catalysts 2 and 3-Fe are superior to the Co alkyl derivative 3-Co.
While the presence of an ortho-methyl substituent on the arene
ring in 1-(o-toluoyl)piperidine does not appear to impact catalytic
6
7
96
(92)
6
4
80]
76
1
4
22
25
4
1
90,
95[b]
(85)
performance for
2 and 3-Fe relative to the parent 1-
benzoylpiperidine substrate, the conversion to product
decreases substantially (from 98% conversion to 59%) in the
case of 3-Co (Table 2, Entry 1). The presence of electron
donating (methoxy) or withdrawing (chloro) groups in the para
position of the arene ring does not have a significant effect on
reactivity (Table 2, Entries 2 and 5), though it should be noted
that the reaction times necessary to achieve high conversion to
product increased from 1 to 4 h for these substrates. The
presence of halide substituents in the ortho position of the arene
ring has a much greater impact on catalytic performance for all
three pre-catalysts. In the case of an ortho-fluoro substituent
8
9
60[c]
30[c]
24
24
-
-
-
-
-
-
-
-
[a] Yield of product (average of two runs at 80 °C, unless otherwise stated)
determined by use of gas chromatography against an internal standard
(dodecane); in the case of incomplete reactions, the mass balance
corresponds primarily to unreacted starting material. Numbers in
parentheses are isolated yields following flash chromatography. [b] 5 mol%
2, RT, 24 h. [c] 10 mol% 2, 110 °C, toluene, 24 h.
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Overall, these results reveal that while pre-catalysts 2 and 3-
Fe are both generally effective in the reduction of tertiary amides
with PhSiH3 under relatively mild conditions, 2 outperforms the
alkyl derivative 3-Fe in a number of instances, and is more
broadly effective in such transformations. In an effort to further
probe the efficacy of pre-catalyst 2, we also evaluated its
reactivity under room temperature conditions. Such room
temperature reactivity is exceedingly rare for the reduction of
tertiary amides by use of 3d transition-metal catalysts.[10, 11b, 12c]
Indeed, at a loading of 5 mol% 2, the efficient reduction of a
number of amide substrates was observed over the course of 24
h at room temperature (Table 1, Entries 1-4; Table 2, Entries 1,
2, and 7). The sterically demanding substrate N,N-
diisopropylbenzamide proved challenging to reduce under such
room temperature conditions (Table 1, Entry 5), which is not
surprising given that heating at 80 °C for 24 h was shown to be
necessary in order to achieve high conversion to product with a
5 mol% loading of 2. The hydrosilylation of secondary amide
substrates was also challenging for pre-catalyst 2. The reduction
of N-methylbenzamide and N-benzylbenzamide with PhSiH3
were each attempted under more forcing conditions, increasing
the reaction temperatures to 110 °C and doubling the pre-
catalyst loading to 10 mol%. Despite these changes, only 60%
conversion to the corresponding secondary amine was observed
for N-methylbenzamide (Table 2, Entry 8) and 30% conversion
was obtained for N-benzylbenzamide (Table 2, Entry 9),
highlighting the difficulty of this transformation; under similar
conditions (5 - 10 mol % 2, 80 °C, 24 h), complex reactivity
leading to multiple unidentified products was observed in the
attempted reduction of the primary amides benzamide and
nicotinamide. Although isolated reports on the Fe-catalyzed
hydrosilylation of such secondary amide substrates have
appeared in the literature previously,[5a, 6b, 7] Fe-based catalysts
for the hydrosilylative reduction of primary and secondary
amides are not well established.[28]
While a direct comparison of the amide hydrosilylation
catalysis reported herein with previously reported Fe catalysts
(where useful substrate scope is established) is difficult due to
slight differences in reaction conditions and choice of
hydrosilane, the reactivity observed for 2 with respect to tertiary
amides is certainly on par with the best of such catalysts.
Particularly noteworthy is the ability of 2 to mediate amide
hydrosilylation under room temperature conditions. To the best
of our knowledge, this is the first example of an Fe-based
catalyst to do this in the absence of photochemical activation.
Also of note is the efficient reduction of N,N-
diisopropylbenzamide using 2. While pre-catalyst 2 requires
heating at 80 °C to achieve high conversion to product for this
substrate (Table 1, Entry 5), the hydrosilylation of amides that
feature 2° alkyl substituents at nitrogen is a considerable
synthetic challenge, with most Fe catalysts restricted to 1° alkyl
substitution at this position. Limited examples of hydrosilylation
of N,N-diisopropylbenzamide (or structurally related species that
feature 2° alkyl substituents at nitrogen) by an Fe-based catalyst
have been reported previously. Beller and co-workers[5a]
reported a 59% isolated yield of N,N-diisopropylbenzylamine
using an Fe3(CO)12 pre-catalyst (30 mol % Fe loading) and
polymethylhydrosiloxane (PMHS, 8 equiv.) as the reductant after
heating at 100 °C for 24 h. Sortais and Darcel[7] also evaluated
the reduction of this substrate using the N-heterocyclic carbene
complex [CpFe(CO)2(IMes)]I (5 mol %) as a pre-catalyst and two
equiv. of PhSiH3. An isolated yield of 73% N,N-
diisopropylbenzylamine was reported following heating at 100 °C
for 24 h under photolysis conditions. We have also previously
reported on the hydrosilylation of N,N-diisopropylbenzamide
using an N-phosphinoamidinate Fe hexamethyldisilazide
complex that is structurally related to 2.[12c] In the latter example,
the use of 5 mol % Fe loading and two equiv. PhSiH3 relative to
amide led to 56% conversion to N,N-diisopropylbenzylamine
following heating at 75 °C for 18 h. The results reported herein
thus represent a notable improvement for Fe catalysis with
respect to both catalyst loading and relatively mild reaction
conditions.
While Fe-based catalysts are advantageous due to the ready
availability of this inexpensive transition metal, examples of
highly competitive 3d transition metal catalysis for amide
hydrosilylation have also been reported. With respect to room
temperature reactivity involving a demonstrated substrate scope,
our recently reported N-phosphinoamidinate Mn pre-catalyst[10]
was among the first examples of such hydrosilylative reduction
of tertiary amides, achieving >90% conversion to the amine
product for N,N-dimethyl- and N,N-dibenzylbenzamide, as well
as 1-benzoylpiperidine under comparable conditions (2 mol%
Mn, one equiv. PhSiH3, 18 h) to those reported herein.
Impressive
Co-catalyzed
room
temperature
amide
hydrosilylation has also recently been reported by Khalimon and
co-workers,[11b] who achieved high conversion for the reduction
of substrates such as N,N-dimethylacetamide, N,N-
dibenzylacetamide, and N-methyl-2-pyrrolidinone using 5 mol%
Co(acac)2 in combination with 5.5 mol% dpephos (bis[(2-
diphenylphosphino)phenyl] ether) and 1.5 equiv. PhSiH3. The
room temperature hydrosilylation of N,N-diisopropylbenzamide
was also achieved under these conditions, affording a 93%
isolated yield of the amine product after 17 h. We also recently
reported a Ni-catalyzed example of hydrosilylation of N,N-
diisopropylbenzamide under ambient conditions using two equiv.
of PhSiH3 (5 mol% Ni, 18 h, >95% conversion to N,N-
diisopropylbenzylamine).[12c] Such catalysts, specifically Co and
Ni examples, have also shown some aptitude for the
hydrosilylation of secondary amide substrates, although such
reactivity typically requires more forcing conditions.[28]
Conclusions
The synthesis of new low-coordinate Fe(II) and Co(II) complexes
supported by a bidentate, monoanionic, anilido phosphine ligand
featuring bulky cyclohexyl substituents at phosphorus and 2,6-
iPr2C6H3 substitution at nitrogen has been described. A three-
coordinate (P,N)Fe-hexamethyldisilazide (2) complex, and four-
coordinate (P,N)Fe- (3-Fe) and (P,N)Co-alkyl (3-Co) complexes
were structurally characterized and were evaluated as pre-
catalysts for the hydrosilylative reduction of amides with PhSiH3.
In general, the Fe-based catalysts outperformed the Co-based
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10.1002/cctc.201900550
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N-(2-dicyclohexylphosphinophenyl)-2,6-diisopropylaniline
(1).
catalyst 3-Co for the reduction of a variety of tertiary amide
substrates under analogous reaction conditions (5 mol% catalyst,
80 °C, 1-24 h). Between the two Fe-based catalysts, the Fe
hexamethyldisilazide complex (2) appeared to be more broadly
effective in such reactions, and was shown to mediate the
reduction of N,N-dibenzylbenzamide at a reduced loading of 1
mol% to achieve near quantitative formation of tribenzylamine in
1 h (80 °C). Most importantly, complex 2 also proved effective
for the hydrosilylation of tertiary amides under ambient
conditions (5 mol%, 24 h), which is a unique example of room
temperature amide hydrosilylation mediated by an Fe catalyst
without the need for photochemical activation. While some
evidence for the hydrosilylation of secondary amides was also
observed, the challenging nature of such reactions resulted in
relatively low conversion to the corresponding secondary amines,
even under forcing conditions (10 mol% Fe, 110 °C, 24 h).
Following a modified literature procedure,[30] Pd(OAc)2 (0.19 g, 0.85
mmol) and 1,1’-diphenylphosphinoferrocene (0.42 g, 1.01 mmol) were
combined with 10 mL toluene in a 250 mL Teflon-sealed reaction vessel
and stirred for 5 min at room temperature. Subsequently, NaOtBu (2.28 g,
23.7 mmol) and HPCy2 (3.71 mL, 16.9 mmol) were added to the reaction
mixture. 5.62 g (0.0169 mol) N-(2-bromophenyl)-2,6-diisopropylamine
(prepared according to literature procedure[29]) was also added to the
reaction mixture and toluene was added to bring the total reaction
volume to approximately 40 mL. The mixture was stirred at 110 °C for 18
hours after which time volatiles were removed in vacuo and the resulting
residue was triturated with 3  10 mL pentane. The residue was
extracted into 50 mL of a ca. 1 : 20 mixture of Et2O in hexanes and
filtered through a Celite and silica gel plug to yield a dark red filtrate. The
solvent was removed from the filtrate in vacuo, and the residue was
triturated with 3  5 mL pentane before being extracted with ca. 30 mL
hexanes and filtered once again through a plug of Celite and silica gel.
The filtrate was concentrated in vacuo and cooled to −35 °C yielding 1 as
a beige crystalline material which was washed with 3  1 mL cold
pentane. Additional 1 could isolated through serial crystallization from
hexanes and washing the resulting crystals with cold pentane yielding
4.94 g 1 overall as a beige solid (11.0 mmol, 65% yield). Single crystals
suitable for X-ray diffraction were isolated from a concentrated pentane
solution of 1 at room temperature; additional crystallographic detail is
available in the Supporting Information and the deposited CIF (CCDC
1905151). 1H NMR (500 MHz, benzene-d6): δ 7.34 – 7.19 (overlapping
resonances, 3 H, Harom + NH), 6.97 (t, 1 H, JHH = 7 Hz, Harom), 6.69 (t, 1 H,
The reactivity of the Fe-based pre-catalyst 2 in tertiary amide
hydrosilylation is generally comparable to that of the best
previously reported Fe catalysts with respect to substrate scope.
In addition, while the large majority of previously reported 3d
transition metal catalysts require reaction temperatures on the
order of 100 °C and/or photochemical activation, the efficacy of
2
under room temperature conditions offers a significant
advance in the development of efficient Fe-based catalysts that
operate under mild conditions. Given that amides are among the
most challenging carbonyl species to reduce, as well the
widespread use of amide reduction protocols in the synthesis of
pharmaceuticals, agrochemicals, and fine chemicals, the
development of efficient, Fe-based catalysts for this application
is highly relevant. Although the Fe pre-catalyst (2) reported
herein is not without limitations, specifically in the hydrosilylation
of secondary amide substrates, our future efforts will focus on
applying a ligand design strategy to optimize the reactivity of
such Fe catalysts for increasingly challenging amide reductions.
3
JHH = 7 Hz, Harom), 6.32 (m, 1 H, Harom), 3.40 (sept, 2 H, JHH = 7 Hz,
CHMe2), 2.00 (m, 4 H, PCy), 1.77 (m, 4 H, PCy), 1.65 (m, 2H, PCy), 1.58
(m, 2H, PCy), 1.45 – 1.09 (overlapping resonances, 22 H, PCy + CHMe2;
the CHMe2 resonances were identified as doublets at 1.23 and 1.15 ppm,
3JHH = 7 Hz). 13C{1H} NMR (125.8 MHz, benzene-d6): δ 154.3 (d, JPC
1
=
20 Hz, Carom), 147.9 (Carom), 136.9 (Carom), 133.5 (CHarom), 130.6 (CHarom),
2
127.6 (CHarom), 124.2 (CHarom), 117.4 (CHarom), 116.1 (d, JPC = 14 Hz,
Carom), 111.6 (CHarom), 33.6 (d, J = 10 Hz, PCy), 30.9 (d, J = 16 Hz, PCy),
29.2 (d, J = 6 Hz, PCy), 29.0 (CHMe2), 27.5 (d, J = 13 Hz, PCy), 27.3 (d,
J = 8 Hz, PCy), 26.8 (PCy), 25.0 (CHMe2), 23.0 (CHMe2). 31P{1H} NMR
(202.5 MHz, benzene-d6): δ -27.2. Anal. Calcd for C30H44N1P1: C, 80.13;
H, 9.86; N, 3.12. Found: C, 80.08; H, 10.12; N, 3.10.
(P,N)FeN(SiMe3)2 (2). A solution of LiN(SiMe3)2 (0.22 g, 1.33 mmol) in ca.
5 mL Et2O was added dropwise to a suspension of FeBr2 (0.14 g, 0.67
mmol) in ca. 5 mL Et2O. The resulting mixture was then stirred for 30 min.
at room temperature. Subsequently, a solution of 1 (0.30 g, 0.67 mmol) in
ca. 5 mL Et2O was added dropwise to the stirring reaction mixture. A
color change to yellow-orange was observed as the mixture was stirred
for 2 h at room temperature. The volatile components of the reaction
mixture were removed in vacuo and the remaining residue was extracted
with ca. 5 mL of benzene. The benzene extracts were filtered through
Celite. The filtrate solution was collected and the benzene was removed
in vacuo. The remaining orange-yellow solid was washed with 3  0.5 mL
cold (-35 °C) pentane to afford 2 (0.35 g, 0.53 mmol, 79%) as a yellow
solid. X-Ray quality crystals of 2 were grown from a concentrated
pentane solution at -35 °C; additional crystallographic detail is available
in the Supporting Information and the deposited CIF (CCDC 1905152).
μeff (benzene-d6): 5.2 μB (S = 2). 1H NMR (300 MHz, benzene-d6): 80.24,
64.86, 32.51, 31.44, 29.17, -11.87, -12.73, -15.12, -15.60, -19.68, -20.45,
-32.86, -41.63, -43.96, -47.34, -75.87, -80.65. Anal. Calcd. for
C36H61N2PSi2Fe: C, 65.03; H, 9.25; N, 4.21. Found: C, 64.79; H, 9.21; N,
4.18.
Experimental Section
General Considerations. All experiments were conducted under
nitrogen in
a glovebox or using standard Schlenk techniques.
Tetrahydrofuran and diethyl ether were distilled from Na/benzophenone
ketyl. Benzene, toluene, and pentane were first sparged with nitrogen
and subsequently dried by passage through a double-column (one
activated alumina column and one column packed with activated Q-5)
solvent purification system. All purified solvents were stored over 4 Å
molecular sieves. Benzene-d6 was degassed via three freeze-pump-thaw
cycles and stored over 4 Å molecular sieves. N-(2-Bromophenyl)-2,6-
diisopropylaniline was prepared by use of previously reported
methods.[29] Amide substrates were prepared from the corresponding
acyl chlorides by using previously reported conditions.[10] All other
reagents were purchased from Strem or Aldrich and used without further
purification. Unless otherwise stated, 1H, 13C, and 31P characterization
data were collected at 300K, with chemical shifts reported in parts per
million downfield of SiMe4 (for 1H and 13C) or 85% H3PO4 in D2O (for 31P).
1H and 13C NMR chemical shift assignments are based on data obtained
from 13C{1H}, 13C-DEPTQ, 1H-1H COSY, 1H-13C HSQC, and 1H-13C
HMBC NMR experiments. Solution magnetic moments were determined
by use of the Evans method.[21]
(P,N)Fe(py)CH2SiMe3 (3-Fe). A solution of LiCH2SiMe3 (0.021 g, 0.22
mmol) in ca. 5 mL Et2O was added dropwise to a precooled (-35 °C)
stirring suspension of py4FeCl2 (0.049 g, 0.11 mmol) in ca. 5 mL Et2O. A
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10.1002/cctc.201900550
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color change from bright yellow to dark red was observed over the
course of 1-2 min. After ca. 15 min. of stirring at room temperature, the
reaction mixture was again cooled to -35 °C and a solution of 1 (0.050 g,
0.11 mmol) in ca. 5 mL Et2O was added dropwise to the reaction mixture.
The resulting mixture was allowed to stir at room temperature for 18 h
during which time the colour lightened to bright red. The volatile
components of the reaction mixture were removed in vacuo and the
remaining residue was triturated with 3  2 mL of pentane and then
extracted with ca. 5 mL of benzene. The benzene extracts were filtered
through Celite, and the filtrate solution was collected. The benzene
solvent was removed in vacuo and the remaining residue was triturated
with 3  2 mL pentane and washed with 3  0.5 mL of cold (-35 °C)
pentane to afford 3-Fe (0.067 g, 0.10 mmol, 90%) as a bright red solid.
X-Ray quality crystals of 3-Fe were obtained from a concentrated Et2O
solution -35 °C; additional crystallographic detail is available in the
Supporting Information and the deposited CIF (CCDC 1905153). μeff
(benzene-d6): 5.2 μB (S = 2). 1H NMR (300 MHz, benzene-d6): δ 49.88,
37.72, 30.47, 28.60, 20.69, 20.03, 9.24, 3.91, 2.00, -1.02, -3.24, -9.22, -
31.35, -38.38. Anal. Calcd. for C39H59N2PSiFe: C, 69.83; H, 8.87; N, 4.18.
Found: C, 69.61; H, 8.95; N, 4.46.
heated to 80 C for the specified reaction time. The reaction mixture was
then exposed to air and the volatile components removed under vacuum.
The crude residue was then purified by flash column chromatography
employing silica gel or neutral alumina as the stationary phase. The
combined fractions were concentrated under reduced pressure to afford
the corresponding amine.
Characterization data for isolated amine products.
Tribenzylamine. Purified by silica gel column
chromatography (ethyl acetate/hexanes, 1:4), 74%
yield (0.13 g, 0.44 mmol). 1H NMR (300 MHz,
CDCl3):  7.40 (m, 6 H, Harom), 7.30 (m, 6 H, Harom), 7.22 (m, 3 H, Harom),
3.55 (s, 6 H, CH2). 13C{1H} NMR (75.5 MHz, CDCl3):  139.9 (Carom),
128.9 (CHarom), 128.4 (CHarom), 127.1 (CHarom), 58.1 (CH2). Spectral data
are in close agreement with previously reported 1H and 13C{1H} NMR
characterization data for the title compound.[5a]
N,N-Dimethylbenzylamine. Purified by silica gel
column chromatography (ethyl acetate/hexanes, 1:50),
57% yield (0.046 g, 0.34 mmol). 1H NMR (300 MHz,
(P,N)Co(py)CH2SiMe3 (3-Co). A solution of LiCH2SiMe3 (0.13 g, 1.35
mmol) in ca. 5 mL of pentane was added dropwise to a precooled
(-35 °C) stirring suspension of py4CoCl2 (0.30 g, 0.67 mmol) in ca. 7 mL
of pentane. A color change from blue to dark red-brown was observed
during the addition. After ca. 15 minutes of stirring at room temperature,
the reaction mixture was cooled again to -35 °C and a solution of 1 (0.30
g, 0.67 mmol) in ca. 7 mL of pentane was added dropwise. The reaction
mixture was allowed to stir at room temperature for 18 h. The volatile
components of the reaction mixture were then removed in vacuo. The
resulting red residue was triturated with 3  2 mL of pentane and then
extracted with ca. 10 mL benzene. The benzene extracts were filtered
through Celite. The filtrate solution was collected and the solvent was
removed in vacuo. The remaining residue was triturated with 3  2 mL of
pentane and washed with 3  2 mL cold (-35 °C) pentane to afford 3-Co
(0.37 g, 0.55 mmol, 82%) as a red solid. X-Ray quality crystals of 3-Co
were obtained from a concentrated Et2O solution at -35 °C; additional
crystallographic detail is available in the Supporting Information. μeff
CDCl3):  7.33 – 7.29 (overlapping resonances, 4 H, Harom), 7.25 (m, 1 H,
Harom), 3.41 (s, 2 H, CH2), 2.23 (s, 6 H, NMe2). 13C{1H} NMR (75.5 MHz,
CDCl3):  139.1 (Carom), 129.3 (CHarom), 128.4 (CHarom), 127.2 (CHarom),
64.7 (CH2), 45.6 (NMe2). Spectral data are in close agreement with
previously reported 1H and 13C{1H} NMR characterization data for the title
compound.[8]
1-Benzylpiperidine. Purified by neutral alumina column
chromatography (ethyl acetate/hexanes, 1:10), 74%
1
yield (0.078 g, 0.45 mmol). H NMR (300 MHz, CDCl3):
 7.32 – 7.18 (overlapping resonances, 5 H, Harom), 3.46 (s, 2 H, CH2),
2.36 (m, 4 H, CH2), 1.56 (m, 4 H, CH2), 1.42 (m, 2 H, CH2). 13C{1H} NMR
(75.5 MHz, CDCl3):  138.9 (Carom), 129.4 (CHarom), 128.3 (CHarom), 127.0
(CHarom), 64.1 (PhCH2), 54.7 (CH2), 26.2 (CH2), 24.6 (CH2). Spectral data
are in close agreement with previously reported 1H and 13C{1H} NMR
characterization data for the title compound.[5a]
1
(benzene-d6): 2.3 μB (S = ½). H NMR (300 MHz, benzene-d6): δ 30.30,
N-Methyldibenzylamine. Purified by silica gel column
chromatography (ethyl acetate/hexanes, 1:4), 72%
yield (0.091 g, 0.43 mmol). 1H NMR (500 MHz,
25.71, 14.59, 13.60, 11.43, 7.40, 5.30, 2.91. 1.94, 0.61, 0.28, -3.45, -6.53,
-30.68. Anal. Calcd. for C39H59N2PSiCo: C, 69.51; H, 8.82; N, 4.16.
Found: C, 69.22; H, 8.64; N, 4.10.
CDCl3):  7.27 (m, 4 H, Harom), 7.22 (m, 4 H, Harom), 7.14 (m, 2 H, Harom),
3.43 (s, 4 H, CH2), 2.09 (s, 3 H, NMe). 13C{1H} NMR (125.8 MHz, CDCl3):
 139.6 (Carom), 129.1 (CHarom), 128.4 (CHarom), 127.1 (CHarom), 62.1
(CH2), 42.5 (CH3). Spectral data are in close agreement with previously
reported 1H and 13C{1H} NMR characterization data for the title
compound.[7]
General Procedure for the Reduction of Amide Substrates. Amide
(0.2 mmol) and PhSiH3 (0.2 mmol) were combined in an oven-dried one
dram vial equipped with a magnetic stirbar. The pre-catalyst (2, 3-Fe, or
3-Co) was then added as a stock solution (64 mM) in benzene (313 μL
for 10 mol% runs, 156 μL for 5 mol% runs, 31 μL for 1 mol% runs), as
well as an additional 300 L of benzene. The vial was sealed with a
PTFE-lined cap, removed from the glovebox, and heated to 80 C or
110 °C for the specified reaction time. The reaction mixture was then
exposed to air, diluted with 1 mL of dichloromethane, and filtered through
a Celite plug. The filtrate solution was then analyzed by use of gas
chromatography against a dodecane internal standard to measure
conversion of the amide to the corresponding amine.
N,N-Diisopropylbenzylamine. Purified by silica gel
column chromatography (ethyl acetate/hexanes, 17:100),
78% yield (0.089 g, 0.47 mmol). 1H NMR (300 MHz,
CDCl3):  7.40 (m, 2 H, Harom), 7.30 (m, 2 H, Harom), 7.21
(m, 1 H, Harom), 3.67 (s, 2 H, CH2), 3.04 (sept, 2 H, 3JHH = 7 Hz, CHMe2),
3
1.04 (d, 12 H, JHH = 7 Hz, CHMe2). 13C{1H} NMR (75.5 MHz, CDCl3): 
143.5 (Carom), 128.2 (CHarom), 128.0 (CHarom), 126.3 (CHarom), 49.1 (CH2),
48.0 (CHMe2), 21.0 (CHMe2). Spectral data are in close agreement with
previously reported 1H and 13C{1H} NMR characterization data for the title
compound.[5a]
General Procedure for the Isolation of Tertiary Amines. Amide
reductions were conducted on either a 0.4 mmol or 0.6 mmol scale for
the purpose of isolation. The amide substrate (0.4 mmol or 0.6 mmol)
and PhSiH3 (0.4 mmol or 0.6 mmol, respectively) were combined in an
oven-dried one dram vial equipped with a magnetic stirbar. The pre-
catalyst 2 was then added as 312 μL (or 468 μL) of a 64 mM benzene
stock solution, as was an additional 600μL (or 900 L) of benzene. The
vial was sealed with a PTFE-lined cap, removed from the glovebox, and
4-Benzylmorpholine. Purified by neutral alumina
column chromatography (ethyl acetate/hexanes, 1:10),
78% yield (0.083 g, 0.47 mmol). 1H NMR (500 MHz,
CDCl3):  7.34 – 7.28 (overlapping resonances, 4 H, Harom), 7.25 (m, 1
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10.1002/cctc.201900550
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FULL PAPER
Harom), 3.70 (m, 4 H, CH2), 3.49 (s, 2 H, CH2), 2.43 (m, 4 H, CH2). 13C{1H}
NMR (125.8 MHz, CDCl3):  138.0 (Carom), 129.4 (CHarom), 128.4 (CHarom),
127.3 (CHarom), 67.2 (CH2), 63.7 (PhCH2), 53.8 (CH2). Spectral data are
in close agreement with previously reported 1H and 13C{1H} NMR
characterization data for the title compound.[5a]
128.5 (CHarom), 128.3 (CHarom), 127.1 (CHarom), 126.6 (CHarom), 125.7
(CHarom), 124.9 (CHarom), 57.8 (CH2), 52.4 (CH2). Spectral data are in
close agreement with previously reported 1H and 13C{1H} NMR
characterization data for the title compound.[5a]
Dibenzyl(2,2-dimethylpropyl)amine. Purified by silica
gel column chromatography (ethyl acetate/hexanes,
1-[(2-Methylphenyl)methyl]piperidine. Purified by
neutral alumina column chromatography (ethyl
acetate/hexanes, 1:20), 73% yield (0.083 g, 0.44
mmol). 1H NMR (500 MHz, CDCl3):  7.39 (m, 1 H,
3:20), 85% yield (0.091 g, 0.34 mmol). 1H NMR (500
MHz, CDCl3):  7.43 (m, 4 H, Harom), 7.36 (m, 4 H,
Harom), 7.28 (m, 2 H, Harom), 3.66 (s, 4 H, CH2), 2.40 (s,
2 H, CH2), 0.87 (s, 9 H, CMe3). 13C{1H} NMR (125.8 MHz, CDCl3): 
140.4 (Carom), 129.4 (CHarom), 128.3 (CHarom), 127.0 (CHarom), 66.1 (CH2),
61.0 (PhCH2), 33.3 (CMe3), 28.7 (CMe3). Spectral data are in close
agreement with previously reported 1H and 13C{1H} NMR characterization
data for the title compound.[5a]
Harom), 7.26 – 7.25 (overlapping resonances, 3 H, Harom), 3.52 (s, 2 H,
CH2), 2.49 – 2.47 (overlapping resonances, 7 H, CH2 + Me), 1.67 (m, 4 H,
CH2), 1.55 (m, 2 H, CH2). 13C{1H} NMR (125.8 MHz, CDCl3):  137.6
(Carom), 137.3 (Carom), 130.3 (CHarom), 129.9 (CHarom), 126.9 (CHarom),
125.5 (CHarom), 61.7 (Ar-CH2), 54.9 (CH2), 26.3 (CH2), 24.8 (CH2), 19.4
(CH3). Spectral data are in close agreement with previously reported 1H
and 13C{1H} NMR characterization data for the title compound.[31]
Acknowledgements
1-[(4-Chlorophenyl)methyl]piperidine. Purified by
neutral alumina column chromatography (ethyl
acetate/hexanes, 1:10), >99% yield (0.084 g, 0.40
We are grateful to the Natural Sciences and Engineering
Research Council (NSERC) of Canada and Dalhousie University
for their support of this work.
mmol). 1H NMR (500 MHz, CDCl3):
 7.30 – 7.24 (overlapping
resonances, 4 H, Harom), 3.43 (s, 2 H, CH2), 2.36 (broad s, 4 H, CH2),
1.57 (m, 4 H, 2 CH2), 1.44 (m, 2 H, CH2). 13C{1H} NMR (125.8 MHz,
CDCl3):  137.5 (Carom), 132.7 (Carom), 130.6 (CHarom), 128.4 (CHarom),
63.3 (CH2), 54.7 (CH2), 26.2 (CH2), 24.6 (CH2). Spectral data are in close
agreement with previously reported 1H and 13C{1H} NMR characterization
data for the title compound.[31]
Keywords: iron • cobalt • amide • reduction • hydrosilylation
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N,N-Dibenzyl-2-fluorobenzenemethanamine.
Purified by silica gel column chromatography (ethyl
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acetate/hexanes, 1:10), 75% yield, (0.092 g, 0.30
mmol). 1H NMR (500 MHz, CDCl3):  7.52 (m, 1 H,
Harom), 7.38 (m, 4 H, Harom), 7.28 (m, 4 H, Harom), 7.21
– 7.14 (overlapping resonances, 3 H, Harom), 7.08 (m, 1 H, Harom), 6.97 (m,
1 H, Harom), 3.62 (s, 2 H, CH2), 3.56 (s, 4 H, CH2). 13C{1H} NMR (125.8
MHz, CDCl3):  161.6 (d, Carom
CHarom, JCF = 4 Hz), 128.9 (CHarom), 128.5 (d, CHarom, JCF = 8 Hz), 128.4
(CHarom), 127.1 (CHarom), 126.4 (d, Carom, JCF = 14 Hz), 124.1 (d, CHarom
,
1JCF = 245 Hz), 139.7 (CHarom), 131.2 (d,
,
JCF = 3 Hz), 115.3 (d, CHarom, JCF = 22 Hz), 58.3 (CH2), 50.6 (CH2).
19F{1H} NMR (470.5 MHz, CDCl3):  −118.2 (s). Spectral data are in close
agreement with previously reported 1H and 13C{1H} NMR characterization
data for the title compound.[32]
N,N-Dibenzyl-4-methoxybenzenemethan-
amine. Purified by silica gel column
chromatography (ethyl acetate/hexanes, 3:20),
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60% yield (0.076 g, 0.24 mmol). 1H NMR (300 MHz, CDCl3):  7.47 (m, 4
H, Harom), 7.39 – 7.34 (overlapping resonances, 6 H, Harom), 7.30 (m, 1 H,
Harom), 7.27 (m, 1 H, Harom), 6.95 (m, 2 H, Harom), 3.84 (s, 3 H, OMe), 3.61
(s, 4 H, CH2), 3.56 (s, 2 H, CH2). 13C{1H} NMR (75.5 MHz, CDCl3): 
158.8 (Carom), 139.9 (CHarom), 131.8 (Carom), 130.1 (CHarom), 128.9
(CHarom), 128.4 (CHarom), 127.0 (CHarom), 113.8 (CHarom), 58.0 (OMe),
57.4 (CH2), 55.4 (CH2). Spectral data are in close agreement with
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(m, 4 H, Harom), 7.12 - 7.09 (overlapping resonances, 3 H, Harom), 6.82 –
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Entry for the Table of Contents
FULL PAPER
Low-coordinate Fe(II) and Co(II)
complexes supported by a
monoanionic P,N-ligand are
described. A 3-coordinate (P,N)Fe-
amido pre-catalyst proved
Dylan J. Hale, Luke J. Murphy, Robert
McDonald, Michael J. Ferguson, and
Laura Turculet*
Page No. – Page No.
particularly effective for the
Hydrosilylative Reduction of Tertiary
Amides to Amines Catalyzed by N-
(Phosphinoaryl)anilido Complexes
of Iron and Cobalt
hydrosilylation of a variety of tertiary
amides, including examples of room
temperature transformations that are
unprecedented for Fe.
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