Pushing back the limits of hydrosilylation: Unprecedented catalytic reduction of organic ureas to formamidines

Article abstract of DOI:10.1002/cctc.201300653

Pushing back the limits: A novel catalytic transformation has been designed to prepare formamidine derivatives by reduction of substituted ureas with hydrosilanes. Simple iron catalysts based on commercially available iron salts and phosphine ligands prov

Full text of DOI:10.1002/cctc.201300653

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DOI: 10.1002/cctc.201300653
Pushing Back the Limits of Hydrosilylation:
Unprecedented Catalytic Reduction of Organic Ureas to
Formamidines
Jacky Pouessel, Olivier Jacquet, and Thibault Cantat*[a]
Catalytic hydrosilylation of carbonyl functional groups is gain-
ing an increasing interest in synthetic organic chemistry be-
cause it circumvents important limitations of the more classical
hydrogenation or metal-hydride mediated reduction method-
ologies.[1] Indeed, hydrosilanes are practical reducing agents
because they have a mild reduction potential and are less sen-
sitive to moisture than LiAlH4, DIBAL, or NaBH4. Moreover, the
slightly polar and weaker SiÀH bond (bond dissociation energy
(BDE) 92 kcalmolÀ1 in SiH4)[2] is easier to activate than the
strong non-polar HÀH bond (BDE 104 kcalmolÀ1)[3] and hydrosi-
lylation reactions can be promoted using noble metal-free cat-
alysts or organocatalysts under mild reaction conditions, with-
out the need for high-pressure apparatus.[1,4] As a result, cata-
lytic hydrosilylation can achieve highly chemo- and regio-selec-
tive transformations and recent examples include the reduc-
tion of carboxylic acids,[4f,5] esters[4e,6] (to ethers and aldehydes)
and amides.[4e,6f,7] Nonetheless, the methodology still has limi-
tations and, so far, the catalytic hydrosilylation of organic ureas
to formamidines remains unknown. Indeed, the C=O group in
urea derivatives is the least electrophilic function within the
series of carbonyl groups in aldehydes, ketones, esters, amides,
carbonates, carbamates, and ureas.[8,9] This effect primarily re-
sults from strong resonance effects between the vacant p*C
Scheme 1. Synthetic routes for the preparation of formamidines.
they are common building blocks in organic chemistry and uti-
lized as reagents for the synthesis of drugs, ligands and N-het-
erocycles or as protecting groups for primary amines.[12,13] The
reduction of substituted ureas is an attractive route to form-
amidines because symmetric and unsymmetric organic ureas
are easily accessible and stable compounds. They can also be
prepared using CO2 as a carbon(IV) source.[14] In principle, the
catalytic reduction of ureas with hydrosilanes would offer an
attractive alternative to current condensation strategies that
require strong and/or toxic drying agents, such as phosphorus
oxychloride, trifluoroacetic anhydride, or thionyl chloride,[15] as
promoters (Scheme 1). Yet, to the best of our knowledge, such
transformations remain undiscovered. Here we report the first
examples of catalytic hydrosilylation of urea derivatives
(Scheme 1). The reduction takes place under mild conditions
and the iron catalyst selectively promotes reduction to the
formamidine products with no over-reduction to the aminal
derivatives.
=
O
orbital and the vicinal nitrogen lone pairs in urea. As a result,
strong reductants such as alumino- and boro-hydrides have
been utilized so far for the reduction of urea derivatives to for-
mamidines.[10] They, however, also lead to over-reduction to
the aminal derivative, because the formamidine product is
more easily reduced than the urea starting material.[10] In 2011,
Milstein and coworkers were the first to successfully promote
the hydrogenation of organic ureas, utilizing tailor-made ruthe-
nium catalysts.[8] However, the ruthenium catalysts promote
CÀN over CÀO bond cleavage and the resulting formamide in-
termediate is hydrogenated faster than the urea starting mate-
rial, leading to the formation of methanol and free amines
(Scheme 1).[8]
Among organic ureas, N,N’-diphenyl-urea (1a) was selected
as a benchmark substrate for the novel hydrosilylation reac-
tion, because the aromatic substituents are expected to in-
crease the reduction potential of the NÀC(O)ÀN carbonyl func-
tion. Motivated by the urge to develop noble metal-free cata-
lysts, important efforts have been recently devoted to explore
the potential of iron catalysts in reduction processes, with
compelling success.[5a,6d,7a,d,i–j,16] Noticeably, Beller and cowork-
ers have shown that iron hydride complexes supported by
phosphine ligands are able to promote the hydrogenation of
the kinetically stable CO2 molecule to formate derivatives.[16a]
Encouraged by these advances, the catalytic activity of a variety
of iron salts was tested in the hydrosilylation of N,N’-diphenyl-
Substituted formamidines are important chemicals in the in-
dustry for their biological activities and pharmacological prop-
erties (bactericidal, fungicidal and insecticidal).[11] Moreover,
[a] Dr. J. Pouessel, Dr. O. Jacquet, Dr. T. Cantat
CEA, IRAMIS, SIS2M, CNRS UMR 3299
91191 Gif-sur-Yvette (France)
Fax: (+33)1-6908-6640
E-mail: thibault.cantat@cea.fr
Homepage: http://iramis.cea.fr/Pisp/thibault.cantat/index.htm
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300653.
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and 1a is reduced to formamidine 2a with an excellent 98%
yield, using an equimolar mixture of Fe(acac)2 and L4 (5 mol%)
with 1 equiv PhSiH3, after 24 h at 1008C (Entry 11, Table 1). A
somewhat lower 81% yield is observed when Fe(BF4)2·6H2O is
used in place of Fe(acac)2 (Entry 17, Table 1). A change of the
hydrosilane indicates that PhSiH3 is the most reactive reductant
in the hydrosilylation of 1a, among the reactants tested. Al-
though (EtO)3SiH affords 2a in 11% yield using Fe(acac)2 and
L4 (5 mol%), Ph2SiH2, polymethylhydrosiloxane (PMHS) and
Et3SiH are completely inactive in the reduction of 1a (En-
tries 20–23, Table 1). This trend in reactivity tends to support
a mechanism involving the formation of an iron-hydride inter-
mediate in the catalytic hydrosilylation of ureas.[18] THF is the
most profitable solvent for the reaction in Equation (1). In the
less polar Et2O solvent, 2a is formed in 57% yield (with
5 mol% (Fe(acac)2 +L4)), however this yield does not exceed
17% in toluene, pyridine, 1,4-dioxane, or acetonitrile. Notably,
DMF and DMSO are found to be unsuitable for the hydrosilyla-
tion of ureas, because the solvent molecules are themselves re-
active towards reduction. Similarly, the more benign alcohols
(2-propanol or tert-butanol)[19] are readily dehydrogenated in
the presence of PhSiH3 and the iron catalyst. Likely, in the near
future, the development of catalysts able to utilize the less
basic PMHS hydrosilane will circumvent this limitation.
Table 1. Catalytic hydrosilylation of N,N’-diphenyl urea (1a) to formami-
dine 2a, as depicted in Equation (1).
Entry
Catalyst[a]
Ligand
[mol%]
Silane (R3SiH)
[eq]
Yield[b]
[%]
1
2
3
4
5
6
7
8
–
–
–
–
–
–
–
–
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
PhSiH3 (1.0)
Ph2SiH2 (1.5)
PMHS (3.0)
Et3SiH (3.0)
(EtO)3SiH (3.0)
<1
<1
<1
<1
<1
5
46
15
34
15
98
27
20
38
65
42
81
43
FeCl2
FeCl3
FeSO4·7H2O
Fe(acac)3
Fe(acac)2
Fe(BF4)2·6H2O
Fe(acac)2
Fe(acac)2
Fe(acac)2
Fe(acac)2
L1 (20)
L2 (10)
L3 (10)
L4 (5)
L5 (5)
L6 (20)
L1 (20)
L2 (10)
L3 (10)
L4 (5)
L5 (5)
L6 (20)
L4 (5)
L4 (5)
L4 (5)
L4 (5)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Having in hand an efficient catalytic system for the hydrosi-
lylation of 1a, the scope of reactive substituted ureas was ex-
plored, utilizing Fe(acac)2 and L4 (5 mol%) as a catalytic
system, with PhSiH3 as a reductant (Table 2). All the substrates
tested (1a–1o) were found reactive under the applied condi-
tions with conversions ranging from 69 to 98%, with the ex-
ception of the very bulky urea 1k, which is converted to other
products in a modest 49% yield. Noticeably, the nature of the
substituents on the nitrogen atoms has a major influence on
the outcome of the reaction and the yields of formamidines 2
vary from 29 to 98% depending on the substitution scheme
(13% for 2k). Introducing electron-withdrawing groups (Cl and
F) on the aryl rings of N,N’-diphenylurea (1a) has little impact
on the reactivity of the urea and formamidines 2b–2e are ob-
tained in good 56-81% yields (Entries 2–5, Table 2). On the
other hand, electron-donating groups somewhat deactivate
the production of formamidines 2 (Entries 6-8, Table 2). For ex-
ample, in the presence of a methyl group in the meta-position
of the aryl rings, formamidine 2h is obtained in 29% yield
(Entry 8, Table 2). Importantly, carbodiimide 3h is formed in
69% yield in this transformation, accounting for the excellent
98% conversion yield for 1h. Indeed, carbodiimides 3 were
successfully detected in the hydrosilylation of 1, in various pro-
portions depending on the electronic nature of N-groups (5–
69% yields). This reactivity suggests that carbodiimides 3 are
reaction intermediates in the hydrosilylation of 1 to 2 (see
below). Interestingly, unsymmetric N,N’-diarylureas 1i and 1j
are reduced to the corresponding unsymmetric formamidines
2i and 2j in good 74 and 63% yields, respectively (Entries 9
and 10, Table 2). As expected, N,N’-dialkylureas are more diffi-
cult to reduce and lead to the accumulation of the carbodi-
imide intermediate. Nonetheless, formamidines 2l and 2m can
be obtained in 39 and 64% yields, respectively, using 10 mol%
Fe(acac)2
Fe(acac)2
Fe(BF4)2·6H2O
Fe(BF4)2·6H2O
Fe(BF4)2·6H2O
Fe(BF4)2·6H2O
Fe(BF4)2·6H2O
Fe(BF4)2·6H2O
Fe(acac)2
Fe(acac)2
Fe(acac)2
Fe(acac)2
15
<1
<1
<1
11
[a] All catalysts were employed at 5 mol%. [b] Determined by GC/MS
(using biphenyl as an internal standard) and 1H NMR analyses of the
crude mixture.
urea (1a). In the presence of a catalytic amount of FeCl2, FeCl3,
Fe(SO4)·7H2O or Fe(acac)3 (5 mol%), addition of 1 equiv PhSiH3
to a THF solution of 1a led to no reaction and the starting ma-
terials were recovered unreacted after 24 h at 1008C (En-
tries 2–5, Table 1). Nonetheless, iron(II) complexes Fe(acac)2 and
Fe(BF4)2·6H2O exhibit a modest, but promising, catalytic activi-
ty, promoting reduction of 1a to its formamidine derivative 2a
in 5 and 46% yields, respectively, under identical reaction con-
ditions (Entries 6 and 7, Table 1). These preliminary results es-
tablish the feasibility of the catalytic reaction depicted in Equa-
tion (1) (Table 1).
Supporting phosphine ligands were then screened, so as to
increase the catalytic activity of both Fe(acac)2 and Fe-
(BF4)2·6H2O (Entries 8–19, Table 1). Overall, the addition of
phosphines proved highly beneficial and substantially en-
hanced the reactivity and/or stability of the catalyst. Notice-
ably, the tetra-phosphine L4 ligand, originally developed by
King and coll.,[17] leads to the most efficient catalytic systems
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Fe(acac)2 +L4 and 2 equiv PhSiH3. Additionally, the
mixed aryl-alkyl N-substituted ureas 1n and 1o have
a greater electrophilicity and are hydrosilylated to 2n
and 2o in good yields (>65%, Entries 14 and 15,
Table 2). Importantly, in all these examples (Entries 1–
15, Table 2), no over-reduction of the urea 1 to the
aminal derivative was observed. Whereas the reduc-
tion of ureas with metal-hydrides (e.g. LiAlH4 or
NaBH4) has a low selectivity and leads to a mixture of
aminal and formamidine products,[10] this reactivity
clearly highlights the benefit of catalytic
hydrosilylation.
Table 2. Catalytic hydrosilylation of organic ureas 1 to formamidines 2, as depicted in
Equation (2).
Entry Substrate (1)
Conv.[a] Yield of 3[a] Yield of 2[a]
[%]
[%]
[%]
1
2
1a 98
<1
98[b]
Though the results gathered in Table 2 demon-
strate, for the first time, the feasible reduction of
ureas using hydrosilanes, this new method for the
synthesis of formamidines suffers from a modest se-
lectivity, for some of the substrates. As such, mecha-
nistic insights on the nature of the reactive inter-
mediates would be informative for the future design
of more active catalysts. From a mechanistic perspec-
tive, the product distributions obtained in Table 2
suggest the involvement of carbodiimide species 3
as reactive intermediates in the hydrosilylation of or-
ganic ureas 1 to formamidines 2. Though ureas 1 are
stable in the presence of hydrosilanes (Entry 1,
Table 1), they are dehydrated to the corresponding
carbodiimides 3 in the presence of the iron catalyst
(Table 2). This transformation is reminiscent of the de-
hydration of primary amides to nitriles using hydrosi-
lanes[7d] and likely proceeds through dehydrogenative
silylation of both NÀH bonds of the di-substituted
urea and subsequent elimination of the siloxane by-
product. Catalytic silylation of NÀH bonds in amines
is indeed well documented[20] and H2 evolution was
successfully detected by monitoring the hydrosilyla-
tion of 1a by using 1H NMR spectroscopy, in the
presence of Fe(acac)2 +L4 (5 mol%) and 1 equiv
PhSiH3. Hydrosilylation of carbodiimides to N-silylfor-
mamidines is a relatively unexplored transformation.
Only two catalysts were found active for this reaction,
namely PdCl2 and [(PPh3)3RhCl], and they function at
elevated temperature (1508C).[21] Importantly, we
found that the phosphine iron catalysts are able to
promote the hydrosilylation of carbodiimides to
formamidines in high yields. As depicted in Equa-
tion (3), Fe(BF4)2·6H2O+L4 efficiently promotes the
hydrosilylation of both aliphatic (3p and 3q) and aro-
matic (3r) carbodiimides, whereas Fe(acac)2 +L4 is
mostly able to reduced N,N’-di-p-tolylcarbodiimide
(3r) to 2r in 60% yield. Overall, these findings sup-
port the mechanistic proposal presented in
Scheme 2. It relies on the iron-catalyzed dehydration
of urea 1 to carbodiimide 3 promoted by two equiva-
lents of the hydrosilane reagent. Subsequent catalytic
hydrosilylation of 3 affords the N-silylformamidine de-
rivative 2’ which is readily hydrolyzed to 2.
1b 86
5
81
3
4
5
6
1c 69
1d 84
1e 90
1 f 94
13
24
30
47
56
58
60
47
7
8
9
1g 86
1h 98
1i 87
55
69
10
31
29
74
10
11
1j 83
20
36
63
13
1k 49
12
13
14
1l 98[c]
1m 95[c]
1n 92
59[c]
39[c]
64[c]
76
31[c]
16
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In conclusion, we have developed the first catalytic
hydrosilylation reaction of organic ureas. Iron cata-
lysts, consisting of a mixture of an iron(II) salt (Fe-
(acac)2 or Fe(BF4)2·6H2O) with a commercially avail-
able tetra-phosphine ligand, are able to promote the
formation of symmetric and unsymmetric formami-
dines without over-reduction to the aminal deriva-
tives. This protocol has been successfully applied to
17 different ureas with different electronic and geo-
metric properties. The main catalytic pathway was
shown to involve the dehydration of the urea re-
agent to its corresponding carbodiimide derivative
Table 2. (Continued)
Entry Substrate (1)
Conv.[a] Yield of 3[a] Yield of 2[a]
[%]
[%]
[%]
15
1o 88
23
65
[a] Determined by GC/MS (using biphenyl as an internal standard) and 1H NMR analy-
ses of the crude mixture. [b] Average isolated yield for 2a, over three experiments:
90%. [c] Results obtained using Fe(acac)2 (10 mol%) and L4 (10 mol%), in the pres-
ence of 2.0 equiv PhSiH3.
which is subsequently reduced by the hydrosilane. Future ef-
forts will be devoted to increasing the catalytic activity and se-
lectivity of the iron system to utilize inexpensive hydrosilanes,
such as PMHS and TMDS, in this transformation.
Experimental Section
Scheme 2. Proposed mechanistic scheme for the iron-catalyzed hydrosilyla-
tion of ureas 1 to formamidines 2.
Detailed descriptions of experimental and spectroscopic results are
given in the Supporting Information.
Acknowledgements
For financial support of this work, we acknowledge the CEA,
CNRS, ANR, the FP7 Eurotalents Program (PD Fellowship to O.J.),
and the CHARMMMAT Laboratory of Excellence. T.C. thanks the
Fondation Louis D.—Institut de France for its formidable support.
Parts of this work have been filed as a French patent application
FR1259757 by CEA.
The involvement of carbodiimides as reaction intermediates
in the reduction of ureas 1 may limit the scope of reactive sub-
strates to N,N’-di-substituted-ureas and we, therefore, explored
the reactivity of tri-substituted ureas 4 in the new iron-cata-
lyzed hydrosilylation reaction [Eq. (4) and (5)]. Interestingly,
N,N-dimethyl-N’-phenylurea 4a is reduced to 5a in 25% yield
using 10 mol% of Fe(acac)2 +L4 and 1 equiv PhSiH3. Though
modest, this conversion indicates that hydrosilylation of ureas
to formamidines may proceed through different yet conver-
gent pathways and that the direct hydrosilylation of the N-CO-
N carbonyl function is also feasible without prior activation by
dehydration. Replacing the electron donating methyl groups in
4a with an imidazole ring facilitates the hydrosilylation and 4b
is reduced to 5b in 40% yield, without reduction of the imid-
azole ring, using 5 mol% Fe(acac)2 +L4.
Keywords: hydrosilylation · iron · reduction · silanes · ureas
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Received: August 6, 2013
Published online on November 7, 2013
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