Direct catalytic access to N-silylated enamines from enolizable imines and hydrosilanes by base-free dehydrogenative Si-N coupling

Article abstract of DOI:10.1002/chem.201402866

A procedure for the synthesis of otherwise difficult-to-make N-silylated enamines, that is masked enamines derived from primary amines, is reported. The approach is based on formation of a silyliminium ion and subsequent abstraction of the acidified α-proton rather than α-deprotonation of the enolizable imine followed by reaction with an electrophilic silicon reagent. The silicon electrophile, stabilized by a sulfur atom, is generated by cooperative activation of an Si-H bond at the Ru-S bond of a tethered ruthenium(II) thiolate complex. After transfer of the silicon cation onto the imine nitrogen atom, the remaining ruthenium(II) hydride fulfills the role of the base. Deprotonation and release of dihydrogen close the catalytic cycle. The net reaction is a dehydrogenative Si-N coupling of enolizable imines and hydrosilanes. Silicon IN, hydrogen OUT: N-Silylated enamines are protected enamines derived from combinations of primary rather than secondary amines and enolizable carbonyl compounds. This rare class of compounds is now directly available from imines by a dehydrogenative Si-N coupling. The catalyst generates the silicon electrophile by Si-H bond activation and also acts as a base after transfer of the silicon electrophile onto the imine nitrogen atom (see scheme, Ar ^F=3,5-bis(trifluoromethyl)phenyl, Si=R₃Si=triorganosilyl).

Full text of DOI:10.1002/chem.201402866

DOI: 10.1002/chem.201402866
Communication
&
Homogeneous Catalysis
Direct Catalytic Access to N-Silylated Enamines from Enolizable
Imines and Hydrosilanes by Base-Free Dehydrogenative SiꢀN
Coupling
Julia Hermeke, Hendrik F. T. Klare, and Martin Oestreich*[a]
Abstract: A procedure for the synthesis of otherwise diffi-
cult-to-make N-silylated enamines, that is masked enam-
ines derived from primary amines, is reported. The ap-
proach is based on formation of a silyliminium ion and
subsequent abstraction of the acidified a-proton rather
than a-deprotonation of the enolizable imine followed by
reaction with an electrophilic silicon reagent. The silicon
electrophile, stabilized by a sulfur atom, is generated by
cooperative activation of an SiꢀH bond at the RuꢀS bond
of a tethered ruthenium(II) thiolate complex. After transfer
Scheme 1. Preparation of N-silylated enamines from imines by deprotona-
tion/electrophilic substitution (top) and dehydrogenative coupling (bottom).
LDA=lithium diisopropylamide and Tf=trifluoromethanesulfonyl.
of the silicon cation onto the imine nitrogen atom, the re-
maining ruthenium(II) hydride fulfills the role of the base.
Deprotonation and release of dihydrogen close the cata-
lytic cycle. The net reaction is a dehydrogenative SiꢀN
coupling of enolizable imines and hydrosilanes.
sequence. That waste could be avoided in a direct dehydro-
genative coupling of imines and hydrosilanes (1!2, Scheme 1,
bottom), but such a reaction has not been described yet.[8] Our
laboratory recently developed a catalytic dehydrogenative cou-
pling of enolizable carbonyl compounds and hydrosilanes that
directly produces silyl enol ethers along with dihydrogen (not
shown).[9] Referring back to the aforementioned analogy, we
anticipated that this method would be applicable to enolizable
imines.[10] The overall process hinges on the ability of tethered
ruthenium complexes 5+[BArF4]ꢀ[11] (with ArF =3,5-bis(trifluoro-
methyl)phenyl, Scheme 2, top) to formally split the SiꢀH bond
of 6 into a hydride and a sulfur-stabilized silicon cation (5+!
[7+]°!8+, Scheme 2, middle).[12] The activation step [7+]° is
a s-bond metathesis of the SiꢀH and RuꢀS bonds. Intermedi-
ate 8+ is an excellent source of electrophilic silicon, and its
transfer onto Lewis basic sites is facile. With imine 1 as the
Lewis base, silyliminium ion 9+ forms immediately, releasing
the ruthenium hydride 10 (Scheme 2, bottom). Complex 10
usually acts as a base (9+!2, right) rather than a hydride
donor (9+!4, left); this counterintuitive behavior is document-
ed in several dehydrogenative bond-forming reactions,[9,10,12]
and hydride transfer has so far only been seen in the absence
of acidic hydrogen atoms.[13] Abstraction of an acidified a-hy-
drogen atom in 9+ would then afford the desired N-silylated
enamine 2 and dihydrogen. We disclose here the direct syn-
thesis of 2 from enolizable imines 1 based on this strategy.
We began our investigation with testing selected RuꢀS com-
N-Silylated enamines are essentially nitrogen analogs of O-sily-
lated enols (=silyl enol ethers), but the chemistry of this un-
common compound class is by far less developed.[1] The SiꢀN
bond serves as a placeholder for an NꢀH bond, and N-monosi-
lylated enamines are, as such, precursors of otherwise unstable
N-unsubstituted enamines that tautomerize preferentially to
the corresponding imine. Hence, protection of the NꢀH group
opens the door to enamine reactivity from combinations of
primary rather than secondary amines and enolizable carbonyl
compounds.[2] A few broadly applicable procedures for their
preparation are known,[1] including the methods starting from
imines that were elaborated by Ahlbrecht and co-workers
a long time ago (1!2, Scheme 1, top).[3,4] These differ in base
(complete[3a] versus partial[3b] deprotonation) and silicon elec-
trophile, and Method 2 is superior to Method 1 in terms of
chemoselectivity [N- (1!2) preferred over C-silylation (1!3)].
Ahlbrecht’s methods are reliable, and even the use of silyl
triflates is not a limitation anymore since those with typical
substitution patterns are nowadays commercially available.
What might be viewed as a disadvantage is the accumulation
of stoichiometric salt as a result of the deprotonation/silylation
[a] J. Hermeke, Dr. H. F. T. Klare, Prof. Dr. M. Oestreich
Institut fꢀr Chemie, Technische Universitꢁt Berlin
Strasse des 17. Juni 115, 10623 Berlin (Germany)
E-mail: martin.oestreich@tu-berlin.de
plexes 5+[BArF ]ꢀ and various hydrosilanes 6 (Table 1). The
4
choice of 6 is determined by the size of the pocket that both
the thiolate and phosphine ligands create around the RuꢀS
bond, and Me2PhSiH (6a) had been found to be a good fit be-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402866.
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Table 1. Optimization of reaction conditions for the synthesis of N-silylat-
ed enamines 2 from enolizable imines 1.[a]
Entry
R
5+
R1
Imine Hydrosilane [h]
SiꢀH
t
Ratio
2/4/11[c]
Yield
[%][d]
1
Et
Ph
1a
Me2PhSiH
6a
0.25 90:10:0
2aa:4aa:11 aa
97:0:3
quant
5a+
iPr
2
12
60
0.5
12
12
4
quant[e]
(79)[f]
97
5b+
Ar[h]
5c+
Et
5a+
iPr
2aa:4aa:11 aa
84:10:6
2aa:4aa:11 aa
13:86:1
2ba:4ba:11 ba
93:2:5
2ba:4ba:11 ba
0:>99:0
3[g]
4
Bn
1b
Me2PhSiH
6a
quant
90
5
5b+[i]
Ar[h]
5c+[i]
iPr
6
95
2ba:4ba:11 ba
>99:0:0
7
Ph
1a
EtMe2SiH
6b[j]
Et3SiH
6c
MePh2SiH
6d
Ph3SiH
6e
quant
8[l]
Scheme 2. Catalyst (top), cooperative SiꢀH bond activation (middle), and
concept of dehydrogenative enamine formation from imines (bottom).
ArF =3,5-bis(trifluoromethyl)phenyl and Si=R3Si=triorganosilyl.
5b+
2ab:4ab:11 ab
>99:0:0
2ac:4ac:11 ac
0:>99:0
2ad:4ad:11 ad
–
8[k]
9[k]
10[k]
48
48
48
1[l]
fore.[9,10,12a,13] Enolizable imines 1a and 1b were employed as
model substrates, and three representative complexes 5a+
–5c+ with sterically and electronically different phosphine li-
gands were used as catalysts. For N-phenylimine 1a as sub-
strate, low loadings of all three Ru–S complexes 5+ were
found to promote the dehydrogenative coupling at room tem-
perature (entries 1–3). However, the reaction times were mark-
edly dependent on the steric and electronic situation at the
RuꢀS bond in 5+. The electron-rich phosphines form more re-
active complexes than the electron-deficient phosphine[14] with
the deprotonation/loss of dihydrogen event likely to be rate-
determining, but increased steric bulk also decelerates the re-
action (hours with iPr3P versus minutes with Et3P). The chemo-
selectivity was generally high when the reaction was run in an
open system to release dihydrogen (1a!2aa, entries 1–3),[15]
and the reduction path could be completely suppressed using
5b+ (entry 2). It is worth noting that we identified a minor
component, the C-silylated N-silylated enamine 11 aa, as the
result of a twofold dehydrogenative coupling. The N-silylated
enamine 2aa is a nucleophile itself that reacts with the silicon
electrophile to yield a C-silylated silyliminium ion that, in turn,
is deprotonated to reestablish the enamine functional group
(not shown).[16,17] Traces of 11 aa are seen with the bulkier cata-
lysts 5b+ and 5c+ (entries 2 and 3), and these proportions did
not change in the presence of excess hydrosilane 6a (2.0 equiv
or more). However, as evident from the substrate screening
below, 11 forms in substantial amounts from imines with steri-
cally hindered substituents at the imine nitrogen atom. Hence,
imine 1 and enamine 2 compete for the silicon electrophile in
these cases.
0[l]
2ae:4ae:11 ae
[a] All reactions were performed according to the General Procedure (see
the Supporting Information for details). [b] Determined by 1H NMR spec-
troscopy using 1,2,4,5-tetramethylbenzene as internal standard. [c] Deter-
mined by 1H NMR spectroscopy. [d] Combined NMR yield determined by
using 1,2,4,5-tetramethylbenzene as internal standard. [e] In situ forma-
tion of coordinatively unsaturated 5b+[BArF ]ꢀ from the corresponding
4
air-stable chloride complex by treatment with NaBArF furnished the
4
same result within the experimental error; in this way, it was not necessa-
ry to handle oxygen-sensitive catalyst 5b+[BArF ]ꢀ in a glovebox. [f] Iso-
4
lated yield on a 1.5 mmol scale. [g] Reaction with 2.0 mol% catalyst load-
ing results in same ratio and yield in 12 h. [h] Ar=4-FC6H4. [i] Catalyst
loading of 2.0 mol% required for these reaction times. [j] 2.0 equiv due to
its volatility. [k] 808C. [l] Conversion.
(Table 1, entries 4–6). To our surprise, catalysts 5a+ and 5c+
furnished the N-silylated amine (1b!4ba, entries 4 and 6)
whereas 5b+ still provided the desired N-silylated enamine
with excellent chemoselectivity (1b!2ba, entry 5). As a result
of the sterically less hindered benzyl group in 1b, formation of
11 ba was hardly seen, if at all. The effect of the phosphine
ligand on the reaction path is absolutely remarkable: 5b+ with
the bulky electron-donating phosphine produces the N-silylat-
ed enamine (1b!2ba) and 5c+ with the electron-withdraw-
ing phosphine exclusively yields the N-silylated amine (1b!
4ba).
With optimal catalyst 5b+ in hands, we tested representa-
tive triorganohydrosilanes 6b–6e (Table 1, entries 7–10). Et-
Me2SiH is as good as Me2PhSiH (6a and 6b, entries 2 and 7).
Conversely, any other hydrosilane 6c–6e was too bulky to un-
dergo the s-bond metathesis at the RuꢀS bond in 5b+ (en-
tries 8–10). While this was expected for Et3SiH (6c) and Ph3SiH
With N-benzylimine 1b as substrate, the product distribution
changes dramatically under otherwise identical conditions
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(6e),[9,10,12a,13] the inability of 5b+ (with iPr3P) to split the SiꢀH
bond in MePh2SiH (6d) was not. The fact that both 5a+ (with
Et3P) and 5c+ [with (4-FC6H4)3P] do mediate the activation of
this particular hydrosilane[14] is evidence for the importance of
the size of the phosphine ligand in catalysts 5+. Elevated tem-
peratures did not make any difference.
nation step. Adjusting the reaction time also allowed for
decent conversion of imine 1k with an electron-donating
group (67% conversion in days versus full conversion in
hours).
Any aryl group other than phenyl at the imine nitrogen
atom will be an exception rather than the norm. We therefore
continued probing the scope of the dehydrogenative SiꢀN
coupling using a range of differently substituted acetophe-
none-derived N-phenylimines under the optimized reaction
conditions (Table 3). Examples include substitution in the ortho,
meta, and para positions with electron-donating as well as
withdrawing groups. Without exception, the N-silylated enam-
ines 2ma–2za and 2a’a were obtained chemoselectively in
high to excellent yields. Substrates with electron-withdrawing
groups worked perfectly with this catalytic system, for exam-
ple, 2ra, 2ua, 2va, 2xa, and 2a’a. Substrates bearing strongly
electron-donating groups furnished 2ma, 2pa, and 2sa in
good to high yields but required longer reaction times. It was
only these reactions where small amounts of the N-silylated
The optimized procedure was then applied to acetophe-
none-derived imines 1c–1l with various aryl groups at the
imine nitrogen atom (Table 2). Gratifyingly, the N-silylated
amines 4c–4l did not form in any of these examples. However,
the aforementioned twofold dehydrogenative coupling leading
to 11 became a serious side reaction with imines 1 having
a sterically less accessible nitrogen atom (1!2!11). As
a result, N-arylimines 1c–1g with an ortho-substituent or
a peri-position afforded C-silylated N-silylated enamines 11 ca–
11 ga in quantities similar to those of desired 2ca–2ga. Imines
1ha–1la not crowded around the nitrogen atom reacted
smoothly to yield the N-silylated enamines 2ha–2la. CF3-sub-
stituted N-arylimines 1j and 1l reacted faster than the OMe-
substituted congener 1k. Electron-withdrawing substituents at
the N-aryl group make the imine nitrogen atom of 1 less basic
but also destabilize the silyliminium ion intermediate 9+,
thereby facilitating the (potentially rate-determining) deproto-
Table 3. Substrate scope II: Variations of the substitution pattern of ace-
tophenone-derived N-phenylimines.[a]
Table 2. Substrate scope I: Variation of the N-aryl moiety of acetophe-
none-derived imines.[a]
[a] All reactions were performed according to the General Procedure (see
the Supporting Information for details). Conversions and NMR yields were
determined by 1H NMR spectroscopy using 1,2,4,5-tetramethylbenzene as
internal standard. [b] Reaction with 2.0 mol% catalyst loading results in
same ratio and yield. [c] Catalyst loading of 2.0 mol%.
[a] All reactions were performed according to the General Procedure (see
the Supporting Information for details). Conversions and NMR yields were
determined by 1H NMR spectroscopy using 1,2,4,5-tetramethylbenzene as
internal standard.
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Communication
amines 4pa (4%) and 4sa (2%) were detected. Steric hin-
drance again led to C-silylation of the N-silylated enamine (2!
11) albeit to much lesser extent than seen for imines substitut-
ed at the nitrogen atom with a bulky aryl group. Hence, ortho-
aryl- and 1-naphthyl-substituted imines 1m–1o and 1y afford-
ed 2ma–2oa and 2ya in good yields along with 11 ma–11 oa
(ꢁ10%) and 11 ya (19%). We also reacted selected systems
1m–1o with double the amount of hydrosilane 6a to learn
whether the C-silylated N-silylated enamine would form quan-
titatively from hindered intermediates 2ma–2oa, and we were
delighted to see the high-yielding formation of 11 ma–11 oa
(see the Supporting Information for details). These electronic
and steric effects are consistent with the trends discussed for
the various acetophenone-derived N-arylimines (Tables 2 and
3).
nium(II) center, is crucial for obtaining high chemoselectivity in
favor of N-silylated enamine (dehydrogenation path) over N-si-
lylated amine (reduction path). We were able to demonstrate
that structurally diverse acetophenone-derived imines partici-
pate in this reaction. By this, a rare class of enamines that are
made from primary amines now becomes readily available.
Acknowledgements
This research was supported by the Cluster of Excellence Unify-
ing Concepts in Catalysis of the Deutsche Forschungsgemein-
schaft (EXC 314/2). M.O. is indebted to the Einstein Foundation
(Berlin) for an endowed professorship. We thank Dr. Sebastian
Kemper (TU Berlin) for expert advice with the NMR measure-
ments. We are grateful to Timo Stahl and Toni T. Metsꢁnen
(both TU Berlin) for their contributions to the preparation and
characterization of the ruthenium(II) thiolate complexes.
Probing the possibility of diastereocontrol in the formation
of the N-silylated enamine, we subjected imines 1b’ and 1c’ to
the standard reaction conditions (Scheme 3, top). Both 2b’a
and 2c’a formed in quantitative yield and, not unexpectedly,
higher steric demand of the R group had a beneficial effect on
Keywords: chemoselectivity
· dehydrogenative coupling ·
enamine compounds · homogeneous catalysis · SiꢀH bond
activation
[1] S. J. Collier in Science of Synthesis Vol. 33 (Ed.: G. Molander), Thieme,
Stuttgart, 2006, pp. 451–473, and references therein.
[2] For the use of N-silylated enamines as carbon nucleophiles, see: a) W.
Ando, H. Tsumaki, Tetrahedron Lett. 1982, 23, 3073–3076 (C-acylation
with acyl chlorides); b) W. Ando, H. Tsumaki, Chem. Lett. 1983, 1409–
1412 (C-hydroxyalkylation with aldehydes); c) M. Fourtinon, B. De Jeso,
J.-C. Pommier, J. Organomet. Chem. 1985, 289, 239–246 (diastereoselec-
tive C-alkylation with a,b-unsaturated ester).
[3] a) H. Ahlbrecht, D. Liesching, Synthesis 1976, 746–748; b) H. Ahlbrecht,
E.-O. Dꢂber, Synthesis 1980, 630–631.
[4] There are several approaches to the preparation of N-silylated enam-
ines using nitriles as starting materials. Ahlbrecht and Dꢂber also
showed that treatment of a-amino nitriles with Et3N/Me3SiOTf yields
the title compounds after elimination of Me3SiCN.[5] Addition of carbon
nucleophiles to nitriles followed by electrophilic substitution of the in-
termediate lithioenamines with R3SiCl is another practical alternative;[6]
examples of internal delivery of the silicon group were also reported.[7]
[5] H. Ahlbrecht, E.-O. Dꢂber, Synthesis 1983, 56–57.
[6] a) P.-L. Compagnon, F. Gasquez, T. Kimny, Synthesis 1986, 948–952;
b) N. Guillot, Z. Janousek, H. G. Viehe, Heterocycles 1989, 28, 879–886.
[7] a) K. Ohkata, Y. Ohyama, Y. Watanabe, K.-y. Akiba, Tetrahedron Lett. 1984,
25, 4561–4564; b) T. Konakahara, Y. Kurosaki, J. Chem. Res. Synop. 1989,
130–131.
[8] We had become aware of this gap during a mechanistic investigation
where independent preparation of N-silylated enamines was required
but Ahlbrecht’s improved procedure in Ref. [3b] emerged as difficult
due to the nonavailability of the corresponding silyl triflate: J. Hermeke,
M. Mewald, M. Oestreich, J. Am. Chem. Soc. 2013, 135, 17537–17546.
[9] C. D. F. Kçnigs, H. F. T. Klare, Y. Ohki, K. Tatsumi, M. Oestreich, Org. Lett.
2012, 14, 2842–2845.
[10] We had already demonstrated that the same catalyst system promotes
the dehydrogenative SiꢀN coupling of weakly as well as moderately
basic NꢀH groups and hydrosilanes: C. D. F. Kçnigs, M. F. Mꢂller, N. Ai-
guabella, H. F. T. Klare, M. Oestreich, Chem. Commun. 2013, 49, 1506–
1508.
Scheme 3. Additional substrates: Exploring control of the double bond ge-
ometry (top) and chemoselectivity in the reaction of a dialkyl imine
(bottom).
the diastereoselectivity. Finally, we studied an example of a di-
alkyl imine, namely the N-phenylimine of cyclohexanone
(Scheme 3, bottom). For 1d’, the chemoselectivity was not as
high as in all previous cases; N-silylated enamine 2d’a and N-
silylated amine 4d’a formed in a ratio of 80:20. These initial re-
sults are promising though and set the stage for further im-
provement of the catalyst structure to control the alkene ge-
ometry as well as expand the substrate scope.
In summary, we disclosed here a new way for the direct cat-
alytic preparation to N-silylated enamines from enolizable
imines and hydrosilanes in high yields. The reaction proceeds
at room temperature with low catalyst loadings and, impor-
tantly, does not require addition of a base, liberating dihydro-
gen as the sole byproduct. In situ generation of the coordina-
tively unsaturated catalyst allows for performing the reactions
outside a glovebox. The choice of the ruthenium(II) thiolate
catalyst or, more precisely, the phosphine ligand at the ruthe-
[11] Y. Ohki, Y. Takikawa, H. Sadohara, C. Kesenheimer, B. Engendahl, E. Kapa-
tina, K. Tatsumi, Chem. Asian J. 2008, 3, 1625–1635.
[12] a) H. F. T. Klare, M. Oestreich, J.-i. Ito, H. Nishiyama, Y. Ohki, K. Tatsumi, J.
Am. Chem. Soc. 2011, 133, 3312–3315; b) T. Stahl, K. Mꢂther, Y. Ohki, K.
Tatsumi, M. Oestreich, J. Am. Chem. Soc. 2013, 135, 10978–10981.
[13] C. D. F. Kçnigs, H. F. T. Klare, M. Oestreich, Angew. Chem. 2013, 125,
10260–10263; Angew. Chem. Int. Ed. 2013, 52, 10076–10079.
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[14] T. Stahl, H. F. T. Klare, M. Oestreich, J. Am. Chem. Soc. 2013, 135, 1248–
1251.
[17] An alternative pathway to arrive at C-silylated N-silylated enamines
from imines could proceed through the C-silylated enamine (by a mech-
anism similar to that outlined in Ref. [16]) followed by its dehydrogena-
tive SiꢀN coupling (cf. Ref. [10]).
[15] a) The procedure is detailed in the General Procedure in the Supporting
Information; b) The rate of the addition of catalyst 5a+[BArF ]ꢀ was
4
found to effect the chemoselectivity but slow addition secured repro-
ducibility.
[16] A similar reaction of N,N-dialkylated enamines with (C6F5)3SiOTf using
Et3N as base was reported by Dilman and co-workers: V. V. Levin, A. D.
Dilman, P. A. Belyakov, A. A. Korlyukov, M. I. Struchkova, V. A. Tartakov-
sky, Tetrahedron Lett. 2005, 46, 3729–3732.
Received: March 31, 2014
Published online on && &&, 0000
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COMMUNICATION
Silicon IN, hydrogen OUT: N-silylated
enamines are protected enamines de-
rived from combinations of primary
rather than secondary amines and eno-
lizable carbonyl compounds. This rare
class of compounds is now directly
available from imines by a dehydrogena-
tive SiꢀN coupling. The catalyst gener-
ates the silicon electrophile by SiꢀH
bond activation and also acts as a base
after transfer of the silicon electrophile
onto the imine nitrogen atom (see
scheme, ArF =3,5-bis(trifluoromethyl)-
phenyl, Si=R3Si=triorganosilyl).
&
Homogeneous Catalysis
J. Hermeke, H. F. T. Klare, M. Oestreich*
&& – &&
Direct Catalytic Access to N-Silylated
Enamines from Enolizable Imines and
Hydrosilanes by Base-Free
Dehydrogenative SiꢀN Coupling
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