Transformation of RN=CHPh to R(R′3Si)NCH2Ph in the catalytic desulfurization of secondary thioamide with R′3SiH promoted by an iron complex

Article abstract of DOI:10.1002/hc.21175

The reaction of imine (RN=CHPh) with hydrosilane (R′₃SiH) in the presence of CpFe(CO)₂Me (1) revealed the formation of a hydrosilylation product (R(R′₃Si)NCH₂Ph). The findings helped us to understand the reaction mechanism of desulfurization of secondary thioamide catalyzed by 1 to give the corresponding imine and amine as a major and minor product, respectively.

Full text of DOI:10.1002/hc.21175

Heteroatom Chemistry
Volume 25, Number 6, 2014
Transformation of RN=CHPh to
R(Rꢀ3Si)NCH2Ph in the Catalytic
Desulfurization of Secondary Thioamide with
Rꢀ3SiH Promoted by an Iron Complex
Kozo Fukumoto,1 Akane Sakai,2 Toshiaki Murai,3
and Hiroshi Nakazawa2
1Department of Chemistry, Faculty of Education, University of the Ryukyus, Okinawa 903-0213,
Japan
2Department of Chemistry, Graduate School of Science, Osaka City University, Osaka 558-8585,
Japan
3Department of Chemistry and Biomolecular Science, Faculty of Engineering, Gifu University, Gifu
501-1193, Japan
Received 28 February 2014; revised 21 April 2014
reactions of thioformamide and thioamide using
ABSTRACT: The reaction of imine (RN=CHPh) with
a transition metal complex have been reported.
hydrosilane (Rꢀ3SiH) in the presence of CpFe(CO)2Me
Sharma and Pannell showed desulfurization
(1) revealed the formation of a hydrosilylation prod-
of thioformamide (Me2NC(S)H) by hydrosilane
uct (R(Rꢀ3Si)NCH2Ph). The findings helped us to
(R3SiH) in the presence of Mo(CO)6 [1], and we
understand the reaction mechanism of desulfuriza-
reported the desulfurization of thioformamide and
tion of secondary thioamide catalyzed by 1 to give
thioamide in the presence of CpFe(CO)2Me (1)
the corresponding imine and amine as a major and
(Cp = η5-C5H5) [2] to give amine. In the latter case,
ꢁC
minor product, respectively.
2014 Wiley Periodi-
the carbene complex of iron with a silylthiolato
ligand CpFe(CO)(SSiR3)(=CH(NMe2)) was isolated
as an intermediate in the catalytic cycle. Moreover,
we found that secondary thioamides R1HNC(S)R2
reacted with Et3SiH in the presence of a catalytic
amount of 1 to give imine R1N=CHR2 as a major
product and amine HR1NCH2R2 as a minor prod-
uct [3]. We have proposed the reaction mechanisms
for the desulfurization of thioamide (Scheme 1).
The thiocarbonyl group coordinates to the
CpFe(CO)(SiEt3) species (A) in an η2-fashion to give
B. Then, the silyl group migrates from the Fe to
the sulfur atom to form C, followed by the S C bond
oxidative addition to give the iron–carbene complex
D. The C S strong bond cleavage of thioamide is
initiated by the silyl migration from Fe to the sulfur
atom in thioamide in the coordination sphere. This
cals, Inc. Heteroatom Chem. 25:607–611, 2014; View
this article online at wileyonlinelibrary.com. DOI
10.1002/hc.21175
INTRODUCTION
Creation of an effective desulfurization system
catalyzed by a transition metal complex is one of
the important subjects because sulfur compounds
generally serve as catalyst poisons toward transition
metal catalysts. Recently, a few desulfurization
Correspondence to: Hiroshi Nakazawa; e-mail: nakazawa@
sci.osaka-cu.ac.jp.
ꢁC
2014 Wiley Periodicals, Inc.
607

608 Fukumoto et al.
FIGURE
1
Plot of the amounts of PhN=CHPh and
PhHNCH2Ph as a function of time.
SCHEME 1 Reaction sequences from thioamide to amine
and imine.
and triethylsilane (Et3SiH, 1.6 mmol) were charged
in a Schlenk tube, and the solution was heated at
80°C for 24 h (Table 1, entry 1). The 1H NMR spectra
and GC-MS analysis of the products demonstrated
the formation of N-benzylaniline (PhHNCH2Ph, 11%
yield based on PhHNC(S)Ph) as a minor product
and N-benzylideneaniline (PhN CHPh, 73% yield)
as a major product. Similar results were obtained
for MeHNC(S)Ph, although conversion yields were
low (entry 2). In entries 3–5, no amine and a trace
amount of imine were formed, indicating that a
bulkier substituent than a phenyl one on the N atom
of secondary thioamides reduced the conversion into
desulfurization products (amine and imine) presum-
ably due to steric difficulty of the formation of B in
Scheme 1. Deoxygenation of secondary amide did
not occur at all (entries 6–7).
We focused on the reaction shown in entry 1
in Table 1, and followed the reaction with time.
A plot of the amounts of PhN CHPh and that of
PhHNCH2Ph as a function of time is shown in Fig. 1.
The imine increased with time and the yield reached
94% at 15 h after the reaction started, and then de-
creased gradually. In contrast, the amine increased
constantly. The results suggest that the imine is con-
verted into the amine in this reaction system.
type of reaction, that is a silyl-migration–induced
reaction (SiMI reaction), has been applied to
achieve the other strong bond cleavage [4]. Next
step depends on whether Z is a tertiary amino group
(–NRꢀ2) or a secondary amino group (–NHRꢀ). In
the former case, an intermolecular addition of the
Et3Si-H bond to the Fe C bond (E) takes place to
give F (Path X), followed by reductive elimination of
Et3SiSSiEt3, oxidative addition of Et3Si-H forming
G, and reductive elimination of amine (Rꢀ2NCH2R)
to regenerate A. In the latter case, an intramolecular
addition of the H N bond of the NHRꢀ group to the
Fe C portion (H) occurs to give I (Path Y), followed
by reductive elimination of imine (RꢀN CHR),
oxidative addition of Et3Si-H forming J, reductive
elimination of Et3SiSSiEt3, oxidative addition of
Et3Si-H forming K, and reductive elimination of
H2 to regenerate A. According to the reaction
mechanism, it appears that tertiary thioamide gives
amine through Path X and secondary thioamide
is converted into imine through Path Y. However,
our previous results showed that amine in addition
to imine was formed in the reaction of secondary
thioamide [3]. How is the amine formed? One
possible explanation is that parallel reactions
(Paths X and Y) proceed to form amine and imine,
respectively. Another possibility is that the Path Y
is predominant to produce RꢀN CHR exclusively
and the imine is converted into RꢀHNCH2R in this
catalytic system. This paper reports the formation
mechanism of amine in the desulfurization reaction
of secondary thioamide with hydrosilane catalyzed
by 1.
To confirm the conversion of imine into amine
in our reaction conditions, a C6D6 solution contain-
ing 1, MeN CHPh, and Et3SiH in 1:1:10 molar ratio
1
in a sealed NMR tube was heated for 24 h. The H
NMR spectrum of the reaction mixture showed the
formation of Me(Et3Si)NCH2Ph quantitatively (Eq.
(1)). When the reaction mixture was exposed to air,
Me(Et3Si)NCH2Ph disappeared and MeHNCH2Ph
appeared instead. PhN CHPh showed similar re-
sults: it was also converted into Ph(Et3Si)NCH2Ph
in 77% yield, which was converted into PhHNCH2Ph
upon exposure to air. The results support that desul-
furization of secondary thioamide generates imine,
which is converted into N-silylated amine in the
RESULTS AND DISCUSSION
Tetrahydrofuran (THF, 0.43 mL), 1 (0.053 mmol), N-
phenylbenzothioamide (PhHNC(S)Ph, 0.53 mmol),
Heteroatom Chemistry DOI 10.1002/hc

Transformation of RN=CHPh to R(Rꢀ3Si)NCH2Ph in the Catalytic Desulfurization of Secondary Thioamide by an Iron Complex 609
TABLE 1 Desulfurization of Secondary Thioamides with an Iron Catalyst
Entry
Amide
Conversion Yield
Yield of Amine
Yield of Imine
1a
84
11
73
2a
3
26
Trace
0
24
Trace
Trace
4
5
Trace
Trace
0
0
Trace
Trace
6
0
0
0
0
0
0
7
aSee [3].
reaction with Et3SiH and then into amine in the
reaction with water involved in the reaction system.
Several examples concerning hydrosilylation of
imines were reported: nickel [5], titanium [6], boron
[7], ruthenium [8], iron [9], zinc [10], copper [11], yt-
terbium [12], iridium [13], rhodium [14], and organo
compounds [15,16].
(1)
SCHEME 2 Proposed catalytic cycle.
Scheme 2 shows a proposed catalytic cycle
for the reaction of imine with Et3SiH promoted
by 1 to form N-silyl amine. The 16e species,
CpFe(CO)(C(O)CH3), is formed by CO insertion un-
der heating, followed by the oxidative addition of
Et3Si-H and the reductive elimination of MeCHO to
form L. Then, RN CHPh interacts with L in the η2-
CN coordination fashion to give M. The silyl group
migrates from Fe to the N atom to produce the Fe–C–
N three-membered ring (N). The dissociation of the
N coordination and the oxidative addition of Et3Si-
H result in the formation of O. Then, the reductive
elimination of N-silylated amine takes place to re-
generate L.
Finally, the reactions of PhN CHPh with
Me2PhSiH and Ph3SiH in place of Et3SiH in the
presence of 1 were conducted to check the effect
of hydrosilane on the hydrosilylation of the imine
Heteroatom Chemistry DOI 10.1002/hc

610 Fukumoto et al.
(Eq. (2)). The reaction depended on the kind of
hydrosilane: Me2PhSiH resulted in the formation
of the corresponding N-silylated amine in 93%,
whereas Ph3SiH did not produce Ph(Ph3Si)NCH2Ph
at all. Our previous paper [3] reported that the
reactions of secondary thioamide with Me2PhSiH
and Ph3SiH produced exclusively the corresponding
amine and imine, respectively. The results can be un-
derstood reasonably. In the reaction with Me2PhSiH,
the corresponding imine is generated first and then
the imine is converted into the N-silylated amine
rapidly. In contrast, in the reaction with Ph3SiH,
the hydrosilylation of the imine does not proceed.
Therefore, the imine is exclusively detected.
Thermal Reaction of Thioamide with
Triethylsilane and the Methyl Iron Complex in
THF
In catalytic reactions, a solution of 1 (10 mg,
0.053 mmol), Et3SiH (0.26 mL, 1.6 mmol), and
R1NHC(S)R2 (0.53 mmol) in THF (0.43 mL) was
heated at 80°C under a nitrogen atmosphere. Yields
of amine and imine of a solution were determined
by GC analysis. The reactions of PhNHC(O)Ph and
MeNHC(O)Ph were also examined under same con-
ditions.
Hydrosilylation of Imine with Hydrosilane
Catalyzed by the Methyl Iron Complex
In catalytic reactions, a solution of 1 (10 mg,
0.053 mmol), R3SiH (0.53 mmol), and R1N CR2
(0.053 mmol) in C6D6 (0.6 mL) was heated at 80°C
for 24 h under a nitrogen atmosphere. Yields of N-
silylated amine of a solution were determined by 1H
NMR.
(2)
Synthesis of 2-Methoxy-N-(2-methoxyphenyl)
methlbenzothioamide (3)
CONCLUSIONS
In
a
20-mL two-necked flask, Lawesson’s
Secondary thioamide was recently reported to be re-
duced to the corresponding imine and amine in the
reaction with hydrosilane in the presence of a cat-
alytic amount of 1. However, the formation mech-
anism of imine and amine remained unclear. This
paper brought the mechanism of imine and amine
formation to light: Secondary thioamide is converted
into the corresponding imine first, and the imine re-
acts with hydrosilane with the help of an iron cat-
alyst to form the N-silylated amine, and finally the
N-silylated amine is converted into the correspond-
ing amine in the reaction of water existing in the
reaction system.
reagent (0.809 g, 2.0 mmol) was added to
a toluene solution (6 mL) of 2-methoxy-N-(2-
methoxyphenyl)methylbenzamide (1.08 g, 4 mmol)
at room temperature, and the mixture was stirred
at 110–120°C for 1 h. The reaction mixture was
concentrated in vacuo. The residue was purified
by column chromatography on a silica gel using
hexane/EtOAc as eluent to give 2-methoxy-N-(2-
methoxyphenyl)methylbenzothioamide (1.09 g,
95%) as a yellow solid: mp 86–87°C; IR (KBr) 3320,
3029, 2939, 2837, 1733, 1599, 1531, 1481, 1242,
1021, 941, 757 cm−1 1H NMR (CDCl3) δ 3.83 (s,
;
3H), 3.80 (s, 3H), 5.08 (d, J = 5.4 Hz, 2H); 6.8–7.1
(m, 5H), 7.28–7.71 (m, 3H), 9.57(brs, 1H); 13C NMR
(CDCl3) δ MS(EI) m/z 287 (M+); HRMS calcd for
C16H17NO2S: 287.098, found: 287.096.
EXPERIMENTAL
General Remarks
All reactions were carried out under an atmosphere
of dry nitrogen by using Schlenk tube techniques.
THF was distilled from sodium metal. This was
stored under a nitrogen atmosphere. Most chemicals
were commercially available except thioamides 2, 3,
and 4. Thioamides 2 and 4 were synthesized accord-
ing to the literature methods [17]. IR spectra were
recorded on a Perkin-Elmer Spectrum One spec-
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