Sequential addition reactions of lithium aeetylides and grignard reagents to selenoiminium salts leading to 2-propynyl tertiary amines bearing a tetrasubstituted carbon center

Article abstract of DOI:10.1246/bcsj.80.2220

Selenoiminium salts generated in situ from selenoamides and MeOTf were reacted sequentially with lithium aeetylides and Grignard reagents to give 2-propynyl tertiary amines bearing a tetrasubstituted carbon center. The lithium aeetylides used were generated from (trimethylsilyl)acetylene, phenylacetylene, and 1-hexyne. Among them, that obtained from (trimethylsilyl)acetylene most effectively gave the corresponding products in higher yields. As Grignard reagents, alkyl-, vinyl-, allyl-, and benzylmagnesium halides gave the products with high efficiency, whereas the reaction of arylmagnesium halides did not proceed as well. A variety of 4-penteneselenoamides were synthesized by four-component coupling reactions of terminal acetylenes, elemental selenium, allylic bromides, and secondary amines, and subjected to the sequential addition reactions to give 5-amino-l,6-enynes. The diastereoselectivity of the reaction was found to depend on the substituents at the α-position of selenoamides and Grignard reagents. In the reaction of selenoiminium salts with excess Grignard reagents, amines that incorporated two molecules of Grignard reagents were formed in good yields.

Full text of DOI:10.1246/bcsj.80.2220

2220 Bull. Chem. Soc. Jpn. Vol. 80, No. 11, 2220–2225 (2007)
Ó 2007 The Chemical Society of Japan
Sequential Addition Reactions of Lithium Acetylides and Grignard
Reagents to Selenoiminium Salts Leading to 2-Propynyl Tertiary
Amines Bearing a Tetrasubstituted Carbon Center
ꢀ
Toshiaki Murai, Sho Nogawa, and Yuichiro Mutoh
Department of Chemistry, Faculty of Engineering, Gifu University, Yanagido, Gifu 501-1193
Received June 11, 2007; E-mail: mtoshi@gifu-u.ac.jp
Selenoiminium salts generated in situ from selenoamides and MeOTf were reacted sequentially with lithium ace-
tylides and Grignard reagents to give 2-propynyl tertiary amines bearing a tetrasubstituted carbon center. The lithium
acetylides used were generated from (trimethylsilyl)acetylene, phenylacetylene, and 1-hexyne. Among them, that ob-
tained from (trimethylsilyl)acetylene most effectively gave the corresponding products in higher yields. As Grignard
reagents, alkyl-, vinyl-, allyl-, and benzylmagnesium halides gave the products with high efficiency, whereas the reaction
of arylmagnesium halides did not proceed as well. A variety of 4-penteneselenoamides were synthesized by four-com-
ponent coupling reactions of terminal acetylenes, elemental selenium, allylic bromides, and secondary amines, and sub-
jected to the sequential addition reactions to give 5-amino-1,6-enynes. The diastereoselectivity of the reaction was found
to depend on the substituents at the ꢀ-position of selenoamides and Grignard reagents. In the reaction of selenoiminium
salts with excess Grignard reagents, amines that incorporated two molecules of Grignard reagents were formed in good
yields.
The development of multiple-component coupling reactions
MeS
R
S
MeOTf
Et2O
OTf
R''C CLi
leading to valuable compounds is an important issue in modern
synthetic chemistry.1 Recent studies have been focused on new
synthetic routes leading to 2-propynylamines. The in situ con-
densation of aldehydes and amines, followed by the addition
reaction of acetylenic nucleophiles gives the corresponding
amines.2 However, these methods are not suitable for the
reaction of ketones. Therefore, the resulting 2-propynylamines
possess a trisubstituted carbon center adjacent to a nitrogen
atom. On the other hand, those with a tetrasubstituted carbon
center attached to a nitrogen atom are rarely prepared.3 Alter-
natively, we recently developed sequential addition reactions
of organolithium and Grignard reagents to thioiminium salts
derived from thioamides leading to tertiary 2-propynylamines
bearing a tetrasubstituted carbon center (Scheme 1).4 How-
ever, in this reaction, a large excess of Grignard reagents is
used, probably because of the low reactivity of in situ-generat-
ed gem-thioamino compound toward Grignard reagents. Sele-
nium isologues of thioamides, i.e. selenoamides,5 have been
believed to be more reactive than thioamides, but can be han-
dled under air. Therefore, the development of new methods for
the synthesis of selenoamides is still an active area.6 If sele-
noamides are used instead of thioamides in the reaction in
Scheme 1, the reaction is expected to progress to completion
under milder reaction conditions. Furthermore, during the
course of our studies on thio-,7 seleno-,8 and telluroamides,9
we have devised four-component coupling reactions of termi-
nal acetylenes, elemental selenium, amines, and allylic bro-
mides to give 4-alkenyl selenoamides (Scheme 2).10 Applica-
tion of the reactions shown in Scheme 1 to selenoamides
in Scheme 2 enables us to provide unprecedented synthetic
routes to 5-amino-1,6-enynes. We report here the details of
NR'2
R
NR'2
SMe
R'2N
R
R'''
R'2N
R
R'''MgX
R"
R"
Scheme 1.
Se
1) n-BuLi, THF
R''
2) Se
NR'2
RC CH
3) R'2NH
R
R''
X
4)
Scheme 2.
the sequential addition reaction of lithium acetylides and
Grignard reagents to selenoiminium salts.
Initially, to compare the reactivity of selenoamides with that
of thioamides, selenobenzamide 111 was reacted with MeOTf,
followed by the addition of lithium (trimethylsilyl)acetylide
(1.5 molar amount) (2a) and ethylmagnesium bromide (3a)
(Scheme 3). After aqueous workup, 2-propynylamine 4 was
obtained in 77% yield. As expected, the synthesis of 4 was
achieved with 3 molar amount. of 3a, which is in marked con-
trast to the reaction of the thioiminium salt derived from thio-
benzamide 10, where 10 molar amount. of 3a was required. In
this reaction, despite the fact that excess organometallic re-
agents 2a and 3a were used, products in which two molecules
Published on the web November 13, 2007; doi:10.1246/bcsj.80.2220

T. Murai et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 11 (2007) 2221
Table 1. Reaction of Selenoamide 9a with MeOTf, Lithium
Acetylide 2a, and Grignard Reagents 3aÞ
Me3SiC CLi
2a
EtMgBr
3a
Me3SiC CLi
Se
(1.5 molar
amount)
E
MeOTf
2a
MeOTf
NMe2
rt, then
Et2O
Ph
1 E = Se
1' E = S
NMe2
0 °C, then
Et2O
rt, 30 s
0 °C, then
9a
reflux, 6h
rt, 30 s
rt, 0.5 h
rt, 0.5 h
Et
Me2N
4
3a
R
Me2N
10–16
RMgX 3
3 molar
amount
10 molar
amount
1
Ph
77%
–OTf
MeSe
4
SiMe3
SiMe3
+
1'
73%
Ph
NMe2
3
Product
5a
Entry
1c)
Yield %b)
conditions
+
Et–
Me2N
Et
SeMe
Me2N
Me2N
MeSe MgBr
EtMgBr 3a
rt, 2 h
Ph
Ph
10 75%
Me
6a'
SiMe3
SiMe3
6a
SiMe3
SiMe3
Me2N
Scheme 3.
MeMgBr 3b
2c)
rt, 2 h
11 61%
Me2N
R''C CLi
–OTf
SeMe
R'2N
R
MeSe
2
MgCl
3c
rt, 2 h
+
3c)
R
NR'2
6
R''
5
12 74%
Me2N
SiMe3
SiMe3
R'2N
R
O
SeMe
R''
SeMe
R''
CH2=CHMgCl
3d
•
4c)
R
7
rt, 2 h
13 79%
Me2N
8
CH2=CHCH2MgBr
Scheme 4.
5c)
3e
rt, 2 h
of 2a or 3a were introduced were not observed. In the initial
stage of the reaction, the addition of lithium acetylide 2a to
selenoiminium salt 5a gives gem-selenoamino compound 6a.
The substitution reaction of 6a with 3a with the elimination
of a MeSe group proceeds smoothly to form 4. We previously
observed that aqueous workup of the reaction mixture of sele-
noiminium salts 5 and lithium acetylides 2 gave 1-alkenyl ke-
tones 8,12 which were probably formed via the 1,3-rearrange-
ment of 6 to 7 followed by the hydrolysis of 7 (Scheme 4),
but the corresponding ketones 8 were not detected in the reac-
tion in Scheme 3. This suggested that the substitution reaction
of 6a with 3a proceeded effectively. In this substitution
reaction, it is of interest whether it proceeds by SN1 or by
SN2 mechanism. No clear results supporting either mechanism
are obtained at present. However, on the basis of the Grignard
reagents-dependent diastereoselectivity vide infra, iminium
salts 6a0 may be formed prior to the addition reaction of
Grignard reagents.
Next, selenoamides 9 derived from the four-component
coupling reaction in Scheme 2 were subjected to the sequential
reactions. The results of the reaction of 4-penteneselenoamide
9a, lithium (trimethylsilyl)acetylide 2a, and Grignard reagents
3 are shown in Table 1. The reaction of the selenoiminium
salt derived from 9a and MeOTf with 2a went to completion
within 0.5 h below room temperature. On the other hand, the
reaction conditions leading to the desired amines depended
on the substituents on the Grignard reagents. The addition of
aliphatic Grignard reagents, such as methyl, ethyl, and cyclo-
hexylmagnesium halides 3a–3c, proceeded at room tempera-
14 97%
Me2N
SiMe3
SiMe3
Ph
PhCH2MgCl
6
3f
rt, 2 h
15 73%
OMe
MeO
MgBr
Me2N
16 39%
7
3g
reflux, 6 h
SiMe3
a) All reactions were carried out with selenoamide 9a (0.50
mmol), MeOTf (1 molar amount), lithium acetylide 2a (1.5
molar amount), and Grignard reagents 3 (3 molar amount) in
Et2O (5 mL) unless otherwise noted. b) Isolated yield. c) The
reaction was carried out with 9a (1.0 mmol).
ture to give the corresponding amines 10–12 in good yields
(Entries 1–3). The use of vinyl-, allyl-, and benzylmagnesium
halides 3d–3f also took place in a similar fashion under the
same reaction conditions to lead to 13–15 (Entries 4–6). In par-
ticular, crude product 14 prepared from 3e was highly pure and
further purification was not necessary (Entry 5). On the other
hand, the reaction of aromatic magnesium halides required
higher reaction temperatures and longer reaction times. For
example, to the reaction mixture of 9a, MeOTf, and 2a was
added 4-methoxyphenylmagnesium bromide (3g), and the
mixture was stirred at reflux for 6 h (Entry 7). Nevertheless, the
corresponding amine 16 was formed in only 39% yield.

2222 Bull. Chem. Soc. Jpn. Vol. 80, No. 11 (2007)
Sequential Reactions of Selenoiminium Salts
Table 2. Reaction of Selenoamides 9 with MeOTf, Lithium Acetylides 2, and Grignard Reagents 3aÞ
R'C CLi
R"
R2N
Se
R"MgX 3
2
MeOTf
NR2
Et2O
rt, 30 s
0 °C, then
rt, 1.5 h
R'
9
17–21
Selenoamide
3
Product
Yield %b)
Entry
1c)
2
9
Conditions
Et
Me2N
Se
PhC CLi 2b
EtMgBr 3a
rt, then
reflux, 22 h
NMe2
Ph
Ph
9a
Se
17 60%
Me2N
MgBr
2
3
3e
PhC CLi 2b
NMe2
NMe2
rt, then
reflux, 2 h
9a
Se
18 65%
Et
Me2N
EtMgBr 3a
rt, 17 h
n-BuC CLi 2c
Bu-n
9a
Se
19 47%
Et
N
EtMgBr 3a
rt, then
reflux, 2 h
Me3SiC CLi 2a
Me3SiC CLi 2a
4
5
N
9b
Se
SiMe3
SiMe3
20 68%
OMe
Et
N
EtMgBr 3a
rt, then
reflux, 6 h
N
MeO
21 67% (3 : 1)d)
9c
a) All reactions were carried out with selenoamides 9 (0.50 mmol), MeOTf (1 molar amount), lithium
acetylides 2 (1.5 molar amount), and Grignard reagents 3 (3 molar amount) in Et2O (5 mL) unless other-
wise noted. b) Isolated yield. c) Selenoamide 9a (0.75 mmol) was used. d) The yield was determined by
NMR spectra. The product was given as a mixture of two stereoisomers after column chromatography.
A diastereomeric ratio is shown in parenthesis.
Various types of selenoamides 9 and lithium acetylides 2
were also used, and the results are shown in Table 2. The ad-
dition reaction of lithium acetylides 2b and 2c derived from
phenylacetylene and 1-hexyne to the selenoiminium salt de-
rived from 9a was complete within 1.5 h, but longer reaction
times and higher temperatures were necessary for the addition
reaction of Grignard reagents 3a and 3e (Entries 1–3), unlike
the case of 2a shown in Table 1. Generally, lithium acetylide
2a derived from (trimethylsilyl)acetylene showed higher effi-
ciency in the reaction. The substituents on the nitrogen atom of
starting selenoamides 9 also affected the reaction conditions.
For the reaction of 9b and 9c, the addition of Grignard reagent
3a was achieved at reflux (Entries 4 and 5). In the latter case,
two diastereomers were obtained in a ratio of 3 to 1.
The stereochemical aspects of the present reaction were
elucidated with 2- or 3-substituted 4-penteneselenoamides 9d–
9h. The results are shown in Table 3. Due to the high efficien-
cy of the reaction of lithium acetylide 2a as noted above, 2a
was added to the selenoiminium salts derived from 9d–9h.
For 2-phenyl selenoamide 9d, one of two diastereomers was
predominantly formed, and the ratio depended on the Grignard
reagents used (Entries 1–3). The replacement of a N,N-dimeth-
yl group with a pyrrolidinyl group did not improve the stereo-
selectivity (Entry 4). The 2-methyl selenoamide 9f showed
relatively lower selectivity (Entry 5), whereas in the reaction
of 2-cyclohexenyl selenoamide 9g, the formation of only a
1
single stereoisomer was observed in H and 13C NMR spectra
(Entry 6). In contrast to these successful results, 2-t-butyl sele-
noamide 9i did not undergo sequential addition reactions to
give the desired amine 29 (Scheme 5). In place of the Grignard
reagent 3a, a hydrogen atom was introduced to the carbon
atom attached to the nitrogen atom to give 30 in 29% yield.
To determine the origin of the hydrogen atom, a reaction mix-
ture consisting of 9i, MeOTf, and 2a was trapped with D2O,
but the incorporation of deuterium atom was not observed,
and the amine 30 was formed again in 29% yield. This shows
that the formation of 30 was independent of the presence of 3a.
Additionally, the deposition of red selenium was not observed,
which implied that the MeSe group in in situ-formed gem-
selenoamino compound 31 was eliminated as a divalent sele-
nium species. Therefore, 31 may undergo 1,3-shift of the hy-
drogen atom on the methyl group to the carbon atom adjacent
to the nitrogen atom to lead to the formation of 30 and de-
composed products of selenoformaldehyde,13 although further
studies are necessary to clarify this possibility (Scheme 6).
Finally, the reverse sequential addition reaction, i.e. the

T. Murai et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 11 (2007) 2223
Table 3. Reaction of 2- or 3-Substituted 4-Penteneselenamides 9 with MeOTf, Lithium Acetylide 2a, and
Grignard Reagents 3aÞ
R'' Se
R'' R2N
R'''MgX 3
Me3SiC CLi 2a
MeOTf
R'''
NR2
Et2O
rt, 30 s
0 °C, then
9
rt, 0.5~1.5 h
R'
R'
SiMe3
R' = aryl, alkyl; R'' = Hor
R' = H, R'' = aryl
22–28
Grignard reagent
conditions
Products Yieldb)
Entry
1
Selenoamide
Et
Se
Se
Me2N
Ph
EtMgBr 3a
rt, 21 h
22
NMe2
NMe2
68% (9:1)
SiMe3
SiMe3
Ph
9d
9d
Me
Me2N
Ph
23
MeMgBr 3b
reflux, 3 h
2
3
47% (5:1)
Ph
Se
Se
Me2N
Ph
i-PrMgCl 3h
24
NMe2
76% (6:1)
rt, 15 h
9d
SiMe3
SiMe3
Ph
Ph
Et
N
25
4
5
EtMgBr 3a
rt, 22 h
N
71% (7:1)
9e
9f
Ph
Et
Et
Me2N
Se
Se
26
EtMgBr 3a
rt, 21 h
NMe2
NMe2
51% (2:1)
SiMe3
SiMe3
Me2N
27 51%
(single isomer)
6
7
EtMgBr 3a
rt, 21 h
9g
9h
Ph
NMe2
Et
Ph Se
28
EtMgBr 3a
rt, 2 h
87% (1:1)
NMe2
SiMe3
a) All reactions were carried out with selenoamides 9 (0.50 mmol), MeOTf (0.50 mmol), lithium acetylides
2 (0.75 mmol), and Grignard reagents 3 (1.5 mmol) in Et2O (5 mL) unless otherwise noted. b) The products
were obtained as mixture of diastereomers. Although the relative stereochemistry of the products was not
determined, their ratio was determined by 1H NMR spectra of crude products, and is shown in parentheses.
addition of Grignard reagents prior to the addition of lithium
acetylides, was carried out. Grignard reagent 3a (1.5 molar
amount) was added to selenoiminium salt 32 generated from
9a and MeOTf, and the mixture was stirred for 0.5 h. Lithium
acetylide 2a (3 molar amount) was then added to give amine
34 in 58% yield along with amine 10, where both 3a and 2a
were incorporated (Scheme 7). In this reaction, two molecules
of 3a were introduced into the product 34 with high efficiency,
whereas the introduction of 2a proceeded with low efficiency.
This indicated that gem-selenoamino compound 33, which was
formed by the addition of 3a to 32, was highly reactive toward
Grignard reagents. In fact, the reaction of 32 with 3 molar
amount of Grignard reagents effectively gave the products
34–36 which incorporated two molecules of Grignard reagents,
in good to high yields (Scheme 8).
Conclusion
In summary, we have demonstrated the sequential addition
reaction of lithium acetylides and Grignard reagents to seleno-
iminium salts derived from selenoamides and MeOTf. The re-
action proceeded under milder reaction conditions compared
to a similar reaction using thioiminium salts to give tertiary

2224 Bull. Chem. Soc. Jpn. Vol. 80, No. 11 (2007)
Sequential Reactions of Selenoiminium Salts
MeSe –OTf
1. MeOTf / Et2O
Se
Se
MeOTf
+
Et2O
rt, 30 s
rt, 30 s
NMe2
NMe2
2. Me3SiC CLi (1.5 molar amount)
0 °C, then rt, 1.5 h
3. EtMgBr (3.0 molar amount)
NMe2
32
9a
t-Bu
9i
Me3SiC CLi
2a
(3.0 molar
amount)
EtMgBr
3a
(1.5 molar
amount)
rt, 24 h
H
Et
Me2N
Me2N
SeMe
Et
Me2N
+
rt, 2 h
0 °C, then
t-Bu
SiMe3
33
t-Bu
SiMe3
rt, 0.5 h
30 29%
29 0%
Et
Et
Me2N
Me2N
Scheme 5.
+
Et
SiMe3
Scheme 7.
34 58%
H
H
10 16%
H
Se
Me2N
+
30
[H2C=Se]
1. MeOTf / Et2O
t-Bu
SiMe3
Scheme 6.
Se
rt, 30 s
31
NMe2
2. RMgX 3 (3.0 molar
amount)
9a
amines bearing a tetrasubstituted carbon center adjacent to a
nitrogen atom. Application of the present reaction to 4-pen-
teneselenoamides led to 5-amino-1,6-enynes, which are not
readily accessible by other known methods. Substrate-depend-
ent diastereoselectivity was observed, and a bulky substituent,
such as a t-butyl group, disturbed the desired reaction. The or-
der of the addition of two organometallic reagents is important.
When Grignard reagents were added prior to the addition of
lithium acetylides, products that incorporated two molecules
of Grignard reagents, were predominantly formed.
0 °C, then rt, 2 h
R
Me2N
34 R = Et, 81%
35
36
R = CH2CH = CH2, 69%
R = CH2Ph, 95%
R
Scheme 8.
106.7 (CꢃC), 114.2 (CH2=CH), 138.7 (CH2=CH); MS (EI) m=z
208 (Mþ ꢂ CH2CH3); HRMS calcd for C13H24NSi (Mþ ꢂ CH3)
222.1678, found: 222.1709.
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Area (No. 19020020, ‘‘Advanced Molec-
ular Transformations of Carbon Resources’’) from the Ministry
of Education, Culture, Sports, Science and Technology, Japan.
Experimental
Typical Procedure for Sequential Addition Reactions of
Lithium Acetylides and Grignard Reagents to Selenoiminium
Salts. To an Et2O (5.0 mL) solution of selenoamide 9a (0.19 g,
1.0 mmol) was added MeOTf (0.11 mL, 1.0 mmol) at room tem-
perature, and the mixture was stirred for 30 s. To this was added
lithium acetylide 2a, prepared from (trimethylsilyl)acetylene
(0.21 mL, 1.5 mmol) and n-butyllithium (1.6 M hexane solution,
0.94 mL, 1.5 mmol) in Et2O (5.0 mL) at 0 ꢁC, and the resulting
mixture was stirred for 0.5 h at room temperature. Ethylmagne-
sium bromide (3a) (1.0 M THF solution, 3.0 mL, 3.0 mmol) was
then added, and the mixture was further stirred for 2 h at this
temperature. The resulting mixture was poured into a saturated
aqueous solution of NH4Cl, washed with brine, and extracted with
Et2O. The organic layer was dried over MgSO4 and concentrated
in vacuo. The residue was purified by column chromatography on
silica gel (hexane:ethyl acetate = 100:1–10:1) to give 10 (0.175 g,
75%) as a brown oil; IR (neat) 3078, 2959, 2865, 2823, 2783,
2156, 1823, 1641, 1456, 1415, 1378, 1333, 1283, 1249, 1163,
1118, 1094, 1040, 1019, 993, 964, 910, 843, 760, 698, 661, 628,
Supporting Information
Detailed experimental procedures and spectroscopic data for
new compounds; this material is available free of charge on the
web at http://www.csj.jp/journals/bcsj/.
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