Propyne iminium salts by N-alkylation of alkynyl imines

Article abstract of DOI:10.1055/s-1999-3697

N-Alkylation of alkynyl imines 5, 6, and 8 with methyl triflate or triethyloxonium tetrafluoroborate provides the open-chain propyne iminium salts 9 and 10, the related salts 11a-e and 12b where the iminium function is a part of a heteroaromatic ring, and p-phenylene-bis(propyne iminium) salts 13a,c. The method gives access to novel propyne iminium salts in which the C,C triple bond bears an alkyl, SiMe₃, or H substituent.

Full text of DOI:10.1055/s-1999-3697

100
PAPER
Propyne Iminium Salts By N-Alkylation of Alkynyl Imines
Jens Schlegel, Gerhard Maas*
Abteilung Organische Chemie I, UniversitŠt Ulm, Albert-Einstein-Allee 11, D-89081 Ulm, Germany
Fax: +49(731)5022803; E-mail: gerhard.maas@chemie.uni-ulm.de
Received 19 June 1998; revised 24 July 1998
substituent. We report here that N-alkylation of alkynyl
imines is a useful and versatile alternative for the prepara-
tion of propyne iminium salts. Surprisingly enough, this
obvious synthetic strategy has not been used before, ex-
cept for the N-quaternization of rather special alkynyl imi-
nes such as 2-alkynylpyridines.7,8
Abstract: N-Alkylation of alkynyl imines 5, 6, and 8 with methyl
triflate or triethyloxonium tetrafluoroborate provides the open-
chain propyne iminium salts 9 and 10, the related salts 11aÐe and
12b where the iminium function is a part of a heteroaromatic ring,
and p-phenylene-bis(propyne iminium) salts 13a,c. The method
gives access to novel propyne iminium salts in which the C,C triple
bond bears an alkyl, SiMe3, or H substituent.
Syntheses of a few acyclic alkynyl imines have been re-
ported. They were prepared by palladium-catalyzed cou-
pling of imidoyl chlorides with terminal alkynes9,10 as
well as (phenylethynyl)tributyltin,11 by condensation of
alkynyl ketones12Ð14 and acetylenic aldehydes15 with pri-
mary amines, or by reaction of alkyl alkynyl ketones with
N-ethylidenecyclohexanamine.12 We report here that
N-alkyl-substituted alkynyl imines 5aÐf can be obtained
conveniently from imidoyl chlorides 4aÐf and (alk-1-
ynyl) Grignard reagents (Scheme 2). This method also
gives access to the p-phenylene-bis(alkynyl imines) 6a,c
(Tables 1 and 2).
Key words: alkynyl imines, iminium salts, imine alkylation, imi-
doyl halides, C,C coupling with alkynes
A convenient route to open-chain and semicyclic propyne
iminium salts 3 is provided by O-sulfonylation of enami-
no ketones 1 with triflic anhydride followed by thermal or
base-assisted 1,2-elimination of triflic acid from the re-
sulting 3-trifloxypropene iminium salts 21,2 (Scheme 1).
However, this synthetic approach has several limitations.
Thus, enamino aldehydes cannot be converted into propy-
ne iminium salts due to subsequent rapid reaction of salts
2 (R2 = H),3 and treatment of 3-alkyl-3-trifloxypropene
iminium salts (2, R2 = CHR2) with an amine base causes a
deprotonation which initially leads to 1-amino-3-trifloxy-
1,3-dienes rather than to propyne iminium salts.4 So far,
(1-methylbut-2-ynylidene)pyrrolidinium triflate (3, R1 =
R2 = Me) represents the only propyne iminium salt with an
alkyl-substituted C,C triple bond that could be prepared
by the method under discussion; in this case, the conver-
sion 2 → 3 was achieved by thermally induced HOTf
elimination.2
Scheme 1
Since propyne iminium salts have proven to be useful
building blocks in organic synthesis (e.g. precursors to
aminoallenes, 1- and 2-dienamines, and propargyl-
amines1c,5,6), we were interested to circumvent some of the
limitations of the synthetic approach mentioned above
and to enlarge the collection of easily available propyne
iminium salts, especially by addition of systems in which
the C,C-triple bond is terminal or bears an alkyl or silyl
Scheme 2
Synthesis 1999, No. 1, 100Ð106 ISSN 0039-7881 © Thieme Stuttgart á New York

PAPER
Propyne Iminium Salts By N-Alkylation of Alkynyl Imines
101
Table 1 Alkynyl Imines 5, 6 and 8 Prepared (Schemes 2 and 3) and their IR Data
Prod-
uct
Yield
(%)
mp (°C) (solvent)
or
IR (neat or KBr) (cm–1)
bp (°C)/mbar
nC≡C
Other Signals
5a
5b
5c
5d
5e
5f
6a
6c
8a
8b
8c
8d
75
76
66
69
54
74
74
70
50
56
45
80
135/0.018
155/0.01
36–38 (Et2O)
145/0.003
155/0.003
oilb
100–102 (Et2O)
119–120 (Et2O)
120/0.02
112/0.028
114–116c
40–41d
2151
2149
2152
2211
2207
2211
2205
2205
2066
2160
2162
2107
2959, 2899, 1595, 1573, 1449, 1397, 1314, 1277, 1252, 1084, 1075, 1061, 1028
2967, 2933, 2899, 2868, 1591, 1570, 1449, 1314, 1275, 1252, 1100, 1057, 1015
2959, 1593, 1567, 1492, 1448, 1314, 1272, 1252, 1087, 1063, 1028
2957, 2931, 2871, 1596, 1574, 1466, 1447, 1327, 1314, 1284, 1028
2962, 2932, 2869, 1593, 1572, 1448, 1314, 1283
2956, 2930, 2870, 1594, 1571, 1491, 1453, 1314, 1282, 1028
1594, 1571, 1446, 1317, 1299, 1053, 1027
3025, 1586, 1561, 1491, 1446, 1315, 1297, 1054, 1027
2964, 2899, 1549, 1454, 1249, 1206, 1032
2959, 1476, 1428, 1324, 1251, 1161, 1140, 1124, 1064
3046, 2955, 1600, 1496, 1482, 1448, 1392, 1354, 1247
3195 (≡CH), 1472, 1429, 1313, 1121
a
Satisfactory microanalyses obtained: C ± 0.40, H ± 0.23, N ± 0.31. EI-HRMS, m/z: 5c: calcd. 291.1439, found 291.1441; 5f: calcd. 275.1674,
found 275.1673.
b
c
d
Purified by column chromatography (silica gel, Et2O).
Purified by column chromatography (silica gel, CHCl3).
Lit.15 mp 39–41°C.
Table 2 1H and 13C NMR Data of Compounds 5, 6, 8
Pro-
duct
1H NMR (CDCl3/TMS),
d, J (Hz)
13C NMR (CDCl3)
d
5a
0.30 [s, 9 H, Si(CH3)3], 3.63 (s, 3 H, NCH3), 7.34–7.39 (m, 3 H, –0.39 [Si(CH3)3], 43.57 (NCH3), 95.61, 105.86 (C≡C), 127.14,
CHarom), 7.98–8.03 (m, 2 H, CHarom 128.06, 130.19, 137.03 (Carom), 152.20 (C=N)
)
5b
0.30 [s, 9 H, Si(CH3)3], 1.35 (t, J = 7.3, 3 H, CH3), 3.88 (q, J = –0.43 [Si(CH3)3], 15.34 (CH3), 50.73 (NCH2), 95.83, 104.78
7.3, 2 H, NCH2), 7.36–7.41 (m, 3 H, CHarom), 8.01–8.06 (m, 2 H, (C≡C), 127.23, 128.02, 130.11, 137.16 (Carom), 150.17 (C=N)
CHarom
)
5c
5d
5e
0.30 [s, 9 H, Si(CH3)3], 5.06 (s, 2 H, NCH2), 7.20–7.44 (m, 8 H, –0.49 [Si(CH3)3], 59.97 (NCH2), 96.08, 105.59 (C≡C), 126.56,
CHarom), 8.09–8.14 (m, 2 H, CHarom
)
127.43, 127.86, 127.98, 128.16, 130.36, 136.82, 139.66 (Carom),
151.00 (C=N)
0.91 (t, J = 7.0, 3 H, CH3), 1.39–1.61 (m, 4 H, CH2CH2), 2.44 (t, 13.20 (CH3), 18.67, 21.70 (CH2CH2), 30.13 (≡CCH2), 42.96
J = 6.8, 2 H, ≡CCH2), 3.58 (s, 3 H, NCH3), 7.32–7.35 (m, 3 H, (NCH3), 73.33, 101.54 (C≡C), 126.97, 127.74, 129.74, 137.54
CHarom), 7.99–8.04 (m, 2 H, CHarom
)
(Carom), 152.32 (C=N)
0.93 (t, J = 7.3, 3 H, CH3), 1.32 (t, J = 7.3, 3 H, CH3), 1.42–1.63 13.25 (CH3), 15.31 (CH3) 18.72, 21.74 (CH2CH2), 30.16 (≡CCH2),
(m, 4 H, CH2CH2), 2.44 (t, J = 6.8, 2 H, ≡CCH2), 3.83 (q, J = 7.3, 50.26 (NCH2), 73.49, 100.58 (C≡C), 127.12, 127.79, 129.76,
2 H, NCH2), 7.31–7.39 (m, 4 H, CHarom), 7.99–8.04 (m, 2 H, 137.73 (Carom), 150.43 (C=N)
CHarom
)
5f
0.92 (t, J = 7.0, 3 H, CH3), 1.41–1.65 (m, 4 H, CH2CH2), 2.48 (t, 13.33 (CH3), 18.83, 21.81 (CH2CH2), 30.14 (≡CCH2), 59.62
J = 6.9, 2 H, ≡CCH2), 5.01 (s, 2 H, NCH2), 7.17–7.74 (m, 8 H, (NCH2), 73.88, 101.39 (C≡C), 126.41, 127.41, 127.70, 127.87,
CHarom), 8.06–8.11 (m, 2 H, CHarom
)
128.11, 130.09, 137.53, 139.92 (Carom), 151.47 (C=N)
6a
6c
3.71 (s, 6 H, NCH3), 7.39–7.45 (m, 6 H, CHarom), 7.58 (s, 4 H, 43.63 (NCH3), 83.14, 98.03 (C≡C), 122.57, 127.13, 128.19,
CHarom), 8.04–8.09 (m, 4 H, CHarom 130.36, 132.06, 137.12 (Carom), 151.87 (C=N)
)
5.13 (s, 4 H, NCH2), 7.24–7.46 (m, 16 H, CHarom), 7.59 (s, 4 H, 60.15 (NCH2), 83.72, 97.63 (C≡C), 122.60, 126.79, 127.55,
CHarom), 8.12–8.17 (m, 4 H, CHarom
)
127.91, 128.25, 128.39, 130.64, 132.21, 137.12, 139.66 (Carom),
150.92 (C=N)
8a
0.30 [s, 9 H, Si(CH3)3], 1.50 [s, 6 H, C(CH3)2], 7.32–7.48 (m, 3 –0.69 [Si(CH3)3], 22.67 (CCH3), 55.04 (CCH3), 85.55, 88.02
H, CHarom), 7.62–7.70 (m, 1 H, CHarom
)
(C≡C), 120.25, 121.23, 125.98, 127.79, 144.26, 151.22 (Carom),
176.79 (C=N)
8b
8c
0.30 [s, 9 H, Si(CH3)3], 7.39–7.45 (m, 2 H, CHarom), 7.74–7.78 –0.84 [Si(CH3)3], 96.60, 102.72 (C≡C), 120.98, 123.36, 126.04,
(m, 1 H, CHarom), 7.99–8.01 (m, 1 H, CHarom 126.35, 134.81, 147.81 (Carom), 152.36 (C=N)
)
0.21 [s, 9 H, Si(CH3)3], 5.09 (s, 2 H, NCH2), 6.93–7.52 (m, 15 H, –0.52 [Si(CH3)3], 48.62 (NCH2), 93.52, 101.55 (C≡C), 126.82,
CHarom 126.99, 127.62, 127.99, 128.20, 128.42, 128.86, 128.98, 129.52,
130.73, 133.24, 133.87, 134.15, 136.17, 138.36 (Carom
)
)
8d
3.60 (s, 1 H, ≡CH), 7.38–7.54 (m, 2 H, CHarom), 7.80–7.85 (m, 1 76.62, (C≡CH), 83.92 (≡CH), 121.21, 123.72, 126.45, 126.66,
H, CHarom), 8.03–8.08 (m, 1 H, CHarom
)
135.00, 147.27 (Carom), 152.45 (C-2)
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102
J. Schlegel, G. Maas
PAPER
Efforts to synthesize N-phenyl alkynyl imines 5g,h analo-
gously were abandoned since the reactions were slow and
preparatively useless product mixtures were obtained.
However, these compounds have been obtained before by
palladium-catalyzed coupling of imidoyl chloride 4g with
the corresponding alk-1-yne9,10 (Scheme 3). Sonogashira-
type coupling reactions also allowed the preparation of
(trimethylsilyl)ethynyl imines 8aÐc, in which the imine
function is part of a heteroaromatic system (Tables 1 and
2). Related systems, such as 2-[(trimethylsilyl)ethy-
nyl]pyridine16 and (2-alkynyl)imidazoles,17 have been
prepared before by this method.
Scheme 4
actions with Et3OBF4 required a higher temperature to go
to completion. Since they had to be performed in CH2Cl2
solution in order to dissolve the alkylating reagent, the
formed propyne iminium salt remained in solution, and
only the Me3Si-substituted salts 10a,b,g and 12b could be
prepared under these conditions, while the iminium salts
derived from 5dÐf seemed not to be stable in the reaction
medium and gave rise to further reactions such as oligo-
merization. Similarly, 2-ethynyl-1,3-benzothiazole (8d)
underwent clean N-methylation with methyl triflate, while
treatment with Meerwein's salt led to oligomerization.
Scheme 3
Desilylation of 8b provided 2-ethynyl-1,3-benzothiazole
(8d) in good yield. Certainly, this short route to 8d is more
convenient than the published procedure18 which requires
the synthesis and dehalogenation of 2-(2,2-dichloro-1-fluo-
rovinyl)-1,3-benzothiazole.
N-Alkylation of imines 5, 6, and 8 could be achieved with
hard alkylating reagents such as methyl triflate or triethyl-
oxonium tetrafluoroborate (MeerweinÕs salt). All alkynyl
imines 5 and 8 used in this study could be converted
smoothly into the corresponding N-methyl propyne imin-
ium triflates 9 and 11 (Scheme 4, Tables 3 and 4). The
salts readily separated (as a solid or an oil) from the reac-
tion mixture, when the alkynyl imine was added slowly to
a solution of methyl triflate in diethyl ether. The twofold
alkylation with methyl triflate of bis(imines) 6a,c, howev-
er, was cleaner with CH2Cl2 as the solvent. Alkylation re-
In this context, it is interesting to note that alkylation of 2-
ethynylpyridine with alkyl halides in MeCN was used to
prepare polyacetylene derivatives [poly-(N-alkyl-2-ethy-
nylpyridinium salts)],19 while methyl triflate gave the
N-methylpyridinium salt monomer cleanly.8 Mechanistic
investigations20 suggest that the neutral (ethynyl)pyridine
acts as a nucleophilic initiator for the oligomerization of
the N-methylpyridinium salts according to a zwitterionic/
Synthesis 1999, No. 1, 100–106 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
Propyne Iminium Salts By N-Alkylation of Alkynyl Imines
103
Table 3 Propyne Iminium Salts 9–13 Prepareda (Scheme 4)
Prod- R1
uct
R2
R3
Condi-
tionsb
Yield mp
(%)
IR (neat or KBr) (cm–1)
(°C)c
νC≡C
Other Signals
9a
9b
9c
Ph
Ph
Ph
SiMe3
SiMe3
SiMe3
Me
Et
0°C, 2 h
0°C, 2 h
73
61
62
83–85
62–64
74–76
2166
2160
2159
2954, 1633, 1458, 1377, 1344, 1275, 1223, 1153, 1034
2966, 2853, 1614, 1450, 1376, 1344, 1275, 1222, 1151, 1030
2918, 2853, 1605, 1452, 1344, 1276, 1222, 1151, 1030
CH2-
Ph
0°C, 1h;
20°C, 1 h
9d
Ph
Bu
Me
20°C, 2h,
34
oil
2219
2960, 2934, 2873, 1626, 1598, 1450, 1362, 1261, 1224, 1153,
1032
9e
9f
Ph
Ph
Bu
Bu
Et
20°C, 2 h
0°C, 2 h
40
60
oil
oil
2214
2213
2959, 1619, 1449, 1360, 1272, 1224, 1155, 1031
CH2-
Ph
2960, 2933, 1613, 1596, 1450, 1359, 1277, 1224, 1200, 1160,
1031
10ad
10b
Ph
Ph
Ph
SiMe3
SiMe3
SiMe3
Me
Et
60°C, 2 h
60°C, 2 h
60°C, 2 h
70
72
59
74–76
2152
2962, 1598, 1451, 1375, 1339, 1283, 1250, 1166, 1048
2924, 2854, 1591, 1450, 1377, 1341, 1274, 1252, 1051
107–109 2153
112–114 2156
10gd
Ph
3065, 1596, 1581, 1563, 1490, 1449, 1339, 1318, 1269, 1255,
1056
11ae
0°C, 1 h;
70
122–124 2073
2960, 1596, 1460, 1260, 1220, 1030
20°C, 1 h
11be
11ce
11de
12be
20°C, 2 h
20°C, 2 h
20°C, 2 h
20°C, 5 h
95
55
80
93
147–149 2158
116–118 2121
120–122 2113
155–156 2159
1285, 1251, 1221, 1032
1625, 1591, 1498, 1264, 1223, 1153, 1052, 1030
3184 (≡CH), 1466, 1446, 1257, 1224, 1163, 1030
3101, 1498, 1473, 1459, 1435, 1287, 1273, 1252, 1211, 1175,
1056
13ae
0°C, 2 h
0°C, 2 h
60
73
130–132 2203
3055, 1620, 1596, 1361, 1277, 1224, 1155, 1030
3062, 1595, 1448, 1363, 1260, 1150, 1028
13cd,e
70–72
2199
a Satisfactory microanalyses obtained: C ± 0.4, H ± 0.4, N ± 0.4.
b Reactions in CH2Cl2 solution at 60°C were carried out in a Schlenk pressure tube.
c Crystallization from CH2Cl2/Et2O.
d Mixture of E- and Z-isomers with respect to the C=N+ bond.
e For the structures of 11–13, see Scheme 4.
anionic mechanism. Therefore, in order to suppress the yields a mixture in which 10aA predominates. Combined
oligomerization reaction of the formed propyne iminium with the 1H NMR spectroscopic test that pure 9bB did not
salts, the N-alkylation of the alkynyl imines in general undergo cis/trans isomerization at 60¡C, these findings
should be fast and quantitative, and the quick separation suggest that the C=N configuration of alkynyl imines 5a,b
of the formed iminium salts from the reaction solution is exclusively (0¡C) or largely (60¡C) the one where the
may also be helpful.
lone electron pair at N is cis to the phenyl group.
When alkynyl imines 5a,g were alkylated with Et3OBF4 at
60¡C, iminium salts 10a,g were formed as a mixture of di-
astereomers with respect to the C=N+ bond. In contrast,
alkylation with MeOTf at 0¡C gave only one diastereomer
of 9, while the reaction at 60¡C furnished a diastereomeric
mixture again, as was demonstrated for 5b → 9b. A
ROESY NMR experiment carried out on 10a suggested
that 10aA was the dominating isomer. By comparison
with the chemical shifts observed for 10aA/B [d (Me-A)
> d (Me-B) and d (CH2-A) < d (CH2-B), see Table 4], iso-
meric pairs in other cases can be assigned either to series
A or series B. We then conclude that methylation of N-ethyl
imine 5b forms isomer 9bB exclusively at 0oC or predomi-
nantly at 60¡C, while ethylation of N-methyl imine 5a
Different results were also obtained when 8a was treated
with the two alkylating reagents. Again, reaction with
methyl triflate was fast and gave iminium salt 11a cleanly
(Scheme 4). On the other hand, reaction of 8a with
Et3OBF4 in CH2Cl2 at 20oC was much slower and instead
Synthesis 1999, No. 1, 100–106 ISSN 0039-7881 © Thieme Stuttgart · New York

104
J. Schlegel, G. Maas
PAPER
Table 4 1H and 13C NMR Data of Salts 9–13
Pro-
duct
1H NMR (CD3CN/TMS)
δ, J (Hz); isomer A/isomer B
13C NMR (CD3CN)
δ, J (Hz); isomer A/isomer B
9a
0.30 [s, 9 H, Si(CH3)3], 3.61 (s, 3 H, NCH3), 3.92
–1.27 [Si(CH3)3], 46.63 (NCH3), 49.09 (NCH3), 97.11 (NCC≡C), 122.04, (q,
JC,F = 320.8, CF3), 130.27, 130.37, 130.78 (NCC≡C), 134.84, 137.77 (Carom),
163.19 (C=N)
(s, 3 H, NCH3), 7.57–7.76 (m, 5 H, CHarom
)
9ba
A: 0.31 [s, 9 H, Si(CH3)3], 1.40 (t, J = 7.2, 3 H, A: –1.22 [Si(CH3)3], 13.04 (CH3), 45.70 (NCH3), 54.13 (NCH2), 97.50
CH3), 3.87 (s, 3H, NCH3), 3.90 (q, J = 7.2, 2H, (NCC≡C), 129.39, 129.71, 130.53, 131.77, 134.49, 163.94 (C=N); B: –1.34
NCH2); B: 0.31 [s, 9 H, Si(CH3)3], 1.52 (t, J = 7.3, [Si(CH3)3], 12.21 (CH3), 44.19 (NCH3), 57.83 (NCH2), 96.80 (NCC≡C), 122.07
3 H, CH3), 3.60 (s, 3 H, NCH3), 4.28 (q, J = 7.3, 2 (q, JC,F = 320.9, CF3), 130.23, 130.65, 131.41 (NCC≡C), 135.03, 137.81 (Carom),
H, NCH2), 7.60–7.78 (m, CHarom
0.32 [s, 9 H, Si(CH3)3], 3.57 (s, 3 H, NCH3), 5.54
(s, 2 H, NCH2), 7.40–7.84 (m, 10 H, CHarom
)
162.86 (C=N)
9c
–1.38 [Si(CH3)3], 44.34 (NCH3), 55.14 (NCH2), 97.67 (NCC≡C), 122.02 (q,
JC,F = 320.8, CF3), 129.97, 130.16, 130.29, 130.62, 130.78, 130.93, 131.54,
135.36, 138.24, 164.02 (C=N)
)
9d
0.90 (t, J = 7.3, 3 H, CH3), 1.33–1.67 (m, 4 H, 13.66 (CH3), 20.84, 22.65 (CH2CH2), 29.87 (≡CCH2), 46.36, 48.61 (NCH3),
CH2CH2), 2.72 (t, J = 7.0, 2 H, ≡CCH2), 3.56 (s, 3 77.45 (NCC≡C), 122.55 (q, JC,F = 320.3, CF3), 130.03, 130.20, 130.95, 132.25,
H, NCH3), 3.86 (s, 3 H, NCH3), 7.61–7.70 (m, 5 134.54 (Carom), 164.30 (C=N)
H, CHarom
)
9e
9f
0.92 (t, J = 7.1, 3 H, CH3), 1.36–1.74 (m, 7 H, 12.33 (CH3), 13.72 (CH3), 20.86, 22.68 (CH2CH2), 29.84 (≡CCH2), 43.82
CH2CH2, CH3), 2.74 (t, J = 7.0, 2 H, ≡CCH2), 3.60 (NCH3), 57.27 (NCH2), 77.13 (NCC≡C), 130.11, 130.29, 130.59, 131.20,
(s, 3 H, NCH3), 4.28 (q, J = 7.2, 2 H, NCH2), 134.59 (NCC≡C and Carom), 164.10 (C=N)
7.57–7.82 (m, 5 H, CHarom
)
0.89 (t, J = 7.2, 3 H, CH3), 1.35–1.78 (m, 4 H, 13.75 (CH3), 21.07, 22.71 (CH2CH2), 29.82 (≡CCH2), 44.00 (NCH3), 54.80
CH2CH2), 2.85 (t, J = 6.9, 2 H, ≡CCH2), 3.58 (s, 3 (NCH2), 64.65 (NCC≡C), 127.37, 129.78, 129.98, 130.21, 130.54, 130.67,
H, NCH3), 5.56 (s, 2 H, NCH2), 7.34–8.20 (m, 10 131.50, 135.00, 137.84 (NCC≡C and Carom), 163.41 (C=N)
H, CHarom
)
10ab
A: 0.31 [s, 9 H, Si(CH3)3], 1.40 (t, J = 7.3, 3 H, A: –1.22 [Si(CH3)3], 13.04 (CH3), 45.70 (NCH3), 54.13 (NCH2), 97.50
CH3), 3.90 (s, 3 H, NCH3), 3.92 (q, J = 7.3, 2 H, (NCC≡C), 163.94 (C=Ν); B: –1.22 [Si(CH3)3], 12.24 (CH3), 44.13 (NCH3),
NCH2); B: 0.33 [s, 9 H, Si(CH3)3], 1.54 (t, J = 7.3, 57.84 (NCH2), 96.86 (NCC≡C), 162.9 (C=N); further signals: 129.39, 129.71,
3 H, CH3), 3.62 (s, 3 H, NCH3), 4.31 (q, J = 7.3, 2 130.28, 130.53, 130.63, 131.33, 131.77, 134.49, 135.04 (Carom
)
H, NCH2), 7.59–7.79 (m, CHarom
)
10b
0.32 [s, 9 H, Si(CH3)3], 1.41 (t, J = 7.2, 3 H, CH3), –1.25 [Si(CH3)3], 13.05 (CH3), 13.64 (CH3), 51.87 (NCH2), 54.37 (NCH2),
1.57 (t, J = 7.2, 3 H, CH3), 3.94 (q, J = 7.2, 2 H, 97.36 (NCC≡C), 129.44, 130.51, 131.28 (NCC≡C), 131.88, 134.54 (Carom),
NCH2), 4.32 (q, J = 7.2, 2 H, NCH2), 7.63–7.76 163.84 (C=N)
(m, 5 H, CHarom
)
10gc
A: 0.40 [s, 9 H, Si(CH3)3], 1.41 (t, J = 7.3, 3 H, A: –1.29 [Si(CH3)3], 12.19 (CH3), 60.38 (NCH2), 97.77 (NCC≡C), 163.81
CH3), 4.73 (q, J = 7.3, 2 H, NCH2); B: 0.03 [s, 9 (C=Ν); B: –1.57 [Si(CH3)3], 13.35 (CH3), 56.00 (NCH2), 98.68 (NCC≡C),
H, Si(CH3)3], 1.29 (t, J = 7.3, 3 H, CH3), 4.36 (q, 166.20 (C=Ν); further signals: 126.03, 126.68, 128.10, 129.78, 130.49, 130.91,
J = 7.3, 2 H, NCH2), 7.30–7.98 (m, CHarom
0.30 [s, 9 H, Si(CH3)3], 1.53 [s, 6 H, C(CH3)2], –0.70 [Si(CH3)3], 24.30 (CCH3), 55.04 (CCH3), 40.15 (NCH3), 89.07 (NCC≡C),
4.00 (s, 3 H, NCH3), 7.53–8.02 (m, 4 H, CHarom 121.20, 122.00, 124.63, 125.98, 126.80, 147.34, 155.22, 179.63 (Carom
)
131.24, 132.00, 132.26, 133.45, 133.96, 135.11, 135.70, 140.62, 142.37 (Carom)
11a
11b
)
)
0.40 [s, 9 H, Si(CH3)3], 4.32 (s, 3 H, NCH3), 7.79– –1.20 [Si(CH3)3], 39.15 (NCH3), 89.06 (NCC≡C), 118.31, 125.24, 127.37
7.96 (m, 2 H, CHarom), 8.11–8.15 (m, 1 H, CHarom), (NCC≡C), 130.65, 130.80, 131.73, 141.48 (Carom), 153.35 (C-2)
8.25–8.29 (m, 1 H, CHarom
0.30 [s, 9 H, Si(CH3)3], 3.73 (s, 3 H, NCH3), 5.38
(s, 2 H, NCH2), 7.10–7.45 (m, 15 H, CHarom
)
11c
11d
12b
–1.09 [Si(CH3)3], 35.37 (NCH3), 51.60 (NCH2), 85.06 (NCC≡C), 125.80,
128.49, 128.69, 129.42, 129.64, 129.89, 130.28, 130.98, 131.44, 131.50,
)
131.67, 131.85, 133.47, 134.34, 134.61, 134.88 (NCC≡C and Carom
)
4.32 (s, 3 H, NCH3), 5.51 (s, 1 H, C≡CH), 7.72– 39.40 (NCH3), 69.80 (NCC≡C), 106.16 (NCC≡C), 118.38, 121.75 (q, JC,F =307,
7.99 (m, 2 H, CHarom), 8.12–8.18 (m, 1 H, CHarom), CF3), 125.31, 130.87, 131.04, 131.92, 141.50 (Carom), 153.60 (C=N)
8.26–8.33 (m, 1 H, CHarom
)
0.40 [s, 9 H, Si(CH3)3], 1.57 (t, J = 7.3, 3 H, CH3), –1.23 [Si(CH3)3], 14.17 (CH3), 48.67 (NCH2), 88.83 (NCC≡C), 118.38, 125.49,
4.86 (q, J = 7.3, 2 H, NCH2), 7.81–7.99 (m, 2 H, 127.49 (NCC≡C), 130.90, 131.37, 131.92, 140.55 (Carom), 152.60 (C-2)
CHarom), 8.16–8.30 (m, 2 H, CHarom
3.65 (s, 6 H, NCH3), 3.98 (s, 6 H, NCH3), 7.60–
7.90 (m, 14 H, CHarom
)
13a
–
–
)
13cd
A: 3.59 (s, 6 H, NCH3), 5.59 (s, 4 H, NCH2); B:
3.87 (s, 6 H, NCH3), 5.24 (s, 4 H, NCH2), 7.35–
8.06 (m, CHarom
)
a Reaction at 0°C: only A; at 60°C: isomer ratio A:B = 0.36.
b Isomer ratio A:B = 2.6.
c Isomer ratio A:B = 3.6.
d Isomer ratio A:B = 1.4.
Synthesis 1999, No. 1, 100–106 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
Propyne Iminium Salts By N-Alkylation of Alkynyl Imines
105
at 0¡C. The mixture was stirred for 1 h at this temperature, and after
addition of Et2O (100 mL), it was washed with satd aq NaHCO3 so-
lution. The organic layer was dried (Na2SO4) and concentrated. The
residue was distilled in vacuo to give 5a as a colorless oil (5.30 g,
75%) (Tables 1 and 2).
of a propyne iminium salt, 2-ethoxy-3H-indolinium salt
14 and buta-1,3-diyne 15 were formed in an unidentified
reaction sequence and were isolated in high yields
(Scheme 5, Tables 3 and 4). The identity of 14 was estab-
lished not only by its NMR and analytical data, but also by
hydrolytic transformation into 3H-indolin-2-one 16.21
EI-HRMS: m/z = 215.1130; calcd for C13H17NSi: 215.1130.
Compounds 5bÐf, 6a, and 6c were prepared analogously; see Ta-
bles 1 and 2 for yields, physical and spectroscopic data.
2-[(Trimethylsilyl)ethynyl]-1,3-benzothiazole (8b): Typical
Procedure
A solution of 2-iodo-1,3-benzothiazole27 (7b) (40.00 g, 0.153 mol),
(trimethylsilyl)acetylene (40 mL, 0.280 mol), PdCl2(PPh3)2 (0.56 g,
0.8 mmol) and CuI (0.50 g, 2.62 mmol) in Et3N (300 mL) was
stirred at 70¡C for 2 days. The solvent was replaced by CH2Cl2
(200 mL), and after treatment with satd aq NaHCO3 solution, the
organic solution was dried (Na2SO4) and concentrated. The residue
was distilled in vacuo to give 8b as a yellow oil (20.0 g, 56%) (Ta-
bles 1 and 2).
Anal. calcd for C12H13NSSi (231.41): C, 62.27; H, 5.65; N, 6.04.
Found: C, 62.17; H, 5.61; N, 6.05.
Compounds 8a (5 mmol scale) and 8c were prepared analogously;
see Tables 1 and 2 for yields, physical and spectroscopic data.
2-Ethynyl-1,3-benzothiazole (8d)
To a solution of 8b (9.00 g, 37 mmol) in CH2Cl2 (50 mL) was added
KF (5.00 g, 86 mmol) and 18-crown-6 (10.00 g, 37 mmol). After
stirring at r.t. for 2 h, the mixture was washed twice with H2O, and
the organic layer was dried (Na2SO4) and concentrated in vacuo.
Flash chromatography on silica gel (Et2O as eluent) of the residual
brown oil afforded 8d; yield: 5.00 g (80%) (Tables 1 and 2).
Scheme 5
In summary, we have shown that N-quaternization of
alkynyl imines with hard alkylating reagents constitutes a
new, convenient method to prepare propyne iminium
salts, especially those with an alkyl, silyl, or H substituent
at the C,C triple bond, which were not easily accessible
previously. Since a variety of alkynyl imines can be syn-
thesized easily by coupling of imidoyl chlorides with ter-
minal alkynes, the method is open to further substituent
variations.
Anal. calcd for C9H5NS (159.15): C, 67.91; H, 3.14; N, 8.79.
Found: C, 67.93; H, 3.37; N, 8.74.
Dimethyl-[1-phenyl-3-(trimethylsilyl)-2-propyn-1-ylidene]am-
monium Trifluoromethanesulfonate (9a); Typical Procedure
for Alkylation with Methyl Triflate
To a solution of methyl triflate (0.87 mL, 8 mmol) in Et2O (10 mL)
was slowly added a mixture of 5a (1.07 g, 5 mmol) in Et2O (10 mL)
at 0¡C. The reaction mixture was stirred for 1 h at 0¡C. The Et2O
layer was pipetted off, and the residual oil was triturated twice with
Et2O and crystallized from CH2Cl2/Et2O at Ð30¡C. Salt 9a was ob-
tained as white crystals (1.40 g, 73%).
All reactions were carried out in rigorously dried glassware under
an Ar atmosphere. THF was freshly distilled from Na/benzophe-
none ketyl. Et2O was dried over Na/benzophenone ketyl and stored
over 4 • molecular sieves under Ar. CH2Cl2 was dried over P2O5 and
stored over 4 • molecular sieves under Ar. Column chromatography
was performed using silica gel (Macherey & Nagel, 0.063Ð0.2 mm).
NMR spectra were recorded on a Bruker AC 200 spectrometer oper-
ating at 200.13 MHz for 1H and 50.32 MHz for 13C. TMS served as
internal standard for 1H and CDCl3 (d = 77.0) or CD3CN (d = 118.2)
for 13C NMR spectroscopic measurements. IR spectra were obtained
on a Perkin-Elmer IR-883 spectrometer. Elemental analyses were
carried out using a Perkin-Elmer EA-240 instrument. Melting points
were determined in a apparatus after Dr. Tottoli (BŸchi) and are un-
corrected.
Anal. calcd for C15H20F3NO3SSi (379.47): C, 47.47; H, 5.31; N,
3.69. Found: C, 47.45; H, 5.35; N, 3.65.
Compounds 9bÐf and 11aÐd were prepared in the same manner.
Reactions to prepare 13a,c were run in CH2Cl2 at 0oC, and the prod-
ucts were precipitated as brown solids with Et2O. For yields, phys-
ical and spectroscopic data see Tables 3 and 4.
Diethyl-[1-phenyl-3-(trimethylsilyl)-2-propyn-1-ylidene]am-
monium Tetrafluoroborate (10b); Typical Procedure for Alky-
lation with Meerwein's Salt
A mixture of 5b (2.29 g, 10 mmol) and Et3OBF4 (1.89 g, 10 mmol)
in CH2Cl2 (20 mL) was heated for 2 h at 60¡C in a Schlenk pressure
tube. The crude product was precipitated with Et2O. Crystallization
from CH2Cl2/Et2O gave brown crystals (2.50 g, 72%) (Tables 3 and
4).
The following starting materials were prepared according to litera-
ture procedures: 4aÐc,22 7a,23 1,4-diethynylbenzene,24 (trimethyl-
silyl)acetyl-ene,25 methyl trifluoromethanesulfonate.26
N-Methyl-1-phenyl-3-(trimethylsilyl)prop-2-yn-1-imine (5a);
Typical Procedure
Anal. calcd for C16H24BF4NSi (345.03): C, 55.69; H, 7.01; N, 4.05.
Found: C, 55.68; H, 6.98; N, 4.03.
To a solution of EtMgBr, prepared from magnesium turnings
(0.80 g, 33.0 mmol) and bromoethane (2.9 mL, 39.6 mmol), in THF
(30 mL) was slowly added at 0¡C (trimethylsilyl)acetylene
(5.4 mL, 39.6 mmol). The solution was stirred for 30 min and N-
methylchloro(phenyl)methanimine22 (4a) was added in one portion
Compounds 10a,g and 12b were prepared analogously; for yields,
physical and spectroscopic data see Tables 3 and 4.
Synthesis 1999, No. 1, 100–106 ISSN 0039-7881 © Thieme Stuttgart · New York

106
J. Schlegel, G. Maas
PAPER
1-Benzyl-2-iodo-4,5-diphenylimidazole (7c)
To
solution of 1-benzyl-4,5-diphenylimidazole28 (0.85 g,
Acknowledgement
a
This work was supported by the Deutsche Forschungsgemein-
schaft and the Fonds der Chemischen Industrie.
2.7 mmol) in THF (20 mL) at 0¡C was added BuLi (1.6 M in hex-
ane, 2 mL, 3.2 mmol). The solution was stirred for 1 h and then
cooled to Ð78¡C. A solution of I2 (0.80 g, 3.1 mmol) in THF (5 mL)
was added and stirred for 30 min. The reaction mixture was allowed
to reach r.t. The solution was evaporated, triturated with CH2Cl2
(50 mL) and washed with aq Na2S2O3 solution, then with H2O, and
dried (Na2SO4). Evaporation of the solvent and column chromatog-
raphy on silica gel (CHCl3 as eluent) gave a yellow powder; yield:
0.58 g (50%); mp 103Ð104¡C.
References
(1) (a) Maas, G.; Singer, B.; Wald, P.; Gimmy, M. Chem. Ber.
1988, 121, 1847.
(b) Reinhard, R.; Maas, G.; Bohrisch, J.; Liebscher, J. Liebigs
Ann. Chem. 1994,429.
(c) Maas, G.; Reinhard, R.; Neumann, R.; Glaser, M. J. prakt.
Chem. 1996, 338, 441.
IR (KBr): n = 3056, 3028, 1598, 1493, 1447, 1411, 1349, 1325,
1177, 1118, 1070, 1024 cmÐ1.
(2) Rahm, R.; Maas, G. Synthesis 1994, 295.
(3) Singer, B.; Maas, G. Chem. Ber. 1987, 120, 485.
(4) Singer, B.; Maas, G. Chem. Ber. 1987, 120, 1683.
(5) Maas, G.; Mayer, T. Synthesis 1991, 1209.
(6) Brunner, M.; Maas, G. Synthesis 1995, 957.
(7) Kiprianov, A. J.; Dyadyusha, G. G., Zh. Obshch. Khim. 1959,
30, 3647; Chem. Abstr. 1961, 55, 22833.
1H NMR (CDCl3): d = 4.98 (s, 2 H, NCH2), 6.83Ð7.48 (m, 15 H,
CHarom).
13C NMR (CDCl3): d = 50.4 (NCH2), 91.8 (C-2), 126.2, 126.4,
126.6, 127.4, 127.9, 128.5, 128.8, 129.0, 130.2, 130.6, 132.6, 133.6,
136.1, 141.8 (Carom).
Anal. calcd for C22H16N2 (433.90): C, 60.56; N, 3.92; N, 6.42.
Found: C, 60.33; H, 4.11; N, 6.16.
(8) Subramanyam, S.; Chetan, M.S.; Blumstein, A. Macromole-
cules 1993, 26, 3212.
(9) Austin, W. B.; Bilow, N.; Kelleghan, W. J.; Lau, K. S. Y.
J. Org. Chem. 1981, 46, 2280.
(10) Lin, S.-Y.; Sheng, H.-Y.; Huang, Y.-Z. Synthesis 1991, 235.
(11) Kobayashi, T.-A.; Sakakura, T.; Tanaka, M. Tetrahedron Lett.
1985, 26, 3463.
Alkylation of 8a with MeerweinÕs Salt
A solution of 8a (5.00 g, 20.7 mmol) and Et3OBF4 (3.92 g,
20.7 mmol) in CH2Cl2 (30 mL) was stirred for 12 h at r.t. 2-Ethoxy-
1-ethyl-3,3-dimethyl-3H-indolinium tetrafluoroborate (14) was pre-
cipitated with Et2O and recrystallized from CH2Cl2/Et2O; yield: 4.50
g (74%); mp 104Ð106¡C. Evaporation of the mother liquor left crude
1,4-bis(trimethylsilyl)buta-1,3-diyne (15). Purification by column
chromatography on silica gel (petroleum ether as eluent) furnished
colorless crystals, (1.74 g, 90%), mp 109Ð111¡C (Lit29 mp 106Ð
107¡C).
(12) Margaretha, P.; Schršder, C.; Wolff, S.; Agosta, W. C. J. Org.
Chem. 1983, 48, 1925.
(13) Iwasaki, G.; Shibasaki, M. Tetrahedron Lett. 1987, 28, 3257.
Shibasaki, M.; Ishida, Y.; Iwasaki, G.; Iimori, T. J. Org.
Chem. 1987, 52, 3489.
(14) Faust, R.; Gšbelt, B.; Weber, C. Synlett 1998, 64.
(15) Suvorova, I. V.; Stadnichuk, M. D. J. Gen. Chem. USSR
1982, 52, 397. Suvorova, I. V.; Stadnichuk, M. D., Mingale-
va, K. S. ibid. 1983, 52, 716. Piterskaya, Y. L.; Khramchikhin,
A. V.; Stadnichuk, M. D. ibid. 1994, 64, 1614.
(16) Sakamoto, T.; Shiraiwa, M.; Kondo, Y.; Yamanaka, H. Syn-
thesis 1983, 312.
(17) Evans, D. A.; Bach, T. Angew. Chem. 1993, 105, 1414; An-
gew. Chem. Int. Ed. Engl. 1993, 32, 1326.
(18) Okuhara, K. J. Org. Chem. 1976, 41, 1487.
(19) Subramanyam, S.; Blumstein, A.; Macromolecules 1991, 24,
2668.
Treatment of salt 14 in a mixture of CH2Cl2/H2O (30/10 mL) over-
night, evaporation and distillation of the residue at 80Ð82¡C/
0.02 mbar gave N-ethyl-3,3-dimethyl-3H-indolin-2-one (16).
14:
IR (KBr): n = 2984, 2944, 2878, 1598, 1575, 1447, 1392, 1246,
1045 (very broad, BF4) cmÐ1.
1H NMR (CD3CN): d = 1.46Ð1.67 (m, 12 H, 2 × CH2CH3, C(CH3)2),
4.58 and 4.66 (2 q, each 2 H, OCH2, NCH2), 7.62Ð7.91 (m, 4 H,
CHarom).
13C NMR (CD3CN): d = 12.7, 12.8 (CH2CH3), 23.3 [C(CH3)2], 45.7
(NCH2), 57.0 [C(CH3)2], 84.8 (OCH2), 116.7, 124.8, 130.7, 131.8,
141.0, 142.0 (Carom), 185.1 (C-2).
Zhou, P.; Samuelson, L.; Alva, K.S.; Chen, C.-C.; Blumstein,
R. B.; Blumstein, A. Macromolecules 1997, 30, 1577.
(20) Balogh, L.; Blumstein, A. Macromolecules 1995, 28, 25 and
5691.
Anal. calcd for C14H20BF4NO (305.12): C, 55.11; H, 6.60; N, 4.59.
Found: C, 54.83; H, 6.72; N, 4.38.
(21) The 13C chemical shifts of 16 are in close agreement with
those of the N-methyl analogue, but different from those of the
positional isomer of 16, 2-ethoxy-3,3-dimethyl-3H-indole;
see: Dobrowolski, P.; Stefaniak, L.; Webb, G. A. J. Mol.
Struct. 1987, 160, 319.
16:
IR (neat): n = 2968, 2930, 2869, 1713, 1614, 1487, 1469, 1462,
1382, 1361, 1212, 1131 cmÐ1.
(22) Ugi, I.; Beck, F.; Fetzer, U. Chem. Ber. 1962, 95, 126.
(23) HŸnig, S.; Balli, H. Liebigs Ann. Chem. 1957, 609, 160.
(24) MacBride, J. A. H.; Wade, K. Synth. Commun. 1996, 26,
2309.
(25) Cambie, R. C.; Chambers, D.; Rutledge, P. S.; Woodgate, P.
D. J. Chem. Soc. Perkin Trans. 1 1981, 40.
(26) Ranganathan, N.; Storey, B. T. J. Heterocycl. Chem. 1980, 17,
1069.
1H NMR (CDCl3): d = 1.06 (t, J = 7.2, 3 H, CH3), 1.17 [s, C(CH3)2],
3.57 (q, J = 7.2, 2 H, NCH2), 6.67 (d, J = 7.8, 1 H, CHarom), 6.83 (t,
J = 7.4, 1H, CHarom), 6.99Ð7.08 (m, 2 H, CHarom).
13C NMR (CDCl3): d = 12.3 (CH3), 24.0 [C(CH3)2], 34.1 (NCH2),
43.6 [C(CH3)2], 107.8, 121.9, 122.0, 127.2, 135.6, 141.2 (Carom),
180.4 (C=O).
Anal. calcd for C12H15NO (189.24): C, 76.15; H, 7.98; N, 7.40.
Found: C, 76.06; H, 7.97; N, 7.02.
(27) Iversen, P. E. Acta Chem. Scand. 1968, 22, 1690.
(28) Milgrom, L. R.; Dempsey, P. J. F.; Yahioglu, G. Tetrahedron
1996, 52, 9877.
(29) Ballard, D. H.; Gilman, H. J. Organomet. Chem. 1968, 15,
321.
Synthesis 1999, No. 1, 100–106 ISSN 0039-7881 © Thieme Stuttgart · New York