Application of phosphonium salts to the reactions of various kinds of amides

Article abstract of DOI:10.1002/1522-2675(20010516)84:5<1112::AID-HLCA1112>3.0.CO;2-8

The phosphonium salts 1 and 2 prepared from triphenylphosphine and N-halogenosuccinimide proved to be applicable to the conversion of amide compounds. Especially, halogenation of electron-deficient heteroaromatic alcohols with these reagents seems to be a convenient method compared to the halogenation with phosphorus oxyhalides.

Full text of DOI:10.1002/1522-2675(20010516)84:5<1112::AID-HLCA1112>3.0.CO;2-8

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Helvetica Chimica Acta ± Vol. 84 (2001)
Application of Phosphonium Salts to the Reactions of
Various Kinds of Amides
by Osamu Sugimoto*, Miho Mori, Keisuke Moriya, and Ken-ichi Tanji
Laboratory of Organic Chemistry, School of Food and Nutritional Sciences, University of Shizuoka, 52-1 Yada,
Shizuoka 422-8526, Japan
The phosphonium salts 1 and 2 prepared from triphenylphosphine and N-halogenosuccinimide proved to
be applicable to the conversion of amide compounds. Especially, halogenation of electron-deficient
heteroaromatic alcohols with these reagents seems to be a convenient method compared to the halogenation
with phosphorus oxyhalides.
Introduction. ± Amide compounds are important intermediates in organic
chemistry. They can be converted to nitriles, chloroimines, or chloroiminium chlorides
with phosphorus oxychloride (ˆ phosphoric trichloride). Phosphorus oxychloride is,
however, an irritant and is unwieldy on account of its moisture-sensitive nature. Thus,
the development of new reaction modes not dependent on halogenating reagents such
as phosphorus oxyhalide would be advantageous for many chemists (Scheme 1).
Scheme 1. Conversion of Amides with Phosphorus Oxychloride (POCl3)
P-Compounds have a strong affinity for O-atoms and are widely used for the
conversion of alcohols or carboxylic acids (halogenation with phosphorus oxyhalide or
Mitsunobu reaction), or carbonyl compounds (Wittig reaction). Recently, conversion
of carboxylic acids to acyl halides [1], esters [2], and amides [3] with phosphonium salts
1 or 2, which were obtained by the reaction of triphenylphosphine with N-
halogenosuccinimide (Scheme 2), was reported by Froyen. As the phosphonium salts
1 and 2 were presumed to have a chemical reactivity similar to that of a representative
halogenating reagent, phosphorus oxyhalide, their application to the reactions of
various kinds of amides was undertaken.
Results and Discussion. ± At first, the phosphonium salt 1 was applied to the
dehydration of primary amides. A mixture of an amide shown in Table 1 and 1 was

Helvetica Chimica Acta ± Vol. 84 (2001)
1113
Scheme 2. Preparation of Phosphonium Salts 1 and 2
heated to reflux to give the corresponding nitrile in good yield. Best results were
achieved with 2 equiv. of 1 and heating to reflux for 4 h. Both dioxane and
tetrahydrofuran (THF) were suitable as a solvent for this reaction.
Table 1. Dehydration of Amides
R
Solvent
Amount of 1
Reaction time [h]
Yield [%]
p-MeOC6H4
p-MeOC6H4
p-MeOC6H4
p-MeOC6H4
Ph
Ph
PhCH2
PhCH2
dioxane
dioxane
dioxane
THF
dioxane
THF
1 equiv.
1 equiv.
2 equiv.
2 equiv.
2 equiv.
2 equiv.
2 equiv.
2 equiv.
1
4
4
4
4
4
4
4
70
74
92
93
64
72
83
92
dioxane
THF
Next, formylation of 1H-indole with 1 was carried out. A mixture of N,N-
dimethylformamide and 3 equiv. of 1 in THF was heated to reflux for 1 h, followed by
addition of 1H-indole to give 1H-indole-3-carbaldehyde (Scheme 3). This result shows
that the reaction of tertiary amides and 1 gives the corresponding iminium salts.
Scheme 3. Formylation of 1H-Indole
Electron-deficient halogenoheteroaromatics are useful intermediates in organic
synthesis. They react with nucleophiles such as amines or alkoxides to give the
corresponding substituted products, and are lithiated to introduce electrophiles via a
halogen-lithium exchange reaction [4] or a tellurium-lithium exchange reaction [5 ± 7]
(Scheme 4).
While the reactivity of heteroaromatics has been studied in detail, the study of their
basic synthetic reactions like halogenation [8] has remained at a standstill. It is well-
known that heteroaromatic alcohols are chlorinated with phosphorus oxychloride to

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Helvetica Chimica Acta ± Vol. 84 (2001)
Scheme 4. Reaction of Electron-Deficient Haloheteroaromatics (Het-X)
give chloroheteroaromatics [9 ± 12]. Similarly, bromoheteroaromatics can be obtained
with phosphorus oxybromide [12 ± 14]. These reagents are, however, irritants and are
unwieldy on account of their moisture-sensitive nature. Furthermore, phosphorus
oxybromide is too expensive for synthetic applications. We speculated that the
phosphonium salts 1 could be applied to chlorination of heteroaromatic alcohols. In a
preceding paper [15], the convenient halogenation of some heteroaromatic alcohols
with triphenylphosphine and N-halogenosuccinimide was reported. We applied this
method to a number of other heteroaromatic alcohols and developed a one-pot
iodination using a combination of the chlorination and a Cl/I-exchange reaction.
The results of chlorination of quinolin-2(1H)-one with 1 and bromination with 2 are
reported in Table 2. Thus, a mixture of quinolin-2(1H)-one and 1 equiv. of 1 in dioxane
was heated to reflux to give 2-chloroquinoline in 35 ± 39% yield (Entries 1 ± 3). When
the amount of 1 was increased, the yield of 2-chloroquinoline was improved (Entries 4
and 5). Bromination of quinolin-2(1H)-one with 5 equiv. of phosphonium salt 2
resulted in 90% yield of 2-bromoquinoline after 4 h reflux in dioxane (Entry 9). In
THF (b.p. 668) as solvent, the yield was lower (Entries 6 and 10), this halogenation
presumably requiring a certain temperature. Iodination of quinolin-2(1H)-one with N-
iodosuccinimide gave 2-iodoquinoline in low yield. This result may be due to the
instability of the iodinating reagent being heated.
Table 2. Halogenation of Quinolin-2(1H)-one
Entry
X
Solvent
Amount of 1 or 2
Reaction time [h]
Yield [%]
1
2
3
4
5
6
7
8
9
Cl
Cl
Cl
Cl
Cl
Cl
Br
Br
Br
Br
dioxane
dioxane
dioxane
dioxane
dioxane
THF
dioxane
dioxane
dioxane
THF
1 (1 equiv.)
1 (1 equiv.)
1 (1 equiv.)
1 (2 equiv.)
1 (5 equiv.)
1 (5 equiv.)
2 (1 equiv.)
2 (2 equiv.)
2 (5 equiv.)
2 (5 equiv.)
2
4
8
4
4
6
4
4
4
35
39
35
58
63
31
46
71
90
0
10
11

Helvetica Chimica Acta ± Vol. 84 (2001)
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Table 3. Halogenation of Some Heteroaromatic Alcohols
Entry
Substrate
Phosphonium salt 1 or 2
Reaction time [h]
Product
Yield [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
A (X ˆ OH)
A (X ˆ OH)
B (X ˆ OH)
B (X ˆ OH)
C (X ˆ OH)
C (X ˆ OH)
D (X ˆ OH)
D (X ˆ OH)
E (X ˆ OH)
E (X ˆ OH)
F (X ˆ OH)
F (X ˆ OH)
G (X ˆ OH)
G (X ˆ OH)
G (X ˆ OH)
1 (5 equiv.)
2 (5 equiv.)
1 (2 equiv.)
2 (5 equiv.)
1 (2 equiv.)
2 (7 equiv.)
1 (5 equiv.)
2 (5 equiv.)
1 (5 equiv.)
2 (5 equiv.)
1 (5 equiv.)
2 (5 equiv.)
1 (5 equiv.)
2 (2 equiv.)
2 (2 equiv.)
4
4
4
9.5
4
4
7
8.5
4
4
A (X ˆ Cl)
A (X ˆ Br)
B (X ˆ Cl)
B (X ˆ Br)
C (X ˆ Cl)
C (X ˆ Br)
D (X ˆ Cl)
D (X ˆ Br)
E (X ˆ Cl)
E (X ˆ Br)
F (X ˆ Cl)
F (X ˆ Br)
G (X ˆ Cl)
G (X ˆ Br)
G (X ˆ Br)
43
54
74
84
89
71
56
50
88
43
82
49
73
5
4
4
4
1
4
1
The halogenation was then applied to some other heteroaromatic alcohols to
confirm its generality, and was successful with most of the substrates shown in Table 3.
Pyridin-2(1H)-one (A), 2,6-dimethylpyrimidin-4(3H)-one (B), 1,4-diphenyl-1H-pyr-
azolo[3,4-d]pyrimidin-6(7H)-one (C), 1-phenyl-1H-pyrazolo[3,4-d]pyrimidin-4(5H)-
one (D), quinoxalin-2(1H)-one (E), and 4-phenylquinazolin-2(1H)-one (F) were
halogenated with 1 or 2 to give the corresponding halide in good to fair yields
(Entries 1 ± 12). Bromination of quinazolin-4(3H)-one (G) gave 4-bromoquinazoline
in low yield because of the instability of the product (Entries 14 and 15).
As the direct iodination of heteroaromatic alcohols with triphenylphosphine and N-
iodosuccinimide was found to be difficult, the one-pot synthesis of some iodohetero-
aromatics from the corresponding heteroaromatic alcohols by chlorination with 1
followed by Cl/I exchange was developed as shown in Table 4. It should be noted that
the isolation of the intermediary chloro compounds synthesized with 1 is not necessary
in this iodination of heteroaromatic alcohols, whereas the isolation of the chloro
compounds synthesized with POCl3 is mandatory.
Conclusion. ± In this study, the conversion of amides into nitriles, haloimines, or
chloroiminium salts was accomplished with the phosphonium salts 1 or 2. Especially the
halogenation of electron-deficient heteroaromatic alcohols with 1 or 2 seems to be a
convenient and inexpensive method compared to the halogenation with phosphorus
oxyhalides.

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Helvetica Chimica Acta ± Vol. 84 (2001)
Table 4. Iodination of Some Heteroaromatic Alcohols by the Cl/I Exchange Reaction
Substrate (Het-OH)
Product (Het-I)
Yield [%]
quinolin-2(1H)-one
2-iodoquinoline
53
60
31
2,6-dimethylpyrimidin-4(3H)-one (B)
1-phenyl-1H-pyrazolo[3,4-d]pyrimidin-
4(5H)-one (D)
4-iodo-2,6-dimethylpyrimidine
4-iodo-1-phenyl-1H-pyrazolo[3,4-d]-
pyrimidine
quinoxalin-2(1H)-one (E)
2-iodoquinoxaline
41
Experimental Part
1. General. CC ˆ column chromatography. M.p.: not corrected. 1H-NMR Spectra: Hitachi-R-90 H
spectrometer; d in ppm rel. to SiMe4 as an internal standard, J in Hz.
2. Dehydration of Amides: General Procedure (Table 1): N-Chlorosuccinimide (1335 mg, 10.0 mmol) was
added little by little to a soln. of PPh3 (2623 mg, 10.0 mmol) in an appropriate solvent (100 ml) and stirred at r.t.
for 30 min. The amide (5 ± 10 mmol) was added to the suspension, and the mixture was heated to reflux. After
the mixture was neutralized with Et3N, the solvent was evaporated and the residue submitted to CC (SiO2) to
give the corresponding nitriles.
4-Methoxybenzonitrile: CC (SiO2, hexane/AcOEt 5 :1). 1H-NMR (CDCl3): 3.85 (s, MeO); 6.94 (d, J ˆ 9.0,
2 arom. H); 7.59 (d, J ˆ 9.0, 2 arom. H).
1
Benzonitrile: CC (SiO2, hexane/AcOEt 10 :1). H-NMR (CDCl3): 7.40 ± 7.75 (m, 5 arom. H).
Benzeneacetonitrile: CC (SiO2, hexane/AcOEt 5 :1). 1H-NMR (CDCl3) 3.73 (s, CH2); 7.34 (s, 5 arom. H).
3. Formylation of 1H-Indole (Scheme 3): N-Chlorosuccinimide (2003 mg, 15.0 mmol) was added little by
little to a soln. of PPh3 (3934 mg, 15.0 mmol) in THF (150 ml) and stirred at r.t. for 30 min. N,N-
Dimethylformamide (2193 mg, 30.0 mmol) was added to the suspension, and the mixture was heated to reflux
for 1 h. Then 1H-indole (586 mg, 5.00 mmol) was added, and the mixture was heated to reflux for 1 h. The THF
was evaporated, H2O (50 ml) added, and the mixture heated to reflux for 1 h and then alkalinized with NaOH
soln. After extraction with AcOEt (200 ml), the residue of the org. phase was submitted to CC (SiO2, hexane/
AcOEt 3 :2): 653 mg (90%) of 1H-indole-3-carboxaldehyde [16]. White solid. M.p. 196 ± 1978 ([16]: 194 ± 1968).
1H-NMR (CDCl3): 7.15 ± 7.66 (m, H C(5),
(m, H C(4)); 8.55 ± 9.45 (br., NH); 10.07 (s, CHˆO).
H
C(6),
H
C(7)); 7.83 (d, J ˆ 3.1,
H C(2)); 8.17 ± 8.46
4. Halogenation of Heteroaromatic Alcohols: General Procedure (Tables 2 and 3): N-Halogenosuccinimide
was added little by little to a soln. of PPh3 in an appropriate solvent and stirred at r.t. for 30 min. The
heteroaromatic alcohol was added to the suspension, and the mixture was heated to reflux. After the mixture
was alkalinized with Et3N, dioxane and Et3N were evaporated, and the residue was submitted to CC (SiO2) to
give the corresponding halogenoheteroaromatics.
2-Chloroquinoline [17]: CC (SiO2, hexane/AcOEt 6 :1). White solids. M.p. 31 ± 328 ([17]: 37 ± 388).
1H-NMR (CDCl3): 7.26 ± 8.18 (m, 6 arom. H).
2-Bromoquinoline [18]: CC (SiO2, hexane/CH2Cl2 1:1). White solid. M.p. 498 ([18]: 48 ± 498). 1H-NMR
(CDCl3): 7.40 ± 8.13 (m, 6 arom. H).
2-Chloropyridine [19]: CC (SiO2, hexane/CH2Cl2 2 :1). Colorless oil. 1H-NMR (CDCl3): 7.06 ± 7.44
(m, H C(3), H C(5)); 7.65 (td, J ˆ 7.5, 2.0, H C(4)); 8.40 (d, J ˆ 5.0, H C(6)).
2-Bromopyridine [20]: CC (SiO2, hexane/CH2Cl2 3 :2). Colorless oil. 1H-NMR (CDCl3): 7.02 ± 7.36
(m, H C(5)); 7.36 ± 7.71 (m, H C(3), H C(4)); 8.20 ± 8.50 (m, H C(6)).
4-Chloro-2,6-dimethylpyrimidine [21]: CC (SiO2, with hexane/AcOEt 10 :1 ! 3 :1). Yellow oil. 1H-NMR
(CDCl3): 2.48 (s, 1 Me); 2.67 (s, 1 Me); 7.04 (s, 1 arom. H).
4-Bromo-2,6-dimethylpyrimidine: CC (SiO2, hexane/AcOEt 10 :1 ! 2 :1). White needles. M.p. 548.
1H-NMR (CDCl3): 2.46 (s, 1 Me); 2.67 (s, 1 Me); 7.21 (s, 1 arom. H). Anal. calc. for C6H7BrN2: C 38.53,
H 3.77, N 14.98; found: C 38.50, H 3.76, N 14.73.

Helvetica Chimica Acta ± Vol. 84 (2001)
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6-Chloro-1,4-diphenyl-1H-pyrazolo[3,4-d]pyrimidine [22]: CC (SiO2, hexane/CH2Cl2 1:1). Pale yellow
1
needles (recryst. from hexane/AcOEt). M.p. 184 ± 1868. H-NMR (CDCl3): 7.25 ± 7.72 (m, 6 H, Ph); 8.04 ± 8.32
(m, 4 H, Ph); 8.50 (s, H C(3)).
6-Bromo-1,4-diphenyl-1H-pyrazolo[3,4-d]pyrimidine: CC (SiO2, hexane/CH2Cl2 1:1). Colorless needles
(recryst. from hexane/AcOEt). M.p. 1948. 1H-NMR (CDCl3): 7.28 ± 7.82 (m, 6 H, Ph); 8.06 ± 8.40 (m, 4 H, Ph);
8.51 (s, H C(3)). Anal. calc. for C17H11BrN4: C 58.14, H 3.16, N 15.95; found: C 58.30, H 3.14, N 15.83.
4-Chloro-1-phenyl-1H-pyrazolo[3,4-d]pyrimidine [10]: CC (SiO2, hexane/AcOEt 10 :1). White solid. M.p.
131 ± 1328 ([10]: 126 ± 1278). 1H-NMR (CDCl3): 7.32 ± 7.70 (m, 3 H, Ph); 8.07 ± 8.29 (m, 2 H, Ph); 8.34
(s, H C(3)); 8.87 (s, H C(6)).
4-Bromo-1-phenyl-1H-pyrazolo[3,4-d]pyrimidine [23]: CC (SiO2, hexane/CH2Cl2 1 :1). White solid. M.p.
1
137 ± 1398 ([23]: 1358). H-NMR (CDCl3): 7.32 ± 7.69 (m, 3 H, Ph); 8.06 ± 8.30 (m, 2 H, Ph); 8.27 (s, H C(3));
8.81 (s, H C(6)).
2-Chloroquinoxaline [9]: CC (SiO2, hexane/AcOEt 6 :1). Pale yellow solid. M.p. 47 ± 47.58 ([9]: 46 ± 478).
1H-NMR (CDCl3): 7.67 ± 7.93 (m, 2 arom. H); 7.93 ± 8.23 (m, 2 arom. H); 8.78 (s, H C(3)).
2-Bromoquinoxaline [14]: CC (SiO2, hexane/CH2Cl2 3 :2). Pale yellow solid. M.p. 588 ([14]: 58 ± 598).
1H-NMR (CDCl3): 7.62 ± 7.90 (m, 2 arom. H); 7.90 ± 8.23 (m, 2 arom. H); 8.86 (s, H C(3)).
2-Chloro-4-phenylquinazoline [24]: CC (SiO2, hexane/CH2Cl2 1:1). Pale yellow solid. M.p. 111 ± 1128 ([24]:
1
1138). H-NMR (CDCl3): 7.45 ± 8.28 (m, 9 arom. H).
2-Bromo-4-phenylquinazoline: CC (SiO2, hexane/CH2Cl2 1:1). Colorless needles (recryst. from hexane/
AcOEt). M.p. 130 ± 1318. 1H-NMR (CDCl3): 7.44 ± 8.23 (m, 9 arom. H). Anal. calc. for C14H9BrN2: C 58.97,
H 3.18, N 9.82; found: C 58.79, H 3.13, N 9.80.
4-Chloroquinazoline [24]: CC (SiO2, hexane/AcOEt 8 :1). White solid. M.p. 97 ± 988 ([24]: 96.5 ± 97.58).
1H-NMR (CDCl3): 7.78 (qd, J ˆ 8.1, 7.9, H C(6)); 7.91 ± 8.17 (m, H C(7), H C(8)); 8.28 (dd, J ˆ 8.1, 0.8,
H
C(5)).
4-Bromoquinazoline: CC (SiO2, hexane/AcOEt 2 :1). Pale orange prisms (recryst. from hexane). M.p. 95 ±
978 (dec.). 1H-NMR (CDCl3): 7.60 ± 8.33 (m, H C(5), H C(6), H C(7), H C(8)); 8.98 (s, H C(2)). Anal.
calc. for C8H5BrN2: C 45.96, H 2.41, N 13.40; found: C 46.33, H 2.62, N 13.73.
5. Iodination of Heteroaromatic Alcohols by Cl/I-Exchange: General Procedure. N-Chlorosuccinimide was
added to a soln. of PPh3 in dioxane and stirred at r.t. for 30 min. Starting material was added to the suspension,
and the mixture was stirred under the Conditions 1 (see below). NaI and a cat. amount of hydroiodic acid were
added to the mixture, and the mixture was stirred under the Conditions 2 (see below). Dioxane was evaporated
and the residue dissolved in CH2Cl2 and alkalinized with Et3N. The crude product was purified with by CC (SiO2).
2-Iodoquinoline [25]: According to the General Procedure (Conditions 1: reflux, 6 h; Conditions 2: reflux,
10 h), from quinolin-2(1H)-one (726 mg, 5.00 mmol), PPh3 (2623 mg, 10.0 mmol), N-chlorosuccinimide
(1335 mg, 10.0 mmol), and NaI (1500 mg, 10.0 mmol): 670 mg (53%) of yellow solid. M.p. 44 ± 458 ([25]: 52 ±
1
538). H-NMR (CDCl3): 7.29 ± 7.90 (m, 5 arom. H); 8.05 (d, J ˆ 7.6, 1 arom. H).
4-Iodo-2.6-dimethylpyrimidine [26]: According to the General Procedure (Conditions 1: reflux, 9 h,
Conditions 2, reflux, 9 h), from 2,6-dimethylpyrimidin-4(3H)-one (372 mg, 3.00 mmol), PPh3 (1574 mg,
6.00 mmol), N-chlorosuccinimide (801 mg, 6.00 mmol), and NaI (1800 mg, 12.0 mmol): 421 mg (60%) of white
powder. M.p. 1158 ([26]: 1128). 1H-NMR (CDCl3): 2.40 (s, Me C(2)); 2.65 (s, Me C(6)); 7.48 (s, H C(5)).
4-Iodo-1-phenyl-1H-pyrazolo[3,4-d]pyrimidine: According to the General Procedure (Conditions 1: reflux,
8 h, Conditions 2: r.t. 14 h), from 1-phenyl-1H-pyrazolo[3,4-d]pyrimidin-4(5H)-one (637 mg, 3.00 mmol), PPh3
(1574 mg, 6.00 mmol), N-chlorosuccinimide (801 mg, 6.00 mmol), and NaI (900 mg, 6.00 mmol): 300 mg (31%)
1
of white powder (recryst. from hexane/AcOEt). M.p. 1658. H-NMR (CDCl3): 7.24 ± 7.74 (m, 3 H, Ph); 7.99 ±
8.34 (m, 2 H, Ph); 8.11 (s, H C(3)); 8.72 (s, H C(6)). Anal. calc. for C11H7IN4: C 41.02, H 2.19, N 17.39; found:
C 41.07, H 2.18, N 17.21.
2-Iodoquinoxaline [10]: According to the General Procedure (Conditions 1: reflux, 10 h; Conditions 2:
reflux, 6 h), from quinoxalin-2(1H)-one (731 mg, 5.00 mmol), PPh3 (2623 mg, 10.0 mmol), N-chlorosuccinimide
(1335 mg, 10.0 mmol), and NaI (3000 mg, 20.0 mmol): pale yellow powder. M.p. 104 ± 1058 ([10]: 104 ± 1058).
1H-NMR (CDCl3): 7.55 ± 7.93 (m, 2 arom. H); 7.93 ± 8.36 (m, 2 arom. H); 8.99 (s, H C(3)).
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Received February 13, 2001