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REGIOSELECTIVE SYNTHESIS OF 1,2-DIARYLBENZAZETIDENES VIA [2 + 2] CYCLOADDITIONREACTIONS OF BENZYNE WITH AZOMETHINESKewal Krishan Singal a & Jaswinder Kaur aa Department of Chemistry, Punjabi University, Patiala, 147002, IndiaPublished online: 15 Aug 2006.

To cite this article: Kewal Krishan Singal & Jaswinder Kaur (2001): REGIOSELECTIVE SYNTHESIS OF 1,2-DIARYLBENZAZETIDENES VIA [2 + 2] CYCLOADDITION REACTIONS OF BENZYNE WITH AZOMETHINES, SyntheticCommunications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 31:18, 2809-2815

To link to this article: http://dx.doi.org/10.1081/SCC-100105330

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REGIOSELECTIVE SYNTHESIS OF

1,2-DIARYLBENZAZETIDENES VIA

[2Q2] CYCLOADDITION REACTIONS

OF BENZYNE WITH AZOMETHINES

Kewal Krishan Singal* and Jaswinder Kaur

Department of Chemistry, Punjabi University,Patiala 147 002, India

ABSTRACT

[2+2] Cycloaddition reactions of benzyne generated in situwith various substituted azomethines have been carried outleading to the synthesis of 1,2-diarylbenzazetidenes in goodyields with a high degree of regioselectivity.

Benzynes are highly reactive species which are extremely short lived1

and their participation as an intermediate in many reactions is well estab-lished.2,3,4 For the present study, benzyne has been generated in situ bythe aprotic diazotization of anthranilic acid5 and trapped as soon asformed by taking the active carbon–nitrogen double bond as azomethinesin suitable solvents to give [2+2] cycloadducts. The azomethines havebeen synthesized by taking equimolar quantities of various aromaticaldehydes and aromatic amines in ethanol or methanol.6 The work-up ofreaction mixture yielded the crude products which have been recrystallized

2809

Copyright & 2001 by Marcel Dekker, Inc. www.dekker.com

*Corresponding author.

SYNTHETIC COMMUNICATIONS, 31(18), 2809–2815 (2001)

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2810 KRISHAN SINGAL AND KAUR

from suitable solvents. These have been characterized as 1,2-diarylbenzaze-tidenes (I) (Scheme 1) from their melting points, infrared absorption spectra,proton magnetic resonance spectra and mass spectrum. Physical and spec-tral data of 1,2-diarylbenzazetidenes have been given in Tables 1 and 2respectively.

The infrared absorption spectrum of Sr. No. IX (Table 2) shows abroad and strong absorption band at 1605 cm�1 and a sharp and mediumintensity absorption band at 1510 cm�1 (>C–C<), sharp and mediumintensity absorption bands at 1150, 1142 and 1100 cm�1 (>C-N), a strongand sharp absorption band at 1258 cm�1 and a sharp and medium absorp-tion band at 1032 cm�1 (C–O).

The proton magnetic resonance spectrum of Sr. No. XV (Table 2)shows a doublet at d 8.64 (equivalent to one proton) which has beenassigned to proton H50 0 being coupled to H60 0, showing coupling constantJH500�H600

¼ 8:2 Hz which is typical of ortho coupled protons. This signalappears downfield due to presence of an electron-attracting chlorine atompresent on the adjacent carbon atom. A double doublet at d 8.07 (equivalentto one proton) has been assigned to proton H30 0 being coupled to H20 0 andH60 0 showing coupling constant JH300 �H200

¼ 8:2 Hz and JH300 �H600¼ 0:8 Hz

(typical of ortho and para coupled protons). A doublet at d 7.54 (equivalentto one proton) has been assigned to proton H20 0 being coupled to H30 0 havingcoupling constant JH200 �H300

¼ 8:2 Hz. A doublet at d 7.68 (equivalent to oneproton) has been assigned to proton H60 0 being coupled to H50 0 showingcoupling constant JH600 �H500

¼ 8:4 Hz. A doublet at d 8.16 (equivalent to oneproton) has been assigned to proton H60 being coupled to H50 having coup-ling constant JH60�H50

¼ 8:9 Hz. A doublet at d 7.94 (equivalent toone proton) has been assigned to proton H20 being coupled to H60 havingcoupling constant JH20 �H60

¼ 2:7 Hz. A doublet at d 7.45 (equivalent totwo protons) has been assigned to proton H30 being coupled to H50 showingcoupling constant JH30 �H50

¼ 2:7 Hz. A multiplet at d 7.86 (equivalent toone proton) has been assigned to proton H5. Another multiplet at d 7.65

Scheme 1.

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[2Q2] CYCLOADDITION REACTIONS OF BENZYNE 2811

Table

1.

Ph

ysi

cal

Data

of

1,2

-Dia

rylb

enza

zeti

den

es(I

)

Sr.

Mel

tin

gP

oin

tY

ield

Mo

lecu

lar

Ele

men

tal

An

aly

sis

(%)

Fo

un

d(c

alc

d.)

No

Y=

Z=

(�C

)*(%

)fo

rmu

laC

HN

IIH

H166

58

C19H

15N

88.7

5(8

8.7

1)

5.8

0(5

.83)

5.4

9(5

.40)

III

H40 -C

l154

58

C19H

14N

Cl

78.1

8(7

8.2

1)

4.8

2(4

.80)

4.8

4(4

.80)

IVH

40 -O

CH

3179

63

C20H

17N

O83.5

9(8

3.6

2)

5.9

5(5

.92)

4.8

9(4

.87)

VH

40 -O

C2H

5145

63

C21H

19N

O83.7

5(8

3.7

2)

6.2

8(6

.31)

4.6

2(4

.65)

VI

400-O

CH

3H

148

62

C20H

17N

O83.6

5(8

3.6

2)

5.9

0(5

.92)

4.8

1(4

.87)

VII

400-O

CH

340 -C

l195

58

C20H

16N

OC

l74.6

2(7

4.6

5)

4.9

9(4

.97)

4.3

7(4

.35)

VII

I400-O

CH

34-O

CH

3175

65

C21H

19N

O2

79.5

2(7

9.4

9)

5.9

4(5

.99)

4.4

5(4

.41)

IX400-O

CH

340 -O

C2H

5167

65

C22H

21N

O2

79.7

1(7

9.7

5)

6.3

7(6

.34)

4.2

7(4

.22)

X400-O

CH

340 -C

H3

179

61

C21H

19N

O83.7

4(8

3.7

2)

6.3

3(6

.31)

4.6

0(4

.65)

XI

400-O

CH

330 -N

O2

183

55

C20H

16N

2O

372.3

2(7

2.2

8)

4.8

5(4

.81)

8.4

1(8

.43)

XII

400-C

lH

0159

59

C19H

14N

Cl

78.2

4(7

8.2

1)

4.7

8(4

.80)

4.8

5(4

.80)

XII

I400-C

l40 -C

l175

60

C19H

13N

Cl 2

69.9

5(6

9.9

3)

3.9

5(3

.98)

4.3

2(4

.29)

XIV

400-C

l40 -C

H3

169

60

C20H

16N

Cl

78.5

8(7

8.5

5)

5.2

6(5

.23)

4.6

2(4

.58)

XV

400-C

l40 -O

CH

3165

66

C20H

16N

ClO

74.6

3(7

4.6

5)

4.9

2(4

.97)

4.3

8(4

.35)

XV

I400-C

l40 -O

C2H

5178

67

C21H

18N

ClO

75.0

8(7

5.1

1)

5.3

9(5

.36)

4.1

9(4

.17)

XV

II400-C

l30 -N

O2

199

58

C19H

14N

2O

2C

l75.5

2(7

5.4

9)

4.6

7(4

.63)

9.3

3(9

.27)

*A

llth

em

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ng

po

ints

are

un

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ecte

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2812 KRISHAN SINGAL AND KAUR

Table

2.

Sp

ectr

al

Data

of

1,2

-Dia

rylb

enza

zeti

den

es(I

)

Sr.

No

Y=

Z=

IR�n

ujo

l

max

cm�

11H

NM

RdT

MS

cDcl

3

III

H40 -C

l1192,

1090

(>C

-N<

),1600,

1580

(>C

-C<

),1060

(C-C

l)

8.3

5,s

(2H

,H

20 ,H

60 ),7.7

9,

m(4

H,H

4,H

5,H

6,H

7),

7.4

2,

d(5

H,H

200,H

300,H

400,H

500,H

600),

7.2

4,

d(2

H,

H30 ,H

50 ),

7.1

3,

s(1

H,H

2)

IVH

40 -O

CH

31110,

1084

(>C

-N<

),1580,

1556

(>C

-C<

),1354,

1012

(C-O

CH

3)

8.3

8,s

(2H

,H

20 ,H

60 ),7.8

0,

m(4

H,H

4,H

5,H

6,H

7),

7.4

0,

m(5

H,H

200,H

300,H

400,H

500,H

600),

7.0

6,

s(2

H,

H30 ,H

50 ),

6.8

8,

s(1

H,H

2),

3.7

8,

s(3

H,

OCH

3)

VII

400-O

CH

340 -C

l1160,

1142,

1080

(>C

-N<

),

1585,

1560,

1520

(>C

-C<

),1510,

1370

(C-N

O2),

1280,

1020

(C-O

CH

3)

8.6

5,m

(2H

,H

30 ,H

50 ),8.3

5,s

(2H

,H

20 ,H

60 ),7.7

9,m

(4H

,H

4,H

5,H

6,H

7),

7.4

0,

m(4

H,H

200,H

300,H

500,

H600),

7.0

5,

(1H

,H

2m

erges

wit

hm

),4.0

1,

s(3

H,

OCH

3)

IX400-O

CH

340 -O

C2H

51150,

1142,

1100

(>C

-N<

),

1605,

1510

(>C

-C<

),1258,

1032

(C-O

CH

3)

8.2

5,s

(2H

,H

20 ,H

60 ),7.9

8,

m(4

H,H

4,H

5,H

6,H

7),

7.6

0,m

(2H

,H

200,H

600),

7.3

3,d

(2H

,H

300,H

500),

7.2

8,

d(2

H,H

30 ,H

50 ),

6.9

4(s

,1H

,H

2,

mer

ges

wit

hm

),3.8

3,

q(2

H,

OCH

2C

H3),

1.4

0,

t(3

H,

OC

H2CH

3)

XI

400-O

CH

330 -N

O2

1195,

1100,

1070

(>C

-N<

),

1599,

1578

(>C

-C<

),1510,

1370

(C-N

O2),

1280,

1020

(C-O

CH

3)

8.4

5,s

(2H

,H

20 ,H

40 ),7.9

7,m

(2H

,H

50 ,H

60 ),7.3

5,m

(4H

,H

4,H

5,H

6,H

7),

7.3

0,

m(2

H,H

200,H

600),

7.2

4,

d(2

H,H

300,H

500),

7.0

2,

s(1

H,H

2,

mer

ges

wit

hm

),3.9

8,

s(3

H,

OCH

3)

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[2Q2] CYCLOADDITION REACTIONS OF BENZYNE 2813

XIV

400-C

l40 -C

H3

1140,

1085

(>C

-N<

),1598,

1506

(>C

-C<

),1065

(C-C

l)

8.7

0,d

(2H

,H

300,H

500),

8.4

5,m

(2H

,H

20 ,H

60 ),8.2

3,d

d

(4H

,H

5,H

4,H

6,H

7),

7.8

9,

s(2

H,H

200,H

600),

7.5

2,

dd

(2H

,H

30 ,H

50 ),

7.2

5,

s(1

H,H

2),

2.3

0,

s(3

H,

CH

3)

XV

400-C

l40 -O

CH

31160,

1100

(>C

-N<

),1598

(>C

-C<

),1246,

1020

(C-O

CH

3),

1048

(C-C

l)

8.6

4,

d(1

H,H

50 ),

8.1

6,

d(1

H,H

60 ),

8.0

7,

d(1

H,H

300),

7.9

4,

d(1

H,H

200),

7.8

6,

d(1

H,H

5),

7.6

8,

d(1

H,H

600),

7.6

5,

m(2

H,H

4,H

6an

dH

7,

mer

ges

wit

hm

),7.5

4,

d(1

H,H

200),

7.4

5,

dd

(2H

,H

30 ,H

50 ),

7.2

6,

s(1

H,H

2),

4.0

5,

s(3

H,

OCH

3)

XV

I400-C

l40 -O

C2H

51165,

1098

(>C

-N<

),1595

(>C

-C<

),1255,

1203,

1040,

1020

(C-O

C2H

5),

1061

(C-C

l)

8.6

2,

d(1

H,H

500),

8.1

5,

d(1

H,H

60 ),

8.0

5,

d(1

H,H

300),

7.9

6,

d(1

H,H

20 ),

7.8

4,

t(1

H,H

5),

7.6

5,

d(1

H,H

600),

7.6

2,

m(3

H,H

4,H

6,H

7),

7.5

3,

d(1

H,H

200),

7.4

0,

dd

(2H

,H

30 ,

H50 ),

7.2

6,

s(1

H,

H2),

4.2

7,

q(2

H,

OCH

2C

H3),

1.5

7,

t(3

H,

OC

H2CH

3)

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2814 KRISHAN SINGAL AND KAUR

(equivalent to two protons) has been assigned to protons H4 and H6

and proton H7 merges with this multiplet. A singlet at d 7.26 (equivalentto one proton) has been assigned to benzylic proton H2. Another singlet at d4.05 (equivalent to three protons) has been assigned to three protons of themethoxyl group.

In the proton magnetic resonance spectra of these 1,2-diarylbenza-zetidenes the benzylic proton (H2) appears highly downfield on account ofbeing substituted by two aryl rings and an N-aryl ring, which due to theirelectron-withdrawing inductive effect strongly deshield this proton.

The mass spectrum of 1-(40-methoxyphenyl)-2-(40-chlorophenyl)benzazetidene shows the presence of parent molecular ion peak at m/z321 (40.1%). The base peak at m/z 319 (100) results from the loss ofa molecule of hydrogen. Other prominent ion peaks are present at m/z284 (32.4%), 245 (21.8%), 210 (10.5%), 180 (10.6%), 103 (2.0%),77 (27.9%). A probable fragmentation pattern has been given in theFigure 1.

Figure 1.

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[2Q2] CYCLOADDITION REACTIONS OF BENZYNE 2815

Synthesis of 1,2-Diarylbenzazetidenes (II-XVII)

A solution of azomethine (0.01 mol) was added with stirring to a sol-ution of benzyne generated in situ at temperatures ranging between 40� to80�C. The reaction mixture was gently heated for an hour. On work-up thecrude product as a viscous residue was obtained which was crystallized froma benzene-petroleum ether (2:1) mixture to give the products in good yields.These have been recrystallized from suitable solvents to give white crystal-line products.

These cycloaddition reactions are regioselective in nature as from allthe reactions only one isomer has been isolated in each case.

REFERENCES

1. Hoffmann, R.W. Dehydrobenzene and Cycloalkynes, Academic Press:New York, 1967, pp. 200.

2. Matsuda, T.; Mitsuyasu, T. Bull. Chem. Soc. Jpn 1966, 39, 1342.3. Luis, J.V.; Ferrer, R.; Burguete, M.I. J. Org. Chem. 1990, 55, 3808.4. Kitamura, T.; Yamane, M. J. Chem. Soc., Chem. Commun. 1995, 9,

983.5. Friedman, L.; Logullo, F.M. J. Org. Chem. 1969, 34, 3089.6. Layer, R.W. Chem. Rev. 1963, 63, 489.

Received in the Netherlands November 20, 2000

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