Ozonolysis of Pyrroles, Furans, and -Pyrones in Connection with

Ozonolysis of Pyrroles, Furans, and γ-Pyrones in Connection with Reactivity of Bonds in Ring System J. P. WIBAUT

Downloaded by UNIV OF SYDNEY on May 20, 2013 | http://pubs.acs.org Publication Date: January 1, 1959 | doi: 10.1021/ba-1959-0021.ch024

Laboratory of Organic Chemistry, Municipal University, Amsterdam, The Netherlands

Scission products were studied from the ozonolysis of some pyrrole homologs, dimethylfurans, a n d methyl homologs of γ-pyrone. The results show that the pyrroles react not only according to the imino struc ture, but also according to a limiting structure by which the α- a n d β-carbon atoms are linked by a single bond. The dimethylfurans react according to this same limiting structure, as well as according to the classical structure. The study of methyl homologs of γ-pyrone shows that the pyrone ring reacts a c c o r d ing to classical as well as polar structures.

Pyrrole a n d i t s h o m o l o g s s h o w a n a r o m a t i c c h a r a c t e r i n s o m e respects, b u t a r e i n g e n e r a l less s a t u r a t e d i n t h e i r b e h a v i o r t h a n b e n z e n e a n d i t s d e r i v a t i v e s . T o p y r r o l e i t s e l f t h e i m i n e s t r u c t u r e ( I ) is assigned. There are indications that certain pyrrole derivatives c a n react according to a p y r r o l e n i n e structure ( I I or I I I ) . HC

CH

II H

\

C

H C = C H

II /

N

C

HC

I I H

H

2

CH

II

C ^ C H

2

I

H C ^ C H

H

I

II

III

A n g e l i , A n g e l i c o , a n d C a l v e l l o (1), w h e n m a k i n g a m y l n i t r i t e a n d s o d i u m

ethylate

r e a c t w i t h 2 , 5 - d i m e t h y l - o r 2 , 4 - d i m e t h y l p y r r o l e , o b t a i n e d t h e s o d i u m salts of t h e i s o n i t r o s o c o m p o u n d s . T o these t h e y a t t r i b u t e d t h e f o l l o w i n g s t r u c t u r e s : Η

ίί

H C— \

fi"

C

3

N

/

C

~

C

H

3

H

*~

|j

f

H C-C^

HN0

2

^C-CH

3

3

HC

2

NaOCgH^

Ç=NONa

H C-C^

^C-CH

3

3

H

H

\\

Γ ™

H j C - C ^ ^ C H

H Ç = = Ç - C H

3

H C—C^ ^CH 3

N

2

3

HN0

H Ç = Ç - C H

2

NaO^H,

H C-C^ 3

H 153

In OZONE CHEMISTRY AND TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

N /

3

C=NONCL

A D V A N C E S IN

154

CHEMISTRY SERIES


T h e r e f o r e , t h e I t a l i a n i n v e s t i g a t o r s a s s u m e d t h a t these d i m e t h y l p y r r o l e s r e a c t a c c o r d i n g t o a p y r r o l e n i n e s t r u c t u r e . H o w e v e r , t h i s a s s u m p t i o n does n o t c o r r e s p o n d t o t h e r e s u l t s of recent p h y s i c a l m e a s u r e m e n t s . B o n i n o {2) s t u d i e d R a m a n s p e c t r a of p y r r o l e a n d some of i t s h o m o l o g s , b u t d i d n o t find a n y i n d i c a t i o n of t h e presence of t a u t o m e r i c f o r m s . A n a l o g o u s r e s u l t s were o b t a i n e d b y L o r d a n d M i l l e r (9) who m a d e a t h o r o u g h i n v e s t i g a t i o n b a s e d o n R a m a n a n d i n f r a r e d s p e c t r a of p y r r o l e a n d of s o m e p y r r o l e s c o n t a i n i n g d e u t e r i u m a t o m s . T h e r e f o r e , i t w a s of i n t e r e s t t o a p p l y the ozonolysis m e t h o d to p y r r o l e a n d its homologs. S t u d i e s i n c o l l a b o r a t i o n w i t h G u l j é (4, 13) s h o w e d t h a t o z o n i z a t i o n of p y r r o l e a n d i t s h o m o l o g s p r o c e e d s s a t i s f a c t o r i l y i f t h e r e a c t i o n t a k e s p l a c e a t —60° C , w i t h c h l o r o f o r m as t h e s o l v e n t . D e s p i t e t h e l o w r e a c t i o n t e m p e r a t u r e , m o r e o r less r e s i n is s o m e t i m e s f o r m e d , b u t w i t h a l l t h e c o m p o u n d s u n d e r i n v e s t i g a t i o n t h e c h a r a c t e r i s t i c scission p r o d u c t s c o u l d be s e p a r a t e d . T h e m e a s u r e m e n t s i n d i c a t e d t h a t e a c h m o l e c u l e of a p y r r o l e h o m o l o g t a k e s u p a b o u t 1.6 t o 1.7 m o l e s of ozone. A f t e r h y d r o l y t i c scission of t h e r e a c t i o n p r o d u c t , 80 t o 8 7 % of t h e n i t r o g e n b o u n d i n t h e p y r r o l e m o l e c u l e is r e c o v e r e d i n t h e f o r m o f ammonia (methylamine). I f t h e q u a n t i t y of a m m o n i a is a s s u m e d t o be a m e a s u r e of t h e q u a n t i t y of p y r r o l e w h i c h has r e a c t e d w i t h ozone, t h e c o n c l u s i o n c a n be d r a w n t h a t 1 m o l e c u l e of p y r r o l e h o m o l o g has r e a c t e d w i t h 2 m o l e c u l e s of ozone. T h e o t h e r scission p r o d u c t s f o r m e d b y h y d r o l y t i c d e c o m p o s i t i o n of t h e o z o n i d e s a r e d i c a r b o n y l c o m p o u n d s , a c e t i c a c i d , a n d / o r f o r m i c a c i d . I n some cases v e r y s l i g h t q u a n t i t i e s of n i t r i c a c i d a r e f o u n d . T h e d i c a r b o n y l c o m p o u n d s h a v e b e e n i d e n t i f i e d i n t h e f o r m of t h e d i o x i m e s . T h e q u a n t i t a t i v e a n a l y s i s of t h e m i x t u r e of d i o x i m e s is carried out according to a m e t h o d described b y H a a y m a n a n d W i b a u t ( 5 ) . I n a n o t h e r series of e x p e r i m e n t s t h e m i x t u r e of ozonides w a s t r e a t e d w i t h h y d r i o d i c a c i d , w h e r e b y d i c a r b o n y l c o m p o u n d s are f o r m e d . B y reacting the dicarbonyl com p o u n d s w i t h p - n i t r o p h e n y l h y d r a z i n e , t h e c o r r e s p o n d i n g p - n i t r o p h e n y l o s a z o n e s are o b tained. These are isolated f r o m p y r i d i n e b y recrystallization a n d identified b y m i x e d melting point. T h e q u a n t i t a t i v e a n a l y s i s of t h e m i x t u r e of n i t r o p h e n y l o s a z o n e s is carried out b y a chromatographic method. O z o n o l y s i s of p y r r o l e i t s e l f g a v e g l y o x a l i n a y i e l d of 1 5 % ( i s o l a t e d i n t h e f o r m of p-nitrophenylosazone). F o r m i c a c i d a n d a m m o n i a were also f o u n d . T h e s e p r o d u c t s m a y h a v e b e e n f o r m e d a c c o r d i n g t o t h e c l a s s i c a l f o r m u l a of p y r r o l e : HC

CH

II

HC

HC +

II

V

^l\T

20

3

^CH

CH

o < Il 3

HC.

H

II >o

Ν Η

.CH

3

^hydrolysis HC—CH

II

0

II

0

'

Ο

Ο

Il

II

hydrolysis

2HC00H + NH

H C ^ C H

3

H

not isolated O z o n o l y s i s of i V - p h e n y l p y r r o l e g a v e , besides g l y o x a l i n a 3 8 % y i e l d ( i s o l a t e d i n t h e f o r m of t h e p - n i t r o p h e n y l o s a z o n e ) , a n i l i n e i n a 5 8 % y i e l d ( i s o l a t e d i n t h e f o r m of t r i b r o m o a n i l i n e ) . T h i s r e s u l t agrees w e l l w i t h t h e c l a s s i c a l s t r u c t u r e : HC

CH

0 0

3

HC^ /CH N

I C H 6

5

Ο 3

hydrolysis

HC—CH II II Ο Ο

0

II

HC

II

.CH

V

C H

hydrolysis

'6«·5 not isolated 6

5


C H NH 6

5

2

+ 2HCOOH

WIBAUT—PYRROLES,

155

FURANS, 7-PYRONES

A s t h e o z o n i z a t i o n is c a r r i e d o u t a t —60° C , t h e p h e n y l g r o u p i s h a r d l y a t t a c k e d . l - P h e n y l - 2 , 5 - d i m e t h y l p y r r o l e also reacts a c c o r d i n g t o t h e c l a s s i c a l s t r u c t u r e ; i n t h i s case t h e a c e t a n i l i d e h a s b e e n i s o l a t e d as a t r i b r o m i n a t e d d e r i v a t i v e : HCH C

-CH >0

3

HC II Ο

hydrolysis

~ ^ N ^ ""CH,

3

C H 6

CH Ο

C H NHCOCH 6

5

hydrolysis H,C

I C H

5

e

^

CH,

3

+ CH COOH 3

5

I n t h e o z o n o l y s i s of 2 , 4 - d i m e t h y l p y r r o l e , m e t h y l g l y o x a l i s f o r m e d i n 4 1 % y i e l d ( i s o l a t e d i n t h e f o r m o f t h e p - n i t r o p h e n y l o s a z o n e ) , as w e l l as f o r m i c a c i d , a c e t i c a c i d , a m m o n i a ( 7 9 % ) , a n d a s m a l l q u a n t i t y of n i t r i c a c i d ( 0 . 8 % ) , i s o l a t e d i n t h e f o r m of the n i t r o n nitrate. T h i s result is i n good agreement w i t h t h e imine s t r u c t u r e : HC


3

N

—CH

0
0

3

^CH,

H C—C—CH II II Ο Ο

hydrolysis >-

3

+

Ο II HC

NH

Ο II V

CH

hydrolysis >-

3

+ HCOOH

+

3

CH COOH

not isolated

3

T h e f o l l o w i n g e x p e r i m e n t s s h o w t h a t o t h e r s t r u c t u r e s also h a v e t o b e c o n s i d e r e d . I n t h e o z o n o l y s i s of 2 , 5 - d i m e t h y l p y r r o l e ( I V ) , m e t h y l g l y o x a l a n d g l y o x a l a r e f o r m e d i n a d d i t i o n t o a m m o n i a ( 8 3 % ) , f o r m i c a c i d , a n d acetic a c i d . T h e y i e l d of the dioximes is about 2 4 % a n d of p-nitrophenylosazones, 29 t o 3 3 % . F o r t h e molecular ratio, m e t h y l g l y o x a l to glyoxal, values of about 5 to 7 are f o u n d b y a n a l y s i s o f t h e m i x t u r e o f o x i m e s ; a v a l u e of 7 h a s b e e n f o u n d b y a n a l y s i s o f t h e m i x t u r e of p - n i t r o p h e n y l o s a z o n e s . T h e dioximes a n d t h e p-nitrophenylosazones of b o t h dicarbonyl compounds have been isolated i n a pure f o r m a n d were identified b y m i x e d m e l t i n g points w i t h authentic samples. T h e e x p e r i m e n t a l d a t a do n o t w a r r a n t c a l c u l a t i o n of a n a c c u r a t e v a l u e f o r t h e molecular ratio, methylglyoxal-glyoxal. H o w e v e r , i t i s c e r t a i n t h a t t h e m i x t u r e of d i c a r b o n y l c o m p o u n d s consists m a i n l y of m e t h y l g l y o x a l . T h i s c o m p o u n d m u s t b e formed f r o m a structure i n w h i c h the second a n d t h i r d c a r b o n atoms are l i n k e d b y a single b o n d . I n t h i s c o n n e c t i o n one m i g h t c o n s i d e r t h e α-pyrrolenine s t r u c t u r e ( I V a ) f o r m e d f r o m t h e i m i n e s t r u c t u r e b y t h e s h i f t i n g of a p r o t o n . HC-

IV

- CH --IK ^.C—CH

HC=t=CH I \ I

3

3

\ HCO—HCO

H C—C^^C—CH 3

IVa

\

3

H C—CO—CHO 3

I t is impossible to imagine, however, h o w f r o m this structure t h e nitrogen could be s p l i t off as a m m o n i a . I n v e s t i g a t i o n s i n t o t h e o z o n o l y s i s o f p y r i d i n e h o m o l o g s s h o w e d t h a t t h e C = N b o n d does n o t r e a c t w i t h ozone, b u t r a t h e r u n d e r g o e s h y d r o l y t i c s p l i t t i n g i n t h e l a t t e r stage o f t h e r e a c t i o n — i . e . , d u r i n g d e c o m p o s i t i o n o f t h e o z o n i d e . O x i d a t i v e s p l i t t i n g of t h e C = C b o n d i n t h e p y r r o l e n i n e s t r u c t u r e , t o g e t h e r w i t h h y d r o l y t i c s p l i t t i n g of t h e C = N b o n d , w o u l d r e s u l t i n t h e f o r m a t i o n of α - a m i n o p r o p a n a l ( o r α - a m i n o p r o p i o n i c a c i d ) , b u t n o t of a m m o n i a . R e a c t i o n of ozone w i t h 2 - m e t h y l p y r r o l e gives rise t o a s t r o n g r e s i n i f i c a t i o n , e v e n a t a t e m p e r a t u r e as l o w as — 6 0 ° C . H o w e v e r , t h e s c i s s i o n p r o d u c t s , g l y o x a l a n d m e t h y l g l y o x a l , h a v e b e e n i s o l a t e d (7 a n d 2 % , r e s p e c t i v e l y ) . T h e ozonolysis of 2 , 3 - d i m e t h y l p y r r o l e yielded m e t h y l g l y o x a l ( 4 . 6 % ) , d i m e t h y l g l y o x a l ( 1 1 . 5 % ) , a n d a s m a l l q u a n t i t y o f g l y o x a l . T h e m i x t u r e of scission p r o d u c t s contained a m m o n i a ( 8 6 % ) a n d a v e r y slight q u a n t i t y of nitric acid.


A D V A N C E S IN

156

H

TôT

C H 3

H—ky^-CH cleavage α-β

CHEMISTRY SERIES

R = Η

or

R = CH 2

5

3

cleavage β-β

linking

linking

\ CH —C—C—Η II II 0 0

+

3

NH

3

0

-CH

0

0

R=H

3

0

or

R = CH

3


NT R cleavage œ-β

cleavage β-β

linking

\

linking

\

H—C—C—Η II II Ο Ο

CH COCHO

HC—C—Η II II Ο Ο A s i n t h e o z o n o l y s i s of 2 , 3 - d i m e t h y l p y r r o l e , 8 6 % of t h e n i t r o g e n w a s r e c o v e r e d i n t h e f o r m of a m m o n i a ; a r e a c t i o n a c c o r d i n g t o a p y r r o l e n i n e s t r u c t u r e c a n n o t be considered. T h e l - e t h y l - 2 , 3 - d i m e t h y l p y r r o l e whose n i t r o g e n a t o m c a r r i e s a n e t h y l g r o u p c a n n o t be c o n v e r t e d i n t o a p y r r o l e n i n e f o r m . O z o n o l y s i s of t h i s p y r r o l e d e r i v a t i v e , h o w ever, n o t o n l y y i e l d e d m e t h y l g l y o x a l ( 2 . 9 % ) b u t also d i m e t h y l g l y o x a l ( 8 . 6 % ) . T h e o z o n o l y s i s of 1 , 2 - d i m e t h y l p y r r o l e y i e l d e d g l y o x a l ( 5 . 1 % ) a n d m e t h y l g l y o x a l (8.3%). H e r e a g a i n , c o n v e r s i o n of t h e p y r r o l e d e r i v a t i v e i n a p y r r o l e n i n e f o r m is impossible. 3

H

1/9^

H—Je

β\

H

5*—CH N' I CH

+

3

3

H—C—C—H II II Ο Ο

II Ο

I n a l l these cases t h e m e t h y l g l y o x a l or t h e d i m e t h y l g l y o x a l has b e e n f o r m e d f r o m a s t r u c t u r e i n w h i c h t h e « - a n d β - c a r b o n a t o m s are l i n k e d b y a single c a r b o n b o n d . I t is a s s u m e d t h a t t h e p y r r o l e s c a n r e a c t w i t h ozone a c c o r d i n g t o p o l a r v a l e n c e s t r u c t u r e s . A c c o r d i n g t o t h e r e a c t i o n scheme o u t l i n e d b e l o w , w h i c h c o n t a i n s v a r i o u s u n p r o v e d a s s u m p t i o n s , t h e p o l a r f o r m of t h e p y r r o l e h o m o l o g w i l l r e a c t w i t h m o l e c u l e s of ozone, w h i l e o n d e c o m p o s i t i o n of t h e r e a c t i o n p r o d u c t , t h e n i t r o g e n w i l l be s p l i t off as a m m o n i a (or as a p r i m a r y a m i n e ) . HC-CH I I Ο .C—CH H C—C 3

>

polar structure 3

HC= I H C—C* 3

=CH I X—CH

3

Η HC= I H C—C;^ 3

M

=CH I / \ C

HC = C

H

*

=CH

C—CH H,C—C il II Ν Ο Η not isolated

3


157

WlBAUT—PYRROLES, FURANS, 7-PYRONES

HC —^CH

H C = C H

I

1

I

H C—C II Ν Η

C—CH II 0

3

0

3

I

H C-

I

3

3

hydrolysis -CH

3

*

2CH —C—C—Η II II 0 0

+

3

NH

3

F r o m 1 m o l e c u l e of the p o l a r f o r m of 2 , 5 - d i m e t h y l p y r r o l e , 2 m o l e c u l e s of m e t h y l g l y o x a l c a n be f o r m e d . T h i s is i n a g r e e m e n t w i t h t h e e x p e r i m e n t a l f a c t t h a t the o z o n o l y s i s of 2 , 5 - d i m e t h y l p y r r o l e y i e l d s a c o n s i d e r a b l e q u a n t i t y of m e t h y l g l y o x a l .

O z o n o l y s i s of

Furans


T h i s w o r k was c a r r i e d out i n c o l l a b o r a t i o n w i t h J i b b e n (8, 11,

12).

F u r a n a n d i t s m e t h y l h o m o l o g s react r a p i d l y w i t h ozone, e v e n a t a l o w t e m p e r a t u r e (—60° C ) . O n a n a v e r a g e , 1 m o l e of t h e f u r a n s i n v e s t i g a t e d c o n s u m e d 1.6 m o l e s of ozone. T h e ozonides ( w h i c h are e x p l o s i v e ) were d e c o m p o s e d b y r e d u c t i o n w i t h h y d r i o d i c a c i d ; t h e d i c a r b o n y l c o m p o u n d s f o r m e d w e r e d e t e r m i n e d as p - n i t r o p h e n y l o s a z o n e s . T h e ozonolysis of f u r a n itself y i e l d s g l y o x a l ( a b o u t 4 6 % b y w e i g h t ) a n d f o r m i c a c i d . I n t e r e s t i n g r e s u l t s , h o w e v e r , h a v e been o b t a i n e d i n t h e ozonolysis of m e t h y l furans. 2 - M e t h y l f u r a n is r a p i d l y a t t a c k e d b y ozone a t —60° C . G l y o x a l a n d m e t h y l g l y o x a l i n a m o l e c u l a r r a t i o of 5 to 2 a n d a t o t a l y i e l d of 3 5 % were o b t a i n e d as scission p r o d u c t s of t h e h i g h l y e x p l o s i v e o z o n i d e . A m o n g t h e scission p r o d u c t s f o r m e d i n t h e ozonolysis of 2 , 5 - d i m e t h y l f u r a n were f o u n d g l y o x a l a n d m e t h y l g l y o x a l i n a m o l e c u l a r r a t i o of 1 to 1.4 a n d a t o t a l y i e l d of 48%. F o r m i c a c i d a n d acetic a c i d were also f o r m e d . T h i s means that i n the cleaving of t h e f u r a n r i n g a l i n k i n g b e t w e e n a n « - a n d a β - c a r b o n a t o m m a y r e m a i n i n t a c t . HC

o o ,

-zCH

HC>.

HC\ /CH ^ 0 ^

^-C

à

n

HC

CH *

CH

HC^

cleavage if α-β linking H — C — C — Η II II Ο Ο

HCOOH

+

H C

CH COCHO 3

+ H - C — C — Η II II Ο Ο

CH COOH 3

*

*

H C—C*

C H

5C—CH

3

3

I cleavage \β-β linking

H — C — C — Η II II Ο Ο

2HCOOH

ÎC—CH

3

Ο

cleavage α-β H — C — C — Η II II 0 0

4-

l i n k i n g \ ^ c l e a v a g e

2CH COOH 3

β-β

2 C H

3

linking

— C — C — Η II II 0 0

A n a l o g o u s results h a v e b e e n o b t a i n e d i n t h e o z o n o l y s i s of 2 , 3 , 5 - t r i m e t h y l f u r a n , w h i c h y i e l d s m e t h y l g l y o x a l a n d d i m e t h y l g l y o x a l . T h e o z o n o l y s i s of 2 , 3 - d i m e t h y l f u r a n yields three d i c a r b o n y l compounds—glyoxal, m e t h y l g l y o x a l , a n d d i m e t h y l g l y o x a l . T h e s e e x p e r i m e n t a l results m a y be e x p l a i n e d b y t h e s u p p o s i t i o n t h a t t h e f u r a n nucleus c a n react a c c o r d i n g to p o l a r s t r u c t u r e s . T h i s h y p o t h e s i s w o u l d also be i n


158

A D V A N C E S IN CHEMISTRY SERIES

H C 5

U

Î C - C H

3

0 cleavage α-β CH —C—C—Η II II

+

3

0

\^cleavage β-β

linking^/

CHXOOH

+

HCOOH

linking

CH —C-—C—CH II II 3

3

0

0

+

3

H — C — C — Η II II

0

0

a c c o r d a n c e w i t h t h e e x p e r i m e n t a l finding t h a t 1 m o l e of f u r a n c o n s u m e s m o l e c u l e s o f ozone, as a p o l a r f o r m c o n t a i n s o n l y one C = C b o n d . HC

HC=4=CH

ÇH

--•H—~4k

H C—C

C—CH


3

H

3

I

3

\

C — ^ C — C H

0

2CH COOH

2

3

CH3

If H

C

3

HC===C—CH

\\

\ ^0/ C — C H 0

C H

\

I

+

3

HC=±=C—CH

I

'

3

I

H C ^ ^ C - C H ,

3

3

+

3

0

3

CH —C—C—CH II II

0

I

— C — C — Η II II

I

o

0

CH C00H

'

H_C^ ^C—CH

3

— C — C — Η II II

0

be

3

0

S~

/

C — X — C H

3

s " C H

0

t!S

I

H

3

^

+

less t h a n 2

HC===CH

I

\ H—C—C—H H II

0

3

+

H—C—C—Η II II

0

0

0

HCOOH

T h e e x p e r i m e n t s s h o w t h a t t h e w a y these f u r a n h o m o l o g s r e a c t w i t h ozone m a y different f r o m w h a t m i g h t be expected f r o m the classical s t r u c t u r e , a n d t h a t

mesomeric forms p l a y a p a r t . γ-Pyrones

O z o n o l y s i s of

T h e p r o b l e m of t h e s t r u c t u r e of t h e γ - p y r o n e n u c l e u s arose w h e n i t w a s d i s c o v e r e d t h a t t h e c o n v e n t i o n a l f o r m u l a s e s t a b l i s h e d o n t h e b a s i s of d e g r a d a t i o n a n d s y n t h e s i s w e r e u n a b l e t o a c c o u n t f o r a l l t h e p r o p e r t i e s of γ - p y r o n e s . 2 , 6 - D i m e t h y l γ - p y r o n e , f o r i n s t a n c e , does n o t f o r m a p h e n y l h y d r a z o n e ; i t s d o u b l e b o n d s a r e n o t r e d u c e d b y z i n c a n d g l a c i a l acetic a c i d . A b o u t 50 y e a r s ago C o l l i e a n d T i c k l e (3) d i s c o v e r e d t h a t 2 , 6 - d i m e t h y l - y - p y r o n e ( V ) f o r m s w e l l - c r y s t a l l i z e d salts w i t h a c i d s . T h e s t r u c t u r e of these s a l t s i s n o w expressed b y s t r u c t u r a l f o r m u l a V I .

OH I HC" H

3

CH

C - C ^ C -

*CH

HC" -CH

3

H

3

C — ^ C — C H

3

0 VI 2 , 6 - D i m e t h y l p y r o n e i t s e l f has a d i p o l e m o m e n t of 4.6 D , w h i c h v a l u e is i n d e p e n d e n t of t h e t e m p e r a t u r e (7, 10). I f s t r u c t u r e V s h o u l d be assigned t o 2 , 6 - d i m e t h y l γ - p y r o n e , t h e d i p o l e m o m e n t c a l c u l a t e d a c c o r d i n g t o v e c t o r a d d i t i o n w o u l d be 2.2 D ;


WlBAUT—PYRROLES, FURANS,

159

7-PYRONES

f r o m t h e e x p e r i m e n t a l v a l u e of 4.6 D u n i t s i t f o l l o w s t h a t d i m e t h y l - y - p y r o n e i s a m e s o meric f o r m a n d that polar structures have to be t a k e n into account—for instance, t h e limiting structures, V I I .


VII H e r z b e r g a n d W i b a u t (6, 14) h a v e s t u d i e d t h e o z o n i z a t i o n a n d t h e o z o n o l y s i s of γ - p y r o n e a n d i t s m e t h y l h o m o l o g s . A l t h o u g h e x a c t m e a s u r e m e n t s of t h e r e a c t i o n v e l o c i t i e s w e r e n o t p o s s i b l e , i t w a s f o u n d t h a t t h e γ - p y r o n e s r e a c t w i t h ozone a t a f a s t e r r a t e t h a n b e n z e n e o r t o l u e n e . O n t h e o t h e r h a n d , t h e v e l o c i t y of t h e o z o n i z a t i o n of γ-pyrones is m u c h smaller t h a n w o u l d be expected for a c o m p o u n d containing t w o isolated double bonds. A t a t e m p e r a t u r e of —20° C . a n d i n c h l o r o f o r m s o l u t i o n , t h e γ - p y r o n e s t a k e u p 2 molecules of ozone. T h i s m e a n s t h a t o n l y t h e C = C b o n d s r e a c t w i t h ozone, w h i c h w a s also f o u n d w h e n ozone r e a c t e d w i t h o t h e r h e t e r o c y c l i c r i n g s , s u c h as p y r i d i n e , p y r r o l e , a n d f u r a n . T h e o z o n i d e s were d e c o m p o s e d b y r e d u c t i o n w i t h h y d r i o d i c a c i d , w h e r e b y c a r b o n y l c o m p o u n d s a r e f o r m e d , as w e l l as acetic a c i d a n d i n some cases f o r m i c a c i d . B y reacting the carbonyl compounds w i t h p-nitrophenylhydrazine, the corresponding nitrophenylhydrazones or p-nitrophenylosazones are formed. T h e separation of the m i x t u r e of h y d r a z o n e s a n d osazones is c a r r i e d o u t b y a c h r o m a t o g r a p h i c m e t h o d . T h e q u a n t i t a t i v e determination of t h e separated nitrophenylosazones (or hydrazones) is carried out b y means of s p e c t r o g r a p h ^ measurements. T h e o z o n o l y s i s of γ - p y r o n e y i e l d s m e s o x a l d i a l d e h y d e (0.03 m o l e ) , g l y o x y l i c a c i d (0.12 m o l e ) , g l y o x a l (0.12 m o l e ) , a n d f o r m i c a c i d (3.1 m o l e s ) .

0=C—C—-C=0 I II I Η Ο H

+

2HC00H

0=C—COOH I H

+

0=C—C=0 II H H

+ HCOOH

T h e f o r m a t i o n of m e s o x a l d i a l d e h y d e is c h a r a c t e r i s t i c o f t h e c l a s s i c a l s t r u c t u r e ; t h e f o r m a t i o n of g l y o x y l i c a c i d c a n b e m o s t r e a d i l y e x p l a i n e d f r o m a p o l a r s t r u c t u r e . F o r m i c a c i d c a n be f o r m e d f r o m t h e t w o structures. T h e t o t a l y i e l d of fission p r o d u c t s c o r r e s p o n d s t o a b o u t 7 3 % of t h e o r i g i n a l γ - p y r o n e . I n t h e ozonolysis of 2 , 6 - d i m e t h y l - y - p y r o n e t h e following scission products are t o be e x p e c t e d :

HI

0=C—C—C=0 I II I Η Ο H

HI ^ reduction

reduction

+

2CH3COOH

0 = C — COOH H

+

CH COÇ=0 3

+

H


CH C00H 3

A D V A N C E S IN CHEMISTRY SERIES

160


T h e f o l l o w i n g h a v e b e e n i s o l a t e d a n d d e t e r m i n e d q u a n t i t a t i v e l y i n t h e f o r m of t h e p - n i t r o p h e n y l o s a z o n e s : m e s o x a l d i a l d e h y d e (0.03 m o l e ) , g l y o x y l i c a c i d (0.2 m o l e ) , a n d m e t h y l g l y o x a l (0.16 m o l e ) . T h e s e figures r e p r e s e n t m i n i m u m v a l u e s . A c e t i c a c i d (1.5 m o l e s ) a n d f o r m i c a c i d (1.1 m o l e s ) h a v e also b e e n f o u n d . T h e t o t a l y i e l d of s c i s s i o n p r o d u c t s c o r r e s p o n d s t o a b o u t 7 3 % of t h e o r i g i n a l 2 , 6 - d i m e t h y l - y - p y r o n e . M e s o x a l d i a l d e h y d e c a n b e f o r m e d f r o m s t r u c t u r e V ; m e t h y l g l y o x a l is c h a r a c t e r i s t i c of s t r u c t u r e V I I , a n d acetic a c i d c a n be f o r m e d f r o m t h e t w o s t r u c t u r e s . T h e f o r m a t i o n of g l y o x y l i c a c i d c a n b e m o s t r e a d i l y e x p l a i n e d f r o m s t r u c t u r e V I I . H o w ever, t h e f o r m a t i o n f r o m structure V v i a a n a b n o r m a l ozonide rearrangement m u s t n o t be p r e c l u d e d . A l s o , t h e f o r m i c a c i d m a y b e f o r m e d v i a a n a b n o r m a l o z o n i d e r e a r r a n g e m e n t f r o m s t r u c t u r e V . I t f o l l o w s f r o m these e x p e r i m e n t s t h a t 2 , 6 - d i m e t h y l γ - p y r o n e r e a c t s w i t h ozone a c c o r d i n g t o different l i m i t i n g s t r u c t u r e s . I n t h e o z o n o l y s i s of t h e h y d r o c h l o r i d e of 2 , 6 - d i m e t h y l - y - p y r o n e , a g r e a t e r a m o u n t of m e t h y l g l y o x a l — i . e . , 0.22 m o l e — i s f o u n d t h a n i n t h e o z o n o l y s i s of d i m e t h y l p y r o n e itself. I n t h e o z o n o l y s i s of 3 - m e t h y l - y - p y r o n e a n d of t e t r a m e t h y l - y - p y r o n e , scission products are formed w h i c h are characteristic for polar structures. I n t h e ozonolysis of 2 , 3 , 6 - d i m e t h y l - y - p y r o n e p a r t i c u l a r l y i n t e r e s t i n g r e s u l t s w e r e o b t a i n e d . T h e f o l l o w i n g c h a r a c t e r i s t i c scission p r o d u c t s h a v e b e e n i s o l a t e d : p y r u v i c a c i d (0.11 m o l e ) , d i m e t h y l g l y o x a l (0.07 m o l e ) , m e t h y l g l y o x a l (0.08 m o l e ) , g l y o x y l i c a c i d (0.11 m o l e ) , a n d acetic a c i d (1.85 m o l e s ) . T h i s m e a n s t h a t t h e t r i m e t h y l - y - p y r o n e reacts a c c o r d ing to mesomeric polar structures.

CH3COCOOH

+

CH COC=0 3

H

+

CH3COOH

0=C—COOH

+

CH COCOCH 3

3

+

CH COOH 3

H

B e s i d e s these scission p r o d u c t s f o r m i c a c i d (0.6 m o l e ) is f o u n d . T h i s m a y be formed v i a a n a b n o r m a l ozonide rearrangement f r o m the nonpolar s t r u c t u r e . T h e t o t a l y i e l d of fission p r o d u c t s is a b o u t 6 5 % of t h e o r i g i n a l t r i m e t h y l p y r o n e . O z o n i z a t i o n o f T h i o - y - p y r o n e s (6). T h i o - y - p y r o n e a n d 2 , 6 - d i m e t h y l t h i o - y - p y rone r e a c t w i t h ozone a t a s l o w e r r a t e t h a n t h e c o r r e s p o n d i n g γ - p y r o n e s . T h e t h i o - γ p y r o n e s t a k e u p 3 m o l e c u l e s of ozone. N o t o n l y d o t h e C = C b o n d s r e a c t w i t h ozone, b u t o x i d a t i o n of t h e s u l f u r a t o m also occurs. W h e n t h e r e a c t i o n p r o d u c t of t h e o z o n i z a t i o n is t r e a t e d w i t h d i l u t e a l k a l i a n d h y d r o g e n p e r o x i d e , t h e s o l u t i o n c o n t a i n s a l k a l i s u l f a t e i n a n a m o u n t c o r r e s p o n d i n g t o t h e s u l f u r c o n t e n t of t h e t h i o p y r o n e . T h e o z o n o l y s i s of 2 , 6 - d i m e t h y l t h i o - y - p y r o n e y i e l d s m e s o x a l d i a l d e h y d e (0.12 m o l e ) , g l y o x y l i c a c i d (0.04 m o l e ) , m e t h y l g l y o x a l (0.02 m o l e ) , acetic a c i d (0.53 m o l e ) , a n d f o r m i c a c i d (0.82 m o l e ) . T h e s e scission p r o d u c t s h a v e b e e n f o r m e d f r o m m e s o m e r i c structures : Ο"

Ο

o=c—c—c=o

H—C—COOH

4-

CH —C—C=0 3


WlBAUT—PYRROLES, FURANS,

Literature

7-PYRONES

161

Cited

(1) Angeli, Α., Angelico, F., Calvello, E., Atti reale accad. Lincei 11 ( V ) , 16 ( 1 9 0 2 ) ; Gazz. chim. ital. 31 (II), 4 (1901). (2) Bonino, G. B., Manzoni-Ansidei, R., Pratesi, P. Z. phys. Chem. 22B, 21 (1933). (3) Collie, J. N., Tickle, Th., J. Chem. Soc. 75, 710 (1899). (4) Guljé, Α., doctor of science thesis, Municipal University, Amsterdam, 1950. (5) Haayman, P. W., Wibaut, J. P., Rec. trav. chim. 60, 842 (1941). (6) Herzberg, S., doctor of science thesis, Municipal University, Amsterdam, 1955. (7) Hunter, E. C. E., Partington, R., J. Chem. Soc. 1939, 87. (8) Jibben, B. P., doctor of science thesis, Municipal University, Amsterdam, 1954. (9) Lord, R. C., Jr., Miller, F. Α., J. Chem. Phys. 1 0 , 328 (1942). (10) Syrkin, J. K., Wassiliew, V. G., Acta Physicochim. U. R. S. S. 10, 677 (1939). (11) Wibaut, J. P., J. chim. phys. 53, 143 (1956). (12) Wibaut, J. P., Koninkl. Ned. Akad. Wetenschap. Verslag. Gewone Vergader. Afdel. Nat. 59, 93 ( 1 9 5 0 ) .


(13) Wibaut, J. P., Guljé, A. R., Koninkl. Ned. Akad. Wetenschap. Proc. 54B, 330 (1951). (14) Wibaut, J. P., Herzberg, S., Ibid., 56B, 333 (1953). RECEIVED for review M a y 27, 1957. Accepted June 19, 1957.


Ozonolysis of Pyrroles, Furans, and -Pyrones in Connection with

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