OZONE CHEMISTRY AND TECHNOLOGY

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.


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