REACTIONS OF HALOPHOSPHAZENES WITH ALKOXIDES, ARYLOXIDES, AND THIOLATES

Chapter 6 REACTIONS OF H A L O P H O S P H A Z E N E S

WITH

A L K O X I D E S , A R Y L O X I D E S , A N D THIOLATES

A . Introduction T h e r e a c t i o n s t o b e d i s c u s s e d in t h i s c h a p t e r a r e t h e i n t e r a c t i o n s o f h a l o p h o s ­ p h a z e n e s w i t h a w i d e v a r i e t y of a l k o x i d e s , a r y l o x i d e s , o r t h e i r m e r c a p t o a n a ­ l o g u e s t o yield o r g a n o - s u b s t i t u t e d p h o s p h a z e n e s . T h e o v e r a l l g e n e r a l r e a c t i o n scheme can be formulated by the following e q u a t i o n s : 2/i ROH + (NPX )„ -> [NP(OR) ]„ + In HX 2

2

In RSH + (NPX )„ -> [NP(SR) ]„ + In HX 2

2

In practice, the nucleophile R O H or R S H can be varied to include almost any s t a b l e a l c o h o l , p h e n o l , m e r c a p t a n , o r even diol o r d i t h i o l . M a n y r e a c t i o n s use s o d i u m a l c o h o l a t e s , p h e n o l a t e s , o r s o d i u m t h i o l a t e s i n s t e a d of t h e free a l c o h o l o r p h e n o l . I n s u c h cases, as s h o w n b e l o w , s o d i u m c h l o r i d e is p r e c i p i ­ t a t e d as a r e a c t i o n p r o d u c t . W h e n t h e free a l c o h o l , p h e n o l , o r t h i o l is u s e d , a In RONa + (NPX )„ -> [NP(OR) ]„ + In NaX 2

2

b a s e s u c h a s t r i e t h y l a m i n e o r s o d i u m c a r b o n a t e is u s u a l l y e m p l o y e d t o r e m o v e t h e h y d r o h a l i d e . T h e h a l o g e n a t o m , X , c a n b e fluorine, c h l o r i n e , o r b r o m i n e , a n d t h e r e a p p e a r t o b e n o limits t o t h e d e g r e e of p o l y m e r i z a t i o n , n, of t h e h a l o p h o s p h a z e n e u s e d . A r e a c t i o n s o l v e n t is n e a r l y a l w a y s e m p l o y e d . It s h o u l d b e n o t e d t h a t n u c l e o p h i l i c s u b s t i t u t i o n r e a c t i o n s of t h i s t y p e p r o ­ v i d e o n e of t h e easiest r o u t e s t o t h e s y n t h e s i s of o r g a n o p h o s p h a z e n e s . T h e r e 150

B .

E X P E R I M E N T A L

151

C O N S I D E R A T I O N S

a c t i o n s a r e often c l e a n p r o c e s s e s w i t h few side r e a c t i o n s , a n d t h e p r o d u c t s u s u a l l y a r e s t a b l e s o l i d s w h i c h a r e easily purified a n d c h a r a c t e r i z e d .

Un­

d o u b t e d l y , t h i s p r o v i d e s o n e r e a s o n for t h e i m p r e s s i v e n u m b e r o f a l k o x y , a r y l o x y , a n d o r g a n o t h i o p h o s p h a z e n e s r e p o r t e d in t h e l i t e r a t u r e . H o w e v e r , a n o t h e r c o n t r i b u t i n g f a c t o r is c o n n e c t e d w i t h t h e fact t h a t c e r t a i n a r y l o x y a n d fluoroalkoxycyclophosphazenes show very high t h e r m a l a n d hydrolytic stabil­ ities a n d t h i s h a s led t o a d e t a i l e d i n v e s t i g a t i o n o f s u c h d e r i v a t i v e s for a p p l i c a ­ t i o n s in t h e h i g h - t e m p e r a t u r e m a t e r i a l s a n d p o l y m e r

fields.

B . Experimental Considerations T h e following three examples illustrate the techniques t h a t are c o m m o n l y used to prepare aryloxy- a n d alkoxycyclophosphazenes.

1.

S Y N T H E S I S

O F

H E X A P H E N O X Y C Y C L O T R I P H O S P H A Z E N E

1

-

3

T h e o v e r a l l r e a c t i o n s c h e m e is CL

Ρ

^Xl

CkJ C r

||^Cl+6PhONa P

V I

P

\ i

PhCX >

Ρ

PhO^ I

.OPh II^OPh+^NaCl

P h O ^ N ^ O P h II

A solution of s o d i u m p h e n o x i d e in t e t r a h y d r o f u r a n

( 5 0 0 m l ) is p r e p a r e d

f r o m p h e n o l (84.6 g, 0.91 m o l e ) a n d s o d i u m (20.7 g, 0.9 m o l e ) in a n a t m o s p h e r e o f d r y a r g o n . T o t h i s s t i r r e d s o l u t i o n is a d d e d a s o l u t i o n o f h e x a c h l o r o c y c l o t r i p h o s p h a z e n e (I) (52.2 g, 0.15 m o l e ) in t e t r a h y d r o f u r a n ( 5 0 0 m l ) . T h e m i x ­ t u r e is t h e n b o i l e d a t reflux for 8 h o u r s a n d t h e n a l l o w e d t o s t a n d a t 2 5 ° C f o r 56 h o u r s . F i l t r a t i o n of t h e m i x t u r e yields s o d i u m c h l o r i d e (55 g, 1 0 0 % b a s e d o n I ) , a n d e v a p o r a t i o n of t h e filtrate, f o l l o w e d b y p r e c i p i t a t i o n o f t h e r e s i d u e f r o m a c e t o n e i n t o a l a r g e excess of w a t e r , yields 88.5 g o f a w h i t e s o l i d . T h i s is t h e n r e c r y s t a l l i z e d t w i c e f r o m ^ - h e p t a n e a n d w - h e p t a n e - b e n z e n e m i x t u r e s , a n d is e x h a u s t i v e l y d r i e d i n a v a c u u m o v e n a t 6 0 ° C t o yield h e x a p h e n o x y ­ c y c l o t r i p h o s p h a z e n e ( I I ) (83.2 g, 9 3 % ) , m . p . 1 1 2 ° - 1 1 2 . 5 ° C . C h a r a c t e r i z a t i o n h a s b e e n effected b y C , H , N , a n d Ρ m i c r o a n a l y s i s , b y i n f r a r e d s p e c t r o s c o p y , a n d b y m a s s s p e c t r o m e t r y . C o m p o u n d I I is, in fact, a n e x c e l l e n t h i g h - t e m p e r a ­ t u r e m a s s s p e c t r o m e t r y s t a n d a r d b e c a u s e o f its h i g h m a s s ( M W = 693) a n d its h i g h s t a b i l i t y . T h e c o m p o u n d is a w h i t e , c r y s t a l l i n e s o l i d w h i c h is s o l u b l e in b e n z e n e o r t e t r a h y d r o f u r a n . It is h i g h l y r e s i s t a n t t o h y d r o l y t i c d e g r a d a t i o n .

152

6.

2.

R E A C T I O N S

S Y N T H E S I S

O F

W I T H

A L K O X I D E S ,

A R Y L O X I D E S ,

A

N

D

T H I O L A T E S

H E X A K I S ( T R I F L U O R O E T H O X Y ) C Y C L O T R I P H O S P H A Z E N E

4

T h e following equation illustrates the reaction scheme: Cl^

CF CH 0^

^Cl

3

N ^ N CkJ IUC1 + 6 C F C H O N a 3

C K

P

2

2

^OCH CH 2

3

N ^ N >

^ N ^ c i

CF CH 0\ I 3

II^OCH CF

2

2

CF CH 0^ 3

I

^OCH CH

2

2

+ 6 NaCl

3

3

III

S o d i u m ( 2 3 . 8 g, 1 . 0 5 m o l e s ) is a d d e d t o a c o o l e d s o l u t i o n of t r i f l u o r o e t h a n o l ( 1 0 3 . 3 g, 1 . 0 3 3 m o l e s ) in a n h y d r o u s e t h e r ( 4 0 0 m l ) . T o t h i s s t i r r e d , c o o l e d solu­ t i o n is a d d e d d r o p w i s e a s o l u t i o n of h e x a c h l o r o c y c l o t r i p h o s p h a z e n e (I) ( 5 9 . 6 g, 0 . 1 7 2 m o l e ) in a n h y d r o u s e t h e r ( 4 0 0 m l ) . T h e m i x t u r e is t h e n s t i r r e d a t 2 5 ° C for 8 h o u r s , a n d t h e w h i t e p r e c i p i t a t e of s o d i u m c h l o r i d e is filtered off. T h e filtrate is w a s h e d t h o r o u g h l y w i t h w a t e r t o r e m o v e excess s o d i u m t r i f l u o r o e t h o x i d e a n d t h e e t h e r e a l l a y e r is distilled t o r e m o v e excess tri­ f l u o r o e t h a n o l a n d s o l v e n t . T h e r e s i d u e is a v i s c o u s oil w h i c h solidifies o n c o o l i n g . T h i s is s u b l i m e d a t 7 0 ° C / 2 m m t o give h e x a k i s ( t r i f l u o r o e t h o x y ) c y c l o t r i p h o s p h a z e n e ( I I I ) ( 9 6 . 0 g, 7 7 % ) , m . p . 4 9 ° C . I t s h o u l d b e n o t e d t h a t o m i s s i o n of t h e w a s h i n g s t e p for r e m o v a l of s o d i u m t r i f l u o r o e t h o x i d e i n t r o ­ d u c e s t h e d a n g e r of a n e x p l o s i o n d u r i n g t h e s u b s e q u e n t d i s t i l l a t i o n a n d s u b l i ­ m a t i o n s t e p s . T h e c y c l o p h o s p h a z e n e I I I is a w h i t e , w a x y , c r y s t a l l i n e s u b ­ s t a n c e , r e a d i l y s o l u b l e in m a n y o r g a n i c s o l v e n t s . I t h a s a n u n u s u a l l y h i g h t h e r m a l stability a n d is q u i t e s t a b l e t o a t m o s p h e r i c m o i s t u r e o r n e u t r a l a q u e o u s solvents. 7

3.

S Y N T H E S I S

O F

T R I S ( O P H E N Y L E N E D I O X Y ) C Y C L O T R I P H O S P H A Z E N E

T h e o v e r a l l r e a c t i o n s c h e m e is a s follows : ^Cl CI J

t

,C1 + 3 [OT

+6Et3N

Cl

( Ç ^ / ^ vlQ)

+ 6Et3NHC1

C .

S C O P E

O F

T H E

153

R E A C T I O N

A s o l u t i o n o f c a t e c h o l (95 g, 0 . 8 6 4 m o l e ) a n d t r i e t h y l a m i n e (175 g, 1.728 m o l e s ) in t e t r a h y d r o f u r a n (500 m l ) is a d d e d d r o p w i s e t o a s t i r r e d s o l u t i o n o f h e x a c h l o r o c y c l o t r i p h o s p h a z e n e (100 g, 0.288 m o l e ) in t e t r a h y d r o f u r a n ( 1 0 0 0 m l ) a t 3 0 ° C . T h e m i x t u r e is t h e n b o i l e d a t reflux for 2 h o u r s a n d s t i r r e d a t 3 0 ° C f o r 4 8 h o u r s . T h e w h i t e p r e c i p i t a t e is filtered off, d r i e d (350 g ) , a n d t h e n washed thoroughly with water to remove triethylamine hydrochloride. T h e p o w d e r y r e s i d u e is t h e n e x t r a c t e d w i t h b o i l i n g b e n z e n e a n d t h e e x t r a c t s a r e c o o l e d t o yield w h i t e c r y s t a l s o f t r i s ( o - p h e n y l e n e d i o x y ) c y c l o t r i p h o s p h a z e n e (74 g, 5 0 % yield). S u b s e q u e n t r e c r y s t a l l i z a t i o n s f r o m b e n z e n e f o l l o w e d b y s u b l i m a t i o n a t 2 3 0 ° C / 0 . 0 5 m m give m a t e r i a l w i t h a m e l t i n g p o i n t o f 2 4 4 ° 245°C. This c o m p o u n d readily forms channel clathrates when b r o u g h t into c o n t a c t w i t h o r g a n i c l i q u i d s (see C h a p t e r 11). 4.

V A R I A T I O N S

I N

T H E

E X P E R I M E N T A L

T E C H N I Q U E

T w o of t h e a b o v e e x a m p l e s u s e s o d i u m salts o f p h e n o l o r a n a l c o h o l a s t h e nucleophilic reagent, b u t a n u m b e r of modifications t o this process are p o s ­ sible. F o r e x a m p l e , t h e r e a c t i o n o f s o d i u m h y d r i d e w i t h a n a l c o h o l h a s b e e n u s e d for p r e p a r a t i o n o f t h e s o d i u m a l k o x i d e . * D i r e c t r e a c t i o n o f a s o l u t i o n o f a n a l c o h o l o r p h e n o l w i t h t h e h a l o p h o s p h a z e n e in t h e p r e s e n c e o f a s u s p e n s i o n of a n h y d r o u s s o d i u m c a r b o n a t e h a s a l s o p r o v e d t o b e a c o n v e n i e n t t e c h ­ nique. 6

8 ,

7

9

6 ROH + (NPC1 ) + 6 N a C 0 2

3

2

3

-* [NH(OR) ] + 6 NaCl + 6 N a H C 0 2

3

3

S o d i u m h y d r o x i d e pellets c a n b e u s e d w h e n a p h e n o l is a l l o w e d t o r e a c t w i t h a h a l o c y c l o p h o s p h a z e n e in a h i g h - b o i l i n g h y d r o p h o b i c s o l v e n t , s u c h as x y l e n e . Finally, a tertiary organic a m i n e , such as triethylamine or p y r i d i n e , m a y b e e m p l o y e d a s a h y d r o g e n c h l o r i d e a c c e p t o r , a s in t h e equation · · 1 0 - 1 2

2 , 8

9

1

3

-

1

9

6 ROH + (NPC1 ) + 6 Et N 2

3

3

[NP(OR) ] + 6 E t N H C l 2

3

3

N e a r l y a n y u n r e a c t i v e o r g a n i c s o l v e n t m a y b e used for t h e s e i n t e r a c t i o n s , b u t aliphatic ethers, such as diethyl ether or tetrahydrofuran, are particularly use­ ful. A l t h o u g h it is u s u a l t o p e r f o r m s u b s t i t u t i o n r e a c t i o n s of t h i s t y p e in s t a n ­ d a r d g r o u n d glass e q u i p m e n t w i t h e x c l u s i o n o f m o i s t u r e b y d r y i n g t r a p s , it is often n e c e s s a r y t o u s e a glass h i g h - v a c u u m s y s t e m w h e n p y r i d i n e is u s e d a s a solvent. 15

C . S c o p e of the Reaction A w i d e v a r i e t y of a l k o x y - , a r y l o x y - , a l k y l t h i o - , a n d a r y l t h i o p h o s p h a z e n e s can be prepared by the techniques described above. F o r example, the following

154

6.

R E A C T I O N S

W I T H

A L K O X I D E S ,

A R Y L O X I D E S ,

A

N

D

T H I O L A T E S

alkoxy groups have been attached to the phosphazene skeleton: CH 0—, > * ~ C H 0—, > · ' > > w-C H 0—, > z-C H 0—, > n-C H 0—, · ' ' PhCH 0— CH =CHCH 0— Fluoroalkoxy groups can be substituted at p h o s p h o r u s with particular ease, a n d the following are some of t h e substituents t h a t have been u s e d : C F C H 0 — , ' ' C F C H 0 — 4-6,24,25,27,29-31.33 C F C H 0 - , - ' > ' ' ' ' H(CF ) 2

1 8

2

0

2

5

2

2

4

1 8

1 3

2

3

1 9

1 8

2 0

2 4

2

2 5

5

3

2 2

2 0

9

2

7

3

2

2

2

2

4 - 6

3

4

2

5

2

3

CH 0—, - ' - ' H(CF ) CH 0—, 5

6

2 9

3 5

3 6

2

4

2

2 9

2

7

'

3 0

6

2 4

2 4

2 5 t

3 5

1 0

2 7

2 9 - 3 1

3 3

3 4

2

2

6

2 7

2

2

3 6

2 9

2

2 7 - 3 2

2

2 5

H(CF ) CH O—, « H(CF ) CH 0—, ' ' « H(CF ) CH 0—, ' and (CF ) CHO—, 5

2

2 0

7

2 6

8

2

2 9

3 0

3 3

2

3 5

6

>

3 5

2 7

2

3

2

Hexachlorocyclotriphosphazene also reacts with a n e q u i m o l a r a m o u n t of a c e t o x i n e i n t h e p r e s e n c e o f t r i e t h y l a m i n e t o yield p e n t a c h l o r o - 7 V - i s o p r o p y l idenoximinocyclotriphosphazene. T h e p r o d u c t is a solid, m . p . 91 ° C . 3 6 a

CL -HCl

(NPC1 ) + H O N = C M e 2

3

>

2

.ON=CMe

N

2

N

I || C1 P^ /PC1 2

N

2

Aryloxy substitution h a s also been studied in detail a n d a wide range of phenols have been used as nucleophiles. T h e substituents include C H 0—,1-3.24,25,35,37-43 _ eC H 0—, /7-MeC H 0—, m-CF C H 0-, ' m-FC H 0-, m-ClC H 0-, /?-ClC H 0-, * m-CF OC H 0-, o,m,and/?-N0 C H 0-, ' « · V-BrC H 0-, >MeOC H 0-. ' 2 9

6

w

5

1 0

2 9

M

6

4

6

6

6

1 1

7

2

4

2 9

4

4

3 7

4

2 9

6

2 7

4

6

3 7

3

2 9

4

4

3

3

4

6

6

3 7

6

2

4

6

3 7

4

Fewer m e r c a p t o derivatives have been reported t h a n alkoxy o r aryloxy p h o s p h a z e n e s , b u t those which a r e k n o w n cover a wide range of alkyl- a n d a r y l t h i o g r o u p s , a s d e m o n s t r a t e d b y t h e f o l l o w i n g list: C H S - , ' C H S-, ' ' iso-C H S-, ' C H S-, ' ' iso-C H S-, ' t-C H S-, C H S-, ' C H S-, C H S-, C H S-, PhCH S-, ' ' C H S-. ' ' - ' 1 3

2

1 9

3

4 5

4 6

7

3

4 6

4 5

9

6

1 3

6

1 9

1 9

7

1 9

8

3 7

4 5

1 9

4

4 6

U

4 5 , 4 6

1 7

4 5

4 5

9

4

1 9

1 2

2 5

4 5 t

4 6

4 6

9

4

1 9

1 6

l 9 f

5

4 6

1 9

3 3

4 5

4 6

2

4 6

5

T h e a b o v e syntheses lead t o t h e formation of derivatives which possess t w o i n d e p e n d e n t s u b s t i t u e n t s a t e a c h p h o s p h o r u s . H o w e v e r , it is a l s o p o s s i b l e t o p e r f o r m s u b s t i t u t i o n r e a c t i o n s u s i n g d i o l s o r d i t h i o l s in s u c h a w a y t h a t cyclized units are formed at t h e p h o s p h o r u s a t o m s , as shown in t h e following equation ' : 7 , 8

ci

x

1 4 - 1 7

o(

^Cl

C l ^ N ^ C l

H

°

< 0 -

P

- N -

P

-

0

/

R

T h e s e d e r i v a t i v e s a r e especially i n t e r e s t i n g b e c a u s e o f t h e i r b e a r i n g o n s u b s t i ­ t u t i o n m e c h a n i s m s . T h e y a r e c o n s i d e r e d in m o r e d e t a i l in S e c t i o n E 5 o f t h i s chapter. F i n a l l y , i t is p o s s i b l e t o c a r r y o u t s u b s t i t u t i o n r e a c t i o n s b e t w e e n a l k o x i d e s , a r y l o x i d e s , o r m e r c a p t a n s a n d fluoro o r b r o m o p h o s p h a z e n e s . T h e r e a c t i o n s

D .

G E N E R A L

P R O P E R T I E S

A

N

D

U S E S

O F

155

A M I N O P H O S P H A Z E N E S

a r e n o t r e s t r i c t e d t o cyclic t r i m e r s o r t e t r a m e r s . T h u s , a l t h o u g h m o s t o f t h e f o r e g o i n g e x a m p l e s h a v e r e f e r r e d t o r e a c t i o n s i n v o l v i n g cyclic t r i m e r s o r tetramers, ( N P X ) 3 2

ring-sized (NPX ) . 2

8

O R 4

, m o n o p h o s p h a z e n e s react similarly,

cyclic c o m p o u n d s 2 3

Even

very

such

high

as ( N P X ) , 2

molecular

5

43

(NPX ) , 2

weight

6

as d o m e d i u m (NPX ) , 2

and

7

chlorophosphazenes*

(NPCl ) ooo> readily u n d e r g o complete replacement of halogen w h e n treated 2

i5)

with methoxide, ethoxide, trifluoroethoxide, phenoxide, or other nucleophiles.

related

O n l y for t h e c y c l i z a t i o n p r o c e s s e s w i t h d i o l s a r e t h e r e

2 4 , 2 5

m a r k e d m e c h a n i s t i c differences b e t w e e n t h e s u b s t i t u t i o n of cyclic t r i m e r s a n d the higher h o m o l o g u e s .

7 , 2 5

D . General Properties of A l k o x y - , A r y l o x y - , Alkylthio-, and Arylthiophosphazenes In general, phosphazenes of structure [ N P ( O R ) ] „ or [NP(SR) ]„ are a m o n g the m o s t stable p h o s p h o r u s - n i t r o g e n derivatives k n o w n . H o w e v e r , the stabil­ ity t o h e a t a n d h y d r o l y s i s d e p e n d s o n t h e n a t u r e of t h e s u b s t i t u e n t O R o r S R a n d , a l t h o u g h t h i s subject is c o n s i d e r e d in m o r e d e t a i l in C h a p t e r s 5 a n d 12, some generalizations are a p p r o p r i a t e at this point. 2

2

M o s t fully s u b s t i t u t e d a l k o x y - , a r y l o x y - , a l k y l t h i o - , a n d a r y l t h i o ­ p h o s p h a z e n e s a r e s t a b l e , w h i t e c r y s t a l l i n e solids w h i c h a r e r e a d i l y s o l u b l e in o r g a n i c m e d i a . H o w e v e r , s o m e of t h e a l k o x y - o r a l k y l t h i o - s u b s t i t u t e d species such as [ N P ( O E t ) ] , [ N P ( O B u " ) ] , [NP(OMe) ] , or [ N P ( S B r " ) ] a r e c o l o r l e s s l i q u i d s a t r o o m t e m p e r a t u r e . A c o m p r e h e n s i v e s t u d y of t h e effects of mixed substitution a r o u n d the p h o s p h a z e n e ring by K o b e r , Lederle, a n d Ottmann h a s s h o w n t h a t s u b s t i t u t i o n of t h e t r i m e r i c o r t e t r a m e r i c r i n g by both fluoroalkoxy and aryloxy groups lowers the melting point to such a d e g r e e t h a t t h e m a t e r i a l s h a v e useful p r o p e r t i e s a s fire-resistant, l o w - t e m p e r a ­ t u r e h y d r a u l i c fluids o r l u b r i c a n t s . A l k o x y - o r a r y l o x y - s u b s t i t u t e d h i g h p o l y ­ m e r s a r e e l a s t o m e r s o r flexible-film-forming m a t e r i a l s . I n s o m e c a s e s , t h e m a t e r i a l s d o n o t lose t h e i r flexible p r o p e r t i e s u n t i l c o o l e d b e l o w — 8 0 ° C (see C h a p t e r 16). 2

2

3

2 3

2

3 o r 4

2

4 6

5 o r 7

2

3

2 9 , 3 0 , 3 5

2 4 , 2 5 , 2 7

A s m e n t i o n e d e a r l i e r , h e x a p h e n o x y c y c l o t r i p h o s p h a z e n e , [ N P ( O P h ) ] , is e x c e e d i n g l y s t a b l e t o h e a t a n d h y d r o l y s i s , a n d t h i s b e h a v i o r is c h a r a c t e r i s t i c of m a n y aryloxyphosphazenes. A l k o x y p h o s p h a z e n e s (except fluoroalkoxy derivatives) rearrange to cyclophosphazanes when heated, a n d they are also m o r e sensitive t o h y d r o l y s i s t h a n a r y l o x y c o m p o u n d s . M o s t a l k o x y d e r i v a ­ tives a r e q u i t e s t a b l e w h e n s t o r e d a t r o o m t e m p e r a t u r e in a m o i s t a t m o s p h e r e , and fluoroalkoxyphosphazenes are exceptionally stable to heat a n d neutral h y d r o l y s i s . W h e n five-membered cyclized g r o u p s a r e a t t a c h e d t o p h o s p h o r u s , h y d r o l y s i s t a k e s p l a c e r e a d i l y in s o l u t i o n , b u t m a r k e d h y d r o l y t i c s t a b i l i t y is 2

4 7

3

156

6.

R E A C T I O N S

W I T H

A L K O X I D E S ,

A R Y L O X I D E S ,

A

N

D

T H I O L A T E S

d i s p l a y e d b y species w h i c h c o n t a i n six- o r s e v e n - m e m b e r e d r i n g s y s t e m s a t t a c h e d t o p h o s p h o r u s in a s p i r o a r r a n g e m e n t . T h e t r i m e r , [ N P ( O M e ) ] , is s o l u b l e in w a t e r . 7

2

3

A l k o x y p h o s p h a z e n e s h a v e b e e n d e v e l o p e d a s flame r e t a r d a n t s for r a y o n . Godfrey and Schappel reported that w-propoxycyclophosphazenes and trimert e t r a m e r - l i n e a r o l i g o m e r m i x t u r e s a r e p a r t i c u l a r l y effective flame r e t a r d a n t s , especially w h e n t h e p h o s p h a z e n e is e n c a p s u l a t e d in d i s c r e t e cavities w i t h i n e a c h fiber. A p p r o x i m a t e l y 15 wt. % o f t h e p h o s p h a z e n e is effective. 47a

E . Influence of the Nucleophile on Substitution T h e a b o v e i n f o r m a t i o n i l l u s t r a t e s t h e w i d e versatility o f t h i s g e n e r a l s u b s t i ­ t u t i o n r e a c t i o n b u t it i n d i c a t e s v e r y little a b o u t t h e r e a c t i o n m e c h a n i s m . T o o b t a i n a d e e p e r i n s i g h t i n t o t h i s p r o c e s s it is n e c e s s a r y t o e x a m i n e t h e influence of a n u m b e r of f a c t o r s o n t h e s u b s t i t u t i o n . O n e of t h e m o s t i m p o r t a n t of t h e s e is t h e influence of t h e n u c l e o p h i l e , R O o r R S . Useful i n f o r m a t i o n a b o u t t h e r o l e of t h e n u c l e o p h i l e c a n b e o b t a i n e d b y a n e x a m i n a t i o n of i n s t a n c e s of incomplete substitution, evidence concerning the p a t t e r n of substitution, the effects o f c o m p e t i t i o n r e a c t i o n s , t h e influence of o t h e r s u b s t i t u e n t s o n t h e r e ­ a c t i o n , a n d t h e r e s u l t s of c y c l i z a t i o n p r o c e s s e s a t p h o s p h o r u s . E a c h of t h e s e f a c t o r s will b e c o n s i d e r e d i n t u r n . e

1.

D E G R E E

O F

H A L O G E N

0

R E P L A C E M E N T

R e p l a c e m e n t of c h l o r i n e in c h l o r o c y c l o p h o s p h a z e n e s b y u n b r a n c h e d a l k o x y groups takes place m o r e readily t h a n replacement by phenoxy or branched a l k o x y g r o u p s . T h u s , c o m p l e t e r e p l a c e m e n t of c h l o r i n e in ( N P C 1 ) 3 , ( N P C 1 ) , or ( N P C l ) „ o o o occurs readily with methoxide, ethoxide, or «-propoxide, b u t m o r e drastic conditions are required t o achieve complete substitution with isopropoxide or phenoxide i o n s . ~ > I n fact, p a r t l y s u b s t i t u t e d phenoxychlorocyclophosphazenes, such as N P C l ( O P h ) , N P C l ( O P h ) , N P C l ( O P h ) , N P C l ( O P h ) , a n d N P C l ( O P h ) can be isolated from re­ actions which have n o t been forced to c o m p l e t i o n . T h e s a m e is t r u e for reactions between (NPC1 ) and the /7-bromophenoxide i o n . 2

2

4

2

2

1 5 j

2

1 , 2 > 2 5 , 3 7

4 2

4 8

3

3

3

3

3

3

3

4

2

3

3

3

5

3

3

2

4

5

2 , 3 8 - 4 1

3 9

2

3

However, partially substituted alkoxy derivatives, such as N P C l ( O C H C F ) , N P C l ( O C H C F ) , N P C l ( O C H C F ) , or N P C l ( O C H C F ) c a n b e p r e p a r e d if t h e r e a g e n t s t o i c h i o m e t r y is c o r r e c t a n d if t h e r e a c t i o n c o n ­ d i t i o n s a r e sufficiently m i l d . Similarly, b u t o x y d e r i v a t i v e s o f t h e t y p e s N P C l ( O B u ) , N P C l ( O B u ) , a n d N P C l ( O B u ) can be isolated from the reaction of ( N P C 1 ) with butoxide ion at 2 0 ° C . A s i m i l a r p a t t e r n c a n b e d i s c e r n e d for t h e t h i o a l k y l a n d t h i o a r y l d é r i v a 3

3

5

3

3

2

2

3

4

3

3

3

2

3

3

3

4 , 2 8

3

3

5

3

3

4

2

3

3

3

3

2 1

2

3

3

3

2

2

3

4

E .

I N F L U E N C E

O F

T H E

N U C L E O P H I L E

O

N

157

S U B S T I T U T I O N

t i v e s . T h u s , in b o i l i n g b e n z e n e , t h e r e a c t i o n o f ( N P C 1 ) w i t h E t S , « - P r S , H - B U S , o r P h C H S gives h e x a t h i o a l k y l d e r i v a t i v e s , b u t b r a n c h e d r e a g e n t s , s u c h a s f - P r S o r c y c l o h e x y l - S yield o n l y t e t r a s u b s t i t u t e d p r o d u c t s . D r a s t i c r e a c t i o n c o n d i t i o n s a r e r e q u i r e d for t h e p r e p a r a t i o n of [ N P ( S P h ) ] in b o i l i n g b e n z e n e . I t s h o u l d b e n o t e d t h a t t h e r e a c t i o n s o l v e n t e x e r t s a p r o f o u n d effect o n t h e r e a c t i v i t y o f t h i o a l k y l o r t h i o a r y l n u c l e o p h i l e s in t h i s r e a c t i o n . T h e a b o v e r e s u l t s s u g g e s t t h a t t h e steric d i m e n s i o n s of t h e n u c l e o p h i l e in­ fluence t h e d e g r e e o f h a l o g e n r e p l a c e m e n t , w i t h b u l k y side g r o u p s b e i n g t h e m o s t difficult t o i n t r o d u c e a s s u b s t i t u e n t s o n t h e p h o s p h a z e n e s k e l e t o n . 4 6

0

2

0

e

3

0

2

0

0

2

2.

P A T T E R N

O F

H A L O G E N

3

R E P L A C E M E N T

S u b s t i t u t i o n a t p h o s p h o r u s in c y c l o - o r p o l y p h o s p h a z e n e s c a n t a k e p l a c e either by geminal or non-geminal routes (IV or V). F u r t h e r m o r e , non-geminal

Non-geminal s u b s t i t u t i o n c a n r e s u l t in t h e f o r m a t i o n o f b o t h cis ( V I ) a n d trans ( V I I ) i s o m e r s . OR RO/

X

VI

\

X RO

VII

158

6.

REACTIONS WITH ALKOXIDES, ARYLOXIDES, A N D THIOLATES

E x a m i n a t i o n o f t h e p a t t e r n a n d s t e r e o c h e m i s t r y of a l k o x i d e , a r y l o x i d e , o r mercaptide

substitution

has

helped

t o clarify

the

mechanisms

of

these

reactions. T h e r e a c t i o n of h e x a c h l o r o c y c l o t r i p h o s p h a z e n e , ( N P C 1 ) 3 , w i t h 2

sodium

p h e n o x i d e o r s o d i u m / ? - b r o m o p h e n o x i d e p r o c e e d s n o n - g e m i n a l l y in b e n z e n e , acetone, or t e t r a h y d r o f u r a n . ' * 3 8

3 9

Γη a c e t o n e s o l u t i o n , t h e cis s u b s t i t u t i o n

4 0

p r e d o m i n a t e s slightly o v e r t h e trans?* T h e s e c o n c l u s i o n s a r e b a s e d o n t h e w o r k of B e z m a n

McBee,

4 1

Shaw,

3 8

3 9 , 3 9 a

a n d t h e i r c o - w o r k e r s , w h o identified t h e

n o n - g e m i n a l p r o d u c t s b y c o n v e r s i o n of t h e p h e n o x y c h l o r o p r o d u c t s phenoxy dimethylamino * 3 9

4 1

or phenoxy amino derivatives,

38

followed

to by

structural identification by melting p o i n t s a n d p r o t o n N M R spectra. T h e re­ action schemes studied are Cl^

/XI

Cl\

N^ "N Cl

> ^

N

^P

/

O

P

Me N

h

2

N ^ N

P

K O p h

HNMe

> ^ /PC Cr NT ^ O P h

K C 1

Cl

^OPh

x

2

PhO^I P\ Me N^

h c 1

ll/NMe

2

^OPh

2

(Ref. 41)

CL

.OPh

H N.

NaOPh

X

X

PhCK

.OPh

2

Ρ N

^N^

NH

p

3

^OPh

PhO^

cis and trans

OPh

cis and trans (Ref. 38)

T h e partly substituted phenoxy chloro derivatives can be separated by column and thin-layer c h r o m a t o g r a p h y .

3 8 , 3 9

It w a s a l s o s u g g e s t e d t h a t i n v e r s i o n of

configuration probably does not occur during phenoxide a t t a c k .

3 8

T h e s i t u a t i o n is less c l e a r w h e n a l k o x y n u c l e o p h i l e s a r e e m p l o y e d .

For

example, sodium trifluoroethoxide reacts with hexachlorocyclotriphosphazene t o yield N P 3 C l 3 ( O C H C F 3 ) 3 . 3

2

2 8

This material can then be a m m o n o l y z e d and

t h r e e i s o m e r s of t h e c o m p o u n d N P 3 ( N H ) 3 ( O C H C F 3 ) 3

2

2

3

are isolated. T w o of

t h e s e species a r e p r o b a b l y n o n - g e m i n a l cis a n d trans i s o m e r s a n d t h e t h i r d is a geminal f o r m .

2 8

Sodium butylate reacts with hexachlorocyclotriphosphazene

t o give a d i b u t o x y d e r i v a t i v e , N P C l ( O B u ) . 3

3

4

It was suggested b u t n o t

2 1

2

p r o v e d t h a t t h i s c o m p o u n d is n o n - g e m i n a l . By c o n t r a s t , it h a s b e e n s h o w n t h a t t h e i n t e r a c t i o n of h e x a c h l o r o c y c l o t r i ­ p h o s p h a z e n e with ethylthiolate or phenylthiolate ions leads to geminal substi­ tution.

4 6

This information was derived from

3 1

P and H l

N M R s p e c t r a of

N P C l ( S E t ) , N P C l ( S P h ) , a n d N P C l ( S E t ) . H o w e v e r , it s h o u l d b e 3

3

4

2

3

3

4

2

3

3

2

4

E.

159

I N F L U E N C E OF THE N U C L E O P H I L E O N S U B S T I T U T I O N

noted that a n u m b e r of these reactions are h e t e r o g e n e o u s a n d m a r k e d l y s o l v e n t - d e p e n d e n t , a n d s o m e c a r e m u s t b e exercised w h e n i n t e r p r e t i n g t h e data. 3 . E F F E C T OF O T H E R S U B S T I T U E N T S

A d d i t i o n a l i n f o r m a t i o n a b o u t the m e c h a n i s m of chlorine replacement c a n b e o b t a i n e d f r o m t h e influence o f s u b s t i t u e n t s a l r e a d y p r e s e n t o n t h e r i n g . F o r e x a m p l e , it w a s r e p o r t e d b y M c B e e , O k u h a r a , a n d M o r t o n t h a t 1,1-diphenyl3,3,5,5-tetrachlorocyclotriphosphazene (VIII) was phenoxylated and amm o n o l y z e d t o I X . T h e cis a n d trans n o n - g e m i n a l i s o m e r s o f I X w e r e i s o l a t e d in 3 8

Ph/

^Ph

Ph^ / P h

N ^ N

N a 0

CL I C

1

/

LCI

r> P

r>

^ / VIII

Ph

XT

„ >

—NaCl

P

N

C

1

N

^

PhO. I c

Ph/

N

NH

ll/OPh

p.

r

V

ρ

/Ph

N ^ N

3

ll/OPh

> PhO/ I

S

H

Ρ/

2

cis and trans

N -

^ρ

^ N ^ IX cis and trans F

/

P

N

H

2

3 9 . 4 % a n d 3 2 . 2 % yields, r e s p e c t i v e l y , a n d a g e m i n a l i s o m e r w a s a l s o r e ­ c o v e r e d in 4 . 5 % yield. S i m i l a r r e s u l t s w e r e r e p o r t e d b y F o r d , B a r r , D i c k s o n , and Bezman b u t , in this c a s e , t h e b i s d i m e t h y l a m i n o a n d m e t h y l a m i n o d e r i v a t i v e s w e r e identified b y Ρ a n d Η N M R s p e c t r o s c o p y . T h u s , p h e n o x y l a t i o n is still p r e d o m i n a n t l y n o n - g e m i n a l in t h e p r e s e n c e o f a d j a c e n t g e m i n a l phenyl units. 4 2

3 1

1

F i t z s i m m o n s , Hewlett, Hills, a n d S h a w have reported the results of alcoholysis r e a c t i o n s of t h e g e m i n a l d e r i v a t i v e s , N3P3PIÎ2CI4 ( V I I I ) a n d N3P3PI14CI2 u s i n g a l k o x i d e i o n in a l c o h o l s o l v e n t s . M e t h o x i d e , e t h o x i d e , / z - p r o p o x i d e , a n d isopropoxide ions were used as nucleophiles. T h e results suggested t h a t t h e ease of s u b s t i t u t i o n d e c r e a s e s in t h e o r d e r ( N P C 1 ) > N3P3PI12CI4 > N3P3PI14CI2. Phenyl groups thus retard the substitution reaction at adjacent p h o s p h o r u s a t o m s . H o w e v e r , it s h o u l d b e n o t e d t h a t side p r o d u c t s o f t y p e s X o r X I w e r e a l s o f o r m e d in t h i s r e a c t i o n . 2 0

2

Ph/ N^

/Ph ? N

3

Ph/ /Ph

NH

RO/1

N^

? N

NH I / O

Ph/I

R O ^ N ^ O R X

P h ^ N '

P

< O R

XI

N o n - g e m i n a l d i a m i n o g r o u p s apparently d o n o t seriously interfere with t h e r e p l a c e m e n t o f a d j a c e n t c h l o r i n e a t o m s b y f l u o r o a l k o x y u n i t s . F o r e x a m p l e , it has been reported that l,3-diamino-l,3,5,5-tetrachlorocyclotriphosphazene ( X I I ) r e a c t s w i t h t h e s o d i u m salts o f t r i f l u o r o e t h a n o l , 2 , 2 , 4 , 4 - t e t r a f l u o r o - « -

160

6.

R E A C T I O N S

W

I

T

H

A L K O X I D E S ,

A R Y L O X I D E S ,

H N

H N.

2

2

NaOR

II/NH2 P

Cl

A

N

D

T H I O L A T E S

OR ll/NH

-NaCl

-ci

2

OR

RO

XII propanol,

2,2,4,4,4-pentafluoropropanol,

2,2,4,4,5,5,5-heptafluorobutanol,

a n d 2 , 2 , 4 , 4 , 5 , 5 - h e x a f l u o r o b u t a n o l t o r e p l a c e all t h e h a l o g e n a t o m s . 1,1,3,5-tetrakis(dimethylamino)-3,5-dichlorocyclotriphosphazene

3 3

a wide variety of nucleophiles, such as M e O , E t O , « - P r O , / - P r O , e

/ - B u O , *-BuO©, e

rt-C H O , e

5

and C H = C H — C H 2

4.

C O M P E T I T I O N

e 2

H

e

i-CsHnO© « - C H 6

e

Similarly,

reacts with 0

n-BuO , e

O , «-C H 0©, PhCH O , 0

1 3

e

7

1 5

to replace the two non-geminal chlorine a t o m s .

2

4 9

R E A C T I O N S

V e r y few e x a m p l e s h a v e b e e n r e p o r t e d in w h i c h t w o n u c l e o p h i l e s h a v e b e e n allowed to interact simultaneously with a halocyclophosphazene. However, e q u i m o l a r a m o u n t s o f t h e s o d i u m salts of w - f l u o r o p h e n o l a n d t r i f l u o r o e t h a n o l were simultaneously allowed to react with hexachlorocyclotriphosphazene. T h e final p r o d u c t c o n t a i n e d 3 t o 4 t i m e s a s m a n y a r y l o x y a s t r i f l u o r o e t h o x y groups, although the detailed molecular a r r a n g e m e n t was n o t established.

2 9

It has also been s h o w n t h a t the interaction of s o d i u m p h e n o x i d e with hexa­ c h l o r o c y c l o t r i p h o s p h a z e n e in b u t a n o l s o l v e n t r e a l l y c o n s t i t u t e s a c o m p e t i t i o n r e a c t i o n for t h e s u b s t r a t e b e t w e e n s o d i u m p h e n o x i d e a n d s o d i u m b u t o x i d e .

2 1

B o t h p h e n o x y a n d b u t o x y s u b s t i t u e n t s w e r e d e t e c t e d in t h e final p r o d u c t , a l t h o u g h the relative p r o p o r t i o n s of each were n o t reported.

5.

C Y C L I Z A T I O N

R E A C T I O N S

A T

P H O S P H O R U S

W h e n c a t e c h o l is a l l o w e d t o i n t e r a c t w i t h h e x a c h l o r o c y c l o t r i p h o s p h a z e n e in the presence of a solvent a n d a suitable base, cyclization takes place at each p h o s p h o r u s a t o m , t o yield X I I I .

I 4 , 1 5

Triethylamine or anhydrous sodium

XIII

E .

carbonate

are

I N F L U E N C E

O F

satisfactory

pyridine can also be u s e d .

T H E

N U C L E O P H I L E

hydrogen

O

N

chloride

S U B S T I T U T I O N

acceptors,

8 , 1 4 , 1 5

161

although

1 5 , 1 6

Similar high-yield reactions t a k e place between

hexachlorocyclotriphos-

p h a z e n e a n d 2 , 3 - d i h y d r o x y n a p h t h a l e n e o r t o l u e n e - 3 , 4 - d i t h i o l t o give X I V a n d XV.

8

I t is w o r t h w h i l e t o n o t e t h a t t h e cyclic t e t r a m e r i c a n a l o g u e o f X I I I is

XV e x t r e m e l y labile a n d difficult t o i s o l a t e . T h e p r i n c i p a l r e a c t i o n p r o d u c t w h e n t r i e t h y l a m i n e is u s e d a s a b a s e is a p h o s p h o r a n e ( X V I ) f o r m e d b y p h o s p h a z e n e r i n g c l e a v a g e . T h e s a m e p h o s p h o r a n e is a l s o p r o d u c e d d u r i n g f o r m a t i o n of X I I I , p a r t i c u l a r l y w h e n c a t e c h o l is in e x c e s s . 7

1 4 , 1 5

XVI

162

6.

R E A C T I O N S

W I T H

A L K O X I D E S ,

A R Y L O X I D E S ,

A

N

D

T H I O L A T E S

Six- a n d s e v e n - m e m b e r e d s p i r o - a r y l o x y s y s t e m s c a n a l s o b e i n t r o d u c e d a t phosphorus. F o r example, 1,8-dihydroxynaphthalene

and

2,2'-dihydroxy-

b i p h e n y l r e a c t w i t h h e x a c h l o r o c y c l o t r i p h o s p h a z e n e t o yield X V I I a n d X V I I I .

XVII

7

XVIII

C y c l o t e t r a p h o s p h a z e n e a n a l o g u e s of t h e s e t w o c o m p o u n d s h a v e a l s o b e e n prepared. 7

Cyclic derivatives such as X I I I - X V I I I are particularly interesting because of t h e u n e x p e c t e d ease of c y c l i z a t i o n . T h i s is especially so for X V I I I , w h e r e t o r ­ sional m o t i o n s of the P h — P h b o n d m i g h t be expected t o reduce the probability o f c y c l i z a t i o n . H o w e v e r , m a n i f e s t a t i o n s o f r i n g s t r a i n in X I I I - X V apparent

during

hydrolysis

5 0

(see C h a p t e r

degradations to form p h o s p h o r a n e s .

9 , 5 1

5) a n d

during

become

phosphazene

F o r e x a m p l e , it h a s b e e n s h o w n t h a t

X I I I , X I V , and X V form p h o s p h o r a n e X I X when treated with 0 - a m i n o p h e n o l . C o m p o u n d s X V I I a n d X V I I I a r e i n e r t u n d e r t h e s e c o n d i t i o n s (see a l s o C h a p t e r 14).

C y c l i z a t i o n a t p h o s p h o r u s is sterically f a v o r e d b y t h e p r e s e n c e of ortho f u n c t i o n a l g r o u p s o n a rigid p h e n y l r i n g , b u t a l i p h a t i c d i o l s m i g h t b e e x p e c t e d t o cyclize less r e a d i l y b e c a u s e o f t h e g r e a t e r o p p o r t u n i t i e s for c r o s s - l i n k i n g between r i n g s w i t h t h e s e l i g a n d s . H o w e v e r , it h a s b e e n s h o w n t h a t s p i r o - t y p e

E .

I N F L U E N C E

O F

T H E

N U C L E O P H I L E

O

N

163

S U B S T I T U T I O N

derivatives are formed w h e n hexachlorocyclotriphosphazene reacts with the s o d i u m o r t r i e t h y l a m i n e salts o f H O C H ( C F ) C H O H 2

C H O H . Species such as X X are isolated.

2

CF —CF 2

CH

2

C F

2

— C H — C H

2

—

P

2

2

2

CH

^CH —Ο ) C ^ _ 2

2

2

C

H

2

- C F

P

C-(CH ON0 )

N O C H

3

\\^0—CU —CF

^ N ^ ^ 0 — C H XX

2

2

2

ο

H C

( 0

2

2

— I

2

HOCH (CF )

I

ο C F

or

2

2

CH

2

I

2

Similarly, 2,2-bis(nitroxymethyl)-

5

2

2

2

2

2

I | | ^ 0 - C H ^ - P ^ P ^ ^ C ( C H 2

O

O

C

H

O N 0

2

2

)

2

XXI

3 - n i t r o x y p r o p a n - l - o l r e a c t s w i t h h e x a c h l o r o c y c l o t r i p h o s p h a z e n e in p y r i d i n e to form derivative X X I . the

interaction

of

1 7

Experiments have also been reported which involve

hexachlorocyclotriphosphazene

with

ethylene

glycol,

p r o p a n e - 1 , 2 - d i o l , b u t a n e - 1,2-diol, a n d c y c l o h e x a n e - l , 2 - d i o l in t h e p r e s e n c e o f pyridine.

5 2 ,

5 3 , 5 3 a

Spirocyclic

phosphazenes

such

as

XXII

are

formed.

CH —CHR 2

I

I

RHC—ON**

Np__ H

x

C

H C—Ο

2

Ο—CHR

2

XXII

A n a l o g o u s cyclic t e t r a m e r s h a v e a l s o b e e n d e s c r i b e d .

5 2

Pornin

5 2

reported

t h a t spiro c o m p o u n d s were also formed when propane-1,3-diol or butane-1,4d i o l r e a c t e d w i t h h e x a c h l o r o c y c l o t r i p h o s p h a z e n e in p y r i d i n e a t — 10°C, b u t Matuszko and C h a n g

5 3

observed that at higher temperatures (200°-210°C) the

p r i n c i p a l p r o d u c t s w e r e cyclic e t h e r s f o r m e d b y d e h y d r a t i o n of t h e d i o l .

HO(CH ) OH 2

4

(NPCI ) 2

—

Ho 2

3

H C 2

H C 2

X

Q

CH

2

/ C H

2

164

6.

REACTIONS W I T H ALKOXIDES, ARYLOXIDES, A N D THIOLATES

2-Aminoethanol (NPCl ) .

has been reported

to form

spirocyclic derivatives

with

5 3 a

2

3

In a similar m a n n e r , t h i o a m i d e s such as d i t h i o o x a m i d e , a n d thiosemicarbazide apparently

react with hexachlorocyclotriphosphazene

c h l o r o e t h a n e t o yield t h e s p i r o d e r i v a t i v e s s h o w n b e l o w .

in d r y

tetra­

T h i o u r e a reacts as

5 4

Ck

J

N*

. P /

^P^

Cl Cl HS—C=NH

I HS - C

NH

HN==C-

C=NH

I

NH

II

N H

NH

NH

C

I

S

NH

il

^N

/ N H - C — S .

I

S.

II / S — C — N H .

^ P ^

;NH

/ P ^

^ N H — C — N

S—C—NH'

NH

C—S

NH

S—C

NH

NH

t h e species H S — C ( N H ) = N H t o give h e x a - s u b s t i t u t e d p r o d u c t s o f s t r u c t u r e [NP(SC(NH )=NH) ] . 2

5 4

2

2

3

I t s h o u l d b e n o t e d t h a t s p i r o c y c l i c p h o s p h a z e n e s a r e of i n t e r e s t a s i n t e r ­ m e d i a t e s for p o l y m e r s y n t h e s i s (see C h a p t e r s 15 a n d 16).

F . Influence of the H a l o g e n in Halophosphazenes M u c h of t h e w o r k w h i c h h a s b e e n r e p o r t e d f o r t h e a l c o h o l y s i s , p h e n o l y s i s , a n d m e r c a p t a n o l y s i s of c h l o r o p h o s p h a z e n e s is believed t o b e a p p l i c a b l e a l s o t o f l u o r o p h o s p h a z e n e s a n d b r o m o p h o s p h a z e n e s , b u t v e r y little e x p e r i m e n t a l w o r k h a s b e e n r e p o r t e d for t h e s e l a t t e r h a l o p h o s p h a z e n e s . H o w e v e r , M a o , Dresdner, and Y o u n g

3 6

showed that ( N P F ) 2

3

and ( N P F ) 2

s o d i u m salt o f 1 , 1 , 7 - t r i h y d r o d e c a f l u o r o h e p t a n o l in a

4

react with the

fluorocarbon

solvent to

yield t h e a p p r o p r i a t e f u l l y - s u b s t i t u t e d c y c l o p h o s p h a z e n e s . S i m i l a r l y , P a d d o c k and

co-workers

reported

2 3

that

species

of

structure

[NP(OMe) ] _ , 2

5

8

[ N P ( O C H C F ) ] _ , and [ N P ( O P h ) ] _ could be prepared from the a p p r o ­ 2

3

2

5

8

priate fluorocyclophosphazenes.

2

5

8

Niecke, T h a m m , and Glemser have pre­

pared mono-methoxy, ethoxy, phenoxy, and from ( N P F ) . 2

3

5 7 a

phenylthio-fluorophosphazenes

H .

E F F E C T

O F

T H E

165

S O L V E N T

G. Influence of Phosphazene Ring or Chain Size I n g e n e r a l , it a p p e a r s t h a t m a n y of t h e r e a c t i o n s t h a t h a v e b e e n d e s c r i b e d for hexachlorocyclotriphosphazene are equally applicable to cyclic tetramers, ~ h i g h e r cyclic o l i g o m e r s , ' a n d very h i g h polymers. However, some noticeable exceptions are also found. F o r i n s t a n c e , a l t h o u g h t h e cyclic t r i m e r ( N P C 1 ) 3 r e a c t s r e a d i l y w i t h c a t e c h o l in t h e p r e s e n c e of b a s e t o yield a s p i r o c y c l o p h o s p h a z e n e ( X I I I ) , t h e h i g h p o l y m e r ( N P C l ) degrades instead to p h o s p h o r a n e (XXVI) and a m m o n i a . The cyclic t e t r a m e r i c s p i r o p h o s p h a z e n e c a n p e r h a p s be i s o l a t e d u n d e r very critical c o n d i t i o n s b u t , h e r e t o o , t h e p r e d o m i n a n t r e a c t i o n is d e g r a d a t i o n t o p h o s ­ p h o r a n e . E x p l a n a t i o n s for t h i s b e h a v i o r c a n b e f o r m u l a t e d in t e r m s of t h e g r e a t e r skeletal flexibility of t h e t e t r a m e r a n d h i g h e r h o m o l o g u e s . 2 - 6 , 2 9 , 3 0 , 3 1 , 3 5

3 7 , 4 8 , 5 2

2 4 , 3 3

5 5

2 4 , 2 5

2

1 5

2

n

5 2

1 5

1 5

Special p r o b l e m s a r e a l s o i n v o l v e d w h e n a l k o x i d e s o r a r y l o x i d e s i n t e r a c t w i t h very h i g h m o l e c u l a r w e i g h t p o l y ( d i c h l o r o p h o s p h a z e n e ) . In t h e first p l a c e , l o n g e r r e a c t i o n t i m e s a n d h i g h e r t e m p e r a t u r e s a r e r e q u i r e d t o effect complete substitution on a high molecular weight polymer. F o r example, total s u b s t i t u t i o n of ( N P C 1 ) b y m a n y a l k o x i d e s is c o m p l e t e in less t h a n 12 h o u r s i n e t h e r a t 2 5 ° C o r in 4 h o u r s in a b o i l i n g e t h e r - t o l u e n e s o l u t i o n . C o m p l e t e s u b s t i t u t i o n of t h e h i g h p o l y m e r r e q u i r e s a t least 16 h o u r s r e a c t i o n a t 7 0 ° 80°C. W h e n bulky substituents such as p h e n o x y are employed, m o r e d r a s t i c c o n d i t i o n s a r e r e q u i r e d for t o t a l p o l y m e r s u b s t i t u t i o n . T h i s r e s u l t s f r o m t h e fact t h a t p o l y m e r c h a i n s a r e often extensively c o i l e d in s o l u t i o n , a n d c o n s i d e r a b l e steric h i n d r a n c e m u s t b e s u r m o u n t e d b e f o r e a n u c l e o p h i l e c a n p e n e t r a t e sufficiently close t o t h e s k e l e t o n t o effect s u b s t i t u t i o n . A s e c o n d p r o b l e m e n c o u n t e r e d w i t h t h e s u b s t i t u t i o n of h i g h p o l y m e r s is t h a t t h e t e m ­ p e r a t u r e m u s t n o t b e so h i g h t h a t d e p o l y m e r i z a t i o n t o cyclic o l i g o m e r s is i n d u c e d (see C h a p t e r 15). 2 4 , 2 5

2

3

4

5

2 4 , 2 5

2 5

H . Effect of the Solvent A v a r i e t y of a n h y d r o u s s o l v e n t s h a v e b e e n u s e d for s u b s t i t u t i o n r e a c t i o n s of t h i s t y p e . T h e s e i n c l u d e d i e t h y l e t h e r , tetrahydrofuran, ' dioxane, benzene, °toluene, xylene, "· 25,44,46,48 a c e t o n e , methyl ethyl k e t o n e , dimethylformamide, fluorocarbon-i-butylamine, pyridine, ~ a n d excess of t h e a l c o h o l s o r t h i o l s b e i n g used as r e a g e n t s for s u b s t i t u t i o n . A n u m b e r of p r a c t i c a l c o n s i d e r a t i o n s m u s t b e t a k e n i n t o a c c o u n t w h e n choosing a reaction solvent. T h u s , a n h y d r o u s conditions must be maintained d u r i n g s u b s t i t u t i o n t o a v o i d h y d r o l y s i s of t h e p h o s p h o r u s - h a l o g e n b o n d s , a n d 4 , 2 8 , 4 6 , 5 6

2 , 2 5 ,

4 3

2 , 2 3 , 2 5 ,

3 7 , 3 8 ,

4

4 2 , 4 6

8 , 1 5

5 , 3 0 , 4 0 ,

4 3

4 2 , 4 6 ,

4 8

1 0 ,

4 3

36

2 , 1 6

1 8 ,

2 5 , 3 9 , 4 2 , 4 6

4 3 ,

4 6 , 5 2 , 5 3

1 8 , 2 0 , 2 1 ,

4 6

4 6

166

6.

R E A C T I O N S

W I T H

A L K O X I D E S ,

A R Y L O X I D E S ,

A

N

D

T H I O L A T E S

hydrophilic solvents such as tetrahydrofuran, dioxane, d i m e t h y l f o r m a m i d e , a n d p y r i d i n e m u s t b e e x h a u s t i v e l y d r i e d b e f o r e u s e . T h e s a m e is, of c o u r s e , t r u e for t h e a l c o h o l s o r m e r c a p t a n s u s e d as r e a g e n t s a n d s o l v e n t s . B o i l i n g x y l e n e , b e n z e n e , o r t o l u e n e is u s e d a s r e a c t i o n m e d i u m w h e n solid p o t a s s i u m o r s o d i u m h y d r o x i d e is e m p l o y e d a s a b a s e , since w a t e r f o r m e d d u r i n g t h e r e a c t i o n is r a p i d l y r e m o v e d b y d i s t i l l a t i o n . T h i s c a n influence t h e r e a c t i o n in c u r i o u s w a y s . T h u s , w h e n p h e n o x y l a t i o n of N3P3CI4PI12 t a k e s p l a c e in b e n z e n e , the râ-non-geminal i s o m e r of N j P j C ^ P l h i O P h ^ is f o r m e d . I n t e t r a h y d r o ­ f u r a n , f r o m w h i c h w a t e r is n o t r e m o v e d by a z e o t r o p i n g , b o t h cis a n d trans isomers are f o r m e d . 4 2

A s e c o n d i m p o r t a n t s o l v e n t influence is c o n n e c t e d w i t h s o l v e n t p o l a r i t y a n d w i t h t h e solubility of s o d i u m salts in t h e m e d i u m . F o r e x a m p l e , a p r e f e r r e d sol­ v e n t is o n e in w h i c h t h e s o d i u m a l k o x i d e , a r y l o x i d e , o r m e r c a p t i d e is s o l u b l e , b u t in w h i c h s o d i u m c h l o r i d e is i n s o l u b l e . E t h e r s often fall i n t o t h i s c a t e g o r y . T h e influence of t h e s o l v e n t o n t h e r e a c t i o n r a t e o r m e c h a n i s m is less easy t o determine. It seems reasonable to assume that the m o r e polar solvents should a c c e l e r a t e t h e r a t e of s u b s t i t u t i o n b y f a c i l i t a t i n g i o n i z a t i o n of a l k o x i d e , a r y l ­ o x i d e , o r m e r c a p t i d e i o n s . S o m e e v i d e n c e in f a v o r of t h i s view h a s b e e n p r o ­ v i d e d b y t h e r e s u l t s of C a r r o l l a n d S h a w for t h e a l k y l t h i o l y s i s a n d a r y l t h i o l y s i s of h e x a c h l o r o c y c l o t r i p h o s p h a z e n e . T h u s , fully m e r c a p t a l a t e d i s o p r o p y l thio-substituted p r o d u c t s were obtained when the solvent was dimethyl­ f o r m a m i d e , f o u r c h l o r i n e a t o m s w e r e r e p l a c e d in t e t r a h y d r o f u r a n , b u t o n l y t w o w e r e r e p l a c e d in b e n z e n e . In g e n e r a l , t h e e a s e of s u b s t i t u t i o n falls off w i t h sol­ v e n t c h a n g e s in t h e o r d e r : d i m e t h y l f o r m a m i d e a t 100°C > b o i l i n g d i g l y m e > boiling tetrahydrofuran > boiling benzene > boiling diethyl ether. This order r o u g h l y p a r a l l e l s t h e e x p e c t e d d e c r e a s i n g s o l v a t i o n of Na® in t h e r e a g e n t , b u t it s h o u l d b e r e m e m b e r e d t h a t t h e r e a c t i o n s a r e h e t e r o g e n e o u s in all o f t h e s e m e d i a e x c e p t d i m e t h y l f o r m a m i d e , a n d r e a g e n t solubility effects m a y t h u s b e r e s p o n s i b l e for t h e differences. It is i n t e r e s t i n g t o n o t e t h a t t h e s e s o l v e n t effects b e c o m e m a n i f e s t as a n influence o n t h e number of h a l o g e n a t o m s r e p l a c e d a n d n o t s i m p l y as a n effect o n t h e r e a c t i o n r a t e . F o r i n s t a n c e , in d i e t h y l e t h e r a t room temperature, ethane-, propane-, butane-, or isobutanethiol bring about r e p l a c e m e n t of t w o c h l o r i n e a t o m s o n l y , a n d f u r t h e r s u b s t i t u t i o n d o e s n o t n o r m a l l y o c c u r if t h e r e a c t i o n t i m e o r r e a g e n t c o n c e n t r a t i o n is i n c r e a s e d . H o w ­ ever, after 7 d a y s in b o i l i n g e t h e r , a t e t r a k i s ( e t h y l t h i o ) d e r i v a t i v e c a n b e i s o l a t e d . I n b o i l i n g t e t r a h y d r o f u r a n , fully s u b s t i t u t e d e t h a n e - , « - p r o p a n e - , « - b u t a n e - , a n d b e n z y l t h i o l d e r i v a t i v e s w e r e f o r m e d , b u t b r a n c h e d a l k a n e t h i o l a t e s (isopropyl or cyclohexyl) gave only tetrakis derivatives. O t h e r nucleophilic substi­ t u t i o n s , s u c h a s a l c o h o l y s i s o r a m i n o l y s i s , d o n o t s h o w t h i s s t r i k i n g sensitivity t o different s o l v e n t s . 45,

4 6

4 6

Z h i v u k h i n , T o l s t o g u z o v , a n d L u k a s h e v s k i h a v e e x a m i n e d t h e initial r a t e of t h e r e a c t i o n b e t w e e n h e x a c h l o r o c y c l o t r i p h o s p h a z e n e a n d sodium 2 2

I.

I N F L U E N C E

O F

T H E

167

B A S E

m e t h o x i d e in several different s o l v e n t s u s i n g a p o t e n t i o m e t r i c t e c h n i q u e . T h e initial r a t e in d i m e t h y l f o r m a m i d e w a s f o u n d t o b e m u c h m o r e r a p i d t h a n in acetone, dioxane, butanol, d i o x a n e - a c e t o n e , or d i o x a n e - m e t h y l ethyl ketone mixtures. H o w e v e r , the almost i n s t a n t a n e o u s replacement of u p to

four

c h l o r i n e a t o m s w h i c h o c c u r r e d in d i m e t h y l f o r m a m i d e m u s t b e c o n t r a s t e d w i t h t h e u l t i m a t e b u t s l o w e r r e p l a c e m e n t o f all six h a l o g e n a t o m s in t h e o t h e r sol­ vent systems.

I. Influence of the B a s e I n t h e m a j o r i t y o f r e a c t i o n s , a b a s e o r s o d i u m salt m u s t b e p r e s e n t b e f o r e a n a l c o h o l , p h e n o l , o r t h i o l will i n t e r a c t c l e a n l y w i t h a c h l o r o p h o s p h a z e n e . O n l y w h e n r e a c t i o n s of t h i s t y p e a r e e m p l o y e d for t h e h i g h - t e m p e r a t u r e c u r i n g o f cross-linked p o l y p h o s p h a z e n e resins m a y a base be absent, b u t even here t h e p r e s e n c e of a h y d r o g e n c h l o r i d e a c c e p t o r m a r k e d l y facilitates t h e p r o c e s s . I n t h e a b s e n c e of a b a s e , a l k a n e t h i o l s o r p h e n y l t h i o l d o n o t u n d e r g o s u b s t i t u t i o n w h e n heated with chloro- or a l k o x y c y c l o p h o s p h a z e n e s . Benzyl alcohol reacts w i t h ( N P C 1 ) in d i o x a n e in t h e a b s e n c e of a b a s e t o yield a h y d r o x y p h o s p h a zane a n d benzyl c h l o r i d e . 4 6

2

3

5 6 3

T h e u s e of s o d i u m salts of a l k o x i d e s , a r y l o x i d e s , o r t h i o l s h a s b e e n m e n ­ tioned t h r o u g h o u t this chapter. Reactions involving such reagents are usually c l e a n a n d r a p i d , a n d t h e y yield s o d i u m c h l o r i d e a s a n easily r e m o v e d side p r o ­ d u c t . S o d i u m salts of a l c o h o l s , p h e n o l s , o r t h i o l s c a n b e p r e p a r e d b y t h e a d d i ­ t i o n of m e t a l l i c s o d i u m t o a s o l u t i o n of t h e r e a g e n t in e t h e r o r t e t r a h y d r o f u r a n . W h e n strongly acidic alcohols or phenols are employed, p o t a s s i u m or s o d i u m h y d r o x i d e s c a n b e u s e d in situ for salt f o r m a t i o n . H o w e v e r , t h e s e m e t h o d s a r e u n s u i t a b l e w h e n t h e a l c o h o l o r p h e n o l u n d e r g o e s side r e a c t i o n s w i t h s o d i u m o r caustic alkali. U n d e r these circumstances, milder conditions are employed, w i t h t h e u s e of s o d i u m c a r b o n a t e , p y r i d i n e , o r t r i e t h y l a m i n e a s t h e h y d r o g e n c h l o r i d e a c c e p t o r . T h i s is e s p e c i a l l y s o f o r t h e f o r m a t i o n o f s p i r o c y c l i c p h o s ­ phazenes such as as X I I I , X I V , X V I I , X V I I I , a n d X X I . T h e d e s i g n a t i o n o f t h e b a s e a s a " h y d r o h a l i d e a c c e p t o r " in t h e s e r e a c t i o n s is n o t strictly v a l i d , since few r e a c t i o n s of t h i s t y p e t a k e p l a c e a t all in t h e a b s e n c e of a b a s e . I t is m o r e r e a s o n a b l e t o view t h e b a s e a s a c a t a l y s t f o r f a c i l i t a t i n g i o n i z a t i o n of t h e a l c o h o l o r p h e n o l t o a l k o x i d e o r a r y l o x i d e i o n ( X X I I I ) . ROH + Et N ^ R O + Et NH 0

3

3

XXIII S o d i u m c a r b o n a t e p r o b a b l y functions partly as a h y d r o h a l i d e acceptor t o re­ m o v e hydrohalide liberated d u r i n g a very slow direct phenolysis process

168

6.

R E A C T I O N

S

W I T

H

A L K O X I D E S

,

A R Y L O X I D E S

,

A

N

D

T H I O L A T E

S

6 RO H + (NPC1 ) - > [NP(OR) ] + 6 HC l 2

3

2

3

HCl + N a C 0 - » NaC l + N a H C 0 2

3

3

XXIV ( X X I V ) . H o w e v e r , i t i s a l s o likel y t h a t t h e i n t e r a c t i o n o f a p h e n o l w i t h s o d i u m c a r b o n a t e result s i n th e f o r m a t i o n o f smal l a m o u n t s o f s o d i u m p h e n o l a t e , which the n reac t b y th e conventiona l r o u t e : ROH + N a C 0 ^ RON a + N a H C 0 2

3

3

R e a c t i o n s p e r f o r m e d wit h pyridin e a s a bas e ar e especiall y sensitiv e t o trace s of m o i s t u r e .

1 5

F u r t h e r m o r e , pyridin e apparentl y form s a n isolabl e crystallin e

complex wit h hexachlorocyclotriphosphazene.

5 2

J . Influenc e o f Temperatur e The interactio n o f som e nonfluorinate d alcohol s o r alkoxide s wit h chloro phosphazenes m u s t b e carrie d ou t a t th e lowes t practicabl e temperatur e t o avoid rearrangemen t o f th e alkoxycyclophosphazen e t o a n oxophosphazan e (XXV).

5 7

(se e C h a p t e r 13) . R e a r r a n g e m e n t s o f t h i s k i n d o c c u r s l o w l y e v e n a t [NP(OR) ] 2

-ï^ >

3

[N(R)—P(0)O R]

3

XXV r o o m temperature , whe n O R i s methox y o r e t h o x y . F o r thi s reason , th e reac 7

tion o f sodiu m methoxid e wit h hexachlorocyclotriphosphazen e i n b e n z e n e methanol solutio n i s carrie d ou t a t 0 ° C . Th e interactio n o f ethanol , pyridine , 2

and

hexachlorocyclotriphosphazene s

fl-butanol

i s conducte d

at

0°-5°C.

2

Whe n

replace s ethanol , th e reactio n take s plac e a t r o o m t e m p e r a t u r e .

2

M i l d r e a c t i o n c o n d i t i o n s a r e a l s o p r e f e r r e d fo r t h e f o r m a t i o n o f s o m e s p i r o cyclophosphazenes, suc h a sX I I I ,

1 4

-

1 5

XXI,

1 7

o rX X I I

5 2

t o a v o i d sid e r e a c t i o n s .

F o r example , th e reactio n o f ( N P C l ) r 4 wit h catecho l i n pyridin e solutio n i s 2

conducted a t - 8 ° t o 3 0 ° C . * 1 5

3 o

5 2

However, mor e drasti ctemperatur e condition s ca n b euse d whe n th e nucleo p h i l e i s i s o p r o p o x y , b e n z y l o x y , p h e n o x y , o r fluoroalkoxy,

an d temperature s i n

the 30°-120° C rang e m a y b e used . Aryloxy - an d

fluoroalkoxyphosphazenes

do no t rearrang e thermall y t o oxophosphazanes . I t shoul d b e note d tha t a t 2 5 0 °C fluoroalkoxy

group s ca n b e exchange d fro m on e molecul e t o a n o t h e r .

5 7 1 3

W h e n d i m e t h y l f o r m a m i d e i s u s e d a s a s o l v e n t fo r t h e i n t e r a c t i o n o f m e r c a p t a n s o r t h e i r s o d i u m s a l t s , t h e t e m p e r a t u r e m u s t b e k e p t b e l o w 100° C t o a v o i d degradation reactions.

4 6

L .

T H E

R E A C T I O N

169

M E C H A N I S M

K. Monophosphazenes and Related Compounds All of the foregoing analysis has centered a r o u n d the nucleophilic substitu­ tions of halocyclophosphazenes or their linear high polymers. However, m o n o p h o s p h a z e n e s are k n o w n to u n d e r g o similar reactions. F o r example, Zhmurova, Voitsekhovskaya, and Kirsanov have shown that P-trichloro-Narylmonophosphazenes (phosphinimines) (XXVI) react with sodium pheno x i d e t o yield t h e a p p r o p r i a t e t r i p h e n o x y d e r i v a t i v e s ( X X V I I ) . Such tri4 3

A r N = P C l + 3 PhONa

ArN=P(OPh) + 3 NaCl

3

3

XXVI

XXVII

phenoxymonophosphazenes are formed from both the m o n o m e r i c trichlorom o n o p h o s p h a z e n e , as shown above, or from the dimeric (cyclodiphosphazane) f o r m . H o w e v e r , t h e t r i p h e n o x y d e r i v a t i v e s exist o n l y a s m o n o m e r s a n d n o t a s cyclic d i m e r s , p o s s i b l y for s t e r i c r e a s o n s . T h i s r e a c t i o n h a s b e e n c a r r i e d o u t for a n u m b e r o f P - t r i c h l o r o m o n o p h o s p h a z e n e s ( X X V I ) in w h i c h t h e g r o u p A r is P h ; o , m-, / ? - M e C H ; 3 , 5 - M e C H ; o-, m-, / ? - C l C H ; 2 , 4 - C l C H ; 3 , 5 - C l C H ; 2 , 4 , 6 - C l C H ; o-, m-, / ? - B r C H ; 2 , 4 - B r C H ; 2 , 4 , 6 - B r C H ; / ? - M e O C H ; / ? - E t O C H ; o, m-, / 7 - N 0 C H ; 2 , 4 - ( N 0 ) C H ; 2 , 6 - C l - 4 - N 0 C H . T h e c o m p o u n d s in w h i c h A r is 2 , 4 , 6 - C l C H ; 2 , 4 , 6 - B r C H ; / ? - N 0 - C H ; 2 , 4 - ( N 0 ) C H ; o r 2 , 6 - C l - 4 - N 0 C H a r e c r y s t a l l i n e s o l i d s , b u t t h e rest a r e v i s c o u s l i q u i d s . E x c e p t in t h e c a s e w h e r e A r is p h e n y l , t h e y c a n n o t b e distilled a t r e d u c e d p r e s ­ s u r e w i t h o u t d e c o m p o s i t i o n . All t h e s e m a t e r i a l s a r e h y d r o l y z e d r a p i d l y b y w a t e r o r m o i s t a i r t o f o r m d i p h e n y l e s t e r s of a r y l a m i d o p h o s p h o r i c a c i d s , b y t h e reaction 6

6

3

6

6

2

4

6

2

6

4

2

6

6

2

3

2

3

2

4

2

6

6

3

3

6

3

3

6

6

3

2

2

2

2

6

2

2

6

4

3

2

6

4

2

6

4

2

2

6

4

2

2

6

4

2

ArN=P(OPh) + H 0 -> ArNH—PO(OPh) + PhOH 3

The compounds

2

SC1 P—N=PC1 2

2

3

and S P ( O P h ) — N = P C 1

sodium phenoxide according to the s c h e m e s

2

3

react

with

5 8 , 5 9

S=PC1 —N=PC1 + 5 NaOPh -> S=P(OPh) —N=P(OPh) + 5 NaCl 2

3

2

3

S=P(OPh )—N=PC1 + NaOPh -> S = P ( O P h — N = P ( O P h ) + 3 NaCl 2

3

2

3

I t h a s b e e n s h o w n t h a t m o n o p h o s p h a z e n e s of s t r u c t u r e C 1 C — C 1 C — N = P C 1 r e a c t w i t h p r i m a r y a l c o h o l s a t 2 0 ° - 2 5 ° C t o yield c o m p o u n d s of s t r u c t u r e C1 C—C1 C—N=PCl (OAlk). However, these materials are unstable and d e c o m p o s e with elimination of alkyl halide. 3

2

3

6 0

3

2

2

L . T h e Reaction M e c h a n i s m T h e n a t u r e of the reaction m e c h a n i s m d u r i n g alcoholysis, phenolysis, or thiolysis is i n t i m a t e l y c o n n e c t e d w i t h q u e s t i o n s a b o u t t h e m e c h a n i s m s o f t h e

170

6.

R E A C T I O N S

W I T H

A L K O X I D E S ,

A R Y L O X I D E S ,

A

N

D

T H I O L A T E S

h y d r o l y s i s , a m i n o l y s i s , m e t a t h e t i c a l e x c h a n g e , a n d s k e l e t a l d e g r a d a t i o n of h a l o p h o s p h a z e n e s . T h e s e l a t t e r m e c h a n i s m s a r e c o n s i d e r e d in C h a p t e r s 5 , 7 , 8, a n d 14. T h e a v a i l a b l e e v i d e n c e a b o u t t h e m e c h a n i s m o f a l c o h o l y s i s , p h e n o l y s i s , a n d t h i o l y s i s is i n c o m p l e t e a n d f r a g m e n t a r y , b u t several p e r t i n e n t facts c a n b e d i s c e r n e d a n d t h e s e a r e listed b e l o w . (a) T h e d i r e c t r e a c t i o n b e t w e e n a h a l o p h o s p h a z e n e a n d a n a l c o h o l , p h e n o l , o r t h i o l in t h e a b s e n c e of a b a s e is a s l o w r e a c t i o n w h i c h often yields side p r o ­ d u c t s o r d e c o m p o s i t i o n species r a t h e r t h a n s u b s t i t u t e d p h o s p h a z e n e s . H o w ­ ever, w h e n s o d i u m alkoxides, aryloxides, or thiolates are used, o r w h e n a s t r o n g b a s e is e m p l o y e d w i t h t h e a l c o h o l o r p h e n o l , t h e r e a c t i o n is r a p i d . T h i s s u g g e s t s t h a t a l k o x i d e , a r y l o x i d e , o r t h i o l a t e i o n s a r e t h e r e a c t i v e species d u r i n g " n o r ­ m a l " substitution. This has been confirmed by the kinetic studies of S o r o k i n and Latov,

6 1

w h o showed that, when sodium ethoxide reacts with ( N P C 1 ) 2

N P C l 4 ( O B u ) 2 , t h e n u c l e o p h i l e is E t O 3

3

0

3

or

and not E t O N a . The ionization to

e t h o x i d e i o n is e n h a n c e d in h i g h d i e l e c t r i c c o n s t a n t s o l v e n t s a n d t h i s l e a d s t o i n c r e a s e d s u b s t i t u t i o n r a t e s in s u c h m e d i a . (b) B o t h t h e d e g r e e of s u b s t i t u t i o n a n d t h e p a t t e r n of h a l o g e n r e p l a c e m e n t a r e sensitive t o t h e steric c h a r a c t e r i s t i c s of t h e n u c l e o p h i l e . T h u s , b r a n c h e d alkoxides, branched alkylthiolates, or phenoxides cause total substitution only w i t h difficulty. P h e n o x i d e i o n s u b s t i t u t e s n o n - g e m i n a l l y , p o s s i b l y b e c a u s e of steric i n f l u e n c e s . (c) T h e p o s s i b i l i t y a l s o exists t h a t t h e n o n - g e m i n a l r e p l a c e m e n t p a t t e r n observed with phenoxide and some alkoxides m a y result from electron supply from alkoxide or aryloxide to p h o s p h o r u s , which lowers the reactivity of a C l — Ρ — O R u n i t b e l o w t h a t of a C l — Ρ — C l u n i t . I n v i e w of t h e r e s u l t s o f polarographic experiments with alkoxy- and a r y l o x y p h o s p h a z e n e s ,

6 2

it s e e m s

u n l i k e l y t h a t r e s o n a n c e effects i n v o l v i n g t h e p h e n y l g r o u p s a r e r e s p o n s i b l e f o r t h e e l e c t r o n s u p p l y . H o w e v e r , d o n a t i o n of t h e o x y g e n l o n e - p a i r e l e c t r o n s t o ­ w a r d p h o s p h o r u s is p o s s i b l e . ( d ) A l k y l t h i o l a t e i o n s s u b s t i t u t e g e m i n a l l y , r a t h e r t h a n n o n - g e m i n a l l y , in s p i t e of t h e l o w e r e l e c t r o n e g a t i v i t y of a l k y l t h i o s u b s t i t u e n t s t h a n groups.

4 6

It has been s u g g e s t e d

4 6

chloro

t h a t the high polarizability of the C l — Ρ — S R

u n i t c o m p a r e d w i t h t h a t of t h e C l — Ρ — C l g r o u p i n g m a y b e t h e r e a s o n for t h i s behavior. (e) G e m i n a l cyclization

to form

spirocyclophosphazenes

occurs

w h e n five-, six-, o r s e v e n - m e m b e r e d r i n g s a t p h o s p h o r u s a r e i n v o l v e d .

readily 8 , 1 5

This

i m p l i e s t h a t , w h e n g e m i n a l s u b s t i t u t i o n is not r e t a r d e d b y steric effects, t h e r e a r e n o s e r i o u s e l e c t r o n i c influences w h i c h i n h i b i t t h e g e m i n a l p r o c e s s . S o m e c o n ­ firmation

of t h i s view is p r o v i d e d b y t h e fact t h a t w h e n ( N P C 1 ) r e a c t s w i t h 2

catechol, 2,3-dihydroxynaphthalene, 2,2'-dihydroxybiphenyl,

3

1,8-dihydroxy-

n a p h t h a l e n e , o r t o l u e n e - 3 , 4 - d i t h i o l , n o n o n - g e m i n a l , u n c y c l i z e d species, s u c h as X X V I I I , are i s o l a t e d . * 8

1 5

L .

T H E

R E A C T I O N

171

M E C H A N I S M

XXVIII (f ) W h e n b u t o x i d e i o n r e p l a c e s c h l o r i n e in h e x a c h l o r o c y c l o t r i p h o s p h a z e n e , t h e r e a c t i o n r a t e d e c r e a s e s in t h e o r d e r ( N P C 1 ) > N P C l ( O B u ) > l N P C l ( O B u ) > N P C l ( O B u ) , w i t h t h e a c t i v a t i o n e n e r g y i n c r e a s i n g in t h e o r d e r 10.0, 11.3, 14.5, a n d 17.2 k c a l / m o l e , r e s p e c t i v e l y . If t h i s r a t e d e c r e a s e is t h e result of electron supply from b u t o x y g r o u p s to the ring, an S l - t y p e mecha­ n i s m is p r e c l u d e d since e l e c t r o n s u p p l y w o u l d e n h a n c e t h e i o n i z a t i o n of Ρ — C l t o P ® C l a n d , t h u s , i n c r e a s e t h e r a t e . S e c o n d - o r d e r k i n e t i c s (first o r d e r in b o t h p h o s p h a z e n e a n d a l k o x i d e ) a r e , in fact, o b s e r v e d a n d t h i s s u g g e s t s a n S 2 t y p e of mechanism. (g) If t h e m e c h a n i s m d o e s p r o c e e d b y a n S 2 - t y p e p r o c e s s , t h e r e a r e t w o possibilities for t h e a p p r o a c h of t h e n u c l e o p h i l e t o p h o s p h o r u s p r i o r t o a t t a i n ­ m e n t of t h e t r a n s i t i o n s t a t e . T h e first m e c h a n i s m is o n e i n v o l v i n g a side a t t a c k o n p h o s p h o r u s in t h e p l a n e of t h e r i n g , a s s h o w n in X X I X . E i t h e r r e t e n t i o n o r 2

2

3

3

3

3

3

3

5

3

3

4

3

61

N

e

N

N

Cl = N

X

/Cl

+

Θ OR

•

θ n > — O R

>

= N \

Cl

/Cl

Cl

e

XXIX i n v e r s i o n of c o n f i g u r a t i o n c o u l d , t h u s , o c c u r d e p e n d i n g o n t h e c h a r a c t e r o f t h e l e a v i n g g r o u p . T h e l o w e n t r o p y of a c t i v a t i o n v a l u e for b u t o x i d e i o n a t t a c k o n h e x a c h l o r o c y c l o t r i p h o s p h a z e n e h a s b e e n cited a s e v i d e n c e for t h i s t y p e of process. 6 1

A second, a n d m o r e logical, a p p r o a c h to a trigonal b i p y r a m i d a l transition s t a t e is via a b a c k s i d e a t t a c k , l e a d i n g t o i n v e r s i o n of c o n f i g u r a t i o n , a s s h o w n in XXX. OR' OR' e

=Nci

-Λ^Υ

ι---Cl

XXX

O K

—

: "OR eci

172

6.

R E A C T I O N S

W I T H

A L K O X I D E S ,

A R Y L O X I D E S ,

A

N

D

T H I O L A T E S

N o u n a m b i g u o u s e v i d e n c e for o r a g a i n s t i n v e r s i o n of c o n f i g u r a t i o n h a s y e t been p r o d u c e d . H o w e v e r , this type of m e c h a n i s m seems reasonable o n theor­ etical g r o u n d s since t h e a t t a c k i n g n u c l e o p h i l e c o u l d a p p r o a c h p h o s p h o r u s a l o n g t h e ζ axis o f a n unfilled d o r b i t a l . A d d i t i o n a l e v i d e n c e f o r t h i s t y p e of m e c h a n i s m is p r o v i d e d b y t h e r e s u l t s o f r e a c t i o n s b e t w e e n n u c l e o p h i l e s s u c h a s H O " or M e O a n d spirocyclic aryloxyphosphazenes. T h u s , attack by R O ~ o n a u n i t s u c h a s X X X I c a n n o t t a k e p l a c e b y a n y r o u t e w h i c h forces t h e t w o 2

z

-

-

XXXI

XXXII

Ρ — Ο — P h u n i t s t o m o v e t o a x i a l p o s i t i o n s in t h e t r a n s i t i o n s t a t e ( X X X I I ) , a n d t h e side a t t a c k m e c h a n i s m is, t h u s , p r o h i b i t e d . A s d i s c u s s e d e a r l i e r in

OR

OR

XXXIII

XXXIV

C h a p t e r 4, n u c l e o p h i l i c a t t a c k b y H O ~ ( a n d R O " ) o n s p i r o a r y l o x y p h o s p h a z e n e s w i t h five-membered r i n g s a t p h o s p h o r u s , a s in X X X I I I , is m u c h m o r e rapid than attack on molecules with seven-membered rings or two i n d e p e n d e n t s u b s t i t u e n t s a t p h o s p h o r u s . T h i s is c o n s i s t e n t w i t h t h e e a s e w i t h w h i c h t h e five-membered r i n g c a n a s s u m e t h e a x i a l a n d e q u a t o r i a l p o s i t i o n s ( X X X I V ) , thus allowing the Ο — Ρ — Ο b o n d angle to a p p r o x i m a t e to 90°-95°. T h u s , t h e e n e r g y of t h e t r a n s i t i o n s t a t e w o u l d b e l o w e r e d b y t h e p r e s e n c e of a five-membered ring at p h o s p h o r u s a n d the rate would be enhanced. In the a b s e n c e of e v i d e n c e t o t h e c o n t r a r y it m u s t b e a s s u m e d t h a t a t t a c k b y R O ~ fol­ l o w s t h e s a m e p a t h w a y a s a t t a c k b y H O , i.e., b y a n S 2 b a c k s i d e a t t a c k . T h e m a i n differences b e t w e e n s u b s t i t u t i o n b y H O a n d R O ~ w o u l d b e e x p e c t e d w h e n t h e g r o u p , R, is b u l k y o r h i g h l y p o l a r i z a b l e . 5 0

-

N

-

R E F E R E N C E S

173

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