62
Phosphorus Chemistry Downloaded from pubs.acs.org by NATL UNIV OF SINGAPORE on 03/27/17. For personal use only.
Contributions to the Chemistry of N-Phosphoryl Phosphazenes L. R I E S E L , E. H E R R M A N N , A . PFÜTZNER, J . STEINBACH, and B. T H O M A S Departments of Chemistry, Humboldt University of Berlin and Technical University Dresden, GDR
N-phosphorylated phosphazenes are characterized by P = N - P O - s e quences. N-dialkoxyphosphoryl trialkoxyphosphazenes ("pentaesters"), (R'O)2PO-N=P(OR)3, especially those with different alkyl groups are biologically active compounds. Tetraesters of imidodiphosphoric a c i d , (RO)2PO-NH-PO(OR)2, formally derived from the former by replacing one alkyl group by a hydroxyl group, are known as indeed good chelate ligands (1). Complex compounds with ligands of this kind are used for separating metal ions by extraction methods. Esters of N-phosphoryl phosphazenes are usually formed by the Staudinger reaction which requires the handling of the extremely toxic phosphoric acid ester azides (eq. 1). F o r developing new synthetic (R'O)2P(O)N + P ( O R ) 3
3
-->
(R'O) P(O)-N=P(OR) + 2
N2
3
(1)
approaches to obtain N-phosphoryl phosphazenes we thoroughly studied both the reaction of C l 3 P = N - P O C l with O-nucleophiles and reactions for P=N-P-bridge formation which avoid handling phosphoryl azides. 2
Solvolysis of C l 3 P = N - P O C l 2 . P2NOCl5 reacts with alcohols in the molar ratio 1:1, forming alkoxydichlorophosphazenes (eq. 2; n=l). Cl2OP-N=PCl3 + n R O H - - > C l 2 O P - N = P C l 3 - n ( O R ) n +
n
HCl
(2)
With excessive alcohol a further substitution occurs at the same phosphorus atom, though due to by-reactions to a considerably smaller extent. For instance in the reaction with ethanol (EtO)Cl P^N-POCU is formed in 97 % yield and (EtO) ClP=N-POCl in 75 % yield but (EtO) P=NP O C l only to an extent of less than 15 %, even when using a great excess of ethanoi. These by-reactions are alkylation reactions (eq. 3), phosphazene-phosphazane rearrangements (eq. 4) and olefin eliminations (eq. 5). In all cases imidodiphosphoryl compounds are formed. ?
2
2
0097-6156/81/0171-0297$05.00/0 © 1981 American Chemical Society
298
PHOSPHORUS CHEMISTRY
(RC0X P=N-P(O)X
2
(RO)X P=N-P(0)X
2
(RO)X P=N-P(0)X
2
2
2
Phosphorus Chemistry Downloaded from pubs.acs.org by NATL UNIV OF SINGAPORE on 03/27/17. For personal use only.
2
+ Nuc" ~
» Nuc-R +
[X P(0)~NHP(0)X ] " 2
2
fi»X P(0)-NR-P(0)X
R
2
(4)
2
X P(0)-NH-P(0)X 2
2
(3)
+ olefin
(5)
X denotes C I o r O R . T h e e x c l u s i v e s u b s t i t u t i o n at the p h o s p h a z e n e s i d e of the m o l e c u le as w e l l as these t h r e e types of b y - r e a c t i o n s c a n be u n d e r s t o o d by t a k i n g i n t o c o n s i d e r a t i o n the f o l l o w i n g m e s o m e r i c form of N - p h o s p h o r y l p h o s p h a z e n e s : ( R O ) X P - N = P ( - O ) X . T h e e l e c t r o p h i i i c n a t u r e of the p h o s p h a z e n e p h o s p h o r u s atom d e t e r m i n e s the s u b s t i t u t i o n o r d e r and e f fects a s t r o n g tendency f o r f o r m i n g a d o u b l e bond between p h o s p h o r u s and o x y g e n c o n n e c t e d w i t h a p a r t i a l t r a n s f e r of the e l e c t r o p h i i i c p r o p e r t i e s to the a l k y l g r o u p . T h e r e f o r e a l k o x y p h o s p h a z e n e s act as f a i r l y good a l k y l a t i o n agents f o r n u c l e o p h i l e s s u c h as H C 1 , R O and P h N M e f o r m i n g R C 1 , R 0 and P h N R M e , r e s p e c t i v e l y . In the a b s e n c e of e x t e r n a l n u c l e o p h i l e s s e l f - a l k y l a t i o n of the m o l e c u l e i s p o s s i b l e , i n w h i c h the e l e c t r o p h i i i c a l k y l g r o u p w i l l a t tack both the p h o s p h o r y l o x y g e n atom and the n i t r o g e n a t o m . A s o n l y i n the l a t t e r c a s e a s u b s t a n c e without a l k y l a t i n g p r o p e r t i e s i s formed s e l f - a l k y l a t i o n f i n a l l y r e s u l t s i n the r e a r r a n g e m e n t of O - a l k y l into N - a l k y l compounds. N - a l k y l a t i o n takes place p a r t i c u l a r l y r e a d i l y in the c a s e of m o n o a l k o x y p h o s p h a z e n e s , ( R O ) C l P = N - P O C U , w i t h R = M e , E t and C H ^ C / T i - ( 2 ) . M o n o a l k o x y p h o s p h a z e n e s h a v i n g d i f f e r e n t a l k y l g r o u p s ( R i C H , i s o - C g H , C H , C - H , C g H ) form t h e ^ N - a l k y l r e a r r a n g e m e n t p r o d u c t s o n l y at rtigner Temperatures (about 75 C ) and to an e x t r e m e l y s m a l l extent (about 1 % ) . " P e n t a e s t e r s " c a n be r e a r r a n g e d i n t o N - a l k y l d e r i v a t i v e s at about 1 5 0 ° C ( e q . 4; X = O R ; R = E t , B u , H e x ) . T h i s r e a r r a n g e m e n t i s c a t a l y s e d by a l k y l i o d i d e s . e
e
2
2
2
+
2
2
2
c
n
A f t e r a l l t h i s r e a c t i o n c a n a l s o be r e g a r d e d as a c h l o r i n e - o x y g e n e x c h a n g e . S u c h e x c h a n g e g e n e r a l l y seems to o c c u r i n r e a c t i o n s of N phosphoryl t r i c h l o r o p h o s p h a z e n e s with O - n u c l e o p h i l e s . T h i s is demon s t r a t e d by the r e a c t i o n s of Ρ Ν Ο Ο w i t h d i m e t h y l f o r m a m i d e and d i m e t h y l s u l f o x i d e ( e q . 6 , 7) as w e l l as by the fact that a l l attempts to synthesize N-dialkoxyphosphoryl trichlorophosphazenes, (RO)~P(0)-N= PC1 have f a i l e d so f a r . B o t h i n the r e a c t i o n of ( E t O ) P O N C 1 w i t h P C n ( e q . 8) and i n the r e a c t i o n of ( E t O ^ P ( O ) N H S i M e w i t h P C I ( e q . 9 i s o m e r i c diethoxyphosphazene ( E t O ) C l P = N - P O C l is obtained instead of t r i c h l o r o p h o s p h a z e n e as s h o u l d be e x p e c t e d . O n l y N - d i p h e n ?
ς
V
2
2
Cl P=N-POCl
2
+ Me NCHO
^|Me N=CClH]
Cl P=N-POCl
2
+ (CH ) SO
^
3
3
(EtO) P(0)NCl 2
2
3
2
+ PC1
(EtO) PONHSiMe 2
3
2
2
[ c ^ P O - N - P O C l J " (6)
C H ^ S - C H ^ l + (Cl PO) NH 2
^(EtO) ClP=N-POCl 2
3
+ PC1
+
5
_ f RC
(EtO) ClP=N-POCl 2
2
2
2
+ C l
2
+ Me SiCl 3
(7) (8) (9)
o x y p h o s p h o r y l t r i c h l o r o p h o s p h a z e n e , ( P h O ) P ( 0 ) - N = P C U , (3) and p r o b a b l y compounds h a v i n g d i f f e r e n t a r y l g r o u p s seem to be s t a b l e . 2
62.
RiESEL ET AL.
N-Phosphoryl
299
Phosphazenes
Due to the by-reactions (eq. 3 - 5 ) alcoholysis of CUP=N-POCl does not yield the desirable "pentaesters" (RO)^P=N-Pu(OR) . Depending on molar ratio, temperature and time the reaction mixtures contain mono-, di-and trialkoxyphosphazenes, tetrachloride and several esterchlorides of imidodiphosphoric acid. In the presence of a great excess of alcohol the final products are exclusively tetraesters of imidodiphosphoric acid, (RO) PO-NH-PO(OR) . Indeed these can be converted into "pentaesters" t>y reacting with diazoalkanes (eq. 10) (2), which, however, is not a convenient way for synthesizing "pentaesters" . 2
2
Phosphorus Chemistry Downloaded from pubs.acs.org by NATL UNIV OF SINGAPORE on 03/27/17. For personal use only.
2
(RO) PO-NH-PO(OR) + R'N 2
2
(RO) P=N-PO(OR) + N
2
3
2
(10)
2
Using alcoholates instead of alcohols "pentaesters" (4,5) and trialkoxyphosphazenes, (RO)~P=N-POCl , can be obtained in good yields. However, this method? does not allow to synthesize pure "pentae s t e r s " with different alkyl groups. 9
Formation of P=N-P-Bridges. A simple reliable synthesis of Nphosphorylated phosphazenes which avoid the handling of the dangerous phosphoryl azides consists in a direct reaction of d i - and t r i a l k y l phosphites and carbon tetrachloride with sodium azide in a single step procedure (eq. 11). The phosphoryl azides formed intermediately i n stantly react with the trialkylphosphites present. Therefore, their ( R O ) P + ( R O ) P ( 0 ) H + CC1 3
2
— (ROX^N-PCXOR'^
+ NaN^
4
+ CHC1 + NaCl
(11)
3
concentration is kept small all the time. In contrast to the classic Atherton-Todd reaction addition of amines is not necessary. In the absence of trialkylphosphite no reaction occurs; the formation of phosphoryl azides does not occur either. We used phosphites of aliphatic and aromatic alcohols and synthesized about 40 different "pentaesters" with yields up to 95 %. As by-products CI C-P(0)(OR) and small amounts of rearrangement products, (RO) Pîfo)-NR-P(0)COR ) » (
