89 Monocyclic Phosphoranide and Phosphoranoxide Anions P ( V ) Oxyphosphorane C a r b a n i o n — P ( I V ) Ylide A l k o x i d e Tautomerism ITSHAK GRANOTH, RIVKA ALKABETS, EZRA SHIRIN, YAIR MARGALIT and PETER BELL Israel Institute for Biological Research, Ness-Ziona 70450, Israel
Pentacoordinate hydroxyphosphoranes are likely intermediates or transition states in substitution reactions at tetracoordinate phosphorus (1). Recently, stable hydroxyphosphoranes (2, 3) and their conjugate bases - metal phosphoranoxides (4, 5) have been isolated. Spectroscopic evidence (4 - 7) for equilibria between Ρ(IV) compounds and hydroxyphosphoranes have been reported. Ob servation (8) and isolation (9) of Ρ(IV) TBP phosphoranide species have also been announced. A l l these phosphoranes are stabilized by several features dominated by their spirobicyclic nature. Monocyclic Phosphoranide Anion. The intramolecular oxida tive addition of hydroxyalkyl phosphites, which gives P-H phos phoranes, is well known (10). Some P-H phosphoranes are so stable that the open-chain P(III) tautomers cannot be detected spectroscopically or even by attempted H2O2 oxidation (8). Thus, it is surprising to find no evidence for an equilibrium between phosphine alcohol 1 and its closed-ring tautomer phosphorane 2. Phosphine 1 is quaternized by alkyl halides giving phosphonium halides such as 3. These in turn are converted to alkoxyphosphoranes, such as 4, by NaH (Scheme I). Phosphine 1 shows the expected P NMR δP-11.1 (THF), typi cal (11) of ortho substituted triphenylphosphine. Addition of LiAlH4 to this solution gives molecular hydrogen and two signals in the P NMR spectrum at -11.1 and -30.3. This observation sug gests that deprotonation of jL leads to a relatively slow equili brium of phosphine alkoxide _5 (major component) and phosphoranide This mixture and alkyl halides give phosphoranes such as 4_. The instability of phosphorane 2_ is surprising because accommodates four features which are known to stabilize hypervalent compounds (12). The central phosphorus atom in the TBP 2_ bears two highly apicophilic and three poorly apicophilic ligands (2). The phosphorus atom is contained in a five-membered ring linking an apical oxygen to an equatorial aromatic ring carbon (12). The gem-dimethyl conformational effect favors a closed ring structure. The increased stability of phosphoranide 6·, com3l
3 1
0097-6156/81/0171-0435$05.00/0 © 1981 American Chemical Society
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PHOSPHORUS CHEMISTRY
Scheme I
4,
δΡ-60.7
6_,
δΡ-30.3
p a r e d w i t h i t s c o n j u g a t e a c i d _2, i s a n a l o g o u s t o t h e e n h a n c e d s t a b i l i t y o f r e p o r t e d p h o s p h o r a n o x i d e a n i o n s (3_4_» 5) . T h i s may r e s u l t from t h e i n c r e a s e d e l e c t r o n e g a t i v i t y d i f f e r e n c e between a p i c a l and e q u a t o r i a l l i g a n d s i n t h e s e TBP b a s e s , compared w i t h t h e i r respective conjugate acids. P h o s p h o r a n e - Y l i d e T a u t o m e r i s m . The l a b i l i t y o f α p r o t o n s i n a l k y l p h o s p h o r a n e s i s n o t known. D e u t e r i u m e x c h a n g e o f t h e b e n z y l p r o t o n s i s n o t o b s e r v e d , e v e n i n t h e p r e s e n c e o f NaOD i n D2OCD3SOCD3. D e p r o t o n a t i o n o f 4_ i n THF b y C H g L i a t room t e m p e r a t u r e is fast. T h i s r e d s o l u t i o n i s shown b y v a r i a b l e t e m p e r a t u r e 31p NMR t o c o n t a i n a n e q u i l i b r i u m m i x t u r e o f p h o s p h o r a n e 7_ and y l i d e 8_ (Scheme I I ) . T h i s m i x t u r e a n d CH3I g i v e p h s o p h o r a n e j ) . s
Scheme I I
9,
6P-54.9
89.
GRANOTH E TAL.
Phosphoranide
and Phosphoranoxide
J
Anions
437
±
Monocyclic Phosphoranoxide Anion. The P NMR c h e m i c a l s h i f t v a l u e and l i n e w i d t h o f 10 i s s o l v e n t and pH s e n s i t i v e , s u g g e s t i n g t h e e q u i l i b r i a shown i n Scheme I I I . The s t r u c t u r e o f 11 i s c o n f i r m e d b y p r e p a r a t i o n and c h a r a c t e r i z a t i o n o f t h e a n a l o gous s t a b l e BF4"" s a l t . D e p r o t o n a t i o n o f 1Ό b y NaH i n THF i s much f a s t e r than t h a t o f i t s para isomer. This deprotonation i s f o l lowed b y b r o a d e n i n g , g r a d u a l u p f i e l d s h i f t and r e s h a r p e n i n g o f the P NMR s i g n a l f r o m 38.5 t o -31.0 ppm. The l a t t e r , somewhat b r o a d l i n e i s c o n s i s t e n t w i t h s t r u c t u r e 14 i n e q u i l i b r i u m w i t h a s m a l l c o n c e n t r a t i o n o f 13. 3 1
Scheme I I I
The e q u i l i b r a t i n g 14 and 13^ r e a c t s u r p r i s i n g l y f a s t with CH3I, g i v i n g ether L5 as the s o l e product. The para isomer o f 13 does not r e a c t with CH3I i n THF a t room temperature during 24 hours. The e x t r a o r d i n a r y r a p i d formation of 15, presumably from 13, may r e s u l t from the increased r e a c t i v i t y of 13 enabled by i n t r a m o l e c u l a r s o l v a t i o n of the metal by the phosphine oxide oxygen. A l t e r n a t i v e mechanisms, such as a methoxyphosphorane intermediate, cannot be r u l e d out a t t h i s stage.
Literature Cited 1. Westheimer, F.H. Acc. Chem. Res. 1968, 1, 70. 2. Segall, Y . ; Granoth I. J . Am. Chem. Soc. 1978, 100, 5130. 3. Granoth, I.; Martin, J.C. J . Am. Chem. Soc. 1979, 101, 4618.
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4. 5. 6. 7. 8. 9. 10. 11. 12.
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CHEMISTRY
Granoth, I.; Martin, J.C. J . Am. Chem. Soc. 1978, 100,5229. Munoz, Α.; Garrigues, B; Koenig, M. J.C.S. Chem. Comm. 1978, 219. Gallagher, M.; Munoz, Α.; Gence, G.; Koenig, M. J.C.S. Chem. Comm. 1976, 321. Ramirez, F . ; Nowakowsky, M; Marecek, J . F . J . Am. Chem.Soc. 1977, 99, 4515. Granoth, I.; Martin, J.C. J . Am. Chem. Soc. 1979, 101, 4623. Garrigues, B.; Koenig, M.; Munoz, A. Tetrahedron Lett. 1979, 4205. Burgada, R. Bull. Soc. Chim. France 1975, 407. Grim, S.O.; Yankowsky, A.W. Phosphorus and Sulfur 1977, 3, 191. Martin, J . C . ; Perozzi, E.F. J . Am. Chem. Soc. 1974, 96, 3155.
RECEIVED
July 7, 1981.
