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61 Dihydrophenophosphazines via the Interaction of Diarylamines and Phosphorus Trichloride: Applications and Limitations H A R O L D S. F R E E M A N and L E O N D . F R E E D M A N Department of Chemistry, North Carolina State University, Raleigh, NC 27650
In 1971 i t was shown (1) that the interaction of diphenylamine and phosphorus trichloride at 210° (followed by treatment of the reaction mixture with water) yields not only the previously described phosphine oxide 1 but also the spirophosphonium chloride 2.
The mechanism of formation of the latter compound (a derivative of PV) from phosphorus trichloride has not been elucidated, but the following mechanism suggested (2) for the formation of 1 seems reasonable:
0097-6156/81/0171-0291$05.00/0 © 1981 American Chemical Society
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Studies (2,3) have also been reported of the reaction of phosphorus trichloride with diarylamines containing p-methyl or p-chloro substituents. In every case, the expected ring-substituted derivatives of 1 and 2 were obtained after the reaction mixture was treated with water. The interaction of N-phenyl-O-toluidine and phosphorus trichloride at 200°C also gave a reaction mixture from which the expected phosphine oxide was isolated (3). None of the corresponding spirophosphonium chloride, however, could be obtained. The failure to isolate this substance can not be explained simply by the presence of an ortho substituent in the diarylamine, since i t had been previously found that a 34% yield of a spirophosphonium chloride can be obtained via the interaction of N-phenyl-1-naphthylamineand phosphorus trichloride (2). No dihydrophenophosphazine derivatives at a l l were obtained by the interaction of di-O-tolylamine and phosphorus trichloride at 200°C (3). It was possible, however, to obtain a small yield of the expected phosphine oxide (isolated as the phosphinic acid) by converting di-O-tolylamine to the corresponding phosphoramidous dichloride, (O-MeC6H4)2NPCl2, dehydrohalogenating the latter substance, and treating the reaction mixture with water. The present paper is concerned with the reaction between phosphorus trichloride and the meta-substituted diarylamines listed in Table I. The TABLE I DIARYLAMINES STUDIED H
3a
R
i
-
3b
R
i
-
3c
R
i
-
3d
R
i
-
R
i
-
3e
3-CF
3
3,5-(CF ) 3
3,5-Me
2
3-Me 3-CF
3
2
R
2
R
2
R
2
R
2
R
2
= H = H = H = 3-CF
3
= 3-CF
3
s u b s t i t u e n t s i n c l u d e both the a c t i v a t i n g , ortho, p a r a - d i r e c t i n g methyl group and the d e a c t i v a t i n g , m e i a - d i r e c t i n g t r i f l u o r o methyl group. Each of the amines was heated with phosphorus t r i c h l o r i d e at 220-250°C f o r about 17 h, and the r e a c t i o n mixtures were then t r e a t e d with water. The f i r s t three amines (3a-3c) y i e l d e d modest amounts of phosphine oxides but no
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Dihydrophenophosphazines
293
spirophosphonium c h l o r i d e s , while the other two amines gave no organophosphorus compounds at a l l . A phosphine oxide was obtained from the f o u r t h amine (3d), however, v i a dehydrohalogenation o f the phosphoramidous d i c h l o r i d e a t 220°C. The l a s t amine (3e) could a l s o be converted to a phosphoramidous d i c h l o r i d e , but the l a t t e r substance could not be dehydrohalogenated to a dihydrophenophosphazine d e r i v a t i v e even a t 255°C. An H NMR study of the phosphine oxides obtained from amines 3a and 3d i n d i c a t e d that each oxide c o n s i s t s of a s i n g l e isomer. Of the two p o s s i b l e i s o m e r i c products from 3a, only 3-trifluoromethyl-5,10-dihydrophenophosphazine 10-oxide (4a) was a c t u a l l y i s o l a t e d . The % NMR spectrum o f t h i s substance i n deuterated DMSO showed a s i n g l e peak f o r the N-H group and two s i g n a l s assigned to H and (Jp_H ^ s i g n a l was about 13 Hz). S i m i l a r l y , an a n a l y s i s o f the ^H NMR spectrum of the phosphine oxide obtained from amine 3d i n d i c a t e d that only 3-methy1-7-trifluoromethyl-5,10-dihydrophenophosphazine 10-oxide (4b) was present; no evidence f o r the formation of any o f the three other p o s s i b l e isomers was obtained. ±
o
r e
a
c
n
a
4a
R = Η
4b
R = Me
S t e r i c f a c t o r s probably play a key r o l e i n determining the r e g i o s p e c i f i c i t y o f the r e a c t i o n s l e a d i n g to the phosphine oxides 4a and 4b. An intermediate i n v o l v e d i n both cases i s probably o f type 5, i n which r i n g c l o s u r e p r e f e r e n t i a l l y occurs para r a t h e r than ortho to the bulky CF3 group. The pre f e r e n t i a l formation o f 5b from the corresponding phosphor amidous d i c h l o r i d e i s undoubtedly a s s o c i a t e d with the f a c t that Η
5a
R = Η
5b
R = Me
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the 6 - p o s i t i o n (para to the methyl group) i s l e s s hindered than the 2 - p o s i t i o n (which i s ortho to both the methyl group and the NHC H CF group). The r e s u l t s obtained i n t h i s i n v e s t i g a t i o n suggest that dihydrophenophosphazine s y n t h e s i s from a diarylamine r e q u i r e s that a t l e a s t one r i n g o f the amine must be free o f a metad i r e c t i n g group. I f t h i s requirement i s f u l f i l l e d , the PCI2 group bonded to the n i t r o g e n o f the phosphoramidous d i c h l o r i d e can migrate to an ortho p o s i t i o n of that r i n g , and c y c l o dehydrohalogenation w i l l subsequently occur. The r e g i o s p e c i f i c i t y noted i n the two cases where more than one i s o m e r i c product seemed p o s s i b l e i n d i c a t e s that s t e r i c f a c t o r s must be important i n determining both the s i t e to which the PCI2 group of the phosphoramidous d i c h l o r i d e migrates and the s i t e a t which r i n g c l o s u r e occurs. 6
4
3
Literature Cited 1. 2. 3.
J e n k i n s , R.N.; Freedman, L.D.; Bordner, J . Chem. Commun. 1971, 1213. Jenkins, R.N.; Freedman, L.D. J . Org. Chem. 1975, 40, 766. B u t l e r , J.R.; Freeman, H.S.; Freedman, L.D. Phosphorus S u l f u r 1981, 9, 269.
RECEIVED
July 7,
1981.
