Ind. Eng. Chem. prod. Res. Dev. 1981, 20, 77-79
77
Organofluorophosphazenes. A Short Review Christopher W. Allen Depertment of Chemistry, University of Vemwnt, Wlngfon, Vemwnt 05405
The reactions of fluorophosphazenes, (NPF2),, with organolithium and Grignard reagents are reviewed. Both substitution
and degradation processes are observed and the factors favoring each are discussed. The capability of producing organofluorophosphazenes with a wide range of substituents and geometrical configurations exists. These materials have been characterized by a variety of physical methods. Some possibilities of the applications potential for organophosphazenesas thermally stable and/or fire retardant materials are mentioned.
The chemistry of cyclic and polymeric phosphazenes, (NPX2),, is a rich and varied one which continues to attract interest in the chemical community (1-4). Traditionally, reactions of the phosphazenes have primarily involved nitrogen or oxygen bases leading to the appropriate amino, alkoxy or aryloxy derivatives (1,3). In recent years, however, it has been shown that the reactions of organometallic reagents with fluorophosphazenes lead to the formation of a wide variety of organophosphazenes. These compounds are of interest for a number of reasons. The reactions involved in the synthesis of organofluorophosphazenes are among the most complex involving phosphazenes and consequently an understanding of these processes is essential to an understanding of phosphazene chemistry. The organophosphazenes are expected to exhibit superior thermal stability compared to the amino and alkoxy (or aryloxy) derivatives (2,4). This observation can be related, in part, to the cross-linking (5) and rearrangement reactions (6) available to the nitrogen and oxygen based derivatives (2, 4). Finally, many organophosphazenes have the potential for further synthetic transformations which lead to new and interesting organophosphazenes and organophosphazene polymers (7). The preponderance of organofluorophosphazenesover the chloro analogues results from the limited number of routes available for the synthesis of organochlorophosphazenes. Studies of the reactions of organolithium and organomagnesium reagents with chlorocyclophosphazenes demonstrate the occurrence of simple substitution (8,9),ring contraction and rearrangement (8,IO), and primarily degradation processes (8, 10-12). These results are rationalized by a model developed by Shaw (11) wherein the phosphazene ring nitrogen atoms coordinate to the metal atom in the organometallic reagent. The resulting drain of electron density from the phosphorusnitrogen bond allows for ring opening reactions. Consequently, the major routes to organochlorophosphnes are the aminolysis of organochlorophosphoranes,R,PCl&, (n = 1,2), and the Friedel-Crafts reaction, both of which have severe limitations (1). Recently, Allcock and co-workers have shown that Grignard reactions catalyzed by certain copper complexes represent a promising route to a broad range of organochlorophosphazenes (13). In the fluorophosphazenes, the strong electron-withdrawing effect of the fluorine atoms results in reduced ring nitrogen atom basicity, and consequently degradative processes in reactions with organometallic reagents are less prevalent. This observation coupled with a facile synthesis of fluorophosphazenes (14) opens the way for the prepa(NPC12), + 2nNaF
cmo,
(NPF2), + 2nNaC1
Table I. Synthesis of Disubstituted Organofluorocyclotriphosphazenes,N,P, F4R, isomer distribution" cis- transreagent 2,2 2,4 2,4 C,H,Li 6 70 24 75 25 o-tolyl Li C6H5MgBr 100 p-( CH,),NC,H,Li 50 50 p-(CH3 )2NC6H4MgBr 50 50 CH3Li 100 100 (CH,),Cfi CH,CH=CHLi 100 100 CH,=C( OC,H,)Li C,H5C=CLi 100 100 (CH,),SiC=CLi
ref 16 16 18 22 22 25 26 27 28 29 30
Expressed as percent of total cyclic product isolated.
ration of a wide variety of organofluorophosphazenesas demonstrated by the work of Moeller (15). The most extensive series of organofluorocyclophosphazenes are the aryl derivatives of hexafluorocyclotriphosphazene, N P p 6 , which are conveniently prepared from the appropriate organometallic reagents. in these N3P3F6+ nArM N3P3F6-,Ar,, + nMF Ar = C& (15-18),C a s ( I g ) , 3,5-C6H3Dz (2O), p-C6H4X;X = F (19,21),C1 (21), OCH3 (21), N(CHJ2 (22), CH3 (15, 16);M = Li, MgBr
-
reactions range from 30 to 70%. Lower degrees of substitution (n = 1,2)have been more thoroughly examined than higher ones (n = 3-6). While extensive substitution can be effeded with phenyllithium (15) Grignard reagents give increased amounts of degradation with increased organometallic to phosphazene ratios (22). Mixed aryl/ phenylphosphazenes can be prepared via the FriedelCrafts reaction in which a s P F A r group is converted to a =P(C&15)Ar group. It has been shown that the partial
X = H (17,23),F (21), C1 (21), OCH3 (21), N(CH312 (22), CH3 (21) NnCk N(C9Hda
~ , ~ - N ~ P ~ F ~ [ C ~ H ~ N.c( C H ~ ) Z I ~ 2,2,4,4-N3P3F4(C~H6)z[C6H4N(CH3)2I2 (22) derivatization of polydifluorophosphazene,(NPF2),, can be effected with phenyllithium (24). Consequently, the potential for preparation, via the organolithium reaction, of poly(organofluoro)phosphazenes with a variety of organic substituents exists. There are three poesible isomers (excluding enantiomers) for the disubstituted phosphazenes N3P3F4R2(Figure 1). An examination of Table I will show an almost bewildering array of isomeric combinations resulting from the reactions of organometallic reagents with N3P3FB.The isomer distribution in aryl fluorocycbtriphosphanes can be significantly modified by changes in either the metal (18) or the aryl function (22) in the organometallic reagent. The 0 1981 American Chemical Society
found in the aryl systems. The reactions of a variety of alkyllithium reagents with N3P3F6have been examined. N3P3F6+ nRLi N3P3FG-,R, + nLiF R = CH3 (25,36), n-C4Hg (37), t-C4Hg (26), C6Hll (38)
-
2,2 (geminal)
cis-2,b (non-geminal)
trans-2.4 (non-geminal)
Figure 1. Isomeric disubstituted fluorocyclotriphosphazenes. CH
e?,
C6H5
/
N;P,4N-161. F\I F/p\
8 pm l+F--153.9
pm
/p\F
