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Thermal rearrangement reactions of methoxycyclophosphazenes
INORG. NUCL. CHEM. LETTERS Vol.15, pp. 109-112 Pergamon Press Ltd. 1979. Printed in Great Britain
THERMAL REARRANGEMENT REACTIONS OF I~THOXYCYCLOPHOSPHAZEhTES K.S.Dhathathreyan, S.S. Krishnamurthy and A.R.Vasudeva Murthy Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bang,lore-560012, India Robert A. Shaw and Michael Woods, Department of Chemistry, Birkbeck College (University of London), M~let Street, London WCIE 7HX, U.K. (Received
6
December 1978; received for publication 13 December 1978)
Alkoxycyclophosphazenes find use as flame retardants for textile fibres (1). The thermal rearrangement of alkoxycyclophosphazenes (2) merits a detailed study in view of its relevance to understanding the transformations assoCiated with this flame retardant behavio~r. We report here our preliminary results on the thermal behaviour of some methoxycyclophosphazenes and the 1H, 31p and 13C ~IR spectroSCOpic data for the products. The rearrangement of the N~P3(O~e) 8 _
methoxycyclophosphazenes,
(1) and N4P4(OMe)8(II) to the respective oxocyelo-
phosphazanes, N3~e3P303(O~e) 3 (III) (70% yield) and N4Me4P404(OMe)4 (IV) (80%) takes place at 150-160°C under reduced pressure (1-2 ram). The rearrangements can be monitored by TLC and are complete in 3.5 h and 6.5 h respectively.
If the heating is prolonged (>7 h) or carried out
in air, the yields of the oxocyclophosphazanes (III or IV) decrease and a resinous material is obtained which is insoluble in organic solvents. Compound IV is a mixture of two isomers (IVa and IVb) which can be separated by fractional crystallisation. The yields of these two products are in the ratio 7s3 as estimated visually from TLC; isomer IVa has a higher Rf value. Crystallographic data (cell dlmensions)reveal that these isomers (IVa and IVb) are identical to the compounds obtained in the rearrangement of N4P4(OMe)8(II) catalysed by methyl iodide (3). A previous report has indicated that the decamethoxy derivative, NsP 5 (OMe)lo(V) undergoes decomposition on heating (4). We observe that N5P5(OMe)Io(V) undergoes IO9
II0
Thermal Behaviour of Methoxycyclophosphazenes
complete rearrangement to oxocyclopentaphosphazanes * when heated at 150 ° rudder reduced pressure (1-2 mm) for 3 h. TLC shows the presence of three components with very close Rf values and the absence of starting material (V). Attempts to separate the mixture have not yet been successful. The 1H NMR spectrum (270 MHz) of the cyclotriphosphazane, N3Me 3P303(OMe) 3(III) in deutercchloroform shows that -OM_~e and -]~_~e protons are in two environments (each in the ratio ls2)
L 3-5
8
j
~0
8
3.0
(a) (b) FIG.I (a) IH m,m spectrum (270 ~ z , CDC13) and (b) 31p(1H} NMR spectrum (36.43 ~ z ,
CDC13) of N3Me3P3(O)3(OMe)3(III ).
[Fig.l(a)]. A corresponding non-equivalence of O-13CH 3 [6 ,, 53o88(2), 53.63(1)] and N-13CH 3 r6 = 33.57(1), 31.22(2)] is observed in the 13C {1H} l~IR spectrum (67.9 MHz; ref. T~,~S]. These observations cain be explained on the basis of the crystallographic data for compound III (5); the distorted boat structure in the solid is presumably retained in solution. The non-equivalence of protons and carbon nuclei is probably due to the magnetic anisotropy associated with the phosphoryl groups (6). I~ is interesting to note that the phosphorus nuclei are in two environments as revealed by the AB 2 spectrum [Fig.l(b)]. This spectrum appears to be the first * The intensities of -OCH_3 and -NCH 3 multlplets (centred at 3.85 and 3.10 6 respectively) are in the ratio lsl.
Thermal Behaviour of Methoxycyclophosphazenes example reported for compounds of this type.
The shifts of
the phosphorus nuclei for the oxocycl~phosphazane
(iII)
(6p 9.4, 6.6) are considerably upfield to that of hexamethoxycyclotriphosphazene, N3P3(OMe) 6 (Sp 21.7). The 1H ~!,[R spectrum of compound III has also been recorded in chl~robenzene at 34, 60 and 99°C.
The separa-
tions of the non-eqaivalent -OChi3 a~d -~CH_3 signals are only marginally decreased (1-2 iz) wi~h increasing temperature, which i~dicates the rigidity of the six-membered cyclophosphazane ring in solution. From the variable temperature ii~,li~data, the enthalpy of activation for scr~llbling or inversion of the cyclotriphosphazane ring can be calculated approximately by a standard procedure (7) and is >20-25 kcal/mole. The 1H ~IR spectra of the cyclotetraphosphazanes, ~4~le4P404(OMe)4 (IVa, IVb) show only a single environ!nen~ for both -OCH~ and -NCH~ protons. ~ The 31p {1H} spectra are singlets at 8.03 ~nd 7.47 6 respectively. The 13C {IH} ~ R spectra in CDCl~ ~_lso show single environments for -O13CHx_~ and -N13CH~_ ~ for 13
6
each isomer ~(IVa): -O C_H3, = 5~.90;6 - ~ (iVb): -O13CH3, 6 = 54.07; -~I3c_H3, = 34.28].
6 = 33.90
The 13C
spectrum of a cyclotetraphosphazane, N4Me4P404(ON~e)4 obtained from the thermal rearrangement of compound II has been reported recently by Mochel and Cheng (8) and the -~:13Cix j
doublet observed at ca. 36.8 6 is ascribed to two slightly different methyl environments in the molecule. Our results suggest that the doublet is due to the presence of two isomers [(IVa) and (IVb)] in the rearranged sample. We have also studied the rearrangement reactions of some me thoxy (alkyl amino )- and me thoxy (phenyl)-cyclo tripho sphazene s. The geminal compounds, N3P3(0Me)4Ph2 axad N3P3(0Me)4(NHBut)2 readily rearrange to give N-methylcyclophosphazanes as indicated by the 1H NkIR spectra of the products. TLC shows the absence of any starting materials.
The nongeminal derivative,
2,2,4, 6~ 2-trans-4-N3P3(OMe)4(NMe2) 2 undergoes rearrangement very slowly. Even after heating a sample at 150 ° (1-2 mm pressure) for 7 h, most of the starting material (75%) could The -O-C~3 ~id -N-C~3 doublets exhibit pronounced virtual coupling. The chemical shifts e~d 3J'(P-H) values are in general agreement with those reported (3a). However, the 3J~(P-H) values for the more abundant isomer (IVa) are approximately 1.5 Hz lower than our values.
If2
Thermal Behaviour of Methoxycyclophosphazenes
be recovered after distillation under vacuum.
The brown
viscous, oily residue (~ 10%) has a 1H NMR spectrum anticipated for a rearranged product (signals observed for =P(0)0Me, -NMe and -NM_~2 groups). Further investigations are being carried out to characterlse the products fully and to elucidate the effect of substituents on the thermal rearrangement of alkoxypho sphaz ene s. ACKNOWLEDGEMENT We thank the U.G.C. (New Delhi) and 0.D.M. (U.K.) for support; the Bangalore I~MR Facility for 1H (270 MHz) and 13C spectra; ULIRS (King's College, London) for 31p spectra and Mr. M. Krishnlah and Professor K. Venkatesan for the measurement of cell dimensions. REFERENCES 1. L.E.A. GODFREY and J.W. SCHAPPEL, Ind. Eng. Chem. Prod. Res. Develop. 9, 426 (1970). 2. B.W. FITZSIk~0NS, C. HEWLETT and R.A. SHAW, J. Chem. Soc.
4459 (1964). 3. (a) G.J. BULLEN, N.L. PADDOCK and D.J. PATMORE, Acta Cryst. 33B, 1367 (1977). (b) G.J. BULLEN, Personal Communication. 4. F. P~ALLE, Ric. Scl. Rend., Sez. A. 8, 1134 (1965). Chem. Abst. 64, 15729 b_ (1966). 5. G.B. ANSELL and G.J. BULLEN, J. Chem. Soc. (A) 3026 (1968). 6. F. RA~IREZ, A.V. PATWARDHAN, N.B. DESAI and S.R. HELLER, J. Amer. Chem. Soc. 87, 549 (1965). 7. A. ALLERHAND, H.S. GUTOWSKY, J. JONAS and R.A. M~INZER, J. Amer. Chem. Soc., 88, 3185 (1966). 8. V.D. M O C ~ L and T.C. CHENG, Macromolecules ll, 176 (1978).
THERMAL REARRANGEMENT REACTIONS OF I~THOXYCYCLOPHOSPHAZEhTES K.S.Dhathathreyan, S.S. Krishnamurthy and A.R.Vasudeva Murthy Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bang,lore-560012, India Robert A. Shaw and Michael Woods, Department of Chemistry, Birkbeck College (University of London), M~let Street, London WCIE 7HX, U.K. (Received
6
December 1978; received for publication 13 December 1978)
Alkoxycyclophosphazenes find use as flame retardants for textile fibres (1). The thermal rearrangement of alkoxycyclophosphazenes (2) merits a detailed study in view of its relevance to understanding the transformations assoCiated with this flame retardant behavio~r. We report here our preliminary results on the thermal behaviour of some methoxycyclophosphazenes and the 1H, 31p and 13C ~IR spectroSCOpic data for the products. The rearrangement of the N~P3(O~e) 8 _
methoxycyclophosphazenes,
(1) and N4P4(OMe)8(II) to the respective oxocyelo-
phosphazanes, N3~e3P303(O~e) 3 (III) (70% yield) and N4Me4P404(OMe)4 (IV) (80%) takes place at 150-160°C under reduced pressure (1-2 ram). The rearrangements can be monitored by TLC and are complete in 3.5 h and 6.5 h respectively.
If the heating is prolonged (>7 h) or carried out
in air, the yields of the oxocyclophosphazanes (III or IV) decrease and a resinous material is obtained which is insoluble in organic solvents. Compound IV is a mixture of two isomers (IVa and IVb) which can be separated by fractional crystallisation. The yields of these two products are in the ratio 7s3 as estimated visually from TLC; isomer IVa has a higher Rf value. Crystallographic data (cell dlmensions)reveal that these isomers (IVa and IVb) are identical to the compounds obtained in the rearrangement of N4P4(OMe)8(II) catalysed by methyl iodide (3). A previous report has indicated that the decamethoxy derivative, NsP 5 (OMe)lo(V) undergoes decomposition on heating (4). We observe that N5P5(OMe)Io(V) undergoes IO9
II0
Thermal Behaviour of Methoxycyclophosphazenes
complete rearrangement to oxocyclopentaphosphazanes * when heated at 150 ° rudder reduced pressure (1-2 mm) for 3 h. TLC shows the presence of three components with very close Rf values and the absence of starting material (V). Attempts to separate the mixture have not yet been successful. The 1H NMR spectrum (270 MHz) of the cyclotriphosphazane, N3Me 3P303(OMe) 3(III) in deutercchloroform shows that -OM_~e and -]~_~e protons are in two environments (each in the ratio ls2)
L 3-5
8
j
~0
8
3.0
(a) (b) FIG.I (a) IH m,m spectrum (270 ~ z , CDC13) and (b) 31p(1H} NMR spectrum (36.43 ~ z ,
CDC13) of N3Me3P3(O)3(OMe)3(III ).
[Fig.l(a)]. A corresponding non-equivalence of O-13CH 3 [6 ,, 53o88(2), 53.63(1)] and N-13CH 3 r6 = 33.57(1), 31.22(2)] is observed in the 13C {1H} l~IR spectrum (67.9 MHz; ref. T~,~S]. These observations cain be explained on the basis of the crystallographic data for compound III (5); the distorted boat structure in the solid is presumably retained in solution. The non-equivalence of protons and carbon nuclei is probably due to the magnetic anisotropy associated with the phosphoryl groups (6). I~ is interesting to note that the phosphorus nuclei are in two environments as revealed by the AB 2 spectrum [Fig.l(b)]. This spectrum appears to be the first * The intensities of -OCH_3 and -NCH 3 multlplets (centred at 3.85 and 3.10 6 respectively) are in the ratio lsl.
Thermal Behaviour of Methoxycyclophosphazenes example reported for compounds of this type.
The shifts of
the phosphorus nuclei for the oxocycl~phosphazane
(iII)
(6p 9.4, 6.6) are considerably upfield to that of hexamethoxycyclotriphosphazene, N3P3(OMe) 6 (Sp 21.7). The 1H ~!,[R spectrum of compound III has also been recorded in chl~robenzene at 34, 60 and 99°C.
The separa-
tions of the non-eqaivalent -OChi3 a~d -~CH_3 signals are only marginally decreased (1-2 iz) wi~h increasing temperature, which i~dicates the rigidity of the six-membered cyclophosphazane ring in solution. From the variable temperature ii~,li~data, the enthalpy of activation for scr~llbling or inversion of the cyclotriphosphazane ring can be calculated approximately by a standard procedure (7) and is >20-25 kcal/mole. The 1H ~IR spectra of the cyclotetraphosphazanes, ~4~le4P404(OMe)4 (IVa, IVb) show only a single environ!nen~ for both -OCH~ and -NCH~ protons. ~ The 31p {1H} spectra are singlets at 8.03 ~nd 7.47 6 respectively. The 13C {IH} ~ R spectra in CDCl~ ~_lso show single environments for -O13CHx_~ and -N13CH~_ ~ for 13
6
each isomer ~(IVa): -O C_H3, = 5~.90;6 - ~ (iVb): -O13CH3, 6 = 54.07; -~I3c_H3, = 34.28].
6 = 33.90
The 13C
spectrum of a cyclotetraphosphazane, N4Me4P404(ON~e)4 obtained from the thermal rearrangement of compound II has been reported recently by Mochel and Cheng (8) and the -~:13Cix j
doublet observed at ca. 36.8 6 is ascribed to two slightly different methyl environments in the molecule. Our results suggest that the doublet is due to the presence of two isomers [(IVa) and (IVb)] in the rearranged sample. We have also studied the rearrangement reactions of some me thoxy (alkyl amino )- and me thoxy (phenyl)-cyclo tripho sphazene s. The geminal compounds, N3P3(0Me)4Ph2 axad N3P3(0Me)4(NHBut)2 readily rearrange to give N-methylcyclophosphazanes as indicated by the 1H NkIR spectra of the products. TLC shows the absence of any starting materials.
The nongeminal derivative,
2,2,4, 6~ 2-trans-4-N3P3(OMe)4(NMe2) 2 undergoes rearrangement very slowly. Even after heating a sample at 150 ° (1-2 mm pressure) for 7 h, most of the starting material (75%) could The -O-C~3 ~id -N-C~3 doublets exhibit pronounced virtual coupling. The chemical shifts e~d 3J'(P-H) values are in general agreement with those reported (3a). However, the 3J~(P-H) values for the more abundant isomer (IVa) are approximately 1.5 Hz lower than our values.
If2
Thermal Behaviour of Methoxycyclophosphazenes
be recovered after distillation under vacuum.
The brown
viscous, oily residue (~ 10%) has a 1H NMR spectrum anticipated for a rearranged product (signals observed for =P(0)0Me, -NMe and -NM_~2 groups). Further investigations are being carried out to characterlse the products fully and to elucidate the effect of substituents on the thermal rearrangement of alkoxypho sphaz ene s. ACKNOWLEDGEMENT We thank the U.G.C. (New Delhi) and 0.D.M. (U.K.) for support; the Bangalore I~MR Facility for 1H (270 MHz) and 13C spectra; ULIRS (King's College, London) for 31p spectra and Mr. M. Krishnlah and Professor K. Venkatesan for the measurement of cell dimensions. REFERENCES 1. L.E.A. GODFREY and J.W. SCHAPPEL, Ind. Eng. Chem. Prod. Res. Develop. 9, 426 (1970). 2. B.W. FITZSIk~0NS, C. HEWLETT and R.A. SHAW, J. Chem. Soc.
4459 (1964). 3. (a) G.J. BULLEN, N.L. PADDOCK and D.J. PATMORE, Acta Cryst. 33B, 1367 (1977). (b) G.J. BULLEN, Personal Communication. 4. F. P~ALLE, Ric. Scl. Rend., Sez. A. 8, 1134 (1965). Chem. Abst. 64, 15729 b_ (1966). 5. G.B. ANSELL and G.J. BULLEN, J. Chem. Soc. (A) 3026 (1968). 6. F. RA~IREZ, A.V. PATWARDHAN, N.B. DESAI and S.R. HELLER, J. Amer. Chem. Soc. 87, 549 (1965). 7. A. ALLERHAND, H.S. GUTOWSKY, J. JONAS and R.A. M~INZER, J. Amer. Chem. Soc., 88, 3185 (1966). 8. V.D. M O C ~ L and T.C. CHENG, Macromolecules ll, 176 (1978).