and l. - university of guelphch (n = 3, 4) and me(nhme)ph4p3n2ch, formed by a rearrangement in which...

The deprotonation and rearrangement of N-methyl methylphosphazenium quaternary salts: a novel synthetic route to cyclic azaphosphorins

RICHARD T. OAKLEY AND NORMAN L. PADDOCK' Department of Chemistry, University ofBritish Columbia, 2075 Wesbrook Place, Vancouver, B.C., Canada V6T lW5

Received February 2, 1977

RICHARD T. OAKLEY and NORMAN L. PADDOCK. Can. J. Chem. 55, 3651 (1977). The N-methyl methylphosphazenium salts (NPMe,),,,.MeI and N3P3Ph4Me2.MeI can be

deprotonated by a variety of bases to yield the novel azaphosphorins Me2,-,(NHMe)P,N,- ,- CH (n = 3, 4) and Me(NHMe)Ph4P3N2CH, formed by a rearrangement in which the methylated nitrogen atom is displaced from the PN ring by the initially produced exocyclic methylene group. The 'H nmr spectra of the azaphosphorins indicate a rapid proton exchange between the endocyclic carbon and the exocyclic nitrogen, which can be slowed by the addition of an auxiliary base. When KO-t-Bu reacts with the quaternary salts, nucleophilic attack competes with proton removal, and the linear oxides (NHMe)(PMe2N).PMe20 (n = 2-4) have been isolated from these reactions. The azaphosphorins Me2,-l(NHMe)P,N,-lCH (n = 3, 4) are hydrolysed in aqueous ethanol to give the cyclic oxides Me2,- ,(O)P,N,_ ,CH2, and react with methyl iodide by a proton transfer reaction to givelthe hydroiodides Me2.- ,(NHMe)P.N,- ,- CH.HI. Their reaction with benzoyl chloride leads to the derivatives Me7(NHMe)P4N3CCOPh and Me7(NMeCOPh)P3N2CCOPh, the initial substitution on carbon indicating that it is the primary basic centre. Model calculations of x-electron energies suggest that both the azaphosphorin rearrangement and the proton exchange reactions depend on the relative orbital electronegativity of the ring and exocyclic atoms, the less electronegative atom being more stable in the endocyclic position.

RICHARD T. OAKLEY et NORMAN L. PADDOCK. Can. J. Chem. 55,3651 (1977). Les sels de N-methyl methylphosphazeniun~ (NPMe2)3,4.MeI et N,P3Ph4Me2 .Me1 peuvent

&tre dCprotonCs par une variete de bases conduisant a de nouvelles azaphosphorines Me2.- I -

(NHMe)P.N.-,CH (n = 3, 4) et Me(NHMe)Ph4P,N2CH, formees par rearrangement, oh l'atome d'azote mCthylC est dCplacC du cycle PN par le groupe methylene exocyclique forme initialement. Le spectre rmn 'H des azaphosphorines indique un Cchange rapide de protons entre le carbone endocyclique et l'azote exocyclique, qui peut &re ralenti par I'addition d'une base auxiliaire. Lorsque le KO-t-Bu rtagit avec les sels quaternaires, une attaque nucleophile concurrence l'abstraction du proton et il resulte de ces reactions des oxydes lintaires (NHMe)(PMe2N),PMe,0 (n = 2-4). Les azaphosphorines Me2.-,(NHMe)P.N,.. ,CH (n = 3, 4) sont hydrolyskes dans l'ethanol aqueux pour conduire aux oxydes cycliques Me2,-1- (O)P,N,-,CH2, et elles reagissent avec I'iodure de mCthyle par une reaction de transfert de protons pour conduire aux hydroiodures Me2,- ,(NHMe)P,N, - lCH. HI. Elles rtagissent avec le chlorure de benzoyle pour conduire aux derives Me7(NHMe)P4N3CCOPh et Me7(NMeCOPh)P3N2CCOPh; la substitution initiale sur le carbone indique que celui-ci est le premier centre basique. Des calculs modeles des energies pour les electrons-n suggerent que tant le rearrangement des azaphosphorines que les reactions d'echange de protons dependent de l'electron6gativite relative des orbitales du cycle et des atomes exocycliques, I'atome le moins Clectrontgatif ttant plus stable en position endocyclique.

[Traduit par le journal]

Introduction we have investigated the reaction of N-methyl The formation of carbanions from methyl- meth~l~hosphazenium salts (NPMe2);MeI with

phosphazenes by their reaction with strong bases (3). As expected, these salts can be depro- bases (1) parallels the acidic behaviour of simple tOnated a bases, as are such alkylphosphine oxides (2) and exemplifies the phosphonium as MezP(NMez)2+X- (4). many chemical similarities that exist betweell However, the final products of such reactions cyclic phosphazenes (X,PN), and simple phos- indicate the occurrence of an unusual skeletal phoryl compounds X,PO. In a further study of rearrangement, thereby illustrating that al- the acidic properties of the methylphosphazenes, though many properties phosphazenes tor-

respond to those of mononuclear phosphorus 'Author to whom correspondence should be addressed. compounds, certain chemical features are di-

3652 CAN. J . CHEM . VOL. 5 5 , 1977

rectly related to their cyclic structure. The reaction leads to several new azaphosphorin derivatives.

Results Reactions of Quaternary Methylphosphazenium

Salts with Bases We have used chiefly the bases potassium

tert-butoxide (KO-t-Bu) and sodium hexa- methyldisilylamide. Both are appreciably soluble in hydrocarbons, which are the most convenient media for achieving a clean separation of the products, and both have been used successfully in the deprotonation of phosphonium salts (5-8). The reaction with the silylamide is the simpler. In boiling octane or toluene, NaN(SiMe,), reacts smoothly with the quaternary iodides N,P,Me, Me1 and gem-N,P,Ph,Me, - MeI, (1, R = Me, Ph) removing a proton from one of the P-methyl groups adjacent to the quaternized nitrogen atom. The final products are not the expected ylids 2, but rather the novel azaphosphorins 4 (a, R = Me; 6, R = Ph), formed from the ylids by a rearrangement in which the methylated nitrogen atom is displaced from the ring, the resulting exocyclic imine (3) then tautomerizing to give the aromatic amine form 4. An exactly analogous reaction occurs with N4P4Me,. Me1 (5), to give the azaphosphorin 8.

Potassium tert-butoxide reacts both as a base, removing a proton from an exocyclic group, (as in Scheme l), to form an azaphosphorin, and as a nucleophile attacking a phosphorus atom adjacent to the quaternized nitrogen atom, to form a linear oxophosphazene. The products obtained from the reactions of the monoquaternary salts (NPMe?), ,, . Me1 and the diquaternary salt (NPMe2),-2MeSO3F with KO-t-Bu collectively demonstrate both possibilities, and indicate that the relative importance of the two processes is strongly dependent on the size and charge of the cyclic phosphazenium cation.

In the reaction of (NPMe,), .Me1 with KO-t-Bu, proton abstraction does not occur. Instead, the tert-butoxide ion effects a nucleophilic attack on phosphorus, and cleaves the PN skeleton (Scheme 2). The subsequent elimination of isobutene or (less probably) an ether (9, 10) then produces the novel linear oxophosphazene (NHMe)(PMe,N),PMe,O (9) in high yield (92%). The reaction of the tetrameric salt (NPMe,),.MeI with KO-t-Bu shows a marked contrast to the one just described. Nucleophilic

CH3 I- H3C\p/ /CHI

H3C\ f H 2

N/p\N/ CH3 N/ + I, +"i' Base - I I

-HI R-P, +P-R R' \R

1 2

I

II I R-P, +P-R R' \R

attack of tert-butoxide ion still occurs, to give the open chain oxide (NHMe)(PMe2N),PMezO (10) but the yield of this product is low (-5%). The principal product (-80%) is the cyclic azaphosphorin 8, formed presumably by a deprotonation reaction and a phosphazene- azaphosphorin rearrangement. The reaction of the diquaternary ion [ ( N P M ~ , ) , . ~ M ~ ] ~ + with KO-t-Bu reflects the effect of an increase in charge on the tetrameric ring. Nucleophilic attack on phosphorus now predominates, the only product of the reaction, the linear oxophosphazene

OAKLEY AND PADDOCK 3653

CH3 H3C\ /CH3 H3C, I,,OtBu

/P+ /CH3 N//P N/CH3

fi' +"; KOtBu (NHMe)(PMe2N),PMe20 9 F I II - +

H3C-P /P-CH3 H3C-P, P-CH3 / \N' \ /\N' \ O(tBu), or Me2CCH2

\ / ,P\ f H 3

N \+? ,,CH3 KOtBu H3C\// P - (NHMe)PMe,NPMe,O 11

H3CA\; & \ C H ~ + / \P' O(tBu), or Me2CCH2

(NHMe)(PMe2N)PMe20 ( l l ) , indicating that ring cleavage occurs twice.

Spectra and Structures The structures of the azaphosphorins 4 and 8

have been established spectroscopically (see Table 1 for numerical details). The 'H nmr spectrum of 4b in benzene (Fig. la) shows the expected features, the resonances of the MeP, the MeN, the NH, and the CH protons all being clearly distin- guishable. The N-methyl signal is similar in position and appearance to the equivalent resonance in N3P3(NHMe), (G,(NMe) = 2.57 ppm (ll)), for which no 'H-lH coupling is observed. The chemical shift of the CH proton is close to that found in other phosphorins (12, 13), and as such is at lower field than in C-alkyl ylids R3P=CHR' (6,(CH) = -0.5 to - 1.0 ppm (14)) in which the anionic nature of the ylidic carbon atom has a large shielding effect on the CH proton. It is at higher field than in those ylids which are stabi-

lized by electron withdrawing substituents on carbon (e.g. FH(CH) in Ph,P=CHC(O)Me is 3.68 ppm (1 5)).

The lack of resolution of the CH resonance into the expected triplet is explicable in terms of a rapid proton exchange between nitrogen and carbon, a process which has already been observed for methylene diphosphine dioxides in the presence of aniline (16), and which, in the present case, corresponds to the tautomeric change between the imine and anline forms 3 and 4. Similar effects have been observed elsewhere; the sulphur ylid 12 has been shown to exist in solution in two tautomeric forms (17), and the azaphosphorin MeS(NPPh,),CH (13), whose broad CH singlet at 6, 1.6 ppm is not resolved into the expected triplet even at -30°C (13), may well undergo a similar exchange.

In the present system, proton transfer is believed to be promoted by traces of acid, as is found for simple ylids (13, 14, 18, 19) and, as in

TABLE 1. 'H nmr parametersa and vibrational frequenciesb of azaphosphorin derivatives

Compound 4a 8 4b4

.S(ppm) in C6D6, reference internal tetramethylsilane. J(PH) (Hz) in parentheses. bv(cm-') from Nulol mull spectra, assignments tentative. 'No 'H-'H coupling observed (at amb~ent temperature). 'Unresolved multi~let. 'G(Phenyl) -7.C8.0 ppm. SAssignment confirmed by ir spectrum of Me,(NHCD3)P4N3CH,v(N-C)1037 cm-',

cf. v(N-C) in MeNH2, 1044 cm-I, CD3NH2, 973 cm-' (52, 53). sS(CH) = 1.23, S(NH) = 1.76 ppm (broad singlets).

CAN. J . CHEM. VOL. 55 , 1977

H

H,C N-CH, \ /

P 4 ~ ~ ' c - n

1 11 (CH3&P --) H3c-j!'\N,'';c2

H3C (CH3)>P -+

( C Y ) N +

d V

4 "

I I I I I I I I I I

45 40 35 30 25 20 15 1 0 3 5 30 25 20 15 10 8 ( ppm 8 (ppm)

FIG. 1 . 'H nmr spectra of azaphosphorins 4b(a, 100 MHz, C6D6; b, 220 MHz, C5D5N) and 4a(c, 100 MHz, C6D6; d, 100 MHz, 3'P decoupled).

ylids (19), can be suppressed by the addition of in the NHMe group and also the 'H-31P an auxiliary base. For example, the 'H nmr coupling of the CH p r ~ t o n . ~ spectrum of 4b in pyridine (Fig. lb) is better The corresponding methyl derivative 4a is resolved than the same spectrum run in benzene more basic, sufficiently so to deconlpose chloro- (Fig. la), and shows clearly the 'H-'H coupliiig form, and proton exchange from carbon to

nitrogen is consequently more rapid. Its 'H nmr

CH3 CH3 spectrum in benzene (Fig. lc and d) shows no I I

S=CH2 NH or CH signals, even when pyridine is added.

[ I 1 ~ s - c H 3 - a The other structural features, however, are observed; the resonances of the PMe group, the

J two inequivalent PMe, groups, and the NMe 1 group are readily distinguished. The NH and CH

H resonances are also absent from the 'H nmr 12 spectrum of the corresponding eight-membered

~h ~h ring compound 8 but, for all the azaphosphorins I Ph listed in Table 1, the presence of the NHMe

/-$ flZP\C,H group is confirmed by the observation of a H3C-S C-H r H2C=S

\ / N=P \ N= P, ' 'H ?he dependence of the chemical shifts of the NH and

I ' ~ h I Ph CH resonances on the nature of the solvent, and on Ph Ph temperature, is noteworthy. Further work is required to

13 relate this dependence to the imine-amine equilibrium.

OAKLEY AND PADDOCK

TABLE 2. 3iPo and 'Hb nmr parameters and vibrational frequenciesc of linear oxophosphazenes MeNH(PMeZ),PMezO

n = 1 2 3

%(ppm), CDCla, reference external P406. Phosphorus atoms lettered alphabetically from the oxygen atom. Positive shifts upfield of P406.

bG(ppm), CD3CN, reference internal TMS; J(PH)(Hz) in parentheses. <(cm-'), Nujol mulls, assignments tentative. dJ(PH) (long range) -1.0 Hz. =In Hz.

v(N-H) band in their infrared spectra. Its frequency (-3200 cm- l ) is similar to the value found for it in N-alkyl phosphoramidates (20). The high values of v(N-C) indicate a single rather than a double exocyclic PN bond. In N-alkylphosphoramidates, for example, v(N-C) is found in the region of 1020-1220 cm-I (20), whilst in N-methyl phosphinimines (21, 22), its value is much lower (848 cm-' in Ph3PNMe (21)).

The 'H and 31P nmr spectra (Table 2) of the compounds NHMe(PMe,N),PMe,O (n = 1, 2, 3) indicate an open chain structure, since all the phosphorus atoms, and PMe, protons, are inequivalent. Shielding of the phosphorus atoms is expected to decrease with increasing distance from oxygen, because of the polarization of charge towards the more electronegative end of the molecule, and, as in the analogous phenyl compounds (23), the 6, values are so assigned. Hydrogen bonding is indicated for all the compounds listed in Table 2 by the low value of v(P=O) (1 170 cm-I in Me3P0 (24)).

Reactions of Azaphosphorins The chemical properties of the azaphosphorins

reported here are primarily those of a strong base but, depending on the position of the equilibrium between the imine and amine tautomeric forms, the compounds can be regarded as being related either to phosphorus ylids R3PCHR1 or to imines R3PNR1. In some

respects their behaviour is similar to that of A,-phosphorins (25); they do not, for example, react with molecular oxygen (as do simple ylids (26, 27)), nor do they undergo the Wittig reaction, the P=C bond being insufficiently polar. In the reaction of 4a and 8 with aqueous ethanol, the exocyclic P-N bond is broken rather than the endocyclic P=C bond, to give the cyclic oxides 14, 15; the ready loss of the exo-

cyclic methylamino group from these molecules is in contrast to the hydrolytic stability of amino- phosphazenes. In related molecules, cleavage of the P=C bond can occur, as in the hydrolysis of the azaphosphorin MeC(NPPh,),CH (12). Both reaction paths are exemplified in the hydrolysis of the di-imine PhN=PPh,CH,PPh,P=NPh. In basic solution, the expected diphosphine dioxide is produced, but acidic hydrolysis results in the cleavage of a central P-C bond, and the formation of the simple oxides MePh,PO and (NHPh)Ph,PO (28). The course of the reaction

3656 CAN. J . CHEM. VOL. 55. 1977

TABLE 3. 'H nmr parameters" of cyclic phosphazene oxides 14, 15

Compound 6(CHz) 6(MeP) 6(MezP)b 6(Me2P')b 6(MezP")b

14 1.95(13.5)' 1.96(13.5) 1.61(14.0) 1.48(15.0)d -

2.11(13.5) 1.53(15.0)

15 2.31(12.5) 1.94(13.0) 1.63(14.0) 1.54(13.0)" 1.47(14.0) 1.56(14.0) 1.47(13.0)

'S(ppm), CDCI,, reference internal TMS. J(PH) (Hz) in parentheses. bRing positions of P, P', P" uncertain. "ABXY pattern partially obscured by P-methyl resonances; J(PH) values approximate. J(H,H,) = 13.5 Hz. dlnequivalent geminal methyl groups displaying second order coupling. eInequivalent geminal methyl groups.

evidently depends sensitively on the conditions used.

Like their parent azaphosphorins, the oxides 14, 15 can be represented by either of two tautomeric forms 16a, b. However, the appearance of

H H H

~ e , 7-H Me \ 0

[2] ~ e - P / \ P - M ~ a Me-P I I I I1 I

characteristic methylene signals in their 'H nmr spectra (Table 3), and the absence of a v(0-H) band from their infrared spectra, indicates that the equilibrium favours the oxide structure 16b. In the six-membered ring, the methylene protons are magnetically inequivalent, as expected, and give rise to an AB proton coupling pattern. In the eight-membered ring, the two protons are accidentally equivalent, and give rise to a simple triplet (Fig. 2).

Unlike other azaphosphorins (12), and simple ylids R,PCHRr (14, 29-32), which react with methyl iodide to give C-methylated phosphonium salts, the azaphosphorins reported here react to give the corresponding C-hydroiodides, 17 and 18 (X = I). Presumably the initial

H I I

H3C\ /N-Me p /HH N / + \ ~ /

I1 I R-P, gP-R / N \

N R R / \

methylation is immediately followed by a proton transfer reaction with a second mole of azaphosphorin (eq. 3), as is found in the alkylation of

3.0 25 20 1.5 10 8 ( P P ~ )

FIG. 2. lH nmr spectra of the phosphazene oxide 15 (100 MHz, CDCI,); (A) direct, (B) 31P decoupled.

phosphinimines (33). Protonation on carbon rather than on nitrogen is confirmed by the 'H nmr spectra (Table 4) of the salts, which show a characteristic AB proton coupling pattern for the inequivalent methylene protons. The presence of a positive charge on the rings further facilitates the interpretation of their spectra; the charge distribution is less uniform than in the neutral compounds, and the various signals are better separated, as is illustrated in Fig. 3, which shows the 'H nmr spectra of 17 (X = I).

OAKLEY AND PADDOCK

TABLE 4. ' H nmr parametersa of the azaphosphorin hydrohalides 17, 18

17 (R = Me) 18 17 (R = Ph) ---

Parameter X = I X = C1 X = I X = C1 X = I*

G(MeN) 2.65(13.5) 2.63(13.5) 2.62(13.0) 2.57(13.5) 2.19(14.5)

J (HH) 5 .5 5.5 5.5 6.0 5.5

W H N ) 5.85 6.03 4.24 5.37 4.51

G(MeP) 1.99(13.0) 1.99(13.5) 1.96(13.5) 2.04(13.5) 1.83(14.5)

G(MeZP)" 1.86(13.5) 1.79(14.0)d 1.79(13.5) 1.78(13.5) - 1.73(14.0)

G(MeZPf)" 1.58(14.0) 1.53(14.0) 1 .53(13.5)d 1 .58(15.0)d -

1.49(13.5) 1.51(15.0)

6(MezP'?, - - 1.49(13.5) 1.51(15.0) -

6(CHz)(HA)e 3.74 4.09 3.75 3.96 3.77 (15 .0 , l l .O) (14 .0 , l l .O) (15.5,12.5) (15.0, 12.0) *

G(CH,)(H,)" 2.78 2.27 3.05 2.86 3.55 (13.5, 13.5) (14.0, 13.5) (15.5, 12.5) (15.0, 13.0)

J ( H A H B ) ~ 15.5 15.0 15.5 15.0 15.0 .S(ppm), CDCI3, reference internal TMS; J(PH) (Hz) in parentheses. bPhenyl reglon omltted. CRelat~ve positions of P P' P uncertain dInequ~valent methyl grouds on same phosphorus atom. eABXY pattern, partially obscured; J(PH) approximate. *J(PH) not assigned. In CD3CN, H A , , equivalent, S = 3.44 ppm, J(PH): 13.0Hz. gln Hz.

The appearance and position of the resonance of the N-methyl protons is similar to that found in aminophosphonium salts (34), exhibiting both 31P-1H and 'H-'H coupling. However, the addition of one drop of D,O to a solution of 17 (X = I) in CDCI, brings about the immediate collapse of 'H-'H couplings, and the loss of the NH resonance (Fig. 3B), indicating that proton exchange between the solvent and the exocyclic nitrogen is rapid. The methylene protons are expected to be less acidic, and their exchange with protic solvents is consequently slower, but the effect is nonetheless observable. When 17

(X = I) is dissolved in D,O, the methylene resonance appears as a simple triplet which, upon allowing the solution to stand, becomes less intense and finally vanishes (after about 2 h).

In the reaction of the azaphosphorins with methyl iodide, the neutral product 20 cannot be isolated. It apparently undergoes a second methylation and proton transfer reaction, the overall yield of hydroiodide being such that 3 mol of azaphosphorin are converted into 2 mol of hydroiodide. In order to determine the position of primary substitution on the azaphosphorin ring (carbon or nitrogen), the reactions of 4a and 8 with benzoyl chloride have been examined. In the case of the eight-membered ring, the reaction yields the C-benzoyl derivative 22. The reaction of the six-membered ring is more vigorous and, as with its reaction with methyl iodide, cannot be stopped at the first stage. Instead, the initially formed C-benzoyl derivative reacts with a second mole of benzoyl chloride, to give the N-benzoyl C-benzoyl compound 21.

As in the case of their parent azaphosphorins, the 'H nmr spectra of the benzoylated derivatives 21 and 22 (Table 5) reveal very little separation of the P-methyl resonances, thereby indicating

CAN. J . CHEM. VOL. 5 5 , 1977

(CHJ2 P -1

H-N L

I

FIG. 3 . 'H nmr spectra (100 MHz) of the azaphosphorin hydroiodide 17 (X = I) in CDCI,; ( A ) direct, (B) with one drop of D,O added, (C) 31P decoupled.

the uniformity of the charge distribution within the ring. For the tetrameric compound, 'H-lH coupling in the NHMe group is now observed (as it is in simple N-methyl amides); proton exchange is obviously suppressed by the low

basicity of the nitrogen. The shielding of the N-methyl protons in 21 (S,(NMe) = 2.50 ppm) is greater than in N-methyl benzanilide (8,- (NMe) = 3.40 ppm, CDCI,, reference internal TMS), suggesting that the diffusion of lone pair density from nitrogen to phosphorus is limited.

The infrared spectra of these compounds are too complex to allow a detailed interpretation, but their carbonyl stretching frequencies (Table 5) are well isolated and easily identified. For both the six- and the eight-membered ring, the low value of the carbonyl frequency is similar to that found in other acylated ylids (35-37), and reflects the extent to which resonance such as that depicted in eq. 4 weakens the C=O double bond. The lower frequency found in the six-


TABLE 5. IH nmr parametersa and carbonyl stretching frequenciesb of azaphosphorin benzoyl derivatives 21, 22

Compound F(MeN) F(MeP) 6(MezP)' v(C-0)

'6(ppm), CDCI,, reference internal TMS; J(PH) (Hz) in parentheses. bv(C=O) (cm-1). Nujol mull. <No equivalent methyl groups. *J(HH)(MeNH) = 5.5 Hz; 6(NH) = 2.70 ppm. 'Unresolved PMe, PMez multiplet near 6 = 1.55 ppm.

membered ring suggests that electron release onto the C-benzoyl group is greater for that ring size. The amide carbonyl frequency of 17 (1642 cm-') is similar to that found in simple tertiary amides (e.g. v(C=O) in PhCONMe, is 1640 cm- ' (38), and in PhCONMePh3 is 1653 cm- I).

In these latter compounds, the presence of an N-phenyl group increases v(C=O). the com- petitive electron withdrawing effect of the aromatic ring reducing the influence of the ionic form Ph-N+=C-0- (40). The relatively low value of the amide carbonyl frequency in 17 is consistent with the 'H nmr evidence indicating that conjugative interactions between phosphorus and the exocyclic nitrogen are limited.

Discussion N-Ethyl phosphazenium salts (41), like their

pyridine analogues, react with bases to yield the neutral compound and ethylene. Such an elimination reaction is not possible for the N- methyl compounds, and their principal reactions with bases are deprotonation and nucleophilic attack. In the pyridine series, a 2-methyl group is deprotonated to give a methide, which is alkylated on carbon rather than on nitrogen, thereby conserving ring aromaticity (42). When

which is enhanced by the strength of the carbonyl b0lld.

In the reactions reported here, deprotonation and nucleophilic attack both occur, but the balance between them is determined by the reagent and by the ring size. Open chain oxides are produced when KO-t-Bu is used, primarily because the elimination of isobutene or dibutyl ether allows the formation of a strong phosphoryl bond. The size of the ring has a marked effect on the relative yield of cyclic and linear products. The 'P nmr shifts of (NPMe,), ,,a Me1 (43) suggest that the partial positive charge on the phosphorus atoms adjacent to the N- methyl group is greater in the six-membered ring compound, nucleophilic attack and the formation of the linear oxide being thereby promoted, as found.

Deprotonation of the N-methyl methylphosphazenium salts occurs, as expected, at the 2- position, and the consequent rearrangement is probably aided by the electrophilic character of the phosphorus atoms adjacent to the alkylated nitrogen atom. Although the detailed mechanism is unknown, it seems likely to involve a four- centre interaction (as shown) similar to that of

an electron-withdrawing substituent is present in the 2- (or 4-) position, the susceptibility of the the Wittig reaction., A related mechanism has

molecule to nucleophilic attack at these positions been suggested (44) for the base catalyzed

is increased. and. in basic solutions. such salts rearrangement of (Me3PCSiMe2),, in which a

are hydrolysed to'pyridones (42), the'stability of 4The possibility that the reaction is bimolecular, so

3Present work, CCl, solution; Nujol mull, 1650 cm-'; reducing strain in the transition state, has been suggested cf. 1641 cm-I (39). by a referee, and cannot at present be excluded.

3660 CAN. J. CHEM. VOL. 55, 1977

methylene group enters the ring. A rearrangement in the opposite sense, but also involving a 4-centre mechanism, occurs during the ammono- lysis of bis(dipheny1phosphino)methanein carbon tetrachloride to give (MePPh2NPPh2NH2)C1 (12), and may account for the apparent migration of a methyl group during the chloramination of bis(diphenylphosphino)methylamine, which yields the salt [NHMePPh2NPPh2NH2]C1 (45).

In the latter two examples, the driving force of the reaction is probably the formation of the resonance stabilized (P=N=P)+ cation. The phosphazene-azaphosphorin system is more complicated, in that both a cyclic 7c-system and an exocyclic n-bond are involved, but we can nevertheless understand the rearrangements that take place by simple model calculations of the x-energy changes. The three related molecular forms are shown in Fig. 4 which shows how the 7c-electron energies of the three forms depends on the assumed Coulomb parameter of carbon. Isomer a is the e ~ o c ~ c l i c ~ y l i d 6 formed by the deprotonation of the quaternary cation, b is

FIG. 4. x-Electron energies (eV, arbitrary zero) of the three isomers shown, as a function of the assumed Coulomb parameter of carbon. The values of ap(2 eV) and a, (- 1 eV) used are not critical, but have been found useful for the description of other properties of (NPMe,),. All resonance parameters were taken to be - 1 eV, independent of overlap, in both out-of-plane and in-plane x-systems.

the product of its direct rearrangement to an exo-imide 7, and c results from the f ~ ~ r t h e r proton migration to give the azaphosphorin 8. Form c is unique in restoring the cyclic delocalization of the initial quaternary compound 5.

Although the energetic advantage of the delocalization may be important, the calculations show that it may be outweighed by a high localized 7c-energy in an exocyclic bond in a or b.

The general result is that the exocyclic position is favoured for the more electronegative atom. To take the extreme cases first, if a, < - 1, (the value for nitrogen), a is the most stable form, and no rearrangement would be expected. On the other hand, if a, < -0.7 (approximately), rearrangement should take place, but should stop at the exo-imide stage b. In both cases the molecule is stabilized by a strong exocyclic 7c-

bond. For intermediate values of a,, the situation is more interesting. The azaphosphorin c is now slightly more stable than the exo-ylid a, because cyclic delocalization is now possible; if localized bonds are assumed, the azaphosphorin is never favoured over the exo-ylid. In terms of the simple model used, it is therefore possible to understand the azaphosphorin rearrangement as being driven by the increase of 7c-energy arising from the displacement of the more electronegative atom from the ring, and reinforced by the attainment of cyclic delo~alization.~

Although the results of Fig. 4 are qualitative, the assumed parameters are realistic enough to show that the energy differences between the three forms are likely to be small, and it is not surprising that proton transfer between the endocyclic carbon and endocyclic nitrogen atoms of the azaphosphorins is fast enough to cause a collapse of the 31P-1H coupling of the CH signal. The results have another application. Since oxygen is more electronegative than nitrogen, the introduction of an exocyclic phosphoryl group in the cyclic oxophosphazenes 14,15 alters the equilibrium in favour of exocyclic 7c-bonding, lowering the energy of the exo-imide b without direct effect on the energies of a and c. I t is consistent with this result that rapid proton

51n a reaction which is apparently similarly dependent on relative electronegativity, 2-arylaminopyrylium chlo- rides can be deprotonated to pyranimines, which then rearrange to pyridones in a reaction closely analogous to the azaphosphorin rearrangement described above. The corresponding reaction of the thiapyrylium iodides stops a t .the thiapyraninline stage (51).

OAKLEY P iND PADDOCK 3661

migration is not observed in the cyclic oxophosphazenes described above. The general concepts are further supported by the structures of the phosphazenes N,P,Ph,(OH) and N4P4Ph,(OH), (non-geminal) for which hydroxy- rather than oxy-structures have been suggested (46, 47). From the present point of view, the potential tautomeric equilibrium is that between forms b and c of Fig. 4, N being substituted for CH in the ring, and 0 for NCH, in the exocyclic position. The increased electronegativity of the ring atom would tend, following the diagram, to stabilize the cyclically delocalized form c. The energy balance is sensitively dependent on the relative electronegativities of the relevant atoms. The same feature is found in pyridine chemistry; 2- and 4-hydroxy pyridines normally exist in the pyridone form, but when the oxygen is replaced by a less electronegative atom, as in 2-amino- pyridine, the aromatic amino-form, rather than the exocyclic imide, is the more stable.

Experimental Sodium metal, hexamethyldisilazane, methyl iodide,

and benzoyl chloride were commercial products. Potas- sium tert-butoxide and methyl fluorosulphate were also obtained commercially. The former was purified by sublimation in vacuo, the latter by distillation under nitrogen. Hexane and octane were of reagent grade. Toluene, diethyl ether, and acetonitrile were dried by distillation from sodium, lithium aluminium hydride, and calcium hydride, respectively. All of the preparations were carried out under an atmosphere of nitrogen.

Quaternary Salts The monoquaternary salts (NPMe2)3,4. Me1 were

prepared by the reaction of the neutral methylphosphazenes with methyl iodide (43). The preparation of the salt (NPMe2),.2MeC1 by the reaction of Me2PC13 and MeNH2.HCI has already been reported (48); the diquaternary salt (NPMe2)4.2MeS03F used here was prepared by treating (NPMe2)4 with two equivalents of MeS0,F in acetonitrile solution. The product was precipitated quantitatively from the solution by the addition of diethyl ether, and purified by recrystallization from acetonitrile: mp 231-233°C; 'H nmr spectrum, 100 MHz (6, CDCI,, int. TMS), 2.84 (6H, triplet, J p H = 11.5 Hz), 1.95 (24H, doublet, J p H = 13.7 Hz); ir spectrum v(P=N) 1275, 1330 cm-I ; v(C-N), 1070 cm-'. Anal. calcd. for C10H30FZN406SZP4: C 22.7, H 5.7, N 10.6; found: C 22.9, H 5.9, N 10.6. gem-Dimethyltetraphenylcyclotri- phosphazene N3P3Ph4Me2 was prepared by the reaction of (NH2PPh2NPPh,NH2)CI with Me2PC13 (49) in chlorobenzene, the neutral compound being liberated from its hydrochloride by treatment with triethylamine; mp 142-144°C (lit. (50) 140-142°C). The methiodide was prepared from it by heating it in methyl iodide for 72 h. The precipitated salt was recrystallized from ethanol and dried at 60"C/0.01 Torr for 7 h to remove all the solvent: mp 226-227°C (dec.); 31P nmr spectrum (6, CDCI,, ext.

P406), 64.1 (IP, PMe2), 81.0 (lP, PPh2 adjacent to NMe), 93.5 (lP, remote PPh,); 'H nmr spectrum (6, CDCI,, int. TMS), 3.07 (3H, triplet, J,, = 11.0 Hz), 2.22 (6H, doublet, J p H = 14.0 HZ), 7.0-8.0 (20H, broad unresolved multiplet); ir spectrum; v(P=N), 1220, 1260 cm-I ; v(C-N), 1070 cnl-l. Anal. calcd. for C27H291N3P3: C 52.7, H 4.75,120.6, N 6.8; found: C 52.9, H 5.0,120.4, N 6.8.

Reaction of (NPMe2)4. Me1 with NaN(SiMe,) , Sodium (0.20 g, 8.7 mmol) was added to a slurry of

(NPMe2)4.MeI (3.54 g, 8.0 mmol) in 200 ml octane. About 0.5 ml of HN(SiMe,), was injected into the reaction mixture at the reflux temperature, and, to com- pensate for possible losses by evaporation, again at 24 h intervals to a total reaction time of 72 h. The mixture was cooled and filtered and the solvent distilled from the filtrate to leave a white solid, which was purified by sublimation at -- 130"C/0.01 Torr and recrystallization from hot hexane to give colourless hygroscopic prisms of 8 (2.21 g, 7.0 mmol, 88%); mp 104-105°C. Anal. calcd. for C9NZ6N4P4: C 34.3, H 8.6, N 17.8; found C 34.4, H 8.5, N 17.6.

Reaction of (NPMe,) ,- Me1 with NaN(SiMe3), In a similar reaction, sodium (0.35 g, 15.3 mmol) and

(NPMe,),.MeI (5.11 g, 13.9 mmol) were allowed to react with HN(SiMe3)2 in octane (200 ml). After 96 h, the mixture was filtered while still hot and the solvent distilled from the filtrate. The white solid product was purified by sublimation at --13O0C/0.01 Torr and recrystallization from hot hexane to give colourless hygroscopic plates of 4a, (2.68 g, 11.2 mmol, 80%); mp 140- 142°C. Anal. calcd. for C7Hz0N3P3: C 35.15, H 8.4, N 17.6; found: C 35.4, H 8.5, N 17.6.

Reaction of Me2Ph4P3N3. Me1 with NaN(SiMe3), Sodium (0.06 g, 2.6 mmol), MeZPh4P3N3. Me1 (1.34 g,

2.2 mmol) dried as before to remove EtOH and HN- were allowed to react together in 60 ml boiling

toluene for 24 h. The mixture was filtered and the solvent distilled off. The residual white solid was purified by recrystallization from benzene-octane to give colo~irless prisms of 46, (0.95 g, 2.0 mmol, 93%); mp 147-149°C. Anal. calcd. for C,,H2,N3P3: C 66.1, H 5.3, N 8.9; found: C 65.9, H 5.55, N 8.65.

Reaction of (NPMe2)4. Me1 with KO-t-Bu Finely powdered (NPMe2)4.MeI (3.00 g, 6.8 mmol)

was added to a slurry of KO-t-Bu (0.84 g, 7.4 mmol) in 100 ml hexane, and the mixture heated under reflux for 24 h. The solvent was distilled from the filtered solution to leave a white solid (1.82 g) which was sublimed at N 1 30°C/0.01 Torr on to a water cooled cold finger. The sublimate (8) was recrystallized from hot hexane (1.71 g, 5.4 mmol, 80%) and identified by comparison of its ir spectrum with that of an authentic sample, and by its mp, 104-105°C. The residual oil in the sublimation vessel solidified into a white solid, which was purified by recrystallization from hot hexane to give colourless hygroscopic cubes of 10 (0.11 g, 0.33 mmol, 5%); mp 72-74°C. Anal. calcd. for C9HZ8N40P4: C 32.5, H 8.5, N 16.9; found C 32.6, H 8.6, N 16.9.

Reaction of (NPMe,) 3 . Mel with KO-t-Bu Similarly, (NPMe,),.MeI (1.38 g, 3.8 mmol) was

3662 CAN. J. CHEM. VOL. 55, 1977

allowed to react with a slurry of KO-t-Bu (0.50 g, 4.5 mmol) in 50 ml boiling hexane. After 24 h, the mixture was filtered and the solvent removed from the filtrate to leave a colourless oil. Sublimation of this oil at -- 130°C/ 0.01 Torr on to a cold finger at -78°C yielded 9 as a white hygroscopic solid (0.89 g, 3.4 mmol, 92%), which was recrystallized as colourless plates by cooling a con- centrated solution of it in pentane to -23°C; mp 35- 37°C. Anal. calcd. for C7HzzN30P3: C 32.7, H 8.6, N 16.3; found: C 32.6, H 8.5, N 16.0.

Reaction of (NPMeZ),.2MeSO3F with KO-t-Bu In a similar experiment, a slurry of KO-t-Bu (1.30 g,

11.6 mmol) and (NPMez),.2MeS03F (2.70 g, 5.1 mmol) was heated to reflux in 60 ml hexane. After 24 h, the solution was decanted from the pasty residue and the solvent distilled off to yield a white solid, which was recrystallized from hot hexane to give colourless feather- like crystals of 11 (0.93 g, 5.0 mmol, 50%); mp 73-75'C. Anal. calcd. for C5H16NzOPz: C 33.0, H 8.9, N 15.4; found: C 33.3, H 8.9, N 15.25.

Hydrolysis of Azaphosphorins 4a, 8 In separate experiments, samples of 4a and 8 (0.3-0.4 g)

were dissolved in 25 ml of a 50150 ethanol-water mixture. After 24 h, the solvent was removed at room temperature in vacuo to leave a white solid. Recrystallization from benzene of the solid from 4a gave colourless hygroscopic blocks of 14, mp 184-186°C. Anal. calcd. for C6H17NZ- OP3: C 31.9, H 7.6, N 12.4; found: C 32.2,H7.6,N 12.3. Recrystallization from hot hexane of the solid from 8 gave colourless feather-like crystals of 15; mp 140- 143°C. Anal. calcd. for C8HZ3N30P4: C 31.9, H 7.7, N 13.95; found: C 32.1, H 7.9, N 14.0.

Reaction of Azaphosphorins 4a, b,8, with Methyl Iodide The addition of an excess of methyl iodide to a solu-

tion of 4a (1.04 g, 4.33 mmol) in ether resulted in the precipitation of the hydroiodide 17 (R = Me) (0.99 g, 2.7 mmol). Similarly, the azaphosphorins 46 (0.155 g, 0.32 mmol) and 8 (0.44 g, 1.4 mmol) yielded, respectively, the hydroiodides 17 (R = Ph) (0.14 g, 0.23 mmol) and 18 (0.43 g, 1.0 mmol). The neutral products of the transylidation reactions could not be isolated. The air- stable crystalline hydroiodides were recrystallized from acetonitrile-toluene: 17 (R = Me), mp 194-195°C. Anal. calcd. for C7HzlIN3P3: C 22.9, H 5.8, I 34.6, N 11.45; found: C 23.0, H 5.9, 134.2, N 11.3; 17 (R = Ph), mp 248-252°C. Anal. calcd. for C27H2yN3P31: C 52.7, H 4.75, N 6.8; found: C 52.7, H 4.75, N 6.9; 18, mp 155-156°C. Anal. calcd. for CyH271N4P4: C24.45, H 6.15, 128.7, N 12.7; found: C 24.5, H6.3, 128.55, N 12.6.

Reaction of Azaphosphorins 4a, 8, with Benzoyl Chloride A solution of benzoyl chloride (0.21 g, 1.5 mmol) in

20 ml ether was added dropwise to a stirred solution of 4a (0.54 g, 2.25 mmol) in 100 ml ether. After 3 h the mixture was filtered, leaving the hydrochloride of 17 (R = Me) (0.45 g, 1.64 mmol): mp 194-197°C. Anal. calcd. for C7HzlC1N3P3: C 30.5, H 7.7, CI 12.9, N 15.2; found: C 30.2, H 7.8, C1 12.6, N 15.0. Distillation of the solvent from the filtrate left the crude dibenzoyl derivative 21 (0.26 g, 0.59 mmol), which was recrystallized from hot benzene-octane as colourless blocks: (dec.) 181-

184°C. Anal. calcd. for CZ1HZ8N3OZP3: C 56.4, H 6.3, N9.4; found: (256.1, H 6.2, N9.6.

In another experiment, a solution of benzoyl chloride (0.12 g, 0.9 mmol) in 10 ml ether was added dropwise to a stirred solution of the azaphosphorin 8 (0.56 g, 1.8 mmol) in 50 ml ether. A fine white precipitate was immediately formed. After 3 h the mixture was filtered, and the hygroscopic solid recrystallized from acetonitrile-benzene to give small colourless cubes of the hydrochloride of 18 (0.26 g, 0.7 mmol): mp 193-195°C. Anal. calcd. for CyH27C1N4P4: C 30.8, H 7.8, N 16.0; found: C 30.9, H 7.7, N 15.9. The solvent was distilled from the filtrate to leave a yellow oil which, on drying in vacuo, solidified to a yellow, air stable powder. Re- crystallization of this solid from hot hexane yielded yellow mica-like plates of the benzoyl derivative 22: mp 133-134°C. Anal. calcd. for C16H30N40P4: C 46.0, H 7.2,N 13.4; found: C45.7,H 7.3,N 13.1.

Acknowledgements We are grateful to the National Research

Counci l of Canada for financial support and for an NRCC Scholarship (to R.T.O.), a n d t o Dr. A. A. Grey, o f the Ontar io Research Foundat ion, f o r the 220 MHz spectra.

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and l. - university of guelphch (n = 3, 4) and me(nhme)ph4p3n2ch, formed by a rearrangement in which...

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