Phosphorus Chemistry

63 Conjugation in Phosphazenes: Pyrrylphosphazenes and Phosphazenyl Carbanions 2

K. D. GALLICANO, R. T. O A K L E Y 1 , R. D. S H A R M A , and N . L. P A D D O C K

Downloaded by UNIV OF PITTSBURGH on February 19, 2016 | http://pubs.acs.org Publication Date: November 11, 1981 | doi: 10.1021/bk-1981-0171.ch063

Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada

The bonding system in phosphazenes i s closely analogous to that in phosphoryl compounds, in which conjugative P-C inter actions require electron release from the substituent. Although there i s only weak conjugation between phenyl and phosphoryl groups (1), the electronic spectra of tris(pyrryl)phosphine oxides show a strong low frequency band attributable to pπ-dπ bonding between the pyrryl and phosphoryl groups (2); i t s nature, for acylpyrroles, has been confirmed theoretically (3). In the expectation that phosphazenyl and phosphoryl groups would behave similarly, we have prepared the 1-methyl-2-(fluoro phosphazenyl)pyrroles NnPnF2n-1.C4H3NMe (n = 3-6; 2,a-d) and have measured their electronic spectra. Figure 1 illustrates possible modes of conjugation to phosphoryl and N3P3 rings. Like the pyrrylphosphine oxides, the fluorophosphazenylpyrroles show a band characteristic of pπ-dπconjugative charge transfer, and an interaction of this type i s equally well established for the two series. The numerical results are shown in Table I. Table I:

Cpd.

Electronic spectra of fluorophosphazenylpyrroles.

λ(ε)

1b

213(6.7)

2a(n=3)

237(10.9)

2b(n=4)

240(12.4)

a

Cpd.

λ(ε) -

a

λ(ε)

λ(ε)

2c(n=5)

235(11.5)

220(9.9)

220(10.1)

2d(n=6)

240(11.8)

220(10.1)

220(10.4)

3c

243(12.9)

218(8.4)

C

λ(nm) and ε(χ10-3) at maxima. b1-methylpyrrole. tris(1methylpyrrol-2-yl)phosphine oxide (2). 1Current address: Department of Chemistry, University of Calgary, Alberta, Canada. 2Current address: Department of Chemistry, Simon Fraser University, British Columbia, Canada.

In Phosphorus Chemistry; Quin, Louis D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


302

PHOSPHORUS CHEMISTRY

The wavelength of the charge t r a n s f e r band i n c r e a s e s i n the s e r i e s phosphazenyl < phosphoryl < formyl (289.5 nm)(3), so that the degree of c o n j u g a t i o n increases i n that o r d e r , the first two being comparable. The s l i g h t a l t e r n a t i o n i n the phosphazenyl s e r i e s , if real, i s c o n s i s t e n t w i t h the t r a n s f e r of charge to an antibonding π-level of the phosphazene r i n g . Ground s t a t e s t a b i l i s a t i o n of 2b i s demonstrated by i g s molecular s t r u c t u r e (4) ( F i g u r e 2), the P-C bond being 0.06 A shorter than i n phenylphosphazenes (50 . Conjugative i n t e r a c t i o n s have important p r e p a r a t i v e conse quences ( 6 ) . P r i m a r i l y , deprotonation of an alkylphosphazene i s promoted by c o n j u g a t i o n i n the carbanion so formed, and a number of new phosphazene d e r i v a t i v e s have been made by the r e a c t i o n of the l i t h i o - d e r i v a t i v e s w i t h e l e c t r o p h i l e s ; some a r e i l l u s t r a t e d i n F i g u r e 3. The r e a c t i o n s of the monocarbanion formed from gem-N3P3Ph4Me2 are the s i m p l e s t , and compounds of Type 4 have been obtained i n h i g h y i e l d . Rings can be j o i n e d , e i t h e r by the use of a d i f u n c t i o n a l h a l i d e , or through a c o u p l i n g r e a c t i o n , i n v o l v i n g CuC£/02(7), to g i v e 5. The t r i c a r b a n i o n formed from N3P3Me$ g i v e s the expected d e r i v a t i v e s N3P3Me3(CH2R)3; both c i s c i s - c i s and c i s - t r a n s - t r a n s stereo-isomers of the tribromod e r i v a t i v e have been i s o l a t e d . T e t r a s u b s u b s t i t u t e d d e r i v a t i v e s 6 a r e e a s i l y obtained from N4P4Meg, and π-electron c a l c u l a t i o n s show that a n t i p o d a l d i s u b s t i t u t i o n i s to be expected f o r e l e c t r o p o s i t i v e s u b s t i t u e n t s , as found f o r R = Me3Si, Me ( 7 ) . Monocarbanions are not o b t a i n e d , p r i n c i p a l l y because the i n t r a - r i n g exothermic e l e c t r o s t a t i c i n t e r a c t i o n s are reduced l e s s by charge t r a n s f e r from the second y l i d i c group than the f i r s t ; i f the added base i s strong enough to remove the f i r s t proton, i t w i l l a l s o remove the second. Normally e l e c t r o n e g a t i v e s u b s t i t u e n t s f a c i l i t a t e f u l l s u b s t i t u t i o n , but i f the withdrawal i s strong enough, charge t r a n s f e r may remove a l l charge from the y l i d i c groups o f , e.g., N 4 P 4 M e 4 ( C H 2 R ) 2 (CH2"") 2 » so that i t i s no longer a n u c l e o p h i l e . T h i s e v i d e n t l y occurs i n the r e a c t i o n of N4P4Me4(CH2Li)4 w i t h e t h y l benzoate, which gives a h i g h y i e l d of the v i c i n a l d i d e r i v a t i v e (Figure 3, 8); the r e a c t i o n cannot be made to go further. The e f f e c t of the s u b s t i t u e n t can be modelled as a p e r t u r b a t i o n of the l o c a l Coulomb parameter dp, and Figure 4 shows t h a t , as t h i s phosphorus atom i s made more e l e c t r o n e g a t i v e , the favoured mode of s u b s t i t u t i o n changes from a n t i p o d a l to v i c i n a l , and the y l i d i c charge decreases, becoming zero near the p o i n t where the v i c i n a l isomer i s the more s t a b l e . The d i c a r banion i s then no longer a n u c l e o p h i l e , and the r e a c t i o n stops. T h i s i s e v i d e n t l y what happens i n the r e a c t i o n of N 4 ? 4 M e 4 ( C H 2 " ) 4 with e t h y l benzoate.



GALLiCANO E T A L .

F

Figure 1.

F

Conjugation

C

in

303

Phosphazenes

F

F

d

Some canonical forms involving conjugation in l-methylpyrrol-2-yl atives. Valence shell expansion is implied in b and d.

deriv-

Canadian Journal of Chemistry

Figure 2. The molecular structure of 2b (4), P(l)C(l) = 1.765(5) Â, P(l)F(l) - 1.561(4) A; mean length of other P-F bonds - 1.536 A. Bonds from P(l) to N(l), N(4), lengthened bv 0.027(12) A.


304

PHOSPHORUS CHEMISTRY CH

CH CHoLi 3

CHoR

3

\V

Ν

RX Ph—R

/


^ Ph

Ph

Ph

Ph

4 Ph

Ph Ph'

Ν

Ρ,

ν ./

:P^ I^Ph

Ph-

Ph

Ph

Me

CH R

CH R

Me

?

2

^P

Phosphorus Chemistry

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