Synthesis and Properties of Phosphaalkenes

the lone pair at phosphorus, intermediate between those in tertia ry phosphines and_X3-phosphorins. In line with this interpretation the IR-spectrum (...
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82 Synthesis and Properties of Phosphaalkenes

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T. A . V A N DER KNAAP, T. C. K L E B A C H , F. VISSER, R . LOURENS, and F. B I C K E L H A U P T Vakgroep Organische Chemie, Vrije Universiteit, De Boelelaan 1083, 1081HVAmsterdam, The Netherlands

Three years ago, i t was shown that phosphaalkenes (1), which are not substituted at the P=C double bond by hetero atoms and owe their thermal s t a b i l i t y mainly to steric hindrance around this bond can be obtained by base-induced elimination of HC1 from 2 (1). The predominant role of steric hindrance was obvious from the i n ­

1

fluence of the groups R on s t a b i l i t y : 1 with R = mesityl (= Mes) or 2,6-dimethylphenyl resulted in stable, isolable compounds, while those with R = 2-methylphenyl or phenyl were too unstable for iso­ lation. For this reason i t was expected that trimesitylphosphaethene (1b) would be a particularly stable phosphaalkene. However, in this case the amount of the steric hindrance is apparently so large as to prevent the formation of 1b. Even 2b (R =R =R =mesityl) could not be obtained in the usual way by treatment of 3 with HC1; i n ­ stead, the interesting phosphonium salt 4 ( p-NMR: δ=31.9 ppm, JpH = 574 Hz) was formed, providing, as a spin-off, insight into the mechanism of transformations of the type R2PX + HY -> R PY. The primary product 4, in this case is presumably for steric reasons prevented from further reaction via phosphoranes such as 5 to form 2a. 1

1

2

3

31

1

2

Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PHOSPHORUS CHEMISTRY

402

However, 2b c o u l d be s y n t h e s i z e d by r e a c t i n g 6^ w i t h d i m e s i t y l m e t h y l p o t a s s i u m ; e l i m i n a t i o n o f H C l c o u l d n o t be a c h i e v e d u n d e r a v a r i e t y o f c o n d i t i o n s , p r o b a b l y due t o t h e i n a c c e s s i b i l i t y o f t h e p r o t o n i n 2b f o r b a s e . MesPCl

+ KCHMes

2

—HCl * » MesP=CMes

» 2b

2

6

2

lb

S i m i l a r l y , a t t e m p t e d e l i m i n a t i o n o f H C l f r o m 1^ t o f u r n i s h 8^ was unsuccessful. Cl P-CHMes 2

ClP=CMes

2

7

2

—^—>

lb

8

With r e s p e c t t o the s t r u c t u r e of phosphaalkenes, c o n s i d e r a b l e p r o g r e s s has b e e n a c h i e v e d by t h e X - r a y c r y s t a l s t r u c t u r e d e t e r m i ­ n a t i o n o f Ja (2) and o f i t s C r ( C 0 ) c o m p l e x 2 (3) . B o t h t h e s t r u c ­ t u r a l d a t a and t h e c l o s e s i m i l a r i t y o f s p e c t r a l d a t a c o n f i r m t h a t La as s u c h and as a l i g a n d i n £ has e s s e n t i a l l y t h e same s t r u c t u r e of a p l a n a r , n o n - d e l o c a l i z e d phosphaethene (Table 1). 5

Table 11. S e l e c t e d

s p e c t r a l and

3

.—Z^ ,C(C6H ) CH Λ . r 5

HH Com­ pound

31 la

233.06

2

I'

N X

13

p

l a : X = lone p a i r

0

3

NMR: P=C (6 i n ppm) c

La and j j i .

s t r u c t u r a l data of

X=Cr(C0)

5

Bond d i s t a n c e s (in pm)

bond angle ( i n °)

P=C

C=P-C

P-C

193.37 J =43.5Hz

170

184

190.94 J =32.3Hz

168

182

P-Cr

108.7

P C

237.3

236

109.8

P C

The P=C bond l e n g t h s o f 170 and 168 pm, r e s p e c t i v e l y , a r e c l e a r ­ l y s h o r t e r than those of d e l o c a l i z e d systems which t y p i c a l l y range f r o m 172 t o 176 pm. I n v i e w o f t h e w e l l - k n o w n t e n d e n c y o f t r i c o o r d i n a t e p h o s p h i n e s t o w a r d s s m a l l bond a n g l e s , i t i s n o t s u r p r i s i n g t o f i n d t h e C=P-C bond a n g l e i n l a (108.70°)to be more a c u t e t h a n e x p e c t e d f o r p u r e s p 2 - h y b r i d i z a t i o n , w h i l e i n t h e f i r s t row a n a l o g o f Uif N - m e s i t y l b e n z o p h e n o n e - i m i n e , t h e C=N-C a n g l e i s 120.8° 0 4 ) . The s t r u c t u r a l d a t a o f j a i m p l y a r e l a t i v e l y h i g h s - c h a r a c t e r i n

Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

VAN D E R K N A A P

82.

ET A L .

Phosphaalkenes:

Synthesis and Properties

403

the lone p a i r a t phosphorus, i n t e r m e d i a t e between those i n t e r t i a ­ r y phosphines and_X3-phosphorins. I nline with this interpretation t h e I R - s p e c t r u m (vco=1955 cm"1) and t h e s t r u c t u r a l d a t a o f (CrC t r a n s 1 8 7 pm, C - O t r a n s 113 pm) show l a t o be a l i g a n d o f i n t e r m e ­ d i a t e b a s i c i t y and π-acceptor s t r e n g t h . F i n a l l y , a p r o g r a m was i n i t i a t e d t o i n v e s t i g a t e t h e c h e m i c a l r e a c t i v i t y o f JLa. E a r l i e r e x p e r i m e n t s h a d shown (J_) t h a t t h e p o l a ­ r i t y o f t h e P=C bond i n p o l a r r e a c t i o n s i s r e v e r s e d a s compared t o t h a t o f t h e N=C bond i n i m i n e s ; t h i s i s f u r t h e r e x e m p l i f i e d b y t h e following reactions : :

la

HCl

2a

»

^excess HCl 5 + C H ^ C ^ ) ^

i ^ / M e s ^ - c H C ^ H )Λ OH ' H

C

*

>

0

Mes-?-CH(C H ) 6

5

2

H

2°2. H

0 )

Mes-P-CH(C,H )„ OH

H

3 CH 0Na

Mes-P-CH(C,H,.) 3

3

0

C

H

'·) EtOH, 2·) 0

0 Mes-P-CH(C,H.) OEt

1.) n - B u L i 2·) D 0

Mes-P-CD(C,H,) _ è

6

2

6

2

n

0 )

9

1

3

5

5

9

2

9

2

u

R e m a r k a b l e a r e t h e r e a c t i o n s w i t h O 2 , S g , and B r 2 , i n w h i c h f o r m a l l y t h e P=C bond o f l a i s c l e a v e d : la.

^

polymer

+

0=C(C H )

^

polymer

+

S=C(C H )

5£l^MesPBr2

+

6

5

6

5

2

2

Br C(C H ) 2

6

5

2

R e a c t i o n s w i t h d i e n e s and c a r b o n y l compounds d i d e i t h e r n o t o c c u r b e l o w 150°C ( e . g . 2 , 3 - d i m e t h y l b u t a d i e n e , c y c l o p e n t a d i e n e , t e t r a c h l o r o - a - p y r o n ; a c e t a l d e h y d e ) , o r were a c c o m p a n i e d b y decom­ p o s i t i o n (2,3-dicarbomethoxybutadiene, hexachlorocyclopentadiene, 1 , 3 - d i p h e n y l i s o b e n z o f u r a n ; a c r o l e i n ) . However, 1 , 3 - d i p o l e s r e a c ­ ted r e a d i l y t o give w e l l - d e f i n e d a d d i t i o n products :

Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

404

PHOSPHORUS CHEMISTRY

/r-° C6H5 V

la

Mes-cJ-0

250C, r a p i d

M e S

C

" ^P^C H 6

5

Mes ,N~C H Mes-P. \-C H 6

ΤΈι*

6

t H 6

la

+

*~

C.H_N Ό J

5

11 5

5

80 0

J

N=N C H -N' V 6 5

\CS2 24h

C

6

5

\

p

12

^ C

H

6

H

8% 11

5

Mes

1H20»0

2

p 13 Mes-f-CH(C H ) ,C H NHC6H5 6

5

2

6

IS

+

«*Η > ™ 5

2

300c,

2

24h

>

S

5

C H5 6

Mes The s t r u c t u r e a s s i g n m e n t s o f 10 (δ-^Ρ: 9.3 ppm; δ 1 C: 98.8 ppm i J p c = 30 H z ) , P-Ç-0; 160.6 p p m ^ J p c = 45 H z ) , P-C=N) and J i (δ 31P: 47.36 ppm; 6 1 3 c : 47.75 ppm ( l j p = 14.7 H z ) ; 64.06 ppm ( 2 j = 20.5 Hz) f o l l o w f r o m t h e s p e c t r a l d a t a . I n t h e r e a c t i o n b e t w e e n La and p h e n y l a z i d e , t h e s t r o n g s o l v e n t dependence o f b o t h t h e r a t e and t h e p r o d u c t f o r m a t i o n i s r e m a r k a b l e . I n C^E^ ( a s i n C D C I 3 ) , the r e a c t i o n i s f a s t , a f f o r d i n g t h e i m i n o - y l i d JM a s t h e s o l e p r o d u c t (631p: 18.8 ppm; 6 1 3 c : 68.32 ppm ( l j = 166?5 H z ) ; 68.26 ppm ( 1 J = 166.5 H z ) ; c a . 1:1 m i x t u r e o f c i s / t r a n s i s o m e r s ) . I n C S 2 , t h e r e ­ a c t i o n i s s l o w ; b e s i d e s 8% Π , t h e m a j o r p r o d u c t i s t h e c y c l o a d d u c t \2 (631p: 140.8 ppm; o ^ C : 6Î.1 ppm ( 1 J c = 49 H z ) ) ; t h e s t r u c t u r e o f \2 i s f u r t h e r c o n f i r m e d b y i t s c o n v e r s i o n t o Γ3. Once f o r m e d , Π[ i s n o t c o n v e r t e d t o \2.; t h u s , b o t h compounds a r e o b v i o u s l y formed by d i f f e r e n t mechanisms. 3

1

C

P

P C

C

P C

P

Literature Cited

1. Klebach, Th.C.; Lourens, R.; Bickelhaupt, F.; J. Am. Chem. Soc. 1978, 100, 4886. 2. Stam, C.H.; personal communication. 3. Klebach, Th.C.; Lourens, R.; Bickelhaupt, F.; Stam, C.H.; Van Herk, Α.; J. Organometal. Chem. 1981, 210, 211. 4. Bokkers, G.; Kroon, J.; Spek, A.L.; Acta Cryst. Β 1979, 35, 2351. RECEIVED

July 7,

1981.

Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.