1 Phosphorus-Carbon Compounds with pπ-pπ Bonds Opening Lecture R. APPEL
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Anorganisch Chemisches Institut der Universität, 5300 Bonn, Gehard-Domagk-Strasse 1, FRG
In this lecture some new routes to phosphorus-carbon com pounds with P-C multiple bonds, found i n connection with our i n vestigations on reactions of tertiary phosphanes with chlorinated carbon compounds, such as tetrachloromethane, hexachloroethane, phosgene, and isocyanide dichlorides are reported. Furthermore some stereochemical problems concerning this type of compound will be discussed. Action of tetrachloromethane on trimethylsilyl-substituted methyl-diphenylphosphanes causes quantitative chloroform elimina tion with formation of trimethylsilylated (TMS) methylenechloro -diphenylphosphoranes. Heating the b i s t r i m e t h y l s i l y l substituted
compound causes spontaneous gas evolution of TmsCl at 120°C. The product i s identified by elemental analysis, molecular mass de termination, and the characteristic 31p nmr s h i f t . I t is a greenish-yellow l i q u i d which can be distilled i n vacuo without decomposition. The general a p p l i c a b i l i t y of this synthesis, based upon migration of an organyl substituent from phosphorus to the ylid-carbon i s restricted so far to P-aryl substituted com pounds. Another route to a number of theoretically interesting com pounds of two-coordinate phosphorus i s the reaction of P-trimethy l s i l y l - s u b s t i t u t e d phosphanes with phosgene. The reaction pro ceeds v i a several isolable intermediates. The first i s the phosphino-substituted methylene-phosphane, which is generated by silyl-group migration from phosphorus to oxygen. Treatment with further phosgene then causes further elimination of CO and chlorotrimethylsilane, yielding a compound with a P-P bond which 0097-6156/81/0171-0001$05.00/0 © 1981 American Chemical Society In Phosphorus Chemistry; Quin, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
2
PHOSPHORUS CHEMISTRY
4PhPTms
2
+ 2C1 C0
4
T
m
s
C
1
2PhP= C-PPhTms
2
+
ci co
TmsCljCO
2
Ψ
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OTms PhP—C —PPh 1 I I PhP — C — PPh i OTms no l o n g e r shows t h e c h a r a c t e r i s t i c -^P nmr s h i f t o f t w o - c o o r d i n a t e p h o s p h o r u s . An x - r a y s t r u c t u r e d e t e r m i n a t i o n w h i c h was c a r r i e d o u t by Dr. H a l s t e n b e r g a n d P r o f e s s o r H u t t n e r a t K o n s t a n z , showed t h a t i t i s t h e f i r s t 2,3,5,6-tetraphosphabicyclo[2.2.0]h e x a n e , w h i c h c o n t a i n s a n a s y m m e t r i c and d i s t o r t e d b i c y c l o h e x a n e s k e l e t o n w i t h two s e t s o f e q u i v a l e n t p h o s p h o r u s atoms i n d i f f e r ent environments. Thus f a r t h e b i c y c l o h e x a n e s k e l e t o n c a n o n l y be o b t a i n e d i n t h e r e a c t i o n o f COCI2 w i t h p h e n y l - b i s ( t r i m e t h y l silyl)phosphane. I n i t i a l l y t-butyl-bis(trimethylsilyl)phosphane indeed r e a c t s analogously t o g i v e t h e p h o s p h i n o - s u b s t i t u t e d m e t h y l e n e p h o s p h a n e , w h i c h c a n be i s o l a t e d a s a p u r e s u b s t a n c e a n d w h i c h c a n be c o n v e r t e d t o t h e P-H p h o s p h i n e by m e t h a n o l . What happens i n t h e s e c o n d s t e p w i t h f u r t h e r p h o s g e n e , h o w e v e r , d e pends v e r y much upon t h e r a t e o f a d d i t i o n . I f theaddition oc c u r s v e r y s l o w l y a t -80°, t h e f i v e - m e m b e r e d r i n g s y s t e m w i t h a C-C d o u b l e bond i s f o r m e d , w h i l e r a p i d m i x i n g o f s o l u t i o n s o f b o t h components i n t h e m o l a r r a t i o 1:1 a f f o r d s a f i v e membered r i n g c o n t a i n i n g f o u r p h o s p h o r u s atoms b r i d g e d by a CO m o i e t y . TmsO
OTms \
c
/ t-Bu-P "
t-Bu-p
c
\
/
P-t-Bu
t-Bu-P
X
p-jt-Bu
\ P-t-Bu
N
P / ~ ~ c ' " I II _t-Bu 0 The r e a c t i o n o f p h e n y l ( b i s t r i m e t h y l s i l y l ) p h o s p h i n e w i t h p h e n y l i s o c y a n i d e d i c h l o r i d e , t h e a z a - a n a l o g u e o f phosgene y i e l d s tetraphosphahexadiene according t o elemental a n a l y s i s , c r y o s c o p i c m o l e c u l a r mass d e t e r m i n a t i o n , a n d 31p nmr s t u d i e d . To u n d e r s t a n d t h e i n t e r e s t i n g s t r u c t u r a l p r o b l e m o f t h i s compound, we must l o o k i n d e t a i l a t t h e 31p m r s p e c t r u m . A t a m b i e n t t e m p e r a t u r e t h e s u b s t r u c t u r e o f an AA XX 4 - s p i n system i s observed. The downf i e l d s i g n a l s a t 258 ppm a r e a s s i g n e d t o t h e t w o - c o o r d i n a t e p h o s p h o r u s a n d t h e u p f i e l d m u l t i p l e t a t -12.3 ppm a r i s e s f r o m t h e t r i v a l e n t phosphane m o i e t y . Between b o t h g r o u p s t h e r e i s a n
f
f
In Phosphorus Chemistry; Quin, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
1.
APPEL
Phosphorus-Carbon
4PhPTms + 2Cl C=NPh 2
2
Compounds
with p ^ p *
Bonds
3
PhNTms ! -4TmsCl > 2PhP=C-PPhTms + Cl C=NPh 2
- 2TmsCl - PhNC
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PhNTms PhNTms \ / PhP=C-P-P-C=PPh ι ι PhPh
d i f f u s e a b s o r p t i o n a t 115.7 ppm which sharpens on heating to 60°C while the other s i g n a l s broaden. The s i g n i f i c a n c e of the broad s i g n a l can be e l u c i d a t e d by c o o l i n g to -70°C. In a d d i t i o n to the two sharp m u l t i p l e t s already mentioned, a second 4-spin A A X X system appears, the l e f t h a l f i n d i c a t i n g two-coordinate phosphorus and the r i g h t , a t -3.2 ppm., the diphosphane Ρ atoms. Renewed heating to ambient temperature r e s u l t s i n coalescence of the two inner m u l t i p l e t s at 115 ppm. Thus the process i s r e v e r sible. On f u r t h e r c o o l i n g to about -80°C, however, the outer m u l t i p l e t s l a b e l l e d 5a t o t a l l y disappear. The reason f o r t h i s i s p r e c i p i t a t i o n due to i n s u f f i c i e n t s o l u b i l i t y . Low temperature f i l t r a t i o n of the c r y s t a l s and r e d i s s o l v i n g them a t 30° again gives the complete 31p nmr spectrum as before. F i r s t l y , according to elemental a n a l y s i s and molecular mass determination, the com pound i s homogeneous, that i s to say, the broad a b s o r p t i o n i s f
f
In Phosphorus Chemistry; Quin, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
PHOSPHORUS CHEMISTRY
4
c a u s e d by an i s o m e r . F u r t h e r m o r e t h e s p l i t t i n g i n t o a s e c o n d AA'XX s y s t e m a t -70° i n d i c a t e s a d i a s t e r e o m e r i c t e t r a p h o s p h a hexadiene, s i n c e both the c h a r a c t e r i s t i c m u l t i p l e t s of diphosphane- and m e t h y l e n e p h o s p h a n e - p h o s p h o r u s c a n be i d e n t i f i e d . An e x p l a n a t i o n , which e l u c i d a t e s the r e v e r s i b l e temperature-depen d e n t c o a l e s c e n c e o f t h e i n n e r s i g n a l s and a l s o r a t i o n a l i z e s why o n l y one s t e r e o i s o m e r d i s p l a y s t h i s phenomenon i s now g i v e n . A t t e m p t s t o e x p l a i n t h e c o a l e s c e n c e by m i g r a t i o n o f t h e s i l y l g r o u p b e t w e e n t h e n i t r o g e n and p h o s p h o r u s atom o r by a r o t a t i o n a r o u n d t h e C-N o r P=C d o u b l e - b o n d a r e n o t s u p p o r t e d by the e x p e r i m e n t a l d a t a . We came t o t h e c o n c l u s i o n t h a t a p e r i c y c l i c r e a c t i o n o c c u r s , w h i c h i s a n a l o g o u s t o t h e Cope r e a r r a n g e ment f o r h e x a d i e n e - 1 , 5 . I n a [ 3 . 3 ] - s i g m a t r o p i c r e a c t i o n o f t h e t e t r a p h o s p h a h e x a d i e n e , t h e bond between t h e two p h o s p h o r u s atoms b r e a k s w i t h s i m u l t a n e o u s f o r m a t i o n o f t h e P=C d o u b l e bond and a new P-P bond. The o r i g i n a l nmr s i g n a l s o f t h e Ρ atoms w i t h t h e c o o r d i n a t i o n number 2 and 3 must c o l l a p s e i n t h e m i d d l e b e c a u s e the r e a r r a n g e m e n t i s a s y m m e t r i c a l one and b e c a u s e i t o c c u r s on the nmr t i m e s c a l e . The f a c t t h a t o n l y one d i a s t e r e o m e r shows c o a l e s c e n c e c a n be e x p l a i n e d a s f o l l o w s . As i s w e l l known, s y m m e t r i c a l , d i f f e r e n t l y s u b s t i t u t e d d i p h o s p h a n e s w h i c h a r e com p a r a b l e t o o u r t y p e i n s u b s t i t u t i o n have two c e n t e r s o f c h i r a l i t y . T h e i r s y n t h e s i s u s u a l l y y i e l d s a 1:1 p r o p o r t i o n o f t h e meso f o r m and r a c e m i c m i x t u r e , w h i c h c a n be c h a r a c t e r i z e d by t h e i r d i f f e r ent 31p nmr s h i f t s . I n o u r c a s e t h e o u t e r s e t o f m u l t i p l e t s must be a s s i g n e d t o one d i a s t e r e o m e r and t h e i n n e r s e t t o t h e o t h e r . From t h e o b s e r v a t i o n t h a t t h e s p e c t r u m o f t h e c r y s t a l s f i l t e r e d a t l o w t e m p e r a t u r e shows two s i g n a l g r o u p s a g a i n a t room t e m p e r a t u r e , we must c o n c l u d e t h a t t h e c o n f i g u r a t i o n a l e q u i l i b r i u m between t h e meso f o r m and r a c e m i c m i x t u r e i s p r o m p t l y a c h i e v e d . The f a s t exchange i n t h e ^ l p nmr s p e c t r u m o f one i s o m e r a t room t e m p e r a t u r e , w h i c h i s n o t o b s e r v e d f o r i s o m e r 5 a , l e a d s t o the c o n c l u s i o n t h a t o n l y one f o r m f u l f i l l s t h e s t e r e o c h e m i c a l d e mands o f t h e C o p e - r e a r r a n g e m e n t . T h i s i s the racemic m i x t u r e as shown. I f t h i s i s c o r r e c t , t h e c r y s t a l s f i l t e r e d a t l o w t e m p e r a t u r e , w h i c h show no c o a l e s c e n c e , s h o u l d be t h e meso f o r m . The x-ray a n a l y s i s of these c r y s t a l s v e r i f i e s t h i s a s s e r t i o n s i n c e the s u b s t i t u e n t s a t t h e P-P bond a r e i n d e e d t r a n s t o one a n o t h e r . Of c o u r s e , we must be c a r e f u l when t r a n s f e r r i n g c o n c e p t i o n s f r o m c a r b o n compounds t o o t h e r e l e m e n t s . N e v e r t h e l e s s t h e r e a r e i n t e r a c t i o n s between e l e c t r o n s o f t h e 2p and e l e c t r o n s o f t h e 3p l e v e l which i s c o n s i d e r a b l y higher. T h e r e f o r e we l o o k e d f o r f u r t h e r p r o o f o f t h i s h y p o t h e s i s . The f o l l o w i n g e x p e r i m e n t seemed t o be a l i n k i n g one between t h e f i e l d s o f c a r b o n and p h o s p h o r u s c h e m i s t r y . We t r e a t e d s u c c i n i c a c i d d i c h l o r i d e w i t h b i s ( t r i m e t h y l s i l y l ) s i l y l p h e n y l phosphane. A c c o r d i n g t o o u r h y p o t h e s i s a p r i m a r y h a l o s i l a n e c o n d e n s a t i o n f o l l o w e d by a s i l y l m i g r a t i o n and f o r m a t i o n o f t h e P=C d o u b l e b o n d , and f i n a l l y a [3.3]s i g m a t r o p i c rearrangement to the diphosphane should occur. In the 31p nmr s p e c t r u m . We o b s e r v e a h a l o s i l a n e c o n d e n s a t i o n t o
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1
In Phosphorus Chemistry; Quin, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
1.
APPEL
Phosphorus-Carbon
2ΡΗΡτ
PhCEP -TmsCl
2
-HCl s p e c t r o m e t e r d i r e c t l y c o n n e c t e d t o t h e p y r o l y s i s a p p a r a t u s , we r e a d i l y s u c c e e d e d i n s y n t h e s i z i n g t h e m e t h i n e p h o s p h a n e on a p r e p a r a t i v e s c a l e and c h a r a c t e r i z e d i t f u r t h e r . The PC t r i p l e bond i n t h e p h o s p h a - a l k y n e h a s been c o n f i r m e d b y i t s c h a r a c t e r i s t i c •^C and P nmr d a t a a s w e l l a s by s t e p w i s e H C l a d d i t i o n . The phospha-alkene i s obtained f i r s t , which i s transformed t o benz y l d i c h l o r o p h o s p h a n e by a s e c o n d m o l e o f H C l . By ^ P nmr s p e c t r o s c o p i c s t u d i e s we c o u l d show t h a t b e n z y l d i c h l o r o p h o s p h a n e c a n be d e h y d r o c h l o r i n a t e d by t e r t i a r y a m i n e s i n r e v e r s a l o f i t s f o r mation r e a c t i o n . I n a d d i t i o n t o the Ε isomer, t h e Ζ isomer i s a l s o formed. Y e t H C l a d d i t i o n t o t h e phospha-alkyne does n o t p r o d u c e any t r a n s - c o m p o u n d . T h i s c a n be e x p l a i n e d i n t e r m s o f a s t e r e o s p e c i f i c c i s - a d d i t i o n t o t h e t r i p l e bond. 3 1
In Phosphorus Chemistry; Quin, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
Downloaded by UNIV OF CHICAGO on July 24, 2013 | http://pubs.acs.org Publication Date: November 11, 1981 | doi: 10.1021/bk-1981-0171.ch001
1.
APPEL
Phosphorus-Carbon
Compounds
with ρπ-ρ*· Bonds
The d i r e c t vacuum p y r o l y s i s o f b e n z y l d i c h l o r o p h o s p h a n e a l s o y i e l d s some p h o s p h a - a l k y n e i n a d d i t i o n t o a l a r g e amount o f u n i d e n t i f i e d side products. In a d d i t i o n to the phenylphosphaa c e t y l e n e w h i c h has a h a l f - l i f e o f 7 m i n u t e s a t 0°C, we c o u l d p r e p a r e t h e c o n s i d e r a b l y more s t a b l e t r i m e t h y l s i l y l d e r i v a t i v e w h i c h has a h a l f - l i f e a t room t e m p e r a t u r e o f 50 m i n u t e s . The _t-butyle d e r i v a t i v e i s e n t i r e l y s t a b l e , as Becker reported recent iy. To sum up, we c a n s t a t e t h a t t o d a y a s u r p r i s i n g l y g r e a t num b e r o f s t a b l e compounds w i t h PC d o u b l e bonds i s known. A t t h e moment we have s e v e r a l methods f o r t h e f o r m a t i o n o f t h i s d o u b l e bond. I n a d d i t i o n t o t h e d e h y d r o h a l o g e n a t i o n o f h a l o g e n o p h o s p h i n e s , t h e t h e r m a l e l i m i n a t i o n o f t r i m e t h y l c h l o r o s i l a n e and t h e e l i m i n a t i o n o f h e x a m e t h y l d i s i l o x a n e can be u t i l i z e d . Prior to e l i m i n a t i o n , the s i l y l group can m i g r a t e from phosphorus t o the o x y g e n , n i t r o g e n , o r s u l f u r o f a n α-carbonyl o r h e t e r o c a r b o n y l f u n c t i o n s u b s e q u e n t t o i n i t i a l TmsCl a d d i t i o n t o o r c o n d e n s a t i o n with silylphosphanes. M o s t i m p o r t a n t l y , t h e r e i s no l o n g e r a n y reason f o r the p r e p a r a t i v e chemist to b a l k a t 2ρ -3ρ i n t e r a c tions. V e r y c l e a r l y t h e l ^ C nmr s p e c t r a s u p p o r t t h i s v i e w , s i n c e t h e y beyond any d o u b t show t h a t t h e c a r b o n i n t h e m e t h y l i d e n e p h o s p h a n e s i s s p ^ - h y b r i d i z e d and s p - h y b r i d i z e d i n t h e p h o s p h a alkynes. The e x i s t e n c e o f Ε and Ζ i s o m e r s a t t h e PC d o u b l e bond can a l s o be demonstrated. Moreover, the f l u c t u a t i n g s t r u c t u r e of the tetraphosphahexadiene i s a b s o l u t e l y analogous to p u r e l y o l e f i n i c compounds. Whether t h e hopes w i l l be f u l f i l l e d t h a t t h e s e s t u d i e s o f t h e l a s t f i v e y e a r s have t u r n e d t h e f i r s t p a g e s o f a new c h a p t e r o f p h o s p h o r u s - c a r b o n c h e m i s t r y s t i l l r e m a i n s t o be s e e n . W h e t h e r , f o r i n s t a n c e , i t w i l l be p o s s i b l e t o c o n s t r u c t c o n j u g a t e d PC s y s t e m s s u i t a b l e f o r D i e l s - A l d e r r e a c t i o n s , i s n o t y e t certain. E x p l o r a t i o n of the c o o r d i n a t i o n chemistry o f these new s p e c i e s may a l s o p r o v e f r u i t f u l . π
R E C E I V E D July 13,
π
1981.
In Phosphorus Chemistry; Quin, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
