109 Isotope Effects in Amination Reactions of Chlorocyclophosphazenes
Phosphorus Chemistry Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/28/19. For personal use only.
J. M. E . G O L D S C H M I D T , R . H A L E V I , and E .
LICHT
Department o f Chemistry, Bar-Ilan U n i v e r s i t y , R a m a t - G a n 52100 Israel
The general substitution reactions of hexahalogenocyclotriphosphazenes, Ν Ρ X (X=F,Cl,Br; Y=Substituent) 3
3
6
can yield up to 12 products. Three of these arise from n = 1, 5 and 6 and the other 9 from n = 2, 3 and 4, for each of which gem-, cis- and trans- isomers can exist. As an example of these react ions, amination of the hexachloro-compound, Ν P Cl has been part icularly extensively investigated, mostly to isolate and character ize the products formed (1). In reactions which can lead to iso mers, mixtures of them are generally formed, though typically the quantity of one of them greatly exceeds that of the rest. In this behavior, called stereoselectivity, the operation of characteristic substitution patterns was revealed. Attempts to rationalize these isomeric preferences mechanistically have been widespread and many factors have been considered. Because the amination reactions have been demonstrated to be kinetically controlled (2), our experiments aimed at elucidating the stereoselectivity mechanistically have focused on detailed kinetic studies. The general mechanism that has been proposed (3,4) for the first stage of substitution of Ν Ρ CI by amine leading to (I) with X = C l , Y = NR (R = alkyl or H) and n = 1 is 3
3
3
3
6
6
2
This proposal rests on a combination of 3 chief experimental find ings. (1) The order of the reactions (5-9) is never less than one with respect to the concentration of the phosphazene and the amine requiring that both of these be involved in the rate-determining, or a prior, step. (2) The existence of a second-order term in the concentration of amine in non-polar solvents arising from base catalysis by amine (7). (3) The effect of the leaving group, the rate of reaction increasing in the order F < CI < Br (5). Corrobatorive evidence for the proposed mechanism comes from the values of the activation parameters measured in THF in which most of our 0097-6156/81/0171-0529$05.00/0 © 1981 American Chemical Society
PHOSPHORUS CHEMISTRY
530
TABLE I K i n e t i c D a t a i n T.H.F. f o r R e a c t i o n s N
P
3 3
C 1
6
+
2
R
i
R
2
Me H Me Me Piperidine Bu H
H
N
R
R
y
i a t 30 C
N
2
k
2
P
3 3
dm^mol ^*sec ^
C 1
N R
R
5 1 2 AH*
+
H
2
k J mol
23.2+0.6 11.5±0.2 1.33±0.02 0.0163±0.0003
N
R
R
+
1 2 AS*
C
JK^mol"
2.911.7 7.112.1 12.112.1 28.811.7
1
1
Refs
-20516 -19718 -18816 -18416
9 3 4 4
w o r k was p e r f o r m e d b e c a u s e o f t h e s i m p l e o v e r a l l s e c o n d - o r d e r i n v a r i a b l y found i n i t because t h e s o l v e n t a c t s as a base ( 3 ) . Some t y p i c a l v a l u e s o f ΔΗ* and AS* a p p e a r i n T a b l e I . The v a l u e s o f ΔΗ* a r e a l w a y s r a t h e r l o w w h i l s t v a l u e s o f AS* a r e so v e r y l o w t h a t i t i s a l w a y s r a t e - c o n t r o l l i n g . The b u l k o f t h e ΔΗ* i s a s c r i b e d t o t h e f i r s t p r e - e q u i l i b r i u m w h i l s t AS* i s l a r g e l y a s s o c i a t e d w i t h t h e s o l v a t i o n o f t h e CI i o n b e i n g formed i n the rate-determining step. This crude d i v i s i o n o f the a c t i v a t i o n p a r a m e t e r s i s s u p p o r t e d b y t h e d a t a on t h e s t e r i c e f f e c t s o f t h e a l k y l g r o u p s o f t h e n u c l e o p h i l e ( T a b l e I ) . The r e d u c t i o n i n the r a t e o f r e a c t i o n w i t h i n c r e a s i n g s i z e o f t h e a l k y l groups r e s u l t s o n l y f r o m i n c r e a s e s i n ΔΗ* , t h e v a l u e s o f AS* b e i n g v i r t u a l l y c o n s t a n t , t h i s l a t t e r f a c t r e f l e c t i n g t h e common f i n a l s t e p i n a l l t h e r e a c t i o n s (4_) . T h e r e b e i n g d i s a g r e e m e n t on t h e d e p r o t o n a t i o n s t e p , i t a l s o h a v i n g been c o n s i d e r e d c o n c e r t e d w i t h CI i o n d e p a r t u r e (_3), we c a r r i e d o u t c o m p a r a t i v e s t u d i e s u s i n g p i p e r i d i n e and N - d e u t e r o p i p e r i d i n e , ( T a b l e I I ) . The c o m b i n a t i o n o f a second-order term i n p i p e r i d i n e c o n c e n t r a t i o n ( i n toluene) w i t h the absence o f a measurable i s o t o p e e f f e c t i n both s o l v e n t s s t u d i e d TABLE I I Rate C o n s t a n t s f o r Amines and N - D e u t e r a t e d Analogues i n R e a c t i o n s (10) N P,C1, + 2H(D)NR R Q
R
i
1
R
2
• N P C1-NR.. R
0
Solvent
0
T°C
Piper idine
T.H.F.
30
Piper idine
Toluene
0
T.H.F.
30
Η k_ = dm
mol
sec
0
. a 2
+
+ H ( D ) N R . R C1~"
0
0
HNR- R, 1 . 1
k
b
-
2.1311.3 xlO
1.71 10.5
0.0163 10.0003
-
k
mol
= dm
0
DNRk 3 k
7.41 10.19
0
a
2
1
k 2
I
k
7.40 10.18
k 3
-
1.7 2.l6i0.6 10.3 xlO 3
0.0163 10.0002 sec
b
k
-
109.
GOLDSCHMiDT
Amination
E T AL.
531
of Chlorocyclophosphazenes
i s i n t e r p r e t e d as supporting the s e p a r a t i o n o f the f a s t p r e - e q u i l i b r i u m deprotonation from the CI i o n departure step. G e m i n a l and n o n - g e m i n a l ( c i s and t r a n s ) i s o m e r s can b e f o r m e d i n the second stage o f the s u b s t i t u t i o n , l e a d i n g t o ( I ) w i t h X = C l , Y = NR^ and η = 2. A t t h i s s t a g e o f s u b s t i t u t i o n g e m i n a l i s o mers a r e o n l y o b t a i n e d w i t h p r i m a r y a m i n e s ( e x c e p t a z i d i r i n e ) a n d t h i s i s accommodated b y t h e f o l l o w i n g p r o p o s e d conjugate-base Η NR ^NHR + NR / -cr μ ρ ι » { f f e ¥ W 2 2 W «Cl ( k a s e ) 2 4 3 ^ 7 7 ^ P
C
1
P
C
1
N
P
C
=
N
R
(
M
)
1
mechanism. A l t h o u g h k i n e t i c a l l y u n c o n f i r m e d , t h i s mechanism i s s u p p o r t e d b y t h e e n h a n c e d gem-isomer f o r m a t i o n o b s e r v e d on a d d i t i o n o f base (11). With r e s p e c t t o the 2 non-geminal isomers which are the e x c l u s i v e p r o d u c t s f o r m e d w i t h s e c o n d a r y a m i n e s , and w h i c h a r e p r o d u c e d i n v a r y i n g q u a n t i t i e s t o g e t h e r w i t h t h e gem-isomer w i t h p r i m a r y a m i n e s , t h e g e n e r a l mechanism o f t h e i r f o r m a t i o n r e s e m b l e s t h a t o f the f i r s t stage o f s u b s t i t u t i o n , but the r e a c t i o n i s slower and p r o c e e d s a t t h e u n s u b s t i t u t e d p h o s p h o r u s atom b e c a u s e o f m e s o m e r i c charge t r a n s f e r from the s u b s t i t u e n t amino-group ( 3 ) . W i t h a l l amines the t r a n s - i s o m e r s p r e d o m i n a t e i n the non-geminal r e a c t i o n s ( 1 2 ) . T h r e e e f f e c t s h a v e been a d v a n c e d t o e x p l a i n t r a n s - p r e f e r e n c e (1) The c i s - e f f e c t ( 1 3 ) , (2) t h e s t e r i c e f f e c t (14) a n d (3) t h e s u b s t i t u e n t s o l v a t i n g e f f e c t (SSE) ( 9 , 1 5 ) . To t e s t t h e s e p r o p o s a l s we p e r f o r m e d a k i n e t i c s t u d y i n THF o f c i s - a n d t r a n s - i s o m e r f o r m a t i o n a t d i f f e r e n t t e m p e r a t u r e s f o r t h e r e a c t i o n s shown i n T a b l e I I I and e v a l u a t e d t h e v a l u e s o f t h e a c t i v a t i o n p a r a m e t e r s ( T a b l e III). TABLE I I I f
1
f
N P Cl NMe 3
3
5
f
2
A c t i v a t i o n Parameters f o r Reactions (15) + 2HNMe , ( N M -2'2 e ), 2 2 T r a n s } N3J P3C l4 C i s
+
0
Cis-Isomer ΔΗ*
8.8
AS*
-239
H
N
M
e
+
C
1
0
Trans-Isomer
± 0.5
28.5
± 1.3
± 8
-159
± 8
"
Units kJ
mol -1 ,-1 JK mol
T h e s e v a l u e s c a n o n l y b e r e c o n c i l e d w i t h SSE i n w h i c h t h e f o l l o w i n g t r a n s i t i o n s t a t e s t r u c t u r e which i s p a t e n t l y ίΟ.-Η-, s t e r i c a l l y i m p o s s i b l e f o r the r e a c t i o n t h a t \ leads t o the c i s - i s o m e r i s p o s t u l a t e d . This i s - - - \ . ci(2> formed by r e p r o t o n a t i o n on the a m i n o - s u b s t i t u e n t pli^N^ \ a f t e r p r i o r d e p r o t o n a t i o n o f tljie p r i m a r y p r o d u c t 2 f o r m e d . The f o r m a t i o n o f a n H C I i o n p a i r , r a t h e r t h a n a ' b a r e CI as i n the c i s - r e a c t i o n , i n the r a t e - d e t e r m i n i n g step accounts f o r t h e h i g h e r v a l u e s o f A S * i n t h e t r a n s - r e a c t i o n compared w i t h the c i s - r e a c t i o n . S u p p o r t i n g e v i d e n c e f o r t h i s p r o p o s a l comes f r o m the r e s u l t s o f e x p e r i m e n t s ( T a b l e IV) i n w h i c h t h e r a t i o s o f nong e m i n a l i s o m e r i c p r o d u c t s u s i n g p i p e r i d i n e and N - d e u t e r o p i p e r i d i n e were determined. The a v e r a g e r a t i o ( t r a n s / ( c i s + t r a n s ) ) ^ R
Ν
2
N
p
x
N
R
1
532
PHOSPHORUS CHEMISTRY
(trans/(cis + trans)) is equal to 0.95. The reduced amount of trans-isomer found with the deuterium compound adds support for the SSE mechanism involving a Η-bonded transition state structure. However a full study of the kinetics of the cis- and trans- re actions at various temperatures using normal and N-deuteropiperidine needs to be carried out to validate this conclusion. TABLE IV N-Deuterium Effect on Ratio of Trans to Total Νon-Geminal Isomers in Reaction N P C1 NC H 3
3
5
5
10
+ 2H(D)NC H 5
1 - HNC H 5
10
• N ^ C ^ ( N C ^ ) + H (D^N^H^Cl""" 2
2 « DNC H
1()
5
Tr
Exp t.
/ ans I |Cis+Trans4
1 2 3 4 5
82.6+1.6 79.0+2.7 75.7±1.1 72.310.4 75.612.1
Xl00
10
I Trans | Icis+TransJ^ 79.711.1 73.611.5 72.712.0 70.713.7 71.512.2
£
2 / 100
l
0.964 0.932 0.959 0.978 0.946
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Krishnamurthy, S.S.; Sau, A.C.; Wood, M. Adv. Inorg. Chem. Radiochem. 1978, 24, 41. Friedman, N.; Goldschmidt, J.M.E.; Sadeh, U.; Segev, M. J. Chem. Soc. Dalton Trans. 1981, 103. Goldschmidt, J.M.E.; Licht, E. J . Chem. Soc. A 1971, 2429. Goldschmidt, J.M.E.; Licht, E. J . Chem. Soc. Dalton Trans. 1979, 1012. Moeller, T.; Kokalis, S.G.J. Inorg. Nucl. Chem. 1963, 25, 1397. Krishnamurthy, S.S.; Sundaram, P.M. Inorg. Nucl. Letters 1979, 15, 367. Capon, B.; Hills, K.; Shaw, R.A. J . Chem. Soc. 1965, 4059. Eliahu, S. M.Sc. Dissertation, Bar-Ilan University, 1969. Goldschmidt, J.M.E.; Licht, E. J . Chem. Soc. Dalton Trans. 1972, 728. Licht, E. Ph.D. Dissertation, Bar-Ilan University, 1971. Gabay, Z.; Goldschmidt, J.M.E. J . Chem. Soc. Dalton Trans. 1981 (in press). Biran, Z.; Goldschmidt, J.M.E. J . Chem. Soc. Dalton Trans. 1979, 1017. Keat, R.; Shaw, R.A. J . Chem. Soc. A 1966, 908. Schmutz, J . L . ; Allcock, H.R. Inorg. Chem. 1975, 14, 2433. Goldschmidt, J.M.E.; Goldstein, R. J . Chem. Soc. Dalton Trans. 1981 (in press).
RECEIVED
July 7, 1981.
