Ligand Effects on the Reaction of Alkoxide Ions with

rate analysis of the reaction of 0, S-dimethyl N-( 1 -phenylethyl)- .... Splitter and Calvin (9) defined for the effect of ligands on the pyramidal in...
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Ligand Effects on the Reaction of Alkoxide Ions with Organophosphorus Derivatives Containing Multiple Leaving Groups K E N N E T H Ε. DEBRUIN, C H A R L E S Ε. E B E R S O L E , M O R G A N M . H U G H E S , and D A V I D M . JOHNSON Department of Chemistry, Colorado State University, Fort Collins, C O 80523

Nucleophilic displacement reactions on tetracoordinate phos­ phorus compounds containing both alkoxy and a l k y l t h i o ligands proceed with widely varying stereochemistry. In the particular case of oxyanions as nucleophiles, hydrolysis of phosphonium salts (1-3) alkoxide displacements on phosphonothioates (2, 4-6) and alkoxide displacements on phosphorothioates (5, 6) have been observed to displace the a l k y l t h i o ligand with retention, inver­ sion, and retention stereochemistry respectively. Presumably, these stereochemical differences reflect changes i n the structure of the k i n e t i c a l l y formed trigonalbipyramid intermediates with inversion resulting when the a l k y l t h i o ligand i s co-axial with the attacking nucleophile while retention requires the a l k y l t h i o group to be i n the equatorial position (equation 1). The varying stereochemistry therefore implies that the non-displaced ligands (A and Β i n equation 1) have a major influence on r e l a t i v e ener­ gies of the t r a n s i t i o n states leading to the two intermediates.

Suggestive that the o r i g i n of the stereochemical crossover from displacement with inversion i n the phosphonothioate system (A = Ph, Me; Β = O, S) to retention i n the phosphorothioate sys­ tem (A = OR, Β = O) is a function of the electronegativity of the ligand A, is the observation that phosphoramidothioates (A = NRR') undergo displacement of the a l k y l t h i o ligand with net inversion but considerable racemization (7, 8). The NRR' group i s of intermediate electronegativity between Ph and OR and appears to give intermediate stereochemistry between inversion and retention. However, no mention i s made of possible OR displacement which was observed i n the reaction of phosphonothioates (A = Ph) and would underestimate the amount of intermediate 2 formation or whether the racemization may have occurred after product formation. 0097-6156/81/0171-0543$05.00/0 © 1981 American Chemical Society

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

544

PHOSPHORUS CHEMISTRY

We h a v e c a r r i e d o u t a d e t a i l e d p r o d u c t , s t e r e o c h e m i s t r y , and r a t e a n a l y s i s o f the r e a c t i o n of 0,S-dimethyl N - ( 1 - p h e n y l e t h y l ) phosphoramidothioate w i t h sodium ethoxide i n e t h a n o l a c c o r d i n g to t h e Scheme b e l o w . C o n c e n t r a t i o n s o f a l l f o u r s p e c i e s were f o l ­ l o w e d b y gas c h r o m o t o g r a p h y and t h e s t e r e o c h e m i s t r y o f t h e i n i t i a l d i s p l a c e m e n t p r o d u c t s were d e t e r m i n e d and c o r r e c t e d f o r i s o m e r i z a t i o n by i s o l a t i o n o f v a r i o u s t i m e i n t e r v a l s . ^ - n m r a n a l y s i s of

0 H

OEt

A — Ρ — SMe

1

\

OMe OEt

3

0 II ]

A — Ρ — OMe ι OEt

SMe

\ Isomeriz,

-OMe

0

II

0 M

OEt

A — Ρ — OEt 1 SMe

3

-OMe

^OEt

A — Ρ — OEt 1 OEt

SMe

t h e d i a s t e r e o m e r i c P-OMe o r P-SMe g r o u p s a f f o r d e d i s o m e r r a t i o and e x t r a p o l a t i o n t o t i m e z e r o gave t h e i n i t i a l r e a c t i o n s t e r e o ­ c h e m i s t r y ( 7 ) . The r e s u l t s a r e i n d i c a t e d i n T a b l e I and c o m p a r e d t o p h o s p h o n o t h i o a t e and p h o s p h o r o t h i o a t e s y s t e m s . It appears t h a t t h e c o m p e t i t i o n b e t w e e n t h e two modes o f a t t a c k by e t h o x i d e i o n on p h o s p h o r a m i d o t h i o a t e s i s v i r t u a l l y i d e n t i c a l t o t h a t f o r p h o s p h o n o t h i o a t e s and n o t i n t e r m e d i a t e i n b e h a v i o r . Table I . P r o d u c t s and C o m p e t i t i o n s f o r F o r m i n g I n t e r m e d i a t e i n the R e a c t i o n of A l k o x i d e Ions w i t h T h i o a t e E s t e r s of Phosphorus SMe

f

' OMe

A

e

-

OR

?"

e

OR

OR

%

Products

%

Products

Ph

OMe

85

100%-SMe

15

100%-OMe

PhCHMeNH

OEt

80

100%-SMe

20

95%-OMe 5%-SMe

iPrO

OEt

0

100

100%-SMe

pN0 Ph

OEt

62

100%- SMe

38

100%-OMe

Ph

OEt

78

100%- SMe

22

100%-OMe

pMe NPh

OEt

83

100%- SMe

17

100%-OMe

2

Ref, 4

5

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

112.

DeBRUIN E T A L .

Ligand

Effects

on

Alkoxide

Ions

Reaction

545

As a f u r t h e r e v a l u a t i o n o f t h e p o s s i b l e e l e c t r o n e g a t i v i t y c o n t r o l by an e q u a t o r i a l l i g a n d on r e l a t i v e s t a b i l i t i e s o f t h e t r a n s i t i o n s t a t e s l e a d i n g t o a t t a c k by a l k o x i d e i o n c o - a x i a l w i t h an SMe g r o u p v s a n OMe g r o u p , we i n v e s t i g a t e d t h e p r o d u c t s f r o m the r e a c t i o n o f sodium e t h o x i d e w i t h p a r a - s u b s t i t u t e d 0 , S - d i raethyl p h e n y l p h o s p h o n o t h i o a t e s . A s s u m i n g c o m p l e t e i n v e r s i o n o f c o n f i g u r a t i o n , p r o d u c t r a t i o s c o r r e c t e d a s above a c c o r d i n g t o t h e Scheme r e f l e c t modes o f a t t a c k . The r e s u l t s a r e l i s t e d i n Table I . C l e a r l y , for large e l e c t r o n i c v a r i a t i o n s i n l i g a n d A ( r a t e c o n s t a n t s s p a n f o u r powers o f 10) a m i n o r v a r i a t i o n i n modes o f i n t e r m e d i a t e f o r m a t i o n i s o b s e r v e d ; a g a i n suggestive that t h e e l e c t r o n e g a t i v i t y o f l i g a n d A i s a minor c o n t r i b u t o r t o d e t e r m i n i n g c o m p e t i t i v e a t t a c k s by a n u c l e o p h i l e . I n t h e absence o f s p e c i f i c l i g a n d - l i g a n d i n t e r a c t i o n s con­ t r o l l i n g c o m p e t i t i v e modes o f r e a c t i o n , t h e k i n e t i c a l l y p r e f e r r e d i n t e r m e d i a t e from a t t a c k by a n u c l e o p h i l e on a t e t r a c o o r d i n a t e o r g a n o p h o s p h o r u s e s t e r must r e f l e c t t h e r e l a t i v e a f f i n i t i e s o f l i g a n d s t o occupy an a x i a l s i t e v e r s u s e q u a t o r i a l s i t e s . To e v a l u a t e t h e s e a f f i n i t i e s , we h a v e measured t h e r a t e s o f r e a c t i o n o f s o d i u m m e t h o x i d e i n m e t h a n o l w i t h a v a r i e t y o f compounds. The r e s u l t s a r e given i nTable I I as logarithm o f r a t e constants f o r a t t a c k b y m e t h o x i d e c o - a x i a l w i t h l i g a n d Y. A c o m p a r i s o n o f t h e r a t e c o n s t a n t s k(Y=SMe) a n d k(Y=0Me) f o r p l a c i n g SMe v s OMe i n t h e a x i a l p o s i t i o n o f t h e t r a n s i t i o n s t a t e r e s p e c t i v e l y i n d i c a t e s a c a . 50-100 f o l d r a t e p r e f e r e n c e by t h e SMe g r o u p f o r a l l compounds s t u d i e d . Therefore, the o r i g i n o f t h e s t e r e o c h e m i c a l c r o s s o v e r between p h o s p h o n o t h i o a t e s and p h o s p h o r o t h i o a t e s l i e s n o t i n t h e k i n e t i c a f f i n i t i e s o f t h e two g r o u p s f o r a n a x i a l p o s i t i o n . I n t h e p h o s p h o n o - s y s t e m (A=Ph) r e p l a c i n g a n e q u a t o r i a l g r o u p B=0Me b y B=SMe p r o d u c e s a c a . 10 f o l d r a t e a c c e l e r a t i o n w h i l e i n t h e p h o s p h o r o - s y s t e m (A=Me0) r e p l a c i n g B=0Me b y B=SMe p r o d u c e s a c a . 500 f o l d r a t e acceleration. Thus, p h o s p h o n o t h i o a t e s k i n e t i c a l l y p r e f e r t o p l a c e t h e SMe g r o u p a x i a l (100/10) w h i l e p h o s p h o r o t h i o a t e s p r e f e r p l a c i n g t h e SMe g r o u p e q u a t o r i a l (500/100) c o n s i s t e n t w i t h t h e stereochemical results. Table I I . Rate Constants f o r Reaction of O r g a n o p h o s p h o r u s E s t e r s w i t h Sodium M e t h o x i d e (0.116M) i n M e t h a n o l a t 20.0°. ABP(0)Y A Β

Y=0Me

Ph Ph Ph Ph MeO MeO

-2.50 -3.19 -3.02 -4.19 -2.84 -5.60

Me Ph SMe OMe SMe OMe

log

k Y-SMe -0.32 -1.52 -0.84 -2.23

σ(Α)

σ(Β)

-1.1 -1.1 -1.1 -1.1 -2.5 -2.5

-0.6 -1.1 +0.9 -2.5 +0.9 -2.5

-1.7 -2.2 -0.2 -3.6 -1.6 -5.0

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

546

PHOSPHORUS CHEMISTRY

To evaluate whether the 10 fold or the 500 fold rate accel­ eration observed by replacing B=0Me by B=SMe is the "normal" effect, we have adopted as a model the σ (inv) constants of Splitter and Calvin (9) defined for the effect of ligands on the pyramidal inversion barriers of amines. A parallel between amines and phosphorus barriers has been demonstrated (10). Pre­ sumably, in the inversion processes, hybridization changes in the bond between the center atom and a ligand upon undergoing pyra­ midal inversion would resemble the changes in placing a ligand into the equatorial position upon forming a trigonal-bipyramid intermediate from tetrahedral reactants in the absence of steric factors. Since a phenyl ligand has a resonance acceleration to pyramidal inversion which would be absent in nucleophilic attack at tetracoordinate phosphorus, we estimated the σ constant for phenyl (-1.1) to be intermediate between NH (-1.6) and Me(-0.6). A plot of the logarithm of the rate constants (Y=0Me) against the sum of the σ constants for ligands A and Β gives a good linear correlation for a l l compounds except 0,S-dimethyl phenylphosphonothioate (A=Ph, B=SMe) which reacts by a factor of 50 slower than predicted. Although the validity of this treatment may be in doubt, the results suggest that an alkylthio has a net rela­ tively higher affinity for equatorial placement compared to an alkoxy group upon nucleophilic attack at tetracoordinate phos­ phorus. Thus, attack of the nucleophile co-axial with an alkoxy group should be preferred. The reason for preferred attack of alkoxide ion co-axial with the alkylthio group in phosphonothio­ ates and phosphoramidothioates is at present unresolved but may suggest these compounds are not undergoing rate limiting forma­ tion of a pentacoordinate intermediate. 2

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

De'Ath, N. J.; E l l i s , K.; Smith, D. J . H.; Trippet, S. J.C.S. Chem. Commun. 1971, 714. Farnham, W. B.; Mislow, K.; Mandel, N.; Donohue, J . J.C.S. Chem. Commun. 1972, 120. DeBruin, Κ. Ε.; Johnson, D. M. J . Am. Chem. Soc. 1973, 95, 4675. DeBruin, Κ. Ε . , Johnson, D. M. J. Am. Chem. Soc. 1973, 95, 7921. Cooper, D. B.; Hall, C. R.; Harrison, J . M.; Inch, T. D. J.C.S. Perkin I, 1977, 1969. Inch, T. D.; Lewis, G. J . Carbohydrate Res. 1975, 45, 65. Hall, C. R.; Inch, T. D. Tetrahedron Lett. 1977, 3765. Hall, C. R.; Inch, T. D. J.C.S. Perkin I, 1979, 1646. Splitter, J . S.; Calvin, M. Tetrahedron Lett. 1973, 4111. Baechler, R. D.; Andose, J . D.; Stackhouse, J.; Mislow, K. J . Am. Chem. Soc. 1972, 94, 8060.

RECEIVED

June 30, 1981.

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