Chirality Induction in Coordination Complexes - ACS Symposium

6

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 24, 2016 | http://pubs.acs.org Publication Date: May 27, 1980 | doi: 10.1021/bk-1980-0119.ch006

Chirality Induction i n C o o r d i n a t i o n Complexes

A. M. SARGESON Research School of Chemistry, The Austrialian National University, P.O. Box 4, Canberra Act 2600, Australia

Over the past twenty years the conformations and steric effects in chelate ring systems have been examined in considerable detail. Structural studies along with equilibrium measurements, some kinetic studies and conformational analyses have given us a better insight into the steric interactions within the chelates and between the chelates (1). For five membered chelates such as coordinated 1,2 diaminopropane

λ

CoR(-)pn

8 CoR(-)pn

the axial-equatorial nature of the substituents on the C atoms is sufficiently pronounced(2) that the axial methyl groups interact with axial substituents on the metal ion and this conformation is rarely if ever observed (1,2). In this way the conformation of the chelate is controlled as λ for Co(R)(-)pn (1,2). Tris chelate systems of this type show a marked preference for the isomer where the C-C axes of the individual chelates are near parallel to the C3 axis of the complex ion, e.g.∆[Co(R)(-)pn ]3+ (lel ) is ~15 fold more stable than Λ[Co(R)(-)pn ] (ob ) where the C-C axes are oblique to the C axis of the ion (1), Substituents on the Ν atoms, however, do not show such a pronounced conformational effect in terms of axial-equatorial orientation but they appear to interact more strongly with the substituents on the metal ion. For example, sarcosinate ion chelates with a substantial specificity in the [Co(en)2sarcosinato] ion (3). 0-8412-0538-8/80/47-119-115$05.00/0 © 1980 American Chemical Society 3

3+

3

3

3

3

2+

Douglas and Saito; Stereochemistry of Optically Active Transition Metal Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


116

STEREOCHEMISTRY OF TRANSITION METALS

I n t h e c o n f i g u r a t i o n w h e r e t h e CH i s poised over the ethylenedi.amine c h e l a t e i t i n t e r a c t s q u i t e s t r o n g l y whereas i n the a l t e r n a t i v e c o n f i g u r a t i o n t h e CH3 g r o u p l i e s i n t h e s p a c e b e t w e e n t h e two "en" c h e l a t e s (4) - I n t h i s s i t u a t i o n t h e c o n f o r m a t i o n o f t h e " e n " r i n g s do n o t seem t o be i m p o r t a n t and t h e amino a c i d shows v e r y l i t t l e c o n f o r m a t i o n a l c h a r a c t e r . The e q u i l i b r a t i o n between such i o n s a r i s e s from a base c a t a l y z e d removal of the proton at the c h i r a l Ν center. The pK of such p r o t o n s i s ~15-17. In a c i d i c s o l u t i o n they are k i n e t i c a l l y i n e r t and t h e c h i r a l i t y at Ν i s therefore preserved. S u b s t i t u t i o n on t h e C atoms o f s u c h amino a c i d c h e l a t e s does not g i v e such a pronounced e f f e c t . Analogous b i s ( e t h y l e n e d i a m i n e ) amino a c i d c h e l a t e s y s t e m s show r a t h e r l i t t l e d i s c r i m i n a t i o n b e t w e e n t h e two d i a s t e r e o i s o m e r s . F o r example, t h e r a t i o o f t h e R and JS i s o m e r s i n t h e Λ c o n f i g u r a t i o n i s a b o u t e q u a l f o r a l a n i n e and 2:1 f o r v a l i n e ( 5 ) . I t can be a r g u e d t h a t t h e b u l k o f t h e R g r o u p i s i m p o r t a n t and t h a t t h e e f f e c t o f t h e i s o - p r o p y l g r o u p i s due t o i t s r e l a t i v e o r i e n t a t i o n i n r e l a t i o n t o c h e l a t e r i n g s 1 and 2 . T h i s e f f e c t i s r e l a t e d to t h a t of the s a r c o s i n a t o i c n except t h a t t h e s u b s t i t u e n t i s now more d i s t a n t f r o m t h e o t h e r c h e l a t e s and the e f f e c t i s l e s s . 3


6.

Coordination

SARGESON

Complexes

117

2+ NH, HN 2


;coC

OH"

H.N-

H

2

NH

2

M

R = CH

«

3

R = iPr m

0

r

e

MS) ^

( f f ) /

/^(S) = I O ± 0 - 2

^

( / ? , /

A.(S) = ' - 7 ± O I 4

S t a b l e

less stable

Much o f t h i s f a c t a n d r a t i o n a l e i s f a i r l y o l d h i s t o r y a n d i t i s i n t r o d u c e d h e r e t o s e t t h e s c e n e f o r some new s p e c i f i c i t y w h i c h i s n o t e q u i l i b r i u m c o n t r o l l e d b u t k i n e t i c a l l y c o n t r o l l e d . The work h a s a r i s e n from i n t r a m o l e c u l a r o r g a n i c r e a c t i o n s promoted by m e t a l i o n s a n d t h e u s e o f c o o r d i n a t e d n u c l e o p h i i e s . We h a v e seen* f o r example^ t h a t bound 0H~ a t a m e t a l c a n p r o v i d e a h i g h l o c a l c o n c e n t r a t i o n o f t h e r e a g e n t a t n e a r n e u t r a l pH. The bound n u c l e o p h i l e i s s t i l l p o t e n t even t h o u g h i t i s somewhat m o d i f i e d b y the m e t a l . The a b i l i t y o f t h e m e t a l i o n t o a c t i v a t e a n d p r o t e c t o r g a n i c m o l e c u l e s h a s a l s o b e e n s e e n t o b e s u b s t a n t i a l . I t was l o g i c a l t h e n t h a t t h e s t r u c t u r a l and c o n f o r m a t i o n a l p r o p e r t i e s s h o u l d b e p u t t o g e t h e r w i t h t h e r e a c t i v i t y a s p e c t s t o d e s i g n some stereospecif i c syntheses. M o s t o f t h e c h e m i s t r y h a s b e e n done w i t h C o ( I I I ) c o m p l e x e s l a r g e l y b e c a u s e t h e l i g a n d s do n o t e x c h a n g e r a p i d l y w i t h t h e m e t a l i o n and t h e complexes remain i n t a c t f o r t h e l i f e times of t h e r e a c t i o n s considered. I n t h i s way t h e e f f i c a c y o f t h e p r o c e s s e s c a n be a s s e s s e d f r e e f r o m t h e c o m p l i c a t i o n o f ligand-metal i o n e q u i l i b r i a . The c o m p l e x e s o n t h e w h o l e a r e cheap t o make a n d t h e methods a r e f a i r l y r o u t i n e and w e l l d e f i n e d . Stereospecific hydration of olefins The e f f i c a c y o f c o o r d i n a t e d n u c l e o p h i i e s h a s b e e n e s t a b l i s h e d f o r t h e i n t r a m o l e c u l a r h y d r o l y s i s o f numerous s u b s t r a t e s ; f o r e x a m p l e , c o o r d i n a t e d amino a c i d e s t e r s ( 5 ) , amino n i t r i l e s (6,7_)



118


and 2 - b r o m o e t h y l a m i n e ( 8 ) . The same p r o s p e c t f o r c y c l i z a t i o n e x i s t s w i t h c o o r d i n a t e d o l e f i n s (9,10) a n d two s y s t e m s h a v e been c o n s t r u c t e d t o examine i t a s f o l l o w s :

C00CH

3

methyl maleate

methyl fumarate

The a d d i t i o n o f OH a t t h e o l e f i n i c c e n t e r c r e a t e s a c h i r a l c e n t e r a t c a r b o n i n c o n j u n c t i o n w i t h t h e c h i r a l c e n t e r a t c o b a l t . Two d i a s t e r e o i s o m e r s a r e t h e r e b y p r o d u c e d . A d d i t i o n o f t h e bound OH i s v e r y r a p i d f o r t h e m a l e a t o s y s t e m a n d e s s e n t i a l l y pH i n d e p e n d e n t b e t w e e n pH 8-10. I n t h i s r a n g e t h e d i a s t e r e o i s o m e r r a t i o i s ~2:1 a s shown b e l o w . However, a t h i g h e r pH v a l u e s t h e r a t e becomes f i r s t o r d e r i n OH" a n d i n 0.1 M NaOH a t 25° t h e h a l f l i f e f o r t h e p r o d u c t i o n o f m a l a t e i s ~3 s e c ( a t l e a s t 1 0 - f o l d f a s t e r t h a n t h e u n c o o r d i n a t e d h y d r a t i o n ) . Under t h e s e c o n d i t i o n s t h e r e a c t i o n becomes much more s t e r e o s p e c i f i c w i t h a 9:1 r a t i o o f diastereoisomers. W h i l e i t i s c l e a r t h a t i n t h e pH i n d e p e n d e n t r e g i o n , H a d d i t i o n a t t h e β c a r b o n atom i s t h e r a t e - d e t e r m i n i n g s t e p , i t l o o k s a s i f i n t h e h i g h base r e g i o n d e p r o t o n a t i o n o f t h e CoOH e n t i t y a n d a d d i t i o n o f Co-0 c o u l d b e r a t e - d e t e r m i n i n g . 6

+

H

OH

,0CH

3

t i 3sec, 01 M OH", 25°



6.

SARGESON

Coordination

Complexes

119

The same p a t t e r n was o b s e r v e d f o r t h e f u m a r a t e c o m p l e x b u t t h e r a t e s were about 1 0 0 0 - f o l d slower. The i n c r e a s e i n s p e c i f i c i t y i s the i n t e r e s t i n g and s u r p r i s i n g f e a t u r e . F o r a n a l o g o u s amino systems, t h e r a t i o o f t h e diastereoisomers Ç 5 ) at equilibrium i s a b o u t 2:1 o r l e s s w i t h t h e same s t r u c t u r a l r e l a t i o n s h i p s a s t h e analogous malate products. I t appears, t h e r e f o r e , that the p r o d u c t r a t i o o f t h e pH i n d e p e n d e n t p a t h r e s e m b l e s t h e e q u i l i b r i u m s i t u a t i o n w h e r e a s t h e p a t h d e p e n d e n t on b a s e i s f a r f r o m t h a t c o n d i t i o n . An e x p l a n a t i o n f o r t h e i n c r e a s e c o u l d a r i s e i f Co-0" a d d i t i o n was r a t e - d e t e r m i n i n g . In the t r a n s i t i o n states f o r the g e n e r a t i o n o f t h e two i s o m e r s c o n f i g u r a t i o n ( a ) w i l l b e more c o m p r e s s e d t h a n t h a t o f ( b ) b y v i r t u e o f t h e non-bonded i n t e r a c t i o n s b e t w e e n t h e s u b s t i t u e n t s o n t h e o l e f i n and a d j a c e n t C o ( e n ) chelate. T h i s s t e r i c c o m p r e s s i o n w i l l b e much more e v i d e n t i n t h e t r a n s i t i o n s t a t e s than i n t h e product malates.

The o t h e r s p e c i f i c i t y f e a t u r e w h i c h i s i n t e r e s t i n g i n t h i s r e a c t i o n i s t h e e x c l u s i v e f o r m a t i o n o f t h e five-membered c h e l a t e r e l a t i v e t o t h e p o s s i b l e six-membered c h e l a t e . Clearly the ester g r o u p exo t o t h e c h e l a t e i s g o v e r n i n g t h e a d d i t i o n and t h e c a r b o x y l bound t o t h e m e t a l i o n h a s no i n f l u e n c e . The s t e r e o c h e m i s t r y f o r t h e a d d i t i o n o f Co-0~ o r Co-OH r e q u i r e s t h e o l e f i n and bound c a r b o x y l π o r b i t a l e t o b e e s s e n t i a l l y o r t h o g o n a l . It f o l l o w s t h e r e f o r e t h a t t h e r e i s a m i n i m a l i n t e r a c t i o n between t h e two. The same r e s t r i c t i o n does n o t h o l d f o r t h e e s t e r g r o u p exo to t h e c h e l a t e and t h e o l e f i n i s t h e r e b y s e n s i t i z e d . T h i s i s an i n t e r e s t i n g r e s t r i c t i o n p l a c e d on t h e r e a c t i o n where t h e m e t a l i o n o r g a n i z e s t h e s t e r e o c h e m i s t r y o f t h e r e a c t a n t s and t h e r e w i l l be o t h e r e x a m p l e s o f s u c h c o n s t r a i n t s where t h e m e t a l s a r e i n v o l v e d . Not o n l y i s 0H~ a p o t e n t n u c l e o p h i l e i n t h i s c o n t e x t b u t s o i s NH2" bound t o t h e m e t a l i o n . The pentaammine m a l e a t o e s t e r c o m p l e x e s o f t h e t y p e Co(NH3) OOCCH=CHCOOR r e a d i l y r e a c t i n b a s i c aqueous s o l u t i o n t o g i v e t h e c h e l a t e d a s p a r t a t e e s t e r complex ( 1 1 ) . 2+

5


120


„0R

NH

2+

NH

3

CH

3

II

-NHa

H N>. 3

OH;

. N H

H N. 3

2

H

^

C

H

:CoCT H NT

H N'

3


3

NH

CH

NH

3

3

II

CH

0^

"OR

/ ,OR

NH

3

H

2

^CH

2+

2

H Nk 3

H N' 3

NH

3

Analogous r e a c t i o n s w i t h the monodentate maleato d i a n i o n , however, do n o t o c c u r and t h i s i s n o t s u r p r i s i n g o r g a n i c c h e m i s t r y s i n c e the t e r m i n a l a n i o n should d e a c t i v a t e the o l e f i n to n u c l e o p h i l i c attack. I t became i n t e r e s t i n g , t h e r e f o r e , t o e x a m i n e t h e c h e l a t e d m a l e a t o c o m p l e x w h e r e b o t h c a r b o x y l a t e i o n s a r e bound t o t h e m e t a l c e n t e r and w h e r e b o t h s h o u l d h a v e some e s t e r - l i k e q u a l i t y . I t transpires that [(en) Co maleato] reacts r a p i d l y i n b a s i c aqueous s o l u t i o n s ( 1 2 ) . The r a t e i s f i r s t o r d e r _ i n h y d r o x i d e i o n (-V = k [ m a l e a t o c o m p l e x ] [0H~~], w h e r e k = 0.45 M *s a t 25°, μ=1.0 NaClOi*). The i n i t i a l p r o d u c t i s a s u b s t i t u t e d a s p a r t i c a c i d t e r d e n t a t e w h i c h s u b s e q u e n t l y decomposes i n t h e b a s i c medium w i t h cleavage of a c a r b o x y l a t e r e s i d u e from the C o ( I I I ) i o n ( V = k [ u n s t a b l e i s o m e r ] [OH ] , k=3.7M ^ " ^ a t [0H~] 25°, μ=1.0 NaClOt»). We presume i t i s t h e s p e c i e s shown, b u t t h i s i s n o t c e r t a i n y e t . An i n t e r e s t i n g a s p e c t o f t h i s r e a c t i o n i s n o t o n l y t h e s p e e d o f t h e i n t r a m o l e c u l a r a d d i t i o n but the s p e c i f i c i t y of i t . Only the i s o m e r shown a p p e a r s t o be f o r m e d . The o t h e r i s o m e r p o s s i b l e , w h e r e t h e d e p r o t o n a t e d e t h y l e n e d i a m i n e n u c l e o p h i l e , - N H ( - ) , adds a t t h e o t h e r c a r b o n atom o f t h e m a l e a t e i o n , d o e s n o t seem t o be p r o d u c e d i n d e t e c t a b l e q u a n t i t i e s . S u p e r f i c i a l l y t h e r e seems no s p e c i a l reason f o r the d i s c r i m i n a t i o n . Moreover the isomer which +

2

1



6.

SARGESON

Coordination

Complexes

121

unstable isomer

Two sites for addition i s n o t o b s e r v e d i s b y f a r t h e most s t a b l e , xt can be s y n t h e s i z e d from C o ( I I ) , en and 2 - a m i n o e t h y l a s p a r t a t e by o x i d a t i o n w i t h O2 i n t h e p r e s e n c e o f c h a r c o a l and t h e e q u i l i b r i u m p o s i t i o n l i e s h e a v i l y i n favor o f t h e isomer which i s not observed i n t h e k i n e t i c s y n t h e s i s . So t h i s i s a n e s p e c i a l l y i n t e r e s t i n g i n s t a n c e o f c h i r a l i n d u c t i o n a t a c a r b o n atom w h e r e the k i n e t i c r o u t e g i v e s , a p p a r e n t l y exclusively, the less stable isomer and t h e e q u i l i b r i u m r o u t e l e a d s a l m o s t


122


e x c l u s i v e l y t o t h e s t a b l e f o r m whose s t r u c t u r e h a s b e e n d e t e r m i n e d .


+

Structure of the stable isomer (W.L. Steffen) The a p p a r e n t r e a s o n s f o r t h i s s p e c i f i c i t y n e e d some amplification. An examination of the problem u s i n g D r e i d i n g m o d e l s r e v e a l s some i n t e r e s t i n g f e a t u r e s f o r t h e a p p r o a c h o f t h e o l e f i n to the nucleophile. I f t h e r e i s a r e q u i r e m e n t f o r one c a r b o x y l a t e i o n t o b e c o - p l a n a r and t h e r e f o r e c o n j u g a t e d w i t h t h e o l e f i n t h e n t h e r e a r e two d i s c r e t e c o n f o r m a t i o n s t o b e a c h i e v e d . B o t h c a n b e r e a l i z e d b u t one o f them i s s u b s t a n t i a l l y more f a v o r a b l e than t h e o t h e r f o r t h e a d d i t i o n o f t h e n u c l e o p h i l e . The two p o s s i b i l i t i e s a r e d e p i c t e d ,

Orientations for the addition of Amido ion to chelated maleate So t h e c o m b i n a t i o n o f t h e e l e c t r o n i c r e q u i r e m e n t f o r a c t i v a t i o n o f t h e o l e f i n and t h e r e s t r i c t i o n w h i c h t h a t p l a c e s o n t h e s i t e f o r a d d i t i o n of the n u c l e o p h i l e i s an i n t e r e s t i n g aspect of the demands w h i c h c h e l a t i o n makes o n t h e r e a c t i o n . I t should be m e n t i o n e d , o f c o u r s e , t h a t t h e r e a c t i o n does n o t o c c u r a t a l l w i t h


6.

Coordination

SARGESON

Complexes

123

t h e f r e e m a l e a t e i o n a n d e t h y l e n e d i a m i n e b u t i t does w i t h t h e d i e s t e r (13). So t h e a c c e l e r a t o r y e f f e c t o f c h e l a t i n g t h e d i a n i o n must e x c e e d 1 0 and p r o b a b l y i s g r e a t e r t h a n 1 0 . Not only i s t h e s p e c i f i c i t y i n t e r e s t i n g b u t t h e r e a c t i o n s c o n d u c t e d i n t h i s way are e x t r a o r d i n a r i l y fast. The p r o c e s s e s a r e r e l e v a n t a s m o d e l s f o r some e n z y m e - c a t a l y z e d h y d r a t i o n , d e h y d r a t i o n and a m i n a t i o n - d e a m i n a t i o n r e a c t i o n s . The s p e c i f i c i t y was a l s o o b s e r v e d i n l i q u i d ammonia a n d d i m e t h y l s u l f o x i d e .


6

Stereo

1 0

S p e c i f i c i t y d i r e c t e d by the

Chelates

I m i n e f o r m a t i o n i n c o o r d i n a t e d c h e l a t e s h a s been known f o r a v e r y l o n g t i m e b u t what h a s n o t b e e n a l l t h a t c l e a r u n t i l r e c e n t l y i s t h e s t a b i l i t y o f t h e c o o r d i n a t e d i m i n e when t h e i m i n e g r o u p i s e s s e n t i a l l y exo t o t h e c h e l a t e r i n g . Systems s u c h a s

f o r example, h a v e a n e x t r a o r d i n a r y s t a b i l i t y , e s p e c i a l l y t o w a r d s c o n c e n t r a t e d a c i d s ( 1 4 ) . They a r e , h o w e v e r , more v u l n e r a b l e t o b a s e s , presumably because o f the ease w i t h w h i c h n u c l e o p h i l e s add at the imine carbon center. A c e t a l d e h y d e condenses r e a d i l y w i t h Co ( I I I ) amine s y s t e m s (14) a n d t h e r e i s a s u b s t a n t i a l a n a l o g o u s l i t e r a t u r e w i t h d i v a l e n t m e t a l i o n amine s y s t e m s ( 1 5 ) . Much o f the l a t t e r a r o s e from the C u r t i s complexes formed by d i s s o l v i n g [Ni(en)3]

i n acetone.

2 +

I n t h e c o u r s e o f c o n d e n s i n g t h e o x a l a t o complex, [ 0 ο ( ε η ) ( ΰ 0 θ ] a n d CH CHO, i n b a s i c s o l u t i o n , a n a c e t a l d i m i n e c o m p l e x was i d e n t i f i e d a n d a f t e r p r o l o n g e d e x p o s u r e t o t h e a l d e h y d e a n d base, a c o m p l e x was i s o l a t e d where two a l d i m i n e r e s i d u e s had condensed t o g e t h e r . This self-condensation has t o +

2

2

3



124

o c c u r on t h e complex and i t appears t o be s t e r e o s p e c i f i c i n s e v e r a l senses. The r e a c t i o n must t a k e p l a c e a f t e r t h e c o n d e n s a t i o n o f two CH3CHO m o l e c u l e s a d j a c e n t t o e a c h o t h e r on d i f f e r e n t e t h y l e n e d i a m i n e r e s i d u e s . F o r one, t h e C H g r o u p h a s t o b e syn t o t h e C o ( I I I ) i o n a n d i n t h e o t h e r anti. The syn m e t h y l i s d e p r o t o n a t e d i n t h e b a s i c medium a n d t h e c a r b a n i o n p r o d u c e d a t t a c k s t h e i m i n e c a r b o n o f t h e o t h e r a l d i m i n e . The r e s u l t i s a q u a d r i d e n t a t e w h e r e t h e c h i r a l C c e n t e r h a s b e e n f o r m e d s t e r e o s p e c i f i c a l l y b y t h e way t h e i m i n e s a r e o r i e n t e d t h r o u g h t h e c h e l a t e s . Commencing w i t h Δ-[Co(en)2ox] , o n l y o n e i s o m e r s h o u l d b e o b s e r v e d . 3


+

The same c o n d e n s a t i o n d o e s n o t o c c u r w i t h e i t h e r m e s i t y l o x i d e o r a l d o l a n d t h e r e f o r e i t i s presumed t h a t t h e o n l y f e a s i b l e p a t h i s t h e one d e s c r i b e d . T h e o r i e n t a t i o n o f t h e n u c l e o p h i l e i s i d e a l f o r t h e i n t r a m o l e c u l a r condensation and i t i s obvious t h a t d e p r o t o n a t i o n o f t h e o t h e r m e t h y l group would n o t l e a d t o any reaction. S i m i l a r l y , c o n d e n s a t i o n o f CH3CHO a t t h e two a d j a c e n t Ν c e n t e r s b o t h t r a n s t o t h e o x a l a t o group would n o t l e a d t o t h e intramolecular reaction. I n s h o r t , the c h e l a t e s themselves have directed thespecificity. I t i s tempting t o argue t h a t t h e C u r t i s condensations w i t h [ N i ( e n ) 3 ] i n acetone t a k e p l a c e by an analogous r o u t e and s u b s e q u e n t l y r e a r r a n g e t o t h e p l a n a r c o n d i t i o n about the N i i o n . So far> a t t e m p t s t o c a r r y o u t a n a l o g o u s r e a c t i o n s w i t h trans-[Co(en)2X2] complexes have n o t been s u c c e s s f u l b u t t h e n e g a t i v e r e s u l t does n o t mean a g r e a t d e a l . +

2

2 +

+

S t e r e o s p e c i f i c a d d i t i o n o f CN" a t a c h e l a t e d i m i n e - o r b i t a l steering I m i n e s bound t o some m e t a l i o n s a r e a c t i v a t e d t o a t t a c k b y n u c l e o p h i i e s ( 1 6 , 17) p r o v i d e d t h e d o n a t i o n f r o m t h e m e t a l d e l e c t r o n s t o t h e empty π* o r b i t a l e o f t h e i m i n e i s n o t s u b s t a n t i a l ( 1 8 ) . A good e x a m p l e o f a n a c t i v a t e d c h e l a t e d i m i n e i s t h a t d e r i v e d f r o m p y r u v a t o - i m i n e bound t o t h e C o ( I I I ) i o n :

In t h i s condition the imine i s susceptible t o very rapid a d d i t i o n of c a r b a n i o n s such as(->CH «02(16)and (-)CH(COCH ) (17). P r e s u m a b l y t h e a c t i v i t y a r i s e s b e c u a s e t h e m e t a l i o n i m p a r t s some i m i n i u m c h a r a c t e r t o t h e c h e l a t e . A t t h e same t i m e t h e m e t a l p r e v e n t s p r o t o n a t i o n o f t h e i m i n e and s t a b i l i z e s t h e imine c h e l a t e . F o r example, s p e c i e s o f t h i s t y p e a r e s t a b l e i n 6 M HC1. Cyanide i o n s h o u l d a d d i n t h e same manner a s t h e c a r b a n i o n s a n d we h a v e i n v e s t i g a t e d t h i s r e a c t i o n using the pyruvato imine b i s ( e t h y l e n e 2

3

2


6.

SARGESON

Coordination


diamine)cobalt(III)

Complexes

i o n (19) shown

125 below:

I t i s l i k e l y t h a t CN" adds r e v e r s i b l y a t t h e i m i n e c e n t e r . Other s t u d i e s would i n d i c a t e t h a t t h e r e i s l i t t l e preference f o r a d d i t i o n on one s i d e o f t h e p l a n a r c h e l a t e r e l a t i v e t o t h e o t h e r , even though t h e C o ( I I I ) c e n t e r i s c h i r a l . The s t a b i l i t i e s o f a n a l o g o u s amino a c i d c o m p l e x e s (5) and t h e r e d u c t i o n o f t h e c h e l a t e d i m i n e by t h e BR\~ i o n b o t h show l i t t l e s p e c i f i c i t y ( 2 0 ) . However, t h e s u b s e q u e n t r e a c t i o n o f c o o r d i n a t e d a m i d e i o n w i t h t h e amino a c i d n i t r i l e i s a n o t h e r m a t t e r . Once f o r m e d , t h e a m i d i n e q u a d r i d e n t a t e i s s t a b l e i n d i l u t e a c i d and base. Moreover, t h e l e a s t s t a b l e c o n f i g u r a t i o n i s t h e p r e f e r r e d p r o d u c t . The s t r a i n i n t h e bound a m i d i n e m o i e t y -CH2-N = C(NH2)-C- f o r t h i s i s o m e r i s much g r e a t e r t h a n t h a t i n t h e k i n e t i c a l l y l e s s - p r e f e r r e d p r o d u c t w h e r e t h e a m i d i n e m o i e t y -CH2-N = C ( N H ) - C - i s c l o s e t o b e i n g planar. The s t r a i n d i f f e r e n c e i s r e f l e c t e d i n t h e e q u i l i b r i u m p o s i t i o n f o r t h e two i s o m e r s w h i c h l i e s h e a v i l y t o w a r d s t h e i s o m e r l e a s t f a v o r e d by t h e k i n e t i c r o u t e . 2


STEREOCHEMISTRY OF


126

TRANSITION METALS

B o t h CN" and 0H~ a p p e a r t o be i n v o l v e d i n t h e r a t e l a w and t h e i n v o l v e m e n t o f CN" c o u l d be accommodated by a p r e - e q u i l i b r i u m . A f t e r a d d i t i o n o f CN" t o t h e i m i n e , OH" a b s t r a c t s a p r o t o n f r o m t h e amine and t h e c o o r d i n a t e d amide i o n a t t a c k s t h e n i t r i l e t o g e n e r a t e t h e a m i d i n e . The s t e r e o s p e c i f i c i t y o f t h e c o n d e n s a t i o n p r e s u m a b l y a r i s e s f r o m t h i s amide a t t a c k and t h e p r e f e r e n c e o f one s i t e o v e r t h e o t h e r n e e d s some e x p l a n a t i o n . D r e i d i n g models of the a n t i c i p a t e d a c t i v a t e d complexes i n d i c a t e a s u b s t a n t i a l d i f f e r e n c e b e t w e e n t h e two o r i e n t a t i o n s . These are d e p i c t e d below l o o k i n g down t h e -CN a x i s a t t h e o r i e n t a t i o n o f t h e g r o u p s a r o u n d t h e bound a m i d e i o n . In the o r i e n t a t i o n which l e a d s to the p r e f e r r e d isomer, the d e p r o t o n a t e d o r b i t a l p o i n t s d i r e c t l y a t the n i t r i l e C atom. I n t h e o r i e n t a t i o n w h i c h l e a d s t o t h e l e a s t abundant isomer, the n i t r i l e i s l e s s f a v o r a b l y d i s p o s e d between t h e Ν p r o t o n and t h e d e p r o t o n a t e d o r b i t a l . We presume i t i s t h i s " o r b i t a l s t e e r i n g " e f f e c t which accounts f o r the s t e r e o s p e c i f i c i t y . I n terms of the e n e r g e t i c s , of c o u r s e , o n l y a d i f f e r e n c e of about 1.3 k c a l / m o l e i n t h e f r e e e n e r g i e s o f a c t i v a t i o n w o u l d be enough t o accommodate t h e r e s u l t s so t h e e f f e c t c o u l d be f a i r l y s u b t l e .

A n o t h e r i n t e r e s t i n g f a c e t of t h i s c h e m i s t r y i s the b e h a v i o r o f t h e most a b u n d a n t a m i d i n e i n c o n c e n t r a t e d HC&. Under t h e s e c o n d i t i o n s t h e c a r b o x y l a t e g r o u p i s c l e a v e d and t h e amino a c i d amine g r o u p u n d e r g o e s an edge - d i s p l a c e m e n t so t h a t C£~ e n t e r s a t t h e s i t e o r i g i n a l l y o c c u p i e d by t h e amine g r o u p . The r e s u l t i n g t e r d e n t a t e i s m e r i d i o n a l and a t pH5 i n aqueous s o l u t i o n , t h e c h l o r o complex s l o w l y g i v e s the " l e s s abundant" amidine isomer q u a n t i t a t i v e l y , a l b e i t w i t h an i n v e r t e d c o n f i g u r a t i o n o f t h e e t h y l e n e diamine l i g a n d s about the c o b a l t ( I I I ) i o n . The q u a n t i t a t i v e edge d i s p l a c e m e n t and t h e q u a n t i t a t i v e i n v e r s i o n a b o u t c o b a l t a r e b o t h u n u s u a l f a c e t s o f c o b a l t ( I I I ) s u b s t i t u t i o n c h e m i s t r y and merit closer investigation. Stereospecific

Carbinolamine Formation

Amino a c e t a l d e h y d e i s a r e l a t i v e l y awkward r e a g e n t i n o r g a n i c c h e m i s t r y m a i n l y because i t condenses w i t h i t s e l f r a t h e r r e a d i l y . The p r o b l e m can be overcome somewhat by c o o r d i n a t i n g t h e amine t o a m e t a l i o n w h i c h renders i t l e s s a c c e s s i b l e to the aldehyde group.


6.

SARGESON

Coordination

Complexes

127

The amino a c e t a l d e h y d e p r o t e c t e d a s t h e d i m e t h y l a c e t a l r e a c t s r e a d i l y w i t h a-[Co t r i e n C l 2 ] i o n t o g i v e a n a m i n o c h l o r o c o m p l e x w h i c h o n t r e a t m e n t w i t h a c i d y i e l d s t h e c o o r d i n a t e d a l d e h y d e shown.


+

Regio- and Stereospecific Aldehyde Condensation

The a l d e h y d e r e a d i l y u n d e r g o e s a b a s e c a t a l y z e d c o n d e n s a t i o n w i t h a coordinated t r i e t h y l e n e t e t r a m i n e nitrogen center t o give a c a r b i n o l a m i n e ( 2 1 ) . The s p e c i f i c i t y a s p e c t s o f t h i s c o n d e n s a t i o n are s i g n i f i c a n t . The c o o r d i n a t e d C I " d i r e c t s t h e c o n d e n s a t i o n e x c l u s i v e l y t o t h e amine c e n t e r trans t o i t a n d t h e c a r b i n o l a m i n e p r o d u c e d i s s t e r e o s p e c i f i c i n t h e o r i e n t a t i o n shown. The r e g i o n a l s p e c i f i c i t y can be accounted f o r by t h e f a c t t h a t the N-protons t r a n s t o c o o r d i n a t e d C l ~ a r e much more a c i d i c t h a n t h e o t h e r s (>100 f o l d ) . The s p e c i f i c i t y o f t h e c a r b i n o l a m i n e , h o w e v e r , i s more d i f f i c u l t t o accommodate. I n t h e s t r u c t u r e o f t h e p r o d u c t t h e OH a p p e a r s h y d r o g e n bonded t o t h e p r o x i m a l a p i c a l amine g r o u p . T h i s a t t a c h m e n t may w e l l d e v e l o p i n t h e t r a n s i t i o n s t a t e a n d D r e i d i n g m o d e l s i n d i c a t e t h a t i t i s a most f a v o r a b l e c o n f i g u r a t i o n . The same o p p o r t u n i t y does n o t o c c u r f o r t h e a l t e r n a t i v e c o n f i g u r a t i o n of t h e carbinolamine moiety. I t appears, t h e r e f o r e , t h a t b o t h t h e s u b s t i t u e n t e f f e c t and i n t r a m o l e c u l a r Η-bonding c o u l d b e p o w e r f u l a i d s i n d e t e r m i n i n g the s p e c i f i c i t y a t r e a c t i o n s i t e s .



128


C h i r a l m e t a l i o n cages The c o n s t r u c t i o n o f l a r g e f u s e d r i n g s y s t e m s l i k e t h e cryptâtes (22) requires rather sophisticated organic synthesis and i t has o c c u r r e d t o a number o f c h e m i s t s t h a t t h e p r o b l e m m i g h t be s i m p l i f i e d by c o o r d i n a t i n g a m e t a l i o n so t h a t t h e problem i s reduced to l i n k e d s m a l l r i n g syntheses. This s t r a t e g y has now b e e n a p p l i e d t o t h e s y n t h e s i s o f t h e n i t r o g e n a n a l o g u e s o f t h e p o l y e t h e r cryptâtes ( 2 4 ) . The s y n t h e s i s a r o s e f r o m c o o r d i n a t e d i m i n e c h e m i s t r y o f t h e t y p e d e s c r i b e d p r e v i o u s l y (16,17) and t h e d i s c o v e r y o f t h e s y n t h e s i s o f a d i o x a c y l c a m q u a d r i d e n t a t e on a m e t a l c e n t e r ( 2 5 ) . i.e.

U s i n g a t r i s ( e t h y l e n e d i a m i n e ) c o m p l e x and ammonia as t h e b a s e i n s t e a d o f OH", t h e p r o s p e c t o f m a k i n g a t r i g o n a l cap was c o n c e i v e d and a c h i e v e d as f o l l o w s :

The cage c o n f e r s i n t e r e s t i n g p r o p e r t i e s on t h e m e t a l compared w i t h the p a r e n t t r i s ( e t h y l e n e d i a m i n e ) . For example, C o does n o t e x c h a n g e w i t h t h e C o ( I I ) c a g e i n 24 h r s a t 25° e v e n t h o u g h Co u s u a l l y e x c h a n g e s i t s l i g a n d s on t h e m i c r o s e c o n d t i m e s c a l e . Moreover, the C o ( I I ) complex r e t a i n s i t s c h i r a l i t y a t l e a s t over 2 +

2 +



6.

SARGESON

Coordination

Complexes

129

two h o u r s w i t h o u t m e a s u r a b l e r a c e m i z a t i o n . The c h i r a l i t y o f t h e c o m p l e x i s o f s p e c i a l i n t e r e s t f r o m t h e p o i n t o f view o f s t e r e o s p e c i f i c i t y s i n c e i t has seven c h i r a l c e n t e r s , y e t i f t h e s y n t h e s i s i s conducted w i t h one c h i r a l form o f t h e t r i s ( e t h y l e n e d i a m i n e ) c o m p l e x , o n l y one i s o m e r i s p r o d u c e d o f t h e 16 p o s s i b l e . T h i s e x t r a o r d i n a r y s p e c i f i c i t y n e e d s some e x a m i n a t i o n a n d t o do t h a t t h e mechanism o f s y n t h e s i s h a s t o b e c o n s i d e r e d i n some d e t a i l . The f i r s t s t e p i s o b v i o u s l y t h e c o n d e n s a t i o n o f f o r m a l d e h y d e w i t h t h e bound e t h y l e n e d i a m i n e T h i s r e q u i r e s d e p r o t o n a t i o n o f t h e bound amine c e n t e r f o l l o w e d b y a t t a c k o f t h e c o o r d i n a t e d amide i o n a t t h e c a r b o n y l c e n t e r t o generate the carbinolamine. E l i m i n a t i o n of water leads t o t h e coordinated imine which i s then s u s c e p t i b l e t o a d d i t i o n of ammonia t o g i v e t h e gem-diamine shown. A d d i t i o n o f a n o t h e r CH2O u n i t t o give another imine i s f o l l o w e d by i n t r a m o l e c u l a r a t t a c k by t h e gem-diamine t o make t h e f i r s t s i x - m e m b e r e d r i n g . Addition o f a n o t h e r CH2O m o l e c u l e t o g i v e a n o t h e r i m i n e g r o u p a n d a n o t h e r i n t r a m o l e c u l a r a t t a c k , t h i s t i m e b y t h e r i n g s e c o n d a r y amine g r o u p , l e a d s t o t h e s y n t h e s i s o f t h e f i r s t c a p . The p r o c e s s i s t h e n




130

r e p e a t e d t o complete the cage. C l e a r l y t h e a m m o n i a - f o r m a l d e h y d e r e a c t i o n competes w i t h t h e process but, u s i n g an excess o f these reagents, the condensation can b e made a l m o s t q u a n t i t a t i v e w i t h r e s p e c t t o t h e t r i s ( e t h y l e n e d i a m i n e ) c o m p l e x . The s p e c i f i c i t y i s d e c i d e d b y t h e c h i r a l i t y o f the p a r e n t t r i s ( e t h y l e n e d i a m i n e ) complex s i n c e t h i s d e c i d e s t h e o r i e n t a t i o n o f t h e gem-diamine a n d s u b s e q u e n t a d d i t i o n s o f t h e amino g r o u p t o t h e a d j a c e n t i m i n e . U n l e s s t h e gem-diamine i s o r i e n t e d i n the a p i c a l p o s i t i o n , condensation t o g i v e the cap i s prohibited. The Δ o r Λ c o n f i g u r a t i o n o f t h e e t h y l e n e d i a m i n e c h e l a t e s then d e c i d e s the o r i e n t a t i o n o f the secondary p r o t o n i f t h e amino m e t h y l e n e m o i e t y i s r e q u i r e d t o b e a p i c a l A ( S ) o r A ( R ) . T h e s e a r e some o f t h e b e s t e x a m p l e s we h a v e e n c o u n t e r e d o f c h i r a l i t y s y n t h e s i s organized by m e t a l i o n s . Further s t u d i e s w i l l a s c e r t a i n whether the o r i g i n s o f t h e s p e c i f i c i t y are a s d e s c r i b e d b u t i t i s c l e a r t h a t s u b s t i t u e n t s on the m e t a l can d i r e c t t h e s i t e o f t h e c o n d e n s a t i o n a n d t h a t t h e m e t a l c o n s t r a i n s t h e i n t r a m o l e c u l a r c y c l i z a t i o n s t o modify the e f f e c t s a n t i c i p a t e d f r o m t h e r e g u l a r o r g a n i c c h e m i s t r y . The a c t i v a t i n g e f f e c t s o f the m e t a l i o n , the use o f coordinated n u c l e o p h i i e s , t h e p o s s i b i l i t y o f s p e c i f i c i t y and t h e p r o t e c t i n g a n d o r g a n i s i n g c a p a c i t y o f the m e t a l center should a l l be u s e f u l f o r i n o r g a n i c and o r g a n i c s y n t h e s i s .

Literature Cited 1.

For examples see Buckingham D.A. and Sargeson A.M, 'Topics in Stereochemistry (Eds) Allinger N.L. and Eliel E.L. Wiley-Interscience, New York, Vol. 6 p.219, and references therein. '

2.

Corey E . J . and Bailar J.C. J . Am. Chem. Soc. (1959) 81, 2620

3.

Fujita M. Yoshikawa Y. and Yamatera H. Bull. Chem. Soc. Japan, (1977) 50, 3209 and references therein.

4.

Halpern B. Turnball K.R. and Sargeson A.M. (1966) 88, 4630.

5.

Buckingham D.A. Marzilli L.G. and Sargeson A.M. J. Am. Chem. Soc. (1967) 89, 5133.

6.

Buckingham D.A. Foster D.M. and Sargeson A.M. J. .Am. Chem. Soc. (1970) 92, 6151.

7.

Buckingham D.A. Foxman B.M. Sargeson A.M. J. Am. Chem. Soc. (1970) 94, 6151.

J . Am. Chem. Soc.

and Zanella A.

Buckingham D.A. Sargeson A.M. and Zanella A. J. Am. Chem. Soc. (1972) 94, 8246.


6. SARGESON

Coordination Complexes

131

8.

Buckingham D.A. Davis C.E. and Sargeson A.M. J . Am. Chem. Soc. (1970) 92, 6159

9.

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(1976) 229 11.

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13.

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14.

Gainsford A.R. Snow M.R. Springborg J . and Taylor D. (to be published).

15.

Curtis N.F. Coord. Chem. Rev. (1968) 3, 3.

16.

Harrowfield J.M. and Sargeson A.M. J . Am. Chem. Soc. (1974) 96, 2634. Golding B.T. Harrowfield J.M. Robertson G.B. Sargeson A.M. and Whimp P.O. J . Am. Chem. Soc. (1974) 96, 3691 Evans I.P. Everett G.W. and Sargeson A.M. J . Am. Chem. Soc. (1976) 98, 8041

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(1978) 647.

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Harrowfield J.M. (unpublished work).

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Herlt A . J . Sargeson A.M. Springborg J . Taylor D. Geue R. and Snow M.R. J . Chem. Soc. Chem. Comm. (1976) 285.

RECEIVED October 22, 1979.


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