Circular Dichroism Spectra of Square Planar Complexes Containing

5

Circular D i c h r o i s m Spectra of Square Planar Complexes Containing Prochiral Olefins and T h e i r

Stereoselective

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Olefin Exchange

KAZUO SAITO Chemistry Department, Faculty of Science, Tohoku University, Sendai, 980 Japan

The relationship between the absolute configuration and CD spectrum has been widely discussed, but mostly for octahedral complexes. (1) Asymmetric coordination of η -olefins was first demonstrated by Cope et al. (2) with trans-[PtCl (1-phenylethylamine)(trans-cyclo-octene)] and extended by Paiaro and Panunzi (3) by the preparation of a pair of diastereoisomers, such as transdichloro(R or S-α-phenylethylamine) (trans-2-butene)platinum(II) , trans-[PtCl (1-phenylethylamine) (tbn)]. Since then several complexes with such an asymmetry have been prepared, and the relationship between the absolute configuration and CD pattern has been discussed for platinum(II) complexes. (4) Scott and Wrixon (5) reported that S,S-η and R,R-η2 configuration give CD peaks with positive and negative signs in the d-d transition region at ca. 27,000 cm-1. Less information is available for the complexes with other metal ions, and only palladium(II) (5) and iron(0) (6) complexes were discussed. A change in the kind of olefin does not cause significant changes in absorption spectra, so long as the other ligands remain unchanged. Hence the replacement of one olefin ligand by another cannot be detected by absorption spectrometry. Olefin exchange in platinum(II) complexes is an important elementary reaction related to their catalytic action in homogeneous systems, but kinetic studies have not been made because of such experimental difficulty. NMR studies gave only limited information. Measurement of the change in the CD spectrum of the complexes with prochiral olefins in the presence of an excess of free prochiral or non-prochiral olefins enables the estimation of the substitution rate. This method is useful for examining the influence of other ligands upon the rate of olefin exchange, (e.g. trans effect, (7)), but is also useful for elucidating the stereo-selctivity involved in the olefin exchange. This paper deals with the relationship between the absolute configuration of r) -olefins and the CD pattern of new platinum(II) 2

2

2

2

2

0-8412-0538-8/80/47-119-091$06.00/0 © 1980 American Chemical Society Douglas and Saito; Stereochemistry of Optically Active Transition Metal Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

92

and rhodium(I) complexes of the types shown i n Figure 1, and the source of s t e r e o s e l e c t i v i t y f o r o l e f i n exchange.

with

CD P a t t e r n of Rhodium(I) and P l a t i n u m ( I I ) Complexes CD of Rhodium(I) Complexes. Figure 2 shows the v i s i b l e and UV absorption of r | - o l e f i n complexes of rhodium(I). The synthesis of these complexes has been reported elsewhere. (8) I t i s seen t h a t r | - o l e f i n s give absorption peaks or shoulders i n the regions 20,000 t o 30,000 cm-1 and around 40,000 cm" . These are the same regions with those where n . - o l e f i n platinum(II) complexes give c h a r a c t e r i s t i c absorption, which i s exemplified i n Figure 3. (9) (Table I) Figure 4 i l l u s t r a t e s the CD s p e c t r a of a few rhodium(I) complexes. The p a t t e r n i s very much dependent on the other l i g a n d s . Two c h a r a t e r i s t i c and common peaks are seen i n the two regions, where the r ] - o l e f i n s cause absorption peaks:i.e. negative peaks between 20,000 and 30,000 cm"! and l a r g e p o s i t i v e peaks at ca. 40,000 cm" . The p a t t e r n i n the r e g i o n between these two i s too complicated and s e n s i t i v e to the other l i g a n d s and no systematic trend i s seen. Figure 5 shows the d i f f e r e n c e i n CD p a t t e r n between doubly bridged b i n u c l e a r complexes of platinum(II) and rhodium(I) c o n t a i n i n g S,£-£ratts-cyclo-octene (coe), which can be coordinated only i n t h i s c o n f i g u r a t i o n . Whenever one looks a t the peaks at ca. 22,000 cm" , the peaks have opposite s i g n s . On the other hand, the s i g n of the peak a t around 40,000 cm"l i s the same, although the i n t e n s i t y d i f f e r s g r e a t l y . The molar e x t i n c t i o n c o e f f i c i e n t i n the former r e g i o n i s ca. 10 M"lcm"l, and cannot be reckoned to be a p u r e l y d-d t r a n s i t ion. However, the i n f l u e n c e of the c e n t r a l metal i o n i s remarkable. The ε value of the peak at ca. 40,000 cm-1 i s more than 104M~ cm- , and the CD s i g n i s independent of the metal i o n . Hence t h i s must r e f l e c t the absolute c o n f i g u r a t i o n of the p r o c h i r a l o l e f i n i t s e l f . 2

2

1

2

2

1

1

3

1

1

CD of Platinum(II) Complexes. Scott and Wrixon discussed the r e l a t i o n s h i p between the CD and the s t r u c t u r e of various o l e f i n s i n c l u d i n g many terpene d e r i v a t i v e s , but t h e i r s i s l i m i t e d to the low energy d-d t r a n s i t i o n r e g i o n . (5) The CD p a t t e r n i n the r e g i o n 30,000 to 40,000 cm" i s very complicated and cannot be understood s y s t e m a t i c a l l y . We have synthesized various complexes c o n t a i n i n g S,S-trans-2-butene (tbn) or /S'-2-methyl-2-butene (mbn) as a source of asymmetry and other l i g a n d s i n c l u d i n g L - p r o l i n a t e , 4-substituted a n i l i n e s and 4 - s u b s t i t u t e d p y r i d i n e s . Among them [PtCl(0-phenylenediamine)(5-mbn)] i s the f i r s t o p t i c a l l y a c t i v e square planar complexes with p o s i t i v e charge. The CD data are given i n Table I I . 1

Importance of the c i s Ligand. Figure 6 shows the CD of the two geometrical isomers of [ P t C l ( L - p r o l i n a t e ) ( S , S - t b n ) ] . Both have r a t h e r l a r g e negative CD peaks a t ca. 36,000 cm" , but the c i s isomer has one shoulder a t ca. 32,500 cm" , whereas the trans 1

1

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

SAiTO

Square

Planar

Complexes

Figure 2. Absorption spectra of rhodium(I) complexes containing olefins and other ligands in hexane: acac, enolate anion of acetylacetone; coe, cyclooctene; dbm~, enolate anion of dibenzoylmethane.


Figure 3.

UV absorption and CD spectra of [PtCl(L-prolinate)-(tra.ns-2-butene)]: ( λ (+)s8o* ; ( λ (-) 8o ; ( λ vincinal effect. €

3

A€


5.

SAITO

Square Planar

95

Complexes

Table I*Absorption Data o f Rhodium(I) and Platinum(II) Complexes Containing r| -01efins 2

Complexes

V 10 cm" a

b

[Rh(acac)(C0) 1 ' [Rh(acac)(ethylene)2] [Rh(dbm) (ethylene) ] [Rh(acac)(trans-coe)2] [Rh(dbm)(trans-coe) ] [Rh2Cl (trans-coe) ] [PtCl(L-pro)(trans-coe)] f [PtCl(L-pro)(trans-coe)] g K[PtCl2(L-pro)] 2

2

c

c

2

c

2

2

h

a) c) e) g) *)

i n diethylether i n hexane cyclooctene i n acetonitrile shoulders

b) d) f) h)

V

loge

3

25.00 25.00 27.40 25.32 27.40 22.73* 24.8 * 27.03* 23.8 *

3

ΙΟ ^"

1

41.00 41.32 40.00 40.98* 40.00 40.98 40.00 40.00* 40.00

1.91 3.31 3.98 2.95 3.96 2.97 1.54 1.73 1.37

loge 1

3.68 4.00 4.08 3.94 4.38 4.48 3.20 3.32 2.62

enolate anion of acetylacetone enolate anion of dibenzoylmethane L - p r o l i n a t e , data i n Ref. i n Ref.

Figure 4. CD spectra of rhodium(I) and platinum(ll) complexes containing S,Strains-2-butene (tbn) and S^S-trans-cyclooctene (coe) in hexane: ( ), P(C H ) [PtCla(S S-tbn)J in acetonitrile; (- - -) [Rh Cl (S,S-coe)] ( ), [Rh(acac)(S,Scoe) ]; ( ), [Rh(dbm)(S$-coe) ]. 6

9

2

t

2

;

2


5 r



5.

SAITO

Figure 6.

Square Phnar

Complexes

CD spectra of cis- and triins(NJ/)-[PtCl(L-prolinate)-(S,S-tTims-2butene)] in acetonitrile: ( j , cis isomer; ( j , trans isomer.


97


6

5

5

4

4

2

e

d

f

f

f

5

a

h

d

/

,

e

6

c

2

22 9(+0.46) 24 0(+D.76) 23. 6(+0.51) 23, 6(+0.44) 23. 6(+0.41)

3

V/10 cm-l ( Δε )

* 34. 3(--1.90) 39 7(-0. 54) 26 .8(+1.08) ** 37.0( -2 26) 33.2(-0 .57) 27 .6(+0.92) 26 .5(-0.21) 29.4(+0.13) 34. 3(--1.05) 35. K- -1.32) 39 ,5(+3. 20) 28 *0(-0.14) 34. 7(--0.86) 27 .0(-0.28) 34. 3(--0.76) 27 .2(-0.30) 34. 5(--0.70) 27 .K-0.26) ** 28 .3(+0.36) 32.8(-0.03) 37. 0(--0.59) 40 3(-0. 35) 37.4( -1 .19) **35.0(-0 .72) 27 .3(+1.32)

CD peaks

a) D i f f e r e n c e spectrum between trans^Ni//)"[PtCl(L-pro)(5,£-tbn)] and transiN,//)-[PtCl(L-pro)(ethylene)] b) D i f f e r e n c e spectrum between cis(N,//)-[PtCl(L-pro)(S,S-tbn)] and ois(N,//)-[PtCl(L-pro)(C2H4)] c) D i f f e r e n c e spectrum between ois-[PtCl (S,£-tbn) (,9-1-phenyl-ethylamine)] and ois(N,//)- [ P t C l 2 (ethylene) (i5-l-phenylethylamine) ] d) i n a c e t o n i t r i l e h) S-l-phenylethylamine; i n acetone e) i n dichloromethane *) peaks are broad f) a n i l , a n i l i n e ; i n benzene **) shoulders g) i n ethanol

2

3

trans (N,//)- [PtCl (L-pro) (5,5-tbn) ] ' cis (N,//y [PtCl (L-pro) (5,5-tbn) ] b,d P(C H ) [PtCl3(S-mbn)] P(C6H )4[PtCl (£,S-tbn)] trans-[PtCl (S-mbn)(4-Cl-anil)] trans-[PtCl2(S-mbn)(anil)] trans-[PtCl2(S-mbn)(4-CH3-anil)] [PtCl(S-mbn){o-C H (NH )2>1 [ B ( C H ) 4 ] 9 cis-[PtCl2('S' /S -tbn) (5-l-PhEtNH2)]

Complexes

2

Table II. Peaks i n the C i r c u l a r Dichroism Spectra o f n - 0 1 e f i n complexes of Platinum(II)

5.

SAiTO

Square

Planar

99

Complexes

isomer gives one broad peak s h i f t e d t o longer wave lengths. This f a c t suggests the importance o f other l i g a n d s which are c i s and trans t o the o l e f i n . Figure 7 i l l u s t r a t e s the CD patterns o f the mbn complexes c o n t a i n i n g c i s - d i c h l o r o l i g a n d s , and a trans-chloro or 4-substituted a n i l i n e l i g a n d . Despite the d i f f e r e n c e i n b a s i c i t y and even i n the c o o r d i n a t i n g atom, the negative CD peaks a t ca. 34,500 c m have almost equal p o s i t i o n s and i n t e n s i t i e s . I t seems as i f the v a r i a t i o n of the c i s l i g a n d i s more important i n determining the CD p a t t e r n i n t h i s region than the trans l i g a n d . This c o n s i d e r a t i o n i s f u r t h e r v e r i f i e d by comparing the CD o f those complexes having c h l o r i n e and n i t r o g e n i n the c i s p o s i t i o n and other donors i n the t r a n s . (Figure 8) The solvents were chosen i n accordance with the s o l u b i l i t y and the p a t t e r n cannot be compared i n one solvent. Here again the l o c a t i o n o f the negat ive CD peaks are not very d i f f e r e n t r e g a r d l e s s of the t o t a l charge or the presence o f another source o f asymmetry ( S - l - p h e n y l e t h y l amine and p r o l i n a t e , which have asymmetric n i t r o g e n upon coordinat ion) . A l l these f a c t s suggest t h a t the c i s - i n f l u e n c e i s more s i g n i f i c a n t than the t r a n s - i n f l u e n c e f o r determining the l o c a t i o n of CD peaks around 35,000 cm" . Because o f the very strong t r a n s i n f l u e n c e o f the asymmetric o l e f i n , l i g a n d s trans t o the o l e f i n would have only a small i n f l u e n c e on the p l a t i n u m ( I I ) , e s p e c i a l l y when the trans l i g a n d s are mere e l e c t r o n p a i r donors. Figure 9 gives the CD p a t t e r n o f the complexes trans-[PtCl2(£-mbn)(4s u b s t i t u t e d - p y r i d i n e ) ] . The peaks below 30,000 c n r are alomost i d e n t i c a l , but the negative peaks a t ca. 35,000 cm" s h i f t s as the s u b s t i t u e n t on the p y r i d i n e r i n g changes. P y r i d i n e d e r i v a t i v e s can have d^-d-ji i n t e r a c t i o n s with platinum(II) and may perturb the electronic state. -1

1

1

1

Asymmetric Influence from the trans Ligand. As shown l a t e r , when an asymmetric n i t r o g e n i s i n the trans p o s i t i o n of the com plexes o f the type trans (N,//)-[PtCl(L-am)(ethylene)], asymmetry i s introduced by the s u b s t i t u t i o n o f tbn f o r the ethylene i n organic s o l v e n t s . (10.) L-Alanine, L-phenylalanine, and L - v a l i n e f a i l t o introduce asymmetry on the incoming tbn i n the e q u i l i b rated s t a t e . However, L - p r o l i n e , N- and C - s u b s t i t u t e d L - p r o l i n e and even ^ - s u b s t i t u t e d L - v a l i n e induce asymmetry. (Table V) There must be some e l e c t r o n i c i n t e r a c t i o n from the asymmetric n i t r o g e n upon the o l e f i n through p l a t i n u m ( I I ) . Figure 10 shows the CD p a t t e r n o f some o f these complexes. Complexes without asymmetric n i t r o g e n give only very weak CD i n the r e g i o n from 20,000 t o 40,000 cm" . E s p e c i a l l y between 27,000 and 40,000 c m the CD i s much weaker than those with asymmetric nitrogens (Figure 10-D). Since the complexes with L - p r o l i n a t e , L-hydroxyprolinate and αΖΖ-ο-L-hydroxyprolinate have very s i m i l a r patterns t o one another, the asymmetric carbon atoms on the p y r r o l i d i n e r i n g do not seem t o give s i g n i f i c a n t c o n t r i b u t i o n s . (Figure 10-B) On the other hand, i n t r o d u c t i o n o f a methyl o r benzyl s u b s t i t u e n t on the n i t r o g e n changes the CD p a t t e r n t o a 1

-1



100

log ε

Figure 7. UV absorption and CD spectra of phtinum(II) complexes containing S-2-methyl-2-butene (mbn) and 4-substituted anilines: ( ), X = Η in trans[PtCl (S-mbn)(4-X-anline)] (in benzene); (- · ·), X = CI in trans-[PtCl (S-mbn)(4-X-aniline)] (in benzene); ( ), (X = CH ) in trans-[PtCl (S-mbn)(4-X-aniline)] (in benzene); (----), P(C HrJJPtCl^S-mbn)] in dichloromethane. 2

2

3

2

6


5.

SAITO

Square Planar

101

Complexes

Λ / \

25

3 t > X 35

1 AO

Ïï/I0 cm" 3

i M

1

\J

Figure 8. CD spectra of platinum(II) complexes containing S-2-methyl-2-butene and various amines on the cis site: ( ), [PtCl(o-phenylenediamine)(S-mbn)] in ethanol; ( ), cisfN,//)[PtCl(L-prolinate)(S,S-tbn)] in acetonitrile; (-'-'), cis(Cl)[PtCl (S-l-phenylethyhmineXS-mbn)] in acetone. 2



102

Figure 9. CD spectra of phtinum(II) complexes containing S-2-methyl-2-butene (mbn) and 4-substituted pyridines on the trans site in dichloromethane: ( ), X = NH ; ( X = H; and( j , X = C0 C H in trans-[PtCl (4-X-py)(S-mbn)]. 2

2

2

5

2


SAiTO

Square Planar

Complexes

log£

Figure 10. UV absorption (A) and CD spectra of transfN,/'/ )-[PtCl(L-aminocarboxyhte)(ethylene)] in acetonitrile (10). B: ( ), L-prolinate; ( ), L-hydroxyprolinate; (· · -), al\o-L-hydroxyprolinate. C: ( ), N-methyl-L-pro; ( ), N-methyl-L-hyp; (· · -), N-benzyl-L-pro. D: ( ), L-ahninate; ( ), L-phenyhlaninate; ( · · -), L-valinate. E: ( ),N-benzyl-L-Oalinate. f



104

marked extent, e s p e c i a l l y i n the region from 25,000 to 40,000 cm"! (Figure 10-C). In p a r t i c u l a r l y tf-benzyl-L-valinate gives a l a r g e negative peak at ca. 34,000 c i r r i , which i s s i m i l a r to that of Nb e n z y l - L - p r o l i n a t e (Figure 10-E). This f a c t i n d i c a t e s a marked s t e r e o s e l e c t i v i t y on c o o r d i n a t i o n of #-benzyl-L-valinate. trans(N,//)-[PtCl(tf-bz-L-val)(ethylene)] can e x i s t as a p a i r of d i a stereoisomers, S(N)S(C) and R(N)S(C). I t s CD spectrum i s very s i m i l a r to t h a t of tf-benzyl-L-prolinato complex, which can have only R{N)S (C) c o n f i g u r a t i o n . The #-benzyl-L-valinato complex seems to be formed almost e x c l u s i v e l y i n the R(N)S(C) form, when Zeise's s a l t undergoes s u b s t i t u t i o n with f r e e #-benzyl-L-valine i n a s l i g h t l y a c i d i c s o l u t i o n . The preference f o r the i?-conf i g u r a t i o n around the coordinated n i t r o g e n adjacent to an 5-carbon i s r a t h e r common f o r octahedral complexes. (11) Molecular model s t u d i e s show that s t e r i c hindrance between the s u b s t i t u e n t s on the n i t r o g e n and 5-carbon would be r e s p o n s i b l e f o r the s e l e c t i v i t y . Figures 10-B and -C i n d i c a t e that the CD curves i n the 33,000 and 37,000 c i r r i regions have reversed s i g n s . On the assumption of the a d d i t i v i t y law, the d i f f e r e n c e s of CD's between the #-benzylL - p r o l i n a t o and L - p r o l i n a t o , and between #-benzyl-L-valinato and L - v a l i n a t o complexes are p l o t t e d against the wave number i n Figure 11-A. S i m i l a r p l o t s of όΔε'ε bentween #-methyl-L-prolinato and L - p r o l i n a t o , and between tf-methyl-L-hydroxyprolinato and Lhydroxyprolinato complexes are shown i n Figure 11-B. The δΔε curves are very s i m i l a r to each other r e g a r d l e s s of the aminocarboxylate moiety. Hence the a d d i t i v i t y law should hold between the c o n t r i b u t i o n s of N-substituent and of the aminocarboxylate, the former being independent of the c h e l a t e framework. Usefulness of the quadrant r u l e f o r the i n t e r p r e t a t i o n of the CD signs of n - o l e f i n complexes of platinum(II) i n 25,000 cirri r e g i o n was demonstrated by Scott and Wrixon (5) . We have a p p l i e d t h i s r u l e f o r i n t e r p r e t i n g the c o n t r i b u t i o n of the asymmetric n i t r o g e n . Figure 12 shows the p r o j e c t i o n of trans(N //)-[PtCl( t f - a l k y l - L - p r o ) ( e t h y l e n e ) ] . The square plane i s represented by the h o r i z o n t a l l i n e , and the Pt-N bond i s perpendicular to the paper plane. The asymmmetric n i t r o g e n i s beneath platinum(II) (large dotted c i r c l e ) . The c o n t r i b u t i o n of the minus quadrant at below l e f t s i d e behind the paper should depend on the s i z e of the s u b s t i tuent on n i t r o g e n ( t r i a n g l e ) . With an increase i n s i z e of t h i s s u b s t i t u e n t (H, methyl and benzyl) the c o n t r i b u t i o n of t h i s minus component should increase to give the c a l c u l a t e d curves shown i n Figure 11. The UV absorption curves of a l l the present complexes have peaks with e=ca.ΙΟ^ΙΓ^-αχΤ from 31,000 to 45,000 cm" . Denning, Hartley and Venanzi assigned the absorption bands of Zeise's s a l t i n t h i s region t o d-π*(ethylene) t r a n s i t i o n . (12) We have observed CD peaks with Δ ε s -1.3 and +3.3 a t ca.35,000 and 39,500 cm" for the tetraphenylphosphonium s a l t of [PtCl3 (S,S-tbn) ] " i n a c e t o n i t r i l e . (13) The peak a t 35,000 cm" must correspond to the same t r a n s i t i o n as that of the main CD band of Figure 10 (and Figure 2

r

1

1

1

1

1


5.

SAiTO

Square

Planar

105

Complexes

-0.5}

Figure 11. Difference in CD between two complexes of the type transfN,//)[PtCl(L-aminocarboxylate)( ethylene ) with and without substituent on the nitrogen (10)

Figure 12.

Projection of the square planar complexes (10): A, tr3u\s(N,//)-[PtCl(-substituted L-am)(ethylene)]; B, S,S-trans-2-fewiene moiety.



106

1 1 ) . Figure 12 a l s o shows the p r o j e c t i o n of t h i s Zeise-type complex; the C-C moiety i s placed across the square plane o f platinum(II) behind the paper. A common c o n t r i b u t i o n o f the minus r e g i o n t o the lower l e f t behind the paper seems t o predominate f o r the Zeise-type and the present complexes. The d-n* t r a n s i t i o n may be perturbed by the asymmetric n i t r o g e n trans t o ethylene t o give a marked CD peak i n t h i s r e g i o n . The c i s - i s o m e r cis (N,//)-[PtCl(L-pro)(ethylene)] gives only a weak CD i n t h i s r e g i o n (Figure 1 3 ) .

S t e r e o - s e l e c t i v i t y on O l e f i n Exchange Selectivity

on the Exchange of Prochiral

Olefins.

Substitut-

2

ion o f o l e f i n s f o r the coordinated r | - o l e f i n was f i r s t s t u d i e d k i n e t i c a l l y by PMR spectroscopy f o r the complex [ P t C l ( a c a c ) ( C 2 H 4 ) ] ( acac, enolate anion o f a c e t y l a c e t o n e ) . (14) S t e r e o - s e l e c t i v i t y i n such a r e a c t i o n was pointed out f i r s t by C o r r a d i n i , Paiaro and Panunzi f o r the e q u i l i b r i u m o f cis-[PtCl2(5-amine)(olefin)] i n organic s o l v e n t , the i ? - c o n f i g u r a t i o n being p r e f e r r e d by 5 t o 50 %. (15) Many s t u d i e s have d e a l t with the s e l e c t i v i t y o f r e a c t i o n s o f coordinated l i g a n d s , (16) but nothing has been r e ported concerning the s t e r e o s e l e c t i v i t y f o r the s u b s t i t u t i o n o f coordinated o l e f i n s . We examined s e v e r a l years ago the r a t e o f the f o l l o w i n g r e a c t i o n s by use o f CD measurements and the i s o t o p i c l a b e l l i n g method. 3

trans (N,//)-[PtCl(L-pro)(S,S-tbn[ #])]

+ tbn

trans (N,//h [PtCl (L-pro) (R,R or 5,5-tbn)]

3

+ tbn[ #]

(1)

and found a s i g n i f i c a n t s e l e c t i v i t y i n favor o f s u b s t i t u t i o n with r e t e n t i o n o f c o n f i g u r a t i o n (Table I I I ) . Table I I I Second Order Rtae Constants o f the S u b s t i t u t i o n of 2-Butene f o r the 5,5-2-Butene i n trans(N,//)[PtCl(L-prolinate)(5S -tpans-2-butene[3iï] )] i n Acetone. (17) #

,

Olefin

trans - 2-butene*

eis-2-butene

Temp/°C -λ -1 -1 k _/io M s k. / i o " V s

8.0

-20.0

8.0

-20.0

347

70.9±7.6

6.2

0.9

70.2±4.1

32.3

5.6

ISO

* The c a l c u l a t e d k , -3 -1 -1 roo

3.1 χ 10

M

s

and k

r o n i

a t 8.0°C are 29.1 and

, r e s p e c t i v e l y , the r a t i o k ^ g j / f c

^

being 9.4.


5.

SAITO

Square

Vlanar

Complexes

107

The i d e n t i c a l r a t e o f s u b s t i t u t i o n o f non-prochiral cis-2-butene f o r the coordinated S,5-tbn measured by the two methods i n d i c a t e s the absence o f other r e a c t i o n s such as l o c a l proton exchange between

Circular Dichroism Spectra of Square Planar Complexes Containing

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