Reactions of Coordinated Ligands and Homogeneous Catalysis

9 Reactivity of Hydroxyl Groups in the Bis(2-hydroxyethyliminodiacetato)chromium(III) Ion RONALD A. KRAUSE and STEVEN D. GOLDBY Chemical Research Department, Central Research Division, American Cyanamid Co., Stamford, Conn. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 10, 2017 | http://pubs.acs.org Publication Date: January 1, 1962 | doi: 10.1021/ba-1963-0037.ch009

1

Several reactions, generally capable of acetylating alcohols, were attempted on the anionic com plex [Cr(HO-A) ]-; most failed to acetylate the 2

uncoordinated hydroxyl groups.

Refluxing in a

glacial acetic acid-acetic anhydride mixture led to extensive decomposition of the complex but pro duced a small quantity of the diester, [Cr(AcO -A) ]-, 2

isolated as the water-insoluble

salt, (Η Ο) [Cr(AcO-A) ]. 3

2

oxonium

The reaction of (Me N) 4

[Cr(HO-A) ] with ketene in refluxing acetonitrile 2

produced the diester; the ketene reaction, how ever, is unusually

slow.

It is concluded that

charge on the complex molecule may not be an important factor governing attack on uncoordi nated hydroxyl groups, and a mechanism for the ketene reaction is discussed.

T h e r e a c t i v i t y o f a h y d r o x y l b e t a t o a c o o r d i n a t e d a m i n o g r o u p h a s b e e n t h e sub ject o f s e v e r a l p a p e r s .

K e l l e r a n d E d w a r d s (4)

ethylenediamine) cobalt (III)

chloride

g r o u p to b e c o m p l e t e l y u n r e a c t i v e , B a u e r , a n d B a i l a r (I)

i n v e s t i g a t e d tris ( 2 - h y d r o x y e t h y l -

and found

the

uncoordinated

even i n refluxing acetyl chloride.

h a v e r e - e x a m i n e d this system m o r e recently.

hydroxyl Drinkard,

They

discuss

t h e f a c t t h a t t h e f u n c t i o n a l groups o f t h e a t t a c k i n g m o l e c u l e s b e a r a p a r t i a l p o s i tive charge; consequently, attack o n a positively c h a r g e d c o m p l e x m o l e c u l e s h o u l d be u n f a v o r a b l e . It w a s o f interest t o u s t o e x a m i n e t h e p r o b l e m o f a c e t y l a t i o n

of an — O H

g r o u p b e t a to a c o o r d i n a t e d a m i n o g r o u p i n a c o m p l e x m o l e c u l e b e a r i n g a n e g a t i v e charge.

T h e acetylation o f the u n c o o r d i n a t e d h y d r o x y l groups o f the b i s ( 2 -

h y d i O x y e t h y l i m i n o d i a c e t a t o ) < j h r o m i u m ( I I I ) i o n ( I ) is d e s c r i b e d b e l o w .

» H O - C H

2

C H

2

i

1

γ —^JCr

N ^

6

(Cr(HO-A) ] " 2

y

Present address, D e p a r t m e n t o f C h e m i s t r y , U n i v e r s i t y of C o n n e c t i c u t , Storrs, C o n n .

143

Busch; Reactions of Coordinated Ligands Advances in Chemistry; American Chemical Society: Washington, DC, 1962.

ADVANCES IN CHEMISTRY SERIES

144 Experimental of K [ C r ( H O - A ) ]

Preparation

2

and (Me N)[Cr(HO-A) ]. 4

A solution of

2

23.2 g r a m s ( 0 . 3 6 m o l e ) of K O H i n w a t e r w a s a d d e d to a n a q u e o u s slurry o f 4 2 . 5 grams (0.24 mole) of H O - A H . T h i s w a s a d d e d to a solution of 3 1 . 9 grams (0.12 m o l e ) of c h r o m i u m (III) c h l o r i d e 6 - h y d r a t e i n water; t h e total v o l u m e o f w a t e r used was 500 m l . After digestion o n the hot plate for a f e w minutes, the dark r e d solution w a s neutralized to p H 7 ( p H p a p e r ) u s i n g 7.76 grams (0.12 m o l e ) of K O H i n 2 0 m l . o f w a t e r . T h e s o l u t i o n w a s filtered, e v a p o r a t e d o n a s t e a m b a t h t o a p p r o x i m a t e l y 4 5 0 m l . , a n d a l l o w e d to s t a n d o v e r n i g h t . After the product h a d b e e n i s o l a t e d b y filtration i t w a s w a s h e d t h r e e t i m e s w i t h w a t e r a n d d r i e d i n v a c u o over P 0 . Yield: 34.5 grams. E v a p o r a t i o n o f t h e filtrate to 2 0 0 m l . a f f o r d e d a n a d d i t i o n a l 9.5 grams of p r o d u c t . 2

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2

5

T h e complex was recrystallized b y dissolving 44.0 grams of the crude m a terial i n 150 m l . of h o t water. A f t e r c o o l i n g , t h e p r o d u c t w a s i s o l a t e d as a r e d c r y s t a l l i n e s o l i d i n t h e s a m e m a n n e r as a b o v e . Y i e l d : 31.2 grams (58.9%). Calculated for [ K C r C H O N ] : C , 32.7; H , 4.12; N , 6.35; C r , 11.8. Observed: C , 32.12, 31.99; H , 4.05, 4.40; N , 6.63; C r , 10.9. 1

2

1

8

1

0

2

T h e t e t r a m e t h y l a m m o n i u m salt, ( M e N ) [ C r ( H O - A ) ] , w a s p r e p a r e d f r o m t h e p o t a s s i u m salt b y i o n e x c h a n g e o n A m b e r l i t e I R - 1 2 0 i n t h e t e t r a m e t h y l a m m o n i u m cycle. Calculated for [ C r C H o 0 N 3 ] : C , 40.4; H , 6.36; N , 8.84. Observed: C , 40.53; H , 6.39; N , 8.67. A c e t y l a t i o n of K [ C r ( H O - A ) ] i n G l a c i a l A c e t i c A c i d . F o u r grams (0.00906 m o l e ) of K [ C r ( H O - A ) ] w a s refluxed i n 2 0 0 m l . of glacial acetic a c i d a n d 3 0 0 m l . of a c e t i c a n h y d r i d e f o r 2 h o u r s . F i l t r a t i o n a f f o r d e d 1.68 grams of starting m a t e r i a l ; t h e filtrate w a s e v a p o r a t e d to a v i s c o u s b r o w n o i l ( r o t a r y evaporator, steam bath) a n d d i l u t e d w i t h a solution of 3 m l . of concentrated nitric a c i d i n 5 0 m l . of water. A f t e r standing overnight the pink, crystalline solid was isolated b y filtration a n d d r i e d i n v a c u o o v e r P O , . Yield: 0.63 g r a m ( 2 3 . 7 % o n basis of 2.32 grams of complex dissolved). Analysis: Calculated for ( H 0 ) [ C r ( A c O A ) ] : C , 38.00; H , 4.96; N , 5.55. O b s e r v e d : C , 37.65; H , 5.24; N , 5 . 3 5 . T h e o x o n i u m salt m a y b e r e c r y s t a l l i z e d b y d i s s o l v i n g i n t h e s t o i c h i o m e t r i c a m o u n t of aqueous potassium bicarbonate a n d reprecipitating b y the addition of nitric acid. Preparation of Ketene. K e t e n e w a s p r e p a r e d b y the pyrolysis of acetone 4

i e

3

2

1 0

2

2

2

3

r

2

using a generator identical to that described i n the literature

T h e generator

(3).

was c a l i b r a t e d f o r ketene y i e l d b y p a s s i n g the effluent gas t h r o u g h s t a n d a r d s o d i u m hydroxide. Acetylation of ( M e 4 N ) [ C r ( H O - A ) 2 ] with Ketene. T e n grams (0.0209 mole) of ( M e N ) [ C r ( H O - A ) ] was m i x e d w i t h 550 m l . of acetonitrile a n d refluxed. T h r o u g h this r e f l u x i n g m i x t u r e k e t e n e w a s p a s s e d f o r 6 h o u r s ( 0 . 1 5 9 m o l e per hour, or 0.955 m o l e ) . A f t e r r e f l u x i n g f o r a n a d d i t i o n a l 18 h o u r s t h e s o l u t i o n w a s filtered ( 0 . 1 4 g r a m o f s t a r t i n g m a t e r i a l r e c o v e r e d ) a n d t h e n e v a p o r a t e d t o dryness. T h e solid was redissolved i n 100 m l . of water a n d 7 m l . of concentrated nitric a c i d was a d d e d . T h e p i n k p r e c i p i t a t e w h i c h f o r m e d w a s i s o l a t e d b y filtration, w a s h e d w i t h alcohol a n d ether, a n d then d r i e d i n v a c u o over P O r , . Yield: 10.17 grams ( 9 5 . 8 % ) . A n a l y s i s : C a l c u l a t e d f o r ( H 0 ) [ C r ( A c O - A ) ] : C , 38.00; H , 4.96; N , 5.55. Observed: C , 36.45; H , 5.38; N , 5.41. A l t h o u g h the carbon a n a l y s i s is n o t i d e a l , t h i s c o m p o u n d a p p e a r e d t o b e i d e n t i c a l t o t h e o x o n i u m salt isolated a b o v e ( a p p e a r a n c e , i n f r a r e d s p e c t r u m , a n d c o n v e r s i o n to t h e p o t a s s i u m salt). C o n v e r s i o n of ( H O ) [Cr( A c O - A ) ] t o P o t a s s i u m Salt. A solution c o n t a i n i n g 0 . 3 0 g r a m ( 0 . 0 0 3 m o l e ) o f p o t a s s i u m b i c a r b o n a t e i n 10 m l . o f w a t e r w a s a d d e d t o a slurry of 1.00 g r a m ( 0 . 0 0 1 9 7 m o l e ) of ( H O ) [ C r ( A c O - A ) ] i n 5 0 m l . of water. T h e a d d i t i o n of 8 0 0 m l . of absolute ethanol to t h e resulting solution c a u s e d a p i n k p r e c i p i t a t e t o f o r m , w h i c h w a s i s o l a t e d b y filtration, w a s h e d w i t h absolute ethanol a n d ether, a n d air-dried. Y i e l d : 0.72 g r a m ( 6 9 . 8 % ) . Analysis. Calculated for K [ C r ( A c O - A ) ] : C , 36.53; H , 4.21; N , 5.33. Observed: C, 36.67; H , 4.57; N , 5.27. Attempted Reaction of K [ C r ( H O - A ) ] with A c e t y l C h l o r i d e i n Water. One g r a m (0.00227 mole) of K [ C r ( H O - A ) ] was dissolved i n 50 m l . of water a n d c o o l e d to 4 ° C , a n d 0 . 2 9 g r a m o f p o t a s s i u m h y d r o x i d e ( 0 . 0 0 4 5 m o l e ) w a s a d d e d . 4

2

2

8

s

2

2

a

2

2

2

2


Κ R AU SE AND GOLDBY

145

Reactivity of Hydroxy] Groups

T o this s o l u t i o n w a s a d d e d 0.35 g r a m ( 0 . 0 0 4 5 m o l e ) of a c e t y l c h l o r i d e i n 10 m l . o f c h l o r o f o r m . A f t e r s t i r r i n g f o r 15 m i n u t e s t h e a q u e o u s a n d o r g a n i c p h a s e s w e r e s e p a r a t e d , t h e f o r m e r c o n c e n t r a t e d to 1 0 m l . , a n d t h e r e d , c r y s t a l l i n e s o l i d i s o l a t e d b y filtration, w a s h e d w i t h w a t e r , a n d d r i e d i n v a c u o o v e r P 0 . Yield: 0.44 g r a m . 2

5

T h e filtrate w a s e v a p o r a t e d to d r y n e s s . Yield: 0.81 g r a m . Infrared exami n a t i o n of b o t h fractions s h o w e d the a b s e n c e of ester a n d c o n f i r m e d the i d e n t i t y of t h e first f r a c t i o n as s t a r t i n g m a t e r i a l . T h e s e c o n d fraction a p p e a r e d to b e starting material a n d potassium acetate. A t t e m p t e d R e a c t i o n of K [ C r ( H O - A ) ] w i t h B e n z o y l C h l o r i d e i n W a t e r . T o a solution of 2.00 g r a m s ( 0 . 0 0 4 5 4 m o l e ) of K [ C r ( H O - A ) ] i n 200 m l . of w a t e r , c o o l e d to 6 ° C , w e r e a d d e d 1 2 . 8 g r a m s o f b e n z o y l c h l o r i d e a n d a s o l u t i o n of 2.00 g r a m s of p o t a s s i u m b i c a r b o n a t e i n 20 m l . of water. This mixture was stirred i n a n ice b a t h for 30 minutes a n d then for an a d d i t i o n a l h o u r out of the bath. T h e solution was acidified w i t h concentrated nitric acid, a n d then filtered to r e m o v e b e n z o i c a c i d . F r o m t h e filtrate o n l y s t a r t i n g m a t e r i a l ( i n f r a r e d s p e c t r u m ) a n d its d e c o m p o s i t i o n p r o d u c t s c o u l d b e i s o l a t e d . 2


2

A t t e m p t e d Reaction of K [ C r ( H O - A ) ] w i t h A c e t y l C h l o r i d e . O n e g r a m of K [ C r ( H O - A ) ] a n d 5 0 m l . o f a c e t y l c h l o r i d e w e r e p l a c e d i n a flask a n d r e f l u x e d for 24 hours. A t the e n d of this t i m e K [ C r ( H O - A ) ] was r e c o v e r e d u n c h a n g e d ( i n f r a r e d s p e c t r u m ) ; the l i q u i d p h a s e was colorless, i n d i c a t i n g i n s o l u b i l i t y of the complex. 2

2

2

A t t e m p t e d Reaction of K [ C r ( H O - A ) ]

with Acetyl Chloride in D M F .

2

One

g r a m ( 0 . 0 0 2 2 6 m o l e ) of K [ C r ( H O - A ] was m i x e d w i t h 150 m l . of hot D M F a n d 1 m l . (excess) of a c e t y l c h l o r i d e was a d d e d . T h e solution turned blue within 1 m i n u t e a n d w i t h i n 3 0 m i n u t e s all s o l i d a p p e a r e d to h a v e d i s s o l v e d . A f t e r s t a n d i n g o v e r n i g h t the s o l u t i o n w a s e v a p o r a t e d to a g r e e n o i l w h i c h g a v e n o i n d i c a t i o n of t h e p r e s e n c e < )f e s t e r ( i n f r a r e d s p e c t r u m ) . 2

A t t e m p t e d R e a c t i o n of ( M e N ) [ C r ( H O - A ] w i t h A c e t y l C h l o r i d e i n A c e t o nitrile. In 2 0 0 m l . of acetonitrile w e r e p l a c e d 0.50 gram (0.00105 mole) of ( M e N ) [ C r ( H O - A ) ] , 1 m l . o f p y r i d i n e , a n d 18 d r o p s ( e x c e s s ) o f a c e t y l c h l o r i d e . T h e solution was w a r m e d a n d w i t h i n a f e w m i n u t e s the color c h a n g e d f r o m r e d to green. O n e v a p o r a t i o n a green oil was o b t a i n e d , the i n f r a r e d s p e c t r u m of w h i c h g a v e n o i n d i c a t i o n o f t h e p r e s e n c e o f ester. 4

4

2

2

A t t e m p t e d R e a c t i o n of K [ C r ( H O - A ) ] w i t h A c e t i c A c i d . Two grams ( 0 . 0 0 4 5 4 m o l e ) of K [ C r ( H O - A ) ] w a s d i s s o l v e d i n 150 m l . of g l a c i a l acetic a c i d a n d refluxed, slowly distilling solvent. A f t e r several hours of s u c h treatment the s o l u t i o n w a s a l l o w e d to s t a n d o v e r n i g h t a n d t h e n e v a p o r a t e d t o a v i s c o u s , b l u e o i l . B y d i s s o l v i n g t h i s i n w a t e r a n d c o l l e c t i n g f r a c t i o n s as t h e a q u e o u s s o l u t i o n w a s e v a p o r a t e d , a l l s t a r t i n g m a t e r i a l w a s r e c o v e r e d u n c h a n g e d ( i d e n t i f i e d b y its i n f r a r e d s p e c t r u m ). 2

2

A t t e m p t e d R e a c t i o n of ( M e N ) [Cr( H O - A ) ] with Acetic Anhydride in Acetonitrile. S e v e n - t e n t h s g r a m ( 0 . 0 0 1 4 7 m o l e ) of ( M e N ) [ C r ( H O - A ) . , ] w a s m i x e d w i t h 2 5 0 m l . of acetonitrile a n d 1 m l . ( 0 . 0 0 9 m o l e ) of a c e t i c a n h y d r i d e . A f t e r r e f l u x i n g f o r 1 h o u r a s m a l l q u a n t i t v of s t a r t i n g m a t e r i a l w a s r e c o v e r e d b y filtration a n d t h e s o l u t i o n w a s c o n c e n t r a t e d to 15 m l . T h e s o l i d w h i c h c r y s t a l l i z e d w a s i s o l a t e d b y filtration, w a s h e d w i t h a c e t o n i t r i l e a n d e t h e r , a n d a i r - d r i e d . In f r a r e d e x a m i n a t i o n p r o v e d t h i s c o m p o u n d to b e s t a r t i n g m a t e r i a l . 4

2

4

R e a c t i o n s of [ C r ( A c O - A ) ] - . T w o grams of ( H 0 ) [ C r ( A c O - A ) ] (freshly r e p r e c i p i t a t e d ) w a s m i x e d w i t h 15 to 2 5 m l . o f s o l v e n t a n d r e f l u x e d f o r c a . 4 h o u r s . S o m e s o l v e n t w a s r e m o v e d b y d i s t i l l a t i o n a n d e x a m i n e d b y m e a n s of v a p o r p h a s e c h r o m a t o g r a p h y for a n e w component. T h e results are s u m m a r i z e d b e l o w : 2

3

2

Solvent

New Component in Distillate

Methanol Ethanol E t h y l propionate

M e t h y l acetate E t h y l acetate E t h y l acetate

T h e complexes were not isolated f r o m these reactions. W h e n 1.5 g r a m s o f K [ C r ( A c O - A ) ] w a s r e f l u x e d w i t h

15

ethanol

i n the

2

for 4

hours

m e a n s of v a p o r p h a s e

no n e w

components

could be

detected

m l . of

absolute

distillate

chromatography.


by


146

Vapor Phase Chromatography. V P C data were obtained on a Perkin E l m e r Vapor Fractometer, M o d e l 154D, using an 0 column on Celite, at 15 pounds of helium pressure. The temperature was varied depending on the components to be separated, but was ca. 9 2 ° . In each experiment a mixture of knowns was run for comparison purposes. Infrared Spectra. Infrared spectra were obtained on a Perkin E l m e r M o d e l 21 spectrophotometer using mineral oil and halocarbon mulls. Discussion Several attempts to prepare typical alcohol derivatives of [ C r ( H O - A ) ] ~ were unsuccessful; these include Schotten-Bauman conditions (acetyl chloride i n chloroform shaken w i t h cold, aqueous alkaline solution of complex), refluxing in acetyl chloride, i n glacial acetic acid, acetyl chloride i n hot dimethyl formamide, acetyl chloride and pyridine i n acetonitrile, and acetic anhydride i n acetonitrile. The failure of refluxing acetyl chloride to effect acetylation brings to m i n d the work of Keller and Edwards (4); the acetyl chloride system is completely hetero geneous and consequently not conducive to reaction. However, even the homo geneous reaction of [ C r ( H O - A ) ] " ~ w i t h acetyl chloride i n acetonitrile failed to give a measurable quantity of the diester. Although refluxing i n glacial acetic acid failed to acetylate the complex, using a solvent of glacial acetic acid-acetic anhydride led to the formation of a low yield of the diester. T h i * compound was isolated as the water-insoluble oxonium salt, ( H 0 ) [ C r ( A c O - A ) ] . The insolubility of this salt i n water was surprising; examination of its infrared spectrum shows the asymmetric carboxyl stretching mode (at 1630 c m . ) to be ca. 20 c m . lower than that i n the corre sponding tetramethylammonium salt. This indicates an interaction between co ordinated carboxyl and H 0 + , and is probably the cause of the insolubility of (H 0) [Cr(AeO-A) ]. O n heating, glacial acetic acid solutions of [ C r ( H O - A ) ] ~ change color to dark blue; this color change probably indicates ligand displacement, according to


2

2

2

3

- 1

- 1

3

2

3

2

[Cr(HO-A) ] ~ + 3 H O A c 2

->

[Cr(HO-A)(OAc) ] ~ + H O - A H + 3

2

3

Free ligand could undergo acetylation, recoordinate, and give the diester which was isolated. This system may not involve a ligand reaction. Refluxing free ligand w i t h acetic acid-acetic anhydride gives a brown oil similar to that obtained in these experiments involving the complex. Consequently, it was deemed desirable to operate i n a very weakly co ordinating solvent if possible. Since acetonitrile is a m u c h poorer donor solvent than glacial acetic acid, and since the tetramethylammonium salt of the chro m i u m (III) complex is soluble i n acetonitrile, this appeared to be an ideal solvent for running ligand reactions. However, initial experiments indicated that neither acetic anhydride nor acetyl chloride w o u l d acetylate the complex i n this solvent. Ketene is k n o w n to be a very active acetylating agent. This reagent reacted w i t h the complex, [ ( C H ) N ] [ C r ( H O - A ) ] , i n acetonitrile solution to produce the diester. However, ketene, w h i c h normally reacts w i t h alcohols very quickly, reacted slowly i n this case (Table I ) . In a l l ketene reactions a large excess of ketene was passed through the solution; the critical factor determining yield appears to be the total time of refluxing. Because of the reaction conditions employed (poor donor solvent, absence of nucleophilic reagents for ligand displacement) it seems certain that a ligand reaction is occurring i n this system, and not acetylation of displaced ligand as is probable i n the acetic acid-acetic anhydride system. 3

4

2


KRAUSE AND GOLDBY Table I.

Reactivity of Hydroxyl

Reaction of (Me N)[Cr(HO-A) ] with Ketene 4

[Gr(HO-A) ] - + 2 C H 2

Equiv. Ketene"/ Equiv. Complex

2

2

= C = 0

[Gr( A c O - A ) ] 2

Reflux Time, Hours

Yield, %

2Vi

31.6 47.4 75.9 75.9

18 35 91 83

33A 24 24

I n a l l experiments 3.0 grams of c o m p l e x was dissolved i n 550 m l . of refluxing acetonitrile.

a


147

Groups

Because of the difficulty encountered i n acetylation of the complexed alcohol, it was of interest to see if the ester complex behaves i n a normal fashion. R e fluxing ( H 0 ) [ C r ( A c O - A ) ] i n methanol or ethanol caused methyl or ethyl acetate to be formed, while refluxing i n ethyl propionate formed ethyl acetate. W h e n the potassium salt was used i n place of the oxonium salt no transesterification was observed; this could be due to the necessity of acid catalysis or a difference in solubility i n these essentially heterogeneous systems. T h e oxonium salt, ( H 0 ) [ C r ( A c O - A ) ] , appears to have typical ester reactivity. T h e structure of the " a l c o h o l " complex, [ C r ( H O - A ) ] , has been elucidated by means of infrared spectra ( 5 ) ; one asymmetric carboxyl stretching mode near 1650 c m . implies that all four carboxyl groups are equivalent and coordinated. Coordination of the two nitrogen atoms then satisfies the hexa-coordinate require ment of chromium ( I I I ) , dictating that the alcohol group must remain uncoordi nated; the O H stretching mode for these groups at 3380 and 3300 c m . is i n the region for uncoordinated hydroxyls. I n the ester, [ C r ( A c O - A ) ] ~ , the ab sence of absorption i n the 3 4 0 0 - c m . region indicates the loss of alcohol groups, while the new b a n d at 1740 c m . verifies the presence of ester. One asymmetric carboxyl stretching mode, again i n the 1 6 5 0 - c m . region, substantiates the belief that the coordination sphere remains unchanged (Table I I ) . Further verification of the identity of the coordination spheres i n [ C r ( H O - A ) ] ~ a n d [Cr(AcO-A) ] is found i n the fact that the visible-ultraviolet absorption spectra of these compounds are nearly identical ( K [ C r ( H O - A ) ] A . 516 (e 43.7), 365 ( 31.5); K [ C r ( A c O - A ) ] A . 506 ( 4 6 . 1 ) , 358 ( c 36.8))'. If the former compound involved coordination of the alcohol groups, one w o u l d expect a greater shift i n the spectrum than that w h i c h is observed. 3

2

3

2

2

_

- 1

- 1

2

- 1

- 1

_ 1

2

2

_

2

e

2

Table II.

[Cr(AcO-A) ] 2

K[Cr(AcO-A) ] 2

K[Cr(HO-A) ] 2

(CH^N[Cr(HO-A) _

3380 m 3300 m

stretch

2

3380 m

3100 v b , w

H 0^ 3

—COOR

— C O O "

m a x

c

Infrared Spectra in the 4000- to 1 5 0 0 - C m . i Region

Assignment OH

m a x

(asymin.)

v b , very b r o a d ;

s, strong;

2980 2960 1735 1660 1645 1620

w w s sh sh s

m , medium;

2980 2960 1740 1695 1675 1660 1650 w, weak;

w w s sh sh s s

2940 w

3040 w 2950 w

1695 sh 1650 s 1630 s

1665 s 1645 s

s h , shoulder.

If charge o n the complex molecule were an important factor i n acetylating uncoordinated hydroxyl groups, one w o u l d expect anionic species to react rapidly.



14α

T h e p r e s e n t w o r k s e e m s to i n d i c a t e t h a t c h a r g e o n t h e m o l e c u l e is n o t a n i m p o r t a n t consideration f r o m the

standpoint of a l c o h o l reactivity;

electrolyte

nature

large effect o n s o l u b i l i t y , of course, a n d f r o m this s t a n d p o i n t c o u l d affect Choline

salts

[( C H ) N C H C H O H ] 3

3

2

are

acylated.

Here

the

positive but

the

A p p a r e n t l y m o r e t h a n c h a r g e is i n v o l v e d i n

the

c h a r g e u n d o u b t e d l y c o n t r i b u t e s t o h y d r o l y t i c i n s t a b i l i t y o f t h e ester a l c o h o l i o n is r e a d i l y a c y l a t e d .

a

(2),

2

readily

has

reactivity.

reactivity of the u n c o o r d i n a t e d h y d r o x y l groups i n [ C r ( H O - A ) ] . _

2


The

slow acetylation

inclined

to

suggest

quently

rendered

of the

that this

hydroxyl group

group

inactive.

is d i f f i c u l t to

is c o o r d i n a t e d

Such

a

to

metal

possibility requires

either

n u m b e r of 7 f o r c h r o m i u m (III)

or d i s p l a c e m e n t of c a r b o x y l a t e

tion

the

sphere

by

group would

hydroxyl.

In

still b e present

latter

instance

an

to r e a c t w i t h k e t e n e

A

c o o r d i n a t i o n n u m b e r of 7

infrared spectrum

droxyl

groups.

The

[Cr(HO-A) ]~

indicates

does

similarity

of

the

2

appear

to

be

uncoordinated

i n the

former.

coordination functional

COOH)

and

an

case.

l i k e l y f o r c h r o m i u m ( III ) ;

We

absorption

and hence,

must

also,

uncoordinated

seek

of

identical

hydroxyl

an

hy

spectra

is f u r t h e r e v i d e n c e o f t h e

character of the d o n o r groups i n b o t h c o m p o u n d s ,

is

conse

T h i s is n o t t h e

visible-ultraviolet

(above)

_

a

uncoordinated

that this c o m p o u n d c o n t a i n s

and [ C r ( A c O - A ) ]

2

not seem

One

ion and

f r o m the coordina

( C O O - or

h y d r i d e s h o u l d be detected i n the crude reaction product.

the

explain.

the

groups

explanation

not

i n v o l v i n g c o o r d i n a t i o n o f t h e h y d r o x y l o x y g e n to c h r o m i u m ( I I I ) . Another

effect

charge o n the

to

be

considered

complex molecule,

c h r o m i u m (III)

is a n

effect;

although

is n e g a t i v e ,

2

e x e r t s its e f f e c t o n t h e f r e e

the a l c o h o l p r o t o n .

inductive

[Cr(HO-A) ]~~,

the

total

it is p o s s i b l e t h a t

O H group, increasing the acidity

" A c i d i c " a l c o h o l s , s u c h as p h e n o l , m a y b e a c e t y l a t e d

of

readily,

so t h a t t h i s d o e s n o t s e e m to b e a p l a u s i b l e e x p l a n a t i o n f o r t h e v e r y s l o w

ketene

reaction. I n t e r a c t i o n of t h e a l c o h o l g r o u p s w i t h s o l v e n t m u s t a l s o b e c o n s i d e r e d . f o r m of solvent interaction does not appear considers pound;

the

w i d e variety

water,

of solvents

dimethyl formamide,

investigated

glacial

acetic

for

acetylation

acid,

of

acetonitrile,

and

as)

acetyl

agents.

It is d i f f i c u l t to c o m p a r e o u r r e s u l t s i n t h e s e v a r i o u s m e d i a , s i n c e d i f f e r e n t agents were

(or

one com

( in a hetrogeneous

systems

were all e m p l o y e d w i t h

this

chloride

acetylating

system)

Some

to b e a g o o d e x p l a n a t i o n w h e n

acetylating

u s e d , b u t o u r lack of success i n a n u m b e r

of

different

i n d i c a t e s t h a t p e r h a p s t h e s a m e e f f e c t is o p e r a t i n g i n a l l o f t h e m .

e x p l a n a t i o n f o r this o b s e r v e d l a c k of r e a c t i v i t y

should account

for the

Any

observa

tions i n a l l of these systems. S t e r i c h i n d r a n c e d o e s n o t , at tively slow rate of reaction. to

be

free

of

the

remainder

first,

s e e m to b e a p o s s i b l e c a u s e of the

M o l e c u l a r models indicate of

the

molecule,

and

the

H O C H

presumably

C H

2

rela group

2

available

for

acetylation. O n e difference between the readily acylated choline ion a n d is t h a t t h e

latter has n o n b o n d i n g d-orbitals c o n t a i n i n g electrons.

proton were the o x y g e n hanced,

to h y d r o g e n - b o n d to o n e of these orbitals, the w o u l d not b e

contrary

to

difficultly accessible the

face

of

the

our

lowered

The

i n a t r a n s i t i o n state.

octahedron

(by

forming

proton,

If t h e a

2

If t h e

however,

should be might

alcohol proton were

hydrogen

_

alcohol

steric a v a i l a b i l i t y of

a n d its n u c l e o p h i l i c c h a r a c t e r

observation.

[Cr(HO-A) ]

bond

to

a

d - o r b i t a l ) it c o u l d b e s h i e l d e d b y t h e d o n o r g r o u p s at t h e o c t a h e d r o n

be

en only

located

in

nonbonding apexes

T h i s t r a n s i t i o n state c o u l d d e c o m p o s e to g i v e k e t e n e a n d t h e s t a r t i n g c o m p l e x ,


(II). or,

KRAUSE AND GOLDBY

Reactivity of Hydroxyl

149

Groups

0.

> II

L m o r e s l o w l y , t h e ester.

A r e a c t i o n i n v o l v i n g a t r a n s i t i o n s t a t e o f t h i s sort m i g h t

be greatly facilitated b y t h e presence


A

principal objection

of a p r o t o n catalyst.

to this m e c h a n i s m

b o n d t o a n o n b o n d i n g cZ-electron.

is t h e r e q u i r e m e n t

of a h y d r o g e n

A l s o , it d o e s n o t e x p l a i n t h e l a c k of reactivity

w i t h acetylating agents w h e r e n o p r o t o n transfer

is r e q u i r e d — e . g . , a c e t y l

and

phenomenon

acetic acid.

U n f o r t u n a t e l y , this interesting

chloride

of extremely

slow

reactivity remains unexplained.

Acknowledgment T h e authors thank their colleagues i n the C h e m i c a l Research S t a m f o r d for m a n y stimulating discussions d u r i n g the course

Department

of this w o r k .

at

They

thank D a r y l e H . B u s c h for suggesting the possibility that the alcohol proton m a y h y d r o g e n - b o n d to a d-orbital.

Literature Cited (1) (2) (3) (4) (5)

Drinkard, W.C.,Bauer, H. F., Bailar, J.C.,Jr.,J.Am. Chem. Soc. 74, 215 (1952). Freese,C.,Ciencia (Mex.) 13, 143 (1953). Hanford, W. E., Sauer, J. S., Org. Reactions 3, 132 (1946). Keller, R. N., Edwards, L. J.,J.Am. Chem. Soc. 74, 215 (1952). Krause, R. Α., unpublished manuscript.

RECEIVED October 8, 1962.


Reactions of Coordinated Ligands and Homogeneous Catalysis

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