Photochemical Reactions in Oriented Systems - American Chemical

17 Photochemical Reactions in Oriented Systems V. R A M A M U R T H Y

Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: May 14, 1985 | doi: 10.1021/bk-1985-0278.ch017

Department of Organic Chemistry, Indian Institute of Science, Bangalore 560 012, India

Modification of chemical reactions through the use of constrained and/or organized media has attracted a great deal of attention recently. Results from our laboratory in this direction which include a study of photochemical reactions in solid state and in cyclodextrins are presented here. A study of solid state photochemical behavior of coumarins has provided information regarding subtler aspects of topochemical postulates of photodimerization. Results pertaining to geometrical criteria for photodimerization and "chloro" as a crystal engineering group are discussed. As a part of an attempt to correlate chemical reactivity with molecular packing in the solid state, photooxidation of diarylthioketones in the solid state has been investigated. The observed differences in the reactivity of these crystals are rationalized in terms of crystal packing. Though cyclodextrins have been extensively studied, very few photochemical reactions involving molecules complexed to cyclodextrins have been examined. In this connection, the u t i l i t y of cyclodextrins in bringing about selectivity in photochemical reactions through the study of excited state behavior of olefins and aryl alkyl ketones has been demonstrated in our laboratory. The photochemistry and p h o t o p h y s i c s o f o r g a n i c m o l e c u l e s i n the c r y s t a l l i n e s t a t e and i n the o r g a n i z e d a s s e m b l i e s has a t t r a c t e d c o n s i d e r a b l e a t t e n t i o n ( 1 - 4 ) . C o n t r o l o f s t e r e o and r e g i o c h e m i s t r y i n photoc h e m i c a l r e a c t i o n s through the use of c o n s t r a i n e d media such as molec u l a r and l i q u i d c r y s t a l s , monolayers, m i c e l l a r a s s e m b l i e s , i n c l u s i o n complexes and s i l i c a g e l s u r f a c e s has opened new v i s t a s i n photochemi s t r y . During the l a s t few years our group has been i n v e s t i g a t i n g the e n v i r o n m e n t a l p e r t u r b a t i o n s on p h o t o c h e m i c a l r e a c t i o n s ( 5 - 1 9 ) . Such s t u d i e s have been concerned w i t h s o l i d s t a t e , m i c e l l a r and i n c l u s i o n complexes. The g o a l i s t o a c h i e v e s e l e c t i v i t y i n photochemi c a l r e a c t i o n s u s i n g these u n u s u a l environments and t o understand the f e a t u r e s c o n t r o l l i n g such s e l e c t i v i t y . T h i s a r t i c l e which summarizes some of our r e s u l t s i s d i v i d e d i n t o two p a r t s : the f i r s t p a r t d e a l s w i t h p h o t o c h e m i c a l d i m e r i z a t i o n of coumarins and p h o t o o x i d a t i o n o f 0097-6156/85/ 0278-0267$06.00/ 0 © 1985 American Chemical Society

In Organic Phototransformations in Nonhomogeneous Media; Fox, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

ORGANIC PHOTOTRANSFORMATIONS IN NONHOMOGENEOUS MEDIA

268

t h i o k e t o n e s i n t h e s o l i d s t a t e . I n the second p a r t , s e l e c t i v i t i e s o b t a i n e d i n the photochemical r e a c t i o n s of o l e f i n s and a r y l a l k y l ketones u s i n g c y c l o d e x t r i n complexes are d e s c r i b e d .


S o l i d State

Photochemistry

P h o t o d i m e r i z a t i o n of Coumarins i n t h e S o l i d S t a t e . S t u d i e s by Schmidt and h i s - c o - w o r k e r s on cinnamic a c i d s have demonstrated t h a t p h o t o d i m e r i z a t i o n i n t h e s o l i d s t a t e are s t r i c t l y c o n t r o l l e d by t h e p a c k i n g arrangement o f the molecule i n the c r y s t a l s (20,21). Schmidt has drawn a t t e n t i o n t o t h e f a c t t h a t n o t o n l y must the double bonds of t h e r e a c t i v e monomers of cinnamic a c i d be w i t h i n 4.2 Â they must a l s o be a l i g n e d p a r a l l e l f o r d i m e r i z a t i o n t o o c c u r . F o l l o w i n g t h e i n i t i a l o b s e r v a t i o n w i t h m e t h y l m-bromo-cinnamate w h e r e i n the r e a c t i v e double bonds a r e r o t a t e d w i t h r e s p e c t t o each o t h e r by 28° a few examples have been r e p o r t e d which support t h e p a r a l l e l i s m requirement f o r p h o t o d i m e r i z a t i o n (22,23). On the o t h e r hand, a few cases have a l s o been r e p o r t e d where exact p a r a l l e l i s m between r e a c t a n t double bonds has n o t been adhered t o and y e t p h o t o d i m e r i z a t i o n o c c u r s (242 7 ) . I t i s c l e a r t h a t a r e e x a m i n a t i o n o f the s u b t l e r a s p e c t s of t h e t o p o c h e m i c a l p o s t u l a t e s i s e s s e n t i a l . I n s p i t e of growing i n t e r e s t i n o r g a n i c r e a c t i o n s i n t h e c r y s t a l l i n e s t a t e , the u t i l i t y of such r e a c t i o n s as a s y n t h e t i c t o o l i s l i m i t e d by the d i f f i c u l t y of a c h i e v i n g the d e s i r e d type of c r y s t a l s t r u c t u r e i n any g i v e n case, f o r t h e f a c t o r s t h a t c o n t r o l t h e c r y s t a l p a c k i n g are n o t f u l l y understood. T h e r e f o r e , scope undoubtedly e x i s t s f o r " e n g i n e e r i n g o r g a n i c c r y s tals. I n t h i s c o n n e c t i o n we have embarked on a s y s t e m a t i c c r y s t a l l o g r a p h i c and p h o t o c h e m i c a l study of a l a r g e number of s u b s t i t u t e d coumarins. A study of a l a r g e number of coumarins (28 i n t o t a l ) p r o v i d e d an o p p o r t u n i t y t o d e r i v e i n f o r m a t i o n r e g a r d i n g v a r i o u s a s p e c t s of p h o t o d i m e r i z a t i o n i n the s o l i d s t a t e . Of t h e s e , r e s u l t s p e r t a i n ing t o p a r a l l e l i s m c r i t e r i a f o r p h o t o d i m e r i z a t i o n and " c h l o r o " as a c r y s t a l e n g i n e e r i n g group are p r e s e n t e d h e r e . S o l i d s t a t e p h o t o d i m e r i z a t i o n of 7-methoxycoumarin i s p a r t i c u l a r l y important i n c o n n e c t i o n w i t h t h e p a r a l l e l i s m c r i t e r i a f o r phot o d i m e r i z a t i o n i n the s o l i d s t a t e . The c r y s t a l s o f 7-methoxycoumarin are t r i c l i n i c w i t h a = 6.834(3), b = 20.672(4), c = 12.600(7) Α, α 108.19(3), β = 9 5 . 2 3 ( 4 ) , γ - 95.22(3), space group Ρτ and Ζ = 4. Xr a y c r y s t a l s t r u c t u r e a n a l y s i s shows t h a t the p o t e n t i a l l y r e a c t i v e double bonds of the monomer m o l e c u l e s w i t h i n the asymmetric u n i t a r e r o t a t e d by 65° w i t h r e s p e c t t o each o t h e r a l t h o u g h the c e n t e r t o cen t e r d i s t a n c e between the double bonds i s 3.83 Â ( F i g u r e 1 ) . However, the dimer y i e l d w i t h i n t w e n t y - f o u r hours of i r r a d i a t i o n of the c r y s t a l l i n e 7-methoxycoumarin was 90%. The s t r u c t u r e of t h e dimer was c o n f i r m e d t o be syn h e a d - t a i l by X-ray s t u d i e s . We note from t h e p r o g r e s s o f the d i m e r i z a t i o n w i t h r e s p e c t t o time o f i r r a d i a t i o n t h a t 7-methoxycoumarin behaves v e r y much l i k e t h e ones i n which t h e r e a c t i o n i s topochemical. I t was observed t h a t t h e r e was no p e r c e p t i b l e evidence f o r i n d u c t i o n p e r i o d i n t h e coumarins ( i n c l u d i n g 7-methoxycoumarin) which are b e l i e v e d t o be t o p o c h e m i c a l l y f a v o r a b l e f o r d i m e r i z a t i o n whereas s i g n i f i c a n t i n d u c t i o n p e r i o d was n o t i c e d i n cases where the p h o t o r e a c t i o n o r i g i n a t e d a t d e f e c t s . However as seen i n F i g u r e 1 t h e two double bonds, a l t h o u g h w i t h i n t h e r e a c t i v e d i s t a n c e , are n o t s u i t a b l y j u x t a p o s e d f o r d i m e r i z a t i o n . T h e r e f o r e , presence o f a c e r t a i n degree of i n h e r e n t o r i e n t a t i o n a l f l e x i b i l i t y of the mole11


Photochemical

Reactions

in Oriented

Systems


RAMAMURTHY

F i g u r e 1. D i s p o s i t i o n of the R e a c t i v e Double Bonds C ( 3 ) - C ( 4 ) and C(3')-C(4') of 7-Methoxycoumarin i n the Asymmetric U n i t .



270


c u l e s i n the c r y s t a l l a t t i c e has t o be invoked t o e x p l a i n the topo c h e m i c a l n a t u r e of d i m e r i z a t i o n . Two dimers namely a n t i head-head and syn h e a d - t a i l c o u l d r e s u l t i f t h e r o t a t i o n of the m o l e c u l e s i s a l l o w e d i n the c r y s t a l l a t t i c e . F o r m a t i o n of a n t i head-head dimer would r e q u i r e a t o t a l r o t a t i o n of 115° whereas t h e syn h e a d - t a i l would r e q u i r e 65°. I t i s q u i t e l i k e l y t h a t the uv r a d i a t i o n absorbed by t h e r e a c t i n g m o l e c u l e s i s s u f f i c i e n t t o a l l o w the m o l e c u l e s t o undergo t h e r e q u i r e d r o t a t i o n p r o v i d e d the motion i s c o - o p e r a t i v e and extends through the c r y s t a l . However, i t seems e s s e n t i a l t o p o s t u l a t e an i n h e r e n t f l e x i b i l i t y w i t h i n the c r y s t a l l a t t i c e f o r these m o l e c u l e s t o undergo r o t a t i o n as t h i s would a l l o w us t o understand the l a r g e y i e l d of the dimer. T h e r e f o r e , i n o r d e r t o e s t i m a t e t h e i n h e r e n t o r i e n t a t i o n a l f l e x i b i l i t y o f the m o l e c u l e s i n the c r y s t a l l a t t i c e , l a t t i c e energy c a l c u l a t i o n s were c a r r i e d o u t u s i n g a computer program WMIN developed by Busing ( 2 8 ) . Much t o our s u r p r i s e the energy c a l c u l a t i o n s r e v e a l e d the presence of o r i e n t a t i o n a l f l e x i b i l i t y i n t h e ground s t a t e f o r both the m o l e c u l e s i n t h e asymmetric u n i t . Indeed a t o t a l r o t a t i o n of about 20° i n the d i r e c t i o n t o generate syn headt a i l dimer i n the ground s t a t e i s p o s s i b l e w i t h o u t much i n c r e a s e i n the l a t t i c e energy (ΔΕ ~ 9.8 k c a l mole ) from the minimum energy p o s i t i o n as determined by X-ray c r y s t a l l o g r a p h y . With the i n c r e a s e i n a t t r a c t i v e f o r c e s between the r e a c t i v e m o l e c u l e s upon e x c i t a t i o n , one may expect t h a t the motion of the m o l e c u l e s so as t o a c h i e v e a maximum π o v e r l a p w i l l become p o s s i b l e . We propose t h a t r o t a t i o n , i n a d d i t i o n t o t h a t i s a v a i l a b l e i n the ground s t a t e as i n d i c a t e d by t h e l a t t i c e energy c a l c u l a t i o n s , t o g e n e r a t e the syn h e a d - t a i l dimer i s a c h i e v e d due t o t h e i n t e r a c t i o n of the e x c i t e d and ground s t a t e mole c u l e s . I n summary, the mechanism o f p h o t o c h e m i c a l d i m e r i z a t i o n of 7-methoxycoumarin i n v o l v e s a t o t a l r o t a t i o n o f 65°, w i t h i n the c r y s t a l l a t t i c e . Thus i t i s demonstrated t h a t t o p o c h e m i c a l d i m e r i z a t i o n of n o n - p a r a l l e l double bonds i s p o s s i b l e once t h e freedom f o r m o t i o n becomes a v a i l a b l e due t o e x c i t e d s t a t e i n t e r a c t i o n between m o l e c u l e s . An aspect o f our d e t a i l e d study on p h o t o d i m e r i z a t i o n of cou m a r i n s i n the s o l i d s t a t e concerns u n d e r s t a n d i n g t h e f a c t o r s t h a t a f f e c t the m o l e c u l a r p a c k i n g and i d e n t i f y i n g the groups t h a t may be of v a l u e i n b r i n g i n g about the p h o t o r e a c t i v i t y . S u b s t i t u e n t s such as h y d r o x y l , m e t h y l , c h l o r o , a c e t o x y and methoxy were u t i l i z e d t o e n g i neer the c r y s t a l s of coumarins towards p h o t o r e a c t i v i t y . Of these a c e t o x y and c h l o r o were found t o be u s e f u l e n g i n e e r i n g groups. R e s u l t s p e r t a i n i n g t o c h l o r o are d i s c u s s e d below. The i m p o r t a n t p o i n t s t h a t emerge from the s t u d i e s on the f i v e c h l o r o c o u m a r i n s i n the s o l i d s t a t e are t h e f o l l o w i n g , (a) A l l c h l o r o s u b s t i t u t e d coumarins ( 7 - c h l o r o , 6 - c h l o r o , 4 - c h l o r o , 4-methyl 7c h l o r o and 4-methyl 6 - c h l o r o coumarins) undergo d i m e r i z a t i o n i n t h e s o l i d s t a t e . W h i l e f o u r o f these g i v e syn head-head dimer as t h e p h o t o p r o d u c t , 4 - c h l o r o c o u m a r i n g i v e s a m i x t u r e of a n t i - h e a d - t a i l and syn h e a d - t a i l i n poor y i e l d s (~ 2 5 % ) . (b) Two of them namely 4c h l o r o and 4-methyl 6 - c h l o r o coumarins e x h i b i t s i g n i f i c a n t i n d u c t i o n p e r i o d f o r d i m e r i z a t i o n s u g g e s t i n g t h a t the d i m e r i z a t i o n i n these cases i s non-topochemical i n n a t u r e , (c) Consistent with t h e i r p h o t o c h e m i c a l b e h a v i o r , the p a c k i n g arrangements, as r e v e a l e d by X-ray c r y s t a l l o g r a p h i c a n a l y s i s , a r e f a v o r a b l e f o r d i m e r i z a t i o n o n l y i n the c r y s t a l s o f 6 - c h l o r o , 7 - c h l o r o and 4-methyl 7 - c h l o r o coumarins ( F i g u r e 2 ) . Syn head-head dimers a r e the d i r e c t consequence of these


Photochemical

Reactions

in Oriented

Systems

271


RAMAMURTHY

Monoclinic a - 4.037 A b « 9.759 À β" 90.73* Ζ » 2

c = 10.922 A

F i g u r e 2. P a c k i n g Arrangement o f (a) 7-Chlorocouraarin and (b) 4-Methyl 7 - c h l o r o c o u m a r i n .

Figure 2 continued




F i g u r e 2. P a c k i n g Arrangement o f ( c ) 4-Chlorocoumarin and (d) 4-Methyl 6 - c h l o r o c o u m a r i n .



17.

RAMAMURTHY

Photochemical

Reactions

in Oriented

Systems

273

p a c k i n g arrangements. Double bonds i n 4 - c h l o r o and 4-methyl 6-chloro coumarins, a c c o r d i n g t o X-ray c r y s t a l l o g r a p h i c a n a l y s e s , are not s u i t a b l y o r i e n t e d f o r d i m e r i z a t i o n . I t i s noteworthy t h a t , whereas coumarin does not undergo d i m e r i z a t i o n i n the s o l i d s t a t e , a l l the f i v e c h l o r o c o u m a r i n s undergo p h o t o d i m e r i z a t i o n . I t i s s i g n i f i c a n t t h a t i n the t h r e e cases the p e r p e n d i c u l a r d i s t a n c e between the c l o s e s t neighbors v a r y from 3.45 t o 4.45 Â w h i l e i n coumarin c r y s t a l s i t i s as l a r g e as 5.67 A. A l t h o u g h the use of " c h l o r o " as an e f f e c t i v e s t e e r i n g d e v i c e was o r i g i n a l l y r e c o g n i z e d by Schmidt, no s y s t e m a t i c study was r e p o r t e d (29,30). T h e r e f o r e , i t was f e l t t h a t the s y s t e m a t i c s i n the mode of p a c k i n g i n c r y s t a l s t r u c t u r e s c o n t a i n i n g c h l o r o group a t t a c h e d t o a r o m a t i c r i n g s i s worthy of i n v e s t i g a t i o n . The e x p e r i m e n t a l i n f o r m a t i o n f o r our a n a l y s i s was taken from Cambridge C r y s t a l l o g r a p h i c Data Base ( V e r s i o n , December 1981). Out of a t o t a l of 132 s t r u c t u r e s , o n l y 22 d i d not c o n t a i n any C l . . . C I i n t e r a c t i o n w i t h i n C l . . . C I d i s tance of

Photochemical Reactions in Oriented Systems - American Chemical

Recommend Documents