1 The Interaction between the Ring-Oxygen p-Type Lone Pair and Adjacent σ-Bonds i n Pyranose Derivatives SERGE
DAVID
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Laboratoire de Chimie Organique Multifonctionnelle, Universitéde Paris-Sud, 91405 Orsay (France)
Bâtiment 420,
In this lecture, t h e a u t h o r will a t t e m p t n o t t o m i x two kinds o f a p p r o a c h t o t h e p r o b l e m o f t h e "Origin and Consequences of the Anomeric Effect", on one side t h e h a r d facts o f experimental d a t a and calculations conducted w i t h clearly-defined a p p r o x i m a t i o n s , and on t h e o t h e r side, semi-empirical theories w h i c h a r e more easily g r a s p e d and p u t t o use with c o m p l e x molecules. Discussion o f t h e s e is reported to the last section. To b e g i n with, t h e e m p h a s i s will be laid on t h e straightforward utilisation of molecular-orbital calculations to an understanding o f some physical and c h e m i c a l properties of pyranoses. An ab initio STO-3G w a v e f u n c t i o n calculation, including all electrons, has b e e n d e v e l o p e d with t h e aid of a p r o g r a m (1) t o provide a theoretical basis for the following discussion of experimental results. The model, w h i c h is simplified, but represents an actual molecule, 2-chlorotetrahydropyran, contains the skeletal six-membered ring o f a p y r a n o s e , with a chlorine atom linked t o C-2 as t h e only substituent. Calculations were first made for the two possible chair conformations of this molecule (Table I), one with an equatorial, and the other one with an axial carbon—chlorine bond, with the same carbon—chlorine bond length in both cases. A calculation has also been made with a slightly longer C—CI bond in the axial case, to follow the known experimental trend according to which axial bonds are longer than equatorial ones (2). The b a s i c a s s u m p t i o n s a r e g i v e n i n T a b l e I , where t h e ()> s a r e t h e r e l e v a n t a t o m i c o r b i t a l s o f t h e component atoms. Thus t h r e e s e t s o f r e s u l t s were o b t a i n e d , w h i c h c o r r e s p o n d to the e q u a t o r i a l c o n f o r m a t i o n , t o the a x i a l c o n f o r m a t i o n w i t h t h e same bond l e n g t h s , and t o t h e a x i a l c o n f o r m a t i o n w i t h a l o n g e r C—CI b o n d . The c o m p u t a t i o n y i e l d e d t h e C-[j m a t r i x and t h e e n e r g i e s o f e a c h m o l e c u l a r o r b i t a l . The a x i a l c o n f o r m e r was f o u n d t o be more s t a b l e t h a n t h e e q u a t o r i a l one by 1.2 K c a l , when t h e l e n g t h o f i t s C - C l bond was 177 pm, and by 3.7 K c a l when t h e l e n g t h o f t h e C - C l bond was 182 pm, i n a p p r o x i m a t e agreement x i t h e x p e r i m e n t a l e v i d e n c e ( 3 ) . H e n c e f o r w a r d , when d e a l i n g w i t h the a x i a l conformer, o n l y the f i g u r e s c o r r e s p o n d i n g t o the f
0-8412-0470-5/79/36-087-001$05.00/0 © 1979 American Chemical Society Szarek and Horton; Anomeric Effect ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
2
ANOMERIC E F F E C T
TABLE I
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CALCULATION OF MOLECULAR ORBITALS OF 2-CHLOROTETRAHYDROPYRAN IN ITS TWO POSSIBLE CONFORMATIONS
GEOMETRY Bond a n g l e s : 109°28' S t a n d a r d bond l e n g t h s f o r C-H, C-C, and C-0 bonds C - C l bonds : e q u a t o r i a l , 177 pm ; a x i a l , 177 and 182 pm MOLECULAR ORBITALS *k
=
E
C
l
i k *
i
5
e
n
e
r
§
y
B o n d i n g : k = 1, 2, ..., 32
E
k Antibonding
: k = 3 3 , ...,64
METHOD Ab i n i t i o
ST0-3G, g i v i n g a l l C.
and E
Szarek and Horton; Anomeric Effect ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
1.
DAVID
Interaction
in
Pyranose
3
Derivatives
geometry w i t h r e l a x e d C - C l bond w i l l be c o n s i d e r e d . We s h a l l now d i s c u s s some o t h e r p r o p e r t i e s o f t h e m o l e c u l e w h i c h can be d e r i v e d i n a f a i r l y s i m p l e manner f r o m t h e Cj_j coefficients. N u c l e a r Q u a d r u p o l e Resonance
Frequencies
The NQR f r e q u e n c i e s o f h a l o g e n a t e d compounds a r e r e l a t e d i n a v e r y s i m p l e manner t o t h e v a l e n c y p o r b i t a l p o p u l a t i o n s on t h e h a l o g e n atom by t h e Townes and D a i l e y f o r m u l a (4_) . F o r a c a r b o n c h l o r i n e bond, t h e 3 5 1 n u c l e u s NQR f r e q u e n c y c a n be s e t e q u a l t o : C
(b-a) MHz
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v = 55
(1)
In t h i s e q u a t i o n , a i s the p o p u l a t i o n of the 3 p c h l o r i n e o r b i t a l c o n t r i b u t i n g t o t h e C - C l bond and b i s t h e a v e r a g e p o p u l a t i o n o f t h e two 3 p and 3py o r b i t a l s on c h l o r i n e . Then i n t h e c a s e o f t h e two p o s s i b l e c o n f o r m e r s o f 2 - c h l o r o t e t r a h y d r o p y r a n t h e computed -^Cl-NQR f r e q u e n c i e s o f e q u a t o r i a l and a x i a l c h l o r i n e a r e f o u n d t o be r e s p e c t i v e l y V = 41.2 MHz and V = 37.9 MHz. The d i r e c t c h e c k i n g o f t h e s e f i g u r e s by measurement i s n o t p o s s i b l e . The a c t u a l , experimental f r e q u e n c i e s f o r per-(9a c e t y l a t e d g l y c o p y r a n o s y l c h l o r i d e s l i e , on t h e a v e r a g e , 6 MHz l o w e r . However, many a p p r o x i m a t i o n s a r e i n v o l v e d i n t h e d e r i v a t i o n o f t h e Townes and D a i l e y f o r m u l a , so i t i s g e n e r a l l y c o n s i d e r e d as s a f e r t o compare v e r y s i m i l a r compounds. The d i f f e r e n c e b e t w e e n t h e c a l c u l a t e d f r e q u e n c i e s f o r t h e two c o n f o r m a t i o n s o f 2 - c h l o r o tetrahydropyran i s : z
x
e q
Av = v
e q
- V
= 3.5
a x
MHz
a x
(2)
Such i s t h e o r d e r o f t h e f r e q u e n c y d i f f e r e n c e we may expect b e t w e e n t h e two anomers, a x i a l and e q u a t o r i a l , o f a g l y c o s y l c h l o r i d e w i t h one c o n f i g u r a t i o n . L o o k i n g a t F i g u r e 1, i t may be seen t h a t observed resonances f o r p e r - 0 - a c e t y l a t e d g l y c o p y r a n o s y l c h l o r i d e s a r e d i s t r i b u t e d i n t o two n o n - o v e r l a p p i n g g r o u p s . The a v e r a g e v a l u e s o f t h e f r e q u e n c i e s o f t h e two g r o u p s d i f f e r by 2.6 MHz, i n q u a l i t a t i v e agreement w i t h e q u a t i o n ( 2 ) . Now NQR f r e q u e n c i e s can o n l y be m e a s u r e d on s o l i d s a m p l e s , and t h e s o l i d s t a t e s t r u c t u r e i s known w i t h c e r t a i n t y f o r o n l y t h r e e compounds i n F i g u r e 1. N e v e r t h e l e s s , d i s c u s s i o n o f i n d i v i d u a l c a s e s l e a v e s l i t t l e d o u b t t h a t compounds i n t h e h i g h e r f r e q u e n c y g r o u p have e q u a t o r i a l c a r b o n - c h l o r i n e b o n d , and t h o s e i n t h e l o w e r , a x i a l ones. In the case of hexopyranosyl d e r i v a t i v e s , i t i s g e n e r a l l y a c c e p t e d t h a t t h e t e n d e n c y o f t h e s i d e - c h a i n t o a d o p t an e q u a t o r i a l p o s i t i o n i s d o m i n a t i n g i n s o l u t i o n , i m p l y i n g t h a t the compounds a d o p t the £i(D) o r conformations. This e f f e c t i s o b s e r v e d even when i t means t h a t f o u r ( 9 - a c e t y l g r o u p s must a d o p t a x i a l p o s i t i o n s , as i n t h e c a s e o f t h e penta-0-acetyl-a-Di d o p y r a n o s y l c h l o r i d e ( 5 ) . I n t h e c a s e o f a-compounds,the p r e s e n c e 4
Szarek and Horton; Anomeric Effect ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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ANOMERIC E F F E C T
r-XYLO S-
