9 Alkaline Degradation of a Nonreducing Cellulose. Model: 1,5-Anhydro-cellobiitol R A L P H E. B R A N D O N ,
LELAND
R. S C H R O E D E R ,
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The Institute of Paper Chemistry, Appleton, Wis.
and D O N A L D C .
JOHNSON
54911
Abstract Degradations o f 1 , 5 - a n h y d r o - c e l l o b i i t o l at 160-180°C in oxygen-free, 0 . 5 - 2 . 5 N NaOH i n v o l v e cleavage o f both the g l y c o s y l -oxygen bond (80-90%) and the oxygen-aglycon bond ( 1 0 - 2 0 % ) . Cleavage o f the oxygen-aglycon bond y i e l d s 1,5:3,6-dianhydro-D-galactitol (50-100%) and u n i d e n t i f i e d products (0-50%) from the a g l y con, and is b e l i e v e d t o occur by an S 1 mechanism. Cleavage o f the glycosyl-oxygen bond y i e l d s 1 , 5 - a n h y d r o - D - g l u c i t o l (100%) from the aglycon. The r e a c t i v e i n t e r m e d i a t e , 1,6-anhydro-β-D-glucopyranose, is formed from the g l y c o s y l moiety i n ca. 35% o f the cleavages o f the glycosyl-oxygen bond and, hence, its formation i s not as s i g n i f i c a n t as i s u s u a l l y presumed. Glycosyl-oxygen bond cleavage does not appear t o occur by a s i n g l e mechanism and i s probably governed by both S 1 c B (2') and SN1 mechanisms. In cont r a s t t o degradations o f 1 , 5 - a n h y d r o - c e l l o b i i t o l , degradations o f 1,5-anhydro-2,3,6-tri-O-methyl-cellobiitol form ca. 65% 1,6-anhydro-β-D-glucopyranose from glycosyl-oxygen bond cleavage. The i m p l i c a t i o n s o f these r e s u l t s with r e s p e c t t o a l k a l i n e cleavage of g l y c o s i d i c bonds i n c e l l u l o s e are d i s c u s s e d . N
N
Introduction High-temperature a l k a l i n e processes i n v o l v i n g c e l l u l o s i c m a t e r i a l s can r e s u l t i n a s i g n i f i c a n t l o s s o f weight and a d r a s t i c decrease i n the degree o f p o l y m e r i z a t i o n o f the c e l l u l o s e (1X3). The weight l o s s has been a t t r i b u t e d p r i m a r i l y t o endwise degradat i o n ("peeling") o f the p o l y s a c c h a r i d e w h i l e the d r a s t i c r e d u c t i o n i n the degree o f p o l y m e r i z a t i o n has been a t t r i b u t e d p r i m a r i l y t o random cleavage o f g l y c o s i d i c bonds (l). While anaerobic a l k a l i n e degradations o f a r y l g l y c o s i d e s j[) and a l k y l g l y c o s i d e s (£) have been i n v e s t i g a t e d e x t e n s i v e l y , and mechanisms have been proposed f o r these r e a c t i o n s , the mechanism o f a l k a l i n e cleavage o f g l y c o s i d i c l i n k a g e s j o i n i n g monosaccharide u n i t s i n o l i g o - o r p o l y s a c c h a r i d e s has r e c e i v e d l i t t l e 125
In Cellulose Technology Research; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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RESEARCH
a t t e n t i o n . I t i s g e n e r a l l y assumed (2_, 3., 6) f o r c e l l u l o s e that a l k a l i n e cleavage o f the 3-1 ,U-glycosidic linkages occurs by a neighboring group mechanism i n which the aglycon i s d i s p l a c e d by the conjugate base o f the t r a n s - 2 - h y d r o x y l group. The mechanism i s analogous t o the mechanism proposed by McCloskey and Coleman (χ) t o account f o r the a l k a l i n e l a b i l i t y o f a r y l t r a n s - 1 , 2 - g l y c o pyranosides, and which has subsequently been e x t r a p o l a t e d , a l b e i t questionably (^), t o a l k y l glycopyranosides. Best and Green (8_) concluded that the data f o r the a l k a l i n e degradation o f methyl 3 - e e l l o b i o s i d e was c o n s i s t e n t with such a mechanism, and a more recent k i n e t i c a n a l y s i s o f these data by L a i (£) presumably strengthens t h i s c o n c l u s i o n . In t h i s paper we report t h e r e s u l t s o f a study o f the mecha nism o f degradation o f a nonreducing c e l l u l o s e model; 1,5-anhydroU-0-(3-D-glucopyranosyl)-D-glucitol, I_ ( 1 , 5 - a n h y d r o - c e l l o b i i t o l ) ; at l60-l80°C i n aqueous, oxygen-free sodium hydroxide (0.5-2.5N). A u x i l i a r y studies o f degradations o f a p a r t i a l l y - m e t h y l a t e d de r i v a t i v e o f I_, 1,5-anhydro-4-O ( 3-D-glucopyranosyl )-2,3,6-tri-0_methyl-D-glucitol (1,5-anhydro-2,3,6-tri-O-methyl-cellobiitol) are a l s o reported. r
ι Results Product Analyses. The product d i s t r i b u t i o n f o r a l k a l i n e deg radations o f 1^ depended on the r e a c t i o n c o n d i t i o n s . S t a b l e , neu t r a l products i d e n t i f i e d were 1,5-anhydro-D-glucitol ( I I , 80-90$), l , 5 : 3 , 6 - d i a n h y d r o - D - g a l a c t i t o l ( I I I , 8-11$), 1,5-anhydro-D-gulitol (IV, v o l ) t o p r e c i p i t a t e excess s i l v e r i o n , and analyzed by g . l . c . (Conditions B ) . A f t e r 5^· h r , the r e a c t i o n mixture was f i l t e r e d and the r e s i d u e was r i n s e d with CHC1 (100 ml). The combined f i l t r a t e s were washed with NaHCO (150 ml) and water (lOO ml), d r i e d ( C a C l ) , and evapo r a t e d , i n vacuo » t o a t h i c k s i r u p . The s i r u p was a c e t y l a t e d with p y r i d i n e - a c e t i c anhydride (38) and separated i n t o crude mono- and d i s a c c h a r i d e f r a c t i o n s on a s i l i c a g e l column (Grace Grade 950, 60-200 mesh, 275 g; 25 χ 1000 mm) e l u t e d with chloroform-ethyl acetate (2:1, v o l ) . The d i s a c c h a r i d e f r a c t i o n was deacetylated (30) and f r a c t i o n a t e d on a s i l i c a g e l column (lOO g, 25 χ 500 mm) with chlorοform-methanol (7:1, v o l ) t o g i v e V I I as the f i r s t d i saccharide component e l u t e d . Since V I I could not be induced t o c r y s t a l l i z e , i t was d r i e d i n vacuo t o an amorphous s o l i d (32$ y i e l d ) , [a]§ 15-3° (H 0). (Found: C, 1+8.9; H, 7-6. Ci H Oio r e q u i r e s : C, 1+8.9; Η, 7-7$·)
Downloaded by CLARK UNIV on February 8, 2015 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0010.ch009
3
3
2
5
2
5
2 8
A c i d h y d r o l y s i s o f VII gave V I I I and a,3-D-glucose as d e t e r mined by g . l . c . (Conditions C) f o r the p e r - O - t r i m e t h y l s i l y l ethers.. The 3-configurâtion o f the g l y c o s i d i c l i n k a g e o f V I I was confirmed by a doublet ( H - l , δ 1+.1+5 ppm, Ji» » 7-0 Hz) i n i t s n.m.r. spec trum (D 0) which was c h a r a c t e r i s t i c o f an anomeric proton a s s o c i ated with a β-D-glueopyranosidic bond (1+0) and s i m i l a r t o the H-1 doublet o f I_ (δ 1+.51 ppm, J i » 7.0 Hz). A c e t y l a t i o n o f V I I with a c e t i c anhydride-pyridine (38) gave l,5-anhydro-2 ,3 ,l+ , 6 ' - t e t r a - O - a c e t y l ^ ^ ^ - t r i - ^ - m e t h y l - c e l l o b i i t o l ; m.p. 80-80.5°C (from MeOH), [α]§* 15-7° (CHC1 ). MlU 18.6° ( C H C I 3 ) . (Found: C, 51-3; H, 6.8. C H 6 0 n , r e q u i r e s : C, f
j 2
2
F
f
s 2
!
!
!
3
23
3
51.5; H, 6.7$.) N-Butyl β-D-glucopyranoside (XXII). D e a c e t y l a t i o n (30) o f nb u t y l tetra-O-acetyl-3-D-glucopyranoside (hi) gave XXII; m.p. 6768°C ( f r o m E t O A c ) , [a]g° - 36.7° ( H 0 ) . L i t e r a t u r e : m.p. 66-67°C [ a ] - 37.1*° (H 0) (1+2). Cyclohexyl 3 - c e l l o b i o s i d e (XXIII). Reaction o f XIX with cyclohexanol (l+l) y i e l d e d c y c l o h e x y l 3 - c e l l o b i o s i d e heptaacetate (XXIV) (62$); m.p. 202-203.5°C (from EtOH), [a]§ - 25-7° (CHCI3)· (Found: C, 53-5; H, 6.5. C H 0 i 8 r e q u i r e s : C, 53-5; H, 6.5$·) Deacetylation (30) o f XXIV gave XXIII; m.p. 206.5-207-5°C (from EtOH), [α]§ - 26.3° (H 0). (Found: C, 51.2; H, 7-6. C i e H 0 i i r e q u i r e s : C, 50.9; H, 7.6$.) The n.m.r. spectrum (D 0) o f XXIII contained two anomeric proton doublets (δ 1+.53 Ppm> J 6.9 Hz and δ 1+.59 ppm, J 7.5 Hz), both c h a r a c t e r i s t i c o f 3-glucopyranosidic 2
D
5
2
5
3 2
l f 6
5
2
3 2
2
In Cellulose Technology Research; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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bonds, thus confirming the β-configuration o f the cyclohexoxy sub stituent. Product A n a l y s i s . The presence of I I , I I I , IV, V, and VI i n r e a c t i o n mixtures was demonstrated by g . l . c . a n a l y s i s o f the pert r i m e t h y l s i l y l ethers (Conditions D) and p.c. The a n a l y s i s and i d e n t i f i c a t i o n of I I I by g.l.c.-mass spectrometry i s described i n d e t a i l elsewhere (10). In a d d i t i o n , I I I was i s o l a t e d from a l a r g e - s c a l e degradation of I_ (ça. 8 g) i n 2.5N NaOH at 170°C (71-5 hr). The r e a c t i o n s o l u t i o n was deionized (Amberlite IR-120 and Amberlite MB-3) and concentrated i n vacuo. The a c e t y l a t e d mixture was separated i n t o crude mono- and d i s a c c h a r i d e f r a c t i o n s on a s i l i c a g e l column (Grace Grade 950, 60-200 mesh, 275 g; 25 x 1000 mm) using chloroform-ethyl acetate (2:1, v o l ) as the eluant. The t r a i l i n g monosaccharide f r a c t i o n s were deacetylated and separated on s i l i c a g e l (35 g> 12 χ 500 mm) using chloroform-methanol (5:1> v o l ) t o y i e l d t . l . c . pure I I I which had i . r . and n.m.r. s p e c t r a v i r t u a l l y i d e n t i c a l with those of an authentic sample. The n.m.r. spectrum of the a c e t y l a t e d I I I was a l s o i d e n t i c a l with that o f known I I I d i a c e t a t e . K i n e t i c A n a l y s i s . A stock s o l u t i o n of 2.50N NaOH was p r e pared under a n i t r o g e n atmosphere from carbon d i o x i d e - f r e e , t r i p l y - d i s t i l l e d water (1+3). The other NaOH s o l u t i o n s were prepared from the stock s o l u t i o n u s i n g s i m i l a r water and, when a p p r o p r i a t e , NaOTs and Nal. A l l a l k a l i n e s o l u t i o n s were s t o r e d under n i t r o g e n in paraffin-lined bottles. The r e a c t o r system, described i n d e t a i l elsewhere (10), con s i s t e d of a type 316 s t a i n l e s s s t e e l r e a c t o r (100-ml c a p a c i t y ) from which samples (ca. 1 ml) could be withdrawn, and an o i l bath equipped with a Bronwell constant temperature c i r c u l a t o r which could maintain the bath w i t h i n 0.2°C o f the d e s i r e d temperature. Oxygen was desorbed from the r e a c t o r by heating the disassem b l e d r e a c t o r under vacuum (ça. 0.05 mm Hg, 1 0 5 - H 0 ° C , 2k-h& h r ) . The r e a c t o r was cooled under vacuum, and loaded (0.001 mole of r e actant; 100 ml of NaOH s o l u t i o n ) and assembled i n a n i t r o g e n atmosphere . The r e a c t o r was connected to the sampling system and immersed i n the o i l bath. The i n i t i a l sample f o r the a r b i t r a r y zero time was taken a f t e r the r e a c t o r had come t o the d e s i r e d temperature (
