Hydroxypropyl Lignins and Model Compounds Synthesis and

Chapter 33

Hydroxypropyl Lignins and Model Compounds Synthesis and Characterization by Electron-Impact Mass Spectrometry

Downloaded by UNIV OF CINCINNATI on May 31, 2016 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0397.ch033

John A . Hyatt Research Laboratories, Eastman Chemicals Division, Eastman Kodak Company, Kingsport, TN 37662

A n iterative method for the controlled rational syn thesis of chain-extended hydroxypropyl derivatives of both aliphatic and aromatic hydroxyl compounds, based on oxymercuration of allyl ethers, is described. It is shown that the degree of chain extension in such com pounds can be determined by electron impact mass spectrometry. Degradation of hydroxypropylated lignins by catalytic hydrogenolysis followed by capillary gas chromatography-mass spectrometry is demonstrated to be a method for establishing the presence and degree of chain extension in the hydroxypropyl lignin polymers. O n e a t t r a c t i v e a p p r o a c h , pioneered b y G l a s s e r a n d co-workers (1), t o t h e conversion o f l i g n i n i n t o a higher-value m a t e r i a l comprises r e a c t i o n o f l i g n i n w i t h propylene oxide or ethylene oxide t o produce a n a l i p h a t i c p o l y o l s u i t able for use i n p r e p a r i n g network p o l y m e r s such as p o l y u r e t h a n e foams a n d plastics. L i g n i n s are c o m p l e x m i x t u r e s o f m o d e r a t e - t o - h i g h m o l e c u l a r weight p h e n y l p r o p a n o i d p o l y m e r s w h i c h bear phenolic a n d p r i m a r y a n d secondary a l i p h a t i c h y d r o x y l groups (2), a n d conversion o f t h i s m a t e r i a l f r o m , i n effect, a m i x t u r e o f different types o f alcohols t o a substance h o mogeneous i n terms o f h y d r o x y l type a n d r e a c t i v i t y g r e a t l y improves i t s u t i l i t y as a p o l y o l . Hydroxypropylation of lignins can be carried out under a variety of c o n d i t i o n s , b u t the most successful approach has i n v o l v e d r e a c t i o n o f l i g n i n w i t h propylene oxide i n t h e presence o f basic c a t a l y s t s ; t h e l i g n i n is sus p e n d e d i n toluene or dissolved i n aqueous base ( 3 , 4 ) . T h e v a r i a t i o n o f p o l y o l properties w i t h p r e p a r a t i v e m e t h o d (4) brings u p a f u n d a m e n t a l question o f h y d r o x y p r o p y l l i g n i n s t r u c t u r e : I n t h e r e a c t i o n o f l i g n i n s w i t h propylene oxide ( F i g . 1), w h a t is the p r o d u c t d i s t r i b u t i o n a n d degree o f c h a i n extension o f a l k o x y p r o p y l groups? A r e there a significant n u m b e r o f 0097-6156/89/0397-0425$06.00/ϋ © 1989 American Chemical Society

Glasser and Sarkanen; Lignin ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

426

LIGNIN: PROPERTIES AND MATERIALS

hydroxypropylated aliphatic hydroxy Is in the product, or does most of the propylene oxide reside on the phenolic hydroxyls of the lignin nucleus? Although techniques such as N M R spectroscopy can provide valuable information about the structure of hydroxypropyl lignins (4), we perceive a need for the development of synthetic methodology for model compounds which incorporate the structural features thought to be present in hydrox ypropyl lignins. Such compounds can be used for studies of reaction kinetics as well as for confirming spectral assignments.


Synthesis and Mass Spectroscopy of M o d e l Compounds Although it is a simple matter to attach a single hydroxypropyl residue to a phenolic hydroxyl, the controlled synthesis of chain-extended hydrox ypropyl ethers is much more difficult (Fig. 2) (5). T h e difficulty is that of selectively alkylating an aliphatic hydroxyl on the substrate in the pres ence of aliphatic hydroxyls on hydroxypropyl groups. Thus we required a method for the preparation of compounds of the type shown in Figure 2 where η is precisely known and controlled (rather than being a statistical range of values). Our solution to this synthetic problem was the development of an iterative technique for preparing hydroxypropyl ethers from allyl ethers via oxymercuration-reduction. Figure 3 illustrates the process for the preparation of a series of three chain-extended hydroxypropyl derivatives of 2,6-dimethoxyphenol. Conversion of phenol 1 to the allyl ether 2 under phase-transfer conditions (6) was followed by oxy mer cur at ion (7) to give the intermediate organomercurial 3, which was reduced without isolation to give hydroxypropyl ether 4 in 64% overall yield. Ether 4. was then allylated to provide 5, which upon oxymercuration-reduction af forded hydroxypropyl derivative 6. One further iteration of the allylationoxymercuration-reduction sequence yielded the hydroxypropyl compound 7. We have found that this sequence can be generally applied to the syn thesis of hydroxypropyl derivatives of alcohols and phenols; yields are uni formly acceptable and products of the desired degree of chain extension can be prepared completely free of lower and higher oligomers. The compounds shown in Table I were prepared using this chemistry. In the course of characterizing the hydroxypropyl ethers prepared in this study, we noticed an intriguing pattern in the mass spectral fragmenta tion of these compounds. As shown in Table I, the degree of chain extension is reflected in the fragmentation pattern: Alcohols modified by a single hy droxypropyl ether unit have a strong M-58 peak, ethers of chain extension degree of 2 show a loss of m/e 116 and 58, and tripropylene glycol deriva tives (degree of chain extension = 3) lose m / e 174, 116, and 58. Figure 4 shows the mass spectrum of compound 6. The loss of m / e 116 for this dipropylene glycol ether leads to the base peak at m / e 168, and the loss of m / e 58 to m / e 226 is the second most important fragmentation. In the case of all of the compounds in Table I, loss of the entire propylene glycol ether side chain provided either the base peak of the mass spectrum or the second strongest peak present. It is clear from this data that the length


33.

HYATT

427

Hydroxypropyl Lignins and Model Compounds

CHJJOH

CH-O-Aryl-

j-O-CH

CH-O-ArylCH3CH—»CH

-O-CH 2


Various Conditions

F i g u r e 1. H y d r o x y p r o p y l a t i o n o f l i g n i n .

Ο

/\

CH OH 2

CH3-CH—CH

OCH3

CH OH 2

2

CH3O

OCH3

OH

Ο

/\ CH3-CH—CH

2

(Value of η is not controllable) F i g u r e 2. H y d r o x y p r o p y l a t i o n o f phenols a n d alcohols.



Γ

M

3

N

OH

/-CH O

o

>

'-\

CH -/

C H

Ν

C H

O

*

/-CH HO

3-\

\ 3

CHaO^^^OCHa

Γ

3

2

2

2

3

' 51%

NaBH4

C

H

3

N

° CH -(

3

M

4

Oh

O ^ Y ^

Γ

4

2

CH HgOAc

NaBH -OH 76%

O

Ο ^ Ο ^ ^ Ν ^ ^ OCH3

1. CH = CH-CH Br 2. Hg(OAc)

2

XX

F i g u r e 3. I t e r a t i v e synthesis o f h y d r o x y p r o p y l ethers.

3

50%

4

3. NaBH

2

OCH

OCH CH = CH

CHaO'Ç^ 2

84%

OH

3

1

^OCH

CH3O^ ^ y ^ O C H a

3

CH 0^

2

H 0, THF

2

PTC

2

Hg(OAc)

2

CH = CH-CH Br


3

H

s

2 3

ο

00

6


33.

HYATT


Table I.

154

Electron Impact Mass Spectral Fragmentation of Hydroxypropyl Lignin Model Compounds 212

8


429

Glasser and Sarkanen; Lignin ACS Symposium Series; American Chemical Society: Washington, DC, 1989. Figure 4. Mass spectrum of Compound 6.


ο

1

S

33.

HYATT


431

o f a n oligo(propylene glycol) c h a i n a t t a c h e d t o a n o r g a n i c a l c o h o l c a n be d e t e r m i n e d b y e x a m i n a t i o n of i t s mass s p e c t r u m .


Degradation and Analysis of H y d r o x y p r o p y l Lignin T h e results described above i n d i c a t e d t h a t , i f a w a y c o u l d be f o u n d t o degrade a h y d r o x y p r o p y l l i g n i n t o v o l a t i l e fragments s u i t a b l e for a n a l y s i s b y c a p i l l a r y gas c h r o m a t o g r a p h y / m a s s s p e c t r o m e t r y , m u c h c o u l d be l e a r n e d a b o u t the d i s t r i b u t i o n a n d l e n g t h of h y d r o x y p r o p y l residues present o n the l i g n i n . T h e k n o w n a b i l i t y t o degrade l i g n i n s b y hydrogenolysis (8), c o u p l e d w i t h the r e d u c t i v e s t a b i l i t y o f s i m p l e a l k y l a n d a r o m a t i c ethers, p r o m p t e d us t o e x a m i n e the scheme s h o w n i n F i g u r e 5. T w o c o n t r o l e x p e r i m e n t s were first c o n d u c t e d . I n the first, m o d e l c o m p o u n d 7 was subjected to the hydrogenolysis c o n d i t i o n s a n d t h e n a n a l y z e d b y V P C / M S . D e s p i t e hydrogenolysis at 285°C over 5 % P d / C c a t a l y s t u n der 5000 p s i H 2 , the m o d e l c o m p o u n d was recovered i n t a c t ; n o evidence of r i n g r e d u c t i o n or h y d r o x y p r o p y l c h a i n cleavage was seen i n the mass spect r u m . I n the second c o n t r o l e x p e r i m e n t , a s a m p l e of m e t h a n o l organosolv l i g n i n ( h a r d w o o d ) was h y d r o g e n a t e d under the same c o n d i t i o n s a n d the v o l a t i l e fractions recovered b y h i g h v a c u u m d i s t i l l a t i o n . V P C / M S a n a l y s i s o f the v o l a t i l e fractions ( w h i c h a m o u n t e d t o 2 8 - 3 8 % o f the l i g n i n charged) l e d t o i d e n t i f i c a t i o n o f the c o m p o u n d s l i s t e d i n T a b l e I I . It is c r u c i a l t o note t h a t none o f the c o m p o u n d s p r o d u c e d b y hydrogenolysis o f u n m o d i fied l i g n i n d i s p l a y e d a mass s p e c t r a l f r a g m e n t a t i o n of m / e 58 or a m u l t i p l e thereof. T h u s we have established t h a t h y d r o x y p r o p y l a t e d l i g n i n moieties s u r v i v e the hydrogenolysis c o n d i t i o n s , a n d t h a t the hydrogenolysis p r o d ucts of u n m o d i f i e d l i g n i n are free of s t r u c t u r e s w h i c h w o u l d interfere w i t h mass s p e c t r a l i d e n t i f i c a t i o n of h y d r o x y p r o p y l a t e d c o m p o n e n t s . W h e n the h y d r o g e n o l y s i s / d i s t i l l a t i o n / V P C / M S sequence was c a r r i e d o u t o n a s a m p l e of l i g n i n w h i c h h a d been m o d i f i e d b y r e a c t i o n w i t h p r o p y lene oxide i n toluene i n the presence of K O H / 1 8 - c r o w n - 6 c a t a l y s t , inspect i o n o f the r e s u l t i n g mass s p e c t r a for c o m p o u n d s h a v i n g m / e 58 f r a g m e n t a t i o n s ( a n d m u l t i p l e s thereof) led to i d e n t i f i c a t i o n of the m a t e r i a l s l i s t e d i n T a b l e I I I . It w i l l be seen t h a t several m o n o - h y d r o x y p r o p y l a t e d m a t e r i a l s a n d three d i - h y d r o x y p r o p y l ( c h a i n extension degree = 2) c o m p o u n d s were f o u n d . N o p r o d u c t s h a v i n g h y d r o x y p r o p y l chains longer t h a n degree 2 were f o u n d ; t h u s i t appears t h a t t h i s p a r t i c u l a r m o d i f i e d l i g n i n contains m a n y short h y d r o x y p r o p y l chains, r a t h e r t h a n a few chains o f greater degree of extension. S i m i l a r a n a l y s i s of a l i g n i n m o d i f i e d b y h y d r o x y p r o p y l a t i o n i n aqueous base y i e l d e d o n l y p r o d u c t s b e a r i n g single h y d r o x y p r o p y l residues. T h i s a p p a r e n t lack of c h a i n extension is of course consistent w i t h the fact t h a t the h y d r o x y p r o p y l a t i o n was c a r r i e d out under c o n d i t i o n s f a v o r i n g r e a c t i o n o n l y o n phenolic h y d r o x y l groups. A r e a s of u n c e r t a i n t y i n t h i s m e t h o d o f a n a l y z i n g m o d i f i e d l i g n i n s i n clude the p r o b l e m t h a t l i t t l e m o r e t h a n o n e - t h i r d of the l i g n i n is converted t o v o l a t i l e p r o d u c t s by the hydrogenolysis r e a c t i o n ; the n o n - v o l a t i l e f r a c t i o n cannot be a n a l y z e d b y V P C / M S . T h i s w i l l c o n s t i t u t e a difficulty o n l y i f the


432

LIGNIN: PROPERTIES AND MATERIALS Hydroxypropyl Lignin

1

H Pd/c, >250 ,>3000 PSI H, lf

Hydrogenated Hydroxypropyl Lignin

I

High Vacuum Distillation

Volatile Fractions Capillary VPC Mass Spectral Analysis


M-58, M-116, M-174,...etc F i g u r e 5. H y d r o x y p r o p y l l i g n i n d e g r a d a t i o n a n d a n a l y s i s scheme.

Table II. Products identified in the VPC/MS Analysis of the Volatile Fraction from Unmodified Lignin

Note: None of these compounds showed m/e M-58, or multiples thereof.


33.

HYATT


433

Table III. Compounds Identified in the VPC/MS Analysis of a Hydrogenated Hydroxypropyl Lignin Peak No.

m/e

Fragmentation

Structure


CH

786

226

3

M-15, -58

C2H5

CH

2

818

854

873

168

M-15, -58

???

Continued on next page


434

LIGNIN: PROPERTIES AND MATERIALS

Continued


Table III.


33.

HYATT


435

d i s t r i b u t i o n a n d degree of c h a i n extension o n the p a r t of the l i g n i n molecules w h i c h r e m a i n n o n - v o l a t i l e differ f r o m t h a t o n t h e p a r t o f the molecules w h i c h are converted t o v o l a t i l e fragments. W h i l e t h i s seems a n u n l i k e l y scen a r i o , i t i s a t present i m p o s s i b l e t o establish the p o i n t w i t h a n y certainty. A second area o f u n c e r t a i n t y arises f r o m the fact t h a t the m e t h o d has been tested o n l y w i t h m o d e l c o m p o u n d s u p t o degree of c h a i n e x t e n s i o n = 3 (i.e., c o m p o u n d 7). W e d o not yet k n o w whether longer h y d r o x y p r o p y l residues w o u l d b e degraded b y these procedures; t h i s p o i n t w i l l be established i n t i m e b y the synthesis o f a d d i t i o n a l m o d e l c o m p o u n d s .


Summary W e have developed a s i m p l e , i t e r a t i v e s y n t h e t i c m e t h o d for the p r e p a r a t i o n of h y d r o x y p r o p y l derivatives of phenolic a n d a l i p h a t i c alcohols w h i c h allows complete d e f i n i t i o n a n d c o n t r o l o f the degree o f c h a i n extension i n t h e p r o d ucts. T h i s m e t h o d o l o g y has been a p p l i e d t o the p r e p a r a t i o n o f a series o f l i g n i n m o d e l c o m p o u n d s h a v i n g h y d r o x y p r o p y l c h a i n e x t e n s i o n degrees o f 13. W e have s h o w n t h a t the degree of chain extension i n such c o m p o u n d s c a n be u n a m b i g u o u s l y defined b y analysis o f t h e i r electron i m p a c t mass spect r a . I t has furthermore been d e m o n s t r a t e d t h a t such c o m p o u n d s are stable to a h y d r o g e n o l y s i s / d i s t i l l a t i o n / V P C / M S sequence w h i c h , w h e n a p p l i e d t o h y d r o x y p r o p y l l i g n i n s , defines the site a n d degree o f h y d r o x y p r o p y l a t i o n o n the v o l a t i l e f r a c t i o n o f the l i g n i n residues. T o p i c s for f u r t h e r i n v e s t i g a t i o n i n c l u d e d e t e r m i n a t i o n of the degree o f c h a i n e x t e n s i o n at w h i c h the m e t h o d breaks d o w n , a n d the n a t u r e o f the l i g n i n segments w h i c h are not rendered v o l a t i l e b y hydrogenolysis. Acknowledgment W e t h a n k P r o f . W . Glasser for p r o v i d i n g several h y d r o x y p r o p y l a t e d phenols a n d l i g n i n samples used i n t h i s study. Literature C i t e d 1. Rials, T.; Glasser, W . Holzforschung 1986, 40, 353, and references cited therein. 2. Sarkanen, K . ; Ludwig, C . Lignins. Occurrence, Structure, and Reactions; Wiley-Interscience: New York, 1971. 3. Wu, L.; Glasser, W . J. Appl. Polym. Sci. 1984, 29, 1111. 4. Glasser, W.; Barnett, C.; Rials, T.; Saraf, V . J. Appl. Polym. Sci. 1984, 29, 1815. 5. Gee, G . ; Higginson, W . ; Levesley, P.; Taylor, K . J. Chem. Soc. 1959, 1338. 6. Starks, C.; Liotta, D . Phase Transfer Catalysis; Academic Press: New York, 1978. 7. House, H . Modern Synthetic Reactions; 2nd ed.; W . A . Benjamin: New York, 1972; pp. 387-396. 8. Hoffmann, P.; Schweers, W . Holzforschung 1975, 29, 74. RECEIVED March 17,1989


Hydroxypropyl Lignins and Model Compounds Synthesis and

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