SO2 H2. O

+

Sulfonation

Condensation

-ROH

5 6

Scheme 4.34 Reaction of b-O-4 aryl ether structures (according to [11]).

Phenolic pinoresinol structures (Scheme 4.35) are opened and the intermediate

benzylium cation undergoes an intramolecular electrophilic aromatic substitution

at C6 of the adjacent aromatic ring. This intramolecular condensation process is

favored due to the close proximity of the adjacent ring, the a-carbon of the side

chain being subsequently sulfonated. Nonphenolic pinoresinols are less reactive.

HC

HC

H C 2

O CH

O CH2

CH

OH

OH

OMe

OMe

SO2

OH

OMe

SO3H

OH

OH

HO

OMe

H2. O

9 10

Scheme 4.35 Reaction of pinoresinol structures (according to [11]).

Phenolic phenylcoumaran (Scheme 4.36) structures also show the possibility for

condensation reactions if the reactive centers are close enough to the benzylium

cation. Possible routes for the formation of 12 by opening the hetero-ring, recyclization

and sulfonation are discussed in more detail by Gellerstedt and Gierer [6].

410 4 Chemical Pulping Processes

CH2OH

O

OH

OH

OMe

MeO

O

SO3H

OH

OMe

CH2OH

MeO

SO2

11 12

H2O

Scheme 4.36 Reaction of phenolic phenylcoumarans under acidic sulfite conditions [5].

Sulfonation of other positions than Ca has been demonstrated with Ca

–-carbonyl

compounds (see Scheme 4.39) and 1,2-diarylpropane structures (b–1, cf.

Scheme 4.37). The latter is converted into stilbene structures upon elimination of

formaldehyde, or to the corresponding c-sulfonated product after elimination of

water and addition of sulfite to the allylic carbonium ion (Scheme 4.38) [5]. The

formaldehyde can be further oxidized by hydrogen sulfite to carbon dioxide and

water.

CH2OH

OH

OH

OMe

RO OMe

OH

OH

OMe

RO OMe

-HCOH

13 14

Scheme 4.37 Reaction of b–1 structures to stable stilbenes [5].

CH2OH

OH

OH

OMe

OMe

CH2+

OH

OH

OMe

OMe

-H2O

HSO3

-

H

+

CH2SO3H

OH

OH

OMe

OMe

15 16 17

Scheme 4.38 Reaction of stilbenes [5].

4.3 Sulfite Chemical Pulping 411

Phenylpropane a-carbonyl-b-arylether structures react also by elimination of

water from the c-hydroxyl group and addition of hydrogen sulfite to the generated

electrophilic center (Scheme 4.39).

OH

OMe

O

O

HO OH

OMe

H

+

OH

OMe

HO

O

CH2

OH

OMe

-H2O

OH

OMe

HO

O

OH

OMe

HO3S

HSO3

+

-

18 19 20

Scheme 4.39 Sulfonation of phenylpropane a-carbonyl-b-arylether structures [5].

Coniferylaldehyde units are sulfonated at the a-position via the allylic carbonium

ion, which is formed after addition of a proton, in a Michael-type addition.

Sulfonation of the c-carbon is only observed under neutral sulfite cooking conditions

with coniferyl alcohol and with coniferylbenzoate at a pH of 3–4 [12].

OR

OMe

CHO

OR

OMe

CHOH

OR

OMe

CHO

HO3S

H

+ + HSO3

-

+

+

21 22 23

Scheme 4.40 Sulfonation of the a-position of coniferyl aldehyde-type structures.

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