Secondary hydroxyl groups terminal

Scheme 4 outlines the synthesis of key intermediate 7 in its correct absolute stereochemical form from readily available (S)-(-)-malic acid (15). Simultaneous protection of the contiguous carboxyl and secondary hydroxyl groups in the form of an acetonide proceeds smoothly with 2,2 -dimethoxypropane and para-toluene-sulfonic acid and provides intermediate 26 as a crystalline solid in 75-85 % yield. Chemoselective reduction of the terminal carboxyl group in 26 with borane-tetrahydrofuran complex (B H3 THF) affords a primary hydroxyl group that attacks the proximal carbonyl group, upon acidification, to give a hydroxybutyrolactone. Treat-... 

Subjection of intermediate 16 to the action of 3 n aqueous HC1 in THF results in the formation of monocyclic lactol 14. In the presence of aqueous acid, the internal acetal grouping in intermediate 16 is hydrolyzed and lactol 14 is produced after the liberated secondary hydroxyl group attacks the terminal aldehyde carbonyl positioned five atoms away (see intermediate 15). Protection of the free aldehyde function in 14 with 1,1-dimethylhydrazine proceeds smoothly under dehydrating conditions and affords intermediate 13 in an overall yield of 72 %. 

Thanks to the difference of reactivity between primary and secondary hydroxyl groups, mono and ditelomers corresponded only to the reactivity of the terminal alcohols (Fig. 3). 

Polyethers. Three structurally different polyether diols are mainly used poly(1,2-oxypropylene)diol (PPG), poly(1,2-oxybutylene)diol (B-2000), both terminated essentially by secondary hydroxyl groups, and poly (l,4-oxybutylene)diol, a considerably more reactive component owing to primary hydroxyl termination. Of these the poly(oxypropylenes)... 

The reaction rates of the various ingredients must be understood to follow the probable course of the reaction. Certain polyols may have secondary hydroxyl groups that are very much slower in reaction than the primary hydroxyl groups. The preparation of a prepolymer will allow the secondary hydroxyls to fully react. The reaction of a 100-percent 2,4 TDI is much faster than the standard 80 20 TDI isomer blend. The reaction rate between the terminal NCO and chain extender can be adjusted to allow the prepolymer mix to fill the mold and allow entrapped air to escape before the material gels. 

Dicyandiamide (DICY) is a solid latent catalyst that reacts with both the epoxy terminal groups and the secondary hydroxyl groups. DICY has the advantage that it only reacts with the epoxy resins on heating beyond an activation temperature, and once the heat is removed, the reaction stops. It is widely used with epoxy resins where long shelf life (up to... 

Short-chain diols are used as chain extenders, especially when both hydroxyl groups are primary. Secondary hydroxyl groups react more slowly with isocyanates but first can be capped with ethylene oxide (EO) to give the primary hydroxy-terminated derivatives. 

As an immediate consequence, the oligo-polyols containing terminal primary hydroxyl groups are more reactive in the reaction with isocyanates than the oligo-polyols having only secondary hydroxyl groups. The primary hydroxyl content of an oligo-polyol is an important characteristic, because it is possible to determine the potential reactivity with isocyanates. 

The most important characteristics of industrially produced polyether diols and triols homopolymers of propylene oxide are presented in Tables 4.4 and 4.5. All the polyether PO homopolymers, diols or triols have mostly secondary hydroxyls as terminal groups (94-96 % secondary hydroxyls) (see Figure 4.14). 

This structure was used successfully for continuous slabstock flexible PU foams. The polyethers have a MW of 3000-3600 daltons (hydroxyl number of 46-56 mg KOH/g), a viscosity of 550-650 mPa-s, have practically only terminal secondary hydroxyl groups and contain around 10-12% EO internally distributed. 

Of course the polyether polyol having terminal hydroxyl groups is not inert in propoxylation reactions. It was observed experimentally that, due to the presence of primary hydroxyl groups in all solid polyols, PO reacts preferentially with the solid polyol and not with the polyether which generally has only secondary hydroxyl groups. On the other hand, the superior reactivity of short chains, as compared with long chains, is another element which makes PO react preferentially with the solid polyol. For example, in the presence of amines used as catalysts, the polyether reacts with only 10-15% of the total PO used for synthesis. 

It is very interesting that a resorcinol diol based on propylene carbonate has 85% secondary hydroxyl groups and 15% primary hydroxyl groups. In the resorcinol diols based on propylene carbonate and ethylene carbonate (terminal units), the terminal primary hydroxyl is around 50% [33, 34]. 

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