Reactions of phenolic ester alcohol with aliphatic isocyanates - Transcarbamoylation of phenolic to aliphatic urethane: A C-13-NMR study

Reactions of aliphatic isocyanates with a phenolic ester alcohol (PHEA) wer e investigated using C-13-NMR spectroscopy. PHEA has two reactive sites: a phenolic -OH group and a secondary aliphatic -OH group. Both -OH groups rea ct with the isocyanate groups. With an organotin catalyst, dibutyltin dilau rate (DBTDL), the aliphatic -OH group reacts first. With a tertiary amine c atalyst, 1,4-diazabicyclo[2.2.2]octane (DABCO), or triphenylphosphine (Ph3P ) or even in the absence of a catalyst at room temperature (RT) the phenoli c -OH group reacts first. With the organotin catalyst, the reactions are ge nerally complete in a day at RT. With DABCO or triphenylphosphine or DNNDSA catalysts, the reactions are almost complete only in 3-4 days at RT in eth yl acetate or acetonitrile. Uncatalyzed reactions are slower. With an acid catalyst such as dinonylnaphthalenedisulfonic acid (DNNDSA), both -OH group s react with the isocyanate. When equimolar quantities of PHEA and hexameth ylenediisocyanate (HDI) polymerize at RT or reflux in the presence of a cat alyst, both -OH groups react, with the phenol reacting slowly. Upon refluxi ng, the phenolic -OH-based urethane slowly rearranges (transcarbamoylation) to the aliphatic -OH-based urethane. DABCO and Ph3P catalysts effect this rearrangement at a much slower rate than does the acid catalyst. In the pre sence of a catalytic amount of DBDTL in a refluxing solvent, this rearrange ment is complete in 2 h. By refluxing the phenolic-OH-based urethane in iso propanol, the mechanism of transcarbamoylation was found to be intermolecul ar. The mechanism is likely to involve deblocking of the phenolic urethane and subsequent reaction of the isocyanate generated, with the aliphatic -OH group. This conclusion was confirmed by differential scanning calorimetry (DSC) experiments. (C) 2000 John Wiley & Sons, Inc.