The structure and thermal properties of novel DOPO-containing 1,3-benzoxazines

Three DOPO (9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide)-containing benzoxazin (3a–3c) were successfully prepared from 2-hydroxybenzaldehyde, p-substituted aniline, and DOPO by a three-step procedure. Since the phosphorus and the adjacent aliphatic carbon are both chiral centers, two pairs of enantiomers exist in 3a, 3b, or 3c. Compound 3a′, which is a pair of enantiomers existing in 3a, was isolated using recrystallization in ethyl acetate solvent. All the structures were confirmed by FT-IR, NMR, and MS spectra. The X-ray analysis indicated that the 3a′ is the RR/SS enantiomers and belongs to the monoclinic crystal system, its space group being P2₁/c. The thermal properties were investigated by DSC and TG. It has been found that the peak temperature of thermally induced ring-opening reaction was around 260–270 °C. Compounds (3a–3c, 3a′, and 3c′) had two similar thermal processes from 50 to 600 °C in TG, whereas the compound 3b′ had three thermal processes. The char yields of 3a–3c were 18.10, 16.84, and 16.34 %, respectively, while those of 3a′–3c′ were 26.00, 31.80, and 19.21 %, respectively. The results indicated that compounds 3a′–3c′ had better char properties than 3a–3c.