Tetrazole ethers from lignin model phenols: synthesis, crystal structures, and photostability

Abstract

The phenolic OH moiety in lignin is one of the key functional groups responsible for the photo-induced yellowing of mechanical wood pulps and papers. To evaluate new protective groups for the stabilization of lignin phenols, the model compounds, 2-methoxy-4-propylphenol (1) and 4-hydroxy-3-methoxyacetophenone (2) were reacted with 5-chloro-1-phenyl-1H-tetrazole to give the corresponding tetrazole ethers 1' and 2', respectively, that were then studied for their photostability. The synthesis of these ethers was more efficient than that of alkyl ethers because of less hydrolysis of the alkylating agent under the reaction conditions. Compounds 1' and 2' were fully characterized, including X-ray crystal structure analyses. Crystals of 1' and 2' were monoclinic of space groups P21/c and P21/a, respectively. For 1': a = 9.8679(6), b = 16.708(1), c = 10.2841(6) Å, β = 109.732(5)°, Z = 4. For 2': a = 7.7212(2), b = 27.350(5), c = 14.569(3) Å, β = 101.30(2)°, Z = 8. The structures were solved by direct methods and refined by full-matrix least-squares procedures to R = 0.039 (Rw = 0.046) for 2029 reflections with I [Formula: see text] 3σ(I) (for 1'), and to R = 0.055 (Rw = 0.073) for 3634 reflections with I [Formula: see text] 3σ(I) (for 2'). Compound 2' was much more photostable than the precursor phenol on exposure to strong fluorescent light, while 1' was only slightly more stable than the precursor phenol.Key words: alkylation, ethers, lignin model compounds, methylation, phenols, yellowing inhibition.