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

The phenolic OH moiety in lignin is one of the key functional groups respon sible 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-methoxyac etophenone (2) were reacted with 5-chloro-1-phenyl-1H-tetrazole to give the corresponding tetrazole ethers 1' and 2', respectively, that were then stu died for their photostability. The synthesis of these ethers was more effic ient than that of alkyl ethers because of less hydrolysis of the alkylating agent under the reaction conditions. Compounds 1' and 2' were fully charac terized, including X-ray crystal structure analyses. Crystals of 1' and 2' were monoclinic of space groups P2(1)/c and P2(1)/a, respectively. For 1': a = 9.8679(6), b = 16.708(1), c = 10.2841(6) Angstrom, beta = 109.732(5)(o) , Z = 4. For 2': a = 7.7212(2), b = 27.350(5), c = 14.569(3) Angstrom, beta = 101.30(2)(o), Z = 8. The structures were solved by direct methods and re fined by full-matrix least-squares procedures to R = 0.039 (R-w = 0.046) fo r 2029 reflections with I greater than or equal to3 sigma (I) (for 1'), and to R = 0.055 (R-w = 0.073) for 3634 reflections with I greater than or equ al to3 sigma (I) (for 2'). Compound 2' was much more photostable than the p recursor phenol on exposure to strong fluorescent light, while 1' was only slightly more stable than the precursor phenol.