Acac-Promoted Rearrangement of an Alkylaluminophosphonate Tetramer to a Decamer

Reaction of MeP(O)(OH) 2 with t Bu 3 Al at low temperature and subsequent trimethylsilylation of the crude reaction product with Me 3 SiNMe 2 yielded the cyclic dimer [ t Bu 2 AlO 2 P(OSiMe 3 )Me] 2 ( 1 ). In contrast, reaction of MeP(O)(OH) 2 with t Bu 3 Al in refluxing toluene/THF yielded a mixture of [ t BuAlO 3 PMe] 4 ( 2 ), [ t BuAlO 3 PMe] 6 ( 3 ), and [ t BuAlO 3 PMe] 10 ( 4 ). Crystallization and sublimation of the crude mixture gave compound 2 in 54% yield. Small quantities of pure 3 were obtained by fractional crystallization of the remaining reaction products. Crystalline 3 was also obtained by slow diffusion of a CHCl 3 solution of acetylacetone into a solution of 2 in poly(ethylene oxide)/CHCl 3 gel. It was not possible to obtain reproducible yields of pure 4 by fractional crystallization of the crude mixture of 2 − 4 . Instead, compound 4 was obtained in 86% yield by an acetylacetone-promoted rearrangement of 2 . In the absence of acetylacetone, CDCl 3 solutions of 4 rearrange to a mixture of 2, 3, and 4 over a period of days. The role of acetylacetone in the rearrangement of 2 to 4 has not been elucidated. Compounds 1 − 4 were characterized by multinuclear ( 1 H, 13 C, 31 P) NMR spectroscopy, infrared spectroscopy, mass spectrometry, and elemental analysis. The molecular structures for compounds 2 − 4 were determined by X-ray crystallography.