Acac-Promoted Rearrangement of an Alkylaluminophosphonate Tetramer to a Decamer

Abstract

Reaction of MeP(O)(OH)2 with tBu3Al at low temperature and subsequent trimethylsilylation of the crude reaction product with Me3SiNMe2 yielded the cyclic dimer [tBu2AlO2P(OSiMe3)Me]2 (1). In contrast, reaction of MeP(O)(OH)2 with tBu3Al in refluxing toluene/THF yielded a mixture of [tBuAlO3PMe]4 (2), [tBuAlO3PMe]6 (3), and [tBuAlO3PMe]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 CHCl3 solution of acetylacetone into a solution of 2 in poly(ethylene oxide)/CHCl3 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, CDCl3 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 (1H, 13C, 31P) NMR spectroscopy, infrared spectroscopy, mass spectrometry, and elemental analysis. The molecular structures for compounds 2−4 were determined by X-ray crystallography.