1,5-Migrations of silicon between oxygen centers in silyl β-diketones

A series of acyclic silyl diketonates, (CH₃)₃Si(dik), where dik = dipivaloyl-methanate, diisobutyrylmethanate and hexafluoroacetylacetonate, along with a new series of cyclic acetylacetonates of the type CH₃(-CH₂(CH₂)_xCH₂-)Si(acac), where x = 1, 2, and 3, have been shown to possess enol ether structures in which the acyl group is positioned cis or trans to the silyl group. The ratios of cis to trans configurations in the (CH₃)₃Si(dik) series increase from <0.02 to >50.0 in the order of diketonate substituents CF₃ < CH₃ <i-C₃H₇ < t-C₄H₉. The ratios for the CH₃(-CH₂(CH₂)xCH₂-)Si(acac) compounds increases in the order χ = 1 > 2 > 3. These data are interpreted in terms of incipient pentacoordination of silicon by the carbonyl oxygen atom in the cis isomer. Rates of 1,5-migration of silicon between oxygen centers in the cis isomers have been determined by NMR spectroscopy. The dependence of the rearrangement rates on diketonate substituent and angle strain at silicon indicate that the migration process is better viewed as an internal nucleophilic displacement, rather than a sigmatropic shift. 1,5-Migration in the chiral (C₆H₅CH₂)(CH₃)(C₆H₅)Si derivative of dipivaloylmethanate occurs exclusively with retention of configuration at silicon. The large difference in activation energy for migration and inversion (>18 kcal/mol) precludes the possibility of distinguishing between a stepwise and concerted displacement mechanism.