SYNTHESIS AND THE ELECTRONIC-SPECTRA OF THE FIRST BETA-KETOACYLSILANES AND THEIR LITHIUM ENOLATES - NEW INSIGHTS INTO HYPERCONJUGATION IN ACYLSILANES AND THEIR ENOLATES

The reaction of tetramethyl-1,3-cyclobutanedione (1) with R(3)SiLi (R = Me(3)Si or Et) and Et(3)GeLi results in the opening of the cyclobuta nedione ring to give the corresponding beta-ketoacylsilane lithium eno lates 2a-2c, which after aqueous work-up gave the first known beta-ket oacylsilanes 3a and 3b and beta-ketoacylgermane 3c. The first X-ray st ructure of a beta-ketoacylsilane, that of lithium enolate 2a, is repor ted and discussed. The UV-visible spectra of the lithium enolates 2 ex hibits two new transitions: one absorption is ''red'' shifted and the other is ''blue'' shifted (each by about 40-50 nm) relative to the abs orptions of the corresponding beta-ketoacylsilanes. Ab initio molecula r orbital calculations show that the ''red-shifted'' transitions resul t from the presence of a low-lying Rydberg-type antibonding O-Li orbit al, while the ''blue-shifted'' transition results from a weakening (du e to Li+ complexation) of the destabilizing hyperconjugative interacti ons between the oxygen lone pair (n(O)) and the sigma(C-Si) orbital, w hich leads to a lowering of the energy of the filled (n(O)-sigma(C-Si) ) orbital (relative to its energy in the acylsilanes), and thus to a h igher (n(O)-sigma(C-Si))-->pi(C-O) excitation energy than in the corr esponding acylsilanes.