Dinuclear Silicon Complexes of Uracil, Barbituric Acid, and 5,5-Dimethylbarbituric Acid: Hydrolytic Formation of Cyclic Oligomers

Abstract

The synthesis and properties of dinuclear silicon complexes derived from uracil, barbituric acid, and 5,5-dimethylbarbituric acid, through the reaction with (chloromethyl)dimethylchlorosilane and (chloromethyl)dichloro(methyl)silane are described. The oxygen atoms are weakly coordinated to silicon in all three groups of compounds. Mild hydrolysis of the dinuclear compounds leads to formation of cyclic oligomers, with 2-9 monomer units in each, in which the individual units are connected through siloxane groups. Lithium chloride is shown to act as an effective template, restricting oligomerization to dimers and preventing higher-mass product formation.

Study of Binuclear Silicon Complexes of Diketopiperazine at SN2 Reaction Profile

“…Neither peak appears at the chemical shift expected for full penta- or completely tetracoordinated silicon. The first two possibilities consistent with the data are that one silicon is more pentacoordinated than the other, but neither is fully coordinated and they are undergoing slow exchange, or if each is fully tetra- or pentacoordinate, then the peaks have shifted closer together through exchange at a rate close to the coalescence rate, Table .…”

“…The rate of exchange for 3b (X = Cl) and 3c (X = OTF) was found to be intermediate on the NMR time scale at room temperature in CDCl 3 /MeOH. As a result, only one averaged 29 Si broad peak was observed in the spectrum for each ( 3b , c ) of the complexes (Figure Table ), at a chemical shift appropriate for a fully pentacoordinate silicon on one site and a tetracoordinate silicon at the other site undergoing exchange at an intermediate rate on the NMR time scale. , Generally the 29 Si NMR chemical shift in related pentacoordinate silicon compounds is found at about −30 to −40 ppm, while the tetracoordinate precursors have resonances at +25−40 ppm. ,− This again is consistent with the resonances for 3b and 3c being approximately halfway between the two limiting structure resonances. It is also possible that each of the silicon centers are equally partially pentacoordinate, but the very broad peaks strongly suggest exchange.…”

Synthesis of Silaoxazolinium Salts Bearing Weakly Coordinating Anions: Structures and Catalytic Activities in the Aldol Reaction

“…The structural changes from pentacoordinate ( N -amidomethyl)dimethyl(X)silanes to tetracoordinate silaoxazolinium salts are shown by several structural parameters, as summarized in Scheme . For example, the O–Si bond lengths of these silicon compounds reflect faithfully the geometries of the silicon . The ratio of Si···X distances to the sum of van der Waals radii, Si···X/∑vdW, displays the extent of coordination of the anion to the silicon.…”

“…For example, the O−Si bond lengths of these silicon compounds reflect faithfully the geometries of the silicon. 8 The ratio of Si•••X distances to the sum of van der Waals radii, Si•••X/∑vdW, displays the extent of coordination of the anion to the silicon. When Si•••X/∑vdW is larger than 1.0, the silicon center is completely free from the coordination of the anion.…”

“…The structural data of ( N -amidomethyl)dimethyl(X)silanes shown in Scheme illustrate that the Si–O bond lengths of 3 (1.831(2) Å), 4 (1.749 Å), 5 (1.75 Å), and 6 (1.745(3) Å) are longer than that of 2a (1.723(3) Å). The Si–O bond of 2a is apparently covalent, which is very close to the Si–O covalent bond length of 1-oxa-3-aza-5-silacyclopentane (1.722 Å), proving the tetrahedral geometry of the silicon of 2a (Scheme ). The Si···I atomic distances of 3 (3.510(2) Å) and 4 (3.734 Å) are significantly shorter than the sum of van der Waals radii (∑vdW for Si····l = 4.08 Å) to indicate that the silicon atoms are appreciably coordinated by I – .…”