Ring—Sil

Just as an aromatic ring is alkylated by reaction with an alkyl chloride, it is acylated by reaction with a carboxylic acid chloride, RCOC1, in the presence of AICI3. That is, an acyl group (-COR pronounced a-sil) is substituted onto the aromatic ring. For example, reaction of benzene with acetyl chloride yields the ketone, acetophenone. 

The all-silicon version of the cyclobutenyl radical, cyclotetrasilenyl radical 40, was recently reported by Sekiguchi et al. Its synthesis was accomplished by the one-electron reduction of the precursor cationic species, cyclotetrasilenylium ion 20 TPFPB with either t-Bu3SiNa or KCg in Et20 (Scheme 2.33). The four-membered Si4 ring of 40 is nearly planar with the Sil-Si2 and Si2-Si3 bonds... 

The central four-membered ring of cation 3 is folded and the dihedral angle between the Sil-Si2-Si3 and Sil-Si4-Si3 planes is 46.6°. The bond angles at the silicon atoms in the four-membered ring suggest that the silicon atoms Sil, Si2 and Si3 (mean E°Si = 359.8, see Table XIII) have a completely planar trigonal geometry and Si4 has a distorted tetrahedral environment. The Si Si bonds of the cationic... 

Only fibrous zeolites (natrolite, mesolite, scolecite, edingtonite) are in this group. In their topological symmetry there are two T sites, one having a multiplicity 1/4 of the other (from now on indicated as Tl and T2 respectively). These tetrahedra are connected to construct a building block formed by five 4-membered rings (see Fig. la). In the case of order, Tl is occupied by Si (and will be called Sil from now on), whereas T2 splits into two symmetrically independent tetrahedra, one occupied by Si, the other by A1 (called Si2 and A1 respectively). 

In these zeolites the Si/Al ratio is normally very near 1.5 [33]. Therefore, disorder is due to an Al->Si substitution between A1 and Sil. In this case, according to Loewenstein s rule. Si must occupy both A1 sites, and the Si atom substituted by A1 in Sil will occupy one Si2 site, the other Si2 site being occupied by the residual A1 (see Fig. lb). Therefore the crystal structure refinement must give an Al-fraction in site Sil, equal to the Si-fraction in site A1 and two times the Al-fraction in site Si2. These ratios have been found when disorder is low, as in some natrolites, in mesolite and scolecite but when the Al-fraction in Sil reaches values about 10 %, the disorder in the 4-ring formed by the T2 sites increases till it reaches complete disorder, with the same Al-fraction both in Tl and T2 (see Table 4 and Fig. 2). Curiously, complete disorder, which is not present in the majority of so-called disordered zeolites, has sometimes been found in these zeolites which are often taken as an example of order. 

DeBolt and Mark [71] reported that a 30-membered cyclic dimethylsiloxane was not threaded by linear polysiloxane via Method 2, but 38-membered rings were threaded to the extent of 5.0% (neat, molar ratio cyclic/linear=43) in a sil-oxane rotaxane. 

Compound 126 crystallizes in a chair conformation. Because of the inversion center, the atoms N2, Sil, N2a, and Sila lie exactly in one plane. The atoms N1 and Nla are 47 pm above and below this plane, respectively. The sum of angles at N2 amounts to 359.8°. This nitrogen atom, in contrast to the NH-functional N1 with 349.9, is. vp2-hybridized. The angle Si2-N2-Sil of 140.7 is comparatively increased. The average Si—N and N—N bond distances are 175 and 148 pm, close to the expected values of 174 pm for Si—N and 145 pm for N—N.60 The same method of preparation, shown in Eq. (39), led to the formation of the first fluorosilyl functional hydrazine six-membered rings 127 and 128.43 The dilithiated bis(silyl)hydrazines 10 and 12 react with tetrafluorosilane to yield tetrakis-(silyl)hydrazines, which again react with the lithium derivatives. 

Fluorouracil [flure oh YOOR a sil] (5-FU), a pyrimidine analog, has a stable fluorine atom in place of a hydrogen atom at position 5 of the uracil ring. The fluorine interferes with the conversion of deoxyuridylic acid to thymidylic acid, thus depriving the cell of one of the essential precursors for DNA synthesis. 

Silver(I)-catalyzed cyclizations of substituted allenes to heterocydic ring systems induding 2,5-dihydropyrroles have been described previously [4,10]. Moreover, sil-ver(I) salts are known to form stable rr-complexes with terminal acetylenes [11]. On the other hand, on treatment with silver nitrate silylacetylenes were reported to afford silver acetylides [12]. Based on these considerations and additional experimental evidence [5,13], the following mechanism has been proposed forthe sUver(I)-mediated oxidative cydization of homopropargylamines to pyrroles ]5] (Scheme 15.3). 

The four-membered ring is slightly folded. The planes through the atoms Si2-Sil-C2 and Si2-Cl-C2 meet at an angle of 35.9 °. The ring C-C bond distance is 157.7 pm [6]. 

The structure of the deprotonation product 3a, in which the yttrium atom is part of a four-membered Y-Cll-Sil-Nl ring, is shown in Fig. 2. The carbanionic Cll atom shows a distorted trigonal bipyramidal coordination, the axial and equatorial positions being occupied by Y, K [with K-Cll-Y 176.36(18)°, and the two H atoms and Sil, respectively. The potassium cation exhibits a pseudo-tetrahedral coordination by the Cl 1 atom, the centroids of two benzene rings, and one intermolecular agostic contact to the carbon atom of an adjacent trimethylsilyl group. 

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