Silyl derivatives bonds

Silyl-derived protective groups are also used to mask the thiol function. A complete compilation is not given here since silyl derivatives are described in the section on alcohol protection. The formation and cleavage of silyl thioethers proceed analogously to simple alcohols. The Si—S bond is weaker than the Si—O bond, and therefore sulfur derivatives are more susceptible to hydrolysis. For the most part silyl ethers are rarely used to protect the thiol function because of their instability. Silyl ethers have been used for in situ protection of the — SH group during amide formation. ... 

The predictions of the reactivities by the geminal bond participation have been confirmed by the bond model analysis [103-105] of the transition states and the calculations of the enthalpies of activation AH of the Diels-Alder reaction [94], the Cope rearrangement [95], the sigmatropic rearrangement [96], the Alder ene reaction [100], and the aldol reaction [101] as are illustrated by the reactions of the methyl silyl derivatives in Scheme 38 [102], The bond is more electron donating than the bond. A silyl group at the Z-position enhances the reactivity. 

Silicon-transition metal chemistry is a relatively new area. The work of Hein and his associates (1941) on Sn—Co derivatives established the possibility of forming bonds between a Group IVB metal and a transition element 139), but it was another fifteen years before CpFe(CO)2SiMej 203), the first of many silyl derivatives, was synthesized. The interest in these compounds derives from (1) comparison with the corresponding alkyl- and Ge-, Sn-, and Pb- transition metal (M) complexes, including the role of ir-back-bonding from filled d orbitals of M into empty d orbitals on Si (or other Group IVB metal), and (2) expectation of useful catalytic properties from such heteronuclear derivatives. 

Silylation of AN (528b,c,e) with another silylating agent (Me3SiCl/Et3N) gives poorly separable mixtures of unidentified products. However, the reaction of AN (528a) under these conditions produces the silyl derivative of bis-oxime (533), which can be subjected to desilylation to prepare free bis-oxime (534) (491, 497). The stereoselectivity of the reaction with respect to the new C,C double bond is low (E/Z 1.3 1). Silylation of sterically more hindered nitroalkane (528 d)... 

The analytical control of this step is of special importance the alcaline saponification is performed at a relatively low pH in order to prevent cleavage of the silicon-carbon bond. The closer the electron-with-drawing carboxyl group is located to the Si-C-bond, the larger is the danger of scission. Therefore, for the B-silyl carboxylic acid derivatives the pH during saponification should not surpass 10.5 however, at this pH saponification of the methyl ester requires about 1 day, even at 60°C. For the -silyl derivatives, the pH of the reaction mixture is not critical. We therefore now exclusively utilize the latter. 

The chemical modification of fullerenes has received considerable attention in the last decade in order to achieve new applications to material sciences [27]. Fuller-ene-bonded polysilane derivatives might be expected to show high conductivity since Ceo-doped polysilane is found to be a good photoconductor [28]. Therefore, a variety of silylated derivatives have been obtained to date, although the available methods are limited to the photoinduced addition of various silanes to Ceo-... 

Disilathiane has been the subject of several recent studies.1 7 Interest has been centered on the importance of (p-d)7r-bonding in determining both the structure and the base strength relative to the disiloxanes and carbon analogues.2 6 The utility of disilathianes has been demonstrated by their conversion to a wide variety of silyl derivatives by exchange reactions or protolyses.2,7... 

Germyl or silyl derivatives of pentafluoro-substituted benzenes react in the same manner with boron trifluoride-diethyl ether complex without cleavage of the C — Ge or C — Si bonds to yield the 1-substituted heptafluorocyclohexa-1,4-dienes l.43... 

Most of the silyl derivatives of drugs, described in literature till now, are trimethyl-silyl compounds, containing a Si-O, Si-N or Si-C bond between the silicon atom and the drug. 

The preparation of silyl derivatives with a Si—C bond between the silicon atom and the drug is not so easy to realize as compared with Si—O and Si—N systems. In most cases the Si-C bond must be constructed at the beginning of synthesis. This is shown for the triethylsilyl derivative (4) of phenethylamine (3) in Scheme 1 ... 

When the salt [BU4NHAUX2] (X = Cl, Br) was reacted with 2 equiv of the dppm-substituted iron silyl metallate K[Fe Si(OMe)3 (CO)3(dppm)], instead of the expected [Fe—Au—Fe]- silyl derivative anion chain, the complex [AuFe Si(OMe)3 (CO)3(/r-dppm)]2 CH2Cl2 (30) was obtained with a Au—Au and two Au—Fe bonds. It is interesting to point out that complex 30 forms a 10-membered-ring structure in which the Au—Au interaction is reminiscent of the transannular interactions that occur in organic cycles of middle size (8-12). The similar reaction with HgCl2 yielded the expected Fe—Hg—Fe complex (31)96. 

This feature is illustrated experimentally by comparing the vertical n and Tti ionization energies of cyclopentadiene and some silyl derivatives, which have been obtained from the PE spectra14,15, as shown in Table 2. Adjustments in the ionization energies due to silylation are more pronounced for the b (jtt) MO than for the z 2(jT ) MO. This is consistent with a more effective rr/jt interaction in the b MO made possible by the fixed orientation with the trialkylsilyl groups in a -position to a 7r-system are powerful electron donors, which enormously... 

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