Directing groups silyl

Direct 3-silylation of A -substituted indoles has been ellected by reaction of the indoles with trimethylsilyl triflate in the presence of triethylamine[12]. The trimethylsilyl group has also been introduced via 3-lithio-l-(phenylsulfonyl)-indole[13]. 

The silyl group directs electrophiles to the substituted position. That is, it is an ipso-directing group. Because of the polarity of the carbon-silicon bond, the substituted position is relatively electron-rich. The ability of silicon substituents to stabilize carboca-tion character at )9-carbon atoms (see Section 6.10, p. 393) also promotes ipso substitution. The silicon substituent is easily removed from the c-complex by reaction with a nucleophile. The desilylation step probably occurs through a pentavalent silicon species ... 

The ability of the silyl group to direct substitution in competition with other directing groups is also found to depend on the nature of the electrophile (equation 32). 

Silylation of benzyl C-H bonds using hydrosilanes can also be performed with the aid of Ru3(CO)12-catalyst (Table 1) [9]. This silylation occurs only at benzylic CH3 groups. Pyridine, pyrazole, and hydrazones function as good directing groups. Benzylamines, oxime ethers, dimethylanilides, and aryl pyridyl ethers have no activity in this silylation. 

Itami, Yoshida and co-workers demonstrated that this methodology can also be employed in the intermolecular variant of the PK reaction.25 They showed that using a dimethyl(2-pyridyl)silyl group as a directing group allows the reaction to proceed catalytically with respect to the metal complex and gives some control over the regioselective outcome of the reaction. 

The silyl group directs electrophiles to the substituted position that is, it is an ipso-directing group. Because of the polarity of the carbon-silicon bond, the substituted... 

Stork and co-workers investigated silyl acetals as the tether-directing group for preparing the difficult )8-mannopyranoside linkage [90]. The desired mixed silyl acetal precursor was initially prepared in a stepwise fashion analogous to that described earlier, by... 

A new method for the stereocontrolled synthesis of pyrimidine p-D-2 -deoxy-ribonucleosides involves the use of a photolabile directing group at C-2, and is illustrated by the case in Scheme 2. New glycosyl donors that have been employed with good to excellent stereoselectivity in reactions with silylated pyrimidines are the deoxyribofuranosyl phosphoramidate 36 and the thiocarba-mate 37 in this latter case, intramolecular participation by the sulfur is thought to be involved in ensuring P-selectivity. ... 

Pyrimidine was used as an effective directing group for the a-arylation of pyrroles in a reaction between pyrroles 62 and silyl coupling partner 63... 

The silyl group directs attacking electrophiles such as the proton or bromine to the substituted position. It is thus a strongly directing group. Structure-reactivity relationships indicate that the mechanism is similar to an aromatic substitution... 

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