Group 4 silyl compounds

Keywords oligosilyl anions, oligosilyl dianions. Group 4 silyl compounds... 

The chemistry of group 4 silyl compounds is closely associated with the dehydrogenating coupling of hydrosilanes. This reaction was discovered by Harrod and coworkers who converted PhSiHa with a catalytic amount of Cp2TiMe2 to H(PhSiH) H [370]. Later Tilley and coworkers showed that if zirconocene and hafnocene complexes are used in the reaction a catalytic cycle is involved which consists of o-bond metathesis steps [371]. To enhance o-bond metathesis reactivity with Si-H and C-H bonds Cp2Hf(Me)Si(SiMe3)3 and related compounds were treated with B(C6F5)3 to obtain cationic Hf complexes [372]. 

The 6-endo activated epoxy alcohol cyclization process was also expected to play a central role in the annulation of pyran ring G of the natural product (see Scheme 22). Silylation of the free secondary hydroxyl group in compound 131 furnishes, after hydrobora-tion/oxidation of the double bond, compound 132. Swern oxidation of alcohol 132 produces an aldehyde which reacts efficiently with (ethoxycarbonylethylidene)triphenylphosphorane in the presence of a catalytic amount of benzoic acid in benzene at 50 °C, furnishing... 

In each of these cases the angle is wider in solution than it is in the gas phase by about 2°. We believe that this is caused by weak interactions between the cyano groups of the solvent and the silyl groups, interactions which are comparable to those seen in many silyl compounds in the crystalline phase. 

Allylation and subsequent protection of the thus formed hydroxyl group furnishes compound 134, which bears the C-ring skeleton of baccatin HI. Removal of the C-9 silyl group, PhLi treatment of the resulting hydroxyl ketone, and in situ acetylation provides compound 135, which has the C-2 benzoate functionality. In the presence of a guanidinium base, equilibrium between the C-9 to C-10 carbonyl-acetate functional groups can be established. Thus, the desired C-9-carbonyl-C-10-acetate moiety 136 can be separated from the mixture. Compound 136 is then converted to aldehyde 137 via ozonolysis for further construction of the C-ring system (Scheme 7-40). 

In the cathodic reduction of activated olefins, chlorosilanes also act as trapping agents of anionic intermediates. Nishiguchi and coworkers described the electrochemical reduction of a,/ -unsaturated esters, nitriles, and ketones in the presence of Me3SiCl using a reactive metal anode (Mg, Zn, Al) in an undivided cell to afford the silylated compounds [78]. This reaction provides a valuable method for the introduction of a silyl group into activated olefins. 

The proposed mechanism of the oxidative cleavage of S-protecting groups by the chlorosilane/sulfoxide procedure is outlined in Scheme 8. 95 The first reaction is considered to be formation of the sulfonium cation 9 from diphenyl sulfoxide (7) and the oxygenophilic silyl compound 8. The formation of a sulfonium ion of this type is known and has been utilized for the reduction of sulfoxides. 97 Subsequent electrophilic attack of 9 on the sulfur atom of the S-protected cysteine residue leads to the formation of intermediate 10, whereby the nature of the silyl chloride employed should be the main factor that influences the electrophilicity of 9. The postulated intermediate 10 may then act as the electrophile and react with another S-protected cysteine residue to generate the disulfide 11 and the inert byproduct diphenyl sulfide (12). This final step is analogous to the reaction of a sulfenyl iodide as discussed in Section 6.1.1.2.1. 

The size of the silyl groups in compounds R3SiC(N2)COOMe influences their decomposition by copper triflate, dirhodium tetraacetate and dirhodium... 

The IR spectra of /3-sultams are characterized by the bands due to the S02 group at 1340-1295 (S02as) and 1200-1140 cm-1 (S02sym), depending on the substituents (Table 6). The data for both bands for the C-3 and C-4 unsubstituted iV-silyl compounds show a shift to higher wave numbers compared with the data for the iV-benzoyl analogue. The data for the 4-substituted derivatives usually show no or only a small shift of both bands. Nevertheless,... 

PMO precursors can be divided roughly into three classes bis-silylated compounds, which contain (1) short-chained alkylene or alkenylene groups as the organic bridge, (2) those with aromatic bridges, and (3) branched or cyclic multi-silylated compounds. An overview of the constitution formulae of PMO precursors is provided in Figure 3.11. 

Derivatives other than esters and silyl compounds were mostly developed for particular groups of acids, if not for individual substances, containing in addition to carboxylic also other functional groups, in an attempt to obtain derivatives possessing convenient properties, e.g., derivatives of lower acids less volatile than methyl esters, or more stable, or derivatives suitable for selective and sensitive detection. 

Hydrostannation of (l-alkynyl)carbene chromium complexes involves addition to the M = C bond as well as to the C=C bond and is strongly influenced by the substituent at the alkynyl moiety. By the bulky trimethyl-silyl group of compound lg, a 1,1-addition to the carbene carbon atom is promoted with formation of a densely functionalized 1,3-heterobimetal lie propargyl reagent 93 containing both a stannyl and a silyl group. A 3,1-addition is observed with phenyl derivative la and also the vinyl compound lo, which leads to production of an allene 94 and 95, respectively (Scheme 31).111... 

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