Silicon halides tertiary

Electron-withdrawing gronps such as aUcoxy and chloro gronps on silicon activate vinyisiianes toward the addition of secondary and tertiary alkylmagnesinm halides (Scheme... 

Simple (RO)4Si compounds may readily be prepared by the reaction between a chlorosilane and an alcohol (or phenol) (Scheme 41). A problem in this type of reaction is that the HCl formed may cause unwanted side reactions. This may be minimized by blowing dry N2 gas through the reaction mixture or heating the reaction so that HCl is driven off as it is formed, or a tertiary amine such as pyridine may be added to the reaction, which forms an amine hydrochloride salt that precipitates to remove the HCl. Silicon alkoxides may also be made by reaction of a silicon halide and an alkali metal alkoxide (Scheme 41). ... 

In addition to their use for the preparation of specific Uthium enolates, silyl enol ethers are also excellent substrates for actrf-catalysed alkylation. In the presence of a Lewis acid (e.g. TICU, SnCU, BF3 OEt2> they react readily with tertiary alkyl halides to give the alkylated product in high yield. This procedure thus complements the more-common base-catalysed alkylation of enolates which fails with tertiary haUdes. It is supposed that the Lewis acid promotes ionization of the electrophile, RX, to form the cation R+, which is trapped by the silyl enol ether to give the addition product with cleavage of the silicon-oxygen bond. 

A. Nucleophilic Reactions of the P=0 Group.—Tris(trifluoromethyl)-phosphine oxide (33) reacts with hexamethyldisiloxane to give a phos-phorane, whose n.m.r. spectrum at — 140 °C shows non-equivalent trifluoromethyl groups. Although this unusual reaction clearly involves nucleophilic attack of the phosphoryl oxygen on silicon at some stage of the reaction, a full study of the mechanism has not been published. Tertiary phosphine oxides can be converted cleanly into dichlorophos-phoranes (34) by treatment with two moles of phosphorus pentachloride. Alkylation of the sodium salt of tetraphenylmethylenediphosphine dioxide (35) with alkyl halides, in dimethyl sulphoxide, has been reported to... 

Alkyl-substituted silicon peroxides, 78 444 Alkyl sulfates, 23 537 use in cosmetics, 7 849 Alkylsulfones, O-alkylation and, 9 280 Alkyl sulfoxide, micellization of, 24 132t Alkyl tertiary alkyl ethers, 70 575 Alkylthexylboranes, 73 639 Alkyl-thiols, 77 57 4-Alkylthiosemicarbazides, 73 575 Alkyl-tin catalysts, 20 40 Alkyltitanium halides, 25 108—109 Alkyltitaniums, 25 106 higher, 25 116... 

The crux of organic mechanistic stereochemistry may be the Walden inversion, the inversion of stereochemistry about a four-coordinate carbon atom by nucleophilic attack of, for example, a hydroxide ion on an alkyl halide. Many reactions of inorganic molecules follow the same mechanism. In contrast, the dissociative mechanism of tertiary halides to form tertiary carbocatanion intermediates is essentially unknown among the nonmetallic elements silicon, germanium, phosphorus, etc. The reason for this is the generally lower stability of species with coordination numbers of less than 4, together with an increased stability of five-coordinate intermediates. This difference is attributable to the presence of d orbitals in the heavier elements (Chapter 18). 

---