Silyls boron halides

Boron salts of the amine called Silyl-Amino-Boron Compounds (or Aminoboronsilicone compds) are obtd by the reaction between tri-silicylamine and a boron halide such as BC13 at —78°. A variety of these addn compds, some of which are sponty ignitable in air, are formed in this manner, ie, (SiH3)2NBCU or (CH3)a-BN(SiH2Br)2 (Refs 8 26)... 

Secondary aliphatic phosphines are prepared by other methods, which have been described. Secondary phosphines form molecular addition compounds with boron halides. Dehydrohalogenation of diphenylphosphine-boron triiodide (or diphenylphosphine-boron tribromide) gives the dimeric phosphinoborane. The following method of preparation makes isolation of the intermediate addition compound unnecessary. Reaction of dibutyl(trimethy 1-silyl)phosphine with boron tribromide and ehmination of bromotrimethylsilane gives a dimeric phosphinoborane also. ... 

Dihaiosilylenes (SiX2, X = F, Cl), generated in situ from SiX and elemental Si at ca. 1200°C, can be co-condensed at 77 K with boron halides to )deld mixtures of silylated haloboranes (Table 1) e.g., SiFj and BFj gases are co-condensed at 77 K over a 2-h period, after which unwanted volatiles (SiF4 and BF3) are pumped off. Trap to trap distillation separates SijBF, (—48° to —59°C), Si3BF9 (—18° to —28°C), Si4BFn and Si,BFi3 (-10° to -f5°C). 

Conversion to a more facile, sulfur-derived, leaving group can be achieved by treatment with sodium thiosulfate or salts of thio and dithio acids (75,87). Under anhydrous conditions, boron tribromide converts the 3 -acetoxy group to a bromide whereas trimethyl silyl iodide gives good yields of the 3 -iodide (87,171,172). These 3 -halides are much more reactive, even when the carboxyl group is esterified, and can be displaced readily by cyano and by oxygen nucleophiles (127). 

Simultaneous treatment of a carbonyl compound with a Lewis acid and a tertiary amine or another weak base ( soft enolization ) can sometimes be used to generate enolates of sensitive substrates which would have decomposed under strongly basic reaction conditions [434]. Boron enolates, which readily react with aldehydes at low temperatures, can also be prepared in situ from sensitive, base-labile ketones or carboxylic acid derivatives [293, 295, 299]. Unwanted decomposition of a carbanion may also be prevented by generating it in the presence of an electrophile which will not react with the base (e.g. silyl halides or silyl cyanides [435]). 

Straightforward 5-lithiation results using -BuLi with ethyl 3-mcthoxy-l -methyl-17/-pyrazol-4-earboxy late and halides, zincates, or boronic acids can thus be produced <2002SL769>. A neat device allows 3-lithiation of indazole silylation at N(2) allows the desired 3-metallation (Scheme 60) <2006EJ02417>. 

Allyl silanes are rather like silyl enol ethers they react with electrophiles, provided they are activated, for example by a Lewis acid. Titanium tetrachloride is widely used but other successful Lewis acids include boron trifluoride, aluminium chloride, and trimethylsilyl triflate. Electrophiles include acylium ions produced from acid chlorides, carbocations from tertiary halides or secondary benzylic halides, activated enones, and epoxides all in the presence of Lewis acid. In each case the new bond is highlighted in black. 

In contrast to the rare occurrence of the oxidative addition of C-C bonds, the oxidative addition of the nonpolar bonds between two main group atoms, such as boron and silicon, can be facile. The oxidative addition of Si-Si bonds in disilanes and the B-B bond in diborane(4) reagents is likely to be a step in a variety of catalytic reactions, including the additions of disilanes, - diborane(4) reagents, and silaboranes - across olefins and alkynes, the silylation and borylation of arene C-H bonds, - - the borylation of alkane C-H bonds, and the conversion of aryl halides to arylsilanes and arylbo-ronate esters. " ... 

---