Silyl borane

Possible precursors for silyleneboranes might be amino(silyl)boranes with stericaUy bulky amino and silyl groups. For this reason silylboranes of types R Si-B(tmp)X and tmpB-... 

CyHiiBNiSi bis(dimethyl-amino)-(trimethyl-silyl)-borane 6917-95-9... 

The preparation and structure of (tert-butylimino)(tris(trimethyl-silyl)silyl)borane have been reported and whilst the molecule does not contain a boron-carbon bond it is of relevance to the subject of this report. 

The (acyloxy)borane complex 9, readily available from tartaric acid derivative 8, also catalyzes aldol additions of silyl enol ethers34 and silylketene acetals3 5 in an enantioselective manner. Thus,. u -ketones 10 and /Thydroxy esters 12 are available34, as well as a-unsubstituted ketones 1135. 

Organoboranes react with a mixture of aqueous NH3 and NaOCl to produce primary amines. It is likely that the actual reagent is chloramine NH2CI. Chloramine itself,hydroxylamine-O-sulfonic acid in diglyme, and trimethyl-silyl azide " also give the reaction. Since the boranes can be prepared by the hydroboration of alkenes (15-16), this is an indirect method for the addition of NH3 to a double bond with anti-Markovnikov orientation. Secondary amines can be prepared by the treatment of alkyl- or aryldichloroboranes or dialkylchlorobor-anes with alkyl or aryl azides. 

The formation of ethers such as 1806 by EtsSiH 84b can also be catalyzed by trityl perchlorate to convert, e.g., benzaldehyde in 84% yield into dibenzyl ether 1817 [48]. The combination of methyl phenethyl ketone 1813 with O-silylated 3-phenyl-n-pro-panol 1818, in the presence of trityl perchlorate, leads to the mixed ether 1819 in 68% yield [48] (Scheme 12.15). Instead of trityl perchlorate, the combination of trityl chloride with MesSiH 84a or EtsSiH 84b and sodium tetrakis[3,5-bis-(trifluoro-methyl)phenyl]borane as catalyst reduces carbonyl groups to ethers or olefins [49]. Employing TMSOTf 20 as catalyst gives very high yields of ethers. Thus benzaldehyde reacts with O-silylated allyl alcohol or O-silylated cyclohexanol to give the... 

Silyl(pinacol)borane (88) also adds to terminal alkenes in the presence of a coordinate unsaturated platinum complex (Scheme 1-31) [132]. The reaction selectively provides 1,2-adducts (97) for vinylarenes, but aliphatic alkenes are accompanied by some 1,1-adducts (98). The formation of two products can be rationalized by the mechanism proceeding through the insertion of alkene into the B-Pt bond giving 99 or 100. The reductive elimination of 97 occurs very smoothly, but a fast P-hydride elimination from the secondary alkyl-platinum species (100) leads to isomerization to the terminal carbon. 

In this context, it is interesting to note that the first synthesis of 2, 3 -0,0-cyclic phosphorothioate 22a was reported by Eckstein in 1968 [25], He also isolated pure Rp diastereomer by fractional crystallization of the triethylammonium salts [26] and used it as reference to determine the absolute configurations of the other phosphorothioate analogues [27], 2, 3 -0,0-Cyclic H-phosphonate 20a was used as a key substrate for the synthesis of uridine 2, 3 -0,0-cyclic boranophosphate 27. Silylation of H-phosphate 20a gave the phosphite triester 25 (two diastereomers). Its boronation, with simultaneous removal of the trimethylsilyl group, was achieved by its reaction with borane-A.A-diisopropylethylamine complex (DIPEA-BH3). 

The synthesis of chiral liquid-crystalline allenes was reported by Tschierske and co-workers (Scheme 4.10) [14]. An asymmetric reduction of 41 with Alpine borane was a key step to an enantioenriched allene 44. After removal of the silyl group, the allenic alcohol was etherified by the Mitsunobu method to give 45, the first liquid-crystalline allene derivatives. 

The adducts derived from the reaction of a variety of alkyl or alkoxyl substituted silyl radicals with aminoboranes have also been recorded [82]. The silyl radical addition takes place at the boron site to give the aminyl-borane radical (Reaction 5.43) and structural information for this class of radicals has been obtained. 

Thermal decomposition of azidoboranes [Eq. (3)], and of [trimethyl-silyl(trimethylsilyloxy)amino]boranes [Eq. (4)], permits a simple synthesis of symmetric iminoboranes RBNR (17-19). A hot tube procedure at about 300°C and 10 Torr turned out to be useful. The iminoborane iPrBNiPr, for example, was prepared at a rate of 10 g/hour by the azidoborane method. No separation problems are met with this method. Handling the liquid reactants of Eqs. (3) and (4) is hazardous. 

Silyl(silyloxy)amino]boranes decompose in the liquid phase to a mixture of products [Eq. (12)] (18,19). Again, the formation of the... 

The chloroborane reactants of Eqs. (13) and (14) can be aminated to isolable [silyl(silyloxy)amino]boranes, which decompose at 120 and 70°C, respectively, to give a mixture of products, three of which were identified in both cases [Eqs. (15) and (16)] (19). Again, the iminoboranes... 

By no means do all metallocarboranes have the metal atoms occupying vertices of the basic polyhedra. Apart from many derivatives in which o-bonded metal residues occupy exo sites attached to particular skeletal atoms, several metalloboranes and -carboranes are known in which the metal occupies an edge-bridging site, effectively replacing a bridging hydrogen atom of the parent borane. Many are nido species related to BeHio, for example, the /x-silyl and /i-germyl carboranes. 

Table 11. H N.M.R. data for silyl- and methylphosphine ( H3)- and ( H3)-borane adducts (pure liquids at-20 or 25 "C)...

The two lactams obtained, 60 and 63, were silylated in situ followed by borane reduction to give the 2-amino-l,2,5-trideoxy-l,5-imino-D-xylitol (61), and 2-amino-l,2,5-trideoxy-l,5-imino-D-ribitol (64), respectively [79]. 

Enantioselective condensation of aldehydes and enol silyl ethers is promoted by addition of chiral Lewis acids. Through coordination of aldehyde oxygen to the Lewis acids containing an Al, Eu, or Rh atom (286), the prochiral substrates are endowed with high electrophilicity and chiral environments. Although the optical yields in the early works remained poor to moderate, the use of a chiral (acyloxy)borane complex as catalyst allowed the erythro-selective condensation with high enan-tioselectivity (Scheme 119) (287). This aldol-type reaction may proceed via an extended acyclic transition state rather than a six-membered pericyclic structure (288). Not only ketone enolates but ester enolates... 

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