Amino silyl boranes

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-... 

C6HisBBr2NSi2 bis(trimethyl-silyl)amino-dibromo-borane 4267-39-4... 

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

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... 

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]. 

The steric bulk of the silyl groups in acylsilanes influences their asymmetric reduction to give chiral secondary alcohols by borane complexed with )-2-amino-3-... 

The determination of the crystal structures of a series of silyl(amino)boranes shows that the B-Si bond is significantly shortened by the introduction of chlorine substituent at boron and is even Shorter in the presence of a Q atom at the silyl group. This definitely shows that the Cl atoms have a noticeable inductive effect on the length (and strength) of the B-Si single bond. 

The use of CAB as a chiral reagent seems to be more effective for this reaction, which proceeds faster and with higher yields and enantiomeric excess. Kiyooka et al. first described the use of various chiral oxaborolidines, derived from sulfonamides of a-amino acids and borane, in the course of the selective aldol reaction between silyl ketene acetals and aldehydes (Eq. 47) [43a]. Stereoselectivity and yields were relatively high. 

Masamune et al. examined the catalytic activity of several boron Lewis acids derived from BH3 THF and the p-toluenesulfonamides of simple a-amino acids towards the aldol reaction of benzaldehyde with TMS enolate 48 [121]. As a result, the borane catalysts derived from a,a-disubstituted glycine p-tolueriesulforiarriides were found to have high activity. The disubstitution would accelerate the second step (Step II) of the catalytic cycle (Scheme 10.43). On the basis of this observation, they developed chiral borane catalysts 47 c and 47 d, which enable highly enantioselective aldol reactions of KSA and thioketene silyl acetals (84—99% ee with 48). 

Boranes other than those based on a-pinene are particularly useful in allylic transfer to imines to make single enantiomers of unsaturated amines. One good combination is an allylboron compound 80 complexed with an /V-sulfonyl amino alcohol such as 78, derived from nor-ephedrine (see chapter 23) with an /V-silyl imine such as 81. The unsaturated amines 82 are formed in good... 

The steric bulk of the silyl groups in acylsilanes influences their asymmetric reduction to give chiral secondary alcohols by borane complexed with (5)-(-)-2-amino-3-methyl-l,l-diphenylbutan-l-ol . The enantiomeric excess increases from 50% to 94% by replacement of the PhMe2Si group with the PhsSi group. 

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