Boranes stereoselective reactions

P-Allyl-to-(isopinocampheyl)borane exhibits high stereoselectivity in reactions with chiral a-substituted aldehydes.40 The stereoselectivity is reagent controlled, in that there is no change in stereoselectivity between the two enantiomeric boranes in reaction with a chiral aldehyde. Rather, the configuration of the product is determined by the borane. Both enantiomers of (Ipc)2BH are available, so either enantiomer can be prepared from a given aldehyde. 

A relatively new method for thermal radical initiation has gained popularity in synthetic applications. A small amount of Et3B is added to the reaction mixture that has not been rigorously deoxygenated." " The borane reacts with oxygen, apparently generating ethyl radicals. A major advantage for this mode of radical initiation is the low reaction temperatures (as low as —78 °C) that can be achieved when stereoselective reactions are desired." ... 

In many stereoselective reactions, the effect of temperature on the selectivity is as expected, with better results being obtained at lower temperature. A lower tempertaure is often required to increase the selectivitty. From the practical point of view, one of the most attractive feature of this enantioselective reduction is that excellent enantioselectivity is obtained at a relatively high temperature such as room temperature. In some cases, the selectivity of the oxazaborolidine catalyzed borane reduction increases with increasing temperature until an optimal range is reached (30-50 °C) where the selectivity then begins to decrease [76]. Interpretation of this phenomena is not so easy. The amount of catalyst dimer that exists in a temperature-dependent equiUbrium with the monomeric form, might have an effect on the selectivity. 

There are also procedures to transform phosphine boranes into other derivatives by one-pot stereoselective reactions (Scheme 1.8). 

Moreover, you will discover that the hydroboration of (+)-a-pinene is an example of a stereoselective reaction because only one of several possible stereoisomeric products is formed by addition of the borane reagent to only one face of the TT-bond of 65. 

The hydroboration of 65 is a stereoselective reaction in which borane approaches from only one face of the carbon-carbon double bond to give the dialkylborane 66. Provide a rationale for this observed stereoselectivity. 

Direct boronation of benzene derivatives with BCI3 in the presence of activated aluminum or AICI3 provides arylboronic acids after hydrolysis (eq 12). Chloroboration of acetylene with boron trichloride produces dichloro(2-chloroethenyl)borane. Similar reaction with phenylacetylene provides ( )-2-ehlorO 2-phenylethenylborane regio- and stereoselectively. ... 

Allyllithium reagents have also been used in the synthesis of (Z)-y-alkoxyallylboronates 23 2 5. Stereoselectivity is excellent in these reactions since the (Z)-y-alkoxyallyl carbanions prepared by metalation of allyl ethers are stabilized by chelation. The (Z)-y-alkoxyallyl(diisopinocam-pheyl)boranes are prepared at low temperature by an analogous procedure and must be used at — 78 "C otherwise reaction diaslereoselectivity suffers owing to the facile isomerization to the -isomer26. 

Both E- and Z-isomers of the terpene y-bisabolene have been isolated from natural sources. The synthesis of these compounds can be achieved by stereoselective alkene syntheses using borane intermediates. An outline of each synthesis is given below. Indicate the reaction conditions that would permit the stereoselective synthesis of each isomer. 

Reduction of sulfoxide (S)-40 was carried out in toluene using 1.05 equiv of 1M BH3-THF at 10°C, producing the cis-diaryl dihydrobenzoxathiin 12 with complete stereoselectivity. Upon work-up and crystallization from toluene and heptane, the desired product 12 was isolated in 88% yield and over 99% ee (Scheme 5.13). Borane dimethylsulfide led to a slower reaction. To the best of our knowledge, this synthetically useful reaction is unprecedented in the literature [13].1 Investigation... 

Stereoselective reduction of some triazolodiazines (derivatives of ring systems 33 and 37) bearing chiral terpene residues has been elaborated by Groselj el al. <2006TA79>. With catalytic hydrogenation, partial saturation of the six-membered ring was experienced, while reaction with borane-methyl sulfide resulted in formation of triazole-boron complexes. 

The utilization of a-amino acids and their derived 6-araino alcohols in asymmetric synthesis has been extensive. A number of procedures have been reported for the reduction of a variety of amino acid derivatives however, the direct reduction of a-am1no acids with borane has proven to be exceptionally convenient for laboratory-scale reactions. These reductions characteristically proceed in high yield with no perceptible racemization. The resulting p-amino alcohols can, in turn, be transformed into oxazolidinones, which have proven to be versatile chiral auxiliaries. Besides the highly diastereoselective aldol addition reactions, enolates of N-acyl oxazolidinones have been used in conjunction with asymmetric alkylations, halogenations, hydroxylations, acylations, and azide transfer processes, all of which proceed with excellent levels of stereoselectivity. 

Addition reactions of allylic boron compounds have proven to be quite general and useful. Several methods for synthesis of allylic boranes and boronate esters have been developed.36 37 The reaction has found some application in the stereoselective synthesis of complex structures. 

Allyl-bis(isopinocampheyl)borane exhibits high stereoselectivity in reactions with chiral a-substituted aldehydes.39... 

This reaction occurs rapidly at room temperature using a small excess of alkyne and either pinacol- (HBpin) or catecholborane (HBcat). When an excess of borane was used, the Z/E ratio of the products was slowly eroded, eventually attaining a thermodynamic distribution of isomers. Equilibration presumably occurs via addition/elimination of excess Rh-H. Miyaura s method provides a useful synthetic complement to knovm cis-hydroboration methods. Under optimized conditions, good yields and high stereoselectivity (>90 10) were achieved for a variety of alkenylboronates (Table 9.8). The best selectivities were generally obtained with the use of catecholborane and Et3N as an additive. As in related reactions, the presence of base seems to suppress undesired reaction pathways. 

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