Boron acid derivatives boranes, reduction

A number of less hindered monoalkylboranes is available by indirect methods, eg, by treatment of a thexylborane—amine complex with an olefin (69), the reduction of monohalogenoboranes or esters of boronic acids with metal hydrides (70—72), the redistribution of dialkylboranes with borane (64) or the displacement of an alkene from a dialkylborane by the addition of a tertiary amine (73). To avoid redistribution, monoalkylboranes are best used /V situ or freshly prepared. However, they can be stored as monoalkylborohydrides or complexes with tertiary amines. The free monoalkylboranes can be hberated from these derivatives when required (69,74—76). Methylborane, a remarkably unhindered monoalkylborane, exhibits extraordinary hydroboration characteristics. It hydroborates hindered and even unhindered olefins to give sequentially alkylmethyl- and dialkylmethylboranes (77—80). 

Oxidative Heck reactions via Pd(II) C—H functionalization of terminal alkenes with pinacol boranes have been described for the preparation of styrenes and derivatives through electrophilic Pd(II) catalysis (Scheme 3.20). ° Treatment of a functionalized allylic precursor with the Pd(II) catalysts listed facilitated an allylic C—H activation. Subsequent transmetallation of the aryl boronic acid and reductive elimination afforded the desired olefin with excellent stereoselectivity. The scope of the transformation allows for a variety of activating and deactivating substituents on the aryl boronic acid as well as numerous functional groups on the starting alkene. A tandem allylic C—H oxidation/vinylic arylation protocol has also been reported. " ... 

The insoluble polymeric pyridine-borane system (20) has been reported to reduce aldehydes and ketones in high yields at room temperature in the presence of boron trifluoride-etherate, whereas acid derivatives and a,/3-enones are not reduced. The polymer is used in benzene, and reducing power is better if the polymer beads are swollen in benzene before use. The corresponding polymeric 4-pyridyl-borane (21) is sluggish in reductions but reactivity can be increased using a borane complex of the quaternary polymer (22), which is assumed to... 

By similar reduction procedures, amino alcohols can be derived from almost every amino acid. (S)-Valinol [(S)-4], (S)-leucinol [(S)-5], and (S -zm-leucinol [(S) ] are prominent examples of these compounds. A detailed procedure for the reduction of (S)-valine to (S)-valtnol by borane-dimethyl sulfide/boron trifluoride-diethyl ether complex has been reported6, as well as details on the preparation of (5)-leucinol2 and (S)-/m-leucinol ... 

The regioselective reductive-cleavage of the galactose-glucose link in die sialic acid containing trisaccharide derivative 7 with borane-dimethylsulfide complex in the presence of boron trifluoride or by triethylsilane afforded anhydro derivative 8."... 

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