Boranes hydroboration

Such functional groups as OR, OH, NH2, SMe, halogen, and COOR may be present in the molecule," but not groups that are reducible by borane. Hydroboration of enamines with 9-BBN provides an indirect method for reducing an aldehyde or ketone to an alkene, for example. 

Borohydrides, 13 613-621 lithium, 13 620-621 potassium, 13 620 sodium, 13 614—620 in uranium systems, 25 440 Boron (B), 4 132-138. See also Boranes Hydroboration MgB2 entries addition to aluminum wrought alloys, 2 327... 

Hydroboration of isoprene with catecholborane at 1 1 molar ratio in tetrahydrofliran in the presence of 1 molar % of NiCbCdppe) at room temperature was completed in 6 h. An interesting observation is a lower reactivity of 1-decene compared with isoprene under these conditions. It is the first case of higher reactivity of 1,3-diene than 1-alkene in the hydroboration reaction. It is also an indirect indication that borane is not involved in the reaction, since acyclic conjugated dienes are less reactive than 1-alkenes toward borane. Hydroboration of representative conjugated dienes in the presence of NiCUCdppe) was carried out on a preparative scale and the results are shown in Table 2.9 The mono-hydroboration products were isolated by distillation and oxidized to the corresponding unsaturated alcohols. 

Hydroboration with Thexylborane and other Alkyl- and Aryl-boranes Hydroboration with Monohaloboranes... 

Figure 6 Some common disubstituted borane hydroborating agents...

Hydroboration is readily effected with alkenes containing many types of functional group. Where the other functional group is not reduced by borane, hydroboration generally proceeds without difficulty and even some functional groups that react only slowly with borane may be tolerated, for example carboxylic esters. Easily reduced carbonyl groups of aldehydes and ketones, however, must be protected as their acetals, and carboxylic acids as esters. 

An alternative means of resolution depends on the differences in rates of reactions of enantiomers with a chiral reagent. The transition state energies for reaction of one chiral molecule with another can be different for each enantiomer. If a racemic mixture R molecule -h 5 molecule) reacts with an optically active reagent (R reagent), the two transition states R molecule / reagent) and (5 molecule i reagent) bear a diastereomeric relationship to each other. Kinetic resolution is the term used to describe the separation of enantiomers by selective reaction with an optically active reagent. A very useful application of this technique is the resolution of allenes by preferential reaction of one enantiomer with an optically active borane. Hydroboration of the allenes occurs at different rates, and the reaction mixture becomes enriched in the less reactive enantiomer. An allene that has been partially resolved by this technique was presented as an example of an axially dissymmetric molecule in Scheme 2.1 68 entry 2,1,3-dimethylallene). ... 

---