Boranes as reagent

Show how the following compounds can be prepared in high enantiomeric purity using enantiopure boranes as reagents... 

Garbodiimides Diisobutylaluminum hydride as reagent 16, 98 —, reductions with — 16, 98/9 Di(isopinocampheyl)borane as reagent 16,196 Diketals... 

The boron-oxygen mesomeric effect described in the previous section explains the lower reactivity of allylic boronates towards carbonyl compounds compared to that of allylic boranes. The use of Lewis acids, however, allows boronate derivatives, including hindered ones, to react at temperatures comparable to the analogous boranes. As described above (see section Mechanism and Stereochemistry ), the most reactive allylic boronates are those with the most electrophilic boron centers.The nucleophilicity of the y-position of an allylic boron reagent (the position that forms the new C-C bond with the aldehyde) is also important to the reactivity of the reagent. For example, allylic boronates with... 

As with borane, this reagent exists as a dimer. Unlike diborane, the solid dimer has limited stability to the atmosphere, but should be stored in an inert environment. Commercially available solutions should be maintained under inert atmosphere and anhydrous conditions. Due to its steric bulk, 9-BBN is seen to be more selective than diborane. This is even observed in reactions where the 9-BBN group is a non-reacting center.1... 

Reaction of borane (as a complex with dimethyl sulphide) with optically pure (+)-a-pinene (in 15% excess) results in the almost exclusive formation of di-isopinocampheylborane (Ipc2,BH)67 (Section 4.1.7, p.421). The use of this optically pure borane in the hydroboration of the prochiral (Z)-but-2-ene, followed by oxidation, yields (R)-(—)-butan-2-ol (ee 98.1%) (Expt 5.45). Interestingly, monoisopinocampheylborane (IpcBH2) with (Z)-but-2-ene gives (S)-(+)-butan-2-ol, and hence the importance of careful reagent preparation is emphasised.68... 

Dibromo(p-chlorophenyl)borane, as catalyst, 9, 223 Dibromoethynes, in iron cluster compounds, 6, 296 (Dibromomethyl)methyldiphenylsilane, in Grignard reagent preparation, 9, 40... 

The synthesis of 9-alkenyl-9-BBN via hydroboration of terminal alkynes with 9-BBN suffered from the formation of 1,1-diborylalkanes via dihydroboration along with desired monohydroboration products. Alternatively, selective monohydroboration of terminal alkynes with dicyclohexylborane was followed by transmetallation with 9-MeO-9-BBN (Equation (193)).710 Treatment of the hydroboration intermediates with DIBAL-H in the presence of a borane-trapping reagent such as 1-hexene gave 1-alkenylaluminum compounds with complete retention of the (E)-stereochemistry (Equation (194)).711... 

Borane and lithium borohydride are a most useful pair of reducing agents, with opposite selectivities. Japanese chemists used an enzyme to make a single enantiomer of the acid below, and were able to reduce either the ester or the carboxylic acid by choosing lithium borohydride or borane as their reagent. Check for yourself that the lactones (cyclic esters) in black frames are enantiomers. 

These difficulties can be circumvented by application of alkyl- or aryl-substituted boranes as hydroborating agents. The reagents are obtained either by partial hydroboration of hindered alkenes [Eqs. (ae), (af), (bj)-(bl)] or by other methods, e.g., redistribution or reduction - ... 

Benzylic and allylic iodides may readily be coupled in excellent yields under mild conditions via air-induced iodine abstraction using triethylborane as reagent. Mixed coupling. Air passed 45 min. into a mixture of 1 4 benzyl and allyl iodides, and triethylborane in tetrahydrofuran at a rate of 50 ml/min so that the oxygen can be consumed completely by the triethylborane present 4-phenyl-l-butene. Y 72% by GLC. - An excess of the less expensive iodide may be used to increase the conversion to the more valuable material, since the coupling gives a nearly statistical distribution of products. F. e., also prepn. of iodides from boranes, s. A. Suzuki, H. C. Brown et al.. Am. Soc. 95, 1508 (1971). 

---