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In solution, 9-BBN is more susceptible both to hydrolysis and oxidation, and contact with the open atmosphere should be rigorously avoided. However, in contrast to other dialkylborane reagents e. Disiamylborane or Dicyclohexylborane which must be freshly prepared immediately prior to their use, 9-BBN-H dimer is a stable crystalline reagent 1 , 3 which is commercially available in high purity.
This feature of the reagent facilitates the control of reaction stoichiometry at a level unattainable with most borane reagents.
Competitive rate studies have revealed that electron-donating groups enhance the rates within these groups, e. Hydroborations of more substituted alkenes such as a -pinene 8 or 2,3-dimethylbutene 4b with 9-BBN-H are slower k 2 is rate-limiting and require heating at reflux temperature in THF for 2 and 8 h, respectively, for complete reaction to occur. While the monohydroboration of symmetrical nonconjugated dienes with 9-BBN-H is thwarted by competing dihydroboration because these remote functionalities act as essentially independent entities, with nonequivalent sites the chemoselectivity of the reagent can be excellent eq 4.
In contrast to the monohydroboration of allene itself, which gives a 1,3-diboryl adduct, 9-BBN-H is an effective reagent for the preparation of allylboranes from substituted allenes. Medium-ring boracycles are efficiently prepared by the dihydroboration of a , o -dienes with 9-BBN-H followed by exchange with borane.
Unlike most dialkylboranes, 9-BBN-H hydroborates alkenes faster than its does the corresponding alkynes, a feature which leads to the competitive formation of 1,1-diboryl adducts in the hydroboration of 1-alkynes with 9-BBN-H employing a 1: It is important to point out that the preference of 9-BBN-H to hydroborate alkenes in the presence of alkynes can have useful synthetic applications eq Organometallic reagents can provide very useful entries to many B -substituted 9-BBN derivatives.
These are particularly important for organoboranes which cannot be prepared by hydroboration. B -HaloBBN derivatives are effectively vinylated with organotin reagents. Hydroboration of the byproduct alkene gives isomeric B -alkylBBN products. Et 2 O to remove the methoxy moiety. The procedure has also been used for the preparation of cis -vinylBBN derivatives 19 since the normal route to such derivatives based upon the hydroboration of 1-haloalkynes, followed by hydride-induced rearrangement gives ring expansion products competitively with Z halovinylBBNs.
The B -acyloxyBBN derivatives have been employed in conjuction with borohydrides for the reduction of carboxylic acids to aldehydes 25 see Lithium 9-boratabicyclo[3. This process is best accomplished with Acetyl Chloride and provides a highly versatile entry to these organoboranes for subsequent conversions eq Reduction is slow with esters, carboxylic acids, nitriles, and epoxides, and does not occur with nitro compounds, nor with alkyl or aryl halides. Moreover, 9-BBN-H is a highly selective reducing agent for the reduction of enones to allylic alcohols eq As noted earlier, B -substitutedBBN derivatives are available from a variety of sources and organoboranes serve as a versatile entry to other functionalities.
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9-Borabicyclo []nonane, a commercially available reagent, is the most versatile hydroborating reagent to synthesize organoboranes (B-RBBN). Hydroboration and Organic Synthesis: 9-Borabicyclo[]nonane (9-BBN) By Ranjit S. Dhillon (Punjab Agricultural University, Ludhiana.
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