Hydroborating alkenes

Hydroboration.1 The usual hydroboration reagents, BH3THF and BH3-S(CH3)2, are sensitive to oxygen and moisture and require special handling. 1 he complexes of BH3 and phosphorus compounds are generally stable, but much less reactive. The complex of BH3 and triphenylphosphine, m.p. 189°, can be used for hydroboration if activated by addition of methyl iodide (to form a phosphonium iodide) or sulfur (to form a triphenylphosphine sulfoxide). The complex of borane and triphenyl phosphite does not require activation and hydroborates alkenes in a reasonable time in refluxing DME or THF. Trialkyl phosphite complexes are not useful. 

Dimethylborane has been used to hydroborate alkenes regiospecifically. The resulting dimethylalkylborane reacted with ammonium hydroxide and sodium hypochlorite (generation of NH2C1 in situ) to yield isomerically pure alkylamines [21] (Table 2.1). 

The reactions of hindered alkenes with thexylborane are generally accompanied by a significant amount of dehydroboration to give 2,3-dimethyl-2-butene. Thus, a very large excess of the latter is needed in order to produce (monomeric) dithexylborane. Hydroboration of a-pinene (1 1 ratio) with thexylborane results in ca. 15% dehydroboration.However, alkenes of somewhat lower steric requirements, such as simple 1,2-disubstituted ethylenes, can be converted fairly cleanly into the corresponding thexylmonoalkylboranes e.g. equation 13). These are mixed dialkylboranes and behave like other fairly hindered dialkylboranes (Section 3.10.4.3). They can hydroborate alkenes of lower steric requirements to give totally mixed trialkylboranes. 

Catecholborane is somewhat more reactive than (1), though still far less reactive than dialkylbo-ranes. 27 28 It is readily prepared by the reaction of catechol with borane-THF, is stable at 0 "C, and hydroborates alkenes slowly and alkynes (equation 49) more rapidly in refluxing The rate of... 

Organoboranes possessing one B—C bond in the molecule can also be obtained by hydroborating alkenes and alkynes with heterosubstituted borane derivatives, especially dihalogenoboranes, which can be stabilized by complexation with amines, phosphines, ethers and sulfides. The last two are useful complexing agents for both the preparation and the reactions of dihalogenoboranes . The most convenient synthesis of dichloroborane etherate is the reaction of LiBH. with BCl, in Et,0 ... 

Lower dialkylboranes are reactive and hydroborate alkenes rapidly, even without a solvent below 0°C. The reactivity depends on the alkyl group, decreasing primary > secondary > cyclo-alkyl. Regioselectivity approaches that of RjBH with bulky substituents. Unfortunately, mixtures of products are formed because the reagents disproportionate and are in equilibrium with R3B and alkylated diboranes ... 

Dialkylboranes with bulky alkyl groups do not disproportionate so readily and hydroborate alkenes of lower steric hindrance than the parent alkene to give the corresponding unsymmetrical trialkylboranes in high yield and purity. Bis(3-methyl-2-butyl)borane is one such reagent, synthesized by controlled hydroboration of... 

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 1 stoichiometry (eq 8). In some cases, the (fs)-l-alkenyl-9-BBN derivative can be efficiently prepared by employing either a large excess of the alkyne or through the use of 1-trimethylsilyl derivatives. However, these vinylboranes are now perhaps best prepared through the dehydroborylation of their 1,1-diboryl adducts with aromatic aldehydes (eq 8). ... 

General Reactions.- Borane-1,4-thioxane hydroborated alkenes rapidly, and was used to prqKire synthetically useful monoalkyl- or dialkylboranes the 1,4-thioxane byproduct did not interfm-e with with the subsequrat utilisation of these reagents. The addition of catecholbor-ane to 2-substituted l-buten-3-ynes was catalysed by phosphine palladium(0) complexes the... 

Hydroboration of Alkenes. Dibromoborane-dimethyl sulfide hydroborates alkenes directly without need for a decomplex-ing agent (eq 1). ... 

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