Conjugated dienes, reaction with borane

Hydroboration of conjugated diene, e.g., monohydroboration of 1,3-cyclohexa-diene with disiamylborane95 yields the corresponding allylic boranes. A similar reaction with 9-BBN affords B-2-cyclohexenyl-9-BBN in 95% yield (Eq. 41). 

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. 

Conjugated enynes are of importance for themselves, as well as for the synthesis of conjugated dienes. The cross-coupling reaction of 1-alkenyl(disiamyl)boranes (3c) with 1-bromo-l-alkynes (Scheme 2-34) provides conjugated enynes in high yields [45]. 

Related techniques have been developed to prepare (Z-,Z)- (Z-, )- and ( -, )- dienes. Hydroboration of diacetylenes followed by protonolysis is a convenient route to (Z-,Z)-dienes, as in the conversion of 89 to 90. The requisite symmetrical diacetylenes are prepared by oxidative coupling with oxygen and cuprous chloride, as in the conversion of 1-cyclohexylethyne (78) to 89. Unsymmetrical conjugated dienes can be prepared by formation of a diacetylene ate complex, prepared from disiamylmethoxyborane by sequential reaction with different acetylides. A similar borane route to unsymmetrical diacetylenes uses dicyclohexyl methyl-thioborane. ... 

Boron.—Preparation of Organoboranes. The year 1977 was one of consolidation in hydroboration. Detailed investigations of the reactions of 9-borabicy-clo[3,3,l]nonane (9-BBN) with cycloalkenes and non-conjugated dienes were reported in full. The reagent shows a remarkable degree of positional and stereochemical selectivity during hydroboration. Details of the applications of borane-dimethyl sulphide have appeared, and hydroborations using dimethyl... 

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