Synthesis of 2-substituted buta-1,3-dienes from 1,4-dichlorobut-2-yne via organoboranes
The reaction of 1,4-dichlorobut-2-yne 1 with a stoichiometric amount of di-sec-alkylborane 2, prepared by the hydroboration of a sterically hindered internal alkene with BH3 in tetrahydrofuran (THF), gave (Z)-(1,4-dichlorobut-2-en-2-yl)di-sec-alkylborane 3 stereospecifically. Treatment of compound 3 with methyllithium resulted in migration of an alkyl group from the boron atom to the adjacent carbon atom with elimination of two chlorine atoms to provide 2-sec-alkylbuta-1,3-dienes 5a–d. Similar treatment of (Z)-(1,4-dichlorobut-2-en-2-yl)-tert-alkyl-primary-alkylborane, prepared by the successive reaction of BH3 in THF with a tetrasubstituted ethene, relatively hindered terminal alkene, and compound 1, provided highly pure 2-tert-alkylbuta-1,3-dienes 5e and 5f whose alkyl group was derived from the tetrasubstituted alkene. On the other hand, similar treatment of compound 3, derived from a terminal or sterically unhindered internal alkene by a modified hydroboration procedure, provided the corresponding 2-primary-(or 2-sec-)alkylbuta-1,3-dienes 5g–m. 2-(Alk-1-ynyl)buta-1,3-dienes 6a–c were provided by the successive reaction of dibromoborane-dimethyl sulfide with compound 1, alk-1-ynyldiethylaluminium and methyllithium, although the yields were less good. Successive treatment of (Z)-(1,4-dichlorobut-2-en-2-yl)bis-(1,2-dimethylpropyl) borane 3b with alkylthiomagnesium bromide and methyllithium afforded exclusively 2-alkylthiobuta-1,3-dienes 8a–f whose alkylthio group migrated from the boron atom via the borate complex.