Hydroboration of Heterocyclic Olefins

Brown and his coworkers have systematically studied the hydroboration of 3-butenyl [3a], 2-butenyl [3b], and 1-butenyl [4] derivatives containing representative substituents. It was found that organoboranes containing a heteroatom at the (3 position tend to undergo 1,2-elimination (Eq. 5.15) [3]. 

The hydroboration of heterocyclic olefins having oxygen [50-56], sulfur [57], and nitrogen [58-68] are reported. Brown and coworkers [69] have conducted detailed hydroboration studies of many heterocyclics with an endocyclic double bond with borane-methylsulfide BMS, 9-BBN, CtuqBH, and SiajBH and have established the optimum conditions for clean and quantitative hydroboration. The hydroboration of 2,3- and 2,5-dihydrofuran with BMS (3 1 molar ratio) at 25 °C for 1 h afibrds trialkylborane, readily oxidized to 3-hydroxytetrahydro-furan, in excellent yield. But the synthesis of dialkylboranes from these olefins using olefins, BMS in 2 1 ratio is not possible at 0 °C. However, hydroboration of 2,3-dihydrofuran proceeds cleanly with 9-BBN in 1 1 ratio and oxidation affords the desired 3-hydroxytetrahydrofuran, in excellent yield. Similar results are obtained with dicyclohexylborane and disiamylborane (Table 5.14). 

At higher temperatures, the ring ruptures to afford 3-buten-l-ol or diol. At 25 °C 2,5-dihydrofuran has also been hydroborated, and comparative results are summarized [69] in Table 5.14. 

The experimental conditions for the hydroboration of 2-methyl-4,5-dihydro-furan, 2,3-dihydrothiophene and N-(benzyloxycarbonyl)-3-pyrroline have been optimized (Table 5.15) [69], and no ring cleavage is observed under these experimental conditions. 

The hydroboration of 3,4-dihydropyran with BMS at 25 °C affords the product also arising from the ring cleavage, while good yield of 3-hydroxytetrahy-dropyran is obtained with BHj-THF in 2-olefin-BH3-THE However, 9-BBN, ChXjBH, and SiajBH afford 3-hydroxytetrahydropyran in excellent yield (Table 5.16) [69]. 

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Brown, H. C., Prasad, J. V. N. V. Hydroboration of heterocyclic olefins - a versatile route for the synthesis of both racemic and optically... 

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