Heterocyclic olefins hydroboration

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

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). 

The relative rates of hydroboration of representative heterocyclic olefins with 9-BBN have been studied [72] at temperature 25 °C, as most of the large amount of data for relative reactivities with 9-BBN is available at this temperature. These rates of reaction for heterocyclic olefins are compared with SiajBH as the hydroborating agent at 0 °C, which is described by Zweifel and Plamon-don [50]. 

Brown et a/ [1] have elegantly hydroborated a variety of heterocyclic olefins, which on oxidation yield the corresponding alcohols in excellent yields. In the case of heterocyclic olefin containing a double bond a to the heteroatom, the hydroboration reaction is highly regioselective, placing boron at the (3-carbon atom. The use of 9-BBN for the synthesis of variety of heterocyclic alcohols is given in Chart 6.21. 

Certain hydroboration reactions which are slow under normal conditions due to their heterogeneous nature proceed much faster under the effects of ultrasound. Several acyclic and cyclic trienes have been hydroborated with borane or triethylamineborane in THF and the stereochemistry of the products investigated. A systematic study of the hydroboration of representative heterocyclic olefins with four different reagents has been undertaken in order to establish optimum conditions for the reactions. This study has been followed by an investigation into the relative rates of reaction of 9-BBN with oxygen and sulphur containing heterocyclic olefins. ... 

RCM of dienes to cycloalkenes provides a useful method for the syntheses of carbo- and heterocycles and thus has been proved to be extremely effective in total synthesis of various natural products. Usually, however, mixtures of (E)- and (Z)-olefms result. In contrast, ring-closing alkyne metathesis provides a reliable route for synthesis of both (E)- and (Z)-macrocycloalkenes in a stereoslective manner taking advantage of stereoselective partial reduction of resulting cycloalkynes. A Lindlar reduction gives (Z)-cycloalkenes, whereas a hydroboration/ protonation sequence afford ( )-cycloalkenes (Equation (23)). Recently, Trost reported an alternative procedure for the synthesis of (E)-olefins from alkynes through hydrosilylation by a ruthenium catalyst. This procedure converts cycloalkyne 130, for example, to vinylsilane 131 and then to (E)-cycloalkene 132 in a stereoselective manner (Scheme 46)7 ... 

The sulphur and nitrogen heterocycles (80) and (81) have been prepared by hydroboration-carbonylation of bis-olefinic precursors, and the latter case is reported to be the first example of such a synthesis in the presence of a reducible functional group. Another hydroboration-carbonylation sequence has been used in the preparation of methylene cycloalkanes from cycloalkenes, and is shown in Scheme 11. ... 

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