Organoboran, diorganozinc

In the case of primary organoboranes, the exchange reaction is best performed with Et2Zn, whereas less reactive secondary organoboranes require the use of iPr2Zn. Thus, a wide variety of terminal olefins have been converted into primary diorganozincs such as 87-89 (Scheme 2.32). 

Thus, the hydroboration of 1-phenylcyclopentene with (—)-IpcBH, (99% produces, after crystallization, the chiral organoborane 126 with 94% ee. The reaction of 126 with Et2BH replaces the isopinocamphenyl group with an ethyl substituent (50 C, 16 h) and provides, after the addition of i-Pr2Zn (25 °C, 5 h), the mixed diorganozinc 127. Its stereoselective allylation leads to the fraw5-disubstituted cyclopentane 128 in 44% yield (94% ee trans cis = 98 2) see Scheme 43 ° . This sequence can be extended to open-chain alkenes and Z-styrene derivative 129 is converted to the anf/ -zinc reagent 130, which provides after allylation the alkene 131 in 40% yield and 74% ee (dr = 8 92). 

The hydroboration of dienic silyl enol ethers, such as 124 with Et2BH leads to organoboranes that can be converted to new diorganozincs, such as 125 Scheme... 

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