Zinc reagents diorganozincs

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 opening of epoxides with zinc reagents is a difficult reaction. However, activated epoxides such as the glycal epoxides 225 react with diorganozincs in the presence... 

The remaining unreacted (RJ-20 can be trapped with an aldehyde such as pivalaldehyde giving the chiral homopro-pargylic alcohol in 87.5% ee.74 This kinetic resolution has been used to prepare anti/inti-vicinal amino diols in > 95% ee and dr > 40 l.75 In general, diorganozincs are more easily prepared in optically pure form. On another hand, secondary zinc reagents prepared by the direct zinc insertion to secondary alkyl iodides are obtained without stereoselectivity (Scheme 4).76... 

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