Zweifel alkene synthesi

ZSM-5 catalyst ZSM-5 zeolite Zudopenthixol [53772-Zvyagintsevite Zweifel alkene synthesis Zwikker reaction... 

The synthesis of Z-disubstituted double bond was accomplished by Zweifel. The Zweifel s (Z)-alkene synthesis involves the iodination of dialkylvinylboranes in the presence of sodium hydroxide which results in the transfer of an alkyl group from boron and the formation of the (Z)-alkene (Eq. 62) 117118>, although the introduction of alkyl groups is limited. 

The Zweifel trans-alkene synthesis from dialkyl-1-alkenylboranes involves the hy-droboration of 1-halo-l-alkynes with dialkylborane (Eq. 69) 126). It is of interest that the base produces a transfer with inversion 126 127). 

Alkene synthesis (4, 37, 110). Evans et al. have developed variations of the Zweifel boron-mediated olefination reaction that lead to improved results. Boronic ester ate complexes (a) and (b) were used for the rearrangement, which... 

Di- and trisubstituted alkenes. A few years ago Zweifel and coworkers1 reported a stereospecific synthesis of cis-alkenes by treatment of the adduct of a 1-atkyne and a dialkylborane with I, and NaOH (equation I). The reaction is believed to involve an iodonium ion, transfer of one of the R groups, and trans-elimination of I - and BROH. 

Brown et al.3 have now extended the Zweifel synthesis to a stereospecific synthesis of trisubstituted alkenes from an internal alkyne (equation III). Evidently a similar mechanism is involved franr-addition of I+ and OC H 3 followed by trans-elimination of I and BROCH3 resulting in tram-orientation of the two alkyl groupsof the alkyne. 

Zweifel and his co-workers have also introduced a stereospecific synthesis of tri-substituted alkenes (Eq. 75) U7 135), although it has limitations of the use. 

However, the utility of this Zweifel synthesis was limited in the past by the limited availability of dialkylboranes, because direct hydroboration leads cleanly to the formation of dialkylboranes only in the case of relatively hindered alkenes such as 2-methyl-2-butene and cyclohexene. More generally, the hydroboration fails to stop at the R2BH stages. Recent developments have provided a general preparation of a variety of dialkylboranes via the hydridation of dialkylhalo-boranes. Thus, dialkylvinylboranes prepared via the hydridation of dialkylhalo-boranes in the presence of an alkyne, react with iodine under basic conditions to produce disubstituted alkenes (Eq. 58) and trisubstituted alkenes (Eq. 59) of established stereochemistry. These results indicate a mechanism analogous to that... 

Unfortunately, the Zweifel s hydroboration procedure suffers from the same dis-adventage as his synthesis of cis and trans alkenes. It requires R2BH as a reagent. Application of the new synthesis of RjBH via hydridation of R2BX made the procedure general for essentially all R groups available through hydroboration (Eq. 76)... 

Brown, H. C., Snyder, C., Rao, B. C. S., Zweifel, G. 1986. Organoboranes for synthesis. 2. Oxidation of organoboranes with alkaline hydrogen peroxide as a convenient route for the cis hydration of alkenes via... 

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