Disubstituted Z-alkenes

Other disubstituted boranes have also been used for selective hydroboration of alkynes. 9-BBN can be used to hydroborate internal alkynes. Protonolysis can be carried out with methanol and this provides a convenient method for formation of a disubstituted Z-alkene.217... 

Asymmetric epoxidation of disubstituted Z-alkenes using a chiral... 

Disubstituted Z-alkene Disubstituted E-alkene Trisubstituted alkene... 

ASYMMETRIC EPOXIDATION OF DISUBSTITUTED Z-ALKENES USING A CHIRAL SALEN-MANGANESE COMPLEX111... 

Epoxidation using manganese salen complexes is very easy to carry out it occurs under aqueous conditions and commercial house bleach can be used as the oxidant. The results are similar to those reported in the literature Table 6.1 gives other examples of alkenes which can be epoxidized using the same procedure. This method gives good results, especially for disubstituted Z-alkenes but trisubstituted alkenes can be epoxidized as well. 

Table 6.1 Epoxidation of disubstituted Z-alkenes by (5,5)-salen-manganese complex (results according to the literature11,51).

Shi s method gives good results for disubstituted /f-alkenes compared to the Jacobsen epoxidation previously described, which is more specific for disubstituted Z-alkenes. Concerning the epoxidation of trisubstituted alkenes, the epoxidation of 1-phenyl-1-cyclohexene could not be validated because of... 

Diisopinocampheylborane is usually used for the hydroboration of relatively unhindered, disubstituted Z-alkenes. Optical purities of the product alcohols are high (Table 2) while the absolute configuration is consistently the same for similar structures. Models predicting the product configuration in the hydroboration of alkenes with mono- and diisopinocampheylborane have been proposed54-59. The preferred transition state structure for the reaction of (Z)-2-butene with ( — )-diisopinocampheylborane is shown in Section D.2.5.2.1.3. 

For analogous cyclopropanation reactions of trisubstituted and 1,2-disubstituted (Z)-alkenes, ligand (1) is less well suited. In... 

Cross-metathesis will then also afford ethene, which will escape the reaction because of its volatility. Alternatively, a 1,2-disubstituted (Z)-alkene bearing the same substituent in position 1 and 2 may be used as the solution component. 

The highest stereoselectivities are reached with disubstituted (Z) -alkenes (with ee-values up to 99%) as well as trisubstituted alkenes, whereas mono-substituted olefins are poorer substrates. Concerning the mechanism of the Jacobsen epoxidation (see Ref. [21]), asymmetric epoxidation of non-activated olefins can be performed with numerous other transition-metal catalysts [22]. 

In general, non-catalyzed hydrometalation reactions including hydroboration proceed in a sy -fashion. When internal alkynes undergo hydrometalation followed by hydrolysis, disubstituted Z-alkenes can be accessed selectively representative examples of Ti-catalyzed hydroalumination and hydromagnesation are demonstrated in Fig. 15. In the first example, it was noted that the hydroalumination reaction in the absence of the Ti catalyst led to the -isomer [37], In the second example, it was shown that, in addition to hydrolysis, the alkenylmagnesium intermediate could undergo addition to aldehydes and alkyl halides to yield trisubstimted alkene products in high stereoselectivity [38]. 

In the earlier, longer approach to (Z)-and (E)-alkenes, ThxR BH was used iastead of IpcR BH. It is also possible to prepare a-chiral acetylenes and alkanes by this method (76,520). In a shorter synthesis of a-chiral alkynes, a prochiral disubstituted (Z)-a1kene is hydroborated with... 

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the alkyne. This avoids the complications which occur with borane that lead to polymeric structures. Catecholborane is a particularly useful reagent for hydroboration of alkynes.168 Protonolysis of the adduct with acetic acid results in reduction of the alkyne to the corresponding Z-alkene. Oxidative workup with hydrogen peroxide gives ketones via enol intermediates. 

In parallel fashion, compound 267, made from 2,3 5,6-di-<9-isopropyl-idene-D-mannono-1,4-lactone by Reformatsky addition using ethyl bromo-acetate and zinc-silver graphite, undergoes ready elimination when converted to the methanesulfonic ester to give a separable mixture of the E- and Z-alkenes (268).251 Quite a different approach involves treatment of the 1-C-nitromannofuranosyl chloride, derived from the oxime of 2,3 5,6-di-O-isopropylidene-D-mannose, with the anion of diethyl malonate to give the disubstituted alkene 269.252... 

Stereoselective conversion of 3,4-disubstituted-l,2-dithietane 1,1-dioxides 114 into symmetrical (Z)-alkenes 115 has been reported. When substituted 1,2-dithietane 1,1-dioxides 114 having a reactive thiosulfonate group in the four-membered ring were treated with lithium cyanide, the alkenes 115a-d were obtained with high (Z)-selectivity and high chemical yield (Equation 15) <1996BSF515>. 

While the additions to these disubstituted alkenes 272 are in fact faster than the additions to their monosubstituted analogues (and additions to E alkenes are faster than additions to Z alkenes), the product organolithiums are nonetheless rather unstable and, especially with R = SiMe3, tend to deprotonate solvent. In these cases the electrophilic quenches are therefore best carried out at 0 °C. 

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. 

We have seen the relative reactivities of structurally different alkenes towards various dialkylboranes vary over a broad range of differences, e.g., disiamylborane 431 and 9-BBN481 vary in a range of 105. This makes selective hydroboration of the more reactive alkenes possible in the presence of less reactive one. Disubstituted internal (Z)-alkenes are more reactive to disiamylborane than their ( T)-isomers (Eq. 125). Recently, it has been observed that the selectivity is much higher with ThxBHCl SMe2... 

The Ji bond has electron density both above and below the C bond as the parallel p orbitals overlap locking the bond rigid. Hence no rotation is possible about a double bond—the Tt bond must be broken before rotation can occur. One consequence of this locking effect of the double bond is that there are two isomers of a disubstituted alkene. One is called a cis or Z alkene, the other a trans or E alkene. 

The diastereomeric ratio is high only for ( )-disubstituted alkenes and norbornene, but low for cyclic and (Z)-alkenes, reflecting the order of stability of the intermediate complexes. The loss of stereochemical purity is probably due to base-catalyzed nitroso-oxime tautomerization during the reduction. However, the use of diisobutylaluminum hydride, before adding lithium aluminum hydride, allows (3/ , 4S )-3,4-hexanediamine to be obtained from (Z)-3-hexene with d.r. [(3R, 4S )IQR AR )] 90 10. 

The Cope elimination has been used to synthesize a variety of acyclic alkenes. 1,2-Disubstituted acyclic alkenes are usually obtained with the ( )-stereochemistry because of steric hindrance to (Z)-alkene formation. The stereochemistry of trisubstituted alkenes is determined by the configuration of the starting amine oxide and is not usually affected by subsequent isomerization. ... 

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