Alkenes rrans-disubstituted

Asymmetric hydroboration.2 Extensive studies indicate that 1 is the reagent of choice for chiral hydroboration of rrans-disubstituted alkenes and trisubstituted alkenes. The corresponding alcohols are obtained in 72-100% ee and all have the same absolute configuration. Surprisingly, this configuration is the opposite to that obtained by hydroboration with diisopinocampheylborane. 

The Julia olefination reaction is highly regioselective and ( )-stereoselective, providing a valuable alternative to the Schlosser reaction for making rrans -disubstituted olefins. The reaction involves condensation of a metalated alkyl phenyl sulfone with an aldehyde to yield a P-hydroxysulfone, which is then subjected to a reductive elimination to produce the alkene. There are limitations to the preparation of tri- and tetra-substituted alkenes via the sulfone route because the P-alkoxy sulfones derived from addition of the sulfone anion to ketones may be difficult to trap and isolate or they may revert back to their ketone and sulfone precursors. 

An 11 kJ/mol (2.7 kcal/mol) stability difference is typical between a monosubstituted alkene (1-butene) and a trans-disubstituted alkene (rrans-2-butene). In the following... 

A study carried out by Kocienski and Lythgoe flrst demonstrated the trans selectivity of the Julia coupling process. The authors found the i uctive elimination could best be carried out with the acet-oxy or benzoyloxy sulfones. If the lithio sulfone derivative is used for addition to the carbonyl, the reaction can be worked up with acetic anhydride or benzoyl chloride to obtain the alkene precursor. In cases where enolization of the carbonyl is a complication, the magnesium derivative can frequently be used successfully. A modification of the reductive elimination was found to be most effective. Methanol, ethyl acetate/methanol or THF/methanol were the solvents of choice and a temperature of -20 C was effective at suppressing the undesired elimination of the acetoxy group to produce the vinyl sulfone. With these modifications of the original procedure, the ability of the reaction to produce dienes as well as rran.r-disubstituted alkenes was demonstrated, llie diastereoisomeric erythro- and threo-acetoxy sulfones could be separated and it was demonstrated that both isomers were converted to the rrans-alkene. It... 

More highly functionalized products can be obtained, often in better yield, by the ene reaction of a-substituted acrylate esters. EtAlCh is a more effective catalyst than AICI3 for these reactions because it is a Brpnsted base as well as a Lewis acid. The EtAlCh catalyzed reactions of methyl a-bromo- and a-chloro-acrylate with rrans-1,2-disubstituted and trisubstituted alkenes are both regio- and stereo-specific. The major product (13a 85-95%) is formed via transition state (12a) in which the methoxycar-bonyl group is The stereochemistry of the major adduct was established by the stereospecific... 

The first examples of the enantioselective Simmons-Smith cyclopropanations mediated by a chiral catalyst are very recent. Scheme 6.33 shows three catalysts for the cyclopropanation of rrans-cinnamyl alcohol. The most selective appears to be Charette s dioxaborolane (Scheme 6.33c, [120-122], which also affords the highest yield of product, although this procedure is only suitable for small scale.With other olefins, such as cis and trans disubstituted alkenes and P,P-trisubstituted alkenes, the yields are nearly as good and the enantioselectivities are 96-97%. An important finding in this study [120] was that, in addition to the Lewis acid (boron) that binds the alcohol, a second atom to chelate the zinc is also necessary. In the... 

The enantioselectivity of the process, as reflected by enantiomeric excesses, shows a rough conelatioa with the substitution pattern of the alkene, as shown in Figure 7. Highest enantimneric excesses (70-90%) are obtained wiA rrans-l,2-disubstituted alkenes. Trisubstimted (35-80% ee) and monosubstituted (25- % ee) alkenes show greater variation in enantioselectivity while poorest enantioselectivity is observed with c/f-l,2-disubstituted alkenes (<25% ee). 

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