Alkenes stereoselectivity

This procedure illustrates a general method for the preparation of alkenes from the pal 1 adium(Q)-cata1yzed reaction of vinyl halides with organo-lithium compounds, which can be prepared by various methods, including direct regioselective lithiation of hydrocarbons. The method is simple and has been used to prepare a variety of alkenes stereoselectively. Similar stoichiometric organocopper reactions sometimes proceed in a nonstereoselective... 

Diborations of 1,3-dienes are carried out in the presence of platinum(O) catalyst. In the Pt(PPh3)4-catalyzed dibora-tion of 1,3-dienes, 1,4-addition takes place to give (Z)-l,4-diboryl-2-alkenes stereoselectively (Equation (10)).62 In contrast, the Pt(dba)2-catalyzed reaction of 1,3-pentadiene affords 1,2-addition product, in which more substituted C=C bond is left intact (Scheme 5). The use of Pt(dba)2 as catalyst also enables the diborative dimerization of isoprene to occur (Equation (ll)).48... 

With sulfur and triethylamine in dimethylformamide, compounds (86) give (89) (Equation (21)) <89JPR243>. With thiols or KjS, sulfides are produced (e.g., (90) and (91)), which show spasmolytic activity in vitro <86MI 404-01 >. Smooth 5N2-displacements of chlorine occur with phenol, thiophenol, SCN, and thiourea in dimethylsulfoxide. However, with K2CO3 or sodium cyanide, (86) (Ar = Ph) affords the trimers (92) or (93) (Scheme 32). With electron-deficient trans alkenes, stereoselective formation of cyclopropanes (94) was observed (Scheme 32). No reactions occur with maleates <66HCA412>. 

Fig. 16.20. Alkenylation of isomeric alkenylboronic acid esters with isomeric iodo-alkenes stereoselective synthesis of isomeric 1,3-dienes.

How would you transform this alkene stereoselectively into either of the diastereo isomers of the amino-alcohol ... 

Azetidines having a 4-methoxyphenyl, styryl, or 2-furyl group at C(2) and a benzyl or allyl substituent on nitrogen, when exposed to diethylaluminium chloride, give alkenes, stereoselectively through a fragmentation process (Scheme 29) <1999JOC9596>. 

Many oxo-metal complexes efficiently epoxidize alkenes. Stereoselectivity in these epoxidations is most often achieved by precoordination of functional groups in the substrates. If the metallic centers are embedded in a chiral environment that allows stereoselectivity to rely solely on nonbonded interactions, enantioselective epoxidation may be extended to nonfunctionalized alkenes16. 

Bromine azide, prepared by different methods, undergoes addition to alkenes stereoselectively by an ionic mechanism 33- 37 (Table 2). Improved yields are obtained by using azidotrimethyl-silane and A-bromosuccinimide (NBS) in the presence of Nafion-FI as catalyst33. Alternatively, A-bromosuccinimide may be added to the mixture of the alkene and sodium azide in dimeth-oxyethane/water34. Aziridines were prepared stereoselectively by reduction of (i-bromo azides with lithium aluminum hydride34. 

Periodate compounds have been used for the epoxidation of simple olefins too (MIO4, MH4O6, or M2H3IO6 M = Li, Na, K, Rb, Cs, etc.). In alkaline medium, xenon-trioxide epoxidizes alkenes stereoselectively there is no c/s-hydroxylation as when other inorganic oxides are employed. ... 

Alkenic stereoselection in the Still-Wittig rearrangement results presumably from conformational control in a very early transition state (Scheme 2). There, the substituent R prefers equatorial (15) over axial (16) orientation, if the 1,2-allylic strain is smaller than the 1,3-allylic strain, and vice versa. This implies trans products in the former vide supra) and (Z)-products in the latter case vide infra). 

In addition to thermal extrusion, a variety of reagents have been shown to facilitate the loss of sulfur from thiiranes and thiirenes <83HC(42/i)333>. The most frequently encountered reagents for this purpose are trivalentphosphines (both trialkyl and triaryl) which give alkenes stereoselectivity. 

Finally the benzyl group was removed, the primary alcohol oxidised to the aldehyde, and an ii-selective Wittig reaction performed with the enantiomerically pure stabilised ylid 137. No racemisation of either partner occurred and the product was almost pure E at the new alkene. Stereoselective reduction gave (+)-pumiliotoxin B 129 identical to the natural product in all respects, including biological effects. 

Epoxidation. Oxone is used to generate dioxirane from a ketone added to the reaction medium. Such dioxiranes epoxidize alkenes stereoselectively. 2-Cyclo-hexenol gives two epoxy alcohols in a ratio of 77 23 (trans cis). 

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