S alkene

If however a t-butoxide was used as base, only the thermodynamically favored S-alkene 9 was formed, suggesting a ElcB-mechanism in that case. It has... 

Alkynes can often be reduced selectively to the alkene by use of the Lindlar catalyst [Pd on CaCOj, partly poisoned with Pb(OAc)2]. Here again SYN stereoselectivity is observed despite the fact that this will lead to the more crowded, thermodynamically less stable, n s-alkene, i.e. (52) rather than (53). 

In most cases, a multistep synthesis is not a simple string of reaction steps. You need to look ahead (and behind). For example, you may have a perfectly good reaction that forms a c/s-alkene, but that s the wrong answer because two steps later, you need a trans-alkene. 

Acetylenic precursors employed in the syntheses of sugars may be divided into three groups (a) aldehydes (usually in the form of acetals), (b) alkyl alkynyl ethers, and (c) alkynols or alkynediols. Some of them are commercially available (for example, 2-butyne-l,4-diol), and others are prepared by Grignard-type reactions between 1-alkynylmag-nesium halides or lithium alkynes and suitable aldehydes, ketones, or epoxides. In this way, the synthesis of substrates having the desired number of carbon atoms, as well as the necessary functional groups, can be achieved. The next step consists in partial saturation of the triple bond to afford the desired cis- or trans-alkene. ct.s-Alkene systems... 

M- Alkylaziridines can be stereospecifically deaminated to alkenes by reaction with m-chloroperbenzoic acid (70AG(E)374>. The reaction and work-up are carried out in the dark to avoid isomerization of the c/s-alkene, and the mechanism is thought to involve an initial oxidation to an amine oxide followed by a concerted elimination. Aziridine oxides have been generated by treating aziridines with ozone at low temperatures (71JA4082). Two... 

S-methyl dithiocarbonate (76S413). Stereoselective isomerization of 1,2-disubstituted alkenes may be achieved by a sequence such as the following rra/ts-alkene -> bromo-hydrin -> /3-hydroxythiocyanate -> cis-thiirane — c/s-alkene (75TL2709). 

Preparation of cis-alkenes Lindlar s catalyst, which is also known as poisoned catalyst, consists of barium sulphate, palladium and quinoline, and is used in selective and partial hydrogenation of alkynes to produce c/s-alkenes. Hydrogen atoms are delivered simultaneously to the same side of the alkyne, resulting in syn addition (cw-alkenes). Thus, the syn addition of alkyne follows same procedure as the catalytic hydrogenation of alkyne. 

The absorption of symmetrical disubstituted trans-alkenes or tetrasubstituted alkenes may be extremely weak or absent. The c/s-alkenes, which lack the symmetry of the trans structure, absorb more strongly than frans-alkenes. Internal double bonds generally absorb more weakly than terminal double bonds because of pseudosymmetry. 

Lindlar catalyst (Section 9.9) A catalyst for the hydrogenation of alkynes to c/.s-alkenes. It is composed of palladium, which has been poisoned with lead(II) acetate and quinoline, supported on calcium carbonate. 

As an illustration of the stereospecificity of eliminations, the meso compound 4 gives the c/s-alkene 5, whereas the d,l isomers 6 give the trans-alkene 7 with ethoxide. Both reactions clearly proceed by antarafacial elimination ... 

Requires a transition metal catalyst, Pt, Pd, Ni, etc. Addition is supra-facial from least hindered side. Rearrangements can occur. Alkynes are reduced to c/ s-alkenes over Lindlar catalyst, Pd-Pb (Section 11-2). 

Temperature (°C) [C6H6]/[Alkene] Contact Time (s) Alkene Conversion (%)... 

In the system with three CF2 groups, i.e. 22, the radical takes on perfluoroalkyl character and the impact on cyclization rate is magnified still further. The dominant factor which has been credited for giving rise to the high reactivities of perfluoro-n-alkyl radicals in their additions to alkenes, particularly to electron-rich alkenes, is their high electrophilicities. That is, charge transfer interactions, e.g. [(CF3CF2CF2) s (alkene)l5+]] stabilize an early transition state and lower both the enthalpic and entropic barriers to reaction. 

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