From vinyl halides

Vinylic lithium reagents (26) react with silyl peroxides to give high yields of silyl enol ethers with retention of configuration. Since the preparation of 26 from vinylic halides (12-37) also proceeds with retention, the overall procedure is a... 

Axelrad, G., Laosooksathit, S., and Engel, R., A direct synthesis of vinylic phosphonates from vinylic halides, Synthetic Commun., 10, 933, 1980. 

Table 4. Preparation of Fluorinated Vinylzinc Reagents from ( )-Vinyl Halides and Zinc Metal [7/0]...

One requirement is diat the pKa of die new organometallic compound is lower than die pKa of die starting organometallic. This in essence means that the equilibrium is driven to products by die formation of a more stable anion. This metiiod is commonly used to make vinyl lithiums from vinyl halides and alkyl lithiums and aryl lithiums from aryl halides and alkyl litiiiums because the electron pair in an sp2 orbital of a vinyl or aryl litiiium compound is more stable than the electron pah in an sp3 orbital of an alkyl lithium. 

Palladium acetate triarylphosphine complexes catalyze the addition of vinylic groups from vinylic halides to olefinic compounds in the presence of amines. Conjugated dienes are major products from 0,/3-unsaturated acids, esters, or nitriles while unactivated olefinic compounds react best in the presence of secondary amines where allylic amines are major products. The reactions are usually regio- and stereospecific. The synthetic utility of the reaction is illustrated with a wide variety of examples. 

Strong bases like alkyllithiums or drastic reaction conditions are required to generate carbenic species from vinyl halides, thus precluding the presence of many functional groups in the substrate. The vinyl-A3-iodane method produces free alkylidene carbenes under mild conditions, making the reaction compatible with a variety of functional groups. 

E2 elimination from vinyl halides how to make alkynes... 

This coupling is equally applicable to alkyl-, aryl-, and vinyllithium reagents. The products are obtained in generally good yield and in 98-100% isomeric purity. The reaction can also be used to prepare vinyl sulfides from vinyl halides and a lithium thiolate (equation II). 

Most Friedel-Crafts reactions involve carbocation electrophiles. Because the carbocations derived from vinyl halides and aryl halides are highly unstable and do not readily form, these organic halides do not undergo Friedel-Crafts alkylation. 

Hydrogen halide eliminations take place from 1,1- and 1,2-dihaloalkanes (equations 5 and 6), as w ell as from vinyl halides (equations 7 and 8). Dehydrohalogenation... 

The mechanism and stereochemsitry of dehydrohalogenations from vinyl halides have been extensively reviewed - It should however be pointed out that the effects of particular bases and solvents and of temperature cannot always be predicted with confidence as to rate of reaction and product distribution (acetylene, allene, diene). Therefore a variety of combinations of base, solvent and reaction conditions should be tried in order to obtain satisfactory results. 

Dehydrohalogenation by oxygen bases from vinyl halides proceeds readily by trans elimination via an 2-type of mechanism. c/s-Elimination is sluggish or does not occur at all. Thus trans elimination from bromovinyl ethers 2 (R = alkyl) furnishes the acetylenes 3 in a fast reaction in about 90% yield, whereas elimination from 4 (R = alkyl) is very sluggish and yields a mixture of the acetylene 3 and allene 5 (equations 17 and 18). Similar observations were shown in Schemes 1 and... 

It has recently been observed that the fluoride ion can promote elimination from vinyl halides to yield acetylenes. Thus Et4NF in MeCN at 25 C converts compounds 14 to acetylenes IS in 60-97% yields, whereas compounds 16 (R = H) under the same conditions do not react at all compounds 16 (R = Me) afford the allenes 17... 

R = Me) in 30-48% yields. With KF in DMSO even higher yields are obtained. Thus acetylenes 15 are obtained in 50-94% yields from 14 at 80-120 °C, whereas allenes 17 (R = Me) are obtained from 15 (R = Me) in 70-93% yields at 100 °C. Use of KF in the presence of a crown ether increases the rate of elimination. The above results make the fluoride ion an effective elimination agent for the preparation of acetylenes from vinyl halides in which the hydrogen and halogen are trans-related . 

The homocoupling of alkenylhalides was also reported, which demonstrated the interest of the Ni catalyst method in the formation of substituted 1,3-butadienes from vinylic halides [88]. 

Table 3. Vinyluthiums Prepared from Vinyl Halides with Li Metal...

The base-induced dehydrohalogenation of vinyl halides and allyl halides often gives low yields of allenes because of the competing reaction to alkynes alkynes can either be formed by direct elimination from vinyl halides or by isomerization of the allene first formed to the isomeric alkyne. Since it has been established that anti elimination of hydrogen halide from vinyl halides to yield alkynes is much faster than syn elimination, the proper choice of the starting material is often important for a successful allene synthesis. When ( )-4-bromo-4-octene was treated with NaOMe, the sole product was 3,4-octadiene, whereas the conesponding Z-educt yielded 4-octyne (Scheme 66). ... 

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