PREPARATION OF OLEFINS

Organic Reagents. Amine oxides are used ia synthetic organic chemistry ia the preparation of olefins, or phase-transfer catalysts (47), ia alkoxylation reactions (48), ia polymerization, and as oxidizing agents (49,50). 

In addition to being a valuable method for the preparation of olefins, the Cope elimination reaction also gives access to iV,A -disubstituted hydroxylamines. 

The Corey-Winter reaction provides a useful method for the preparation of olefins that are not accessible by other routes. For instance it may be used for the synthesis of sterically crowded targets, since the initial attack of phosphorus at the sulfur takes place quite distantly from sterically demanding groups that might be present in the substrate molecule. Moreover the required vicinal diols are easily accessible, e.g. by the carbon-carbon bond forming acyloin ester condensation followed by a reductive step. By such a route the twistene 10 has been synthesized ... 

This method can also be utilized as a general method for the preparation of olefins with terminal difluoromethylene groups from aldehydes.8 Also, by the substitution of tributylphosphine for triphenylphosphine in this procedure, ketones other than those containing an a-perfluoroalkyl group can be converted to terminal difluoromethylene compounds.9... 

The intramolecular elimination of water from alcohols, which constitutes the usual method for the preparation of olefines, does not proceed so simply, when concentrated acids are used, as the equation... 

A method for the preparation of olefins from primary amines is shown in equation 120. Treatment of 2-(4-bromophenyl)ethylamine (358) with acetic acid, acetic anhydride and sodium nitrite generates the nitroso amide 359, which decomposes to 4-bromostyrene in the presence of rhodium(II) acetate. The procedure is thus a mild, non-basic alternative to the classical Hofmann elimination of amines396,397. 

Well developed is the anionic polymerization for the preparation of olefin/di-olefin - block copolymers using the techniques of living polymerization (see Sect. 3.2.1.2). One route makes use of the different reactivities of the two monomers in anionic polymerization with butyllithium as initiator. Thus, when butyl-lithium is added to a mixture of butadiene and styrene, the butadiene is first polymerized almost completely. After its consumption stryrene adds on to the living chain ends, which can be recognized by a color change from almost colorless to yellow to brown (depending on the initiator concentration). Thus, after the styrene has been used up and the chains are finally terminated, one obtains a two-block copolymer of butadiene and styrene ... 

The design and development of a Z-selective CM reaction faces two major challenges first, the thermodynamic preference for the formation of -olefms renders their Z-counterparts difficult to prepare second, any inherent kinetic Z-preference demonstrated by a catalyst can erode via secondary metathesis if the -pathway is not blocked. Consequently, most efforts to effect stereoselective olefin formation have focused on the preparation of -olefins. 

Similarly, the reaction of 1-chloropentyhnagnesium chloride (56), derived from 1-chloro-l-iodopentane, with a-sulfonyl lithium carbanions (52) affords 1,2-di- or 1,1,2-trisubstituted olefins (57) in moderate to good yields (equation 14). This reaction represents an elegant preparation of olefins from sulfones. 

The acetylenic diol Me2C(OH)-C C-C(OH)Me2 (ac) forms complexes of the types K PtCl3ac] and cis- and 2mns-[PtCl2(ac)(am)] (am = amine), by reactions commonly used in the preparation of olefin complexes 23, 25,... 

The treatment of aliphatic amines with nitrous acid is not a useful method for the preparation of olefins any more than it is for the preparation of alcohols (p. 355), though some olefin is usually formed in such reactions. 

When some straight and branched-chain aliphatic alcohols, such as n-propanol, n-butanol and isopropanol, are subjected to high temperatures, dehydrogenation products predominate over dehydration (51). Presumably the eliminations take place via a six-membered transition state and are catalyzed by hydrogen halides in the homogeneous phase (52) to produce olefins. On the other hand, gas phase dehydration over solid catalysts is a valuable process for the preparation of olefins and ethers. 

Preparation of Olefins by Transition Metal-catalyzed Dehydrogenation... 

What procedures arc available Tor tile preparation of olefin... 

Some preparations of olefinic and acetylenic compounds from olefinic and acetylenic starting materials can, in principle, be classified in either the monofunctional or difunctional sections for example, RCH=CHBr - RCH=CHCOOH, carboxylic acids from halides (Section 25, monofunctional compounds) or carboxylic acid-olefin (Section 322, difunctional compounds). In such cases both sections should be consulted. 

I he preparation of olefins via their thiocarbonate is a stereo-specific elimination reaction which may be used to advantage when a mixture of cis- and frawj-olefins is difficult to sejiarate. However, all the reagents required to prepare the thiocarbonate are not readily available. 

This combination of reactions represents the best general method for the preparation of olefins of unequivocal structure. Many mono-, ... 

The reaction is most useful for the preparation of olefinic, halo, and nitro alcohols from the corresponding substituted aldehydes and ketones. These substituents ate very often affected by other reduction procedures. Excellent directions are found in the preparations of crotyl alcohol (60%), l-bromo-5-hexanol (64%), l-chloco-4-pentanol (76%), /S,/S,/S-trichloroethyl alcohol (84%), methyl-p-chlorophenylcarbinol (81%), and o-nitrobenzyl alcohol (90%). The reaction has also been used in the preparation of certain tetralols and decalols as well as 9-fluo-renylcarbinol (50%). The thiophene and furan nuclei are not reduced. 

Two techniques are commonly used in the preparation of olefinic ethers from halo ethers. The first involves heating a /5-halo ether with fused or powdered potassium hydroxide. This method is typified by the conversion of -phenoxyethyl bromide to phenyl vinyl ether (69%) and, /5 -dichlorodiethyl ether to divinyl ether (61%). In the latter case, yields are improved in the presence of ammonia gas. In the second procedure, an aliphatic or aromatic chloro ether is heated with pyridine to 115°. This method is of value in the preparation of several methoxystyrenes. Chloroalkylation of the aromatic ether is followed by dehydrohalogenation. 

Pyrolysis of steroid esters, particularly benzoates, has been used as a method for the preparation of olefins [143]. The reaction involves a cis hydrogen atom stereospecifically and is interpreted in terms of a cyclic transition state of essentially non-ionic character (i). The specificity of the reaction is such that 5a-cholestan-4jS yl benzoate (2) gave only the A -olefin (3) [118], whereas the epimeric 4a-benzoate (4) gave the... 

Most steroid ketones react readily with tolnene- Siilphonyl hydrazide -CH3 C6H4 S03 NH NH2) to form crystalline derivatives ( tosylhydrazones ) which are valuable intermediates for the preparation of olefins and fully-saturated unsubstituted products. Three modes of reaction of tosyl-hydrazones have become important. 

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