Reactions in Olefins

Fluoroalkylphosphonates may also be deprotonated for use in olefination reactions with aldehydes [77] (equation 64)... 

Table 11. a-and (3-Fluorinated a, 3-Unsaturated Aldehydes in Olefination Reactions [43 ... 

Quinolylmethylphosphonium salts (111) can be prepared directly from the reaction of triphenylphosphine with 2-methylquinolines and iodine,111 and the reaction of 5-chloromethyluracil with triphenylphosphine gives112 the salt (112). Both salts behave normally in olefination reactions. 

The phosphoranium salts obtained in this way are soluble in polar solvents and can be used without isolation in olefination reactions. For example, if triphenylphosphane is reacted with dibromodifluoromethane in diglyme at room temperature, the corresponding bromo(di-fluoro)methyl triphenylphosphonium salt precipitates quantitatively... 

Najera and coworkers [106,107,113] have successfully applied aromatic 3,5-bis(trifluoromethyl)phenyl (BTFP) sulfones in olefination reactions. The sulfones are readily prepared using commercially available 3,5-bis(trifluoromethyl)thiophenol and the respective precursors of the aliphatic or benzylic portion of the molecule. 

The major anomaly in olefin reactions is the superior ability of Fe, Co, and Ni to act as hydrogenation catalysts in comparison with expectations based on the strength of olefin bonding in complexes. Olefins are quite strongly chemisorbed by these metals (5) we must therefore infer that the presence of several metal atoms in proximity sometimes confers a binding ability not possessed by single atoms. Whether this effect is of particular importance with these metals or whether it will prove to... 

Molecular Beam—Transient Species.—In their molecular beam-AES study of C2H4 adsorption on Ni(llO), Zuhr and Hudsonfound that there was an initial interaction process between C2H4 and Ni. In addition, a secondary non-dissociated adsorbed phase was found to co-exist with the initial phase formed at temperatures below 350 °C. They say that this may well be the active phase in olefin reactions on transition metals. 

An additional complication appeared in olefin reactions when the olefin contained allylic hydrogen atoms (or other abstractable hydrogen, cf. ethyl vinyl sulfide). The hydrogen abstraction reaction illustrated to form 1,2-adducts 38 then competes with cycloaddition. Such... 

In other fields of kinetics, the principle is particularly useful in two situations for pathways with repetitive, consecutive addition reactions such as multiple ethoxylation or paraffin oxidation (see examples in Section 5.5) and for networks with coupled parallel steps, as in olefin reactions such as hydrogenation, hydroformylation, and hydrocyanation that involve double-bond migration in concert with conversion to products [17]. Consider as a prototype the network 12.13 of n-heptene hydroformylation, keeping in mind that the arrows represent multistep pathways and that the reactions of higher straight-chain olefins involve still more... 

Other titanium-based olefination reagents have been developed. Eisch used a zinc analog of the Tebbe reagent (688) in a reaction with benzophenone to give 1,1-diphenylethene in 78% yield. Similarly, Clawson et al. used 689 in olefination reactions with ketones and aldehydes. Alkoxytitanium reagents such as 690 have been employed, as in the conversion of cyclohexane carboxaldehyde to 1-cyclohexyl-1,3-butadiene (691), in 86% yield.In this olefination reaction, the (Z)-isomer predominated over the ( ) (96 4 Z/E). 

Miscellaneous Reactions.- A full report has appeared of the use of /er/-butyl 4-diethylphosphono-3-oxobutanthioates (232) in olefination reactions for the preparation of homologated 3-oxobutanthioates and hence of tetramic acid derivatives. The reaction of the phosphonate dianion (234) with the aldehyde (233) is a key step in a total synthesis of the tetramic acid antibiotic ( .)-tirandamycin B (235). Olefinations involving both ylides and phosphonates have been used to construct the triene (236) en route to the spirotetronate subunit of the aglycone of the antitumour antibiotic... 

Acid catalysts are mainly used in alkylation processes, but also for hydration, dehydration, and condensation reactions. In olefin reactions, heterogeneous catalysts are mainly employed for metathesis reactions and the production of polymers. 

In olefination reactions of racemic 2-alkylsubstituted cyclohexanones with the chiral phosphonoacetate 115 obtained from natural D-mannitol, the unreacted ketone could be recovered in enantiomerically enriched form, provided the base was not in too high a concentration [75]. 

Trialkyl phosphono[ CJacetates ((EtOlzPl0) CH2 COOR) The use of trialkyl phospho-no[ C] acetates 159 in olefination reactions (Homer-Wadsworth-Emmons or HWE reactions) is generally superior to the use of the related Wittig reagents. The carbanions, generated by a suitable base, are more nucleophilic than the latter, so that they react even with ketones. Furthermore, they often show an improved stereoselectivity. They provide 100% -configured acrylates when they react with linear aldehydes, and ( )-alkenes... 

As discussed earlier, metal halides have little interaction with the pi electron systems of olefins. The function of the metal halides in olefin reactions is to react with the cocatalyst to greatly accentuate the activity of the protons (alkyl halides functioning by eliminating the hydrogen halide). Protonation of the olefin produces the carbonium ion intermediate requisite for the reaction. 

Carbanions derived from sulfoxides, phosphonates, and phosphonium salts have a rich preparative chemistry. The fact that sulfoxides are configuration-ally stable and partake in a wide range of reactions as nucleophiles has inspired a number of investigators to examine chiral sulfoxides as auxiliaries for asymmetric synthesis [21-23]. The chemistry of phosphonates and phosphonium salts is largely dominated by their use in olefination reactions. Consequently, the applications of chiral phosphorus-stabilized carbanions in asymmetric synthesis are primarily found in the desymmetrization reaction of ketones, leading to optically active olefins. 

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