Alkyl groups in alkenes

Although alkyl groups in general increase the rates of electrophilic addition, we have already mentioned (p. 974) that there is a different pattern depending on whether the intermediate is a bridged ion or an open carbocation. For brominations and other electrophilic additions in which the first step of the mechanism is rate determining, the rates for substituted alkenes correlate well with the ionization potentials of the alkenes, which means that steric effects are not important. Where the second step is rate determining [e.g., oxymercuration (15-3), hydroboration (15-17)], steric effects are important. ... 

Palladium(II) acetate was found to be a good catalyst for such cyclopropanations with ethyl diazoacetate (Scheme 19) by analogy with the same transformation using diazomethane (see Sect. 2.1). The best yields were obtained with monosubstituted alkenes such as acrylic esters and methyl vinyl ketone (64-85 %), whereas they dropped to 10-30% for a,p-unsaturated carbonyl compounds bearing alkyl groups in a- or p-position such as ethyl crotonate, isophorone and methyl methacrylate 141). In none of these reactions was formation of carbene dimers observed. 7>ms-benzalaceto-phenone was cyclopropanated stereospecifically in about 50% yield PdCl2 and palladium(II) acetylacetonate were less efficient catalysts 34 >. Diazoketones may be used instead of diazoesters, as the cyclopropanation of acrylonitrile by diazoacenaph-thenone/Pd(OAc)2 (75 % yield) shows142). 

To sum up, the rate retardation attributed to steric effects of bulky alkyl groups can arise from substituent-electrophile, substituent-substituent and substituent-solvent interactions in the first ionization step of the reaction and also from substituent-nucleophile interactions in the product-forming step. It is therefore not surprising that the usual structure-reactivity correlations or even simpler log/log relationships cannot satisfactorily describe the kinetic effects of alkyl groups in the electrophilic bromination of alkenes. 

Tho main routes for the insertion of fluorinated alkyl groups in the silicones are suggested either by organometallic or by hydrosilylation methods as shown in Scheme 2. Consequently, research can be pursued in two directions (a) synthesis of fluorohalides and alkenes, and (b) introduction in siliconated groups and polymerization. 

Thus monomers such as isobutylene, styrene, methyl vinyl ether, and isoprene undergo polymerization by cationic initiators. The effect of alkyl groups in facilitating cationic polymerization is weak, and it is only the 1,1-dialkyl alkenes that undergo cationic polymerization. 

Electrochemical or Li-benzophenone reduction of Fe(Pc) and Co(Pc) gives [M Pc] and [M (Pc)]2. 198,199 These monovalent metal complexes react with alkyl halides to give organometallic compounds which transfer the alkyl group to alkenes in the presence of Pd11 salts (Scheme 61). [Co PCTS]5- and [Co PCTS]6- reductively bind one and two molecules of oxygen respectively in DMF,198 The second 02 addition is reversible (Scheme 62). 

If one side of the double bond is substituted as in 20 or 21, in which R stands for an alkyl group, an alkene gauche correction of 0.50 kcal mole-1 must be added. Thus the calculated AH° for 2,3-dimethylbut-l-ene (22) is as follows ... 

The reaction is a stereospecific syn addition. The cis alkyl groups in the starting alkene remain cis in the product epoxide. 

This trans-selectivity ultimately results from the fact that trans-alkenes are more stable than their cis-isomers. This energy difference is especially pronounced for the alkenes in Figure 4.3 because they are styrene derivatives. Styrenes with one alkyl group in the trans-position on the alkenyl C=C double bond enjoy the approximately 3 kcal/mol styrene resonance stabilization. This is lost in cis-styrenes because in that case the phenyl ring is rotated out of the plane of the alkenic C=C double bond to avoid the cis-alkyl substituent. However, the transselectivity documented in Figure 4.3 is not a consequence of thermodynamic control. This could occur only for a reversible elimination or if the alkenes could interconvert under the reaction conditions in some other way. Under the conditions of Figure 4.3, alkenes are almost always produced irreversibly and without the possibility of a subsequent cis/frans-isomeriza-tion. Therefore, the observed trans-selectivity is the result of kinetic control. 

Diorgano tellurium oxides are reported to decompose when heated under vacuum5. Such decompositions have been used to convert the alkyl group in alkyl phenyl tellurium oxides to alkenes (see below). The Te —C (aliphatic) bond in alkyl phenyl tellurium oxides is cleaved by 3-chloroperoxybenzoic add6. 

Intermediates (40) react at -110 C with TMS-Cl to give compounds (42), which are stable, readily purified products. The H NMR of (42) is in accord with the assigned stereochemistry and shows that the oxidation of (40) had indeed proceeded with retention of configuration. Reaction of (42) with HF/MeCN gives ( )-alkenes in excellent yields (Scheme 6) with none of the many by-products seen in the original reaction. The ( ) (Z) ratios range from 100 0 to 95 5 and the reaction tolerates NO2, Cl, OMe and alkyl groups in the aromatic aldehyde. ... 

Me4Sn + Decays by progressive loss of Me and MeMe, but with P-H available in the alkyl group, the alkene R(-H) is eliminated, and the hydrides Bu2SnH+ and BuSnH2+, and Sn + are major products from Bu4Sn. 

In a large number of carbene and carbenoid addition reactions to alkenes the thermodynamically less favored syn-isomers are formed 63). The finding that in the above cyclopropanation reaction the anti-isomer is the only product strongly indicates that the intermediates are organonickel species rather than carbenes or carbenoids. Introduction of alkyl groups in the 3-position of the electron-deficient alkene hampers the codimerization and favors isomerization and/or cyclodimerization of the cyclopropenes. Thus, with methyl crotylate and 3,3-diphenylcyclopropene only 16% of the corresponding vinylcyclopropane derivative has been obtained. 2,2-Dimethyl acrylate does not react at all with 3,3-dimethylcyclopropene to afford frons-chrysanthemic add methyl ester. This is in accordance with chemical expectations 69) since in most cases the tendency of alkenes to coordinate to Ni(0) decreases in the order un-, mono-< di- < tri- < tetrasubstituted olefines. 

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