Alkene structure effect

The carbonyl carbon of a ketone bears two electron releasing alkyl groups an aldehyde carbonyl group has only one Just as a disubstituted double bond m an alkene is more stable than a monosubstituted double bond a ketone carbonyl is more stable than an aldehyde carbonyl We 11 see later m this chapter that structural effects on the relative stability of carbonyl groups m aldehydes and ketones are an important factor m then rel ative reactivity... 

Oxidation of unsaturated alcohols in the presence ofTS-1 effect of alkene structure on selectivity... 

The alkene structure and the solvent polarity markedly affect the reaction rate. However, these effects are not easy to rationalize since, as shown in equation 90, one or more intermediates may be involved and each factor can influence the individual rate constants in a different way. It follows that only when the first step is rate determining can the observed rate constant k0bsd t>e interpreted straightforwardly. 

The old and lasting problem of heterogeneous catalysis, the mechanism of alkene hydrogenation, has also been approached from the viewpoint of structure effects on rate. In 1925, Lebedev and co-workers (80) had already noted that the velocity of the hydrogenation of the C=C bond decreases with the number of substituents on both carbon atoms. The same conclusion can be drawn from the narrower series of alkenes studied by Schuster (8J) (series 52 in Table IV). Recently authors have tried to analyze this influence of substituents in a more detailed way, in order to find out whether the change in rate is caused by polar or steric effects and whether the substituents affect mostly the adsorptivity of the unsaturated compounds or the reaetivity of the adsorbed species. Linear relationships have been used for quantitative treatment. 

The oxidation of alkenes by nitrous oxide on silver at 350°C has been studied from the viewpoint of structure effects on rate by Belousov, Mulik, and Rubanik (J40), and very good correlations of Type B have been found with ionization potentials and with the rate of oxidation by atomic oxygen (series 110 and 111). 

Structure effects on the rate of selective or total oxidation of saturated and unsaturated hydrocarbons and their correlations have been used successfully in the exploration of the reaction mechanisms. Adams 150) has shown that the oxidation of alkenes to aldehydes or alkadienes on a BijOj-MoOj catalyst exhibits the same influence of alkene structure on rate as the attack by methyl radicals an excellent Type B correlation has been gained between the rate of these two processes for various alkenes (series 135, five reactants, positive slope). It was concluded on this basis that the rate-determining step of the oxidation is the abstraction of the allylic hydrogen. Similarly, Uchi-jima, Ishida, Uemitsu, and Yoneda 151) correlated the rate of the total oxidation of alkenes on NiO with the quantum-chemical index of delo-calizability of allylic hydrogens (series 136, five reactants). 

Masclet, P., D. Grosjean, G. Mouvier, and J. Dubois. Alkene ionization potentials. Part I Quantitative determination of alkyl group structural effects. J. Electr. Spectrosc. Relat. Phenom. 2 225-237, 1973. 

Steric effects in the alkene structure also affect linearity. As a result, quaternary carbon atoms are rarely formed in hydroformylation45 In contrast, electronic effects in hydroformylation of arylalkenes often result in the predominant formation of the branched aldehyde.6 40 43 46- 8 Styrene has a marked tendency to form 2-phenylpropanal when hydroformylated in the presence of rhodium catalysts. Rhodium complexes modified by biphosphine49 or mixed amino phosphine oxide ligands50 were shown to give the branched aldehyde with high reactivity and selectivity (iso normal ratios <61.5). 

The effect of alkene structure on relative reactivity indicates that a much greater structural change in the alkenic moiety occurs on adsorption than in the change from adsorbed alkene to the transition state of the rate controlling surface reaction. Moreover, where measures indicate appreciable differences in adsorption energy, the more strongly adsorbed compound often exhibits the smaller zero order rate. 

Tolman notes that the high sensitivity of the Ni(0) equilibrium constants to structural modifications of the alkene is due to the low ionization potential of Ni(0) and the resulting small energy separation between the HOMO of the metal and the pi orbital of the alkene. Steric effects of substituents... 

The reaction may be subject to limitations in the alkene structure similar to those of the nitrile process. Besides simple amides, one can also utilize urea and urethanes in this reaction. Demercuration is best effected by using alkaline sodium borohydride in the presence of a primary amine such as Bu"NH2. 

The removal of a molecule of a hydrogen halide from an alkyl halide to yield an alkene is effected under strongly basic conditions, e.g. a concentrated alcoholic solution of sodium or potassium hydroxide or alkoxide. This overall reaction has been submitted to most rigorous mechanistic studies. Most of the factors (temperature, nature of base, structure of substrate, solvent, etc.) which control product composition have been evaluated. It thus appears that under the conditions noted above, an E2 process, in which the participating sites adopt an ann -periplanar conformation leading to an anti-elimination process, is generally favoured. 

Typical transient absorption spectra are shown in Figs. 3 and 4, exhibiting the effect of alkene structure on the electron transfer — H-abstraction ratio for benzophenone as well as the influence of added salt on the SSIP-CIP ratio for benzil. 

SCHEME 15. The effect of alkene structure on relative rates of fluorination... 

Structural effects on the C-basicity of enamines are, however, more complex. Because of the paucity of values for pX H+, we shall anticipate our discussion of the kinetics of C-protonation (see Section III) so that some of the information presented there can be incorporated into the present section. The justification for doing so is that many of the effects that influence the stability of the iminium ion are expected to be operational in the transition state. In particular, the coplanarity of the atoms about the C=N bond in the iminium ion (already preferred for some enamines, but only when geometrically possible24) should be maintained or improved in the transition state in order to maximize p-n overlap (equation 4)25. This means that, besides the ability of the amino nitrogen to bear a positive charge, other factors such as formation of the C=N double bond (with attendant rehybridization at nitrogen), and steric interactions between groups attached to the alkene and amine moieties will be important both in the transition state and in the iminium ion product. 

Alkene brominations have been widely studied by the Dubois and Ruasse group, and the aryl substituent effects and their dependence upon the alkene structures have been precisely analysed, using the Y-T equation as well as other structure-reactivity relationships. The results have been summarized by Ruasse in her review article (Ruasse, 1993). The results of our analysis by the Y-T equation and related equations are summarized in Table 11. 

TABLE 9. Effect of Alkene Structure on Preference for Addition vs. Abstraction by -BuO Radicais at 40°C. Data from (21). 

Enantioselection can be controlled much more effectively with the appropriate chiral copper, rhodium, and cobalt catalyst.The first major breakthrough in this area was achieved by copper complexes with chiral salicylaldimine ligands that were obtained from salicylaldehyde and amino alcohols derived from a-amino acids (Aratani catalysts ). With bulky diazo esters, both the diastereoselectivity (transicis ratio) and the enantioselectivity can be increased. These facts have been used, inter alia, for the diastereo- and enantioselective synthesis of chrysan-themic and permethrinic acids which are components of pyrethroid insecticides (Table 10). 0-Trimethylsilyl enols can also be cyclopropanated enantioselectively with alkyl diazoacetates in the presence of Aratani catalysts. In detailed studies,the influence of various parameters, such as metal ligands in the catalyst, catalyst concentration, solvent, and alkene structure, on the enantioselectivity has been recorded. Enantiomeric excesses of up to 88% were obtained with catalyst 7 (R = Bz = 2-MeOCgH4). 

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