Effect of Alkene Structure

The chelating ether motif used in Hoveyda-type catalysts has become a widely used structural platform in ruthenium metathesis catalyst development [3, 22]. 

Chart 9.2 Compared activation parameters for Grubbs and Grubbs-Hoveyda catalysts [13, 26]. 

Further detailed studies on the kinetics of initiation by the Hoveyda-type catalysts revealed that the structure of the alkene influenced the mechanistic pathways by which the reaction proceeded. For particular alkenes, parallel mechanistic pathways were available [26-28]. The predominant mechanism involves dissociative or interchange IJ mechanisms. More hindered alkenes follow primarily a dissociative mechanism, while styrene is borderline where initiation proceeds, with contributions from both theand dissociative pathways. For smaller alkenes (i.e., those without allylic or homoallylic substitution), the two pathways were found to be operative. Substrate electronics also influenced pathway preference electron-rich or sterically permissive alkenes preferentially reacted via an interchange mechanism, while electron-poor and bulkier alkenes followed a dissociative pathway. 

The free energy of activation was calculated for the reaction of ethylene and EVE with 3a byway of a dissociative, associative, and interchange mechanism and compared to the experimental values (Table 9.8) [14]. The initiation step had the highest AG and was indeed rate-limiting. The predicted values for the interchange mechanism were found to be comparable to the experimental values obtained for the reaction of EVE with 3a. 

Examination of the crystal structures of 3a and 2 showed that there is an open coordination site at the apical position, but it is too hindered for alkene binding. Even if an alkene bound to this site, it would not be in a position for cycloaddition with the Ru=C bond. Weakening of coordination by the 2-(isopropoxy)styrene unit produced precatalysts with faster initiation rates. For example, if the aromatic ring was electron-poor, as in the Grela catalyst 3b, the ether oxygen coordinated more weakly, and the catalyst was found to initiate 12 times faster than 3a (Chart 9.4, below) [29]. If the ether experienced steric strain, as in the Blechert-Wakamatsu complex 17 [30], the catalyst was more active, most Ukely due a dissociative process. 

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

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. 

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... 

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

If the transition state for proton transfer from HCl to the alkene (arrow 5) resembles a carbocation and this step is rate-determining, what should be the effect of alkene structure on the rate of the overall reaction ... 

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). 

Consider the effect of monomer structure on the enthalpy of polymerization. The AH values for ethylene, propene, and 1-buene are very close to the difference (82-90 kJ mol 1) between the bond energies of the re-bond in an alkene and the a-bond in an alkane. The AH values for the other monomers vary considerably. The variations in AH for differently substituted ethy-lenes arise from any of the following effects ... 

In the case of indene and acenaphthylene slightly syn prevalent fluoro methoxy adduct formation was observed, while in the case of 1-phenyl-substituted benzocyclene triads the profound effect of the structure of the alkene and the reaction conditions on the stereochemical result of the reaction was established.76 ... 

Figures 3 to 5 show some effects of hydrocarbon structure on maximum burning velocity (30, 32, Jfi, 72). Figure 3 deals with straight-chain alkanes, alkenes, alkynes, alkadienes, and allenes Figure 4 shows the effects of methyl substitution in four-carbon-atom alkane, alkene, alkyne, and alkadiene molecules. In general, the burning velocity decreases with increased chain length or methyl substitution, except for the alkanes,...

It is well established that steric effects hinder the Cope rearrangement of divinylcyclopropanes. An interesting example of this steric effect is seen in the reaction of 33 with cis- and trans-l-acetoxy-butadiene (Scheme 13). ° The reaction of 33 with trans-1-acetoxy-l, 3-butadiene leads cleanly to the [3+4] annulation product 34 in 67% yield. In contrast, the product from the reaction of 33 with c/j-l-ace-toxy- 1,3-butadiene is the cw-divinylcyclopropane 35 (80% yield), and high temperatures (220 °C) are required to convert 35 to the [3+4] annulation product 36. The effect of alkene geometry on the stereochemistry and the rate of reaction is readily explained by considering the boat transition state for the Cope rearrangement of divinylcyclo-propanes (structure 37). A trans diene substituent (Y) would generate a trans product (34), whereas a cis substituent (X) would lead to a cis... 

The effects of molecular structure, electrolyte and temperature on the rate and extent of adsorption and electrochemical oxidation of hydrocarbons (alkanes, alkenes, alkynes) have been reviewed . ... 

You knowr the mechanism of HBr addition to alkenes, and you know the effects of various substituent groups on aromatic substitution. Use this knowledge to predict which of the following two alkenes reacts faster with HBr. Explain your answer by drawing resonance structures of the carbocation intermediates. 

Chirality center, 292 detection of, 292-293 Eischer projections and, 975-978 R,S configuration of, 297-300 Chitin, structure of, 1002 Chloral hydrate, structure of, 707 Chloramphenicol, structure of, 304 Chlorine, reaction with alkanes, 91-92,335-338 reaction with alkenes, 215-218 reaction with alkynes, 262-263 reaction with aromatic compounds, 550 Chloro group, directing effect of, 567-568... 

The syntheses in Schemes 13.45 and 13.46 illustrate the use of oxazolidinone chiral auxiliaries in enantioselective synthesis. Step A in Scheme 13.45 established the configuration at the carbon that becomes C(4) in the product. This is an enolate alkylation in which the steric effect of the oxazolidinone chiral auxiliary directs the approach of the alkylating group. Step C also used the oxazolidinone structure. In this case, the enol borinate is formed and condensed with an aldehyde intermediate. This stereoselective aldol addition established the configuration at C(2) and C(3). The configuration at the final stereocenter at C(6) was established by the hydroboration in Step D. The selectivity for the desired stereoisomer was 85 15. Stereoselectivity in the same sense has been observed for a number of other 2-methylalkenes in which the remainder of the alkene constitutes a relatively bulky group.28 A TS such as 45-A can rationalize this result. 

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