Alkenes structure-reactivity

Isopentenyl pyrophosphate and dimethylallyl pyrophosphate are structurally sim liar—both contain a double bond and a pyrophosphate ester unit—but the chemical reactivity expressed by each is different The principal site of reaction m dimethylallyl pyrophosphate is the carbon that bears the pyrophosphate group Pyrophosphate is a reasonably good leaving group m nucleophilic substitution reactions especially when as in dimethylallyl pyrophosphate it is located at an allylic carbon Isopentenyl pyrophosphate on the other hand does not have its leaving group attached to an allylic carbon and is far less reactive than dimethylallyl pyrophosphate toward nucleophilic reagents The principal site of reaction m isopentenyl pyrophosphate is the carbon-carbon double bond which like the double bonds of simple alkenes is reactive toward electrophiles... 

The rate of epoxidation of alkenes is increased by alkyl groups and other ERG substituents and the reactivity of the peroxy acids is increased by EWG substituents.72 These structure-reactivity relationships demonstrate that the peroxyacid acts as an electrophile in the reaction. Decreased reactivity is exhibited by double bonds that are conjugated with strongly electron-attracting substituents, and more reactive peroxyacids, such as trifluoroperoxyacetic acid, are required for oxidation of such compounds.73 Electron-poor alkenes can also be epoxidized by alkaline solutions of... 

Because of the absence of any obvious reference value, the p -value of — 3.1 is not readily discussed in terms of charge magnitude or brominebridging at the rate-limiting transition states. For alkene hydration, it is now accepted that the intermediates are carbocations (20). The corresponding structure-reactivity relationship (21) is obtained by using o and [Pg.244]

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. 

Extreme cases were reactions of the least stabilized, most reactive carbene (Y = CF3, X = Br) with the more reactive alkene (CH3)2C=C(CH3)2, and the most stabilized, least reactive carbene (Y = CH3O, X = F) with the less reactive alkene (1-hexene). The rate constants, as measured by LFP, were 1.7 x 10 and 5.0 X lO M s, respectively, spanning an interval of 34,000. In agreement with Houk s ideas,the reactions were entropy dominated (A5 —22 to —29e.u.). The AG barriers were 5.0 kcal/mol for the faster reaction and 11 kcal/ mol for the slower reaction, mainly because of entropic contributions the AH components were only —1.6 and +2.5 kcal/mol, respectively. Despite the dominance of entropy in these reactive carbene addition reactions, a kind of de facto enthalpic control operates. The entropies of activation are all very similar, so that in any comparison of the reactivities of alkene pairs (i.e., ferei)> the rate constant ratios reflect differences in AA//t, which ultimately appear in AAG. Thus, car-benic philicity, which is the pattern created by carbenic reactivity, behaves in accord with our qualitative ideas about structure-reactivity relations, as modulated by substiment effects in both the carbene and alkene partners of the addition reactions. " Finally, volumes of activation were measured for the additions of CgHsCCl to (CH3)2C=C(CH3)2 and frani-pentene in both methylcyclohexane and acetonitrile. The measured absolute rate constants increased with increasing pressure Ayf ranged from —10 to —18 cm /mol and were independent of solvent. These results were consistent with an early, and not very polar transition state for the addition reaction. 

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). 

STRUCTURE-REACTIVITY IN THE HYDROGENATION OF ALKENES. COMPARISONS WITH REDUCTIONS BY DIIMIDE AND THE FORMATION OF A Ni(O) COMPLEX... 

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. 

Nickel affords selective catalysts for the hydrogenation of alkenes, dienes, and alkynes. When catalyzed by C. A. Brown s P-2 nickel, prepared by the reduction of Ni(0Ac)2 with NaBH in ethanol, the individual rates as well as the competitive rates appear to be sensitive to the alkene structure as judged by the reported initial rates of hydrogen addition (ref. 23). Alkene isomerization is relatively slow. Except for the most reactive alkenes such as norbornene, the individual hydrogenations seem to be first order in alkene. This indicates that alkenes are more weakly bound to Ni than to Pt or Pd. Similar selectivities are reported by Brunet, Gallois, and Caubere for a catalyst prepared by the reduction of Ni(0Ac)2 with NaH and t-amyl alcohol in THF (ref. 27). 

These points have been pursued in detail for two reasons. The first is to indicate the level of uncertainty in deriving pATas when the rate of deprotonation falls significantly short of its relaxation limit and the structure-reactivity correlation for the alkene conjugate base of the cation is insufficiently defined. The second is that the identity of the rate constants for 2-propene and 2-butene still imply a difference of 0.3 log units between 2-propyl and 2-butyl cations. In so far as this difference corresponds with the small difference in geminal interaction of the OH groups, the implication is that as measured by their HIAs the two ions have the same stability (cf. discussion on p. 25). In conclusion, the preferred pATR for the 2-propyl cation is listed below with the more secure values for the /-butyl and ethyl cations. 

Other reactions for which a discussion of their structure-reactivity behavior in terms of the PNS has provided valuable insights include nucleophilic addition and substitution reactions on electrophilic alkenes, vinylic compounds, and Fischer carbene complexes reactions involving carbocations and some radical reactions. 

All aspects of the structure, reactivity and chemistry of fluorine-containing, carbon-based free radicals in solution are presented. The influence of fluorine substituents on the structure, the stability and the electronegativity of free radicals is discussed. The methods of generation of fluorinated radicals are summarized. A critical analysis of the reactivities of perfluoro-n-alkyl, branched chain perfluoroalkyl and partially-fluorinated free radicals towards alkene addition, H-atom abstraction, and towards intramolecular rearrangement reactions is presented. Lastly, a summary of the synthetically-useful chemistry of fluorinated radicals is presented. 

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