Relative alkene reactivities

Table 3 Comparison of Relative Alkene Reactivities Determined by Competition Experiments and Measurements of Absolute Rate Constants (CH2CI2/-700 C)...

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... 

Addition of disulfides to carbon-carbon double bonds is catalyzed by ruthenium complexes (Equation (71)).204 Even relatively less reactive dialkyl disulfides add to norbornene with high stereoselectivity in the presence of a catalytic amount of Cp RuCl(cod). Diphenyl disulfide adds to ethylene and terminal alkenes under identical conditions (Equation (72)). 

A modification of an earlier procedure for debromination of v/c-dibromides in the presence of catalytic amounts of diorganotellurides has allowed the synthesis of terminal alkenes and cis- and frani-l,2-disubstituted alkenes from appropriate precursors the relative substrate reactivities suggest that, as for the stoichiometric reaction, the catalytic reaction involves intermediate bromonium ion formation. The Te(IV) dibromides formed in the debrominative elimination are reduced back to the catalysts by either sodium ascorbate or the thiol glutathione. 

The relative rate constants (fe ) do not account for the fact that approach of the nitrile oxide to the 7i-bond can occur from both olefinic diastereofaces with two regioisomeric modes of reaction (Scheme 6.14). In the case of achiral 1-alkenes, only one regioisomer is formed. With chiral dipolarophiles, preference for one of the two is usually found (diastereodifferentiation). The relative diastereofacial reactivity (fejH) is used to evaluate this effect (121). With ethylene, there are four possibilities of attack (two for each face corresponding to the different regio-isomers), and the of each is set as 0.25. In diastereodifferentiating cycloadditions, such as those with a-chiral alkenes, the major isomer generally results... 

Although monosubstituted alkenes usually show relatively high reactivity in nitrile oxide cycloadditions to give 5-substituted-2-isoxazolines in a regioselective... 

Problem 6.31 Compare and explain the relative rates of addition to alkenes (reactivities) of HCl, HBr and HI. 

The structure of Os3(/r-H)2(CO)10 has been established by X-ray8 and neutron diffraction.9 The 46-electron complex displays a relatively high reactivity under mild conditions, associated with a stable triosmium framework and has been extensively studied as a model for the chemisorption of alkenes and alkynes on surfaces and in the catalytic isomerization and hydrogenation of alkenes.10 When supported onto alumina it is a catalyst for the methanation of CO and C02 slightly less efficient than NiOs3(/r-H)3(CO)9(,5-CsH5)>... 

Until recently, knowledge about absolute and relative rates of reaction of alkenes with carbocations was very limited and came almost exclusively from studies of carbocationic polymerizations [119-125]. The situation changed, when it became obvious that reactions of carbocations with alkenes do not necessarily yield polymers, but terminate at the 1 1 product stage under appropriately selected conditions (see Section III.A). Three main sources for kinetic data are now available Relative alkene and carbo-cation reactivities from competition experiments, absolute rates for reactions of stable carbocation salts with alkenes, and absolute rates for the reactions of Laser-photolytically generated carbocations with alkenes. All three sets of data are in perfect mutual agreement, i.e., each of these sets of data is supported by two independent data sets. 

Comparison of the relative alkene reactivites reported in Section III.D.4 with known copolymerization parameters [57] indicates qualitative agreement between the two sets of data. It is thus possible to use the reactivity scales in Section III.D.4, for predicting approximate copolymerization parameters, but the calculation of accurate rate ratios in copolymerization suffers from the fact that alkene reactivities are not completely independent of the nature of the attacking carbocation. This is especially true if the propagation rates get close to the diffusion limit. It is not possible to directly measure propagation rate constants by investigating cationic telomerizations, but extrapolations to such data on the basis of Eq. (23) in Section V are conceivable. 

The FMO model is also valuable in rationalizing several observations about relative reactivities in the Diels-Alder reaction. Reactivity is enhanced with electron-withdrawing groups on the olefin, as shown by the data in Table 11.5 for reaction of cyclopentadiene with cyano-substituted alkenes. Reactivity is also enhanced by any electron-donating groups on the diene, as shown by the data in Table 11.6 for the reaction of some dienes with tetr acy anoethy lene. 

A 64% yield was also observed for the hydrogenation of 1-decyne to 1-decene, showing the relative low reactivity of 1-alkenes under these reaction conditions. Methyl 2-octynylcarboxylate was converted to a mixture of cis/trans/alkmie (98 2 <1). Upon... 

Alkenes are relatively more reactive compounds than alkanes because of the carbon-carbon double bond. They undergo a range of addition reactions. 

Other tertiary amines have been utilized in the MTO-catalyzed epoxidation reactions. Pyrazole was also found to be an effective additive for a variety of alkenes. The use of an equimolar amount of 3-cyanopyridine and pyridine as an additive for the epoxidation of terminal alkenes has been found to be high yielding with little-to-no destruction of the resulting epoxide detected. This system is effective for the epoxidation of a range of alkenes, in particular alkenes of relatively low reactivity (eqs 8-10). Electron deficient alkenes did not perform well under such epoxidation conditions. 

These data were accepted for a long time as proper measures of alkene reactivity in photocycloadditions and were critical elements in the formulation of Corey s famous exciplex mechanism to explain enone photocycloadditions.However, as is now well known, product ratios in photochemical processes reflect relative quantum efficiencies for disappearance of starting materials and/or formation of products and rarely reflect relative rates of reaction of the photoexcited state, particularly when it is a triplet state. ° ... 

The notion that carbocation formation is rate determining follows from our previous experience and by observing how the reaction rate is affected by the shucture of the aUcene Table 6 2 gives some data showing that alkenes that yield relatively stable carbocations react faster than those that yield less stable carbocations Protonation of ethylene the least reactive aUcene m the table yields a primary carbocation protonation of 2 methylpropene the most reactive m the table yields a tertiary carbocation As we have seen on other occa sions the more stable the carbocation the faster is its rate of formation... 

Table 6.3. Relative Reactivity of Alkenes toward Halogenation...

This elimination reaction is the reverse of acid-catalyzed hydration, which was discussed in Section 6.2. Because a carbocation or closely related species is the intermediate, the elimination step would be expected to favor the more substituted alkene as discussed on p. 384. The El mechanism also explains the general trends in relative reactivity. Tertiary alcohols are the most reactive, and reactivity decreases going to secondary and primary alcohols. Also in accord with the El mechanism is the fact that rearranged products are found in cases where a carbocation intermediate would be expected to rearrange ... 

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