Alkenes, addition reactions inductive effects

This chapter and Chapter 10 continue our cataloging of the standard reactions of organic chemistry. To the SnI, Sn2, El, and E2 reactions we now add a variety of alkene addition reactions. Although there are several different mechanisms for additions, many take place through a three-step sequence of protonation, addition, and deprotonation. The following new problems allow you to practice the basics of addition reactions and to extend yourself to some more complex matters. Even simple additions become complicated when they occur in intramolecular fashion, for example. These problems also allow you to explore the influence of resonance and inductive effects, and to use the regiochemistry and stereochemistry of addition to help work out the probable mechanisms of reactions. 

Unsaturated fluorinated compounds are fundamentally different from those of hydrocarbon chemistry. Whereas conventional alkenes are electron rich at the double bond, fluoroal-kenes suffer from a deficiency of electrons due to the negative inductive effect. Therefore, fluoroalkenes react smoothly in a very typical way with oxygen, sulfur, nitrogen and carbon nucleophiles.31 Usually, the reaction path of the addition or addition-elimination reaction goes through an intermediate carbanion. The reaction conditions decide whether the product is saturated or unsaturated and if vinylic or allylic substitution is required. Highly branched fluoroalkenes, obtained from the fluoride-initiated ionic oligomerization of tetrafluoroethene or hexafluoropropene, are different and more complex in their reactions and reactivities. 

The effect of monofluorination on alkene or aromatic reactivity toward electrophiles is more difficult to predict Although a-fluonne stabilizes a carbocation relative to hydrogen, its opposing inductive effect makes olefins and aromatics more electron deficient. Fluorine therefore is activating only for electrophilic reactions with very late transition states where its resonance stabilization is maximized The faster rate of addition of trifluoroacetic acid and sulfuric acid to 2-fluoropropene vs propene is an example [775,116], but cases of such enhanced fluoroalkene reactivity in solution are quite rare [127] By contrast, there are many examples where the ortho-para-dueeting fluorine substituent is also activating in electrophilic aromatic substitutions [128]... 

Measurements of 2o /205 j ijj spin-spin couplings for the organothallium compounds formed in oxythallation reactions provides the first direct evidence for trans addition in the oxythallation of acyclic olefins. Products from the reactions of styrene, u-alkylphenols, propylene, and oct-l-ene were studied. The rate constants for the oxythallation of alkenes shows a reasonable correlation with the first ionization energy of the alkenes. Inductive effects were found to be most important in the oxythallation of RCH=CH2 and R R C=CHa alkenes. ... 

Cracking and disproportionation in the reaction of hexane could be suppressed by the addition of cycloalkanes (cyclohexane, methylcyclopentane, cyclopentane).101 Furthermore, 3-methylpentane and methylcyclopentane also reduced the induction period. These data indicate that reactions are initiated by an oxidative formation of alkene intermediates. These maybe transformed into alkenyl cations, which undergo cracking and disproportionation. When there is intensive contact between the phases ensuring effective hydride transfer, protonated alkenes give isomerization products. 

As an alternative procedure, which results in a shorter induction period but otherwise has no major effect on the reaction, addition of the alkene is postponed until after the reaction of the zinc-copper couple and dibromomethane starts (c. 30 minutes). Sonication is continued for an additional 60 minutes and stirring without sonication for a further 60 minutes. 

The original procedure has been modified by the use of a slow addition of the alkene to afford the diol in higher optical purity, and ironically this modification results in a faster reaction. This behavior can be rationalized by consideration of two catalytic cycles operating for the alkene (Scheme 9.20) the use of low alkene concentrations effectively removes the second, low enantio-selective cycle.145151 The use of potassium ferricyanide in place of A-methylmorpholine-iV-oxide (NMMO) as oxidant also improves the level of asymmetric induction.152153... 

Model studies discussed in previous chapters show that the reactivity of cations and alkenes are very strongly affected by inductive and resonance effects in the substituents. Correlation of the rate constants of addition of benzhydryl cation to various styrenes with Hammett substituted benzhydryl cations to a standard alkene (2-methyl-2-pentene) gave also good correlation and p+ = 5.1 [28]. The large p value signals difficult copolymerizations between alkenes, even of similar structures. Thus, in contrast to radical copolymerization which easily provides random copolymers, cationic systems have a tendency to form either mixtures of two homopolymers or block copolymer (if the cross-over reaction is possible). 

Asymmetric Hydroboration. For reaction with a prochiral alkene, (/ ,/ )-2,5-dimethyl-B-methoxyborolane is liberated from (1) and a standard solution of the corresponding lithium dihydri-doborate in ether is prepared (eq 5). Hydroboration is effected by addition of lodomethane to the solution of dihydridoborate and alkene (eq 6). After oxidation, chiral secondary alcohols of high enantiomeric purity and predictable configuration are obtained from cis, trans, and (risubstituted alkenes. As is the case with other known asymmetric hydroborating agents, 2-methyl-1-alkenes react with low asymmetric induction. 

In order to collect more information about the mechanism of the reaction, we devised three independent experiments in which the three reactants (alkyne, silane, and catalyst) were incubated two by two at 60 °C for 3h, before addition of the third component at 20 °C [35]. The results of these experiments are presented in Figure 5.20. As can be seen, the addition of complex 45 to a mixture of alkyne and silane displays a kinetic profile identical to what was observed previously (curve A). However, when the silane was added to the alkyne, previously incubated with the precatalyst, a considerable reduction of the catalytic activity occurred (curve B). It thus transpires that the alkyne triggers somehow the deactivation of the catalyst. In stark contrast, a dramatic acceleration of the reaction rate, concomitant with the disappearance of the induction period, was observed when the catalyst was heated with the silane prior to addition of the alkyne (curve C). This last effect is reminiscent of what was observed upon repeated addition of fresh reactants during the hydrosilylation of alkenes (see Figure 5.7). Therefore, treating the precatalyst with a silane before adding the alkyne leads to a particularly active and selective catalyst. 

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