Olefinic strain

Rh2(OAc)4 is the most effective and versatile of the three catalysts used. Terminal and non-terminal olefins, strained olefins (norbomene, norbomadiene) and conjugated olefins (styrene) all react in good yield. 

Maier and Schleyer (220) have studied the problem of the stability of bridgehead double bonds in bi- and polycyclic systems. They define an olefinic strain (OS) as the difference in strain energy between the olefin and its parent saturated... 

All the amendments to Bredt s rule that have been presented in the past decade have been more or less violated. One reason for these failures is that the past rules ignored the strain in parts of the molecule other than at the bridgehead double bond. Schleyer defines the strain at the bridgehead double bond, or olefin strain (OS), as the difference in strain between the olefin and the parent hydrocarbon, analogous to for carbonium ions (293). He... 

Strained alkenes may have a combination of the strain in the basic ring system, and the added strain that results from the introduction of the double bond. Thus, it is convenient to define olefinic strain (OS) that may be given as the difference in heat of hydrogenation of the alkene in question and that of a similarly substituted... 

However, in many interesting cases, the requisite experimental data are not available, and the olefinic strain for disubstituted alkenes may be estimated using the calculated energies and the following isodesmic reaction ... 

The calculated energy change for this reaction gives the difference in olefinic strain between the alkene and cyclohexene, and the olefinic strain of the latter is negligible. With tri- and tetrasubstituted alkenes, it is more appropriate to use methylcyclohexane or 1,2-dimethylcyclohexane as the unstrained reactant since it is known that alkyl substitution stabilizes double bonds. The values of olefinic strain for a number of alkenes are summarized in Figure 15.2. Compounds with high OS are expected to be relatively reactive. 

It is interesting to note that in some cases, the olefinic strain is negative indicating that some strain terms are lost on going from the alkane to the alkene. With cyclooctene, for example, some of the torsional interactions and H- H nonbonded... 

Heats of formation, strain energies, and olefinic strain for cycloalkenes. 

There is a group of alkenes in which the olefinic strain has relatively large negative values. These alkenes are termed hyperstable,and are expected to be relatively unreactive. One example is bicyclo[4.4.4]tetradec-l-ene (2) with a predicted OS of —14 kcal/mol. 

Cycloaddition reactions of C=C double bonds are often facilitated by increased olefinic strain. For example, phenyl azide does not react with unstrained alkenes, but does react readily with the smaller tran -cycloalkenes. Many interesting cycloaUcenes are not sufficiently stable to be isolated, or even observed in solution. However, in many cases they can be trapped by reagents that lead to a 3+2] or [4+2] (Diels-Alder) reaction. 

Compound A (25 0 Strain Energy kcal/mol Olefinic Strain kcal/mol... 

TABLE 5. Olefinic strain energy (OS, kcal mol 1) calculated for compounds 31 and 33-35135... 

Olefins, Strained Structure and Reactivity of Nonplanar Carbon-Carbon Double... 

If the released electrophile HS03 is not intercepted during the protodesulfonyla-tion as in Figure 5.5, it attacks the defunctionalized aromatic compound again. In this way an isomer of the original sulfonic acid may be obtained. The best-known example of such an isomerization is the conversion of naphthalene-l-sulfonic acid into naph-thalene-2-sulfonic acid (Figure 5.6). Naphthalene-l-sulfonic acid is destabilized by the so-called peri-interaction, that is, the steric interaction between the C8—H bond of the naphthalene and the substituent on Cl. The peri-interaction is thus a cix-olefin strain. Because naphthalene-2-sulfonic acid does not suffer from this interaction, it becomes the only reaction product under conditions of thermodynamic control. 

Adam W, Oppenlander T (1986) 185-nm Photochemistry of Olefins, Strained Hydrocarbons, and Azoalkanes in Solution, Angew. Chem. Int. Ed. Engl. 25 661-672. 

There are some examples known for the cycloaddition of azomethine ylides with nonactivated olefins such as aryl-substituted olefins, strained olefins, acyclic or cyclic olefins, and electron-rich olefins. Stabilized ylide 79 (R = H, R = Et, R = Me), bearing an ester moiety as the only C substituent, can be successfully trapped with styrene when generated by the deprotonation route (Section II,D) from ethyl sarcocinate and paraformaldehyde under reflux in toluene, to give 194 as a mixture of two regioisomers (86CL973). 

In examining the cycloalkenes, one must first recognize that a double bond has considerable inherent strain. For example, the dimerization of ethylene to give cyclobutane is fairly exothermic (—18 kcal mol" ) and if there were a way to readily overcome orbital symmetry restrictions, cyclobutane would be a very common reagent. However, in the following, we will take the conventional view that ethylene is unstrained. Then, in comparing cycloalkanes and cycloalkenes it is helpful to define olefinic strain (OS) as the difference in strain between the alkene and the corresponding alkane ... 

The bent geometry is at least in part a means of reducing the angle strain. Bicyclo[2.1.0]pent-l(4)-ene also is calculated to have this geometry. Despite the very large olefinic strain, derivatives of both have been formed and trapped as reaction intermediates. ... 

Palladium-based catalysts also bring about cyclopropanations in high-yield. With palladium acetate/CHjNj, styrene , unactivated terminal olefins strained olefins , 1,3-dienesan enamine , as well as a,3-unsaturated carbonyl compounds have been cyclopropanated (Table 1). Contrary to an earlier report, the reaction also works well with cyclohexene if the conditions are chosen appropriately it seems that the notniyst is rapidly deactivated in the presence of this olefin >. Trisubstituted a,p-unsaturated carbonyl compounds were found to be unreactive, and the same is true for the double bonds in diethyl fumarate, maleic anhydride, coumarin and 1,3-dimethyluracil. Whereas the latter two were totally unreactive, [3-1-2] cycloaddition of diazomethane gave pyrazolines in the former two cases. The last entry of Table 1 shows that an allyl alcohol function can still be cyclopropanated, but methylene insertion into the O—H bond is a competing process. 

In cycloolefms of decreasing ring size 6- 3, the strain increases with a concomitant decrease of the C=C-C bond angles. For a more detailed analysis, it is of advantage to consider the olefinic strain (Table 24). 

Relative Rates of Expoxidation by Metachloroperbenzoic Acid in CH2C12 at 0° (227) and Strain (SE) and Olefinic Strain Energies (OS) (kcal/mol) In Dienes 277a, b and 278a, b, the Rate and Strain Energies for the First Double Bond are Given... 

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