Reactive Intermediate Chemistry

728 STRAINED HYDROCARBONS: STRUCTURES, STABILITY, AND REACTIVITY

TABLE 15.2. Calculated Energies and Structures of cisand trans-Cycloalkenes

a In kilocalories per mole (kcal/mol). b Torsional angles in degrees.

c The s bond deviation in degrees from the line of centers, and the p bond deviation from perpendicular to the line of centers.

One way in which the p bonds in these compounds may be examined is by calculating natural bond orbitals (NBO) from the molecular wave function.40 Here, the MOs are localized, and the nature of the localized orbitals may be examined. The p–p localized orbital is found to have decreased occupation as the carbon–carbon bond is distorted, indicating that the p orbitals are being used in part in forming other localized orbitals (i.e., rehybridized). It is also possible to calculate natural hybrid orbitals, and their deviation from the line of centers between the atoms involved. The deviations are included in Table 15.2, and are seen to increase markedly with increased twisting.

Another group of compounds that have a twisted double bond are the bicyclic

compounds with bridgehead double bonds such as 1,2-norbornene (9) and 1,7- norbornene (10).16,41 It has been found that many compounds, such as 11, which

is based on trans-cyclooctene, may be isolated whereas those based on smaller trans-cycloalkenes are usually quite unstable. Some evidence for the formation of 9 has been obtained by trapping the product of the dehalogenation of 1,2-diha- lonorbornanes.42 Here, the simplest view is that the two p orbitals that form the ‘‘double bond’’ in 9 and 10 are roughly perpendicular to each other. However, pyramidalization and rehybridization also are involved. One indication is the reduced localized p-orbital population found in the NBO analysis. Whereas normal alkenes have p populations of 1.96 e, for 9 with OS ¼ 57 kcal/mol, it is 1.921, and for 10 with OS ¼ 86 kcal/mol, it is 1.896. With 9, the deviations of the s and p orbitals from the line of centers are 24 and 19 , respectively, and with 10, the deviations are 34 and 29 .

730 STRAINED HYDROCARBONS: STRUCTURES, STABILITY, AND REACTIVITY

All three compounds are prepared by elimination of bromine from the corresponding bridgehead–bridgehead dibromides. In the case of 3, this may be effected by butyllithium in a hydrocarbon solvent, but for 14 and 15 the elimination must be carried out by potassium vapor in the gas phase followed by trapping the product as an argon matrix at 10–15 K.

4.1. Thermal Reactivity

Small ring hydrocarbons have a wide range of thermal reactivity, with cyclopropane and cyclobutane being quite stable thermally. With these compounds, the thermolysis is known to proceed via initial cleavage of one C C bond giving a diyl,48 which has a relatively high energy.

There often is a correlation between the strain relief in a reaction and the rate of thermolysis, but other factors may also be of importance. The C8 propellanes 16 and 17 have quite different reactivities. Whereas 16 undergoes thermal cleavage at 360 C,49 17 undergoes cleavage at 25 C.50 Aside from the difference in reactivity, the two reactions are essentially the same.

360 °C

16

25 °C

17

One factor that contributes to the higher reactivity of 17 is its strain energy (94 kcal/ mol) that is higher than that of 16 (67 kcal/mol). Here, the products have essentially the same strain and so the strain energies may be compared directly. There is also

another possible factor that may contribute to the reactivity of 17. It is relatively easy to move the bridgehead carbons apart in 17 due to the flexibility afforded by the four-membered rings. Hoffmann and Stohrer51 suggested that there is a special mechanism that will aid ring cleavage if these carbons can be moved apart. Such a mechanism is not possible with 16 because the three-membered ring prevents the bridgehead carbons from moving apart a sufficient amount.

The thermolysis of a series of homologues of 17 has been examined and the free energies of activation were linearly related to the relief of strain in the cleavage of the central C C bond forming a diyl. The compounds related to 16 fell on a different line.49

In some cases, steric interactions can prevent unimolecular reactions. Tetrahedrane (18) has been the subject of a number of studies, and the conclusion is that, if formed, it would rapidly decompose to form two molecules of acetylene.52 However, tetra-tert-butyltetrahedrane (19) is a quite stable substance, and on heating rearranges to tetra-tert-butylcyclobutadiene.53 An orbital symmetry54 analysis of the cleavage of tetrahedrane to acetylene indicates that it involves a torsional motion that in the case of the tert-butyl substituted derivative would bring the tert-butyl groups very close to each other. As a result, this mode of reaction is not possible, and the compound is relatively stable.

t-Bu

t-Bu

The thermolysis of cycloalkenes is often a more facile process than for the cycloalkenes. Cyclobutene undergoes thermolysis at 175 C and yields butadiene in an orbital symmetry controlled reaction as shown by stereochemical studies of substituted cyclobutenes.55

4.2. Dimerization

Dimerization is an important mode of reaction of strained alkenes, and two different modes of reaction may be found. With cyclopropene, an ene reaction occurs to give a dimer,56 and this in turn may further react in the same fashion to give a polymer.

H

H

732 STRAINED HYDROCARBONS: STRUCTURES, STABILITY, AND REACTIVITY

The stereochemistry and regiochemistry of the dimerization of 2-tert-butylcyclo- propenecarboxylic acid has been examined,57 and other examples of this reaction have been reported.58

Another reaction of strained alkenes is cycloaddition to form a cyclobutane derivative. An example is59

2

The initial dimerization presumably leads to a [2.2.2]propellane derivative that undergoes thermolysis to the diene that is the observed product. The dimerization occurs in dilute solutions, but at higher concentrations, polymers are found. This finding is in accord with the proposal that these reactions occur via initial formation of a 1,4-diyl, which closes to form the cyclobutane ring. In this way, the reaction avoids an orbital symmetry disallowed pathway. The diyl could react with the alkene at higher concentrations to give a polymer.

Cubene undergoes a similar reaction in which the initial cycloadduct undergoes further reactions.43 The bridged cyclopropene, bicyclo[4.1.0]hept-1,6-ene gives a rapid ene reaction forming two diastereomeric cyclopropenes, which further dimerize leading to a tetramer.56 This type of reaction appears to be common when the alkene has a high enough energy to make 1,4-diyl formation energetically possible.

Note that the formation of cyclobutane from ethylene has a favorable free energy change at room temperature, but the reaction will not occur because of the large reaction barrier. The free energy change becomes increasingly less favorable as the temperature is raised because of the negative entropy of reaction. If this can be avoided by having the two double bonds in the same molecule, the conversion to a cyclobutane ring can occur on heating.49 All of these reactions probably occur via a 1,4-butane diradical intermediate, just as in the thermal decomposition of cyclobutane to ethylene at higher temperatures.48

450 °C

5. REACTIVITY OF STRAINED HYDROCARBONS TOWARD EXTERNAL REAGENTS

Some strained hydrocarbons are quite reactive, whereas others are not. Strain is one of the factors that control reactivity, but other factors are often important.

5.1. Cycloaddition Reactions

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 trans-cycloalkenes. Many interesting cycloalkenes 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 {3þ2] or [4þ2] (Diels–Alder) reaction.

Most cyclopropenes are good dieneophiles. Bicyclo[3.1.0]hex-1,5-ene and bicy- clo[4.1.0]hept-1,6-ene are examples of strained alkenes that cannot be isolated, but can be trapped by diphenylisobenzofuran.60 A number of bridged bicyclo[1.1.0)but- 1,3-ene derivatives that cannot be isolated also have been trapped via Diels–Alder reactions.61

5.2. Reaction with Free Radicals

The strained saturated hydrocarbons are generally not reactive in free radical additions across C C bonds, although hydrogens may be extracted from many of them by the more reactive free radicals. The propellanes are one group that frequently undergo free radical addition. Although [1.1.1]propellane is quite stable thermally, it does undergo facile free radical addition,22 which frequently leads to oligomers62 and may be related to the nonbonded electron density near the bridgehead carbons. It is interesting to note that this propellane readily adds iodine, and that the iodine may be removed and the propellane recovered via reactions with nucleophiles.63

The cycloalkenes are, of course, reactive in free radical additions just as the open chain alkenes.

5.3. Reactions of Cyclopropanes and Cyclobutanes with Electrophiles

Although cyclopropane and cyclobutane have similar strain energies, they differ markedly in their reactivity toward electrophiles. Thus, whereas cyclopropane reacts readily with bromine to give 1,3-dibromopropane, cyclobutane does not react with bromine.

The reaction of small ring hydrocarbons with acids has been extensively studied. Although cyclopropane does not react with acetic acid, bicyclo[2.1.0]pentane (20) reacts rapidly to give cyclopentyl acetate. On the other hand, bicyclo[2.2.0]hexane (21) does not react. Other small ring hydrocarbons that react rapidly with acetic

734 STRAINED HYDROCARBONS: STRUCTURES, STABILITY, AND REACTIVITY

acid are [3.2.1]propellane (22), bicyclo[1.1.0]butane, and [1.1.1]propellane (3). The compound [2.2.2]propellane appears to be relatively stable toward weak acids.

+ HOAc OAc

20

21

OAc

+ HOAc

22

+ HOAc

In general, high reactivity toward electrophiles is found only with cyclopropane derivatives.

The origin of the difference between cyclopropanes and cyclobutanes in these reactions was suggested by a study of the cleavage of cyclopropane with D2SO4.64 The product 1-propyl hydrogen sulfate had deuterium scrambled among the carbons, and it was found that H2SO4 reacted more rapidly than D2SO4. These data suggest that protonation is rate determining, and that a protonated cyclopropane intermediate is formed in which the deuterium can be scrambled before reacting with a nucleophile to give the product. There is now much evidence for protonated cyclopropane intermediates.65

Calculations have shown that there are two ways in which cyclopropane may be protonated: at one of the carbons (corner protonated) and at one of the C C bonds (edge protonated). They have about the same energy, and are involved in the movement of a proton around the ring. The calculations indicate that cyclopropane is unusually basic for a hydrocarbon. The reason can be seen in the structures of the protonated cyclopropanes (Fig. 15.4).66 The bent bonds permit a proton to bond to a C C bond without coming close to the positively charged carbon nuclei.

Figure 15.4. Calculated structures of protonated cyclopropane and cyclobutane.

Corner protonation leads to a species that could be described as a methyl cation (a known ion) coordinated with a carbon–carbon double bond.

In contrast to cyclopropane, the cyclobutane C C bonds are only slightly bent, so that in order for the proton to form a bond, it must come close to the positively charged carbon nuclei, leading to increased Coulombic repulsion. Similarly, an attempt to bond one of the carbons does not lead to an ion with any apparent stabilization. This attempt to bond to one of the carbons leads to a relatively unstable species, and a slow rate of reaction.66

5.4. Carbon–Carbon Bond Cleavage by Transition Metal Species

The large difference in reactivity between cyclopropanes and cyclobutanes toward most electrophiles arises because there is little strain relief in the rate-determining step. The cleavage of C–C bonds by transition metal species leads to metallocycloalkanes, and if cleavage proceeds to a significant extent in the rate-determining step, the reactivities of cyclopropanes and cyclobutanes should become more comparable.

736 STRAINED HYDROCARBONS: STRUCTURES, STABILITY, AND REACTIVITY

Cyclopropanes are cleaved by a variety of transition metal derivatives. The first observation was that PtCl4 reacts to give a platinocyclobutane.67 Cyclobutanes appear to be less reactive, but cubane reacts with [Rh(CO)2Cl]2 leading to C C bond cleavage.68 The subject of C C bond cleavage has been reviewed.69

6. EFFECT OF STRAIN ON PHYSICAL PROPERTIES

6.1. NMR Spectra

Some of the more remarkable effects of strain are found in NMR chemical shifts. Cyclopropane derivatives usually have upfield proton chemical shifts with regard to the corresponding cyclohexane derivatives, whereas cyclobutanes commonly have downfield shifts.70 The upfield shift for cyclopropane protons have sometimes been attributed to a ring current in the three-membered ring,71 but there is little evidence for such a phenomenon. The unusual shift for these protons has proven valuable in demonstrating the presence of a three membered ring.

More information may be obtained from the carbon-13 nuclear magnetic resonance (13C NMR) chemical shifts.72 Cyclopropane has a shift of 2.8 ppm, whereas cyclobutane is found at 22.4 ppm, which is close to that of cyclohexane, 27.0 ppm. The bridgehead carbon of bicyclo[1.1.0]butane is at 3.0 ppm, whereas the methylene group is at 33.0 ppm. The annelation of a third cyclopropane ring, giving [1.1.1]propellane (3) has its bridgehead carbon resonance at 1.0 ppm. whereas the methylene carbon is found at 74 ppm. Thus, on going from cyclopropane to [1.1.1]propellane, the methylene carbon resonance increases by 77 ppm! However, in comparing cyclopropane with the bridgehead carbons of bicyclobutane and [1.1.1]propellane, one finds only a small change in chemical shift. Although the changes in these and other related compounds71 are well documented, there is at present no comprehensive explanation for the changes in 13C NMR chemical shifts.

6.2. Electronic Spectra

The vacuum ultraviolet (UV) spectrum of cyclopropane has been examined in some detail, and its first band is found at a significantly lower energy than for cyclobutane or cyclohexane.73 This result has been attributed to the increase in strain that raises the ground-state energy. All of the transitions in the cycloalkanes appear to have Rydberg character in which one electron is promoted to a high-energy diffuse orbital, leaving an electron deficient core that resembles a radical cation. The spectrum of cyclobutane is similar to that of cyclohexane.

Bicyclo[1.1.0]butane has been studied in some detail.74 Again, all of the states appear to involve Rydberg transitions. The transition energies are at lower energies than those for cyclopropane, which may be a result of the strain in the bicyclobutane ring that further increases the ground-state energy. The spectrum of [1.1.1]propellane has not as yet been subjected to a detailed analysis.