Alkenes nonplanar

It is a commonly accepted belief that the planar excited states of simple alkenes, formed by light absorption or energy transfer, decay very rapidly to more stable nonplanar species.6 The perpendicular geometry is believed to be the stablest configuration of both the lowest singlet and triplet states. In this particular case it is obvious that reactions (10) and (11) are virtually the same process, although, in chemical parlance, reaction (10) is part of an internal conversion and (11) is the last step in a photochemical reaction. 

The triplet state of the alkene is most stable when the p orbitals, which make up the normal tt system of the double bond, are not parallel to one another (Figure 6-17). Therefore, if the energy-transfer process leads initially to a planar triplet, this is converted rapidly to the more stable nonplanar form. The excitation of either the cis or the trans isomer of the alkene appears to lead to a common triplet state, as shown in Figure 28-4. 

Porphycenes 51 bearing substituents at ethylenic carbons have been prepared from 5,5 -acyl bipyrroles 53 (05OL1887) (Scheme 26). However, oxidation of the intermediate annulene 54 is difficult, owing to the presence of the alkyl chains on the alkene bridges. These reduce conformational flexibility, resulting in a near-nonplanar geometry, which resists oxidation and/or aromatization (08CSR215). 

Because of its nonplanar geometry, cyclooctatetraene is not antiaromatic and its hydrogens appear at 5.75 8, a value typical for alkenes. However, the triple bonds of the compound called benzo-l,5-cyclooctadiene-3,7-diyne force this molecule to assume a nearly planar geometry. The pi system of its eight-membered ring contains eight electrons. (Only two of the electrons of each triple bond are part of the conjugated system.)... 

These diastereoselectivities can rationalized by transition state (64 Scheme 9). A pyramidal configuration of the anionic center is assumed (c/. nonplanarity of a benzylic or an allylic anion ). This leaves more space for an equatorial substituent EWG (64) than for an axial EWG (65). The resulting product structure (63) agrees well with the general trends of Table 4. The finer issue — why (64) is preferred over (65) in the case of Z) vs. ( )-alkenes — remains unexplained. 

In summary, the reactivity of strained olefins is different from that of simple alkenes. The wealth of transformations initiated by strained double bonds has recently been reviewed in great detail (276). In this chapter some structural and strain-related features of reactivity observed for olefins with nonplanar double bonds have been presented. It is apparent that in polar reactions as well as cycloadditions of these molecules the release of strain may contribute to enhanced rates, but it is certainly not the controlling factor. In view of the operation of microscopic reversibility, it may be questioned whether the buildup of strain in reverse reactions—eliminations and cycloreversions—is the controlling factor. 

The final chapter in this volume deals with the chemistry of strained (bent and nonplanar) alkenes. Wolfgang Luef and Reinhart Keese have surveyed the recent literature with respect to syntheses and properties. This chapter also reflects the modern tendency to calculate the energies of interesting molecules and to use the calculated values to rationalize properties and to guide syntheses. 

The catalyst adopts a nonplanar, bent structure and asymmetric induction in this process is postulated to arise from side on approach of the alkene from the least hindered side, with the largest substituent pointing away from the bulky substituents of the aromatic ring (Figure 4.5). The approach vector depicted... 

Rondan, N. G., Paddon-Row, M. N., CarameUa, R, Houk, K. A. (1981). Nonplanar Alkenes and Carbonyls A Molecular Distortion which Parallels Addition Steroselectivity. J. Am. Chem. Soc., 103,2436. Ess, D. H. Houk, K. N. (2007). Distortion/Interaction Energy Control of 1,3-Dipolar Cycloaddition Reactivity. J. Am Chem. Soc., 129, 10646-10647. Lopez, S. A., Houk, K. N. (2013). Alkene Distortion Energies and Torsional Effects Control Reactivities, and Stereoselectivities of Azide Cycloadditions to Norbomene and Substituted Norbomenes. J. Org. Chem., 78(5), 1778-1783. Hong, X., Liang, Y, Griffith, A. K., et al. (2013). Distortion-Accelerated Cycloadditions and Strain-Release-Promoted Cycloreversions in the Organocatalytic Carbonyl-Olefin Metathesis. Chem. Sci., 5(2), 471-475. 

The interplay between hyperconjugation and conjugation plays an important role in structures of neutral nonplanar polyenes. Hyperconjugation also contributes to the greater stability of internal alkenes versus their tenninal isomers (Figure 6.34.). It also partially masks the stabilizing effect of conjugation if it is evaluated by the Kistiakowsky approach (vide infra). 

Hydrocarbons contain only C and H atoms, so their physical properties depend on the strength of their dispersion forces. The names of organic compounds have a root for the longest chain, a prefix for any attached group, and a suffix for the type of compound. Alkanes (CnHan+a) have only single bonds. Cycloalkanes (C Hp ) have ring structures that are typically nonplanar. Alkenes (CnHpn) have at least one C=C bond. Alkynes (CnHpn-p) have at least one C=C bond. Aromatic hydrocarbons have at least one planar ring with delocalized tt electrons. 

The directing effect of bulkier alkyl groups is pronounced in ds-alkene due to their nonplanar geometry as compared to trans-alkene. This allyl strain in ds-alkene is termed the strain [7]. Consequently, the tram isomer of 1 -phenyl-propene and tra s-2,4-dimethyl-3-hexene isomers react at a faster rate than do the corresponding ds isomers (Chart 4.3) [6]. 

The special stability and reactivity associated with cyclic delocalization is not unique to benzene and polycyclic benzenoids. Thus, we shall see that other cyclic conjugated polyenes can be aromatic, but only if they contain (An + 2) tt electrons (n = 0, 1, 2, 3,. . . ). In contrast, An tt circuits may be destabilized by conjugation, or are antiaromatic. This pattern is known as Hiickel s rule. Nonplanar systems in which cyclic overlap is disrupted sufficiently to impart alkene-like properties are classified as nonaromatic. Let us look at some members of this series, starting with 1,3-cyclobutadiene. 

Spectral and structural data confirm the ordinary alkene nature of cyclooctatetraene. Thus, the NMR spectrum shows a sharp singlet at S = 5.68 ppm, typical of an alkene. The molecular-structure determination reveals that cyclooctatetraene is actually nonplanar and tub-shaped (Figure 15-17). The double bonds are nearly orthogonal (perpendicular) and not conjugated. Conclusion The molecule is nonaromatic. 

In Summary Cyclic conjugated polyenes are aromatic if their tt electron count is 4n + 2. This number corresponds to a completely filled set of bonding molecular orbitals. Conversely, 4n TT systems have open-shell, antiaromatic structures that are unstable, are reactive, and lack aromatic ring-current effects in NMR. Finally, when steric constraints impose nonplanarity, cyclic polyenes behave as nonaromatic alkenes. 

---