Distorted double bonds

Calculations (53) with our olefin force field (Section 6.2.) are in accord with these results, and give II as 2.84 kcal mole-1 more favourable (AF AH = 2.66) than III. In view of the highly distorted double bonds (113) in II and III and the resulting transferability problems for our force field, the agreement is quite satisfactory. 

Interactions of C-Cl dipoles with iT-electrons in highly chlorinated bicy-clo[2.2.1]heptenes such as 226 were invoked to explain the chemical shifts of C(2), C(3), and C(7) (358). Similarly, remarkably large upheld ya-SCS(OR) values in 227 (n = 2 to 4) were attributed to a back-lobe interaction of the C(l)-O bond with the p-orbital of C(3) in the distorted double bond (cf. 228) (359). 

Shea, K. J. Kim, J. S. Influence of strain on chemical reactivity. Relative reactivity of tor-sionally distorted double bonds in MCPBA epoxidations. J. Am. Chem. Soc. 1992, 114, 3044-3051. 

Computational methodology has been used to accompany or to anticipate experimental results for many classes of compounds. Such results are particularly helpful for transient species, for rationalization of physical and structural properties, and for simulation of reaction pathways and transition states. Semiempirical valence electron (CNDO/MNDO), ab initio, and nonquantum mechanical force field (molecular mechanics) calculations have mainly been used for the examination of structure and stability of moderately strained olefins, whereas many-electron quantum-chemical methods have been used for detailed discussion of electronic aspects. Excellent reviews of molecular mechanics calculations, the principal method used to describe geometrical and energy features in distorted double bond systems, have been written by Osawa and Musso (61). 

Figure 4. Basic structures used for generation of distorted double bonds.

Spectroscopic methods are indispensable tools for the ready identification of the molecular structure of noncrystalline compounds and for the detection of unstable intermediates. Beyond the gross structural features, spectroscopic results contain essential information about structural details. Earlier results in this area have been summarized by Zefirof and Sokolov (183). In this section, the more recent features of distorted double bonds apparent in NMR, IR, UV, and PE spectroscopy are presented. 

For the hydronium-ion-catalyzed hydration of bicyclo[4.2.1]non-l-ene (177) and bicyclo[4.2.1]non-l(8)-ene (176), appreciable solvent isotope effects have been observed. Since these correspond to those found for reactions of unstrained olefins, it was concluded that the hydration proceeds as with unstrained alkenes by a two-step mechanism protonation of the double bond is followed by addition of the nucleophile (152b). The strained olefin with its distorted double bond is higher in energy and more reactive than an unstrained alkene. Hence, the transition state for protonation of Bredt-olefins is expected to be an early one (95). 

According to MNDO and ab initio results for the addition of BH3 to ethylene (1), the reaction is highly exothermic and proceeds via a reactant-like transition state with the double bond elongated to 138-140 pm and little pyramidalization (254). It therefore seems unlikely that release of strain will be noticeable in hydroborations of distorted double bonds. The low regioselectiv-ity observed in hydroboration of 69 and the structurally related 282 may therefore reflect similar hybridizations of both carbon atoms of the double bond. 

Unactivated double bonds do not react readily with nucleophiles. However, additions are observed if the olefin is coordinated to a transition metal many organometallic complexes are known which induce such reactions (263). While this aspect has not been explored with strained olefins, a few examples of direct nucleophilic additions to distorted double bonds have been reported. 

Six-Center Reactions. Apart from the [2 + 4]cycloadditions used for trapping highly strained olefins, systematic studies for probing the nature of distorted double bonds and strain-related features are scarce. The relative rates... 

Structural features of olefins with distorted double bonds have been discussed within the deformation space defined by the eight bond angle deformations. The out-of-plane bond angle distortions are of particular interest because they are involved in addition reactions of the double bond. The symmetrical Blg-type deformation is related to concerted anti-additions, whereas the J3lu-type distortion (cf. Table 1) is appropriate for concerted syn-addition and those reactions that involve three-center intermediates and the formation of transition metal complexes. Twist or torsion is due to the Alu-type oop distortion and may be related to addition reactions, which in principle would lead—in the extreme case of a 90° twist angle—to an eclipsed rather than a staggered arrangement. 

More than one of the three possible oop bond angle deformations are usually apparent in the distorted olefins. Nevertheless, it is possible to detect in most cases the dominant type of distortion. The kinetic stability of olefins with distorted double bonds is enhanced by steric shielding. Hence, distorted double bonds in bicyclic and polycyclic structures may not display the reactivity to be expected if the dominant type of the oop distortion is the selectivity-controlling factor. The strong preference of syn- over anti-addition in Bredt-olefins may be due to this steric shielding. 

The reaction of norbornene was also found to have a lower activation barrier to form the metallacyclobutane than the reactions of cyclopentene, cycloheptene, and Z-cyclooctene. This can be attributed to the pre-distorted double bond of norbornene, which requires less distortion to achieve the pyramidalized geometry in the transition state than that needed for planar olefins [43]. 

A common example of the Peieds distortion is the linear polyene, polyacetylene. A simple molecular orbital approach would predict S hybddization at each carbon and metallic behavior as a result of a half-filled delocalized TT-orbital along the chain. Uniform bond lengths would be expected (as in benzene) as a result of the delocalization. However, a Peieds distortion leads to alternating single and double bonds (Fig. 3) and the opening up of a band gap. As a result, undoped polyacetylene is a semiconductor. 

For all three diallyl phthalate isomers, gelation occurs at nearly the same conversion DAP prepolymer contains fewer reactive allyl groups than the other isomeric prepolymers (36). More double bonds are lost by cyclisation in DAP polymerisation, but this does not affect gelation. The heat-distortion temperature of cross-linked DAP polymer is influenced by the initiator chosen and its concentration (37). Heat resistance is increased by electron beam irradiation. 

The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffiaction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. ... 

Compare geometries of the cis and trans cycloalkenes. Are the double bonds incorporated into the trans compounds significantly more distorted than those incorporated into the analogous cis cycloalkenes Consider carbon-carbon bond lengths and the twisting and/ or puckering of the double bond. Are any distortions greater in trans-cycloheptene than in trans-cyclooctene ... 

The niobium atom has a slightly distorted octahedral coordination. Interatomic distances between the niobium atom and the two oxygen atoms in trans positions, O-Nb-O are 1.81 and 2.14 A. The niobium atom is shifted from the base plane of the octahedron by 0.23 A, and this shift, in adjacent chains, is in opposite directions. Pakhomov and Kaidalova [204] concluded that the shorter Nb-O bond (1.81 A) is an intermediate between a single and double bond. 

The results of our calculations based on both the static and dynamic theories show that most of the nonbenzenoid cyclic conjugated systems examined exhibit in a greater or lesser degree a marked double-bond fixation. The static theory indicates that even in benzene there exists a hidden tendency to distort into a skewed structure and that such a tendency is actually realized in [4n-f-2] annulenes larger than a certain critical size. In nonalternant hydrocarbons bond distortion is a rather common phenomenon. Fulvenes, fulvalenes and certain peri-condensed nonalternant hydrocarbons undergo a first-order bond distortion, and... 

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