Electronic strain

A variety of other highly-strained electron-rich donors also form colored complexes (similar to homobenzvalene) with various electron acceptors, which readily undergo thermal cycloadditions (with concomitant bleaching of the color).209 For example, Tsuji et al.210 reported that dispiro[2.2.2.2]deca-4,9-diene (DDD), with an unusually low ionization potential of 7.5 eV,211 readily forms a colored charge-transfer complex with tetracyanoquinodimethane (TCNQ). The [DDD, TCNQ] charge-transfer complex undergoes a thermal cycloaddition to [3,3]paracyclophane in excellent yield, i.e.,... 

Under these conditions only one mixed [6,6]-triazoline-azahomofullerene isomer 25 was formed, which is explained by the fact that 24 behaves as a strained electron-poor vinylamine. The significantly highest Mulliken charge of 0.06 (AMI) is located at C-1 and C-6, and the lowest of-0.07 at C-2 and C-5 (Figure 10.10). The most negatively polarized N atom of the azide (AMI) is that bearing R. A kinetically controlled attack of the azide, therefore, leads predominantely to 25. 

Organism (strain) Electron acceptor Concentration max. Degradation rate Reference... 

Another factor, besides strain, that can destabilize a molecule is an unfavorable electronic interaction, for example that manifested as antiaromaticity, in, say cyclobutadiene [39], but unlike strain, electronic interactions can be favorable and stabilizing. Minyaev et al. proposed that orbital interactions between the two capping (CH)n and the adjacent C moieties stabilize [3 n]- and [4,n]-prismanes by CT-overlap of rr-type ring orbitals, and precedent for snch stabihzation in simple... 

Macrocycles 10 and 11 were converted efficiently to yield [6]CPPA and [8] CPPA for the first time (17% overall yield, 4 1 12 13) [16]. Owing to their strained, electron-rich nature, these belts were observed to oxidize upon storage and [6] CPPA was shown to decompose explosively upon heating in air. Interestingly, the [4]CPPA precursor 9 was found to undergo only addition of tert-butoxide or THF (when used as solvent) at the reactive site (Fig. 8) [15]. 

The predominant reaction mode of Cm is that of a strained, electron deficient alkene. Addition reactions typically occur in a 1,2 fashion across the 6-6 bonds, adding across the olefin and not opening up the basic Cm ring system. Representative of such reactions are the carbene addition and the Diels-Alder reaction shown in the first two examples of Figure... 

The calculated bond distances in the C-C bonds in [60]fiillerene reveal significant differences between the [5,6]- and [6,6]-bonds with values of 1.45 and 1.38 A, respectively. Because of the mixed character of 1,3,5-cyclohexatrienes and [5] radialenes, Qo behaves as a highly strained electron-deficient olefin whose chemical reactivity is mainly driven by strain relief. Therefore, addition reactions have been widely used [13]. Interestingly, although similar reactivity patterns have also been observed for higher fullerenes, the chemical reactivity tends to decrease significantly with their size [14-16]. 

Mechanisms can be complex. / ow you begin will likely determine whether you will reach the correct solution. Sometimes the steps in a possible sequence will be readily reversible, so the pathway may not lead to any stable structure. In this case you will simply have to start over and try to find a new trail that points to a more productive direction. Alternatively, two sequences may both end in reasonable compounds, but one may be much faster than the other and provide the predominant or exclusive product. Or, your arrow pushing may generate a molecule that is energetically too unfavorable to be feasible (e.g., too strained, electron deficient, and the Uke). Your /nformation base is the critical component of evaluating these situations. In particular ... 

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