Transannular electronic interactions

The electronic spectra of the [2.2]paracyclophanes provide valuable information regarding the extent and mechanisms of transannular electronic interactions in strained 7t-electron systems. A rigid system like the [2.2]paracyclophane molecule is of great value as a model for checking theoretical data with a view to interpretating U V spectra of sterically hindered molecules ( overcrowded compounds ). 

Transannular electronic interactions ("inter-ring iz overlap ) would also appear to play a role, as evidenced by the absorption at X = 244 nm. 

Dialkyl 1,3-diacetyIenes usually absorb in the ultraviolet region, Amax 226, 239 and 253 nm. Of a series of cyclic tetraacetylenes (8), two higher homologues, 8c and 8d, show absorption spectra similar to that of acyclic compounds, whereas the trimethylene homologue (8b) demonstrates strikingly different features in its absorption spectrum (0 a red shift of 9nm and (//) appearance of a new band at 246 nm. This is attributed to the marked transannular electronic interaction in 8b, in... 

A series of [n]paracyclophadiynes or [tw.nlparacyclophadiynes (12) with different numbers of methylenes was prepared for the study of the transannular electronic interaction between a diacetylene unit and a benzene ring . The absorption spectra of 12 show features which are obvious when compared with that of a reference compound 15c (Figure 1) (/) disappearance of vibrational fine structures with decrease of the methylene number, (//) bathochromic shift of the longest wavelength bands and (ii7) appearance of a new band at 233 nm for 12c and at 227 nm for 12d,... 

The importance of transannular electronic interactions is beautifully shown in a reaction found by S. Misumi, T. Ogawa and T. Kaneda [Eq. (22)], when transannular interaction between a diyne system and an aromatic ring results in completely unusual reactivity [3 a]. 

A further factor influencing the transannular interaction is represented by the steric and electronic interaction of substituents on... 

The absorption spectrum of a carbazole-acceptor cyclophane shows transannular 7t-7t electronic interaction and little charge transfer interaction, while the fluorescence spectrum exhibits intramolecular exciplex emission around 525 nm <2003CL910>. 6,13-Bis(9-ethyl-9//-carbazol-3-yl)pentacene (a pentacene derivative bearing two carbazole moieties) displays an absorption spectrum that consists of the bands due to the carbazole (352 nm) and the pentacene moieties (483, 523, 561, 605 nm), indicating that the electronic transition level of pentacene is not affected by the carbazole chromophore <2006MI185>. 

A large number of cyclic acetylenes have been synthesized to investigate the proximity interactions between two triple bonds or between a triple bond and other unsaturated systems. Such transannular proximity interactions have been observed in some cyclic acetylenes of medium ring size. However, no appreciable evidence of the interaction was detected in the electronic spectra of diacctylenes 1 2 - and 3... 

However, on top of the inclusion phenomena, cyclophanes have more interesting properties to offer. In his first publication on cyclophanes, Cram [13] had already expressed his opinion about some peculiarities to be expected in cyclophanes, which he outlined as follows a) electronic interaction between aromatic rings placed face to face , b) the resulting influence on substitution reactions in the aromatic rings by transannular electronic effects, c) intramolecular charge transfer complexes and d) ring strain, steric strain and transannular strain. These effects have been studied on the parent compounds in detail [3]. 

Moreover, the second-order nonlinear susceptibility (Pz) of DPN derivatives is higher than the value of related DPM derivatives, due to the n-n interaction in the gable conformation. Both transannular electronic transitions and hyperpoiarizability are Franck-Condon processes, which are favored in DPN, because the probability of a photon impacting the gable conformation in a free rotor is negligible. 

Transannular interaction via the electron-delocalization mechanism was found, but lessened by 10-15% for the ligand superhyperfine splitting and 30-35% for the hyperfine splitting (62) in the epr spectrum. The crystal structure of [VOS2CNEt2)2] shows that the molecular core has the expected C2V symmetry [V-0 = 159.1(4), V-S = 138.7(2)-241.0(2) pm] (63). Magnetic and spectral data provided evidence for a tetragonal, pyramidal structure (VII) for these complexes. Like many other coordinatively unsaturated, metal... 

Through an interaction of the two singly occupied ji orbitals, a transannular six-center four-electron 71 -ji bond with a formal bond order of 0.67 results... 

As shown in Table 6 and Figure 1, the oxidation potentials of 2-substituted norbornadienes (1), 2-substituted bicyclo[2.2.2]octa-2,5-dienes (2) and 4-substituted [2.2]paracyclophanes (3) clearly indicate that the transannular interaction between two double bonds contributes already at the stage of the first electron transfer. Namely, in compounds 1-3, the electron is transferred from the unsaturated bond which is not substituted by the electron-withdrawing group, Figure 1 shows the... 

The electronic and photoelectron spectra of these types of molecules can only be rationalized in terms of appreciable transannular interactions. The transannular resonance integral has been estimated to be about 40% of that between adjacent p orbitals in benzene. It would seem, therefore, that [130] is more correctly referred to as homonaphthalene and [131] as homoanthracene , etc. It is of interest to note that our probes for homoaromaticity correctly discern the importance of homoconjugation in [130] (Williams et al., 1988). Similar significant transannular interactions are also evident in homoazulene [133] (see Scott et al., 1985 Scott, 1986). 

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