1.3- Dipoles fulvenes

Here the charges as indicated are no longer the same for each carbon atom. Actually these hydrocarbons have a dipole moment (azulene 1.0 D, fulvene 1.2 D). 

Nucleophiles attack C-6, since this is the site of highest LUMO coefficient. Similarly, nucleophilic 4 n addends should cycloadd across the 2 and 6 positions, with the more nucleophilic end of the 4 rr addend attacking C-6. By contrast, electrophiles attack C-2, the site of highest HOMO coefficient, and electrophilic 4 n addends can only add across C-2 and C-3, due to the node at C-6. The presence of the second highest occupied MO (SHOMO) of the fulvene complicates this simple picture, and substituents may drastically change the HOMO shape, as described below. Nevertheless, this picture does help explain, at least qualitatively, the varying periselec-tivity of diene and 1,3-dipole cycloadditions to fulvenes. 

We have described the cycloadditions of a variety of dienes, ranging from cyclo-pentadiene to cyclopentadienones with alkyl and aryl fulvenes80-82. In these cases, only the [4 + 2] cycloadducts across the 2 and 3 positions are observed. Similarly, 1,3-dipoles such as nitrones and nitrile oxides add in this fashion, as well. We discovered the first authentic [6 + 4] cycloaddition of a fulvene in 197083. The cycloadditions of tropone to fulvenes, which we originally suggested involved [6-fulvene + 4-tropone] cycloadditions, now appear to be [6-tropone + 4-fulvene] cycloadditions8... 

We predicted that electron-rich 1,3-dipoles in addition to diazomethane, such as nitrile ylides, would react in this fashion as well, and in 1978, Padwa and Nobs reported just such a reaction, shown in the second equation in Fig. 30. This reaction also gives some [4 + 2] adduct, in which the most nucleophilic terminus of the dipole has added to the 6-position of the fulvene. We have also found that electron-rich dienamines add to fulvenes in high yield (third equation of Fig. 30)85 These results... 

Cycloadditions between substituted fulvenes and 1,3-dipolar reagents have been the subject of considerably less study than the corresponding diene-mediated chemistry, paralleling the situation seen in the tropone series. For the most part, however, the reactivity patterns formulated in the fulvene-diene studies are repeated in the analogous 1,3-dipole reactions. 

A further illustration of the electronic influence of substituents on the course of reactions with 1,3-dipoles can be seen in the cycloaddition of an amino-substituted fulvene to benzonitrile oxide. Adduct (76) was obtained in 60% yield when the electron-rich dipolarophile 6-dimethylaminofulvene was exposed to benzonitrile oxide. The structure of the resultant product can be conveniently explained by an initial [6... 

The mode of 1,3-dipolar cycloaddition of a methylenecyclopropene depends upon the nature of the dipole and the C(4) substituent of the fulvene. Normally, the initial... 

The reactions of fulvenes (373) also provide examples where the longest conjugated system available is not always the one involved in cycloadditions, but this time frontier orbital theory is rather successful in accounting for the experimental observations. The orbital energies and coefficients are illustrated in Fig. 4-71, where it can be seen that there is a node through C-l and C-6 in the HOMO. The result is that when a relatively unsubstituted fulvene might react either as a n6 or as a n2 component with an electron-deficient (low-energy LUMO) diene or dipole, it should react as a n2 component because... 

A primitive approach to treating the solvent effect was made with 6,6-diheterosub-stituted fulvenes by calculating the gas-phase barriers with the CNDO/2 method and the solvent stabilization of ground and transition states in the reaction field formalism with a spherical cavity. The result was found to be very sensitive to the cavity size and to the calculated ground-state and transition-state dipole moments, and no reliable numerical data could be obtained. The calculated gas-phase barriers were too high, although they fell in the correct order. 

Thiazolo[3,4-2l] benzimidazole [-Cg/. 1,3-Diphenylthiazolo [3,4-fl] benzimidazole (245), containing quadrivalent S, reacts as a thiocarbonyl ylide dipole with fulvene and tropone systems. 

MCP, the archetype of nonalternant hydrocarbon known as the fulvenes, is a molecule of considerable theoretical and experimental interest [82-90]. It was synthesized and characterized for the first time in 1984 Billups et al. [84] and Staley and Norden [85] independently. An ultraviolet (UV) spectrum was included in the latter study. The electric dipole moment and heavy-atom molecular structure were reported two years later [88]. The experimental value of the dipole moment, p = 1.90 0.02 D, remarkably large for such a small molecule, which has been related to the strong transfer of 7r-electron density out of the ring due to the two double bonds interacting in a T-type arrangement. The... 

Thus X-ray crystallography indicates that the double bonds are not significantly different from normal double bonds [211], Earlier dipole measurements on fulvene gave a value of -I-ID (212] a later value obtained from microwave spectroscopy is only 0-49D [213]. H-n.m.r. spectroscopy, and especially a consideration of... 

This conclusion is supported also by LCAO-HMO calculations. It can be shown that the molecular orbitals of [ ] fulvenes and [n, w] fulvalenes belong either to the A2 representation of the point group C21, or to its representation. In the molecular orbitals of [/i] fulvenes which belong to A2, all the electron density is concentrated in the ring. The A -> transition in triafulvene is from a Bi molecular orbital to an A2 molecular orbital. Thus, in the lowest unoccupied molecular orbital, all the electron density is concentrated in the three-membered ring. Likewise, in 1,2-diphenyltriafulvene, upon A - Fi transition, the calculated dipole moment diminishes. 

Consider the case of triapentafulvalene (XIV) (calicene), a combination of tria-fulvene and pentafulvene. Hexaphenyltriapentafulvalene (XV) [37] was taken as a representative. This compound has the highest experimental dipole moment in the ground state ever recorded in a hydrocarbon - 6.3D (benzene, 30°C) [37]. A similar value was obtained in the case of the l,2-dimenthyl-3,4,5,6-tetraphenyl derivative [38]. 

A. Pullman In all fairness, it must be said that the quantum chemical computations, as a rule, and quite generally, are much less satisfactory, for excited states properties than for ground state properties. This is true particularly when quantitative evaluation of a property is requested. This inconvenience appears in all its fullness for non-benzenoid hydrocarbons because the dipole moment is a striking example which does not occur in the benzenoid series, but it is true for any quantity related to excitation. As concerns qualitative evolutions of various properties, theory has been extremely successful in the past in the very case of the fulvenes and has even been instrumental in the development of the field since it has often preceded experiment and served as an incentive for the chemist to synthetize the compounds. (See the numerous series of papers by ourselves and Dr Ernst Bergmann in the early 1950 s.)... 

One might think this a trivial result on the grounds that the carbon atoms in a neutral hydrocarbon obviously must be neutral—but this would be an incorrect intuition. Nonalternant hydrocarbons are polar and do have non vanishing dipole moments. Fulvene (5) and azulene (6) are good examples, each having dipole moments of about 1 D (Debye). In the case of an odd AH ion, the total Tc-electron density at atom i is 1 + ah for an anion and 1 -... 

Fulvenes, cyclic molecules with odd number of carbon atoms in the ring, represent one of the fundamental categories of cross-conjugated systems (see Chapters 1 and 6). According to the ring size of these molecules, the compounds are classified astriafulvenes 1, pentafulvenes 2, heptafulvenes 3, and so on (Scheme 7.1). Owing to their dipole moments, fulvenes possess unique reactivity, different from that of open-chain polyolefins and aromatic compounds. In 1989, Neuenschwander... 

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