Annulenes, bridged methano

In general, bridged methano[10]annulene derivatives are a very attractive class of compounds for investigating non-trivial aromatic systems. Being quite easy to be synthesized and showing a somewhat tunable conjugation pattern [84] due to the... 

Heteroannulenes electrophilic reactions, 7, 726 tr-excessive synthesis, 7, 727 nucleophilic reactions, 7, 727 Hetero[l IJannulenes structure, 7, 715 Hetero[ 12]annulenes pyridine-like methano-bridged, 7, 715 Hetero[ 13]annulenes unrestricted structure, 7, 716... 

The preparation of l,6-oxido[10]annulene, described simultaneously by Sondheimer and Shani4 0 and by Vogel, Biskup, Pretzer, and Boll, is illustrative of the rather general synthesis of aromatic 1,6-bridged [10]annulenes starting from 1,4,5,8-tetrahydronaphthalene. Apart from the present compound, the following bridged [10]annulenes have thus far been obtained by this approach l,6-methano[10J-annulene,2-8-9 the ll,ll-dihalo-l,6-methano[10]annulenes,9 10 and 1,6-imino [ 1 OJannulene.11... 

Thus, l,6-methano[10]annulene (77) and its oxygen and nitrogen analogs 78 and 79 have been prepared and are stable compounds that undergo aromatic substitution and are diatropic. For example, the perimeter protons of 77 are found at 6.9-7.3 5, while the bridge protons are at —0.5 5. The crystal structure of 77 shows that the perimeter is nonplanar, but the bond distances are in the range 1.37-1.42A. It has therefore been amply demonstrated that a closed loop of 10 electrons is an aromatic system, although some molecules that could conceivably have such a system are too distorted from planarity to be aromatic. A small distortion from planarity (as in 77) does not prevent aromaticity, at least in part because the s orbitals so distort themselves as to maximize the favorable (parallel) overlap of p... 

Alder-Rickert cleavage was also examined as a potential route for 100 and 102, but the required precursors could not be synthesized. Bis-methano[14]annulene 105 added dicyanoacetylene in the wrong sense, and did not yield 106, which could have served as precursor of 102. Similarly, the attempted Diels-Alder reaction of the bridged cyclopropafl OJannulene 107 with dlmethoxycarbonylacetylene, which should furnish 108, produced only decomposed material. This is consistent with the observation of Halton and Russel that 107 does not add to 4-phenyl-1,2,4-tria-zoline-3,5-dione. In order to circumvent the low reactivity of 107 in cycloaddi-... 

The NMR and UV characteristics of 58a and b were shown to be consistent with a heavily buckled atropic frame and one whose hetero-atomic unit is directed syn to the methano bridge.60 It is also interesting to note in this connection that, whereas thia-annulene (58b) does differ rather significantly in some aspects of its NMR and UV spectra from... 

Prior to the work of Cheng et al. Rubin et al. reported a similar [4 + 4] pho-tocyclization reaction [299]. Rubin has shown that a cyclohexadiene derivative underwent a very facile photochemically promoted rearrangement to the stable bridged bisfulleroid 198 (Scheme 76). This process occurs via the initial [4 + 4] photoadduct (not observed), which undergoes a thermally allowed [2 + 2 + 2] cycloreversion to afford a bis-methano[12]annulene structure 198. Alternatively, compound 198 can be obtained by photolysis of the allylic alcohol 197 under reflux and acidic conditions in good yields. The allylic alcohol is obtained by acidic... 

The benzannulation technique is also applicable to the benzene homologation and functionalization of annulenes, as well to a quadruple arene modification of dendritic cores. The reaction of chromium carbene functionalized l,6-methano[10]annulene 82 with 3-hexyne under standard conditions afforded a fair yield of the benzannulation product 83 after protection and oxidative work-up (Scheme 32) [75]. The chromium complex 84 evidently partly survived the oxidation conditions using Feln a syn-stereochemistry with respect to the Cr(CO)3 fragment and the methano bridge was suggested on the basis of NMR data, which is in contrast to the preferred formation of anti-annulation products bearing cydophane skeletons [75b]. 

Within the framework of reactions of various propellane substrates with very reactive dienophiles, N-methyl- and iV-phenyl-triazolinediones, l,6-methano[10]annulene itself and many bridge- and ring-substituted derivatives were subjected to such Diels-Alder reactions. All of the products are (more complex) [4.4.1]propellane derivatives. In general it is clear that the rates of reaction are very much slower than those of various tetraenic. 

Finally, if the perturbation due to the substituent as well as its position in the molecular framework is known, MCD spectroscopy can be used to investigate the nature and the ordering of the frontier orbitals. In this way, the relative importance of transannular interactions compared to substituent effects and geometrical distortions for the electronic structure of bridged an-nulenes such as l,6-methano[10]annulene (14) may be assessed by determining the energy ordering of the frontier orbitals of substituted derivatives of 14 from their MCD spectra. This is shown in detail in Example 3.12. 

Methano[10]annulene is rather like naphthalene but with the middle bond replaced by a methylene bridging group. 

This strategy has been of basic importance not only in the field of cascade molecules. In 1978, Vogel et al. synthesized methano-bridged annulene derivates by a repetitive synthetic strategy6 (Scheme 3). The repetitive steps (initial Wittig reaction followed by reduction of the resulting ester groups to the aldehyde) led him to a new class of aromatic compounds. 

In contrast, stabilization of the cycloheptatriene form is achieved by extending it to the 10-Tt-electron aromatic, l,6-methano-[10]annulene 8. Again disubstitution of the methano-bridge (as in 9 10) shifts the equilibrium, so that both forms coexist in measurable amounts/ Aperi-C i2-CU.2 bridge does not alter the enthalpy difference between 8 and its norcaradiene valence tautomer, whereas a / m-CH = CH bridge provides considerable de-stabilization of 8. Consequently 11, that exists in the norcaradiene form, on selective hydrogenation of the peri-double bond gives 12, that exists in the [10]-annulene form. ... 

In the effort to prepare l,5-methano[10]annulene, elimination of HI from iodotetraene 83 was found to give the rearranged pentaene 84 (126). A reasonable explanation for its formation is a walk rearrangement of the intermediate 1,7-bridged bicyclo[5.1.0]octadienyl cation. In this case the walk may be favored by the steric effect of the three-atom linkage, which probably constrains the homotropylium cation to the closest bicyclo[5.1.0] octadienyl form. 

When the parent compound of the bridged [lOjannulene series, l,6-methano-[10]annulene 11>120) 28 (see Table 6), was first prepared, it was necessary to distinguish between three possible structures a) the "double norcaradiene structure 20, b) a rapidly fluctuating system (c.f. cyclooctatetraene), possibly proceeding through low equilibrium concentrations of the double norcaradiene, 21, or c) a delocalized 10 -electron system 22. 

Various substituted l,6-methano-[10]annulenes have been prepared and characterized, and in particular the n.m.r. of such compounds reflects their delocalized nature 17,120,124-127) (a 71-complex formed from reaction of l,6-methano-[10]annulene with hexacarbonylchromium has also been characterized 128>). The X-ray crystallographic analysis 129> of the 2-carboxylic acid derivative 24 shows the bridge angle to be 99.6 °, and thus that the Cio perimeter is even flatter than expected from molecular models. Furthermore, the ten perimeter bonds have lengths which vary over the narrow range 1.38 to 1.42 A, the values being typical of benzenoid aromatic bonds. This lack of appreciable... 

Two bridged, ionic 10 n systems directly analogous to the bridged l,6-methano-[10]annulene discussed earlier, have been studied. Thus the cation 61 and anion 62 (see Table 13) bear a similar relationship to 1,6-methano-[10]annulene, 28, as do the tropylium ion and cydopentadienyl anion to benzene. The n.m.r. spectrum of cation 6117>203> is characterized by low-field absorptions of the peripheral protons (t 0.4 to 1.7) and by... 

The cation possesses a more favourable geometry than 1,6-methano-[10]annulene, being able to incorporate the 10 -system into a more planar structure. It is evidently more stable than the tropylium ion, as it can be formed from its neutral hydrocarbon precursor 57 by a hydride-transfer reaction with tropylium tetrafluoroborate 136>. The anion, on the other hand, involves substantial steric strain and is not as planar as either the cation or the neutral bridged [lOJannulene. Whereas formation of the cation from 57 was seen to be most favourable, the anion precursor 58 is much less acidic than cyclopentadiene, although in the presence of... 

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