1,6-Methano annulene, protonation

Oxido[10]annulene closely resembles l,6-methano[lOJannulene in many of its spectral properties, particularly in its proton magnetic resonance, ultraviolet, infrared, and electron spin resonance spectra,1 but is chemically less versatile than the hydrocarbon analog due to its relatively high sensitivity toward proton and Lewis acids. 

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... 

Warner and Winstein284 have obtained the stable monocation 106 by protonating l,6-methano[10]annulene in HSO3F. Cram and Cram285 have been successful in protonating [2.2]-pura-cyclophane. The initial protonated species 107 rearranges to the meta-protonated species 108 [Eq. (3.40)]. 

Lammertsma and Cerfontain411 have obtained the cyclopropyldicarbinyl dication 197 by diprotonating the l,6-methano[10]annulene in much stronger Magic Acid at 60°C [Eq. (3.49)]. If the same protonation was carried out at — 120°C, they were able to obtain the previously discussed monocation 106.284... 

The third returns the reader to chemi-ionization of cyclopropane derivatives with excited state electron acceptors. In the particular, it has been observed that oxidation of l,6-dimethylbicyclo[4.1.0]hept-3-ene (46) and [4.4.l]propella-2,6-diene (47) result in 2,6-dimethylcycloheptatriene (48) and l,6-methano[10]annulene (49) (equations 28 and 29 respectively). The former compound is conceptually the deprotonation product of the dication wherein the C-C bond between the bridgehead carbons has been totally depleted of electrons. In this case, however, the dication is not expected to be as stable as in the above pyramidal or cyclobutadiene cases and indeed, the product is seemingly not formed via the dication route. The methanoannulene likewise does not arise from the tetracation of its precursor. Rather, proton and electron transfer reactions involving merely radical cations proceed to remove hydrogens sequentially. 

This is mainly the work of Philip Warner who also (cf. Ref. 44) appears to have been introduced into the field of propellanes by Saul Winstein in work concerned with the protonation of l,6-methano[10]annulene, work in which they ruled out the presence of a propellane-type cation". ... 

Although the protons of benzene reside in the deshielded portion of the cone, molecules have been constructed to explore the full range of the effect. The methylene protons of methano[10]annulene (3-3) are constrained to positions above the aromatic IOtt electron... 

Consideration of 9,10-ethano-9,10-dihydronapththalene, 23 122,123 a reasonable model for 20, further supports the delocalized nature of l,6-methano-[10]annulene. The AA BB system of olefinic protons in 23... 

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... 

A potentially anti-homoaromatic species (79, see Table 16) has been generated by protonation of l,6-methano-[10]annulene at the 2-position 224>. As expected, 1,3 overlap is not significant and the compound can be formulated as a classical carbonium ion. In fact the abnormally large H(2a) to H(2a) coupling constant of —24.9 Hz indicates that the 1,3 distance has been maximized in order to avoid the expected destabilization, and thus there is no evidence for a paramagnetic ring current in this system. 

The methano-bridged aza[10]annulene 20 [9], a 10 r-analogue of pyridine, is a stable, yellow compound with a quinoline-like odour and a flattened perimeter (UV Amax 364 nm). The NMR data confirm the aromatic character of 20. The perimeter protons combine the features of a-substituted 1,6-methano[10]annulenes and quinoline (see p 318) the upheld shift of the bridge H-atoms (< ch2 = -0.40 / +0.65) is characteristic. It is caused by the shielding effect of the diatropic heteroarene system. The heteroannulene 20 is less basis (pX = 3.20) than pyridine (pX = 5.23) or quinoline pKa = 4.94). 

Strong acid leads to protonation of 1,6-methano[10]annulene at the 2-position [135] 1,6-oxido[10]annulene is converted by acids... 

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