Naphthalenophanes

Furano( 1,4)naphthalenophane conformation, 4, 538 Furanonaphthalenophanes molecular structure, 4, 5 Furan-2-one, tetrahydro-reactions, 4, 656... 

The inherent plane of chirality in the metal carbene-modified cyclophane 45 was also tested in the benzannulation reaction as a source for stereoselectivity [48]. The racemic pentacarbonyl(4-[2.2]metacyclophanyl(methoxy)carbene)-chromium 45 reacts with 3,3-dimethyl-1-butyne to give a single diastereomer of naphthalenophane complex 46 in 50% yield the sterically less demanding 3-hexyne affords a 2 1 mixture of two diastereomers (Scheme 30). These moderate diastereomeric ratios indicate that [2.2]metacyclophanes do not serve as efficient chiral tools in the benzannulation reaction. 

EM-values for the formation of cyclophane and naphthalenophane diethers by intramolecular Williamson reaction (51a) in 99% Me2SO at 25°C ... 

The aryl groups of the styryl systems need not be unsubstituted, as has been illustrated before for the cyclizations encountered in the synthesis of naphthalenophanes from 120. Indeed cyclization to afford a cyclobutane derivative where methoxy groups are on the adjacent ring position to the vinyl moieties has also been studied. The irradiation of 138 affords the m-cyclophanes 139 and 14065. Further study has sought to evaluate the steric effect of o-methoxy groups in such molecules66. 

Characteristic shifts due to paramagnetic ring currents are also met with in the singly annellated [2]paracyclo[2](l,4)naphthalenophane (35) 16>-... 

The iH—NMR spectra of the recently prepared [2.2](l,4)anthraceno-phanes 36 63> resemble those of the isomeric naphthalenophanes 34. In the anti conformer A, the Ha proton, which lies directly above an anthracene ring, absorbs at much higher field strength than the syn Ha proton. 

The iH—NMR spectrum of [2.2](l,6)naphthalenophane (46) is not consistent with a symmetrical structure 46 b 71>. According to molecular models, the CH2—CH2 bridges in 46 a are not perpendicular but inclined toward the naphthalene planes and skewed with respect to each other. Rotation of the naphthalene nuclei about the axis through the bridgehead atoms can be ruled out. 

Wasserman ef f.62>93> have shown that syn-[2.2](l,4)naphthalenophane (34B) is converted into the anti form in 70% yield when irradiated with light of wavelength 3500 A in benzene for 10 days this reaction also proceeds thermally at 250 °C. Photolysis of the pure anti isomer... 

Wasserman and Keehn ") have also carried out the photosensitized auto-oxidation of anti-[2.2](l,4)naphthalenophane (34A). Irradiation of anti-34 in methanol and simultaneous reaction with singlet oxygen affords the oxidation product 127 in 20% yield. The primary step in the reaction is assumed to be formation of a peroxide (128) whose geometry permits an intra-annular Diels—Alder reaction as second step methanol-ysis then leads to 127 which was isolated. 

Strong additional support for the assignment of the metapara-cyclophane structure 129 was obtained from dynamic NMR studies The temperature dependence of the proton resonance of this compound is analogous to that of the parent [2.2]metaparacyclophane 3>. The multiplets observed for the protons of the p-phenyl nucleus gradually broaden with increasing temperature, disappear completely at 150 °C, and reappear at 180 °C as a midway peak. The isomer of 129, the 13-methoxy[2]paracyclo[2](l,4)naphthalenophane (132), formation of which by the mechanism outlined above seems equally feasible, does not appear to occur. 

A variety of chiral [m.n]cyclophanes has been described, including [2.2](2,6> naphthalenophane (27) 49-50> and the corresponding diene 49), or [2.2](2,5)pyridino-phanes (28)51). In both cases (27, 28) achiral and chiral isomers (a, b) were isolated and their structures assigned mainly by H-nmr spectroscopy. The chiral structure... 

The naphthalenophanes that have been synthesized to date are listed in Table 6, in order of their discovery. The [m.n] isomers for which m,n > 3 have not yet been synthesized. References for the UV absorbance, fluorescence, and other properties of existing naphthalenophanes are given in Table 6. The UV absorption spectra of all the naphthalenophanes are red-shifted and broadened relative to their respective open-chain analogs, similar to the [2.2] and [3.3] paracyclophanes. Moreover, broad and structureless emissions have been observed for the naphthalenophanes in all references cited in Table 6 except one.107) The structural aspects of naphthalenophane photobehavior will be discussed in detail in the following paragraphs. 

The naphthalenophanes that are fully eclipsed, i.e. the sj>n-[2.2](l,4), achiral [2.2](1,5), achiral [3.3](2,6),. n -[3.3](l,4), and syn-[2>,2] A) isomers, share certain traits in absorption and fluorescence. The UV absorbance spectra of these compounds between 260 and 310 nm retain all of the structure shown in the spectra of the open-chain analogs. Also, new absorption shoulders not seen in the open-chain spectra appear strongly at 245 and weakly at 340 nm. The fluorescence peak of these fully eclipsed naphthalenophanes occurs near 22,000 cm-1, as seen in Table 7. This represents a red shift of 2600 cm-1 relative to the solution excimer of the dimethylnaphtha-lenes.71)... 

The remaining naphthalenophanes in Table 6, which are noneclipsed, show little vibrational structure in their UV absorption spectra relative to the open-chain analogs. 

New absorption shoulders appear strongly at 250 and 345 nm. Given this evidence of ground-state interaction, the fluorescence band of the noneclipsed naphthalenophanes should be red-shifted below the peak emission of the dimethylnaphthalene solution excimer. In fact, the emission of chiral [2.2](2,6) naphthalenophane is blue-shifted 900 cm 1, and the emissions of the onh -[2.2](l,4), onfi-[3.3](l,4), and chiral [2.2](1,5)... 

The (1,4) substituted naphthalenophanes undergo [4 + 4] photocycloaddition when irradiated at X > 280 nm, in addition to fluorescence. This photoreaction is competitive with fluorescence, and requires a conformational change that can be suppressed at low temperature 93). The few reports of the lifetime or quantum yield of naphthalenophane fluorescence indicate the effects of photocycloaddition. For the anti-[2.2](1,4) isomer, kpu/ku = 0.021 in cyclohexane 93) the lifetime of syn-[3.3](l,4) naphthalenophane fluorescence was given as 15.3 ns107). Both values are low relative to the naphthalene solution excimer (kpu/kjj 0.2 xD 80 ns 71)), and this may be due in part to the photoreaction of the (1,4) naphthalenophanes. 

In conclusion, there is much to be done in characterizing the photophysics of naphthalenophanes. The fact that the eclipsed isomers emit at lower energies relative to the noneclipsed isomers is in accord with the assignment of the parallel-plane sandwich structure to the naphthalene solution excimer. 

It is clear that the sandwich-dimer studies discussed above apply to P1VN, not P2VN, since no photodimerization has been observed in bis(2-naphthyl)alkanes 10 and ethers 39). Nevertheless, the UV absorbance of naphthyl sandwich dimers, like that expected for [3.4] or [3.5] naphthalenophanes, differs from that of isolated molecules only for X > 325 nm. The same slight difference in UV absorbance probably occurs for excimer-forming sites. 

As noted earlier, the limiting lifetime of pyrene excimer fluorescence from concentrated solutions in PS and PMMA glasses was found to be the same as that of pyrene in cyclohexane solution. There have been no similar studies of naphthyl compounds in rigid glasses. Values of k and Q for the [2,6]-naphthalenophanes have not yet been determined for any solvent system. The bis(2-naphthyl) compounds have not been quantitatively characterized in rigid matrices, probably because excimer fluorescence is weak and difficult to detect under such conditions. Given such limited data, it can only be assumed that the values of QD and kD of 2-naphthyl excimers remain the same in rigid solution as in fluid solution. 

Liang and Mislow s idea of contracting naphthalenes can be formalized if we consider the molecular cell complex rather than the molecular graph of triple layered naphthalenophane. We obtain a cell complex G by replacing each naphthalene by a pair of cells. We shall now prove by contradiction that this mo-... 

Figure 29. The cell complex of triple-layered naphthalenophane together with the arcs ax, a2, and a3.

T. Otsubo, F. Ogura, S. Misumi, Triple-layered [2.2]naphthalenophane. Tetrahedron Lett. 1983, 24, 4851-4854. 

E. Flapan, B. Forcum, Intrinsic chirality of triple-layered naphthalenophane and related graphs. Journal of Mathematical Chemistry 1998, 24, 379-388. 

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