Methano bridges

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... 

Coupling of diazonium betaines with 3-tert-butoxy-l,6-methano[10]an-nulene 275 under mild conditions led to elimination of tert-butyl alcohol with the formation of methano-bridged triazines 276 (88CB1359). 

Because of the better jt-type overlapping of the carbonyl jt orbital with the o bonds of the ethano bridge as compared with that of the methano bridge in 27 (i.e., 9 (dihedral angle, or ZCfff. ) < 9 (dihedral angle or... 

Isaacs, L., Wehrsig, A., and Diederich, F. (1993) Improved purification of C60 and formation of S- and 7i-homoaromatic methano-bridged fullerenes by reaction with alkyl diazoacetates. Helv. Chim. Acta 76, 1231-1250. 

The synthesis of the macrocycles 43 (Scheme 9) is an example of repetitive, highly stereoselective Diels-Alder reaction between bis-dienes 41 and bis-dienophiles 42, containing all oxo or methano bridges syn to one another. The consecutive inter- and intramolecular Diels-Alder reactions only succeed at high pressure. Obviously, both reactions are accelerated by pressure. The macrocycles are of interest in supramolecular chemistry (host-guest chemistry) because of their well-defined cavities with different sizes depending on the arene spacer-units. 

If the oxo (or methano) bridges are not exclusively syn to one another in either the bis-dienophiles or bis-dienes, then the pressure-induced repetitive Diels-Alder reactions (proceeding again highly stereoselectively) produce rigid ribbon-type oligomers on a nanometer scale (Scheme 10 entry 1). Bis-diene 45 reacts less stereoselectively than bis-diene 44 and forms with bis-dienophiles such as 46 the ribbon-type oligomers 47... 

In the reaction of fused aziridines with alkene dipolarophiles, the opportunity for stereoselectivity as well as facial selectivity arises since exo- or entfo-isomers can be formed (Scheme 10). In practice, maleic anhydride 6, A-methyl maleimide and JV-phenyl maleimide each reacted exo-stereoselectively with TV-benzyl aziridine 69 to form adducts of type 71 (Scheme 10b), the stereochemistries of which were confirmed by NOE measurement between Hb and He. Similar reaction of the Y-phenyl aziridine 67 with N-Ph maleimide gave a 1 1 mixture of endo-adduct 72 and exo-adduct 73 (Scheme 10c). Adducts 68, 71-73 all exhibited a low-field methano-bridge proton (Ha) in the range 5 3.06-3.60 confirming the syn-facial stereochemistry of the two bridges. 

Reaction of norbomadiene 74 (in excess) with 7V-benzyl aziridine 67 formed exclusively the all-sy l l-adduct 77. This stereochemistry, confirmed by NOE between Ha and Hb, resulted from attack at the underface of the dipole by the exo-face of the dipolarophile. Similarly, reaction of A -benzyl aziridine 67 with the diacetoxybenzonorbomadiene 30 gave a single adduct 78 (Scheme 11), the symmetrical structure of which was clearly apparent in the ll NMR spectrum. These stereochemical outcomes demonstrated that the transition state (TSa), in which the methano-bridge was adjacent to the (V-substituent, was favoured in the A -benzyl series (X and R small), and in accord with the semiempirical calculations. 

Whereas A -benzyl aziridine 69 reacted with the methano-bridged dipolarophile 39 to give exclusively TSA-product 79, use of the bulkier isopropylidene bridged dipolarophile 38 afforded substantial amounts of the TSB-adduct 84 as well as some of the TSA-product 80, an outcome attributed to unfavourable X)(NR TSa interaction in the latter reaction (X large, R small) (Scheme 12a). 

These isomers resulted from the non-stereoselectivity of the initial coupling process typical of the aza-ACE reactions of the 7-isopropylidene-bridged dipolarophile 38, while molecular weight measurements and the presence of an isopropenyl group in the H NMR of each product supported C,A-methano-bridge formation. Such products were considered to arise via the bond reorganisation depicted by the arrows in adduct 156 in which one of the isopropylidene rc-bonds acted as the nucleophile to attack the methylene carbon of the adjacent A-methoxymethyl group. 

Shortly after this prediction, Schroder (1963) isolated bullvalene. Numerous studies amply demonstrated the facile Cope rearrangement of [84] and its derivatives (see for example Schroder and Oth, 1967 Doering et al., 1967). Theory and experiment agree that, by pinching the methano bridges closer together, the rate of the Cope process increases in the sequence semibullvalene [83] > barbaralane [85] > bullvalene [84] (Dewar and Schoeller, 1971 Anastassiou et al., 1975). 

An interesting synthetic approach (Scheme 46) starts with 3,8-methano-bridged (lO)-annulenones 103, which react with A-vinyliminophosphoranes 104. After Michael-type addition to C-2 and C-11 of the (lO)-annulenone,... 

It was shown that the methano-bridge exerts a strong directive effect that diminishes as the bridge moves from the more central inner positions to more peripheral outer positions. Charge alternation paths in the resulting carbocations were discussed based on the magnitude of A5 C values. It was apparent that both... 

Figure 4.6 Chemical shifts for the bridgehead C atoms and the methylene H-atoms and coupling constants j(CH) for the methano bridge C atoms in H2 for isomers 113a and 113b. The corresponding C atoms resonate in the region between 130 and 150 ppm together with all other sp -carbons of the fullerene sphere. For the 1,6-addition adduct of with (p-methoxy-phenyl)diazomethane, the peak position of the bridgehead C atoms was found by HETCOR analysis to be 138.65 ppm [110. ...

The electronic absorption spectra of Cgo and [5,6] and [6,6] methano-bridged cycloadducts (monoadducts) exhibit almost the same features in the UV region, consisting of three dominant bands at 325, 259 and 219 nm [103, 106, 108, 126,... 

An enantiopure dimer 149 with a l,l -binaphthyl-bridge was prepared via the bis-tosylhydrazone (see Table 4.4, page 130/131) [122], The electronic properties of these dimers, such as the electronic absorphon spectra and cyclic voltammetry, are indistinguishable from those of other methano-bridged fullerenes. CV-data show clearly that the two CgQ-imits of the binaphthyl-dimer are reduced independently [122],... 

The synthesis of a diphenylmethano-bridged fuUerene derivahve with reactive functional groups on the phenyl rings is exemplified by the preparahon of the diphenol derivative 118 (Scheme 4.23). It can be obtained from the corresponding methyl ether by treatment with BBrj in o-dichlorobenzene at 0 °C to room temperature in 94% yield. In contrast to the non-polar diphenyl-methano bridged fullerenes, 118 is soluble and stable in pyridine but sparingly soluble in benzene or toluene. 

Thermal extrusion of N2 from O-benzyl- and O-pivaloyl-protected diazirine yields the corresponding carbenes, which react with in toluene to give the fullerene sugars 333 (Scheme 4.67) [377, 378]. These [2+1] carbene additions cleanly lead to 1,2-methano-bridged sugar monoadducts. 

Each macrocycHzahon could, in principle, lead to a mixture of diastereoisomers, depending on how the EtOOC residues at the two methano bridge C atoms are oriented with respect to each other (in-in, in-out, and out-out stereoisomerism) [92]. Usually, only out-out stereoisomerism has been observed so far. One exception is the in-out isomer 81. 

The structure (88), indicated by spectroscopic methods was confimed for (88 a) by X-ray analysis. Bridging of terminal rings by an internal Friedel-Crafts reaction was also reported by Martin 6). Acid treatment of 1-hydroxymethylpentahelicene (89) resulted in the formation of (90). Wijnberg162) reported the formation of methano-bridged heterohelicenes by treatment of the 1-methyl- and 1-ethylheterohelicene (91) with N-bromosuccinimide (NBS)... 

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