Annulenes, bridged structure

Ando and coworkers23 reacted C(,o with bis(2,6-diisopropylphenyl)silylene generated in situ by the photolysis of the corresponding trisilane. The dark brown powder obtained (58%) exhibits 29Si signal at 8 — 72.74 which, together with other experimental evidence, supports the silirane structure (8a) rather than the bridged annulene structure (8b). 

In this case the bridged annulene structure causes extreme steric hindrance to peripheral conjugation and the ring is also deformed. Hence there is insufficient delocalisation energy to favour the bridged annulene structure, and this, together with steric factors, leads to (XXXII) being the preferred structure. 

Bromine can also undergo addition to the nonaromatic double bond of dibenz[ft,/]oxe-pin.160,161 Oxygen-bridged annulenes that incorporate an oxepin structure have been shown to undergo 1,4-addition of bromine at — 78°C.159... 

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... 

Dienic, trienic, and tetraenic propellanes (tricyclic compounds with a common C —C single bond) are relatively novel compounds that have been used as models for stereochemical studies. Propellanes with fused cyclohexadienc and three- or four-membered rings are in equilibrium with annulenes which have a bridged bicyclic structure. The stability of the annulene ring increases with increasing size2. 

The H NMR spectrum of 7.80 was also recorded by Elix. While it was of insufficient quality to allow an extract assignment of structure vide supra), it did allow Elix to conclude that, as expected given its 4n n-electron formulation, compound 7.80 is not aromatic as judged by the absence of any substantial macrocyclic ring-current effects. This conclusion reached, it should be noted, that the formally 4n + 2 pentaoxa[30]pentaphyrin-(2.2.2.2.2) isomers also failed to display any kind of substantial ring-current effects. This led Elix to conclude that there is no fundamental difference in electronic character between large 4n and 4n + 2 7r-electron oxido-bridged annulenes. ... 

Non-benzoid phanes have always played an important role in cyclophane chemistry and many structures have been prepared with other Hiickel aromatic rings such as azulene or tropone [49]. A cyclophane with two bridged annulene units was recently synthesized by Mitchell [50,51 ]. Dimethyldihydropyrene (76), an excellent NMR probe first introduced by Boekelheide [52], was converted into the dialcohol 77 in three steps. Reaction with adipoyl chloride afforded the large [10.10]cyclophane 78. Unfortunately the conversion of the dialcohol into the corresponding dibromide 79, an obvious precursor to the interesting phane 80, has failed so far. 

Balaban, A.T. 1973. Chemical graphs. XVIII. Graphs of degrees four or less, isomers of annulenes, and nomenclature of bridged polycyclic structures. Rev. Roum. Chim. 18 635-653. 

Other compounds which might be mentioned, because they illustrate applications of the method to distinctly non-planar systems, are the bridged annulenes bicyclo[4,4,l] undecapentaene f 12] and the isomeric[5,3,l] compound [13]. The x-ray structure is known for [12] and the calculated values are in quite good... 

The molecular structure of oxy-bridged annulenes has been studied using a range of semiempirical methods including HF/STO-3G, HF/4-31G, MP2/STO-3G, MNDO, and correlation corrected MNDO <86JA4768>. The barrier to inversion, at the ether oxygen, of l,6-methano-8,13-oxido[14]-annulene (11) was predicted to be 27 kcal mol . ... 

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... 

Dieckmann cyclization thiophenes from, 4, 874-875 Thiaannulenes structure, 7, 715 twice-bridged structure, 7, 718 Thiaannulenes, didehydro-structure, 7, 718 Thia[17]annulenes restricted structure, 7, 720... 

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... 

Another type of face-to-face-arrangement is found in the multi-layered annulene systems [ 14] and [15], Here, unlike the para-cyclophanes, in which the phenyl units are connected by external alkanediyl groups, the stacking of annulene layers is achieved by internal linkages, with the bridge located inside the 7t-clouds. The advantage of the latter structure is that one can... 

Hydrocarbons containing one or more triple bonds in addition to double bonds have been excluded from the tile, as have been radicals (e.g. the allyl radical C3H5 ) and aromatic molecules, i.e. molecules for which more than one unexcited resonance structure (Kekule structure) can be written. Consequently, hydrocarbons such as phenyl-substituted polyenes, or annulenes — bridged or unbridged—have not been included. 

The PMR spectra of substituted derivatives, however, provide additional evidence for the structure of bridged [12]annulenes. A comparison of 92a and 92b reveals a normal control of the cyano group over the chemical shift of the neighboring protons (A = 1.0 ppm), the influence on the remote pyrrolic protons being small (A = 0.11-0.39 ppm). Hence diazapyracylenes should be composed of loosely connected closed-shell moieties (formula 97a) the influence of the substituents on the chemical shift being limited to the substituted part of the molecule.112... 

Three structures may be proposed for diazapyracylene an A/,N-bipyrryl core with two vinyl bridges (307a), a cyclododecahexaene dianion with an internal cross-linked N=N unit (307b) and a peripheral [12]annulene loosely linked to an internal hydrazine unit (307c). Structure (307c) does not correlate well with the isoelectronic pyracylene dianion (308). 

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