Annulenes, dehydro

It is not possible to generalise about the stability of dehydro--annulenes too many factors come into play. Some, for example,... 

These examples typifjf the usual approach to the synthesis of dehydro-annulenes. Much use has been made of large substituent groups,... 

Hetero[13]annulene, bis-dehydro-dimethylated structure, 7, 716 H etero[ 17]annulenes H NMR, 7, 719, 721 restricted structure, 7, 720 Heteroaromatic compounds reactions... 

Guss, J. M. Mason, R. (1973) Reactions of [14]annulene and dehydro[14]annulene with organotricarbonylchromium complexes Crystal and molecular structures of hexacarbonyl-trans-6a,12a-dihydrooctalenedichromium(0), tricarbonyl-1,4-dihydrophenanthrenechromium(O), and tricarbonylphen-anthrenechromium(O), J. Chem. Soc. Dalton Trans., 1834— 1840. 

Another way of eliminating the hydrogen interferences of [14]annulene is to introduce one or more triple bonds into the system, as in dehydro[14]annulene (87). All five known dehydro[14]annulenes are diatropic. Compound 79 can be nitrated or sulfonated. The extra electrons of the triple bond do not form part of the aromatic system but simply exist as a localized bond. The [18]annulene (88)... 

Annulene and dehydro[22]annulene are also diatropic. A dehydroben-zo[22]annulene has been prepared that has eight C=C units, is planar and possesses a weak induced ring current. In the latter compound there are 13 outer protons at 6.25-8.45 8 and 7 inner protons at 0.70-3.45 5. Some aromatic bridged [22]annu-lenes are also known. The [26]annulene has not yet been prepared, but several dehydro[26]annulenes are aromatic.Furthermore, the dianion of 1,3,7,9,13,15, 19,21-octadehydro[24]annulene is another 26-electron system that is aromatic. Ojima and co-workers prepared bridged dehydro derivatives of [26], [30], and [34]annulenes. All of these are diatropic. The same workers prepared a bridged tetradehydro[38]annulene, which showed no ring current. On the other hand, the dianion of the cyclophane 89 also has 38 perimeter electrons, and this species is diatropic. ... 

The [12]annulene (96) has been prepared. In solution this molecule exhibits rapid conformational mobility (as do many other annulenes), so that above a certain temperature, in this case — 150°C, all protons are magnetically equivalent. However, at — 170°C the mobility is greatly slowed and the three inner protons are found at 85 while the nine outer protons are at 68. Annulene 96 suffers from hydrogen interference and is certainly not planar. It is very unstable and above —50°C rearranges to 97. Several bridged and dehydro[12]annulenes are known. 

The results for [16] annulene are similar. The compound was synthesized in two different ways, both of which gave 103, which in solution is in equilibrium with 104. Above -50°C there is conformational mobility, resulting in the magnetic equivalence of all protons, but at — 130°C the compound is clearly paratropic there are 4 protons at 10.565 and 12 at 5.35 5. In the solid state, where the compound exists entirely as 103, X-ray crystallography shows that the molecules are nonplanar with almost complete bond alternation The single bonds are 1.44-1.47 A and the double bonds are 1.31-1.35 A. A number of dehydro and bridged... 

NMR data for aza[18]annulene 234 show a wide separation between the centers of the inner and outer proton lH NMR multiplets (Ad =11 ppm), indicative of a strong diamagnetic ring current.270 271 The bis-dehydro system 235 with 22 jr-electrons in the aromatic ring is diatropic.272... 

Recently, further examples for dehydro Diels-Alder reactions were published, which are believed to proceed via intermediates of the type 205. To explain the formation of products besides those of the type 206, the authors [132] proposed a remarkable reaction cascade and supported it by quantum-chemical calculations. Accordingly, an isonaphthalene of the type 205 undergoes an electrocyclic ring expansion to give a l,2-dehydro[10]annulene derivative, in which a configurational isomerization occurs followed by an electrocyclic ring closure, yielding a further isonaphthalene of the type 205, and aromatization. 

PLANAR DEHYDRO[8]ANNULENES AND OTHER THEORETICALLY INTERESTING MOLECULES... 

This chapter is a review of my research endeavors in planar dehydro[8]annulenes, which subsequently have been broadened into 1,4-endoxide deoxygenation mediated by low-valent titanium, as well as into polysubstituted furans. This review by... 

Pibenzo[a,c]cyclooctene 35 has been known since 1963. In a continuation of my interest in dehydro[8]annulenes, the conversion of 35 to the bis-alkyne 36 was also attempted. However, it was apparent that the presence of the 1,3-diyne unit, together with the expected peri-H-H interaction, would render the planar 36 highly... 

The electronic spectrum of the fractions containing the pure tri-dehydro[18]annulene exhibits the strongest absorption maximum (in benzene) at 342 nm. (e 155,000)3 and the spectroscopic yield, based on the molar extinction coefficient, is 1.17 g. (2.40% from 1,5-hexadiyne). The yield of tridehydro[18]annulene in the mixed fractions, based on the 342 nm. maximum,is 0.27g.(0.55%). Thetridehydro[18]annulene is best stored in solution in the refrigerator. 

Another way of eliminating the hydrogen interferences of [14]annulene is to introduce one or more triple bonds into the system, as in dehydro[14]annulene (79).202 All five known... 

In the fused compounds (241) and (242) the furan ring fails to react as a diene and Diels-Alder reaction with dienophiles occurs on the terminal carbocyclic rings. However, (243) and (244) afford monoadducts with dimethyl fumarate by addition to the furan rings (70JA972). The rates of reaction (Table 2) of a number of dehydroannuleno[c]furans with maleic anhydride, which yield fully conjugated dehydroannulenes of the exo type (247), have been correlated with the aromaticity or antiaromaticity of the products (76JA6052). It was assumed that the transition state for the reactions resembled products to some extent, and the relative rates therefore are a measure of the resonance energy of the products. The reaction of the open-chain compound (250), which yields the adduct (251), was taken as a model. Hence the dehydro[4 + 2]annulenes from (246) and (249) are stabilized compared to (251), and the dehydro[4 ]annulenes from (245) and (248) are destabilized (Scheme 84). 

The prototropic rearrangement of the cyclic tetramer 152, the pentamer 153 and the hexamer 154 on treatment with potassium r-butoxide gave the tetrakis-dehydro[24]- (157) , the pentakisdehydro[30]- (159) and the hexakisdehydro[36]-(161) annulenes . Catalytic half-reduction of the triple bonds in 157 and 159 gave [24]annulene (158) and [30]annulene (160), respectively , although their configurations were not firmly established. 

Formation of substituted products by electrophilic reactions is one of the characteristics of benzenoid compounds. It has been shown that a few dehydro-[4/j + 2]annulenes afford substituted annulenes under strictly limited reaction conditions. Monodehydro[14]annulene, on treatment at room temperature with copper(ii) nitrate-acetic anhydride, oleum-dioxane and subsequent methylation and acetic anhydride-borontrifluoride etherate, yielded monosubstituted products, 169a, 169b and 169c, respectively. The electrophilic reactions must have resulted in substitution of one of the outer protons, but the exact point of attack has not been determined . 

Acetylene-cumulene dehydro[4 ]annulenes offer a tool for the study of antiaromaticity in view of the high conformational stability and the strong diatropicity of the [4n + 2] counterparts. 

The C chemical shifts of sp-hybridized carbon atoms in the paratropic tris-dehydro[16]- (310), -[20]- (311), and diatropic tetra-/-butylbisdehydro[14]- (26Sa) and -[18]- (282) annulenes are summarized in Table 12==. The bisdehydro[4 -l-2]-annulenes give signals ascribable to sp-hybridized carbons at an intermediate field... 

In order to get information on the effect of annelation of a 6tz ring onto macro-cyclic 4 starting material. The reference compound 337 was prepared by the reaction of 334 with maleic anhydride. It was found that the degree of paratropicity decreased in the sequence 337 > 336 > 335 > 333 ci 331. This is the same order as found for the reduction of diatropicity in the annelated bis-dehydro[14]annulenes, 324, 325, 327, 328 and 329. 

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