Annulenes, formation

Boese, R Matzger, A.J Volhardt, K.P.C. Synthesis, crystal structure, and explosive decomposition of 1,2 5,6 11,12 15,16-tetrabenzo-3,7,9,13,17,19-hexa-dehydro[20]annulene formation of onion- and tube-like closed-shell carbon particles. J. Am. Chem. Soc. 1997, 119, 2052-2053. 

The 5,10-epoxy[10]annulene system readily undergoes addition of bromine with formation of an epoxide across the central bond. By dehydrohalogenation the annulene structure can be restored. For example, the dibromo and tetrabromo adducts of 5,10-epoxy[10]annulene can be converted with potassium rm-butoxide to l-bromo-5,10-epoxy[10]annulene (13) and 1,6-dibromo-5,10-epoxy[10]annulene (14), respectively.159... 

It is interesting that attachment of — ( 2)4— and —CH2CH=CHCH2— to benzene results in nearly the same enthalpy of formation change but it is not obvious how fortuitous this equality is we have reasons for considerable skepticism of its validity68. That formation of naphthalene from benzene is accompanied by a lessened enthalpy of formation increase than that of l,6-methano[10]annulene (yet another name for species 90) from tropilidene would appear to be more of a strain than a resonance derived effect. From Roth, we find the resonance energy increase on going from tropilidene to l,6-methano[10]annulene is 55 kJmol-1 and from benzene to naphthalene the increase is nearly the same, nearly 59 kJmol-1. By contrast, the l,5-methano[10]annulene (99) is less stable by 77 kJmol 1 than the species it appears most naturally to be compared with, namely the isomeric 90. 

On the basis of some judicious measurements and relevant estimations, the enthalpy of formation of gas phase [18]annulene was derived97 to be ca 519 22 kJmol-1. From this we conclude that benzene is ca 15 kJmol-1 more aromatic per carbon than [18]annulene. This value seems too large. 

Is this value of 330 kJmol-1 plausible Were pentalene a normal polyene, we would anticipate an enthalpy of formation of ca 4.52,5 + 5.5 or ca 235 kJmol-1. There is thus ca 100 kJ mol-1 of destabilization. Is this due to antiaromaticity since we recognize pentalene as a derivative of planar [8]annulene We think not, for there are two five-membered rings in pentalene each contributing ca 30 kJmol-1 of strain apiece104. 

We suspect fewer problems would have arisen had Oth and coworkers (see Reference 97) decided to perform enthalpy of hydrogenation measurements on [18]annulene. Nonetheless, we note that Oth s suggested value for the enthalpy of formation of benzo-l,3,5-cyclooctatriene is within 2 kJ mol of that estimated summing Roth s enthalpy of formation of 1,3,5-cyclooctatriene and Liebman s (cited in Reference 68) benzoannelation constant. 

We have summed the enthalpy of formation of five ethylenes for the five formal double bonds and 6(5) for the six formal single bonds by analogy to our discussion of [18]annulene. 

The tetrabenzohexadehydro[20]annulene 13 was found by Vollhardt and coworkers1331 to undergo explosive decomposition under formation of graph-itized carbon particles in low yield aside from... 

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. 

Consequently, the reduction of [4n]annu-lenes should be observed at relatively positive potentials with small AE separations for the dianion formation, while the reduction of the [4n + 2]annulenes should occur at more negative potentials with large AE separations for the dianion formation. This is exactly what is observed in (see Table 1). Although benzene [37, 38], the classical Hiickel aromatic with 4 1 + 2 = 6 jr-electrons, is reduced at —3.42 V (vs. Ag/AgCl), the reduction of the [Sjannulene COT occurs at —1.81V. Similarly, the [16]annulene (3) [67, 68] is more easily reduced than the corresponding [ISjannulene (4) [69], although the reduction of the larger jr-systems should be more favorable for electrostatic reasons. 

There have been several developments in this area since this manuscript was prepared. The heat of combustion of corannulene was determined by microbomb combustion calorimetry and its gas-phase enthalpy of formation was estimated at 110.8 kcal/mol. All anionic oxidation states of corannulene were observed by optical absorption, EPR, and NMR spectroscopies. More support for the an-nulene-within-annulene model of the corannulene tetraanion was presented. An alternative pyrolysis route to corannulene was reported, as well as some attempts toward the synthesis of bowl-shaped subunits of fullerenes. And in contrast with previous semiempirical studies," ab initio calculations predicted a general concave preference for the metal cation binding to semibuckminsterfullerene 2%. ... 

Maleic anhydride cycloaddition to furanothia[ll]annulene (28) led to the formation of the triply annulated parent (29) (see p. 715). 

The synthesis of diazapyraceheptylene derivatives (312) is depicted in Scheme 48. Attempts to synthesize the parent compound resulted in the formation of 1,4 8,11-bisimino[14]annulene (315a) (Scheme 49). 

A logical extension of the condensation reactions which yield multidentate nitrogen donors is the formation of macrocyclic ligands. The preference of palladium for square planar coordination makes it an ideal metal for the formation of complexes of these ligands. Thus palladium porphyrins are very stable and resistant to demetallation.281 Both 14- and 16-atom macrocycles have been used to form complexes. The ligand l,8-dihydro-5,7,12,14-tetramethyl-dibenzo[b,i][l,4,8,ll]tetraaza[14]annulene reacts with [PdCl2(PhCN)2] to yield the complex (46). 

The field of metallabenzenes is under active investigation, with numerous reports being published each year. A brief listing of references that have not been cited above is given according to the metal ruthenabenzenes [300-302], platina-benzenes [303,304], iridabenzenes [305], osmabenzenes [306], Haley and coworkers observed that in some instances the formation of metallabenzenes from vinylcyclo-propenes competes with valence-isomeric benzvalenic structures [307, 308] (for a systematic survey of valence isomers of annulenes an earlier three-volume book may be consulted [309]). 

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