Tropones fulvenes

Kinetic analysis shows that the formation of tropone through a hydroxyphenyl-carbene intermediate (which exhibits the lowest activation energy 69.3 kcal/mol) dominates o-QM decomposition process up to 1500 K, with fulvene + CO formation becoming competitive at higher temperatures. In fact, the latter decomposition mode although disfavored by its higher activation enthalpy (75.4 versus 69.3 kcal/mol) becomes competitive due to its more positive activation entropy. 

Due to the high conformational demands which are imposed on higher-order cycloadditions, fulvenes, heptafulvenes and tropones have been mostly applied in uncatalyzed [6 + 4] cycloadditions. The scope of metal-promoted cycloadditions, however, is much broader due to the preorganized orientation of the reactants which are both co-ordinated to the metal center. 

Kato et al. (119) explored reactions of fulvenes with a variety of mesoionic heterocycles. Unfortunately, reactions of miinchnone 38 with several fulvenes afforded complex mixtures in each case, and no identifiable products were reported, although Friedrichsen and co-workers (120-122) previously reported the reaction between mtinchnones and fulvenes to give cycloadducts. Kato et al. (123) also studied the cycloaddition reactions of tropone with several mesoionic heterocycles. Despite heroic efforts, the reaction of tropone with miinchnone 38 was complex and could not be unraveled. However, as described later, the reaction of tropone with isomtlnchnones was successful. Wu et al. (124) effected the cycloaddition between a miinchnone and fullerene-60 (Ceo) to give the corresponding dihydropyrrole in excellent yield. 

Kato et al. observed that mesoionic 339 reacts with fulvenes, (119,151,152) benzocyclobutadiene (114), tropone (123), and triphenylphosphirene (112) to give cycloadducts 342-345, respectively. Only a sampling of products is given as these reactions are quite complex. For example, 2-ferf-butyl-6,6-dimethylfulvene gives a product type different from 342 (151,152). 

We have described the cycloadditions of a variety of dienes, ranging from cyclo-pentadiene to cyclopentadienones with alkyl and aryl fulvenes80-82. In these cases, only the [4 + 2] cycloadducts across the 2 and 3 positions are observed. Similarly, 1,3-dipoles such as nitrones and nitrile oxides add in this fashion, as well. We discovered the first authentic [6 + 4] cycloaddition of a fulvene in 197083. The cycloadditions of tropone to fulvenes, which we originally suggested involved [6-fulvene + 4-tropone] cycloadditions, now appear to be [6-tropone + 4-fulvene] cycloadditions8... 

Cycloadditions between substituted fulvenes and 1,3-dipolar reagents have been the subject of considerably less study than the corresponding diene-mediated chemistry, paralleling the situation seen in the tropone series. For the most part, however, the reactivity patterns formulated in the fulvene-diene studies are repeated in the analogous 1,3-dipole reactions. 

The steric consequences of substitutents at various locations on the tropone addend have also been described for this reaction. Substituents at the 2-position tqiparently provided sufficient steric hindrance to the approach of the fulvene addend that cycloaddition proceeded so as to completely avoid this center. Thus 2-chlcycloaddition products in which die chlorine substituent was always situated on the three-carbon bridge remote from the fulvene moiety in the initial [6 -i- 4] cycloadducts. Furthermore, 2,7-dichlorotropQne failed to cycloadd to substituted fulvenes under any conditions. From these results it is clear diat steric hindrance plays a crucial role in determining the course of fiilvene-tropone... 

Two other bisepoxides derived from fulvene (84) (Scheme 17) <87TL3779> and tropone (86) (Scheme 18) <83TL3l i> have been prepared from the respective endoperoxide precursors with CoTPP under mild conditions without any by-products. 

Thiazolo[3,4-2l] benzimidazole [-Cg/. 1,3-Diphenylthiazolo [3,4-fl] benzimidazole (245), containing quadrivalent S, reacts as a thiocarbonyl ylide dipole with fulvene and tropone systems. 

Small amounts of an analogous [4 + 2]adduct are also formed from tropone itself, but its mode of formation is not certain and may be indirect [211]. Tropone also reacts as a 4tt component in reactions with simple fulvenes [212], but as a 6it component with an isobenzo-fulvene [213]. 

The intramolecular 8 -I- 2-cycloaddition of heptafulvene-fulvene (199) produced the intermediate adduct (200) which converted into the product (201) by 1,5-sigmatropic shifts in 46% yield (Scheme 78). 2-Oxo-27/-cyclohepta[ ]furan derivatives (202) undergo 8 -I- 2-cycloaddition with dienes to yield cycloadducts (203) in high yields (Scheme 79) ° Ab initio calculations on the cycloaddition of tropone with maleic anhydride favour the 4 - - 2-endo pathway while calculations of tropothione with maleic anhydride favour the 8 -I- 2-endo pathway. ... 

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