Tetra cyclopentadienone

This double Claisen-Schmidt reaction takes place under the influence of ethano-lic potassium hydroxide (Expt 7.7) and presumably proceeds in the stepwise manner (cf. formulation in Section 6.12.2, p. 1032). The four aryl groups in tetra-cyclopentadienone effectively stabilise the cyclopentadienone system, which otherwise has only a transient existence and readily undergoes dimerisation by way of a diene-dienophile interaction (Diels-Alder reaction, Section 7.6). The use of tetracyclone as a dienophile for the preparation of 3,4,5,6-tetraphenyl-dihydrophthalic anhydride is noted on p. 1121. 

J 7i-Cycloadducts at the C4 — C5 azepine positions are also formed with 1,2,3,4-tetra-chloro-5,5-dimethoxycyclopentadiene260 261 and with hexachlorocyclopentadiene.261 The reactivity of ethyl l//-azepine-l-carboxylate towards cyclopentadienones has been studied in terms of frontier molecular orbital theory which predicts that dimethyl 2-oxo-4,5-diphenyl-cyclopenta-1 (5),3-diene-1,3-dicarboxylate (20) should be more reactive towards the 1/f-azepine than other more common cyclopcntadienone derivatives.262 In fact, in refluxing benzene, the cyclopentadienone and ethyl 1/f-azepine-l-carboxylate (1) form a mixture of the [4 + 2] n-endo,anti-adduct 22, produced by Cope rearrangement of the initially formed [2 + 4] 7T-adduct 21, and the c.w-adduct 23, a rare example of a [6 + 4] rc-cycloadduct. At room temperature, only the [6 + 4] adduct 23 and a small amount of the [2 + 4] adduct 21 are obtained, the latter rearranging to the [4 + 2] adduct 22 on warming.262 Other [4 + 2] 7r-adducts with cyclopentadienones have been prepared similarly.263... 

Scheme 7). The two-fold Diels-Alder cycloaddition of an excess of tetra-phenylcyclopentadienone (13), which is regarded as a first-generation dendron, to 24 in refluxing o-xylene leads to the second-generation benzilic dendron 25. The dendron is isolated as a pale yellow amorphous powder in 91% isolated yield. The Knoevenagel condensation of 25 with 1,3-diphenylacetone (26) to the corresponding cyclopentadienone dendron 27 is achieved in dioxane and in the presence of tetrabutylammonium hydroxide as a base. Like 25, the cyclopentadi-... 

Such cycloadditions are dependent on the interactions of the azepine HOMO and the diene LUMO. Theoretical consideration of these orbitals reveals that bonding overlap is favourable for C-6—C-7 and C-4—C-5 additions and that, on the basis of secondary orbital interactions, the endo product is favored. Experimentally, however, it is found that additions are periselective and C-4—C-5 addition predominates in the cycloaddition of 1//-azepines with cyclopentadienones, isobenzofurans, tetra- and hexa-chlorocyclopentadienes, 1,2,4,5-tetrazines, a-pyrones and 3,4-diazacyclopentadienones (8lH(15)1569). 

Stable, isolable metallacycles are also obtained from reaction of complexes that serve as sources of the CpCo fragment (e.g. CpCo(PPh3)2) and alkynes. Upon carbonylation diese typically give high yields of cobalt-complexed cyclopentadienones. Direct reaction of CpCo(CO)2 with alkynes is similarly useful. The cycloaddition of di(t-butoxy)acetylene upon photolysis with CpCo(CO)2 is an example (Scheme 5). In all these systems the final complexes lack coordinated CO, and therefore amine oxides are not suitable reagents for liberating the stable cyclopentadienones. Tetra(t-butoxy)cyclopentadienone is accessible on a preparative scale via controlled electrochemical oxidation. Other oxidants such as Cr have been used as well in other systems. 

Matrix irradiation (240nm) of tricyclopentanone (111) yields cyclobutadiene, CO, cyclopentadienone, and acetylene.No evidence for the formation of tetrahedrane (112a) was obtained, in contrast with the previously reported behaviour of the tetra-t-butyl derivative of (111), which gave the tetrahedrane... 

Recently, Mullen et al. have reported a convergent approach134 to poly(phenylene) dendrimers (Scheme 5) similar to the divergent [4 + 2] cycloaddition route they had previously developed.135 The process was initiated by a Diels—Alder reaction between tetra-substituted cyclopentadienone 61 and dialkynyl mono-... 

A number of symmetrically substituted cyclopentadienone metal complexes have been prepared by metalcarbonyl-mediated dimerization of alkynes [23a, 24]. The (tetracyclopropylcyclopentadienone)tricarbonyliron complex 27 can easily be obtained as the major product by ironcarbonyl-mediated dimerization with CO insertion of dicyclopropylacetylene [25]. Upon treatment of the complex 27 with triethylamine N-oxide, the uncomplexed tetracyclopropylcy-clopentadienone 28 apparently is liberated however, in contrast to the kineti-cally sufficiently stabilized tetra-ferf-butylcyclopentadienone 19 (see Scheme 5)... 

Benzyne cycloadditions have been useful in synthesizing sterically crowded naphthalenes . Recent examples include 1,2,4-tri- -butylnaphthalene 207 from cyclopentadienone 206 Tetra- -butylcyclopentadienone, however, turned out to be inert toward benzyne. The synthesis of 210 could be accomplished by adding benzyne to tetra-r-... 

This outline omits details, sometimes at first fmstrating and often baffling, of some reactions, particularly the introduction of the fourth t-butyl group into the cyclopentadienone, and the photochemical isomerization of tetra-/-butylcyclopent-... 

The reactions of alkynes with transition metal complexes often lead to bewildering mixtures of products. Substituted alkynes often produce rj -cyclobutadiene derivatives (p. 269) metal carbonyls also afford products containing cyclopentadienone and quinone ligands, which incorporate two alkyne units together with one or two carbonyl groups. Some of these systems catalyse the cyclotri- or tetra-merization of alkynes to benzenes or cyclooctatetraenes respectively (Fig. 7.12). 

Scheme 6.78. A representation of the low-temperature (Uquid nitrogen) photochemical conversion of 2,3,4,5-tetra(f-butyl)cyclopentadienone to l,2,3,4-tetra(i-butyl)cyclobntadiene.

Acetylene scavenger cyclopentadienone, tetra-phenyl- as - 31, 985s32 1,2,4,5-tetrazine, 3,6-di-(2-pyridyl)- as - 31,985 Acetylenestannanes... 

Decarboxylation.—The most important synthetic applications of photochemical decarboxylation reactions have been to the low-temperature formation of ben-zyne and cyclobutadiene from appropriate precursors.Interesting recent results in this area concern a decarbonylation reaction to form the elusive tetrahedrane skeleton. Irradiation of the cyclopentadienone (184) at low temperature gave the crystalline (m.p. 135 C) and air-stable tetra-t-butyltetra-hedrane (186), presumably by decarbonylation of the intermediate (185) (Scheme 52). ... 

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