Cyclopentadienone

CO. Alkynes will react with carbon monoxide in the presence of a metal carbonyl (e.g. Ni(CO)4) and water to give prop>enoic acids (R-CH = CH-C02H), with alcohols (R OH) to give propenoic esters, RCH CHC02R and with amines (R NH2) to give propenoic amides RCHrCHCONHR. Using alternative catalysts, e.g. Fe(CO)5, alkynes and carbon monoxide will produce cyclopentadienones or hydroquinols. A commercially important variation of this reaction is hydroformyiation (the 0x0 reaction ). 

Use the same method to calculate the dipole moment of cyclopentadienone,. Assume, for the calculation, that the endocyclic double bonds are parallel and the angle at carbon 2 is the same as in eyelopropenone. 

Group IV substituted, 1, 614-620 Cyclopentadiene-spiropyrazolenine photolysis, 5, 251 Cyclopentadienone, 2,3-epoxy-photochromic compound, 1, 385 Cyclopentadienone, tetraphenyl-diepoxide, 7, 191... 

When benzyne is generated in the presence of potential dienes, additions at the highly strained triple bond occur. Among the types of compounds that give Diels-Alder addition products are furans, cyclopentadienones, and anthracene. 

There are four possible pyrazol-3-(Mie structures I-IV. The nomenclature most frequently used in the literature has been taken from Chemical Abstracts, wlwre for example, structure I is named 2,3-dihydropyrazol-3(lW)-(Mie and structure II is named 4,5-dihydropyrazol-5(l//)-one. Compounds I and II have also been named as pyrazolinones. Compounds with structure IV have been referred to as 2,3-diaza-2,4-cyclopentadienones. To avoid this needless confusion, the nomenclature used throughout this review is in accord with the recommendations set forth by lUPAC. 

Cycloheptatrienone is stable, but cyclopentadienone is so reactive that it can t be isolated. Explain, taking the polarity of the carbonyl group into account. 

Interestingly, in the inverse-electron-demand Diels-Alder reactions of oxepin with various enophiles such as cyclopentadienones and tetrazines the oxepin form, rather than the benzene oxide, undergoes the cycloaddition.234 236 Usually, the central C-C double bond acts as dienophile. Oxepin reacts with 2,5-dimethyl-3,4-diphenylcyclopenta-2,4-dienone to give the cycloadduct 6 across the 4,5-C-C double bond of the heterocycle.234 The adduct resists thermal carbon monoxide elimination but undergoes cycloreversion to oxepin and the cyclopenta-dienone.234... 

The reaction of oxepin with dimethyl 5-oxo-2,3-diphenylcyclopenta-l,3-diene-l,4-dicarboxy-late takes a different course. Two products 7 and 8 can be isolated, 7 is the [4 + 2] adduct of the cyclopentadienone across the central C-C double bond of the oxepin, the other, 8, is thought to be a [4+6] cycloadduct across the triene system of the oxepin.237 In boiling benzene, the [4 + 2] adduct 7 undergoes no cycloreversion, but rearranges to the tricyclo[5.3.02,4]deca-5,8-dien-10-one system.237 The [4+6] adduct, however, is stable under thermal conditions. 

Diels-Alder cycloaddition of 2/f-azirines 23 with cyclopentadienones provides 3//-azepines 25 in excellent yields by electrocyclic ring opening, with concomitant loss of carbon monoxide, of the initially formed, nonisolable cycloadducts 24, followed by a [1,5]-H shift in the resulting 2//-azepines.31 108... 

Azepines 25 from Cyclopentadienones and 2//-Azirines 23 General Procedure 31... 

As anticipated, unsymmetrically substituted cyclopentadienones yield mixtures of two isomeric azepines, e.g. 26 and 27.31... 

Subsequently, it was shown that cyclopentadienones are very efficient in trapping 2//-azirines formed transiently during the thermolysis of vinyl azides,109 and that 3//-azepines may be formed by heating the vinyl azide directly in the presence of the cyclopentadienone rather than with the often foul-smelling 2//-azirine.31,109 An example of this procedure is the preparation of the dihydro-3/f-naphth[2,l-fe]azepine 29 by thermolysis of 4-azido-l,2-dihydronaph-thalene (28) with 2,5-dimethyl-3,4-diphenylcyclopentadienone in refluxing toluene. 

The 3//-azepines obtained by cycloaddition of azirines to cyclopentadienones (see Section 3.1.1.1.2.) are thought to arise from the initially formed 2/7-azepines by [1,5]-H suprafacial sigmatropic shifts.31-108 In contrast, 1/Z-azepine 9 results from the thermal rearrangement of the nonisolable 2//-azepine-2-carboxylate 8.13 Presumably, the 1 //-azepine is stabilized, relative to the 3//-isomer, by intramolecular hydrogen bonding between the NH and the adjacent ester group. 

Azcpincs under acid conditions reportedly117-225 yield aniline derivatives although ring contraction to pyridines is more usual. Thus, highly substituted 3//-azepines, e.g. 28, with a vacant 7-position, formed by cycloaddition of 2//-azirines with cyclopentadienones, on heating in acetic acid isomerize rapidly to the correspondingly substituted anilines 29.117... 

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... 

Trithiadiazepyne 1 reacts with some cyclopentadienones to give Diels-Alder adducts 5, which represent remarkably stable norbornadienones. However, on heating the cycloadducts readily lose carbon monoxide to yield benzotrithiadiazepines 6, which are described in the following section.393... 

Other cyclopentadienones give norbornadienones 4 (see previous section), which lose carbon monoxide on heating to give benzotrithiadiazepines 5.393... 

Hofmann elimination to correspondingly substituted cyclopentadienones which, depending on the nature and the nucleophilicity of the base as well as the nature of the substituents RLand Rs, undergo [2+2] or [4+2] cyclodimerization or in situ Michael addition to yield compounds 69, 70, and 71, respectively (Scheme 14) [44,70]. 

Several 4-alkoxy-2,3-diphenyl- and one 4-methoxy-2,3-bis(trimethylsilyl)-substituted cyclopentadienones have been isolated as reasonably stable compounds, see Herndon JW, Patel PP (1997) Tetrahedron Lett 38 59... 

Harano and colleagues [48] found that the reactivity of the Diels-Alder reaction of cyclopentadienones with unactivated olefins is enhanced in phenolic solvents. Scheme 6.28 gives some examples of the cycloadditions of 2,5-bis-(methoxycar-bonyl)-3,4-diphenylcyclopentadienone 45 with styrene and cyclohexene in p-chlorophenol (PCP). Notice the result of the cycloaddition of cyclohexene which is known to be a very unreactive dienophile in PCP at 80 °C the reaction works, while no Diels-Alder adduct was obtained in benzene. PCP also favors the decarbonylation of the adduct, generating a new conjugated dienic system, and therefore a subsequent Diels-Alder reaction is possible. Thus, the thermolysis at 170 °C for 10 h of Diels-Alder adduct 47, which comes from the cycloaddition of 45 with 1,5-octadiene 46 (Scheme 6.29), gives the multiple Diels-Alder adduct 49 via decarbonylated adduct 48. In PCP, the reaction occurs at a temperature about 50 °C lower than when performed without solvent, and product 49 is obtained by a one-pot procedure in good yield. 

Spectroscopic measurements indicate that PCP forms hydrogen bonds with carbonyl oxygen atoms of cyclopentadienone in both the ground and transition states, but the transition state is more effectively stabilized than the ground state, so a rate enhancement is observed. 

These molecules must be regarded as essentially nonaromatic, although with some aromatic character. Tropolones readily undergo aromatic substitution, emphasizing that the old and the new definitions of aromaticity are not always parallel. In sharp contrast to 44, cyclopentadienone (46) has been isolated only in an argon matrix below 38 Above this temperature, it dimerizes. Many earlier attempts to prepare it were unsuccessful. As in 44, the electronegative oxygen atom draws electron to itself, but in this case it leaves only four electrons and the molecule is unstable. Some derivatives of 46 have been prepared. ... 

For a review of cyclopentadienone derivatives and of attempts to prepare the parent compound, see Oglianiso, M.A. Romanelli, M.G. Becker, E.I. Chem. Rev., 1965,65,261. For a monograph on metallocenes, see Rosenblum, M. Chemistry of the Iron Group Metallocenes Wiley NY, 1965. For reviews, see Lukehart, C.M. Fundamental Transition... 

Diels-Alder reactions of cyclopentadienones with alkynes... 

In line with a second novel synthetic principle, the authors further developed the repetitive Diels-Alder procedure, in which monomers containing cyclopentadienone (dienophile) units were reacted with protected/deprotected ethynylene functions (see [31]). In this way, they generated a novel class of highly arylated phenylene dendrimers 46, starting from a 3,3, 5,5 -tetraethynyl-substituted biphenyl core [60]. 

Cyclopentadienone iron alcohol complexes like 37 were generated from the reactions of [2,5-(SiMe3)2-3,4-(CH2)4(ri -C4COH)]Fe(CO)2H (36) and aromatic aldehydes [47]. This process can be used for the iron-catalyzed hydrogenation of aldehydes (Fig. 18 and Fe-H Complexes in Catalysis ). 

Casey CP, Guan H (2009) Cyclopentadienone iron alcohol complexes synthesis, reactivity, and implications for the mechanism of iron-catalyzed hydrogenation of aldehydes. J Am Chem Soc 131 2499-2507... 

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