Cyclopentadienone substituted

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

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

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... 

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... 

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. ... 

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]. 

The ring system can be generated by D-A addition of a substituted cyclopentadienone and an alkyne. A reaction sequence involving addition followed by CO elimination can be used for the synthesis of highly substituted benzene rings.308... 

Cyclic dienes also react readily with ADC compounds although in many cases the initial adducts are not isolable. 1-Substituted pyrid-2-ones give Diels-Alder adducts with ADC compounds, although 2-pyridone itself gives only the substitution product (e.g., 113).174 2-Pyrone gives a 1 2 adduct with PTAD, since the initial adduct (114) rapidly loses C02 to generate a diene which then reacts with more PTAD.175 The initial adducts of ADC compounds with cyclopentadienones,176 and 3,4-dimethyl-1-phenylphos-phole 1-sulfide (115)177 also regenerate a diene by loss of CO and PhP=S, respectively. 

A more complex reaction is involved in the cooligomerization of acetylenes and tert-butyl isocyanide using nickel acetate as the catalyst (Scheme 20)43 the nature of intermediate complexes leading to the formation of 2-cyano-5-terf-butylaminopyrroles has not been established. Cocyclization of tert-butyl isocyanide with coordinated hexafluoro-2-butyne gives rise to coordinated cyclopentadienone anils for molybdenum systems,44 hence the nature of acetylene substitutents and of the organometallic catalyst play crucial roles in these processes. The pyrrole products from the former reaction can be decomposed by sulfuric acid and the overall sequence provides a simple synthesis of 5-amino-2-cyanopyrroles (Scheme 20). 

Gandolfi and coworkers301 studied the periselectivity in the reactions of substituted cyclopentadienones with iV-aryl-8-azahcptafulvcncs. The reactions proved to produce mainly [6 + 4] cycloadducts, along with some [8 + 2] and [4 + 2] cycloadducts, as illustrated by the reaction between azaheptafulvene 488 and cyclopentadienone 489 which... 

Low-temperature photochemical cyclization of alkynes bearing a bulky substituent, mediated by CpCo(CO)2, proceeds with CO insertion to give cyclopentadienone complexes. Higher reaction temperatures lead to cyclotrimerization. The intramolecular variant of this reaction gives the bicyclic cyclopentadienones 139 and 139 (equation 19)142. Cyclization of unsymmetrically substituted diynes with the chiral R CpCo(CO)2 (R = 8-phenylmenthyl) leads to the formation of a mixture of diastereomers modest diastere-oselectivity was found. 

We are able to functionalize our polyphenylene dendrimers via three different methods the use of functionalized cyclopentadienones, polymer-analogous reactions (group conversions), and electrophilic aromatic substitution. 

In case of fluorenone, the situation is reversed. A general bond-order increase is expected when an electron is added to a substituted cyclopentadienone such as fluorenone (Scheme 2.39). 

Substituted cyclopentadienones react with iron carbonyls to form stable, diamagnetic 7r-co triplexes of the type [Fe(CO)3(cyclopentadienone)] (215). The proposed structure is shown in (XX). These complexes undergo reactions typical of metal carbonyls, e.g., displacement of carbon monoxide by tertiary phosphines, but the carbonyl group of the ligand does not show reactions characteristic of a keto-group. These complexes are also formed by interaction of acetylenes with iron carbonyls (see Section VI,C). Interaction of tetracyclone and Fe3(CO)i2 gives unstable complexes which contain the sandwich anion [Fe(tetracyclone)2]2 analogous to the anion (XXV) (215). 

Many complexes of conjugated ketones are also known, such as the iron tricarbonyl complexes of substituted cyclopentadienones 30), although reaction with chromium hexacarbonyl occurs only if phenyl substituents are available for tt complexing 31). A common difficulty of preparing complexes of heterocyclics is the ability of the heteroatom to form o bonds with the metal. 

In the case of tt complexes of substituted cyclopentadienones, such as the iron tricarbonyl derivatives prepared by Weiss and H libel (30), qualitative molecular-orbital theory (20) predicted a considerable reduction of the ketonic carbonyl bond order. It was observed that the ketonic carbonyl frequency dropped by as much as 65 cm-1, in agreement with theory. A similar explanation can also be provided in terms of valence bond theory (Fig. 14). It has been suggested that n complexing of arenes such as benzene results in loss of aromaticity of the ring in contrast to the dicyclopentadienyl... 

Cyclopentadienones.2 Cyclopentadienones are generally not useful synthons because of their ready dimerization. A new synthesis of more stable substituted cyclopentadienones involves cyclization of two substituted alkynes with carbon monoxide by the [2 + 2 + 21cycloadditions shown in equations (I) and (II). 

SCS-MP2 and the new perturbative B2-PLYP density functional methods provide accurate reaction barriers and outperform MP2 and B3-LYP methods when applied to the 1,3-dipolar cycloaddition reactions of ethylene and acetylene.39 Phosphepine has been shown to catalyse the asymmetric 3 + 2-cycloaddition of allenes with a variety of enones (e.g. chalcones) to produce highly functionalized cyclopentenes with good enantiomeric excess.40 The AuPPh3SbF6 complex catalysed the intramolecular 3 + 2- cycloaddition of unactivated arenyne- (or enyne)-yne functionalities under ambient conditions.41 A review of the use of Rh(I)-catalysed 3 + 2-cycloadditions of diaryl-and arylalkyl-cyclopropenones and aryl-, heteroaryl-, and dialkyl-substituted alkynes to synthesise cyclopentadienones for use in the synthesis of natural products, polymers, dendrimers, and antigen-presenting scaffolds has been presented.42... 

Most hydroxy-substituted azaheteroaromatic compounds exist predominately in the NH/carbonyl tautomeric form, and such compounds have been widely discussed. By contrast, aza derivatives of cyclopentadienone, and of quinones, although they have received considerable attention in the last 30 years, have never previously been reviewed. In the present overview, we have attempted to collect the information on such derivatives, as well as including l-azetin-4-ones, i.e., of those azaheterocycles which contain in the ring at least one carbonyl group and one or two nitrogen atoms in the form of C=N or N=N bonds. Many of these compounds are unstable but their importance as reactive intermediates is established and many of them could be used as synthons in the preparation of pharmacologically active compounds. 

On the other hand, a large number of 3,4-diaza-2,4-cyclopentadienone derivatives have been described. The earliest studies about these compounds were carried out by Freeman et al. (69JOCI87, 69JOC194), who synthesized a series of substituted 3,4-diazacyclopentadienones, N-oxides and Af.N -dioxides. Treatment of /)-aryl-a,/3-unsaturated oximes with nitrous acid gave compound (81) because the 3,4-diaza-2,4-cyclopen-tadienone containing adjacent /V-oxide functions was the only structure consistent with all the spectral data. 

The synthetic building block of cyclopentadienones, prepared via appropriately substituted benzils, places certain limitations on the convergent method... 

The cycloaddition reactions of nitrile oxides with several substituted cyclopentadienones led to the formation of only one regioisomer the cyclopenta[2,3-d]isoxazol-4-one (6) structure for five 1 1 adducts was fully supported by X-ray analysis (79JHC731). 

A borderline situation is encountered concerning the treatment of substituted fulvenes and cyclopenladienylides (of the general formula 6) as well as cyclopentadienones (8) in this review. They will not be considered if resonance form 6 seems to be prevalent. However, if the characterization indicates a dominance of the ylidic mesomer 7, reference will be made (see under Fulvenes, cyclopentadienylides in the Appendix). We note that... 

---