Indene anions

Interest in this reaction was revived when the relevance of a carbene mechanism was realized, particularly following the demonstration (cf. SectionI,B) of a similar ring expansion of indene to 2-chloro-naphthalene by dichlorocarbene via the cyclopropane adduct. Indeed, at this time Nakazaki suggested that these reactions occurred by the addition of dichlorocarbene to the indolyl anion and subsequent rearrangement to the indolenine and, with loss of chloride ion, to the quinoline [Eq. (12)]. The preference of dichlorocarbene for... 

The ability of the stable carbene 218 to deprotonate acidic hydrocarbons was examined by NMR in (CD3)2S0.153 Indene (pJta = 20.1) was completely converted to its anion whereas 9-phenylxanthene (pAfa = 27.7) was not measurably deprotonated. The NMR spectra of 1 1 mixtures of 218 with fluorene (pXa = 22.9) and 2,3-benzofluorene (pA"a = 23.5) showed separate absorptions for the hydrocarbons and their anions. From the integration of these spectra, P a = 24.0 for 218 was derived. In THF, 218 failed to deprotonate fluorene but almost completely deprotonated indene. The proton transfer from hydrocarbons to 218 creates ions (ion pairs) from neutral species, which will be less favorable in solvents of lower polarity. 

The cyclopentadiene anion is stabilized by five equivalent resonance structures. The anion is an aromatic anion by virtue of it being a six-jr-electron system. The indenyl anion is stabilized by a total of seven resonance contributors. However, they are nonequivalent and all but one require that the aromatic cloud of the benzene ring is disrupted. Thus, while the negative charge is well delocalized, the resonance stabilization is less than that of the cyclopentadiene system. Thus the proton is not as easily removed, making indene a weaker acid. 

Probably, the same mechanism is involved in the conversion of 2-benzopyrylium formates 10 into indenes 118 on heating in formic acid (Section III,C,2), or in the formation of indanes 214 on heating the fluoro-borate 178 in hydrochloric acid with phosphorus or in glacial acetic acid with phosphonium iodide (71CB2984). The primary step of this conversion includes the addition of a hydride anion then the intermediate isochro-mene of type 107 becomes protonated and takes part in further transformations. 

It has been reported that 1 //-bcnz / indene (1), l//-benz[e]indene (2), l//-benz[/g] acenaphthylene (3), and 1 //-cyclopenta[/]-phcnanthrene (4), possessing planar carbon frameworks with a single secondary C(.sy 3)H2 centre, exhibit modest acidity in the gas-phase and in DMSO.1 The origin of their amplified acidity compared with cyclopentane, cyclopentene, and cyclopentadiene is the more pronounced anionic resonance, which distributes the negative charge over the whole planar carbon skeleton via mobile n-electrons. 

In some cases radical cations may undergo cycloadditions with an acceptor derived intermediate without prior proton transfer. This is observed especially for radical cations without sufficiently acidic protons, although it is not limited to such species. For example, the photoreaction of chloranil with 3,3-dimethylindene results in two types of cycloadducts [141]. In the early stages of the reaction a primary adduct is identified, in which the carbonyl oxygen is connected to the p-position of the indene (type B) in the later stages this adduct is consumed and replaced by an adduct of type A, in which the carbonyl oxygen is connected to the a-position. CIDNP effects observed during the photoreaction indicate that the type B adduct is formed from free indene radical cations, which have lost their spin correlation with the semiquinone anions. 

This mechanistic consideration is in accord with the result from arylation of indene (7) [5]. The arylation occurs exclusively at the 1,3-positions, those with the highest electron density, of intermediary indenyl anions (9). The related arylation of azulene also proceeds exclusively at 1,3-positions, interpreted as electrophilic aromatic substitution, again by an electrophilic aryl-Pd species [10]. 

Neodymium cyclopentadienyl complexes are also obtained in-situ by the addition of Cp-derivatives (e.g. indene, cyclopentadiene, pentamethyl-cyclopentadiene, tetramethylcyclopentadiene, di-tert-butylcyclopentadiene, methylcyclopentadiene and fluorene) to standard Nd-catalyst systems such as NdV/MAO. It can be assumed that the respective cyclopentadienyl-anions are formed by proton abstraction from the Cp-derivatives. In the homopolymerization of BD the addition of Cp-derivatives results in an increase of the 1,2-content of about 4-10%. In addition, the in-situ formed Nd Cp-derivatives... 

One catalyst system which allows for the copolymerization of BD and St is based on NdV and is activated by MAO. It is important to note that this catalyst system does not contain a halide donor. By the addition of cy-clopentadienyl derivatives (e.g. cyclopentadiene, indene, anthracene etc.) St incorporation is increased. It may be speculated that by the influence of MAO a proton is abstracted from the cyclopentadienyl derivatives added and that the resulting cyclopentadienyl-type anions coordinate to the active Nd sites [498,499]. In this way the activity pattern of Nd is changed and copolymerization of BD and St is made possible. As shown in the attached Table 28 increases of St incorporation occur simultaneously with decreases of cis-1,4-contents. 

Kochi et al. [111-113] have utilized time-resolved laser spectroscopy to examine the excited complexes. For the indene (IN)/TCNE system, excitation of the CT complex (532 nm) afforded not only absorption bands assigned to TCNE anion radical and IN cation radical, but also an absorption band due to an unidentified intermediate. Kochi proposed a 1,4-diradical or 1,4-zwitterionic intermediate, and postulated bond-formation as follows [111] ... 

The emission properties of anions as for instance carbanions of indene [28], xanthene or thioxanthene [29], fluorene [30, 31], diphenyl propene or penta-diene [32] were also discussed. It was shown that in certain cases, important changes may occur between ground and excited states, increasing the percentage of SSIP when the relaxed excited state is reached [33], Two different types of... 

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