Chichibabin hydrocarbon

The structure of the parent Chichibabin hydrocarbon has been ascertained by X-ray analysis. The two phenyls of the biphenyl system are coplanar in the solid state on account of the weak repulsions between the central o-protons (Montgomery et al., 1986). 

Early investigations indicated that consisted of less than 0.2% biradical 3, but that compound the Chichibabin hydrocarbon, was 5-10% biradical based upon p to jor-hydrogen conversion or esr. However a more recent esr stud has led to the conclusion that the paramagnetic species in is in fact the dimer. On the other hand because of their reactivity and the impossibility of drawing classical valence bond structures, with no unpaired electrons, the Schlenk hydrocarbons ( > ) were considered to be diradicals. 

Shimizu and Hiyama [129] reported that the palladium-catalyzed double-cross-coupling reaction of 9-stannafluorenes 213 with 1,1-dihaloethylene 214 yields the dibenzofulvene 215 (Scheme 6.57a). The 9-staimafluorene 213, an equivalent of a 1,4-dimetalated reagent, is available from the corresponding 2,2 -dihalobiphenyls. The reaction was applied to the synthesis of the jt-extended fulvalene derivative 217 from 216. The former is a stable benzaimulated Chichibabin hydrocarbon possessing an open-shell diradical character (Scheme 6.57b) [130]. 

In connection with Chichibabin s hydrocarbon it is appropriate to mention that there is little or no resonance between states of different multiplicity.4 Thus the singlet-triplet transition represented by the covalent and diradical structures of the hydrocarbon is a true equilibrium and not an example of resonance. 

The addition product (II) is also called Chichibabin s hydrocarbon and should not be confused with Chichibabin s (diradical) hydrocarbon of formula I. 

We have already seen that triphenylmethyl radical reacts both with itself to form Chichibabin s hydrocarbon (formula II, not the diradical) and with halogens and oxygen. The reactions of the stable radical are important because of the expectation that radicals too unstable to isolate will betray their presence by giving similar products. 

Sodium amalgam rather than sodium is used in the preparation of triphenylmethyl from the halide the mercury prevents the formation of sodium triphenylmethyl and of Chichibabin s hydrocarbon. 

The type of reactivity shown by the formation of Chichibabin s hydrocarbon extends to the activation of substituents other than hydrogen as well. For example, molecular silver will not usually remove fluorine from organic compounds, not even from triphenylmethyl fluoride. Yet the radical parafluorophenyldiphenylmethyl reacts with molecular silver to give silver fluoride.88... 

Historically, it took quite some time before the diradical paradox associated with the Chichibabin [9], Thiele [10] and Schlenk [11] hydrocarbons was resolved by epr experiments and by the reasoning based on these topological considerations (Platz, 1982) only the last hydrocarbon was concluded to have a ground triplet state (Kothe et al., 1970 Luckhurst et al., 1971). 

Dimerization is well known to be a side reaction in the Chichibabin reaction (78RCR1042). In fact, dimerization can be the only product with certain alkylpyridines (79USP4177349 81USP4267335). Under normal Chichibabin conditions, heating a heterogeneous mixture of sodium amide in a hydrocarbon at atmospheric pressure with a branched chain alkylpyridine leads to... 

Early work on the amination of quinoline and substituted quinolines showed that the parent compound gave only a 32% yield of 2-aminoquinoline (78) in aromatic hydrocarbons (20MI1). Many derivatives, including quinoline carboxylic acids and amides, formed amino products more readily. However, amino- and hydroxyquinolines do not participate in the Chichibabin reaction (78RCR1042). 

The case of Chichibabin s hydrocarbon (CflH5)2-CC6H4—CeH4-C (C6H5)a is somewhat more complicated. This hydrocarbon is deeply coloured and readily reacts with oxygen and for these reasons the suggestion was made that it may exist in the form of the bi-radical XIX,... 

The classical Chichibabin reaction is normally to be carried out on heating (>100°C) in a media, which is inert towards sodium amide (aromatic hydrocarbons, AT,iV-diaIkylanilines, a mineral oil), or without any solvent at all. The aminatirm under these heterogeneous conditions is supposed to proceed on the surface of sodium amide particles, which are insoluble in all abovementioned solvents. Vigorous hydrogen gas evolution and intense red color are typical indicators of the reaction progress. 

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