Pentalene derivatives

In Sections III,A,4 and III,B,l,f, examples of ring transformation reactions have been mentioned. Here we shall discuss in detail the interesting case of isomerism between an eight-membered ring with an 8-7r-electron system (thus, in principle, antiaromatic) and a 10-n pentalene derivative (briefly mentioned in Section III,B,4,a). Scheme 14 shows... 

The analogous equilibrium was assumed for pentalene derivative 33 and its monocyclic form 40 (Equation 1) <2003HAC95> as well as for structure 41 and its open form 42 (Equation 2) <1997IJB399>. There are two... 

The reaction of compound 90 with imidoyl halides without added base gives the hydrochloride salts of 1,2,4-thiadiazol-5-imines 95. The corresponding free bases can be liberated from the salts 95 with weak bases such as bicarbonate, but these imines are labile and decompose readily to form the W-cyanamide 96 and sulfur. The free base can be treated in situ with a second electrophile, such as an acid chloride, imidoyl chloride, or aryldiazonium salt, affording pentalene derivatives 97-99. It should be noted that the compound 98 cannot be prepared in a one-pot procedure from compound 90 and 2 equiv of imidoyl chloride in the presence of base, in contrast to the corresponding dioxa compound 91 (Scheme 22) <1981BSB89, 1992JHC1317>. 

The question of aromaticity versus antiaromaticity and delocalized versus localized double bonds in pentalene (2) dates back to 1922, when Armit and Robinson compared it with naphthalene and postulated that the former might be similarly aromatic [32, 33]. While the first synthesis of a non-fused hexaphenylpentalene (38) [30] provided only some clues as to the non-aromatic reactivity of the pentalene skeleton, the tri-tert-butyl derivative 39, prepared and studied by Hafner et al. in great detail [31], gave a better insight. The ring-proton signals of this alkyl-substituted pentalene 39 are shifted upfield compared to those of fulvene (27) and other cyclic polyenes. This observation led to the conclusion that the pentalene derivative 39 should be an antiaromatic species. However, the results did not permit a distinction... 

An unstable and two stable pentalene derivatives that have been observed experimentally. 

A characteristically different mechanism appears to operate in alkyne trimerization systems based on PdCb." Cationic metal complexes are involved in which initial halide transfer to an alkyne carbon is followed by sequential linear insertion of two more alkyne moieties. Metallacyclopentadiene intermediates are not involved in this sequence. Unique to this mechanism is the subsequent ring-closure to a cy-clopentadienylmethyl metal derivative, which, via halide transfer back to the metal, eventually leads to benzene via a bicyclo[3.1.0] system (Scheme 27). Support for this mechanism comes in part from the isolation of methylcyclopentadienyl-derived structures in several cases, including pentalene derivatives from further alkyne insertion followed by a second ring-closure." ... 

Reduction of the ketone moiety in 24 with NaBH4 provides the corresponding alcohol as a 1 1 mixture of diastereoisomers, which is then transferred into the pentalene derivative by a Barton deoxygenation using thiocarbonyl diimidazole and tributyl tinhydride. Finally, desily-lation with AgNOs and KCN in EtOH provides the desired deprotected alkyne 25. 

Finally, we should mention some more complex fused systems with nonalternant rings. A good example is dicyclohepta[cc/, /3]pentalene derivative (methyl 10-ethyldicyclohepta[cc/,g/3]pentalene-5-car-boxylate) for which the hydrocarbon skeletone is shown in Figure 9 ... 

As shown in Scheme 6.37, some transition-metal-assisted ring-closure reactions of 1,2-diethynylbenzenes afford dibenzopentalene derivatives unexpectedly. Diederich s group designed a carbopalladation cascade reaction of easily accessible ew-dibromoolefins 286 and alkynes to furnish monobenzo-287 and mononaphthopentalenes 288 (Scheme 6.72). This successful methodology allows the preparation of asymmetrically substituted pentalene derivatives with only one fused aromatic system [167]. 

The above area has attracted much attention in recent years. Several new synthetic methods for symmetric and asymmetric tt-extended pentalene derivatives... 

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