Alkylidenecycloproparenes

The discovery of alkylidenecycloproparenes as a class of isolable compounds is the most significant event in recent cycloproparene chemistry. Methylidenecyclo-propabenzene (234) combines the structural features of benzocyclopropene (1), methylenecyclopropene (233), and trimethylenecyclopropane (235) in one and the same structure. Although the parent 234 has so far not been isolated, many substituted and annulated derivatives have been synthesized and charaeterized. Their unexpected stability contrasts sharply with that of the benzocyclopropenones 219, which are only observable as transient reaction intermediates. Reviews on the chemistry of alkylidenecycloproparenes are available. ... 

Other approaches to alkylidenecycloproparenes have been attempted without success. Aromatization of appropriate alkylidenecyclopropanes or their precursors could not be realized, and flash vacuum pyrolysis of methylene phthalide and 3-methylene-2-coumaranone afforded rearrangement products rather than alkylidenecycloproparenes via extrusion of 002. The photochemical or thermal decomposition of the sodium salt of benzocyclobutenone p-toluenesulfonyl hydrazone led to products derived from dimerization of the intermediate benzocy-clobutenylidene, or from its reaction with the solvent, but no ring-contracted products were observed. When the adduct of methylene-l,6-methano[10]annulene to dicyanoacetylene (249) was subjected to Alder-Rickert cleavage, phenylacety-lene (250) was formed, which derives reasonably from the parent 234. ... 

Alkylidenecycloproparenes are a subclass of fulvalenes, a family of molecules consisting of two conjugated ring systems, which are connected with a double... 

The CH2 group of cycloproparenes is relatively acidic. Extended HUckel calculations predict that the benzocyclopropenyl anion 294 should be a resonance-stabilized species, contrary to the cyclopropenyl anion 295 or cyclohepta-trienide (296), which are at least potentially antiaromatic. This prediction has been experimentally verified benzocyclopropene (1) may be deprotonated with BuLi, and the intermediate anion 294 has been trapped with trimethylsilane to afford 236. From the rate of hydrolysis of 236, the pX of 1 has been estimated to 36, i.e., some 5 units below that of toluene (pXj = 41). Theoretical calculations (STO-3G) give a pK of 33 for 1. Metallation at the CHj group of cycloproparenes is the key step for the synthesis of alkylidenecycloproparenes (see above) however, it should be noted that so far, no benzocyclopropenyl anions have... 

Almost all reactions of alkylidenecycloproparenes lead to opening of the cyclopropane ring. A notable exception to this is the reversible electrochemical reduction of237 and 240 which leads to the stablfe radical anions 396 and 397, with half-wave potentials of -2.32 and -1.93 V, respectively, and their oxidation to the quasi-stable radical cations 398 and 399 ( i/2(ox) = +0.68 and +0.81). The cations may be further oxidized to the corresponding very short-lived dications. In contrast, the photoelectron spectra of 237 and 240 reveal practically identical first-oxidation potentials of both compounds, which indicates that the difference in half-wave potentials for oxidation (in condensed phase) of 237 and 240 does not exist in the gas phase. This has been attributed to structure-specific solvation energies in the radical cations 398 and 399. °... 

While electrophilic reactions of alkylidenecycloproparenes are generally rapid, the nucleophilic pathways require more vigorous conditions. The only nucleophilic addition of cycloproparenes known is that of /-butoxide across the bridge bond of 237,240 and 241 which leads to the corresponding heptafulvenes 407. ... 

The third method for alkylidenecycloproparene synthesis provides triafulvene derivatives by way of cycloproparenylidene dimerization166. The reaction is limited in scope by the availability of appropriate carbene (carbenoid) precursors and thus far has provided 98a,b only (equation 22). Attempts to extend the method into the naphthalene series167 by using the known89 naphthalene analogue of 97a provides a very sensitive (air/light) deep red material that is fluorescent and, while uncharacterized, is likely the analogue of 98a. 

TABLE 4. Alkylidenecycloproparenes from 1 and 11 via Peterson olefination employing aldehydes and ketones... 

The lithiation of 1 has many synthetic applications beyond alkylidenecycloproparene synthesis. Thus Szeimies and Wimmer232 have employed 6 for the synthesis of a range of derivatives in which the cyclopropabenzenyl framework is maintained (Scheme 14). 

Reactions of the alkylidenecycloproparenes with rhodium(I) and platinum(O) reagents lead to metallacycles (Section VI.B.4). ... 

Unlike cyclopropabenzenones, alkylidenecycloproparenes are a well established class of compounds although parent 950 and its -naphtho[6] analogue have yet to be obtained. The... 

FIGURE 2. Calculated geometry of 9 (bond lengths in A) and polarity of selected alkylidenecycloproparenes... 

The UV spectra of the coloured aryl-substituted alkylidenecycloproparenes show long-wavelength absorption maxima, the positions of which are solvent dependent and in accord with the measured polarities (Table 4, Section II.G)153-157-158,294 297. Removal of the... 

Oxocycloproparenes are especially sensitive to electrophiles and nucleophiles and do not survive in solution but open to esters (Scheme 10, Section II.F)130,131. The chemistry of the alkylidenecycloproparenes is also dominated by reactions that involve ring opening and the compounds are far more sensitive to electrophiles310 than to nucleophiles140. In general. 

Alkylidenecycloproparenes react slowly with nucleophiles and then only under forcing conditions140. Reactions with r-BuOK (refluxing THF, several days) provide heptafulvenes in both the benzene and naphthalene series by nucleophilic addition to the strained bridge bond and not to the exocyclic double bond (equation 36). 

There have been no accounts of alkylidenecycloproparene photolyses, but it has been suggested that the exocyclic propadienone 143 is stable to short-wavelength irradiation516 317. 

Ketone 8 and the naphthalene homologue can be generated in solution " from singlet oxygen addition to various alkylidenecycloproparenes (Section VLB.2). However, attempts to construct 87 from 5,7-dibromodibenzo[ ,c]cycloheptadienone were unsuccessful " and the only known condensed analogue is 88 that has been matrix isolated prior to photodecarbonylation". ... 

---