Benzvalenes, cycloadditions

A mechanism involving benzvalene intermediates has been proposed for the formation of 27-29.37,39 Crossover experiments between various perfluoro(alkylpyridaztnes) and doubly 15N-labeled derivatives ruled out rearrangement mechanisms involving fragmentation or cycloaddition processes.38,39... 

The Diels-Alder reaction of hexakis(trifluoromethyl)benzvalene with furan had been reported 63) but not its stereochemistry. We have examined this reaction and found that it proceeds smoothly with many 1.3-dienes if the steric requirement of the olefin is not large in the transition state where exo-attack occurs as confirmed by X-ray analysis (97)96). Therefore, 2,3-dimethylbutadiene reacts smoothly with hexakis(trifluoromethyl)benzvalene whereas 1,3-pentadiene reacts very slowly and 2,4-hexadiene does not at all. The cycloaddition to cyclobutadiene gives a mixture of endo- and exo-adducts (98)97) ... 

Similarly, vinylcyclopropane (1) gives only [2 -I- 2] cycladdition products with TONE, leaving the three-membered ring intact (see Section 2.2.1.2.2.). Preorientation of the vinylcyclopropane in the synperiplanar or synclinal conformation (rule 1), as realized in the bicyclohexene system 2, should enhance the reactivity towards dienophiles however, this is clearly insufficient. Additional strain (rule 2) or jc-donor substitution (3, rule 3) are further options to enhance the reactivity per se they seem to be insufficient. The combination of two activating principles, however, realized in homofuran (4, rule 1 -t- 3) or benzvalene (6, rule 1 -f 2) sufficiently peps up the reactivity of the embedded vinylcyclopropane system such that homo-diene-type cycloadditions occur (see Section 2.2.1.2.1.). 

Formal [27t + 2ff + 2homo-homo-Diels-Alder) reactions include the cycloaddition of 4-phenyl-l,2-4-triazoline-3,5-dione to benzvalene (7), which served as the key step in the Katz prismane synthesis, and TCNE to 2-exo-methyl-endb-tricyclo[3.2.1.0 ]oct-6-ene (55). ... 

Irradiation of fluorinated furans did not give the valence isomers but, instead, the more familiar ring opening to cyclopropenylketones (130).298 In the presence of alkynes, however, a novel series of (2n + 27r)-cycloadditions occurs (Eq. 102).381 An analog of benzvalene has been obtained from the diphosphabenzene (46).129... 

Systematic studies of the cycloaddition of l,3,4-oxadiazin-6-ones with a variety of alkenes have been carried out. With benzvalene the tetracyclic ketone (110) is the major product (75%) accompanied in some instances (R = Ph, = C02Me) by a small amount of the fused ) -lactone (111) (Scheme 13—path a). Cycloaddition with the less reactive norbomene (Scheme 13—path b) yields ultimately the reduced a-pyrone (112), an enol-lactone <81AG(E)674, 85CB2940, 85JHC61, 90CB203i>. The enol-lactone is found to be produced more quickly and at a lower temperature if cycloaddition is carried out in the presence of trifluoroacetic acid. 

Fowler, F. W. Cycloadditions of N-methoxycarbonyl-2,3-homopyrrole . Angew. Chem. 1971, 83, 148-149. Angew. Chem. Int. Ed. Engl. 1971,10, 135-136. Baldwin, J., Pinschmidt, R. K. jr. The Cycloaddition of Bicyclo2.1.0pent-2-ene with Tetracyanoethylene . Tetrahedron Lett. 1971, 935-938. Christl, M., Brunn und E., Lanzendoerfer, F. Reactions of Benzvalene with Tetracyanoethylene,2,3-Dichloro-5,6-dicyano-p-benzoquinone, Chlorosulphonyl Isocyanate, and Sulphur Dioxide. Evidence for Concerted 1,4-Cycloadditions to a Vinylcyclopropane System . J. Am. Chem. Soc. 1984,106, 373-382. 

Ratio of cyclopentadiene The formation of bicyclopentene is formally an electro-cyclic reaction like that which converts the benzene system to a Dewar benzene [47], and the formation of 3 is crossed cycloaddition like that which converts benzene to benzvalene [48]. Stmctural derivatives of 3 were known prior to the isolation of the parent Masamune prepared 1,5-diphenyl tricyclo[2.1.0.0 Jpentan-3-one by photolysis of a diazocyclopropenyl ketone [49] (X-ray analysis of a derivative confirmed the structure in the face of skepticism [50,51] and Closs and Larrabee prepared 2,4-dimethyltricyclo[2.1.0.0 ]pentane [52] by heating the sodium salt of a cyclobutenyl tosylhydrazone both kinds of reactions are known to generate carbenes or carbe-noids, and presumably involve insertion of a carbene-type carbon into a carbon-carbon double bond ... 

Benzvalene (807) is now readily available and its reactions are being actively explored. Cycloadditions to the double bond can be effected without opening the bicyclobutane. Thus the dichloroketen adduct could be reduced with PhgSnH to (808) and the tosylhydrazone of this treated with a strong non-nucleophilic base to give (809) in 50 % yield. Attempts to open this to (810) by heat or Ag" catalysis failed. ... 

Figure 5.10. The benzvalene-benzene interconversion allowed or forbidden" (For clarity the arrows are drawn to depict the reverse reaction [ 2 + 2] cycloaddition, to which the same Rules apply)...

The Diels-Alder reactivity of (346) has been the subject of further theoretical study, and cycloadditions to perfluoromethylated benzvalenes have been exa-mined. ... 

In a synthesis of benzvalene-3highly strained benzene valence tautomer, is subjected to 1,3-dipolar cycloaddition with fulminic acid H-C=N-0, the parent compound of the nitrile oxide family (generated in situ from formhydroximoyl iodide/NEt3). The cycloadduct 33 on treatment with NaOCH3 affords the a-hydroxy nitrile 35, which is converted to the tosylate and finally to the a,P-unsaturated nitrile 34 by elimination of TosOH with KOtBu. 

Cycloaddition to benzvalenes A --Isoxazoline ring from nitrile oxides... 

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