Cyclobutadienes, cycloaddition

Scheme 79 Snapper s total synthesis of asteriscanolide (116) by sequential intramolecular cyclobutadiene cycloaddition, ring-opening CM (ROCM), and Cope rearrangement [167]...

Limanto, J., Snapper, M. L. Sequential Intramolecular Cyclobutadiene Cycloaddition, Ring-Opening Metathesis, and Cope Rearrangement Total Syntheses of (+)- and (-)-Asteriscanolide. J. Am. Chem. Soc. 2000, 122, 8071-8072. 

Limanto. J. Snapper. M.L. Sequential intramolecular cyclobutadiene cycloaddition, ring-opening metathesis, and cope rearrangement Total syntheses of (-i-)- and... 

Within the cubane synthesis the initially produced cyclobutadiene moiety (see p. 329) is only stable as an iron(O) complex (M. Avram, 1964 G.F. Emerson, 1965 M.P. Cava, 1967). When this complex is destroyed by oxidation with cerium(lV) in the presence of a dienophilic quinone derivative, the cycloaddition takes place immediately. Irradiation leads to a further cyclobutane ring closure. The cubane synthesis also exemplifies another general approach to cyclobutane derivatives. This starts with cyclopentanone or cyclohexane-dione derivatives which are brominated and treated with strong base. A Favorskii rearrangement then leads to ring contraction (J.C. Barborak, 1966). 

Extrapolation from the known reactivity of cyclobutadiene would suggest that azetes should be highly reactive towards dimerization and as dienes and dienophiles in cycloaddition reactions and the presence of a polar C=N should impart additional reactivity towards attack by nucleophiles. Isolation of formal dimers of azetes has been claimed as evidence for the intermediacy of such species, but no clear reports of their interception in inter-molecular cycloaddition reactions or by nucleophiles have yet appeared. 

Intramolecular cycloadditions between cyclobutadiene and an oxygen-tethered unactivated alkene (alkyne) offers an attractive route to benzo[c]furans (Scheme 26, <96JA9196>). 

Until 1992, the only ethynylated cyclobutadiene complexes pertinent in the literature were 22 - 24, prepared by Fritch and Vollhardt using [2 -i- 2]-cycloaddition of suitable polyynes over CpCo(CO)2 [24]. No alkynylated derivatives of 25, however, had been prepared. 

The heptafulvenes 512/513 were believed to originate via dipolar (2 + 2) cycloaddition of a cyclobutadiene to triafulvene CVC2 bond according to the following scheme ... 

Recently, synthetic routes to longer ladderanes with C-C double bonds in the terminal rungs have been improved the [3]-, [5]-, and [7]ladderane derivatives were synthesized by the repeated cycloaddition of cyclobutadiene derivatives,30 and the [w]ladderane derivatives (w = 3,4,5,6,7,9) were synthesized by the successive and alternate cycloaddition with cyclobutadiene and dimethyl acetylenedicarboxylate.31 X-ray crystallographic analysis of the [5]ladderane derivative31 shows a corrugated backbone without twist, in contrast with saturated and unsubstituted ladderanes.22,23... 

Fig. 9 illustrates that the two acetylenic systems become nearly parallel at C1-C6 distances close to 3 A where the constructive overlap of the re-orbital with one of the re -nodes is compensated by a destructive overlap with the other rc -node (Fig. 9, bottom). From a conceptual point of view, the properties of the in-plane re-system at the 3 A threshold bear a striking resemblance to the interaction of the two re-bonds in D2h cyclobutadiene where the re-re interaction is zero and the re-re repulsion is considerable, thus accounting for the extreme instability of this antiaromatic molecule.41 Even more relevant is a comparison with the TS of the symmetry forbidden thermal [2S + 2S] cycloaddition (Fig. 10) which prompted us to call this region antiaromatic .42... 

Tetramethyl- or tetraphenyl- (cyclobutadiene)nickel dihalides undergo reductive ligand substitution with nitrogen donor ligands such as 2,2 -bipyridine or 1,4-diaza-1,3-dienes with the addition of sodium metal237. The 2,2/-bipyridyl ligand is readily displaced and reaction of this complex with a variety of olefins and alkynes leads to cycloaddition reactions with the cyclobutadiene ligand. 

Radiolysis of deca-2,8-diyne (57) results in an interesting cycloaddition, forming a cyclobutadiene radical cation (58 ) at 77 K without requiring annealing at higher... 

Radical cations (94 +) of simple alkynes (e.g., butyne) generated by y-irradia-tion in frozen matrices are stable at 77 K. Upon warming, changes in the ESR spectra support an interesting cycloaddition forming cyclobutadiene radical cations... 

Similarly, cycloaddition of 4-toluenenitrile oxide to cyclobutadiene gave a 2 2 adduct,26 whose configuration was not determined, but it is believed that this adduct was formed from the cycloaddition of nitrile oxide to the C —C double bonds of the major dimer of cyclobutadiene, namely A > -tricyclo[4.2.0.02 5]octa-3,7-diene.26... 

Cyclobutadienes represent very reactive alkenes that undergo both [2 + 2] as well as [4 + 2] cycloadditions. Both the cyclodimerizations, mixed [2 + 2] cycloadditions and Diels-Alder reactions of these reactive species have been reviewed (see Houben-Weyl, Vols. 4/4, p 231 and E 17 f, Section 10B). In most instances the initially formed cyclodimer is tricyclo[4.2.0.02-5]octa-3,7-diene (36) and has the all cis-syn configuration. This is attributed to the concerted [4n -I- 2n] cycloaddition mechanism in which stereochemical control is affected by secondary orbital interactions. 

Other reactive cyclobutadienes including the parent derivative have been reported to undergo cycloadditions with cycloalkenes to give tricycloalkenes. In these instances both syn-and anti-isomers are obtained.11-15... 

Electrophilic alkenes undergo cycloaddition to cyclobutadienes to give the ew/oproducts. 

Isolable sterically congested cyclobutadienes react with electrophilic alkenes under mild conditions. tert-Butyl 2,3,4-tri-tert-butylcyclobutadienecarboxylate undergoes cycloadditions with both tetracyanoethene17 and maleic anhydride1S at — 20 to — 30 °C to give the correspondingly substituted bicyclo[2.2.0]hexenes. 

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