Cyclobutadienes push-pull

A very elegant formation of heptafulvenes from triafulvenes was found by Gompper294 utilizing the push-pull stabilized cyclobutadiene 511 in the case of 1,2-diphenyl-4,4-diacetyl triafulvene (180) the 4-acyl group caused additional ring closure yielding cyclohepta(b)furan 514 ... 

Unlike pyridine (85MI1), tri-rm-butylazete (101) has, as judged from its reactivity, olefinic character [88AG(E)(27)272]. As in the case of cyclobutadiene, the push-pull substitution [69CC240 88AG(E)(24)1437] promotes the stabilization of azete, as has been demonstrated by the isolation of the first thermodynamically stable substituted azete, tris(dimethylamino)azete... 

Fig. 13a-c. Push-pull cyclobutadienes as biradicaloid species [20]. a The two degenerate frontier orbitals localize upon introduction of donor and acceptor substituents as shown and energetically split by the energy gap b. b Ground and S, states can therefore be assigned hole-pair (hp - one doubly occupied, one unoccupied frontier orbital) and dot-dot character (dd - two singly occupied frontier orbitals) similarly as in the case of twisted ethylene (Fig. 7). The energetic order is determined by the interplay of electron repulsion and the orbital energy gap h which depends on the substituents, c In-plane relaxational deformations in Si can lead to an approach of S and S, and thus to fluorescence red shifts or even to photochemical funnels...

The other type of stable cyclobutadiene has two electron-donating and two electron-withdrawing groups,and is stable in the absence of water. An example is 58. The stability of these compounds is generally attributed to the resonance shown, a type of resonance stabilization called the push-pull or captodative... 

Push- Pull-, Hetero- and Other Stabilized Cyclobutadienes.118... 

Stabilization of cyclobutadienes may be achieved, if the cyclobutadiene structure is only one of the important resonance forms. This is the case in the so-called push-pull cyclobutadienes. 

Indeed, push-pull cyclobutadienes of the type 12 are stable isolable substances, with seemingly strong contributions of canonical structure 11c to the ground-state21, 22K An X-ray structural determination of 12 (R = CH3CH2, R =... 

Push-pull stabilized benzocyclobutadienes are not known, but several benzo-cyclobutadienes are isolable as stable transition metal complexes9, notably 29, synthesized by Pettit and his co-workers41. The diene character of the uncomplexed... 

From the spectroscopic data of Sections II,A-C it can be concluded that partially saturated heterocyclic /J-enamino esters possess a specific push-pull stabilization owing to the conjugation of electron-donating amino group and electron-withdrawing ester function.67 Push-pull stabilization is responsible for the remarkable stability of this class of compounds similar stabilization allows isolation of certain antiaromatic cyclobutadienes or nonaromatic o-quinodimethanes.68 2-Aminofurans, -pyrroles, and -thiophenes, because of their extremely large electron density, are generally unstable and sensitive... 

Other push-pull cyclobutadienes have been prepared, including... 

All these push-pull cyclobutadienes react readily with both electrophiles and nucleophiles [72,78,79]. With electrophiles reaction takes place at a ring atom carrying an ester group with nucleophiles ring-opening follows the initial nucleophilic attack, e.g. 

Hess and Sehaad have searched for possible push-pull stabilized cyclobutadienes using their adaptation of the Huckel method. In all the cases investigated, alternate electron-donating and electron-withdrawing groups were more effective in stabilizing the cyclobutadiene than the alternative substitution pattern. The most stable systems found by this method were the fluorinated species (9). 

The resulting dimers of the original phosphacumulene ylides are of particular interest because one of their resonance forms represents another type of push-pull cyclobutadiene. 

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